U.S. patent application number 16/596579 was filed with the patent office on 2020-02-06 for analyte sensor with increased reference capacity.
The applicant listed for this patent is DexCom, Inc.. Invention is credited to Sebastian Bohm, Daiting Rong, Matthew D. Wightlin.
Application Number | 20200037938 16/596579 |
Document ID | / |
Family ID | 49158264 |
Filed Date | 2020-02-06 |
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United States Patent
Application |
20200037938 |
Kind Code |
A1 |
Rong; Daiting ; et
al. |
February 6, 2020 |
ANALYTE SENSOR WITH INCREASED REFERENCE CAPACITY
Abstract
Systems and methods of use for continuous analyte measurement of
a host's vascular system are provided. In some embodiments, a
continuous glucose measurement system includes an electrochemical
sensor incorporating a silver/silver chloride reference electrode,
wherein a capacity of the reference electrode is controlled.
Inventors: |
Rong; Daiting; (San Diego,
CA) ; Bohm; Sebastian; (San Diego, CA) ;
Wightlin; Matthew D.; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DexCom, Inc. |
San Diego |
CA |
US |
|
|
Family ID: |
49158264 |
Appl. No.: |
16/596579 |
Filed: |
October 8, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15370357 |
Dec 6, 2016 |
10470691 |
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16596579 |
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15153586 |
May 12, 2016 |
9517025 |
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15370357 |
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13784523 |
Mar 4, 2013 |
9351677 |
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15153586 |
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12829296 |
Jul 1, 2010 |
8828201 |
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13784523 |
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61222716 |
Jul 2, 2009 |
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61222815 |
Jul 2, 2009 |
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61222751 |
Jul 2, 2009 |
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61706055 |
Sep 26, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 2562/0209 20130101;
A61B 2562/043 20130101; A61B 5/14532 20130101; A61B 2562/125
20130101; A61B 2560/0223 20130101; A61B 5/14865 20130101 |
International
Class: |
A61B 5/145 20060101
A61B005/145; A61B 5/1486 20060101 A61B005/1486 |
Claims
1. An electrochemical sensor for measuring an analyte concentration
in a host, comprising: a working electrode configured to measure a
concentration of an analyte; and a reference electrode comprising a
material that depletes during sensor use, wherein the material is
configured to be regenerated during sensor use, and wherein a rate
of material depletion during sensor use substantially correlates
with a rate of material regeneration during sensor use over a time
period.
2. The electrochemical sensor of claim 1, wherein a correlation
between the rate of material regeneration and the rate of material
depletion is positive.
3. The electrochemical sensor of claim 1, wherein the sensor is
configured to increase the rate of material regeneration responsive
to an increase in the rate of material depletion.
4. The electrochemical sensor of claim 1, wherein the sensor is
configured to decrease the rate of material regeneration responsive
to a decrease in the rate of material depletion.
5. The electrochemical sensor of claim 1, wherein at least a
portion of the reference electrode is covered with an enzyme
layer.
6. The electrochemical sensor of claim 5, wherein the enzyme is an
oxidase enzyme.
7. The electrochemical sensor of claim 1, wherein the analyte is
glucose.
8. The electrochemical sensor of claim 1, wherein the material is
silver chloride.
9. The electrochemical sensor of claim 1, wherein the reference
electrode is formed of a chloridized elongated silver body.
10. The electrochemical sensor of claim 1, wherein the enzyme layer
has a thickness from about 0.01 microns to about 12 microns
thick.
11. The electrochemical sensor of claim 1, wherein a percentage of
a surface area of the reference electrode covered with the enzyme
layer is from about 10% to about 100%.
Description
INCORPORATION BY REFERENCE TO RELATED APPLICATIONS
[0001] Any and all priority claims identified in the Application
Data Sheet, or any correction thereto, are hereby incorporated by
reference under 37 CFR 1.57. This application is a continuation of
U.S. application Ser. No. 15/370,357, filed Dec. 6, 2016, which is
a continuation of U.S. application Ser. No. 15/153,586, filed May
12, 2016, now U.S. Pat. No. 9,517,025, which is a divisional of
U.S. application Ser. No. 13/784,523 filed Mar. 4, 2013, now U.S.
Pat. No. 9,351,677, which is a continuation-in-part of U.S.
application Ser. No. 12/829,296 filed Jul. 1, 2010, now U.S. Pat.
No. 8,828,201, which claims the benefit of U.S. Provisional
Application No. 61/222,716, filed Jul. 2, 2009, U.S. Provisional
Application No. 61/222,815, filed Jul. 2, 2009, and U.S.
Provisional Application No. 61/222,751, filed Jul. 2, 2009. U.S.
application Ser. No. 13/784,523 claims the benefit of U.S.
Provisional Application No. 61/706,055 filed Sep. 26, 2012. Each of
the aforementioned applications is incorporated by reference herein
in its entirety, and each is hereby expressly made a part of this
specification.
FIELD OF THE INVENTION
[0002] Systems and methods of use for continuous analyte
measurement of a host's vascular system are provided. In some
embodiments, a continuous glucose measurement system includes an
electrochemical sensor incorporating a silver/silver chloride
reference electrode, wherein a capacity of the reference electrode
is controlled.
BACKGROUND OF THE INVENTION
[0003] Diabetes mellitus is a disorder in which the pancreas cannot
create sufficient insulin (Type I or insulin dependent) and/or in
which insulin is not effective (Type 2 or non-insulin dependent).
In the diabetic state, the victim suffers from high blood sugar,
which can cause an array of physiological derangements associated
with the deterioration of small blood vessels, for example, kidney
failure, skin ulcers, or bleeding into the vitreous of the eye. A
hypoglycemic reaction (low blood sugar) can be induced by an
inadvertent overdose of insulin, or after a normal dose of insulin
or glucose-lowering agent accompanied by extraordinary exercise or
insufficient food intake.
[0004] Conventionally, a person admitted to a hospital for certain
conditions (with or without diabetes) is tested for blood sugar
level by a single point blood glucose meter, which typically
requires uncomfortable finger pricking methods or blood draws and
can produce a burden on the hospital staff during a patient's
hospital stay. Due to the lack of convenience, blood sugar glucose
levels are generally measured as little as once per day or up to
once per hour. Unfortunately, such time intervals are so far spread
apart that hyperglycemic or hypoglycemic conditions unknowingly
occur, incurring dangerous side effects. It is not only unlikely
that a single point value will not catch some hyperglycemic or
hypoglycemic conditions, it is also likely that the trend
(direction) of the blood glucose value is unknown based on
conventional methods. This inhibits the ability to make educated
insulin therapy decisions.
[0005] A variety of sensors are known that use an electrochemical
cell to provide output signals by which the presence or absence of
an analyte, such as glucose, in a sample can be determined. For
example, in an electrochemical cell, an analyte (or a species
derived from it) that is electro-active generates a detectable
signal at an electrode, and this signal can be used to detect or
measure the presence and/or amount within a biological sample. In
some conventional sensors, an enzyme is provided that reacts with
the analyte to be measured, and the byproduct of the reaction is
qualified or quantified at the electrode. An enzyme has the
advantage that it can be very specific to an analyte and also, when
the analyte itself is not sufficiently electro-active, can be used
to interact with the analyte to generate another species which is
electro-active and to which the sensor can produce a desired
output. Such conventional sensors can employ a silver/silver
chloride reference electrode. Over time, such a reference electrode
becomes depleted, as silver ion is converted to silver metal. As
silver ion is depleted, reference electrode capacity decreases,
reducing the stability of the reference electrode such that glucose
sensor becomes less linear.
SUMMARY OF THE INVENTION
[0006] In a first aspect is provided an implantable electrochemical
sensor for measuring an analyte concentration in a host,
comprising: a working electrode configured to measure a
concentration of an analyte; and a silver/silver chloride reference
electrode, wherein at least a portion of the silver/silver chloride
reference electrode is covered with an enzyme layer, wherein the
enzyme layer is configured, in vivo, to generate hydrogen peroxide
upon exposure to a substrate, whereby the hydrogen peroxide
regenerates silver chloride of the reference electrode such that a
reference capacity of the reference electrode is increased.
[0007] In an embodiment of the first aspect, the enzyme is an
oxidase enzyme.
[0008] In an embodiment of the first aspect, the enzyme is glucose
oxidase.
[0009] In an embodiment of the first aspect, the substrate is
selected from the group consisting of glucose, urate, ascorbate,
citrate, L-lactate, succinate, D-glucose, and ethanol.
[0010] In an embodiment of the first aspect, the analyte is
glucose.
[0011] In an embodiment of the first aspect, the substrate is the
analyte.
[0012] In an embodiment of the first aspect, the reference
electrode comprises a chloridized elongated silver body.
[0013] In an embodiment of the first aspect, the sensor further
comprises a diffusion barrier configured to substantially block
diffusion of hydrogen peroxide between the silver/silver chloride
reference electrode and the working electrode.
[0014] In an embodiment of the first aspect, the diffusion barrier
is a discontinuity of a sensor membrane between the silver/silver
chloride reference electrode and the working electrode.
[0015] In an embodiment of the first aspect, the diffusion barrier
is a spatial diffusion barrier.
[0016] In an embodiment of the first aspect, the diffusion barrier
is a physical diffusion barrier.
[0017] In an embodiment of the first aspect, the diffusion barrier
is a temporal diffusion barrier.
[0018] In a second aspect is provided an electrochemical sensor for
measuring an analyte concentration in a host, comprising: a working
electrode configured to measure a concentration of an analyte; and
a reference electrode, wherein at least a portion of the reference
electrode is covered with an enzyme layer, wherein the enzyme layer
is configured, in vivo, to generate a reference electrode
regenerating species upon exposure to a substrate, whereby the
reference electrode regenerating species regenerates a component of
the reference electrode such that a reference capacity of the
reference electrode is increased.
[0019] In an embodiment of the second aspect, the enzyme is an
oxidase enzyme.
[0020] In an embodiment of the second aspect, the enzyme is glucose
oxidase.
[0021] In an embodiment of the second aspect, the substrate is
selected from the group consisting of glucose, urate, ascorbate,
citrate, L-lactate, succinate, D-glucose, and ethanol.
[0022] In an embodiment of the second aspect, the analyte is
glucose.
[0023] In an embodiment of the second aspect, the substrate is the
analyte
[0024] In an embodiment of the second aspect, the reference
electrode comprises a chloridized elongated silver body.
[0025] In an embodiment of the second aspect, the reference
electrode regenerating species is hydrogen peroxide.
[0026] In a third aspect is provided an electrochemical sensor for
measuring an analyte concentration in a host, comprising: a working
electrode configured to measure a concentration of an analyte; and
a reference electrode comprising a material that depletes during
sensor use, wherein the material is configured to be regenerated
during sensor use, and wherein a rate of material depletion during
sensor use substantially correlates with a rate of material
regeneration during sensor use over a time period.
[0027] In an embodiment of the third aspect, a correlation between
the rate of material regeneration and the rate of material
depletion is positive.
[0028] In an embodiment of the third aspect, the sensor is
configured to increase the rate of material regeneration responsive
to an increase in the rate of material depletion.
[0029] In an embodiment of the third aspect, the sensor is
configured to decrease the rate of material regeneration responsive
to a decrease in the rate of material depletion.
[0030] In an embodiment of the third aspect, at least a portion of
the reference electrode is covered with an enzyme layer.
[0031] In an embodiment of the third aspect, the enzyme is an
oxidase enzyme.
[0032] In an embodiment of the third aspect, the enzyme is glucose
oxidase.
[0033] In an embodiment of the third aspect, the analyte is
glucose.
[0034] In an embodiment of the third aspect, the substrate is the
analyte
[0035] In an embodiment of the third aspect, the material is silver
chloride.
[0036] In an embodiment of the third aspect, the reference
electrode is formed of a chloridized elongated silver body.
[0037] In a fourth aspect is provided a method for measuring an
analyte in a host in vivo, comprising: exposing an electrochemical
sensor to a bodily fluid of a host, wherein the electrochemical
sensor comprises a working electrode and a silver/silver chloride
reference electrode, wherein at least a portion of the
silver/silver chloride reference electrode is covered with an
enzyme layer; receiving a signal from the working electrode,
wherein the signal is indicative a concentration of an analyte in
the bodily fluid; and generating hydrogen peroxide upon exposure of
an enzyme in the enzyme layer to a substrate for the enzyme present
in the bodily fluid, wherein the hydrogen peroxide regenerates
silver chloride of the reference electrode, whereby a reference
capacity of the reference electrode is increased.
[0038] In an embodiment of the fourth aspect, the method further
comprises substantially blocking diffusion of hydrogen peroxide
generated at the reference electrode to the working electrode.
[0039] In a fifth aspect is provided a method for manufacturing an
implantable continuous analyte sensor comprising: depositing an
enzyme-containing material onto a conductive surface, wherein the
conductive surface comprises at least a portion of a reference
electrode, wherein the conductive surface comprises a conductive
material; and drying the deposited enzyme-containing material to
form a membrane that covers the conductive surface, wherein the
membrane is configured to produce a regenerating species that
regenerates the conductive material during sensor use.
[0040] In an embodiment of the fifth aspect, the enzyme-containing
material comprises glucose oxidase.
[0041] In an embodiment of the fifth aspect, the analyte is
glucose.
[0042] In an embodiment of the fifth aspect, the conductive
material comprises silver chloride.
[0043] In an embodiment of the fifth aspect, the conductive
material comprises silver and silver chloride, and wherein silver
chloride is regenerated by the regenerating species.
[0044] In an embodiment of the fifth aspect, the regenerating
species is hydrogen peroxide.
[0045] In a sixth aspect is provided an implantable electrochemical
sensor for measuring a glucose concentration in a host, comprising:
a working electrode configured to measure a glucose concentration;
and a reference electrode comprising silver and silver chloride,
wherein the reference electrode is configured to provide
substantially stable reference potential for up to eight days while
continuously responding to a signal current of about 20 nA from the
working electrode.
[0046] In an embodiment of the sixth aspect, the reference
electrode is configured to provide substantially stable reference
potential for up to sixteen days while continuously responding to a
signal current of about 20 nA from the working electrode.
[0047] In an embodiment of the sixth aspect, at least a portion of
the reference electrode is covered with an enzyme layer.
[0048] In an embodiment of the sixth aspect, the enzyme layer is
configured, in vivo, to generate a reference electrode regenerating
species upon exposure to a substrate, whereby the reference
electrode regenerating species regenerates a component of the
reference electrode such that a reference capacity of the reference
electrode is increased.
[0049] In an embodiment of the sixth aspect, the regenerating
species is hydrogen peroxide.
[0050] Any of the aforementioned embodiments of an aspect may be
employed in connection with one or more other embodiments of an
aspect.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1A is a side-view schematic illustrating an in vivo
portion of an analyte sensor, in one embodiment.
[0052] FIG. 1B is a perspective-view schematic illustrating an in
vivo portion of an analyte sensor, in one embodiment.
[0053] FIG. 1C is a side-view schematic illustrating an in vivo
portion of an analyte sensor, in another embodiment.
[0054] FIG. 1D is a cross-sectional/side-view schematic
illustrating an in vivo portion of an analyte sensor, in another
embodiment.
[0055] FIG. 1E is a side-view schematic illustrating an in vivo
portion of an analyte sensor, in another embodiment.
[0056] FIG. 1F is a side-view schematic illustrating an in vivo
portion of an analyte sensor, in another embodiment.
[0057] FIG. 1G is a side-view schematic illustrating an in vivo
portion of an analyte sensor, in another embodiment.
[0058] FIG. 2A is a perspective-view schematic illustrating an in
vivo portion of an analyte sensor, in one embodiment.
[0059] FIG. 2B is a perspective-view schematic illustrating an ex
vivo portion of the analyte sensor of FIG. 2A, in one
embodiment.
[0060] FIG. 2C is a perspective-view schematic illustrating an in
vivo portion of an analyte sensor, in one embodiment.
[0061] FIG. 3A is a perspective-view schematic illustrating an in
vivo portion of an analyte sensor, in another embodiment.
[0062] FIG. 3B is a cross-sectional schematic illustrating an in
vivo portion of an analyte sensor, in another embodiment.
[0063] FIG. 4A is a perspective-view schematic illustrating an in
vivo portion of an analyte sensor, in one embodiment.
[0064] FIG. 4B is a perspective-view schematic illustrating an in
vivo portion of an analyte sensor, in another embodiment.
[0065] FIG. 4C is a perspective-view schematic illustrating an in
vivo portion of an analyte sensor, in another embodiment.
[0066] FIG. 5A is a perspective-view schematic illustrating an in
vivo portion of a multi-electrode analyte sensor, in one
embodiment.
[0067] FIG. 5B is a perspective-view schematic illustrating an in
vivo portion of a multi-electrode analyte sensor, in another
embodiment.
[0068] FIG. 5C is a perspective-view schematic illustrating an in
vivo portion of a multi-electrode analyte sensor, in another
embodiment.
[0069] FIG. 5D is a perspective-view schematic illustrating an in
vivo portion of a multi-electrode analyte sensor, in another
embodiment.
[0070] FIG. 6A is a cross-sectional schematic of the analyte sensor
of FIG. 1A, taken on line 6-6, in one embodiment.
[0071] FIG. 6B is a cross-sectional schematic of the analyte sensor
of FIG. 1A, taken on line 6-6, in another embodiment.
[0072] FIG. 6C is a cross-sectional schematic of the analyte sensor
of FIG. 1A, taken on line 6-6, in yet another embodiment.
[0073] FIG. 7 is a perspective-view schematic illustrating an in
vivo portion of an analyte sensor, in another embodiment.
[0074] FIG. 8A is a perspective-view schematic illustrating an in
vivo portion of a multi-electrode analyte sensor, in another
embodiment.
[0075] FIG. 8B is a close perspective schematic of the distal
portion of the sensor embodiment illustrated in FIG. 8A.
[0076] FIG. 8C is a front view of the sensor embodiment illustrated
in FIGS. 8A and 8B.
[0077] FIG. 9A is a perspective-view schematic illustrating an in
vivo portion of a multi-electrode analyte sensor, in another
embodiment.
[0078] FIG. 9B is a close perspective schematic of the distal
portion of the sensor embodiment illustrated in FIG. 9A.
[0079] FIG. 9C is a front view of the sensor embodiment illustrated
in FIGS. 9A and 9B.
[0080] FIG. 10A is a perspective-view schematic illustrating an in
vivo portion of a multi-electrode analyte sensor, in another
embodiment.
[0081] FIG. 10B is a close perspective schematic of the distal
portion of the sensor embodiment illustrated in FIG. 10A.
[0082] FIG. 10C is a front view of the sensor embodiment
illustrated in FIGS. 10A and 10B.
[0083] FIG. 10D is a perspective-view schematic illustrating an in
vivo portion of a multi-electrode analyte sensor, in another
embodiment.
[0084] FIG. 11A is a perspective-view schematic illustrating an in
vivo portion of a multi-electrode analyte sensor, in another
embodiment.
[0085] FIG. 11B is a close perspective schematic of the distal
portion of the sensor embodiment illustrated in FIG. 11A.
[0086] FIG. 11C is a front view of the sensor embodiment
illustrated in FIGS. 11A and 11B.
[0087] FIG. 12 is a front-view schematic illustrating an in vivo
portion of a multi-electrode analyte sensor, in another
embodiment.
[0088] FIG. 13A is a front-view schematic illustrating another
embodiment a multi-electrode analyte sensor, during one stage of
sensor fabrication.
[0089] FIG. 13B is a front-view schematic illustrating the
embodiment shown in FIG. 13A, during another stage of sensor
fabrication.
[0090] FIG. 13C is a front-view schematic illustrating the
embodiment shown in FIG. 13A, during yet another stage of sensor
fabrication.
[0091] FIG. 13D is a front-view schematic illustrating another
embodiment a multi-electrode analyte sensor, during one stage of
sensor fabrication.
[0092] FIG. 13E is a front-view schematic illustrating the
embodiment shown in FIG. 13D, during another stage of sensor
fabrication.
[0093] FIG. 13F is a front-view schematic illustrating the
embodiment shown in FIG. 13D, during yet another stage of sensor
fabrication.
[0094] FIG. 13G is a front-view schematic illustrating the
embodiment shown in FIG. 13D, during yet another stage of sensor
fabrication.
[0095] FIG. 14 is a perspective-view schematic illustrating a
fatigue measurement device 1410.
[0096] FIG. 15 is a table summarizing the results of the
performance of test sensors with conventional sensors, with respect
to fatigue life.
[0097] FIG. 16 is a schematic illustrating metabolism of glucose by
Glucose Oxidase (GOx) and one embodiment of a diffusion barrier D
that substantially prevents the diffusion of H.sub.2O.sub.2
produced on a first side of the sensor (e.g., from a working
electrode E2 that has active GOx) to a second side of the sensor
(e.g., to a silver/silver chloride electrode E1 that has active
GOx).
[0098] FIGS. 17A-17D are plots illustrating capacities of reference
electrodes with no enzyme coverage (FIG. 17A), 0.00225 in.sup.2 of
enzyme coverage (FIG. 17B), and 0.00633 in.sup.2 of enzyme coverage
(FIG. 17C). FIG. 17D provides a comparison of average reference
capacities for the reference electrodes of FIGS. 17A-17C.
[0099] It should be understood that the figures shown herein are
not necessarily drawn to scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0100] The following description and examples illustrate some
exemplary embodiments of the disclosed invention in detail. Those
of skill in the art will recognize that there are numerous
variations and modifications of this invention that are encompassed
by its scope. Accordingly, the description of a certain exemplary
embodiment should not be deemed to limit the scope of the
embodiments described herein.
Definitions
[0101] In order to facilitate an understanding of the embodiments
described herein, a number of terms are defined below.
[0102] The term "analyte," as used herein, is a broad term, and is
to be given its ordinary and customary meaning to a person of
ordinary skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to a substance
or chemical constituent in a biological sample (e.g., body fluids,
including, blood, serum, plasma, interstitial fluid, cerebral
spinal fluid, lymph fluid, ocular fluid, saliva, oral fluid, urine,
excretions, or exudates. Analytes can include naturally occurring
substances (e.g., various minerals), artificial substances,
metabolites, and/or reaction products. In some embodiments, the
analyte for measurement by the sensing regions, devices, and
methods is glucose, calcium, sodium, magnesium, potassium,
phosphorus, CO.sub.2, chloride, blood urea nitrogen, creatinine,
pH, a metabolic marker, oxygen, albumin, total protein, alkaline
phosphatase, alanine amino transferase, aspartate amino
transferase, alanine transaminase, bilirubin, gamma-glutamyl
transpeptidase, and hematocrit. However, other analytes are
contemplated as well, including but not limited to acetaminophen,
dopamine, ephedrine, terbutaline, ascorbate, uric acid, oxygen,
d-amino acid oxidase, plasma amine oxidase, Xanthine oxidase, NADPH
oxidase, alcohol oxidase, alcohol dehydrogenase, Pyruvate
dehydrogenase, diols, Ros, NO, bilirubin, cholesterol,
triglycerides, gentisic acid, ibuprophen, L-Dopa, Methyl Dopa,
salicylates, tetracycline, tolazamide, tolbutamide,
acarboxyprothrombin; acylcarnitine; adenine phosphoribosyl
transferase; adenosine deaminase; albumin; alpha-fetoprotein; amino
acid profiles (arginine (Krebs cycle), histidine/urocanic acid,
homocysteine, phenylalanine/tyrosine, tryptophan);
andrenostenedione; antipyrine; arabinitol enantiomers; arginase;
benzoylecgonine (cocaine); biotinidase; biopterin; c-reactive
protein; carnitine; carnosinase; CD4; ceruloplasmin;
chenodeoxycholic acid; chloroquine; cholesterol; cholinesterase;
conjugated 1-.beta. hydroxy-cholic acid; cortisol; creatine kinase;
creatine kinase MM isoenzyme; cyclosporin A; d-penicillamine;
de-ethylchloroquine; dehydroepiandrosterone sulfate; DNA
(acetylator polymorphism, alcohol dehydrogenase, alpha
1-antitrypsin, cystic fibrosis, Duchenne/Becker muscular dystrophy,
glucose-6-phosphate dehydrogenase, hemoglobin A, hemoglobin S,
hemoglobin C, hemoglobin D, hemoglobin E, hemoglobin F, D-Punjab,
beta-thalassemia, hepatitis B virus, HCMV, HIV-1, HTLV-1, Leber
hereditary optic neuropathy, MCAD, RNA, PKU, Plasmodium vivax,
sexual differentiation, 21-deoxycortisol); desbutylhalofantrine;
dihydropteridine reductase; diptheria/tetanus antitoxin;
erythrocyte arginase; erythrocyte protoporphyrin; esterase D; fatty
acids/acylglycines; free .beta.-human chorionic gonadotropin; free
erythrocyte porphyrin; free thyroxine (FT4); free tri-iodothyronine
(FT3); fumarylacetoacetase; galactose/gal-1-phosphate;
galactose-1-phosphate uridyltransferase; gentamicin;
glucose-6-phosphate dehydrogenase; glutathione; glutathione
perioxidase; glycocholic acid; glycosylated hemoglobin;
halofantrine; hemoglobin variants; hexosaminidase A; human
erythrocyte carbonic anhydrase I; 17-alpha-hydroxyprogesterone;
hypoxanthine phosphoribosyl transferase; immunoreactive trypsin;
lactate; lactate dehydrogenase; lead; lipoproteins ((a), B/A-1,
.beta.); lysozyme; mefloquine; netilmicin; phenobarbitone;
phenytoin; phytanic/pristanic acid; progesterone; prolactin;
prolidase; purine nucleoside phosphorylase; quinine; reverse
tri-iodothyronine (rT3); selenium; serum pancreatic lipase;
sissomicin; somatomedin C; specific antibodies (adenovirus,
anti-nuclear antibody, anti-zeta antibody, arbovirus, Aujeszky's
disease virus, dengue virus, Dracunculus medinensis, Echinococcus
granulosus, Entamoeba histolytica, enterovirus, Giardia
duodenalisa, Helicobacter pylori, hepatitis B virus, herpes virus,
HIV-1, IgE (atopic disease), influenza virus, Leishmania donovani,
leptospira, measles/mumps/rubella, Mycobacterium leprae, Mycoplasma
pneumoniae, Myoglobin, Onchocerca volvulus, parainfluenza virus,
Plasmodium falciparum, poliovirus, Pseudomonas aeruginosa,
respiratory syncytial virus, rickettsia (scrub typhus), Schistosoma
mansoni, Toxoplasma gondii, Trepenoma pallidium, Trypanosoma
cruzi/rangeli, vesicular stomatis virus, Wuchereria bancrofti,
yellow fever virus); specific antigens (hepatitis B virus, HIV-1);
succinylacetone; sulfadoxine; theophylline; thyrotropin (TSH);
thyroxine (T4); thyroxine-binding globulin; trace elements;
transferrin; UDP-galactose-4-epimerase; urea; uroporphyrinogen I
synthase; vitamin A; white blood cells; and zinc protoporphyrin.
Salts, sugar, protein, fat, vitamins, and hormones naturally
occurring in blood or interstitial fluids can also constitute
analytes in certain embodiments. The analyte can be naturally
present in the biological fluid, for example, a metabolic product,
a hormone, an antigen, an antibody, and the like. Alternatively,
the analyte can be introduced into the body, for example, a
contrast agent for imaging, a radioisotope, a chemical agent, a
fluorocarbon-based synthetic blood, or a drug or pharmaceutical
composition, including but not limited to insulin; ethanol;
cannabis (marijuana, tetrahydrocannabinol, hashish); inhalants
(nitrous oxide, amyl nitrite, butyl nitrite, chlorohydrocarbons,
hydrocarbons); cocaine (crack cocaine); stimulants (amphetamines,
methamphetamines, Ritalin, Cylert, Preludin, Didrex, PreState,
Voranil, Sandrex, Plegine); depressants (barbituates, methaqualone,
tranquilizers such as Valium, Librium, Miltown, Serax, Equanil,
Tranxene); hallucinogens (phencyclidine, lysergic acid, mescaline,
peyote, psilocybin); narcotics (heroin, codeine, morphine, opium,
meperidine, Percocet, Percodan, Tussionex, Fentanyl, Darvon,
Talwin, Lomotil); designer drugs (analogs of fentanyl, meperidine,
amphetamines, methamphetamines, and phencyclidine, for example,
Ecstasy); anabolic steroids; and nicotine. The metabolic products
of drugs and pharmaceutical compositions are also contemplated
analytes. Analytes such as neurochemicals and other chemicals
generated within the body can also be analyzed, such as, for
example, ascorbic acid, uric acid, dopamine, noradrenaline,
3-methoxytyramine (3MT), 3,4-dihydroxyphenylacetic acid (DOPAC),
homovanillic acid (HVA), 5-hydroxytryptamine (5HT), histamine,
Advanced Glycation End Products (AGEs) and 5-hydroxyindoleacetic
acid (FHIAA).
[0103] The term "baseline" as used herein is a broad term, and is
to be given its ordinary and customary meaning to a person of
ordinary skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to a
substantially constant signal derived from certain electroactive
compounds found in the human body that are relatively constant (for
example, baseline of the host's physiology, non-analyte
related).
[0104] The term "calibration," as used herein, is a broad term, and
is to be given its ordinary and customary meaning to a person of
ordinary skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to the
relationship and/or process of determining the relationship between
the sensor data and the corresponding reference data. In some
embodiments, namely, in continuous analyte sensors, calibration can
be updated or recalibrated over time if changes in the relationship
between the sensor data and reference data occur, for example, due
to changes in sensitivity, baseline, transport, metabolism, and the
like.
[0105] The terms "continuous," "continuously," and "continuous (or
continual) analyte sensing" as used herein in reference to analyte
sensing, are broad terms, and are to be given their ordinary and
customary meaning to a person of ordinary skill in the art (and is
not to be limited to a special or customized meaning), and refer
without limitation to the continuous, continual, or intermittent
(e.g., regular) monitoring of analyte concentration, such as, for
example, performing a measurement about every 1 to 10 minutes. It
should be understood that continuous analyte sensors generally
continually measure the analyte concentration without required user
initiation and/or interaction for each measurement, such as
described with reference to continuous glucose sensors in U.S. Pat.
No. 6,001,067, for example. These terms include situations wherein
data gaps can exist (e.g., when a continuous glucose sensor is
temporarily not providing data).
[0106] The term "count," as used herein, is a broad term, and is to
be given its ordinary and customary meaning to a person of ordinary
skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to a unit of
measurement of a digital signal. For example, a raw data stream or
raw data signal measured in counts is directly related to a voltage
(for example, converted by an A/D converter), which is directly
related to current from the working electrode. In some embodiments,
the terms can refer to data that has been integrated or averaged
over a time period (e.g., 5 minutes).
[0107] The term "crosslink," as used herein, is a broad term, and
is to be given its ordinary and customary meaning to a person of
ordinary skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to the formation
of bonds (e.g., covalent bonds, ionic bonds, hydrogen bonds, etc.)
that link one polymer (or oligomer) chain to another, or to a
process that increases the cohesiveness of one polymer (or
oligomer) chain to another. Crosslinks can be formed, e.g., through
various reactions or processes, e.g., chemical processes initiated
by heat, pressure, catalysts, radiation, and the like.
[0108] The term "distal to," as used herein, is a broad term, and
is to be given its ordinary and customary meaning to a person of
ordinary skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to the spatial
relationship between various elements in comparison to a particular
point of reference. In general, the term indicates an element is
located relatively far from the reference point than another
element.
[0109] The term "domain" as used herein is a broad term, and is to
be given its ordinary and customary meaning to a person of ordinary
skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to regions of a
membrane that can be layers, uniform or non-uniform gradients
(i.e., anisotropic) or provided as portions of the membrane.
[0110] The terms "electrical connection" and "electrical contact,"
as used herein, are broad terms, and are to be given their ordinary
and customary meaning to a person of ordinary skill in the art (and
are not to be limited to a special or customized meaning), and
refer without limitation to any connection between two electrical
conductors known to those in the art. In one embodiment, electrodes
are in electrical connection with (e.g., electrically connected to)
the electronic circuitry of a device. In another embodiment, two
materials, such as but not limited to two metals, can be in
electrical contact with each other, such that an electrical current
can pass from one of the two materials to the other material.
[0111] The terms "electrochemically reactive surface" and
"electroactive surface," as used herein, are broad terms, and are
to be given their ordinary and customary meaning to a person of
ordinary skill in the art (and are not to be limited to a special
or customized meaning), and refer without limitation to a surface
where an electrochemical reaction takes place. As a non-limiting
example, in an electrochemical glucose sensor, a working electrode
measures hydrogen peroxide, H.sub.2O.sub.2, at its electroactive
surface. The hydrogen peroxide is produced by the enzyme-catalyzed
reaction of the analyte detected, which reacts with the
electroactive surface to create a detectable electric current. For
example, glucose can be detected utilizing glucose oxidase (GOX),
which produces hydrogen peroxide as a byproduct. Hydrogen peroxide
reacts with the surface of the working electrode (e.g., the
electroactive surface), producing two protons (2H.sup.+), two
electrons (2e.sup.-) and one molecule of oxygen (O.sub.2), which
produces the electronic current being detected.
[0112] The term "electrical potential" as used herein is a broad
term, and is to be given its ordinary and customary meaning to a
person of ordinary skill in the art (and is not to be limited to a
special or customized meaning), and refers without limitation to
the electrical potential difference between two points in a circuit
which is the cause of the flow of a current.
[0113] The terms "electronics," "sensor electronics," and "system
electronics," as used herein, are broad terms, and are to be given
their ordinary and customary meaning to a person of ordinary skill
in the art (and are not to be limited to a special or customized
meaning), and refer without limitation to electronics operatively
coupled to the sensor and configured to measure, process, receive,
and/or transmit data associated with a sensor, and/or electronics
configured to communicate with a flow control device and to control
and/or monitor fluid metering by a flow control device.
[0114] The term "elongated conductive body," as used herein, is a
broad term, and is to be given its ordinary and customary meaning
to a person of ordinary skill in the art (and is not to be limited
to a special or customized meaning), and refers without limitation
to an elongated body formed at least in part of a conductive
material and includes any number of coatings that may be formed
thereon. By way of example, an "elongated conductive body" may mean
a bare elongated conductive core (e.g., a metal wire) or an
elongated conductive core coated with one, two, three, four, five,
or more layers of material, each of which may or may not be
conductive.
[0115] The term "ex vivo portion," as used herein, is a broad term,
and is to be given its ordinary and customary meaning to a person
of ordinary skill in the art (and is not to be limited to a special
or customized meaning), and refers without limitation to a portion
of a device (for example, a sensor) adapted to remain and/or exist
outside of a living body of a host.
[0116] The term "host," as used herein, is a broad term, and is to
be given its ordinary and customary meaning to a person of ordinary
skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to plants or
animals, for example humans.
[0117] The term "in vivo portion," as used herein, is a broad term,
and is to be given its ordinary and customary meaning to a person
of ordinary skill in the art (and is not to be limited to a special
or customized meaning), and refers without limitation to a portion
of a device (for example, a sensor) adapted for insertion into
and/or existence within a living body of a host.
[0118] The term "interference domain" as used herein is a broad
term, and is to be given its ordinary and customary meaning to a
person of ordinary skill in the art (and it is not to be limited to
a special or customized meaning), and refers without limitation to
any mechanism of the membrane system configured to reduce or
eliminate any kind of interferents or noise, such as constant
and/or non-constant noise.
[0119] The terms "interferents" and "interfering species" as used
herein are broad terms, and are to be given their ordinary and
customary meaning to a person of ordinary skill in the art (and are
not to be limited to a special or customized meaning), and refer
without limitation to effects or species that interfere with the
measurement of an analyte of interest in a sensor to produce a
signal that does not accurately represent the analyte measurement.
In an exemplary electrochemical sensor, interfering species can
include compounds with an oxidation potential that overlaps with
that of the analyte to be measured.
[0120] The term "linear" as used herein is a broad term, and is to
be given its ordinary and customary meaning to a person of ordinary
skill in the art (and it is not to be limited to a special or
customized meaning), and refers without limitation to a sensor's
linear response to an analyte (e.g., glucose) concentration over a
wide concentration range.
[0121] The term "multi-axis bending," as used herein, is a broad
term, and is to be given its ordinary and customary meaning to a
person of ordinary skill in the art (and is not to be limited to a
special or customized meaning), and refers without limitation to a
preference for bending in more than one plane or about more than
one axis.
[0122] The term "noise" as used herein is a broad term, and is to
be given its ordinary and customary meaning to a person of ordinary
skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to a
substantially intermittent signal caused by relatively non-constant
factors (for example, the presence of intermittent noise-causing
compounds that have an oxidation/reduction potential that
substantially overlaps the oxidation/reduction potential of the
analyte or co-analyte and arise due to the host's ingestion,
metabolism, wound healing, and other mechanical, chemical and/or
biochemical factors, also non-analyte related).
[0123] The terms "operatively connected," "operatively linked,"
"operably connected," and "operably linked" as used herein, are
broad terms, and are to be given their ordinary and customary
meaning to a person of ordinary skill in the art (and are not to be
limited to a special or customized meaning), and refer without
limitation to one or more components linked to one or more other
components. The terms can refer to a mechanical connection, an
electrical connection, or any connection that allows transmission
of signals between the components. For example, one or more
electrodes can be used to detect the amount of analyte in a sample
and to convert that information into a signal; the signal can then
be transmitted to a circuit. In such an example, the electrode is
"operably linked" to the electronic circuitry. The terms include
wired and wireless connections.
[0124] The term "potentiostat," as used herein, is a broad term,
and is to be given its ordinary and customary meaning to a person
of ordinary skill in the art (and is not to be limited to a special
or customized meaning), and refers without limitation to an
electronic instrument that controls the electrical potential
between the working and reference electrodes at one or more preset
values.
[0125] The term "processor module," as used herein, is a broad
term, and is to be given its ordinary and customary meaning to a
person of ordinary skill in the art (and are not to be limited to a
special or customized meaning), and refers without limitation to a
computer system, state machine, processor, components thereof, and
the like designed to perform arithmetic or logic operations using
logic circuitry that responds to and processes the basic
instructions that drive a computer.
[0126] The term "proximal to," as used herein, is a broad term, and
is to be given its ordinary and customary meaning to a person of
ordinary skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to the spatial
relationship between various elements in comparison to a particular
point of reference. In general, the term indicates an element is
located relatively near to the reference point than another
element.
[0127] The terms "raw data," "raw data stream," "raw data signal,"
"data signal," and "data stream," as used herein, are broad terms,
and are to be given their ordinary and customary meaning to a
person of ordinary skill in the art (and are not to be limited to a
special or customized meaning), and refer without limitation to an
analog or digital signal from the analyte sensor directly related
to the measured analyte. For example, the raw data stream is
digital data in "counts" converted by an A/D converter from an
analog signal (for example, voltage or amps) representative of an
analyte concentration. The terms can include a plurality of time
spaced data points from a substantially continuous analyte sensor,
each of which comprises individual measurements taken at time
intervals ranging from fractions of a second up to, for example, 1,
2, or 5 minutes or longer. In some embodiments, the terms can refer
to data that has been integrated or averaged over a time period
(e.g., 5 minutes).
[0128] The term "reference electrode capacity" and "reference
capacity," as used herein, are broad terms, and are to be given
their ordinary and customary meaning to a person of ordinary skill
in the art (and are not to be limited to a special or customized
meaning), and refer without limitation to the actual or potential
ability of the reference electrode to maintain a certain stable
reference potential in a response to current signals from a working
electrode. For example, with a silver/silver chloride reference
electrode, the reference capacity can refer without limitation to
how long the reference electrode can maintain sufficient silver
chloride to maintain a certain stable reference potential in a
response to current signals from a working electrode.
[0129] The term "sample," as used herein, is a broad term, and is
to be given its ordinary and customary meaning to a person of
ordinary skill in the art (and it is not to be limited to a special
or customized meaning), and refers without limitation to a sample
of a host body, for example, body fluids, including, blood, serum,
plasma, interstitial fluid, cerebral spinal fluid, lymph fluid,
ocular fluid, saliva, oral fluid, urine, excretions, or
exudates.
[0130] The terms "sensing membrane," "membrane," and "membrane
system" as used herein are broad terms, and are to be given their
ordinary and customary meaning to a person of ordinary skill in the
art (and are not to be limited to a special or customized meaning),
and refers without limitation to a permeable or semi-permeable
membrane that can comprise one or more domains and constructed of
materials of a few microns thickness or more, which are permeable
to oxygen and may or may not be permeable to an analyte of
interest. In one example, the sensing membrane or membrane system
may comprise an immobilized glucose oxidase enzyme, which enables
an electrochemical reaction to occur to measure a concentration of
glucose.
[0131] The terms "sensing region," "sensor", "sensor system," and
"sensing mechanism," as used herein are broad terms, and are to be
given their ordinary and customary meaning to a person of ordinary
skill in the art (and are not to be limited to a special or
customized meaning), and refer without limitation to the region or
mechanism of a monitoring device responsible for the detection of a
particular analyte, or to a device, component, or region of a
device by which an analyte can be quantified.
[0132] The term "sensitivity" as used herein is a broad term, and
is to be given its ordinary and customary meaning to a person of
ordinary skill in the art (and is not to be limited to a special or
customized meaning), and refers without limitation to an amount of
electrical current produced by a predetermined amount (unit) of the
measured analyte. For example, in one embodiment, a sensor has a
sensitivity (or slope) of from about 1 to about 100 picoAmps of
current for every 1 mg/dL of glucose analyte.
[0133] The term "sensor session," as used herein, is a broad term
and is to be given its ordinary and customary meaning to a person
of ordinary skill in the art (and is not to be limited to a special
or customized meaning), and refers without limitation to a period
of time a sensor is in use, such as but not limited to a period of
time starting at the time the sensor is implanted (e.g., by the
host) to removal of the sensor (e.g., removal of the sensor from
the host's body and/or removal of (e.g., disconnection from) system
electronics).
[0134] The terms "substantial" and "substantially," as used herein,
are broad terms, and are to be given their ordinary and customary
meaning to a person of ordinary skill in the art (and are not to be
limited to a special or customized meaning), and refer without
limitation to a sufficient amount that provides a desired
function.
[0135] As employed herein, the following abbreviations apply: Eq
and Eqs (equivalents); mEq (milliequivalents); M (molar); mM
(millimolar) .mu.M (micromolar); N (Normal); mol (moles); mmol
(millimoles); tmol (micromoles); nmol (nanomoles); g (grams); mg
(milligrams); .mu.g (micrograms); Kg (kilograms); L (liters); mL
(milliliters); dL (deciliters); .mu.L (microliters); cm
(centimeters); mm (millimeters); .mu.m (micrometers); nm
(nanometers); h and hr (hours); min. (minutes); s and sec.
(seconds); .degree. C. (degrees Centigrade).
Overview
[0136] The development of reliable, user-friendly in vivo analyte
sensors has been presented with several technical challenges
relating to mechanical properties of the in vivo portion of the
sensor. Sensors designed with an in vivo portion that has weak
strength are more prone to the risk of breakage. Sensors designed
with an in vivo portion that has great strength are often hard, and
thus uncomfortable to the patient wearing the sensor. What has been
desired is a sensor design that has the mechanical properties
(e.g., a certain flexibility/stiffness as measured by Young'
modulus and a certain high yield strength that reduces the risk of
breakage of a bending sensor) that both lend comfort to the user
and mechanical properties (e.g., fatigue life, strength) that
provide durability and robustness to the sensor, thereby minimizing
the risk of breakage. Described herein are sensor embodiments that
overcome these technical obstacles and possesses both the
mechanical properties that allow for comfort to the user and that
minimizes the risk of breakage. For example, in one embodiment, the
sensor comprises an elongated conductive body that has a Young's
modulus of from about 160 GPa to about 220 GPa and a yield strength
of at least 60 kPsi.
[0137] In some embodiments, the sensor is configured and arranged
to monitor a single analyte. However, in other embodiments, the
sensor is configured and arranged to monitor a plurality of
analytes, such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more
different analytes. In yet another embodiment, the sensor is
configured to monitor at least one analyte substantially
continuously and to monitor at least one analyte intermittently.
The analyte that is substantially, continuously monitored and the
analyte that is intermittently monitored can be the same analyte or
a different one.
[0138] In some embodiments, the sensor is configured and arranged
for implantation in a host and for generating in vivo a signal
associated with an analyte in a sample of the host during a sensor
session. In some embodiments, the time length of the sensor session
is from about less than 10 minutes, 10 minutes, 20 minutes, 30
minutes, 40 minutes, or 50 minutes to about 1 hour, 2 hours, 3
hours, 4 hours, 5 hours or longer. In some embodiments, the time
length of the sensor session is from about 1 hour, 2 hours, 3
hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10
hours to about 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16
hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours,
23 hours, 24 hours or longer. In some embodiments, the time length
of the sensor session is from about less than 0.25 days, 0.25 days,
0.5 days, 0.75 days, or 1 day to about 2 days, 3 days, 4 days, 5
days, 6 days, 7 days, 8 days, 9 days, 10 days or longer.
[0139] The analyte sensor can be configured for any type of
implantation, such as transcutaneous implantation, subcutaneous
implantation, or implantation into the host's circulatory system
(e.g., into a vessel, such as a vein or artery). In addition, the
sensor may be configured to be wholly implantable or
extracorporeally implantable (e.g., into an extracorporeal blood
circulatory device, such as a heart-bypass machine or a blood
dialysis machine). U.S. Patent Publication No. 2006-0020187-A1
describes an exemplary continuous analyte sensor that can be used
for transcutaneous implantation by insertion into the abdominal
tissue of a host. U.S. Patent Publication No. 2008-0119703-A1
describes an exemplary embodiment of a continuous analyte sensor
that can be used for insertion into a host's vein (e.g., via a
catheter). In some embodiments, the sensor is configured and
arranged for in vitro use.
[0140] By way of example and not of limitation, a wide variety of
suitable detection methods including, but not limited to,
enzymatic, chemical, physical, electrochemical, immunochemical,
optical, radiometric, calorimetric, protein binding, and microscale
methods of detection, can be employed in certain embodiments,
although any other techniques can also be used. Additional
description of analyte sensor configurations and detection methods
that can be used can be found in U.S. Patent Publication No.
2007-0213611-A1, U.S. Patent Publication No. 2007-0027385-A1, U.S.
Patent Publication No. 2005-0143635-A1, U.S. Patent Publication No.
2007-0020641-A1, U.S. Patent Publication No. 2007-002064-A11, U.S.
Patent Publication No. 2005-0196820-A1, U.S. Pat. Nos. 5,517,313,
5,512,246, 6,400,974, 6,711,423, 7,308,292, 7,303,875, 7,289,836,
7,289,204, 5,156,972, 6,528,318, 5,738,992, 5,631,170, 5,114,859,
7,273,633, 7,247,443, 6,007,775, 7,074,610, 6,846,654, 7,288,368,
7,291,496, 5,466,348, 7,062,385, 7,244,582, 7,211,439, 7,214,190,
7,171,312, 7,135,342, 7,041,209, 7,061,593, 6,854,317, 7,315,752,
and 7,312,040.
[0141] Although certain sensor configurations and methods of
manufacture are described herein, it should be understood that any
of a variety of known sensor configurations can be employed with
the analyte sensor systems and methods of manufacture described
herein, such as those described in U.S. Pat. No. 5,711,861 to Ward
et al., U.S. Pat. No. 6,642,015 to Vachon et al., U.S. Pat. No.
6,654,625 to Say et al., U.S. Pat. No. 6,565,509 to Say et al.,
U.S. Pat. No. 6,514,718 to Heller, U.S. Pat. No. 6,465,066 to
Essenpreis et al., U.S. Pat. No. 6,214,185 to Offenbacher et al.,
U.S. Pat. No. 5,310,469 to Cunningham et al., and U.S. Pat. No.
5,683,562 to Shaffer et al., U.S. Pat. No. 6,579,690 to Bonnecaze
et al., U.S. Pat. No. 6,484,046 to Say et al., U.S. Pat. No.
6,103,033 to Say et al., U.S. Pat. No. 6,512,939 to Colvin et al.,
U.S. Pat. No. 6,424,847 to Mastrototaro et al., U.S. Pat. No.
6,424,847 to Mastrototaro et al., for example. The sensors
described in the above-identified patents documents are not
inclusive of all applicable analyte sensors. It should be
understood that the disclosed embodiments are applicable to a
variety of analyte sensor configurations. It is noted that much of
the description of the embodiments, for example the membrane system
described below, can be implemented not only with in vivo sensors,
but also with in vitro sensors, such as blood glucose meters
(SMBG).
[0142] The sensors of certain embodiments are configured and
arranged for implantation into body structures. In use, the in vivo
portion of the sensor can be bent about one or more axes. This
bending can occur intermittently, which can be frequently or
infrequently, depending upon factors such as the nature of the
implantation site (e.g., the type(s) of surrounding tissue, the
thicknesses of the tissue, etc.), the type or amount of host
activity, and/or the sensor's configuration.
[0143] In one embodiment, the sensor is configured and arranged for
transcutaneous implantation. One exemplary transcutaneous
implantation site is the abdomen, which includes an abdominal wall
with a plurality of layers (e.g., skin, fascia, fat, muscles) that
can move and/or slide transversely with respect of one another
(e.g., in response to movement by the host). The fascia can
sometimes slide, stretch or move small distances across underlying
fat or muscle tissue.
[0144] In some embodiments, when a sensor is transcutaneously
implanted, the in vivo portion passes through the skin and into an
underlying tissue layer. Depending upon the nature of the
implantation site, the sensor may pass through two or more tissue
layers. Consequently, voluntary or involuntary movements by the
host can move the tissue layers, which in turn can apply force to
the implanted sensor. Similarly, when the sensor is implanted into
the host's circulatory system, such as into a vein or artery, and
the host moves his arm, wrist and/or hand, forces may be applied to
the implanted sensor.
[0145] When certain forces (e.g., forces that cause the sensor to
bend in a non-preferred bending axis) are applied to a conventional
sensor, they can cause damage to the sensor and/or the tissue
surrounding the sensor. In contrast, the sensors of some of the
embodiments are configured and arranged to bend and/or flex in
response to forces applied thereto by surrounding tissue and/or
body movements. While not wishing to be bound by theory, it is
believed that the capability of some sensor embodiments to bend or
flex, in response to application of forces by the surrounding
tissue, reduces the risk of host tissue damage and sensor damage,
while still maintaining sensor accuracy.
[0146] In certain embodiments, the sensor is configured and
arranged for a unique combination of strength and flexibility that
enables the sensor to be implanted for at least one, two, three or
more days and to measure at least one analyte after implantation,
while withstanding intermittent and/or repeated bending and/or
flexing about multiple axes, such that the sensors. In some
embodiments, the sensor is configured and arranged to bend and/or
flex at one, two, three or more points along its length (e.g.,
along a length corresponding to the in vivo portion implanted into
the host). Additionally or alternatively, the sensor may be capable
of bending about a plurality of axes (e.g., multi-axis bending)
and/or within a plurality of planes. As is described in greater
detail elsewhere herein, components of the sensor may be treated,
formed and/or combined in a way to achieve the requisite
combination of strength or flexibility that enables certain sensor
embodiments to provide substantially accurate continuous analyte
data, while withstanding a harsh implantation environment for at
least 1, 2, 3 or more days while, at the same time.
[0147] FIGS. 1A through 1C illustrate one aspect (e.g., the in vivo
portion) of a continuous analyte sensor 100, which includes an
elongated conductive body 102. The elongated conductive body 102
includes a core 110 (see FIG. 1B) and a first layer 112 at least
partially surrounding the core. The first layer includes a working
electrode (e.g., located in window 106) and a membrane 108 located
over the working electrode configured and arranged for multi-axis
bending. In some embodiments, the core and first layer can be of a
single material (e.g., platinum). In some embodiments, the
elongated conductive body is a composite of at least two materials,
such as a composite of two conductive materials, or a composite of
at least one conductive material and at least one non-conductive
material. In some embodiments, the elongated conductive body
comprises a plurality of layers. In certain embodiments, there are
at least two concentric (e.g., annular) layers, such as a core
formed of a first material and a first layer formed of a second
material. However, additional layers can be included in some
embodiments. In some embodiments, the layers are coaxial.
[0148] The elongated conductive body may be long and thin, yet
flexible and strong. For example, in some embodiments, the smallest
dimension of the elongated conductive body is less than about 0.1
inches, 0.075 inches, 0.05 inches, 0.025 inches, 0.01 inches, 0.004
inches, or 0.002 inches. While the elongated conductive body is
illustrated in FIGS. 1A through 1C as having a circular
cross-section, in other embodiments the cross-section of the
elongated conductive body can be ovoid, rectangular, triangular,
polyhedral, star-shaped, C-shaped, T-shaped, X-shaped, Y-Shaped,
irregular, or the like. In one embodiment, a conductive wire
electrode is employed as a core. To such a clad electrode, two
additional conducting layers may be added (e.g., with intervening
insulating layers provided for electrical isolation). The
conductive layers can be comprised of any suitable material. In
certain embodiments, it can be desirable to employ a conductive
layer comprising conductive particles (i.e., particles of a
conductive material) in a polymer or other binder.
[0149] In certain embodiments, the materials used to form the
elongated conductive body (e.g., stainless steel, titanium,
tantalum, platinum, platinum-iridium, iridium, certain polymers,
and/or the like) can be strong and hard, and therefore are
resistant to breakage. For example, in some embodiments, the
ultimate tensile strength of the elongated conductive body is from
about 80 kPsi to about 500 kPsi. In another example, in some
embodiments, the Young's modulus of the elongated conductive body
is from about 160 GPa to about 220 GPa. In still another example,
in some embodiments, the yield strength of the elongated conductive
body is from about 60 kPsi to about 2200 MPa. Ultimate tensile
strength, Young's modulus, and yield strength are discussed in
greater detail elsewhere herein. In some embodiments, the sensor's
small diameter provides (e.g., imparts, enables) flexibility to
these materials, and therefore to the sensor as a whole. Thus, the
sensor can withstand repeated forces applied to it by surrounding
tissue. One measurement of the sensor's ability to withstand the
implantation environment is fatigue life, which is described in
greater detail in the section entitled "Multi-Axis Bending." In
some embodiments, the fatigue life of the sensor is at least 1,000
cycles of flexing of from about 28.degree. to about 110.degree. at
a bend radius of about 0.125-inches.
[0150] In addition to providing structural support, resiliency and
flexibility, in some embodiments, the core 110 (or a component
thereof) provides electrical conduction for an electrical signal
from the working electrode to sensor electronics (not shown), which
are described elsewhere herein. In some embodiments, the core 110
comprises a conductive material, such as stainless steel, titanium,
tantalum, a conductive polymer, and/or the like. However, in other
embodiments, the core is formed from a non-conductive material,
such as a non-conductive polymer. In yet other embodiments, the
core comprises a plurality of layers of materials. For example, in
one embodiment the core includes an inner core and an outer core.
In a further embodiment, the inner core is formed of a first
conductive material and the outer core is formed of a second
conductive material. For example, in some embodiments, the first
conductive material is stainless steel, titanium, tantalum, a
conductive polymer, an alloy, and/or the like, and the second
conductive material is conductive material selected to provide
electrical conduction between the core and the first layer, and/or
to attach the first layer to the core (e.g., if the first layer is
formed of a material that does not attach well to the core
material). In another embodiment, the core is formed of a
non-conductive material (e.g., a non-conductive metal and/or a
non-conductive polymer) and the first layer is a conductive
material, such as stainless steel, titanium, tantalum, a conductive
polymer, and/or the like. The core and the first layer can be of a
single (or same) material, e.g., platinum. One skilled in the art
appreciates that additional configurations are possible.
[0151] Referring again to FIGS. 1A-1C, in some embodiments, the
first layer 112 is formed of a conductive material. The working
electrode is an exposed portion of the surface of the first layer.
Accordingly, the first layer is formed of a material configured to
provide a suitable electroactive surface for the working electrode,
a material such as but not limited to platinum, platinum-iridium,
gold, palladium, iridium, graphite, carbon, a conductive polymer,
an alloy and/or the like.
[0152] As shown in FIG. 1B-1C, a second layer 104 surrounds a least
a portion of the first layer 112, thereby defining the boundaries
of the working electrode. In some embodiments, the second layer 104
serves as an insulator and is formed of an insulating material,
such as polyimide, polyurethane, parylene, or any other known
insulating materials. For example, in one embodiment the second
layer is disposed on the first layer and configured such that the
working electrode is exposed via window 106. In another embodiment,
an elongated conductive body, including the core, the first layer
and the second layer, is provided, and the working electrode is
exposed (i.e., formed) by removing a portion of the second layer,
thereby forming the window 106 through which the electroactive
surface of the working electrode (e.g., the exposed surface of the
first layer) is exposed. In some embodiments, the working electrode
is exposed by (e.g., window 106 is formed by) removing a portion of
the second and (optionally) third layers. Removal of coating
materials from one or more layers of elongated conductive body
(e.g., to expose the electroactive surface of the working
electrode) can be performed by hand, excimer lasing, chemical
etching, laser ablation, grit-blasting, or the like.
[0153] In some embodiments, the sensor further comprises a third
layer 114 comprising a conductive material. In further embodiments,
the third layer may comprise a reference electrode, which may be
formed of a silver-containing material that is applied onto the
second layer (e.g., an insulator). The silver-containing material
may include any of a variety of materials and be in various forms,
such as, Ag/AgCl-polymer pastes, paints, polymer-based conducting
mixture, and/or inks that are commercially available, for example.
The third layer can be processed using a pasting/dipping/coating
step, for example, using a die-metered dip coating process. In one
exemplary embodiment, an Ag/AgCl polymer paste is applied to an
elongated body by dip-coating the body (e.g., using a meniscus
coating technique) and then drawing the body through a die to meter
the coating to a precise thickness. In some embodiments, multiple
coating steps are used to build up the coating to a predetermined
thickness. Such a drawing method can be utilized for forming one or
more of the electrodes in the device depicted in FIG. 1B.
[0154] In some embodiments, the silver grain in the Ag/AgCl
solution or paste can have an average particle size associated with
a maximum particle dimension that is less than about 100 microns,
or less than about 50 microns, or less than about 30 microns, or
less than about 20 microns, or less than about 10 microns, or less
than about 5 microns. The silver chloride grain in the Ag/AgCl
solution or paste can have an average particle size associated with
a maximum particle dimension that is less than about 100 microns,
or less than about 80 microns, or less than about 60 microns, or
less than about 50 microns, or less than about 20 microns, or less
than about 10 microns. The silver grain and the silver chloride
grain may be incorporated at a ratio of the silver chloride
grain:silver grain of from about 0.01:1 to 2:1 by weight, or from
about 0.1:1 to 1:1. The silver grains and the silver chloride
grains are then mixed with a carrier (e.g., a polyurethane) to form
a solution or paste. In certain embodiments, the Ag/AgCl component
form from about 10% to about 65% by weight of the total Ag/AgCl
solution or paste, or from about 20% to about 50%, or from about
23% to about 37%. In some embodiments, the Ag/AgCl solution or
paste has a viscosity (under ambient conditions) that is from about
1 to about 500 centipoise, or from about 10 to about 300
centipoise, of from about 50 to about 150 centipoise.
[0155] In some embodiments, Ag/AgCl particles are mixed into a
polymer, such as polyurethane, polyimide, or the like, to form the
silver-containing material for the reference electrode. In some
embodiments, the third layer is cured, for example, by using an
oven or other curing process. In some embodiments, a covering of
fluid-permeable polymer with conductive particles (e.g., carbon
particles) therein is applied over the reference electrode and/or
third layer. A layer of insulating material is located over a
portion of the silver-containing material, in some embodiments.
[0156] In some embodiments, the elongated conductive body further
comprises one or more intermediate layers located between the core
and the first layer. For example, in some embodiments, the
intermediate layer is an insulator, a conductor, a polymer, and/or
an adhesive.
[0157] It is contemplated that the ratio between the thickness of
the Ag/AgCl layer and the thickness of an insulator (e.g.,
polyurethane or polyimide) layer can be controlled, so as to allow
for a certain error margin (e.g., an error margin associated with
the etching process) that would not result in a defective sensor
(e.g., due to a defect resulting from an etching process that cuts
into a depth more than intended, thereby unintentionally exposing
an electroactive surface). This ratio may be different depending on
the type of etching process used, whether it is laser ablation,
grit blasting, chemical etching, or some other etching method. In
one embodiment in which laser ablation is performed to remove a
Ag/AgCl layer and a polyurethane layer, the ratio of the thickness
of the Ag/AgCl layer and the thickness of the polyurethane layer
can be from about 1:5 to about 1:1, or from about 1:3 to about
1:2.
[0158] In certain embodiment, the core comprises a non-conductive
polymer and the first layer comprises a conductive material. Such a
sensor configuration can sometimes provide reduced material costs,
in that it replaces a typically expensive material with an
inexpensive material. For example, in some embodiments, the core is
formed of a non-conductive polymer, such as, a nylon or polyester
filament, string or cord, which can be coated and/or plated with a
conductive material, such as platinum, platinum-iridium, gold,
palladium, iridium, graphite, carbon, a conductive polymer, and
allows or combinations thereof.
[0159] As shown in FIGS. 1C and 1D, the sensor also includes a
membrane 108 covering at least a portion of the working electrode.
Membranes are discussed in detail in the section entitled "Membrane
Configurations."
[0160] FIGS. 3A and 3B illustrate another aspect of a continuous
analyte sensor 100, including an elongated body comprising a
conductive core 110 and an insulating layer 104 at least partially
surrounding the conductive core, a working electrode body 112 in
electrical contact with the conductive core, and a membrane (not
shown) covering the working electrode body. The elongated
conductive body is configured and arranged for multi-axis bending.
In some embodiments, such as the embodiment shown in FIG. 3A, the
sensor includes a single insulated conductive core. However, in
other embodiments, two, three, or more conductive cores are
embedded in a single insulator. For example, FIG. 3B shows an
elongated body having three conductive cores embedded in the
insulator. The conductive core is formed of a conductive material
suitable to electrically connect the working electrode body to
sensor electronics (not shown), and to provide flexible support to
the sensor. The conductive material can include, but is not limited
to, stainless steel, titanium, tantalum, a conductive polymer, an
alloy, or the like. In some embodiments, the conductive core
comprises an inner core and an outer core. In further embodiments,
the inner core comprises a first conductive material, and the outer
core comprises a second conductive material. In alternative
embodiments, the inner core comprises an insulating material (e.g.,
non-conductive material) and the outer core comprises a conductive
material. For example, a non-conductive polymer, such as nylon
filament can be used to form the core, and electrical conduction
(e.g., between the working electrode body and sensor electronics)
is provided by an outer core formed of a conductive material, such
as, for example, stainless steel, titanium, tantalum, a conductive
polymer, an alloy or the like.
[0161] In some embodiments, the insulating layer 104 can be formed
of any of a variety of insulating materials, such as polyurethane,
polyimide, for example, as described elsewhere herein. In some
embodiments, such as those shown in FIGS. 3A and 3B, a window 106
is formed in the insulator, such as by removal of a portion of the
insulator using techniques described elsewhere herein. However, in
other embodiments, no window is formed; rather, the working
electrode body (as described below) is configured to penetrate
through (e.g., by piercing) the insulator and make physical contact
(e.g., electrical contact) with the core. In certain embodiments,
one or more insulating layers can be formed from heat-shrink
material.
[0162] As shown in FIGS. 3A and 3B, the sensor includes a working
electrode body 112, which is formed of any of a variety of
conductive materials, such as, platinum, platinum-iridium, gold,
palladium, iridium, graphite, carbon, a conductive polymer, an
alloy, and the like, for example. The working electrode body
provides the electroactive surface of the working electrode. In
some embodiments, the working electrode body includes a structure
that can be attached to the elongated body, such as by crimping,
clamping, welding, adhesive, and/or the like, such as but not
limited to a C-clip, a washer, a foil, and the like. The working
electrode body is applied to the conductive core, such that the
working electrode body and the conductive core are electrically
connected. In some embodiments, at least a portion of the working
electrode body penetrates (e.g., pierces, intersects) the
insulating layer 104 to physically contact (e.g., touch) the core,
such that the working electrode body and the conductive core are
electrically connected. In other embodiments, the working electrode
body is applied over window 106, such that it makes electrical
contact with the conductive core through the window. In some
embodiments, one or more conductive materials are applied between
the working electrode body and the core to facilitate conductivity
therebetween. In some embodiments, an adhesive, such as a
conductive adhesive, is applied between the working electrode body
and the conductive core, so as to attach the working electrode body
to the core. In some embodiments, additional materials, such as but
not limited to polytetrafluoroethylene (such as is marketed under
the trade name TEFLON.RTM.), are used to attach the working
electrode body to the conductive core.
[0163] In some embodiments, instead of an elongated body having a
plurality of conductive cores embedded in an insulator, the sensor
includes two or more elongated bodies (e.g., bundled and/or twisted
together) with at least one of the elongated bodies having a
working electrode body electrically connected thereto. For example,
FIG. 4C illustrates an in vivo portion of a sensor including three
elongated bodies, wherein each elongated body includes a conductive
core at least partially coated in insulator. Two of the elongated
bodies are shown to include windows, wherein working electrode
bodies can be attached. In an alternative embodiment, windows are
not formed, and the working electrode bodies are C-clip structures
that are crimped about the elongated bodies, wherein the ends of
the C-clips pierce the insulator and make physical (e.g.,
electrical) contact with the underlying conductive cores. In yet
another embodiment, the working electrode body is deposited,
printed, and/or plated on the conductive core (e.g., through the
window).
[0164] In some embodiments, the sensor includes a reference
electrode, and optionally an insulator applied to an ex vivo
portion of the sensor (e.g., a portion of the reference electrode
material exposed to air during implantation), such as described
herein.
[0165] FIG. 4B illustrates another embodiment of an analyte sensor,
including an elongated body comprising an insulator 104, a first
conductive core 110A embedded in the insulator and a second
conductive core 110B embedded in the insulator. The insulator
comprises a first window 106A configured and arranged to expose an
electroactive portion of the first conductive core, wherein the
insulator also comprises a second window 106B configured and
arranged to expose an electroactive portion of the second
conductive core. The elongated body is configured and arranged for
multi-axis bending. A membrane 108 covers the exposed electroactive
portion of the first conductive core. In some embodiments, the
sensor has a relatively small diameter, such as described elsewhere
herein. For example, in some embodiments, the smallest dimension of
the elongated body is less than or equal to about 0.002 inches,
0.004 inches, 0.01 inches, 0.05 inches, 0.075 inches, 0.1 inches,
0.25 inches, 0.5 inches, or 0.75 inches. The elongated body can be
formed using a variety of insulators known in the art. In some
embodiments, the insulator comprises at least one of polyurethane
or polyimide.
[0166] In some embodiments, the electroactive portion of the first
conductive core is a working electrode. Accordingly, in some
embodiments, the first conductive core may be formed of platinum,
platinum-iridium, gold, palladium, iridium, graphite, carbon, a
conductive polymer, or combinations or alloys thereof. In
alternative embodiments, the first conductive core comprises a core
and a first layer, wherein an exposed electroactive surface of the
first layer provides the working electrode. For example, in some
embodiments, the core comprises stainless steel, titanium, tantalum
and/or a polymer, and the first layer comprises platinum,
platinum-iridium, gold, palladium, iridium, graphite, carbon, a
conductive polymer and/or an alloy.
[0167] In some embodiments, the second conductive core provides a
reference electrode, and thus may be formed of a silver-containing
material, such as, a silver wire or a silver-containing polymer,
for example. In some embodiments, the silver is chloridized prior
to being embedded in the insulator. In other embodiments, the
silver is non-chloridized prior to being embedded in the insulator,
and is chloridized after the silver is exposed (e.g., via window
106B).
[0168] In some embodiments, a third conductive core is embedded in
the insulator. For example, in some embodiments, the sensor is a
dual-electrode sensor and the third conductive core is configured
as a second working electrode. In alternative embodiments, the
third conductive core is configured as a counter electrode, in some
embodiments. The third conductive core is a wire-shaped structure
formed of at least one of platinum, platinum-iridium, gold,
palladium, iridium, graphite, carbon, a conductive polymer and an
alloy, in some embodiments. Alternatively, the third conductive
core can include a core and a first layer, such as described
herein. In some further embodiments, the core comprises an inner
core and an outer core, such as described herein.
[0169] In some further embodiments, the sensor includes two or more
additional working electrodes. For example, the sensor can be
configured to detect two or more analytes. Alternatively, some of
the additional working electrodes can be configured as redundant
sensors.
[0170] Referring again to FIG. 4B, in one exemplary embodiment, the
sensor is a glucose sensor configured and arranged for multi-axis
bending, and comprises an elongated body comprising an insulator
104 formed of an insulating material such as a polyurethane or a
polyimide. A platinum or a platinum-iridium wire and a silver wire
110B is embedded in the insulator. The electroactive surface (e.g.,
the working electrode) of the platinum or a platinum-iridium wire
is exposed via window 106A. The reference electrode (e.g., of the
silver wire) is exposed via window 106B. A membrane 108 covers at
least the working electrode. In some embodiments, the membrane may
cover a larger portion of the sensor. For example, in FIG. 4B, the
membrane covers the illustrated in vivo portion of the sensor.
[0171] The sensor can be manufactured using a variety of techniques
and methods known in the art. For example, in one embodiment, the
elongated body is provided with the insulator 104 and with the
first 110A and second 110B conductive cores embedded therein).
Portions of the insulator (e.g., windows 106A and 1060B) are then
removed to expose the working electrode (e.g., located on the first
conductive core, the Pt or Pt/Ir wire) and the reference electrode
(e.g., located on the second conductive core, the Ag wire). In some
embodiments, the step of removing a portion of the insulator to
expose the working electrode and/or the reference electrode
comprises ablating (e.g., laser or UV ablation) a portion of the
insulator. Alternatively, portions of insulator can be removed
manually or using methods known in the art such as grit blasting.
If the silver wire is not provided as a chloridized wire, it can be
chloridized prior to application of a membrane to the sensor. In
some circumstances, the electroactive surfaces are cleaned, using
standard methods known in the art, prior to membrane application.
The membrane 108 is then applied to at least a portion of the
sensor, such as but not limited to the working electrode. For
example, at least the portion of the membrane covering the working
electrode includes an enzyme (e.g., glucose oxidase) selected to
detect the analyte.
[0172] In some embodiments, the membrane is formed of a polymer
having a Shore hardness of from about 70A to about 55D. In some
embodiments, one or more of the membrane domains are formed of
polymers within this hardness range. However, in some embodiments,
only the resistance domain is formed from a polymer having a Shore
hardness of from about 70A to about 55D. While not wishing to be
bound by theory, it is believed that some polymers having a Shore
hardness in this range are sufficiently elastic yet resilient to
withstand multi-axis bending without substantial disruption of the
membrane's function. In some embodiments, the manufactured sensor
has a fatigue life of at least about 1,000 cycles of flexing of
from about 28.degree. to about 110.degree. at a bend radius of
about 0.125 inches. However, the sensors of other embodiments can
have longer fatigue lives (e.g., fatigue lives of about at least
10,000, 20,000, 30,000, 40,000 or 50,000 cycles or more of
flexing).
[0173] FIG. 7 illustrates yet another continuous analyte sensor of
an embodiment. In this particular embodiment, the sensor 700
comprises an elongated body 702 configured and arranged for
multi-axis bending. The elongated conductive body 702 comprises a
nonconductive material, a working electrode 704 located on the
elongated body, a reference electrode 706 located on the elongated
body, and a membrane 108 covering the working electrode. Conductive
pathways 708 and 710 connect the working electrode and the
reference electrode (respectively) to the sensor electronics (not
shown). In further embodiments, the sensor is configured and
arranged such that the fatigue life of the sensor is at least 1,000
cycles of flexing of from about 28.degree. to about 110.degree. at
a bend radius of about 0.125 inches. Some embodiments are
configured to provide longer fatigue lives, as described in the
section entitled "Multi-Axis Bending."
[0174] In some embodiments, the elongated body can be formed out of
any nonconductive material that can be formed into a thin,
elongated structure. In further embodiments, the nonconductive
material is a polymer. The polymer may be a nylon or polyester
filament, string or cord, etc. In some embodiments, the elongated
body is non-planar, such as described herein, and thus has a
non-rectangular cross-section. However, in certain embodiments, the
elongated body is planar. In some embodiments, the smallest
dimension (e.g., the diameter or width) of the elongated body is
less than about 0.004 inches. However, in certain embodiments,
relatively larger or smaller sensor diameters are acceptable, such
as described elsewhere herein.
[0175] The sensor 700 illustrated in FIG. 7 can be manufactured
using a variety of techniques known in the art. In one embodiment,
a method of making a flexible continuous analyte sensor adapted for
in vivo use includes the steps of providing a non-planar elongated
body 702 comprising a nonconductive material, applying a working
electrode 704, the reference electrode 706 and conductive pathways
708, 710 on the elongated body, and covering (at least) the working
electrode with a membrane 108, thereby producing a sensor capable
of multi-axis bending. In some embodiments, the working electrode
is applied by depositing a conductive material (e.g., at least one
of platinum, platinum-iridium, gold, palladium, iridium, graphite,
carbon, a conductive polymer and an alloy) on the elongated body.
In some embodiments, the conductive material is an ink, paint or
paste, and is deposited using thick film and/or thin film
deposition techniques known in the art, such as but not limited to
screen printing, jet printing, block printing, and the like.
However, in some embodiments the working electrode material is
plated on the elongated body, using plating techniques known in the
art, such as but not limited to electroplating. The reference
electrode can be applied by depositing a silver-containing material
on the elongated body. Similar to the working electrode, the
silver-containing material of the reference electrode can be
deposited using thick film and/or thin film deposition techniques,
various printing techniques known in the art, and/or plating. The
membrane is applied to at least the working electrode using
standard techniques. In particular, in some embodiments, the
membrane is applied by applying a polymer having a Shore hardness
of from about 70A to about 55D. As described with reference to
FIGS. 6A-C, the membrane can include a plurality of layers and/or
domains. The outermost domain in certain embodiments is the
resistance domain, which is configured to modulate the amount of
analyte and/or other substances diffusing into and/or through the
membrane. In some embodiments, the step of applying a membrane
comprises forming a resistance domain from a polymer having a Shore
hardness of from about 70A to about 55D. For example, additional
membrane domains (e.g., enzyme, interference, electrode domains,
etc.) can be formed of other polymers, as is known in the art and
described with reference to FIGS. 6A-C. While the sensor can be
manufactured by hand, in some embodiments, at least one step is
semi-automated. In certain embodiments, at least one step is
fully-automated. In some circumstances, two or more steps are
semi-automated or fully-automated.
[0176] Various sensor configurations that can be useful in
connection with certain embodiments are described in U.S. Pat. No.
7,529,574. For example, the analyte sensor can have an active
sensing region that includes an electrochemically active surface
and a membrane system that adheres to or is otherwise situated atop
the electrochemically active surface, wherein one or more
protruding structures of dielectric material may extend outwardly
from the electrochemically active surface (or other surface below
the membrane system) and serve as supportive structure(s) to the
membrane system. This particular configuration can be desirable
when forming the membrane by dip coating a liquid (e.g., a viscous
liquid, or curable liquid) onto the electrochemically active
surface or other underlying surface of the sensor. The protruding
structures are configured in a way such that they can support the
liquid before, during, or after the curing process. The protruding
structures can be in a form of one or more rings having, e.g.,
sharp corners or smooth edges, a square or semicircular cross
section, or any other desired configuration. For example, the
sensor can comprise a platinum wire coated with an insulating
material (e.g., a polyimide layer), and a silver wire can be
wrapped around a portion of this structure. A retractor (e.g., of
stainless steel or other suitable material) can be incorporated
into the sensor. Portions of the insulating material can be
removed, e.g., by laser ablating, with the remaining insulating
material forming the protruding structures (e.g., in the form of
protruding rings). Multiple protruding structures can be spaced
longitudinally along the surface of the platinum wire. After the
laser ablation operation, the platinum wire with protruding
structures is dip coated with one or more membrane layers, e.g.,
one or more layers such as an interference layer, a resistance
layer, an enzyme layer, and the like, as discussed elsewhere
herein. The protruding structures enable additional coating
material to adhere to the wire, e.g., through capillary action,
which can be desirable in forming thicker membranes in fewer steps,
or by reducing the number of dip coating/curing steps necessary to
form a particular thickness of layer.
[0177] U.S. Patent Publication No. 2007-0173711-A1 describes thin
film fabricating techniques that can be employed in the manufacture
of sensors of certain embodiments. In such techniques, a base layer
or substrate (conducting or nonconducting) is subjected to one or
more deposition steps (e.g., metallization steps to form one or
more conductive layers and/or electrode layers, or steps wherein an
electrically insulating layer such as a polyurethane or polyimide
is applied) to form at least a portion of the sensor. For example,
a base layer that is an electrically insulating layer such as a
polyimide substrate can be employed (e.g., self-supporting or
further supported by another material). The base layer can be a
polyimide tape, dispensed from a reel, to facilitate clean, high
density mass production, and/or production of sensors on both sides
of the tape.
[0178] Metallization steps involve application of a conductive
layer onto an insulating layer (or other layer). The conductive
layer can be provided as a plurality of thin film conductive
layers, e.g., a chrome-based layer for chemical adhesion to the
base layer, followed by subsequent formation of a thin film gold-
or platinum-based layer, or a chrome-based top layers on top of the
thin film gold- or platinum-based layer. The conductive layer may
also be formed of gold and/or chrome in different ratios and/or
other adhesive/conductive layers, such as titanium, platinum,
tungsten, or the like. In alternative embodiments, other electrode
layer conformations or materials can be used. The conductive layer
can be applied using electrode deposition, surface sputtering, or
another suitable process step. The electrical circuit of each
conductive layer typically comprises one or more conductive paths
with regions at a proximal end that form contacts and regions at a
distal end that form sensor electrodes. Generally, etching is
performed to define the electrical circuit of each layer.
Alternatively, "lift off" may be used, in which the photoresist
defines a pattern prior to metal sputtering, after which the
photoresist is dissolved away (along with the unwanted metal), and
the metal pattern is left behind. In further embodiments,
photoresisting is performed to protect the metalized pathway and
electrode and photoimaging is performed to cure specified areas.
For example, the conductive layer is covered with a selected
photoresist coating, followed by an etch step resulting in one or
more conductive paths. An electrically insulating cover layer (or
dielectric layer), such as a polymer coating, is then applied over
at least portions of the conductive layer. Suitable polymer
coatings for use as the insulating cover layer include, for
example, non-toxic biocompatible polymers such as polyimide,
biocompatible solder masks, epoxy acrylate copolymers, and the
like. Further, these coatings can be photoimageable to facilitate
photolithographic formation of apertures through to the conductive
layer to expose the electrode. Multiple metallization steps can be
employed to fabricate additional electrodes (e.g., sequentially)
when intervening insulating layers are employed. For example, the
resulting electrodes can be in a staggered configuration, so that
at least a portion of each electrode may be exposed, or the
conductive layers can be directly above each other. Alternatively,
multiple electrodes can be fabricated at the same time (e.g.,
simultaneously) on the same insulating substrate. The conductive
layers can be horizontally displaced from each other. The
electrodes can be further be configured in any way that allows the
electrodes to contact fluid when inserted into a body of a
patient.
[0179] Sensors of embodiments can include conductive layers
alternating with the insulating layers. In between every two
conductive layers there may be an insulating layer that serves to
isolate each conductive layer so that there is no trace
communication between the layers. Apertures can be formed in a top
insulating cover layer, or the top layer can be a conductive layer.
Electrodes can be in a vertical orientation atop each other, or
spaced sideways so that they are not directly on top of each other
(e.g., horizontally displaced). Conductive pathways that lead to
conductive contacts can be similarly positioned. The apertures can
be made through photolithographic development, laser ablation,
chemical milling, etching, or the like. The exposed electrodes
and/or contacts can also undergo secondary processing through the
apertures, such as additional plating processing, to prepare the
surfaces, and/or strengthen the conductive regions.
[0180] Typically, the conductive layers (or electrodes) are formed
by any of a variety of methods known in the art such as
photoresist, etching and rinsing to define the geometry of the
active electrodes. The electrodes can then be made
electrochemically active, for example by electrodeposition of
platinum black for the working and counter electrode, and silver
followed by silver chloride on the reference electrode. The sensor
chemistry layer is then disposed on the conductive layer by a
method other than electrochemical deposition, usually followed by
vapor crosslinking, for example with a dialdehyde, such as
glutaraldehyde, or a carbodiimide.
[0181] One or more sensors can be formed on a rigid flat substrate,
such as a polymer, glass, ceramic, composite, or metal. When
finished, the sensors may be removed from the rigid flat substrate
by a suitable method, such as laser cutting. Other materials that
can be used for the substrate include, but are not limited to,
stainless steel, aluminum, and plastic materials. Flexible sensors
can be formed in a manner which is compatible with
photolithographic mask and etch techniques, but where the sensors
are not physically adhered or attached directly to the substrate.
Each sensor thus comprises a plurality of thin film electrodes
formed between an underlying insulating base layer and an
insulating cover layer.
[0182] A flexible electrochemical sensor can be constructed
according to thin film mask techniques to include elongated thin
film conductors embedded or encased between layers of a selected
insulating material such as polyimide film or sheet. The sensor
electrodes at a tip end of the sensor distal segment are exposed
through one of the insulating layers for direct contact with
patient fluids, such as blood and/or interstitial fluids, when the
sensor is transcutaneously, subcutaneously, or intravenously
placed. The proximal segment and the contacts thereon are adapted
for electrical connection to a suitable monitor for monitoring
patient condition in response to signals derived from the sensor
electrodes. The sensor electronics may be separated from the sensor
by wire or be attached directly on the sensor. For example, the
sensor may be housed in a sensor device including a housing that
contains all of the sensor electronics, including any transmitter
necessary to transmit data to a monitor or other device. The sensor
device alternatively may include two portions, one portion housing
the sensor and the other portion housing the sensor electronics.
The sensor electronics portion could attach to the sensor portion
in a side-to-side or top-to-bottom configuration, or any other
configuration that would connect the two portions together.
[0183] If the sensor electronics are in a housing separated by a
wire from the sensor, the sensor electronics housing may be adapted
to be placed onto the user's skin or placed on the user's clothing
in a convenient manner. The connection to the monitor may be wired
or wireless. In a wired connection, the sensor electronics may
essentially be included in the monitor instead of in a housing with
the sensor. Alternatively, sensor electronics may be included with
the sensor as described above. A wire could connect the sensor
electronics to the monitor. Examples of wireless connection
include, but are not limited to, radio frequency, infrared, WiFi,
ZigBee and Bluetooth. Additional wireless connections further
include single frequency communication, spread spectrum
communication, adaptive frequency selection and frequency hopping
communication. In further embodiments, some of the electronics may
be housed on the sensor and other portions may be in a detachable
device. For example, the electronics that process and digitize the
sensor signal may be with the sensor, while data storage, telemetry
electronics, and any transmission antenna may be housed separately.
Other distributions of electronics are also possible, and it is
further possible to have duplicates of electronics in each portion.
Additionally, a battery may be in one or both portions. In further
embodiments, the sensor electronics may include a minimal antenna
to allow transmission of sensor data over a short distance to a
separately located transmitter, which would transmit the data over
greater distances. For example, the antenna could have a range of
up to 6 inches, while the transmitter sends the information to the
display, which could be over 10 feet away. The overall sensor
height of sensors fabricated by such methods (from base to top
insulating layer) can be on the order of microns (e.g., less than
250 microns, less than 100 microns, less than 50 microns, or less
than 25 microns). The base layer can be about 12 microns and each
insulating layer can be about 5 microns. The conductive/electrode
layers can be several thousand angstroms in thickness. Any of these
layers could be thicker if desired. The overall width of the sensor
can be as small as about 250 microns or less or 150 microns or
less. The length of the sensor can be selected depending upon the
depth and/or method of insertion. For example, for transcutaneous
or subcutaneous sensing, the sensor length may be about 2 mm to 5
mm, or for intravenous sensing up to about 3 cm or more.
[0184] In certain embodiments, the sensor (e.g., sensor 100) is
configured and arranged for multi-axis bending. The term "bending,"
as used herein, is a broad term, and is to be given its ordinary
and customary meaning to a person of ordinary skill in the art (and
is not to be limited to a special or customized meaning), and
refers without limitation to movement that causes the formation of
a curve, or a condition that is characterized as being not rigid or
not straight. In general, a structure capable of multi-axis bending
is configured for substantial bending in (e.g., within, along) two
or more planes (e.g., about two or more axes). In one exemplary
embodiment, with respect to the in vivo portion of a continuous
analyte sensor, there is no preferred bending point or location for
a bend and/or flex to occur. Accordingly, in some embodiments, the
sensor is configured and arranged to bend along a plurality of
planes, such as within 2, 3, 4, 5, 6, 7, 8, 9, 10 or more planes.
In a further embodiment, multi-axis bending includes flexing (e.g.,
curving, bending, deflecting) in at least three directions. For
example, in some embodiments, the sensor is configured to bend
and/or flex in 4, 5, 6, 7, 8, 9, 10 or more directions. In further
embodiment, the sensor is configured and arranged without preferred
bending points and/or locations along its in vivo portion.
Accordingly, in these embodiments, the sensor is configured and
arranged for multi-axis bending at any point along the length of
the sensor's in vivo portion (e.g., non-preferential bending). In
some embodiments, a sensor with multi-axis bending does not have a
preferred bending radius, thereby allowing substantial bending in
360.degree.. Since movements by the host can cause the sensor to
bend, it is believed that multi-axis bending extends sensor
lifetime (e.g., by preventing sensor breakage and/or degradation)
and affords greater host comfort (e.g., by moving/flexing/bending
with, instead of resisting, the host's movements, and/or causing
tissue damage).
[0185] Multi-axis bending of the certain embodiments includes a
combination of strength and flexibility. The material properties of
the components of the in vivo portion of the sensor (e.g., the
elongated conductive body, the conductive core, the insulator
and/or the membrane) and/or the geometry of the in vivo portion of
the sensor impart this combination of strength and flexibility that
enables multi-axis bending to the sensor. Material properties can
be described in a variety of ways known in the art. For example,
tensile strength is the stress at which a material breaks or
permanently deforms. Ultimate tensile strength (UTS) is the maximum
stress a material can withstand when subjected to tension,
compression or shearing, and is the maximum stress on a
stress-strain curve created during tensile tests conducted on a
sensor. Young's modulus (E) is a measure of the stiffness of an
isotropic elastic material, and can be determined from the slope of
a stress-strain curve described above. Yield strength is a measure
of the ability to bend and not snap (e.g., break). Fatigue is a
measure of the progressive and localized structural damage (e.g.,
the failure or decay of mechanical properties) that occurs when a
material is subjected to cyclic loading (e.g., stress). The maximum
stress values are less than the ultimate tensile stress limit, and
may be below the yield stress limit of the material.
[0186] Fatigue life is the number of cycles of deformation required
to bring about failure of the test specimen under a given set of
oscillating conditions. Fatigue life can be determined by fatigue
testing, such as by testing with a device configured to repeatedly
bend, pull, compress and/or twist the device. For example,
fatigue-life testing can be performed on a plurality of sensors and
then the tensile strength and/or Young's modulus mathematically
determined from data collected during the sensor testing. For
example, sensors to be tested can include pre-bent elbows at a
predetermined angle, such as but not limited to into a 10, 20, 30,
40, 50, 60, 70 or 80-degree elbows, wherein the elbows have a bend
radius of about 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045 or
0.05-inches. Using a fatigue-testing machine (e.g., via a Bose
ElectroForce.RTM. 3200 fatigue-testing unit, Bose Corporation, Eden
Prairie, Minn., USA), the elbows can be repeatedly pulled open
and/or pushed closed a predetermined amount, such as but not
limited to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15-mm
or more, and/or through a plurality of deflection ranges, such as
but not limited to at a cycle frequency of about 1, 2, 3, 4, 5, 6,
7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 70, 18, 19 or 20 Hertz. For
example, a peak-to-peak deflection of 4-mm means that the elbow was
pushed in the closed direction 2-mm from its initial condition, as
well as pulled open 2-mm from its initial condition. The number of
cycles (of pulling/pushing) to failure of the device (e.g.,
breaking, buckling, cracking, fraying) can be counted. In one
exemplary embodiment, 60.degree. elbows having a bend radius of
about 0.025-inches (e.g. bent sensors) can withstand at least about
5,000-10,000 cycles of 5-mm peak-to-peak displacement. In another
exemplary embodiment, the elbows can withstand at least about
10,000-70,000 cycles of 4-mm peak-to-peak displacement. In another
exemplary embodiment, the elbows can withstand at least about
1,000,000-10,000,000 cycles of 2-mm peak-to-peak displacement. In
another exemplary embodiment, the elbows can withstand at least
about 100,000-600,000 cycles of 3-mm peak-to-peak displacement.
[0187] These data (above) can be used to calculate the sensor's
tensile strength, Young's modulus, and the like, as is understood
by one skilled in the art. In some embodiments, the sensor is
configured for multi-axis bending to an angle of at least about
60.degree., 70.degree., 80.degree., 90.degree., 100.degree.,
110.degree. or 120.degree. or more. In some embodiments, a sensor
with multi-axis bending does not have a preferred bending radius,
thereby allowing substantial bending in 360.degree. about the
sensor's longitudinal axis. In some embodiments, the sensor is
configured and arranged such that the ultimate tensile strength of
the elongated conductive body is from about less than about 80, 80,
90, 100, 110, 120, 130, 140 or 150 kPsi (551 MPa) to about 160,
170, 180, 190, 200, 210, 220 or 500 kPsi (1517 MPa) or more. In
some embodiments, the Young's modulus of the sensor is from more
than about 165, 165, 170, 175, 180, 185 or 190 GPa to less than
about 195, 200, 205, 210, 215 or 220 GPa. In some embodiments, the
yield strength of the elongated conductive body (e.g., the sensor,
conductive core) is at least about 70, 100, 150, 200, 250, 300,
350, 400, 450, 500, 600, 700, 800, 900, 1000, 1250, 1500, 1750,
2000, 2250, 2500, 2750, or 3000 MPa or more. In some embodiments,
the fatigue life of the sensor is at least about 1,000, 2,000,
3,000, 4,000, or 5,000 cycles or more when the sensor is pre-bent
into an elbow comprising a bend angle of at least 60.degree. and a
bend radius of about 0.05-inches or less. In some embodiments, the
fatigue life of the sensor is at least 1,000 cycles of flexing of
from about 28.degree. to about 110.degree. and a bend radius of
about 0.125-inches.
[0188] The analyte sensors (e.g., electrodes and membrane systems)
of some embodiments are coaxially and/or concentrically formed.
Namely, the electrodes (e.g., elongated conductive bodies) and/or
membrane systems all share the same central axis. While not wishing
to be bound by theory, it is believed that a coaxial design of the
sensor enables a symmetrical design without a preferred bend
radius. In contrast to conventional sensors comprising a
substantially planar configuration that can suffer from regular
bending about the plane of the sensor, the coaxial design of the
certain embodiments do not have a preferred bend radius and
therefore are not subject to regular bending within and/or about a
particular plane (which can cause fatigue failures and the like).
However, non-coaxial sensors can be implemented with the sensor
system of the some embodiments.
[0189] In addition to the above-described advantages, the coaxial
sensor design of some embodiments enables the diameter of the
connecting end of the sensor (proximal portion) to be substantially
the same as that of the sensing end (distal portion). For sensors
configured and arranged for implantation into a host's circulatory
system, this configuration enables the protective slotted sheath to
insert the sensor into a catheter and subsequently slide back over
the sensor and release the sensor from the protective slotted
sheath, without complex multi-component designs. For sensors
configured for transcutaneous implantation, this configuration
enables a needle to implant the sensor and then slide over the
sensor when the needle is withdrawn.
[0190] FIG. 1B is a schematic illustrating an elongated conductive
body 102 (also referred to as the "elongated body") in one
embodiment, wherein the elongated conductive body is formed from at
least two materials and/or layers of conductive material, as
described in greater detail elsewhere herein. In some embodiments,
the term "electrode" can be used herein to refer to the elongated
conductive body, which includes the electroactive surface that
detects the analyte. In some embodiments, the elongated conductive
body provides an electrical connection between the electroactive
surface (e.g., working electrode) and sensor electronics (not
shown). In certain embodiments, each electrode (e.g., the elongated
conductive body, on which the electroactive surface is located) is
formed from a fine wire with a diameter of from about 0.001 inches
or less to about 0.01 inches or more, for example, and is formed
from, e.g., a plated insulator, a plated wire, or bulk electrically
conductive material. For example, in some embodiments, the wire
and/or elongated conductive body used to form a working electrode
is about 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009,
0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04 or 0.045 inches in
diameter.
[0191] In some embodiments, the working electrode (e.g., the
elongated conductive body including an electroactive surface)
comprises a wire formed from a conductive material, such as
platinum, platinum-iridium, palladium, graphite, gold, carbon,
conductive polymer, alloys, or the like
[0192] In some embodiments, the working electrode is formed of
platinum-iridium or iridium wire. In general, platinum-iridium and
iridium materials are generally stronger (e.g., more resilient and
less likely to fail due to stress or strain fracture or fatigue).
While not wishing to be bound by theory, it is believed that
platinum-iridium and/or iridium materials can facilitate
fabrication of a wire with a smaller diameter to further decrease
the maximum diameter (size) of the sensor (e.g., in vivo portion).
Advantageously, with respect to intravascularly-implanted sensors,
a smaller sensor diameter both reduces the risk of clot or thrombus
formation (or other foreign body response) and allows the use of
smaller catheters.
[0193] Referring to FIG. 1B, in some embodiments, the elongated
conductive body 102 comprises at least two concentric layers (e.g.,
a composite structure). In a further embodiment, the elongated
conductive body comprises a core 110 and a first layer 112. The
core is formed from one of the at least two materials referred to
above. For example, the core can be formed of a polymer, a metal,
an alloy and the like. In some embodiments, the core is formed from
a conductive polymer, such as but not limited to polyaniline and
polypyrrole. In some embodiments, a conductive material is added to
(e.g., mixed with and/or applied to) a non-conductive polymer,
whereby the polymer core is rendered conductive. For example, in
some embodiments, one or more conductive metals (e.g., carbon,
gold, platinum, iridium, etc.), such as but not limited to
particles, can be mixed with the uncured polymer, which can be
formed into the core. Alternatively, the core can comprise an inner
core and an outer core, in some embodiments. For example, platinum,
iridium or gold particles can be ion-implanted on the surface of a
polymer inner core, such that the particles form an outer core. For
example, a polymer filament fiber can be ion-implanted with gold,
such that the treated filament fiber is conductive. In some
embodiments, the core is formed from a metal, such as but not
limited to at least one of stainless steel, tantalum, titanium
and/or an alloy thereof. For example, in one embodiment, the core
is formed of an extruded stainless steel, tantalum, titanium and/or
an extruded alloy. In some embodiments, the material of the core is
processed to provide the strength and flexibility necessary for
multi-axis bending. Processing the metal changes its properties,
such as but not limited to by compressing and/or rearranging the
metal's crystalline lattice. For example, tempering can make a
metal less brittle and more springy; hardening can make a metal
hold its shape better. Accordingly, in certain embodiments, the
core is formed of a metal that has been processed to provide the
requisite combination of strength and flexibility (e.g., an
ultimate tensile strength of from about less than 80, 80, 90, 100,
110, 120, 130, 140 or 150 kPsi (551 MPa) to about 160, 170, 180,
190, 200, 210, 220 or 500 kPsi (3297 MPa)) or more. For example, in
some embodiments, the core is formed from a metal that has been
annealed, tempered, normalized, hardened, work-hardened,
full-processed, case hardened, draw air hardened, cold worked
and/or the like, to render it more stiff. In one embodiment, the
core is formed from full-processed platinum. In another embodiment,
the core is formed from work-hardened platinum-iridium.
[0194] In some embodiments, the surface of the elongated conductive
body and/or the core is treated to remove initiation sites (e.g.,
locations/points of irregularity, where sensor breaking tends to
begin), to smooth and/or clean the surface, to prepare it for
application of the next material, and/or the like. Suitable
treatments include but are not limited to electro-polishing,
etching, application of a tie layer, electro-deposition, and
electrostatic deposition.
[0195] In some embodiments, the elongated conductive body (and/or
the core, and/or the sensor) is wire-shaped. However, the
wire-shape can include one of a variety of cross-sectional shapes,
such as but not limited to a circle, an oval, a rectangle, a
triangle, a cross, a star, a cloverleaf, an X-shape, a C-shape, an
irregular or other non-circular configuration, and the like. The
elongated conductive body includes a diameter and/or a smallest
dimension (e.g., width) of about less than 0.002, 0.002, 0.003,
0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.015, 0.02, 0.025,
0.03, 0.035, 0.04 or 0.045 inches or more in diameter. The
elongated conductive body can be provided as a reel and/or extended
lengths that are subsequently processed and/or singularized into
individual sensor lengths.
[0196] In some embodiments, the elongated conductive body 102
comprises a first layer 112 applied to a core 110. In some
embodiments, the first layer is applied to the core such that they
are electrically connected (e.g., in electrical contact, such that
a current can pass therebetween). The first layer can be formed of
a variety of conductive materials, such as but not limited to at
least one of platinum, platinum-iridium, gold, palladium, iridium,
graphite, carbon, conductive polymers and an alloy. In certain
embodiments, the first layer is relatively thin, such as but not
limited to a thickness of from about less than 50, 50, 55, 60, 65,
70, 75, 80, 85, 90, 95 or 100 micro-inches to about 125, 150, 175,
200, 225, 250, 275 or 300 micro-inches, or thicker. As is described
elsewhere herein with greater detail, at least a portion of the
surface of the first layer provides the sensor's electroactive
surface (e.g., working electrode). For example, as described
herein, in some embodiments, the electroactive surface is exposed
through a window formed in the insulator. In some embodiments, the
surface of the applied first layer is treated prior to application
of membrane materials, such as to optimize the surface for membrane
attachment and for function as an electroactive surface. For
example, the surface can be cleaned, smoothed, etched, and the
like. Advantageously, forming the conductive core of an inexpensive
yet strong and flexible inner body with a thin layer of the costly
electroactive surface material enables a substantial reduction in
material costs.
[0197] In some embodiments, a conductive paste comprising a mixture
of material (e.g., an ink) and an enzyme (e.g., glucose oxidase)
may be applied to the layer surrounding the core, or applied
directly to the core. The conductive paste may also include an
interference reducing substance, mediator, or diffusion limiting
polymers. Use of the conductive the paste may reduce or eliminate
the need for certain membrane layers (e.g., the enzyme layer).
[0198] The first layer 112 can be applied to the core 110 using a
variety of manufacturing methods. For example, in some embodiments,
the first layer is co-extruded with the core using known
techniques, such as but not limited to metal-on-metal or
metal-on-polymer extrusion techniques. Some useful co-extrusion
techniques are described in U.S. Pat. Nos. 7,416,802, 7,268,562,
7,153,458, 7,280,879, 5,324,328 and 6,434,430. In one exemplary
embodiment, a stainless steel inner body is co-extruded with a
platinum first layer, such as but not limited to through a die, to
form a thin reel of 0.005-inch diameter wire having a stainless
steel core with a 100-micro-inch layer of platinum thereon.
[0199] In some embodiments, the first layer 112 is applied to the
core 110 (which, in some embodiments, is pre-treated as described
above) using a thin film or thick film technique (e.g., spraying,
electro-depositing, vapor-depositing, dipping, spin coating,
sputtering, evaporation, printing or the like). For example, in one
embodiment, the first layer is applied by dipping the core into a
solution of the first layer material and drawing out the core at a
speed that provides the appropriate first layer thickness. However,
any known thin or thick film method can be used to apply the first
layer to the core, as will be appreciated by one skilled in the
art. Some examples of thin and/or thick film manufacturing
techniques can be found in U.S. Patent Publication No.
2005-0181012-A1, U.S. Patent Publication No. 2006-0036143-A1, U.S.
Patent Publication No. 2007-0163880-A1, U.S. Patent Publication No.
2006-0270923-A1, U.S. Patent Publication No. 2007-0027370-A1, U.S.
Patent Publication No. 2006-0015020-A1, U.S. Patent Publication No.
2006-0189856-A1, U.S. Patent Publication No. 2007-0197890-A1, U.S.
Patent Publication No. 2006-0257996-A1, U.S. Patent Publication No.
2006-0229512-A1, U.S. Patent Publication No. 2007-0173709-A1, U.S.
Patent Publication No. 2006-0253012-A1, U.S. Patent Publication No.
2006-0195029-A1, U.S. Patent Publication No. 2008-0119703-A1, U.S.
Patent Publication No. 2008-0108942-A1, and U.S. Patent Publication
No. 2008-0200789-A1.
[0200] In some embodiments the first layer 112 is deposited onto
the core 110. For example, in some embodiments, the first layer is
plated (e.g., electroplated) onto the core. In one exemplary
embodiment, a thin layer of platinum is plated onto a tantalum core
by immersing the inner body in a platinum-containing solution and
applying a current to the inner body for an amount of time, such
that the desired thickness of platinum first layer is generated
and/or achieved. Description of deposition methods and devices
therefore can be found in U.S. Pat. Nos. 7,427,338, 7,425,877,
7,427,560, 7,351,321 and 7,384,532.
[0201] In still other embodiments, the core 110 is embedded in
insulator and a working electrode body 112 is attached, such that
the core and the working electrode body are electrically (e.g.,
functionally, operably) connected, such as described with reference
to FIGS. 3A and 3B. For example, in some embodiments, a working
electrode body is formed as a foil that is attached to the core,
such as with adhesive, welding and/or an intermediate layer of
conductive material to provide adhesion between the core and the
working electrode body material (e.g., at tie layer). In some
embodiments, multiple layers are applied on top of the core. In
some embodiments, each layer possesses a finite interface with
adjacent layers or together forms a physically continuous structure
having a gradient in chemical composition. In another embodiment,
the working electrode body is a C-clip or snap-ring that is
attached by compression about and/or around the core. In some
embodiments, the working electrode body is attached over a window.
In other embodiments, there is no window, instead, the working
electrode body is configured to pierce the insulator and to
physically contact the underlying core, such that the working
electrode body and the core are operably connected. In some
embodiments, a conductive metal C-clip is attached to the core with
adhesive, welding and/or a tie layer. In yet another embodiment, an
adhesive is attached to the core, followed by wrapping a conductive
foil there-around.
[0202] The elongated conductive body 102 can be manufactured using
a variety of manufacturing techniques. In some embodiments, the
first layer 112 is applied to the core 110 in a substantially
continuous process. For example, in some embodiments, the
manufacturing of the elongated conductive body involves a
reel-to-reel process. In other embodiments, a sheet-fed technique
is used. In some embodiments, application of the first layer to the
core can be by either a semi-automated or fully-automated process.
Automation of some or all manufacturing steps generally requires
the use of one or more machines, such as robotic devices, that are
configured and arranged to perform the manufacturing step(s). In
some embodiments, one manufacturing step can be automated, such as
production of the elongated conductive body. However, in other
embodiments, two or more of the manufacturing steps can be
automated. For example, a device can be configured to perform two
or more of the steps, or two or more devices can perform the steps.
In some embodiments, when multiple devices are used, the devices
are connected, coupled together, interconnected, and linked
functionally and/or physically. For example, in some embodiments,
the product of one device is fed directly into the next device, and
so on. In one exemplary embodiment, a reel of previously
manufactured core, such as a stainless-steel, tantalum or titanium
wire, can be fed substantially continuously through a device
configured to electroplate the core with platinum, gold, carbon or
the like, such that a reel of plated wire is generated. For
example, a manufacturing device and/or system can be configured to
automatically co-extrude stainless-steel and platinum to
generate/produce a reel of wire-shaped elongated conductive body
comprising a stainless-steel core and platinum first layer.
Examples of continuous manufacturing processes can be found in U.S.
Pat. Nos. 6,103,033, 5,879,828, 5,714,391, 7,429,552, 7,402,349 and
7,387,811.
[0203] In a further embodiment, the first layer comprises an
electroactive surface (e.g., the portion exposed through the window
106). The exposed electroactive surface of the first layer is the
working electrode, in some embodiments. For example, if the sensor
is an enzymatic electrochemical analyte sensor, the analyte
enzymatically reacts with an enzyme in the membrane covering at
least a portion of the electroactive surface, which can generate
electrons (e.sup.-) that are detected at the electroactive surface
as a measurable electronic current. For example, in the detection
of glucose wherein glucose oxidase produces hydrogen peroxide as a
byproduct, hydrogen peroxide reacts with the surface of the working
electrode producing two protons (2H.sup.+), two electrons
(2e.sup.-) and one molecule of oxygen (O.sub.2), which produces the
electronic current being detected.
[0204] As previously described with reference to FIG. 1A and as
shown in FIG. 1C, an insulator 104 is disposed on (e.g., located
on, covers) at least a portion of the elongated conductive body
102. In some embodiments, the sensor is configured and arranged
such that the elongated body includes a core 110 and a first layer
112, and a portion of the first layer is exposed via window 106 in
the insulator. In other embodiments, the sensor is configured and
arranged such that the elongated body includes a core embedded in
an insulator, and a portion of the core is exposed via the window
in the insulator. For example, in some embodiments, the insulating
material is applied to the elongated body (e.g., screen-, ink-jet
and/or block-printed) in a configuration designed to leave a
portion of the first layer's surface (or the core's surface)
exposed. For example, the insulating material can be printed in a
pattern that does not cover a portion of the elongated body. In
another example, a portion of the elongated body is masked prior to
application of the insulating material. Removal of the mask, after
insulating material application, exposes the portion of the
elongated body.
[0205] In some embodiments, the insulating material 104 comprises a
polymer, for example, a non-conductive (e.g., dielectric) polymer.
Dip-coating, spray-coating, vapor-deposition, printing and/or other
thin film and/or thick film coating or deposition techniques can be
used to deposit the insulating material on the elongated body
and/or core. For example, in some embodiments, the insulating
material is applied as a layer of from about less than 5, 5, 10 or
15-microns to about 20, 25, 30 or 35-microns or more in thickness.
In some embodiments, the insulator is applied as a single layer of
material. In other embodiments, the insulator is applied as two or
more layers, which are comprised of either the same or different
materials. In some embodiments, the insulating material comprises
at least one of polyurethane, polyimide and parylene. In one
embodiment, the insulating material comprises parylene, which can
be an advantageous polymer coating for its strength, lubricity, and
electrical insulation properties. Generally, parylene is produced
by vapor deposition and polymerization of para-xylylene (or its
substituted derivatives). However, any suitable insulating
material, such as but not limited to a dielectric ink, paste or
paint, can be used, for example, fluorinated polymers,
polyethyleneterephthalate, polyurethane, polyimide, other
nonconducting polymers, or the like. In some embodiments, glass or
ceramic materials can also be employed. Other materials suitable
for use include surface energy modified coating systems such as are
marketed under the trade names AMC18, AMC148, AMC141, and AMC321 by
Advanced Materials Components Express of Bellafonte, Pa. In some
alternative embodiments, however, the conductive core may not
require a coating of insulator. In certain embodiments, the
insulating material defines an electroactive surface of the analyte
sensor (e.g., the working electrode). For example, in some
embodiments a surface of the conductive core (e.g., a portion of
the first layer 112) either remains exposed during the insulator
application or a portion of applied insulator is removed to expose
a portion of the conductive core's surface, as described above.
[0206] In some embodiments, in which the sensor has an insulated
elongated body, a portion of the insulating material is stripped or
otherwise removed, for example, by hand, excimer lasing, chemical
etching, laser ablation, grit-blasting (e.g., with sodium
bicarbonate or other suitable grit), or the like, to expose the
electroactive surfaces. In one exemplary embodiment, grit blasting
is implemented to expose the electroactive surface(s), for example,
by utilizing a grit material that is sufficiently hard to ablate
the polymer material yet also sufficiently soft so as to minimize
or avoid damage to the underlying metal electrode (e.g., a platinum
electrode). Although a variety of "grit" materials can be used
(e.g., sand, talc, walnut shell, ground plastic, sea salt, and the
like), in some embodiments, sodium bicarbonate is an advantageous
grit-material because it is sufficiently hard to ablate, e.g., a
parylene coating without damaging, e.g., an underlying platinum
conductor. One additional advantage of sodium bicarbonate blasting
includes its polishing action on the metal as it strips the polymer
layer, thereby eliminating a cleaning step that might otherwise be
necessary. In some embodiments, the opening in the insulator,
through which the surface of the first layer is exposed, is
referred to as a "window" 106.
[0207] Due to the small sizes of the sensors in some embodiments,
it can be difficult to precisely remove the insulator over one
insulated conductive core without affecting, and possibly removing,
the insulator over an adjacent conductive core or over other parts
of the sensor. However, in some embodiments, the insulator is
configured such that the precision of laser ablation is
substantially improved. For example, in some embodiments, the
insulator is configured such that two different types of lasers can
be used to ablate separate portions of the insulator. For example,
if the insulators of two elongated bodies are different materials
(i.e. one is polyurethane and another is TEFLON.RTM. or another
type of polytetrafluoroethylene), then it is possible to
selectively ablate the insulator off of one of the elongated bodies
and to not remove insulator from the other elongated body in the
same region of the sensor. In some embodiments, the two insulation
materials require different laser parameters for optimal ablation,
such that a first laser setup could be used to ablate a first
material but not the second material, and a second laser setup
could be used to ablate a second material but not the first
material. In another example, for a sensor containing two elongated
bodies, the insulator covering one elongated body can be configured
for laser ablation with an ultraviolet laser (e.g., using a
wavelength of about 200 nm), and the other elongated body can be
configured for laser ablation with an infrared laser (e.g., using a
wavelength of about 1000 nm). In another embodiment, the insulator
materials are selected such that the insulator of a first elongated
body requires a substantially higher laser power to be ablated than
the insulator of a second elongated body. For example, the
insulator over the two elongated bodies can be the same, except
that the insulator of the first elongated body is thicker than the
insulator of the second elongated body. In another example, the
insulator on each of the elongated bodies has a different
thickness, such that a single laser is used to remove the insulator
over both cores, except that the window in the thinner insulator is
formed more quickly than the window in the thicker insulator. For
example, the insulator of one elongated body can be from about
0.0001 inches to about 0.0003 inches in thickness, and the
insulator of one elongated body can be from about 0.0008 inches to
about 0.0010 inches in thickness. In yet another example, a
colorant can be added to the insulator of one of the elongated
bodies, to modify the amount of energy that is absorbed from the
laser. For example, adding a dark colorant or other absorptive
material to the first insulator but not the second insulator can
cause the first insulator to absorb much more energy of the laser
than the non-colored second insulator. In this way, a small amount
of laser energy would ablate one wire but not the other, but a
large amount of laser energy would ablate both. As is understood by
one skilled in the art, the setup of the laser can be adjusted, to
fine-tune the insulator removal process. For example, the laser
pulse width and power level can be adjusted to modify and/or
modulate the amount of insulator removed, the rate of removal,
and/or the like. This principle can be used for assemblies (e.g.,
sensors) of three or more elongated bodies (e.g., cores, wires).
The same principle may be applied to chemical ablation, where
different solvents are required for the different insulation layers
such that they can be selectively ablated. The same principle may
also be used with plasma ablation, where different plasma settings
or amounts of energy are required to ablate the different
materials.
[0208] The electroactive surface of the working electrode is
exposed by formation of a window 106 in the insulator 104. The
electroactive window 106 of the working electrode is configured to
measure the concentration of an analyte. For example, in an
enzymatic electrochemical sensor for detecting glucose, the working
electrode measures the hydrogen peroxide produced by an enzyme
catalyzed reaction of the analyte being detected and creates a
measurable electronic current. For example, in the detection of
glucose wherein glucose oxidase (GOX) produces hydrogen peroxide as
a byproduct, hydrogen peroxide reacts with the surface of the
working electrode producing two protons (2H.sup.+), two electrons
(2e.sup.-) and one molecule of oxygen (O.sub.2), which produces the
electronic current being detected. The sensor can be configured to
detect other analytes by substituting an enzyme that metabolizes
the analyte of interests for GOX, as is understood by one skilled
in the art.
[0209] In the embodiments illustrated in FIGS. 1A, 1C, 2A, 4B, and
5A through 5D, a radial window 106 is formed through the insulating
material 104 to expose a circumferential electroactive surface of
the working electrode (e.g., first layer 112). In other
embodiments, such as those shown in FIGS. 3A, 3B, 4A, 5B and 5D, a
radial or non-radial window 106 is formed (e.g., for electrical
connection to the working electrode body 112) by removing only a
portion of the insulating material 104. Additionally, a section of
electroactive surface of the reference electrode 114 is exposed, in
some embodiments (not shown). For example, the sections of
electroactive surface can be masked during deposition of an outer
insulating layer and/or etched after deposition of an outer
insulating layer. In some embodiments, a plurality of micro-windows
comprises the electroactive surface of the working electrode,
wherein the sum of the micro-window surface areas is substantially
equal to the window 106 electroactive surface area. In certain
embodiments, the plurality of micro-windows are spaced and/or
staggered along a length of the conductive core.
[0210] In some embodiments, the window 106 (or the working
electrode body 112) is sized to provide an electroactive surface
(e.g., working electrode) having an area such that the sensor
functions in the picoAmp range (e.g., when the analyte is glucose,
a sensitivity of from about 1 to about 300 pA per mg/dL glucose, or
a sensitivity of from about 5 to about 100 pA per mg/dL glucose, or
from about 5 to about 25 pA per mg/dL glucose, and or from about 4
to about 7 pA per mg/dL). For an electrode having an electroactive
surface area of about 0.3 mm.sup.2, the current density
(sensitivity divided by surface area) is or from about 17
pA/mg/dL/mm.sup.2 to about 1000 pA/mg/dL/mm.sup.2, or from about 3
pA/mg/dL/mm.sup.2 to about 83 pA/mg/dL/mm.sup.2, or 13
pA/mg/dL/mm.sup.2 to about 23 pA/mg/dL/mm.sup.2. In some
embodiments, the working electrode has a diameter of from about
0.001 inches or less to about 0.01 inches or more, or from about
0.002 inches to about 0.008 inches, or from about 0.004 inches to
about 0.005 inches. The length of the window can be from about 0.1
mm (about 0.004 inches) or less to about 2 mm (about 0.078 inches)
or more, or from about 0.5 mm (about 0.02 inches) to about 0.75 mm
(0.03 inches). In such embodiments, the exposed surface area of the
working electrode is from about 0.000013 in.sup.2 (0.0000839
cm.sup.2) to about 0.0025 in.sup.2 (0.016129 cm.sup.2) (assuming a
diameter of from about 0.001 inches to about 0.01 inches and a
length of from about 0.004 inches to about 0.078 inches). The
exposed surface area of the working electrode is selected to
produce an analyte signal with a current in the picoAmp range.
However, a current in the picoAmp range can be dependent upon a
variety of factors, for example the electronic circuitry design
(e.g., sample rate, current draw, A/D converter bit resolution,
etc.), the membrane system (e.g., permeability of the analyte
through the membrane system), and the exposed surface area of the
working electrode. Accordingly, the exposed electroactive working
electrode surface area can be selected to have a value greater than
or less than the above-described ranges taking into consideration
alterations in the membrane system and/or electronic circuitry. In
certain embodiments of a glucose sensor, it can be advantageous to
minimize the surface area of the working electrode while maximizing
the diffusivity of glucose in order to optimize the signal-to-noise
ratio while maintaining sensor performance in both high and low
glucose concentration ranges.
[0211] In some embodiments, the exposed surface area (e.g.,
electroactive surface) of the working (and/or other) electrode
(e.g., conductive core) can be increased by altering the
cross-section of the electrode itself. For example, in some
embodiments the cross-section of the working electrode can be
defined by a cross, star, cloverleaf, ribbed, dimpled, ridged,
irregular, or other non-circular configuration; thus, for any
predetermined length of electrode, a specific increased surface
area can be achieved (as compared to the area achieved by a
circular cross-section). Increasing the surface area of the working
electrode can be advantageous in providing an increased signal
responsive to the analyte concentration, which in turn can be
helpful in improving the signal-to-noise ratio, for example. In
some embodiments, application of the insulator to the conductive
core can be accomplished by a substantially continuous process,
which can be semi- or fully-automated, such as in a manner similar
to some methods described for formation/manufacture of the
conductive core.
[0212] In some embodiments, the sensor 100 further comprises a
reference electrode 114. The reference electrode 114, which can
function as a reference electrode alone, or as a dual reference and
counter electrode, is formed from silver, silver/silver chloride,
or the like. In some embodiments, the reference electrode 114 is
juxtapositioned and/or twisted with or around at least a portion of
the sensor. For example, in FIG. 2A, the reference electrode is a
silver wire helically twisted and/or wrapped and/or wound around
the working electrode. This assembly of "wires" is then optionally
coated or adhered together with an insulating material, similar to
that described above, so as to provide an insulating
attachment.
[0213] In some embodiments, a silver wire is formed onto and/or
fabricated into the sensor and subsequently chloridized to form
silver/silver chloride reference electrode. Advantageously,
chloridizing the silver wire as described herein enables the
manufacture of a reference electrode with optimal in vivo
performance. Namely, by controlling the quantity and amount of
chloridization of the silver to form silver/silver chloride,
improved break-in time, stability of the reference electrode and
extended life has been shown with some embodiments. Additionally,
use of silver chloride as described above allows for relatively
inexpensive and simple manufacture of the reference electrode.
[0214] Referring to FIGS. 1B-1C, in some embodiments, the reference
electrode 114 comprises a silver-containing material applied over
at least a portion of the insulating material 104. In some
embodiments, the silver-containing material is applied using thin
film and/or thick film techniques, such as but not limited to
dipping, spraying, printing, electro-depositing, vapor deposition,
spin coating, and sputter deposition, as described elsewhere
herein. For example, a silver or silver-chloride-containing paint
(or similar formulation) is applied to a reel of the insulated
conductive core, in one embodiment. In another example, the reel of
insulated elongated body (or core) is cut into single unit pieces
(e.g., "singularized") and a silver-containing ink is pad printed
thereon. In still other embodiments, the silver-containing material
is applied as a silver foil. For example, an adhesive can be
applied to an insulated elongated body, around which the silver
foil is then wrapped in. Alternatively, the sensor can be rolled in
Ag/AgCl particles, such that a sufficient amount of silver sticks
to and/or embeds into and/or otherwise adheres to the adhesive for
the particles to function as the reference electrode. In some
embodiments, the sensor's reference electrode includes a sufficient
amount of chloridized silver that the sensor measures and/or
detects the analyte for at least three days.
[0215] In some embodiments, the sensor is formed from an elongated
body 102 (e.g., elongated conductive body), such as that shown in
FIG. 1B, wherein the elongated body includes a core 110, a first
layer 112, an insulator 104, and a layer of silver-containing
material 114. In some embodiments, such as that shown in FIG. 1C,
the electroactive surface of the elongated body (e.g., also the
(electroactive) surface of the first layer 112) is exposed by
formation of a window 106 through both the silver-containing
material and the insulator. In one exemplary embodiment, the
elongated body of FIG. 1B is provided as an extended length on a
reel that is singularized into a plurality of pieces having a
length (e.g., less than 0.5, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5,
5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12,
12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5,
19, 19.5, 20, 20.5, 21, 21.5, 22, 22.5, 23, 23.5 or 24-inch or
longer lengths) suitable for a selected sensor configuration. For
example, a first sensor configured for transcutaneous implantation
can employ 2.5-inch lengths, while a second sensor configured for
transcutaneous implantation can employ 3-inch lengths. In another
example, a first sensor configured for implantation into a
peripheral vein of an adult host can employ a 3-inch length, while
a second sensor configured for implantation into a central vein of
an adult host can employ a 12-inch length. The window is formed on
each sensor, such as by scraping and or etching a radial window
through the silver-containing material and the insulator such that
the platinum surface is exposed (e.g., the electroactive surface of
the "working electrode"). In some embodiments, a reel of elongated
body is singularized and then the windows are formed. In other
embodiments, the windows are formed along the length of the reel of
elongated body, and then later singularized. In a further
embodiment, additional manufacturing steps are performed prior to
singularization. A membrane 108 is applied to the exposed
electroactive surface (e.g., the working electrode) defined by the
edges of the window, such that the electroactive surface can
function as the working electrode of the sensor to generate a
signal associated with an analyte (e.g., when the sensor is in
contact with a sample of a host). Alternative manufacturing
techniques and/or sequences of steps can be used to produce sensors
having the configuration shown in FIG. 1C, such as but not limited
to masking a portion of the elongated body (or core) prior to
application of the insulator and the silver-containing
material.
[0216] FIG. 1B is an illustration showing layers cut away, but in
the fabrication process the material typically obtained has all
layers ending at a tip. A step of removing layers 104 and 114 can
be performed so as to form window(s). FIG. 1D illustrates the
results of this removal/cutting away process through a
side-view/cross-section. The removal process can be accomplished by
the methods already described or other methods as known in the art.
In one embodiment the removal step is conducted, e.g., by laser
skiving, and can be performed in a reel-to-reel process on a
continuous strand. The removed area can be stepped, for example, by
removing different layers by different lengths (FIG. 1D). In such a
fabrication method involving a continuous strand, the sensors can
be singularized after the removal step. In some embodiments, if the
core is a metal, an end cap may be employed, e.g., by dipping,
spraying, shrink tubing, crimp wrapping, etc., an insulating or
other isolating material onto the tip. If the core is a polymer
(e.g., hydrophobic material), an end cap may be omitted. For
example, in the sensor depicted in FIG. 1D, an end cap 120 (e.g.,
of a polymer or an insulating material) or other structure may be
provided over the core (e.g., if the core 110 is not insulating).
FIG. 1E can be considered to build on a general structure as
depicted in FIG. 1B, in that two or more additional layers are
added to create one or more additional electrodes. Methods for
selectively removing two or more windows to create two or more
electrodes can also be employed. For example, by adding another
conductive layer 122 and insulating layer 124 under a reference
electrode layer 114, then two electrodes (first and second working
electrodes) can be formed, yielding a dual electrode sensor. The
same concept can be applied to create, a counter electrode,
electrodes to measure additional analytes (e.g., oxygen), and the
like, for example. FIG. 1F illustrates a sensor having an
additional electrode 122 (as compared to FIGS. 1B-1D), wherein the
windows are selectively removed to expose working electrodes 112,
122 in between a reference electrode (including multiple segments)
114, with a small amount of insulator 104, 124 exposed
therebetween. FIG. 1G illustrates another embodiment, wherein
selective removal of the various layers is stepped to expose the
electrodes 112, 122 and insulators 104, 124 along the length of the
elongated body.
[0217] In some embodiments, the silver-containing material is
applied to the sensor (e.g., the insulated conductive core) in a
substantially continuous process, such as described elsewhere
herein. Accordingly, in some embodiments, the silver-containing
material is applied in a fully-automated process. In other
embodiments, the silver-containing material is applied in a
semi-automated process.
[0218] Referring to FIGS. 2A to 2B, in some embodiments, the sensor
can be configured similarly to the continuous analyte sensors
disclosed in co-pending U.S. Patent Publication No.
2007-0197889-A1. The sensor includes a distal portion 202, also
referred to as the in vivo portion, adapted for implantation into a
host, and a proximal portion 204, also referred to as an ex vivo
portion, adapted to operably connect to the sensor electronics. In
certain embodiments, the sensor includes two or more electrodes: a
working electrode (e.g., the electroactive surface of the elongated
conductive body 102/first layer 112) and at least one additional
electrode (e.g., electroactive surface), which can function as a
counter electrode and/or reference electrode, hereinafter referred
to as the reference electrode 114. In this embodiment, an insulator
104 is deposited over the conductive core. A radial window 106 is
formed through the insulator, such that the working
electrode/electroactive surface is exposed. The reference electrode
is formed from a silver wire helically wound/wrapped around at
least a portion of the sensor. The silver wire can be chloridized
either before and/or after application to the sensor. The
insulator, which is disposed between the elongated conductive body
and reference electrode, provides electrical insulation
therebetween. A membrane system may be deposited over the
electrodes, such as described in more detail below with reference
to FIGS. 6A-C.
[0219] Although the embodiments of FIGS. 2A to 2B illustrate one
electrode configuration including one bulk metal wire helically
wound around another bulk metal wire, other electrode
configurations are also contemplated. In an alternative embodiment,
the working electrode comprises a tube with a reference electrode
disposed or coiled inside, including an insulator therebetween.
Alternatively, the reference electrode comprises a tube with a
working electrode disposed or coiled inside, including an insulator
therebetween. In another alternative embodiment, a polymer (e.g.,
insulating) rod is provided, wherein the electrodes are deposited
(e.g., electro-plated) thereon. In yet another alternative
embodiment, a metallic (e.g., steel) rod is provided, coated with
an insulating material, onto which the working and reference
electrodes are deposited. In yet another alternative embodiment,
one or more working electrodes are helically wound around a
reference electrode.
[0220] FIG. 3A is a perspective schematic illustrating an
alternative embodiment of the analyte sensor, wherein the working
electrode body 112 is formed separately from the core 110. FIG. 3B
is a cross-sectional view of the sensor of FIG. 3A. In this
embodiment, the core is coated with and/or embedded in the
insulator 104. A portion of the insulator is removed to provide a
window 106 therein. The working electrode body is applied to the
core, such that the working electrode body and the core are in
electrical contact (e.g., functionally connected). In some
embodiments, the working electrode body is formed as a C-clip that
is attached over the window. In this embodiment, the interior
surface of the C-clip (e.g., working electrode body) makes either
direct electrical contact with the exposed surface of the core,
such as by the two members touching, or via indirect contact
through an intervening media placed/applied on the core and/or the
C-clip prior to connection (e.g., an electrically conductive
adhesive, gel, paint or other media). In other embodiments, the
working electrode body is applied to the exposed surface of the
core as a conductive ink, paint or paste, which is subsequently
cured and/or dried. For example, in one embodiment, an ink
containing platinum particles is printed into the window. In
another embodiment, a conductive material, such as a liquid metal,
is applied directly to the exposed surface of the core. In another
embodiment, no window is formed. Rather, the working electrode body
is configured to pierce the insulator and to make physical (e.g.,
electrical, functional) contact with the core. In some embodiments,
such as that shown in FIG. 7, the reference electrode is printed on
the elongated body, using methods known in the art. For example, in
some embodiments, the reference electrode is a silver-containing
ink, paint or paste, such as but not limited to a silver-containing
polymer, that is printed on the elongated body using thin-film
and/or thick-film printing techniques.
[0221] The electrochemical analyte sensors described herein are
configured to generate a signal associated with a concentration of
the analyte in the host. The sensors provide at least one working
electrode and at least one reference electrode. The output signal
is typically a raw data stream that is used to provide a useful
value of the measured analyte concentration in a host to the
patient or doctor, for example. The analyte sensors of certain
embodiments may further measure at least one additional signal. For
example, in some embodiments, the additional signal is associated
with the baseline and/or sensitivity of the analyte sensor, thereby
enabling monitoring of baseline and/or sensitivity changes that may
occur in a continuous analyte sensor over time. Additionally or
alternatively, multiple working electrodes can allow for
measurement of multiple analytes, which may also allow for improved
accuracy in the measurement of glucose, or allow for detection of
certain conditions that can affect sensor accuracy.
[0222] In some embodiments, the sensor comprises a second elongated
conductive body 102 (or a core that can be electrically connected
with a working electrode body). In some embodiments, the second
elongated conductive body is configured as a counter electrode. In
other embodiments, a sensor comprising a second elongated
conductive body (or core) is configured and arranged as a second
working electrode, as described below. In some embodiments, the
sensor comprises at least three elongated conductive bodies (or
cores). The insulating material 104 covers at least a portion of
each of the first and second elongated conductive bodies (or
cores). In some embodiments, the insulating material covering at
least a portion of each of the first and second elongated
conductive bodies (or cores) is unitary, such that the insulating
material covers at least a portion of both the first and second
elongated conductive bodies (or cores). For example, in some
embodiments, the elongated conductive bodies (or cores) are
disposed (e.g., embedded, located) within the same insulator.
[0223] FIG. 3B is a schematic illustrating a cross-section of an
analyte sensor in one embodiment, in which an insulated conductive
body includes a plurality of conductive cores 110A, 110B, and 110C
located (e.g., embedded) in the insulator 104. A surface of core
110B is exposed by window 106, and a working electrode body 112 is
applied to the exposed surface of the core. FIG. 3B shows a single
working electrode body. However, each core can have a working
electrode body attached thereto (e.g., in electrical contact with
it). Accordingly, in some embodiments, a completed sensor includes
one, two, or three working electrode bodies. Similarly, if the
insulated conductive body includes more than three cores, then the
completed sensor manufactured from that insulated conductive body
can include a corresponding number of elongated conductive
bodies.
[0224] In the embodiment shown in FIG. 3B, the working electrode
body is a formed structure and/or body (e.g., a C-clip, wire or
foil) that is attached at the window. In an alternative embodiment,
the working electrode body comprises an amorphous material (e.g.,
an ink, paint or paste) applied to the exposed surface (e.g.,
through the window) and subsequently cured. In another alternative
embodiment, the working electrode body is configured for
application over the insulator (e.g., no window is formed) and to
extend through (e.g., pierce, intersect) the insulator, such that
at least the ends of the working electrode body make electrical
contact with the core. In some embodiments, the core and working
electrode body make direct physical contact, such that an
electrical current can pass therebetween. However, a conductive
intermediary, such as a conductive adhesive, gel, lubricant, paint,
ink or paste is disposed therebetween, such as to enable current
transfer from one component to the other, or to promote attachments
between two incompatible materials (e.g., that will not readily
adhere to each other). In this embodiment, one, some or all of the
inner bodies can be connected and/or attached to a working
electrode body, wherein the sensor includes one, two, three, or
more working electrodes. In embodiments in which two or more
windows are formed in the insulator, the windows are staggered
along a length of the sensor (e.g., the in vivo portion). In other
embodiments, the windows are not staggered along the length of the
sensor. In some embodiments, a silver-containing material is
applied over the insulator, to form a reference electrode. In other
embodiments, a silver wire is wrapped around the sensor, to form
the reference electrode, as described elsewhere herein.
[0225] FIG. 4A is a perspective view of the in vivo portion of an
analyte sensor in another embodiment. In this embodiment, the
insulated elongated body comprises three conductive cores 110A,
110B, 110C located in (e.g., embedded in, coated with) the
insulator 104. In this embodiment, a plurality of windows is formed
in and/or through the insulator, such that each window exposes a
portion of a core. As a non-limiting example, window 106A is formed
in the insulator such that a portion of core 110A is exposed.
Similarly, window 106B is formed in the insulator such that a
portion of core 110B is exposed. The windows can be staggered
and/or non-staggered along the longitudinal length of the sensor.
In a further embodiment, each conductive core includes an inner
core and an outer core, such as described elsewhere herein.
[0226] FIG. 4B is a perspective view of the in vivo portion of an
analyte sensor including an elongated body (e.g., configured and
arranged for multi-axis bending) formed of an insulator 104, first
and second conductive cores 110A, 110B embedded in the insulator,
and a membrane 108. The first conductive core is formed of
platinum, platinum-iridium, gold, palladium, iridium, graphite,
carbon, a conductive polymer and/or an alloy, and a first window
106A is configured and arranged to expose an electroactive portion
of the first conductive core. The second conductive core is formed
of a silver-containing material (e.g., a silver or
silver/silver-chloride wire, or a silver-containing wire-shaped a
silver-containing material body), and a second window 106B is
configured and arranged to expose an electroactive portion of the
second conductive core. In some embodiments, instead of a bulk
metal wire, the first conductive core comprises an inner core and
an outer core. For example, to reduce material costs, the inner
core is formed of a material that is relatively less expensive than
platinum, such as stainless steel, titanium, tantalum and/or a
polymer, and the outer core is formed of a material that provides
an appropriate electroactive surface, such as but not limited to
platinum, platinum-iridium, gold, palladium, iridium, graphite,
carbon, a conductive polymer and/or an alloy. In some embodiments,
the membrane covers the exposed electroactive portion of the first
conductive core. In a further embodiment, the membrane covers the
in vivo portion of the sensor. In some embodiments, a third
conductive core is embedded in the insulator. In some embodiments,
the third conductive core is configured and arranged as a second
working electrode, which can be configured as a redundant working
electrode, a non-analyte signal-measuring working electrode (e.g.,
no enzyme as described below), as a counter working electrode, to
detect a second analyte, and/or the like.
[0227] FIG. 4C is a perspective view of the in vivo portion of an
analyte sensor comprising three insulated conductive bodies,
wherein each insulated conductive body includes a core (e.g., 110A,
110B and 110C) coated with insulator (e.g., 104A, 104B and 104C).
In some embodiments, one or more of the cores is formed of a
material that provides the electroactive surface of the working
electrode, such as but not limited to platinum, platinum-iridium,
gold, palladium, iridium, graphite, carbon, a conductive polymer
and/or an alloy. However, in some embodiments, one or more of the
cores is formed of an inner core and an outer core, wherein a
portion of the surface of the outer core provides the electroactive
surface of the working electrode. In still other embodiments, one
or more of the cores is formed of a material that provides
electrical conduction from the working electrode (e.g., an attached
working electrode body) to sensor electronics. Materials suitable
to provide electrical conduction include, but are not limited to
stainless steel, titanium, tantalum and/or a conductive polymer. In
some embodiments, one or more working electrode bodies 112 are
disposed (e.g., applied, attached, located) on the cores, as
described elsewhere herein. In some embodiments, the cores (e.g.,
coated with insulator) are bundled together, such as by an elastic
band, an adhesive, wrapping, a shrink-wrap or C-clip, as is known
in the art. In other embodiments, the inner bodies (e.g., coated
with insulator) are twisted, such as into a triple-helix or similar
configuration. In one embodiment, two of the cores (e.g., coated
with insulator) are twisted together to form a twisted pair, and
then a third core (e.g., with insulator) and/or elongated
conductive body is twisted around the twisted pair. In some
embodiments, the sensor comprises additional cores (e.g., coated
with insulator).
[0228] FIG. 5A is a perspective view of the in vivo portion a
dual-electrode analyte sensor, in one embodiment. In this
embodiment, the sensor comprises first and second bundled elongated
bodies (e.g., conductive cores) E1, E2, wherein a working electrode
comprises an exposed electroactive surface of the elongated body,
and a reference electrode 114, wherein each working electrode
comprises a conductive core. For example, the first working
electrode comprises an exposed portion of the surface of a first
elongated body 102A having an insulating material 104A disposed
thereon, such that the portion of the surface of the elongated body
(e.g., the working electrode) is exposed via a radial window 106A
in the insulator. In some embodiments, the elongated body comprises
a core and a first layer, wherein an exposed surface (e.g.,
electroactive) of the first layer is the first working electrode.
The second working electrode comprises an exposed surface of a
second core 110B having an insulator 104B disposed thereon, such
that a portion of the surface of the core is exposed via a radial
window 106B in the insulator. A first layer (not shown) is applied
to the exposed surface of the second core to form the second
working electrode. In this embodiment, the radial windows are
spaced such that the working electrodes (e.g., electroactive
surfaces) are substantially overlapping along the length of the
sensor. However, in other embodiments, the working electrodes are
spaced such that they are not substantially overlapping along the
length of the sensor. In this embodiment, the reference electrode
comprises a wire (e.g., Ag/AgCl wire) wrapped around the bundled
conductive cores. However, in some embodiments, the referenced
electrode comprises a layer of silver-containing material applied
to at least one of the conductive cores, such as described with
reference to FIG. 1B.
[0229] FIG. 5B is a perspective view of the in vivo portion of a
dual-electrode analyte sensor, in another embodiment. In this
embodiment, the first and second elongated bodies E1, E2 are
twisted into a twisted pair, such as a helix. The reference
electrode 114 is then wrapped around the twisted pair.
[0230] FIGS. 5C and 5D include views of the in vivo portion of a
dual-electrode analyte sensor, in additional embodiments. In these
embodiments, the first and second elongated bodies E1, E2 are
bundled together with reference electrode 114. Connectors 502, 530
are configured and arranged to hold the conductive cores and
reference electrode together. Alternatively, instead of connectors
502, a tube 530 or heat shrink material can be employed as a
connector and/or supporting member. The tubing or heat shrink
material may include an adhesive inside the tube so as to provide
enhanced adhesion to the components secured within (e.g., wire(s),
core, layer materials, etc.). In such a configuration, the
heat-shrink material functions not only as an insulator, but also
to hold the proximal ends of the sensor together so as to prevent
or reduce fatigue and/or to maintain the electrodes together in the
event of a fatigue failure. In the embodiment depicted in FIG. 5C,
the wires need not be a core and a layer, but can instead comprise
bulk materials. The distal ends of the sensor can be loose and
finger-like, as depicted in FIG. 5C, or can be held together with
an end cap. A reference electrode can be placed on one or more of
the first and second elongated bodies instead of being provided as
a separate electrode, and the first and second elongated bodies
including at least one reference electrode thereof can be bundled
together. Heat shrink tubing, crimp wrapping, dipping, or the like
can be employed to bundle one or more elongated bodies together. In
some embodiments, the reference electrode is a wire, such as
described elsewhere herein. In other embodiments, the reference
electrode comprises a foil. In an embodiment of a dual-electrode
analyte sensor, the first and second elongated bodies can be
present as or formed into a twisted pair, which is subsequently
bundled with a wire or foil reference electrode. Connectors, which
can also function as supporting members, can be configured and
arranged to hold the conductive cores and reference electrode
together.
[0231] In some embodiments, a dual-electrode sensor is configured
and arranged to detect two analytes and/or configured as
plus-enzyme and minus-enzyme electrodes. In certain embodiments of
a dual-electrode analyte sensor, the first working electrode (e.g.,
the electroactive surface of the first elongated body E1) is
configured and arranged to generate a first signal comprising an
analyte component and a baseline, and the second working electrode
(e.g., the electroactive surface of the second elongated body E2)
is configured and arranged to generate a second signal comprising
baseline without an analyte component. In one such dual-electrode
system, a first electrode functions as a hydrogen peroxide sensor
including a membrane system containing glucose-oxidase disposed
thereon, which operates as described herein. A second electrode is
a hydrogen peroxide sensor that is configured similar to the first
electrode, but with a modified membrane system (without active
enzyme, for example). This second electrode provides a signal
composed mostly of the baseline signal, b.
[0232] In some dual-electrode systems, the baseline signal is
(electronically or digitally) subtracted from the glucose signal to
obtain a glucose signal substantially without baseline.
Accordingly, calibration of the resultant difference signal can be
performed by solving the equation y=mx with a single paired
measurement. Calibration of the inserted sensor in this alternative
embodiment can be made less dependent on the values/range of the
paired measurements, less sensitive to error in manual blood
glucose measurements, and can facilitate the sensor's use as a
primary source of glucose information for the user. U.S. Patent
Publication No. 2005-0143635-A1, U.S. Patent Publication No.
2007-0027385-A1, U.S. Patent Publication No. 2007-0213611-A1, and
U.S. Patent Publication No. 2008-0083617-A1 describe systems and
methods for subtracting the baseline from a sensor signal.
[0233] In some alternative dual-electrode system embodiments, the
analyte sensor is configured to transmit signals obtained from each
electrode separately (e.g., without subtraction of the baseline
signal). In this way, the receiver can process these signals to
determine additional information about the sensor and/or analyte
concentration. For example, by comparing the signals from the first
and second electrodes, changes in baseline and/or sensitivity can
be detected and/or measured and used to update calibration (e.g.,
without the use of a reference analyte value). In one such example,
by monitoring the corresponding first and second signals over time,
an amount of signal contributed by baseline can be measured. In
another such example, by comparing fluctuations in the correlating
signals over time, changes in sensitivity can be detected and/or
measured.
[0234] In some embodiments, a substantial portion of the in vivo
portion of the sensor is designed with at least one dimension less
than about 0.004 inches, 0.005 inches, 0.006 inches, 0.008 inches,
0.01 inches, 0.012, 0.015, or 0.02 inches. In some embodiments, in
which the sensor is configured and arranged for implantation into a
host vessel, a substantial portion of the sensor that is in fluid
contact with the blood flow is designed with at least one dimension
less than about 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.008,
0.01, 0.012, or 0.015, inches. As one exemplary embodiment, a
sensor such as described in more detail with reference to FIGS. 5A
to 5E is formed from a 0.004 inch conductive wire (e.g., platinum)
for a diameter of about 0.004 inches along a substantial portion of
the sensor (e.g., in vivo portion or fluid contact portion). As
another exemplary embodiment, a sensor such as described in more
detail with reference to FIGS. 5A to 5E is formed from a 0.004 inch
conductive wire and vapor deposited with an insulator material for
a diameter of about 0.005 inches along a substantial portion of the
sensor (e.g., in vivo portion or fluid contact portion), after
which a desired electroactive surface area can be exposed. In the
above two exemplary embodiments, the reference electrode can be
located remote from the working electrode (e.g., formed from the
conductive wire). While the devices and methods described herein
are suitable for use within the host's blood stream, one skilled in
the art will recognize that the systems, configurations, methods
and principles of operation described herein can be incorporated
into other analyte sensing devices, such as but not limited to
transcutaneous devices, subcutaneous devices, and wholly
implantable devices such as described in U.S. Patent Publication
No. 2006-0016700-A1.
[0235] In addition to the embodiments described above, the sensor
can be configured with additional working electrodes as described
in U.S. Patent Publication No. 2005-0143635-A1, U.S. Pat. No.
7,081,195, and U.S. Patent Publication No. 2007-0027385-A1. For
example, in one embodiment have an auxiliary working electrode,
wherein the auxiliary working electrode comprises a wire formed
from a conductive material, such as described with reference to the
glucose-measuring working electrode above. The reference electrode,
which can function as a reference electrode alone, or as a dual
reference and counter electrode, is formed from silver,
silver/silver chloride, and the like.
[0236] In some embodiments, the electrodes are juxtapositioned
and/or twisted with or around each other; however other
configurations are also possible. In one example, the auxiliary
working electrode and reference electrode can be helically wound
around the glucose-measuring working electrode. Alternatively, the
auxiliary working electrode and reference electrode can be formed
as a double helix around a length of the glucose-measuring working
electrode. The assembly of wires can then be optionally coated
together with an insulating material, similar to that described
above, in order to provide an insulating attachment. Some portion
of the coated assembly structure is then stripped, for example
using an excimer laser, chemical etching, and the like, to expose
the necessary electroactive surfaces. In some alternative
embodiments, additional electrodes can be included within the
assembly, for example, a three-electrode system (including separate
reference and counter electrodes) as is appreciated by one skilled
in the art.
[0237] In some alternative embodiments, additional electrodes can
be included within the assembly, for example, a three-electrode
system (working, reference, and counter electrodes) and/or an
additional working electrode (e.g., an electrode which can be used
to generate oxygen, which is configured as a baseline subtracting
electrode, or which is configured for measuring additional
analytes). U.S. Patent Publication No. 2005-0161346-A1, U.S. Patent
Publication No. 2005-0143635-A1, and U.S. Patent Publication No.
2007-0027385-A1 describe some systems and methods for implementing
and using additional working, counter, and/or reference electrodes.
In one implementation wherein the sensor comprises two working
electrodes, the two working electrodes are juxtapositioned (e.g.,
extend parallel to each other), around which the reference
electrode is disposed (e.g., helically wound). In some embodiments
wherein two or more working electrodes are provided, the working
electrodes can be formed in a double-, triple-, quad-, etc. helix
configuration along the length of the sensor (for example,
surrounding a reference electrode, insulated rod, or other support
structure). The resulting electrode system can be configured with
an appropriate membrane system, wherein the first working electrode
is configured to measure a first signal comprising glucose and
baseline (e.g., background noise) signals and the additional
working electrode is configured to measure a baseline signal only
(e.g., configured to be substantially similar to the first working
electrode, but without an enzyme disposed thereon). In this way,
the baseline signal can be subtracted from the first signal to
produce a glucose-only signal that is substantially not subject to
fluctuations in the baseline and/or interfering species on the
signal.
[0238] In certain embodiments, the analyte sensor is configured as
a dual-electrode system and comprises a first working electrode and
a second working electrode, in addition to a reference electrode.
The first and second working electrodes may be in any useful
conformation, as described in U.S. Patent Publication No.
2007-0027385-A1, U.S. Patent Publication No. 2007-0213611-A1, U.S.
Patent Publication No. 2007-0027284-A1, U.S. Patent Publication No.
2007-0032717-A1, U.S. Patent Publication No. 2007-0093704-A1, and
U.S. Patent Publication No. 2008-0083617-A1. In some embodiments,
the first and second working electrodes are twisted and/or bundled.
For example, two wire working electrodes can be twisted together,
such as in a helix conformation. The reference electrode can then
be wrapped around the twisted pair of working electrodes. In some
embodiments, the first and second working electrodes include a
coaxial configuration. A variety of dual-electrode system
configurations are described with reference to FIGS. 2G through 2H
of the references incorporated above. In some embodiments, the
sensor is configured as a dual electrode sensor, such as described
in U.S. Patent Publication No. 2005-0143635-A1, U.S. Patent
Publication No. 2007-0027385-A1, U.S. Patent Publication No.
2007-0213611-A1, and U.S. Patent Publication No.
2008-0083617-A1.
[0239] In certain embodiments, both of the working electrodes of a
dual-electrode analyte sensor are disposed beneath a sensor
membrane, such as but not limited to a membrane system similar to
that described with reference to FIGS. 6A-C, with the following
exceptions. The first working electrode is disposed beneath an
enzymatic enzyme domain (or portion of the sensor membrane)
including an active enzyme configured to detect the analyte or an
analyte-related compound. Accordingly, the first working electrode
is configured to generate a first signal composed of both a signal
related to the analyte and a signal related to non-analyte
electroactive compounds (e.g., physiological baseline,
interferents, and non-constant noise) that have an
oxidation/reduction potential that overlaps with the
oxidation/reduction potential of the analyte. This
oxidation/reduction potential may be referred to as a "first
oxidation/reduction potential" herein. The second working electrode
is disposed beneath a non-enzymatic enzyme domain (or portion of
the sensor membrane) that includes either an inactivated form of
the enzyme contained in the enzymatic portion of the membrane or no
enzyme. In some embodiments, the non-enzymatic portion can include
a non-specific protein, such as BSA, ovalbumin, milk protein,
certain polypeptides, and the like. The non-enzymatic portion
generates a second signal associated with noise of the analyte
sensor. The noise of the sensor comprises signal contribution due
to non-analyte electroactive species (e.g., interferents) that have
an oxidation/reduction potential that substantially overlaps the
first oxidation/reduction potential (e.g., that overlap with the
oxidation/reduction potential of the analyte). In some embodiments
of a dual-electrode analyte sensor configured for fluid
communication with a host's circulatory system, the non-analyte
related electroactive species comprises at least one species
selected from the group consisting of interfering species,
non-reaction-related hydrogen peroxide, and other electroactive
species.
[0240] In one exemplary embodiment, the dual-electrode analyte
sensor is a glucose sensor having a first working electrode
configured to generate a first signal associated with both glucose
and non-glucose related electroactive compounds that have a first
oxidation/reduction potential. Non-glucose related electroactive
compounds can be any compound, in the sensor's local environment
that has an oxidation/reduction potential substantially overlapping
with the oxidation/reduction potential of H.sub.2O.sub.2, for
example. While not wishing to be bound by theory, it is believed
that the glucose-measuring electrode can measure both the signal
directly related to the reaction of glucose with GOx (produces
H.sub.2O.sub.2 that is oxidized at the working electrode) and
signals from unknown compounds that are in the blood surrounding
the sensor. These unknown compounds can be constant or non-constant
(e.g., intermittent or transient) in concentration and/or effect.
In some circumstances, it is believed that some of these unknown
compounds are related to the host's disease state. For example, it
is known that blood chemistry changes dramatically during/after a
heart attack (e.g., pH changes, changes in the concentration of
various blood components/protein, and the like). Additionally, a
variety of medicaments or infusion fluid components (e.g.,
acetaminophen, ascorbic acid, dopamine, ibuprofen, salicylic acid,
tolbutamide, tetracycline, creatinine, uric acid, ephedrine,
L-dopa, methyl dopa and tolazamide) that may be given to the host
may have oxidation/reduction potentials that overlap with that of
H.sub.2O.sub.2.
[0241] In this exemplary embodiment, the dual-electrode analyte
sensor includes a second working electrode that is configured to
generate a second signal associated with the non-glucose related
electroactive compounds that have the same oxidation/reduction
potential as the above-described first working electrode. In some
embodiments, the non-glucose related electroactive species includes
at least one of interfering species, non-reaction-related
H.sub.2O.sub.2, and other electroactive species. For example,
interfering species includes any compound that is not directly
related to the electrochemical signal generated by the glucose-GOx
reaction, such as but not limited to electroactive species in the
local environment produces by other body processes (e.g., cellular
metabolism, a disease process, and the like). Other electroactive
species includes any compound that has an oxidation/reduction
potential similar to or overlapping that of H.sub.2O.sub.2.
[0242] The non-analyte (e.g., non-glucose) signal produced by
compounds other than the analyte (e.g., glucose) may obscure the
signal related to the analyte, may contribute to sensor inaccuracy,
and is considered background noise. Background noise includes both
constant and non-constant components and is to be removed to
accurately calculate the analyte concentration. While not wishing
to be bound by theory, it is believed that the sensor of some of
the embodiments are designed (e.g., with symmetry, coaxial design
and/or integral formation, and interference domain of the membrane
described elsewhere herein) such that the first and second
electrodes are influenced by substantially the same external and/or
environmental factors, which enables substantially equivalent
measurement of both the constant and non-constant species/noise.
This advantageously allows the substantial elimination of noise on
the sensor signal (using electronics described elsewhere herein) to
substantially reduce or eliminate signal effects due to noise,
including non-constant noise (e.g., unpredictable biological,
biochemical species, medicaments, pH fluctuations, O.sub.2
fluctuations, or the like) known to effect the accuracy of
conventional continuous sensor signals. The sensor includes
electronics may be operably connected to the first and second
working electrodes. The electronics are configured to provide the
first and second signals that are used to generate glucose
concentration data substantially without signal contribution due to
non-glucose-related noise. The electronics can include at least a
potentiostat that provides a bias to the electrodes. In some
embodiments, sensor electronics are configured to measure the
current (or voltage) to provide the first and second signals. The
first and second signals are used to determine the glucose
concentration substantially without signal contribution due to
non-glucose-related noise such as by but not limited to subtraction
of the second signal from the first signal or alternative data
analysis techniques. In some embodiments, the sensor electronics
include a transmitter that transmits the first and second signals
to a receiver, where additional data analysis and/or calibration of
glucose concentration can be processed. U.S. Patent Publication No.
2005-0027463-A1, U.S. Patent Publication No. 2005-0203360-A1, and
U.S. Patent Publication No. 2006-0036142-A1 describe systems and
methods for processing sensor analyte data.
[0243] In some embodiments, the dual-electrode sensor is configured
such that the first and second working electrodes are equivalently
influenced by in vivo environmental factors. For example, in one
embodiment, the dual-electrode sensor is configured for fluid
communication with the circulatory system of the host, such as by
implantation in the host's vein or artery via a vascular access
device (also referred to as a fluid communication device herein)
such as a catheter and/or cannula. When the sensor is contacted
with a sample of the host's circulatory system (e.g., blood), the
first and second working electrodes are configured such that they
are equivalently influenced by a variety of environmental factors
impinging upon the sensor, such as but not limited to non-analyte
related electroactive species (e.g., interfering species,
non-reaction-related H.sub.2O.sub.2, another electroactive
species). Because the first and second working electrodes are
equivalently influenced by in vivo environmental factors, the
signal component associated with the in vivo environmental factors
(e.g., non-analyte related species with an oxidation/reduction
potential that overlaps with that of the analyte) can be removed
from the signal detected by the first working electrode (e.g., the
first signal). This can give a substantially analyte-only
signal.
[0244] In some embodiments, the surface area of the electroactive
portion of the reference (and/or counter) electrode is at least six
times the surface area of the working electrodes. In other
embodiments, the reference (and/or counter) electrode surface is at
least 1, 2, 3, 4, 5, 7, 8, 9 or 10 times the surface area of the
working electrodes. In other embodiments, the reference (and/or
counter) electrode surface area is at least 11, 12, 13, 14, 15, 16,
17, 18, 19 or 20 times the surface area of the working electrodes.
For example, in a needle-type glucose sensor, similar to the
embodiment shown in FIGS. 5A-5E, the surface area of the reference
electrode (e.g., 114) includes the exposed surface of the reference
electrode, such as but not limited to the electrode surface facing
away from the working electrodes.
[0245] As a non-limiting example, in one embodiment, the
dual-electrode analyte sensor comprises a first working electrode
configured to detect the analyte and a second working electrode,
wherein the first and second working electrodes are located on of
two wire elongated conductive bodies E1, E2 twisted together to
form a "twisted pair." The first working electrode is disposed
beneath an enzymatic portion of the membrane (not shown) containing
an analyte-detecting enzyme. For example, in a glucose-detecting
dual-electrode analyte sensor, a glucose-detecting enzyme, such as
GOX, is included in the enzymatic portion of the membrane.
Accordingly, the first working electrode detects signal due to both
the analyte and non-analyte-related species that have an
oxidation/reduction potential that substantially overlaps with the
oxidation/reduction potential of the analyte. The second working
electrode is disposed beneath a portion of the membrane comprising
either inactivated enzyme (e.g., inactivated by heat, chemical or
UV treatment) or no enzyme. Accordingly, the second working
electrode detects a signal associated with only the non-analyte
electroactive species that have an oxidation/reduction potential
that substantially overlaps with that of analyte. For example, in
the glucose-detecting dual-electrode analyte sensor described
above, the non-analyte (e.g., non-glucose) electroactive species
have an oxidation/reduction potential that overlaps substantially
with that of H.sub.2O.sub.2. A reference electrode 114, such as a
silver/silver chloride wire electrode, is wrapped around the
twisted pair. The three electrodes (e.g., working electrodes E1, E2
and the reference electrode 114) are connected to sensor
electronics (not shown), such as described elsewhere herein. In
certain embodiments, the dual-electrode sensor is configured to
provide an analyte-only signal (e.g., glucose-only signal)
substantially without a signal component due to the non-analyte
electroactive species (e.g., noise). For example, the
dual-electrode sensor is operably connected to sensor electronics
that process the first and second signals, such that a
substantially analyte-only signal is provided (e.g., output to a
user). In other exemplary embodiments, the dual-electrode sensor
can be configured for detection of a variety of analytes other than
glucose, such as but not limited to urea, creatinine, succinate,
glutamine, oxygen, electrolytes, cholesterol, lipids,
triglycerides, hormones, liver enzymes, and the like.
[0246] In some embodiments, the analyte sensor substantially
continuously measures the host's analyte concentration. In some
embodiments, for example, the sensor can measure the analyte
concentration every fraction of a second, about every fraction of a
minute or every minute. In other exemplary embodiments, the sensor
measures the analyte concentration at least about every 2, 3, 4, 5,
6, 7, 8, 9, or 10 minutes. In still other embodiments, the sensor
measures the analyte concentration every fraction of an hour, such
as but not limited to every 15, 30 or 45 minutes. Yet in other
embodiments, the sensor measures the analyte concentration about
every hour or longer. In some exemplary embodiments, the sensor
measures the analyte concentration intermittently or periodically.
In one embodiment, the analyte sensor is a glucose sensor and
measures the host's glucose concentration about every 4-6 minutes.
In a further embodiment, the sensor measures the host's glucose
concentration every 5 minutes.
[0247] As a non-limiting example, dual-electrode glucose sensor can
be manufactured as follows. In one embodiment, the conductive cores
are first coated with a layer of insulating material (e.g.,
non-conductive material or dielectric) to prevent direct contact
between conductive cores and the reference electrode 114. At this
point, or at any point hereafter, the two insulated conductive
cores can be twisted and/or bundled to form a twisted pair. A
portion of the insulator on an exterior surface of each conductive
core is etched away, to expose the electroactive surfaces of the
working electrodes. In some embodiments, an enzyme solution (e.g.,
containing active GOx) is applied to the electroactive surfaces of
both working electrodes, and dried. Thereafter, the enzyme applied
to one of the electroactive surfaces is inactivated. As is known in
the art, enzymes can be inactivated by a variety of means, such as
by heat, treatment with inactivating (e.g., denaturing) solvents,
proteolysis, laser irradiation or UV irradiation (e.g., at 254-320
nm). For example, the enzyme coating one of the electroactive
surfaces can be inactivated by masking one of the electroactive
surfaces/electrodes (e.g., temporarily covered with a UV-blocking
material); irradiating the sensor with UV light (e.g., 254-320 nm;
a wavelength that inactivates the enzyme, such as by cross-linking
amino acid residues) and removing the mask. Accordingly, the GOx on
the second working electrode is inactivated by the UV treatment,
but the first working electrode's GOx is still active due to the
protective mask. In other embodiments, an enzyme solution
containing active enzyme is applied to a first electroactive
surface (e.g., first working electrode) and an enzyme solution
containing either inactivated enzyme or no enzyme is applied to the
second electroactive surface (e.g., second working electrode).
Thus, the enzyme-coated first electroactive surface detects
analyte-related signal and non-analyte-related signal, while the
second electroactive surface, which lacks active enzyme, detects
non-analyte-related signal. As described herein, the sensor
electronics can use the data collected from the two working
electrodes to calculate the analyte-only signal.
[0248] In some embodiments, the dual-electrode sensor system is
configured for fluid communication with a host's circulatory
system, such as via a vascular access device. A variety of vascular
access devices suitable for use with a dual-electrode analyte
sensor are described U.S. Patent Publication No. 2008-0119703-A1,
U.S. Patent Publication No. 2008-0108942-A1, U.S. Patent
Publication No. 2008-0200789-A1.
[0249] FIG. 8A is a perspective view of the in vivo portion of
another embodiment of a multi-electrode sensor system 800
comprising two working electrodes and at least one
reference/counter electrode. The sensor system 800 comprises first
and second elongated bodies E1, E2, each formed of a conductive
core or of a core with a conductive layer deposited thereon. In
this particular embodiment, an insulating layer 810, a conductive
layer 820, and a membrane layer (not shown) are deposited on top of
the elongated bodies E1, E2. The insulating layer 810 separates the
conductive layer 820 from the elongated body. The materials
selected to form the insulating layer 810 may include any of the
insulating materials described elsewhere herein, including
polyurethane and polyimide. The materials selected to form the
conductive layer 820 may include any of the conductive materials
described elsewhere herein, including silver/silver chloride,
platinum, gold, etc. Working electrodes 802', 802'' are formed by
removing portions of the conductive layer 820 and the insulating
layer 810, thereby exposing electroactive surface of the elongated
bodies E1, E2, respectively. FIG. 8B provides a close perspective
view of the distal portion of the elongated bodies E1, E2. FIG. 8C
provides a front view of the sensor embodiment illustrated in FIGS.
8A and 8B.
[0250] The two elongated bodies illustrated in FIG. 8A are
fabricated to have substantially the same shape and dimensions. In
some embodiments, the working electrodes are fabricated to have the
same properties, thereby providing a sensor system capable of
providing redundancy of signal measurements. In other embodiments,
the working electrodes, associated with the elongated bodies E1,
E2, may each have one or more characteristics that distinguish each
working electrode from the other. For example, in one embodiment,
each of the elongated bodies E1, E2 may be covered with a different
membrane, so that each working electrode has a different membrane
property than the other working electrode. For example, one of the
working electrodes may have a membrane comprising an enzyme layer
and the other working electrode may have a membrane comprising a
layer having either an inactivated form of the enzyme or no enzyme.
Additional sensor system configurations that are possible with a
plurality of working electrodes (e.g., sensor elements) are
described in U.S. Patent Publication No. 2011-0024307-A1, which is
incorporated by reference herein in its entirety.
[0251] Although not shown in FIGS. 8A-8C, in certain embodiments,
the distal ends 830', 830'' of the core portions of the elongated
bodies E1, E2 may be covered with an insulating material (e.g.,
polyurethane or polyimide). In alternative embodiments, the exposed
core portions 830', 830'' may be covered with a membrane system and
serve as additional working electrode surface area.
[0252] Regarding fabrication of the sensor system illustrated in
FIG. 8A-8C, in one embodiment, two elongated bodies E1, E2 are
provided. As described above, the elongated bodies E1, E2 may be
formed as an elongated conductive core, or alternatively as a core
(conductive or non-conductive) having at least one conductive
material deposited thereon. Next, an insulating layer 810 is
deposited onto each of the elongated bodies E1, E2. Thereafter, a
conductive layer 820 is deposited over the insulating layer 810.
The conductive layer 820 may serve as a reference/counter electrode
and may be formed of silver/silver chloride, or any other material
that may be used for a reference electrode. In alternative
embodiments, the conductive layer 820 may be formed of a different
conductive material, and may be used another working electrode.
After these steps, a layer removal process is performed to remove
portions of the deposited layers (i.e., the conductive layer 820
and/or the insulating layer 810). Any of the techniques described
elsewhere herein (e.g., laser ablation, chemical etching, grit
blasting) may be used. In the embodiment illustrated in FIGS. 8A
and 8B, layers of the conductive layer 820 and the insulating layer
810 are removed to form the working electrodes 802', 802''.
Although in the embodiment shown, layer removal is performed across
the entire cross-sectional perimeter (e.g., circumference) of the
deposited layer, it is contemplated that in other embodiments,
layer removal may be performed across a preselected section of the
cross-sectional perimeter, instead of across the entire
cross-sectional perimeter.
[0253] Contacts 804', 804'' used to provide electrical connection
between the working electrodes and other components of the sensor
system may be formed in a similar manner. As shown, contacts 804'
and 804'' are separated from each other to prevent an electrical
connection therebetween. Because the layer removal process is
performed on each individual elongated body E1, E2, instead of a
single geometrically complicated elongated body, this particular
sensor design (i.e., two elongated bodies placed side by side) may
provide ease of manufacturing, as compared to the manufacturing
processes involved with other multi-electrode systems having other
geometries.
[0254] After the conductive and insulating layers are deposited
onto the elongated body, and after selected portions of the
deposited layers have been removed, a membrane is applied onto at
least a portion of the elongated bodies. In certain embodiments,
the membrane system is applied only to the working electrodes, but
in other embodiments the membrane system is applied to the entire
elongated body. In one embodiment, the membrane system is deposited
onto the two working electrodes simultaneously while they are
placed together (e.g., by bundling), but in another embodiment,
membranes are deposited onto each individual working electrode
first, and the two working electrodes are then placed together.
[0255] FIG. 9A is a perspective view of the in vivo portion of
another embodiment of a multi-electrode sensor system 900
comprising two working electrodes and one reference/counter
electrode. The three electrodes are integrated into one piece. The
sensor system 900 comprises first, second, and third elongated
bodies E1, E2, E3, each formed of a conductive core or of a core
with a conductive layer deposited thereon. In this particular
embodiment, an insulating domain 910 and a membrane layer (not
shown) are deposited on top of an assembly comprising the elongated
bodies E1, E2, E3. The insulating domain 910 binds the three
elongated bodies E1, E2, E3 in close proximity of each other, while
also separating them from direct contact with each other. The
materials selected to form the insulating domain 910 may include
any of the insulating materials described elsewhere herein,
including polyurethane and polyimide, for example. Working
electrode 902' on elongated body E1 and another working electrode
(not shown) on elongated body E2, are formed by removing portions
of the insulating domain 910, thereby exposing electroactive
surface of the elongated bodies E1, E2. Similarly, the reference
electrode 904' on elongated body E3 is also formed by removing
portions of the insulating domain 910, thereby exposing
electroactive surface of the elongated body E3. FIG. 9B provides a
close perspective view of the distal portion of the elongated
bodies E1, E2, E3. FIG. 9C provides a front view of the sensor
embodiment illustrated in FIGS. 9A and 9B.
[0256] As described elsewhere herein, in certain embodiments, the
working electrodes, associated with the elongated bodies E1, E2,
may each have one or more characteristics that distinguish each
working electrode from the other. For example, in some embodiments,
one of the working electrodes may have a membrane comprising an
enzyme layer and the other working electrode may have a membrane
comprising a layer having either an inactivated form of the enzyme
or no enzyme. Additional sensor system configurations that are
possible with a plurality of working electrodes (e.g., sensor
elements) are described in U.S. Patent Publication No.
2011-0024307-A1, which is incorporated by reference herein in its
entirety. In other embodiments, the working electrodes are
fabricated to have the same properties, thereby providing a sensor
system capable of providing redundancy of signal measurements.
[0257] Although not shown in FIGS. 9A-9C, in certain embodiments,
the distal ends 930', 930'', 930''' of the core portions of the
elongated bodies E1, E2, E3, respectively, may be covered with an
insulating material (e.g., polyurethane or polyimide). In
alternative embodiments, one or more of the exposed core portions
930', 930'', 930''' may be covered with a membrane system and serve
as additional working electrodes.
[0258] In one embodiment, fabrication of the sensor system
illustrated in FIGS. 9A-9C involves providing three elongated
bodies E1, E2, E3. As described above, the elongated bodies may be
formed as an elongated conductive core, or alternatively as a core
(conductive or non-conductive) having at least one conductive
material deposited thereon. The E3 reference electrode includes a
core and/or an outer layer that comprises a reference electrode
material, such as, silver/silver chloride, for example. Next, an
insulating layer is deposited onto each of the elongated bodies.
Thereafter, the three elongated bodies E1, E2, E3 (with an
insulating layer thereon) are placed together to form a single
elongated body. Although not required, in some embodiments, the
three elongated bodies E1, E2, E3 may be coated with a
thermoplastic material and fed through an aligning die. Afterwards,
an insulating domain 910 is deposited over this single elongated
body. The deposited domain is then allowed to dry or be cured,
after which an unitary elongated body is formed, in which the three
elongated bodies E1, E2, E3 are encased and held together by
insulating domain 910.
[0259] After these steps, a layer removal process is performed to
remove portions of the insulating domain 910. Any of the techniques
described herein (e.g., laser ablation, chemical etching, grit
blasting) may be used. In the embodiment illustrated in FIGS. 9A
and 9B, portions of the insulating domain 910 are removed to form
the working electrode 902' on elongated body E1, to form a second
working electrode (not shown) on elongated body E2, and to form a
reference electrode 904 on elongated body E3. Contacts 904', 906
used to provide electrical connection between the working and
reference electrodes and other components of the sensor system may
be formed in a similar manner.
[0260] FIG. 10A is a perspective view of the in vivo portion of yet
another embodiment of a multi-electrode sensor system 1000
comprising two working electrodes and at least one
reference/counter electrode. The sensor system 1000 comprises first
and second elongated bodies E1, E2, each formed of a conductive
core or of a core with a conductive layer deposited thereon. An
insulating layer 1010 is deposited onto each elongated body E1, E2.
Furthermore, a conductive domain 1020 and a membrane layer (not
shown) are deposited on top of an assembly comprising the elongated
bodies E1, E2 and the insulating layer. The conductive domain 1020,
binds the two elongated bodies E1, E2 into one elongated body. The
insulating layers 1010 surrounding each elongated body E1, E2
prevents electrical contact between the two elongated bodies E1,
E2. The materials selected to form the insulating layer 1010 may
include any of the insulating materials described elsewhere herein,
including polyurethane and polyimide, for example. The materials
selected to form the conductive domain 1020 may include any of the
conductive materials described elsewhere herein, including
silver/silver chloride and platinum, for example. Working electrode
1002' on elongated body E1 and another working electrode (not
shown) on elongated body E2, are formed by removing portions of the
conductive domain 1020 and portions of the insulating layer 1010,
thereby exposing electroactive surfaces of elongated bodies E1, E2.
The portion of the conductive domain 1020 not removed forms the
reference/counter electrode. FIG. 10B provides a close perspective
view of the distal portion of the elongated bodies E1, E2. FIG. 10C
provides a front view of the sensor embodiment illustrated in FIGS.
10A and 10B.
[0261] As described elsewhere herein, in certain embodiments, the
working electrodes, associated with the elongated bodies E1, E2,
may each have one or more characteristics that distinguish each
working electrode from the other. For example, in some embodiments,
one of the working electrodes may have a membrane comprising an
enzyme layer and the other working electrode may have a membrane
comprising a layer having either an inactivated form of the enzyme
or no enzyme. Additional sensor system configurations that are
possible with a plurality of working electrodes (e.g., sensor
elements) are described in U.S. Patent Publication No.
2011-0024307-A1, which is incorporated by reference herein in its
entirety. In other embodiments, the working electrodes are
fabricated to have the same properties, thereby providing a sensor
system capable of providing redundancy of signal measurements.
[0262] Although not shown in FIGS. 10A-10C, in certain embodiments,
the distal ends 1030', 1030'' of the core portions of the elongated
bodies E1, E2, respectively, may be covered with an insulating
material (e.g., polyurethane or polyimide). In alternative
embodiments, one or more of the exposed core portions 1030', 1030''
may be covered with a membrane system and serve as additional
working electrodes.
[0263] In one embodiment, fabrication of the sensor system
illustrated in FIGS. 10A-10C involves providing two elongated
bodies E1, E2. As described above, the elongated bodies may be
formed as an elongated conductive core, or alternatively as a core
(conductive or non-conductive) having at least one conductive
material deposited thereon. Next, an insulating layer is deposited
onto each of the elongated bodies. Thereafter, the two elongated
bodies E1, E2 (with an insulating layer thereon) are placed
together to form a single elongated body. Although not required, in
some embodiments, the three elongated bodies E1, E2, E3 may be
coated with a thermoplastic material and fed through an aligning
die. Afterwards, a conductive domain 1020 is deposited over this
single elongated body. The coated domain is then allowed to dry or
be cured, after which the one unitary elongated body is formed, in
which the two elongated bodies E1, E2 are encased and held together
by conductive domain 1020.
[0264] After these steps, a layer removal process is performed to
remove portions of the conductive domain 1020 and portions of the
insulating layer 1010. Any of the techniques described herein
(e.g., laser ablation, chemical etching, grit blasting) may be
used. In the embodiment illustrated in FIGS. 10A and 10B, portions
of the conductive domain 1020 and insulating layer 1010 are removed
to form the working electrode 1002' on elongated body E1, to form a
second working electrode (not shown) on elongated body E2, and to
form the reference electrode 1020. Contacts 1004', 1004'' used to
provide electrical connection between the working electrodes and
other components of the sensor system may be formed in a similar
manner.
[0265] With this particular sensor design, because the conductive
domain 1020 is disposed between the contact point between the two
elongated bodies E1, E2, the sensor system's largest
cross-sectional dimension is minimized, as compared to a design in
which both of the elongated bodies were each individually covered
with a conductive layer.
[0266] FIG. 11A is a perspective view of the in vivo portion of
another embodiment of a multi-electrode sensor system 1100
comprising two working electrodes and one reference/counter
electrode. The sensor system 1100 comprises first, second, and
third elongated bodies E1, E2, E3, each formed of a conductive core
or of a core with a conductive layer deposited thereon. In this
particular embodiment, an insulating layer 1110 and a membrane
layer (not shown) are deposited on top of the elongated bodies E1,
E2. The insulating layer 1110 separates the elongated bodies from
each other. The materials selected to form the insulating layer
1110 may include any of the insulating materials described
elsewhere herein, including polyurethane and polyimide. Working
electrodes 1102', 1102'' are formed by removing portions of the
insulating layer 1110, thereby exposing electroactive surface of
the elongated bodies E1, E2, respectively.
[0267] As described elsewhere herein, in certain embodiments, the
working electrodes, associated with the elongated bodies E1, E2,
may each have one or more characteristics that distinguish each
working electrode from the other. For example, in some embodiments,
one of the working electrodes may have a membrane comprising an
enzyme layer and the other working electrode may have a membrane
comprising a layer having either an inactivated form of the enzyme
or no enzyme. Additional sensor system configurations that are
possible with a plurality of working electrodes (e.g., sensor
elements) are described in U.S. Patent Publication No.
2011-0024307-A1, which is incorporated by reference herein in its
entirety. In other embodiments, the working electrodes are
fabricated to have the same properties, thereby providing a sensor
system capable of providing redundancy of signal measurements.
[0268] Although not shown in FIGS. 11A-11C, in certain embodiments,
the distal ends 1130', 1130'' of the core portions of the elongated
bodies E1, E2 may be covered with an insulating material (e.g.,
polyurethane or polyimide). In alternative embodiments, the exposed
core portions 1130', 1130'' may be covered with a membrane system
and serve as additional working electrodes.
[0269] Regarding fabrication of the sensor system illustrated in
FIG. 11A-11C, in one embodiment, three elongated bodies E1, E2, E3
are provided. As described above, the elongated bodies may be
formed as an elongated conductive core, or alternatively as a core
(conductive or non-conductive) having at least one conductive
material deposited thereon. The elongated bodies E1, E2 that
correspond to working electrodes may comprise an elongated core
with a conductive material typically used with working electrodes
(e.g., a core formed of a conductive material like platinum, or a
core plated, coated, or cladded with a conductive material like
platinum). The elongated body E3 that corresponds to a reference
electrode may comprise an elongated core with a conductive material
typically used with reference electrodes (e.g., a core formed of a
conductive material like silver/silver chloride, or a core plated,
coated, or cladded with a conductive material like silver/silver
chloride).
[0270] Next, an insulating layer 1110 is deposited onto each of the
elongated bodies. In some embodiments, the insulating layer 1110 is
formed of a thermoplastic material, thereby allowing the three
elongated bodies E1, E2, E3 to be attached together by a heating
process that permits the insulating layers of the three elongated
bodies E1, E2, E3 to adhere together.
[0271] Thereafter, a layer removal process is performed to remove
portions of the insulating layer 1110. Any of the techniques
described herein (e.g., laser ablation, chemical etching, grit
blasting) may be used. In the embodiment illustrated in FIGS. 11A
and 11B, the insulating layer 1110 is removed to form the working
electrodes 1102', 1102'' and reference electrode 1106. Contacts
1104', 1104'' used to provide electrical connection between the
working electrodes and other components of the sensor system may be
formed in a similar manner. As shown, contacts 1104' and 1104'' are
separated from each other to prevent an electrical connection
therebetween. Because the layer removal process is performed on
each individual elongated body, instead of a single geometrically
complicated elongated body, this particular sensor design (i.e.,
two elongated bodies placed side by side) may provide ease of
manufacturing, as compared to the manufacturing involved with other
sensors having other geometries.
[0272] After the conductive and insulating layers have been
deposited onto the elongated body, and after selected portions of
the deposited layers have been removed, a membrane can be applied
onto at least a portion of the elongated bodies. In certain
embodiments, the membrane system is applied only to the working
electrodes, but in other embodiments the membrane system is applied
to the entire elongated body. In one embodiment, a membrane system
is deposited onto the two working electrodes simultaneously while
they are placed together (e.g., by bundling), but in another
embodiment, a membrane is deposited onto each individual working
electrode, and the two working electrodes are then placed
together.
[0273] In one exemplary embodiment, the two elongated bodies E1, E2
are bundled together first (e.g., by providing adherence between
the insulating layers of the working electrodes) to form a
subassembly and an uncoated silver elongated conductive body E3 is
then adhered to the subassembly to form an assembly including all
three elongated bodies E1, E2, E3. Subsequently, the silver
elongated conductive body E3 can be chlorized to form a
silver/silver chloride reference electrode.
[0274] It should be understood that with any of the embodiments
described herein involving multiple working electrodes, one or more
working electrodes may be designed to serve as an enzymatic
electrode and one or more working electrodes may be designed to
serve as a "blank" working electrode configured to measure
baseline. This configuration allows for subtraction of a signal
associated with the "blank" working electrode (i.e., the baseline
non-analyte related signal) from the signal associated with the
enzymatic working electrode. The subtraction, in turn, results in a
signal that contains substantially reduced (or no)
non-analyte-related signal contribution (e.g., contribution from
interferents).
[0275] As described elsewhere herein, the elongated body may have
any of a variety of cross-sectional shapes. This concept also
applies to multi-electrode sensors. For example, even though the
elongated bodies of the embodiment illustrated in FIGS. 10 A-10C is
shown with a circular or substantially circular cross-sectional
shape, it is contemplated that other shapes may be used. By way of
example, FIG. 10D is a perspective view of the in vivo portion of
one embodiment having a sensor design similar to that of the
embodiment illustrated in FIGS. 10A-10C, except that that this
particular embodiment has a substantially rectangular
cross-section. The rectangular shape may provide advantages in
certain instances. For example, a rectangular shape design may
provide a larger window area per unit length of etching (e.g.,
laser ablation). This results in a larger electrode surface per
unit of length of the elongated body, which in turn, allows for a
sensor with a higher sensitivity, as compared to an equivalent
sensor with a circular cross section. Additionally, the rectangular
cross-section may allow for easier handling (e.g., easier
alignment) during the fabrication operations, such as, extrusion,
dip-coating, etching, and membrane applications (e.g., with a
"drop-on-demand" systems such as ink-jetting.). Furthermore, a
rectangular shape may provide for a more compact cross-section,
which allows for the sensor to be inserted with a needle with a
smaller diameter than an equivalent sensor with a different
cross-sectional shape.
[0276] As described above, in some embodiments, a domain formed of
a conductive material or an insulating material may be used to
encase multiple electrodes so that they are held together to form a
unitary elongated body. In other embodiments, other types of
material may be used instead of (or in addition) to a conductive or
insulating domain to hold together the multiple elongated bodies.
FIG. 12 illustrates one such embodiment. In this particular
embodiment, the multi-electrode sensor system 1100 comprises two
working electrodes associated with elongated bodies E1, E2 and one
reference/counter electrode associated with elongated body E3. Each
of the elongated bodies E1, E2, E3 may optionally include an
insulating layer 1110, portions of which are removed to expose
electroactive portions that correspond to working or reference
electrodes. In addition, the elongated bodies E1, E2 associated
with the working electrodes include a membrane 1140. Although in
this particular embodiment, the elongated body E3 associated with
the reference electrode does not include a membrane, in alternative
embodiments, a membrane covers at least a portion of the reference
electrode. A support structure 1150 is used to hold together the
multiple elongated bodies E1, E2, E3. The support structure 1150
may be made from any of a variety of materials, such as,
polyurethane, silicone, or certain membrane materials. In addition,
the support structure 1150 may contain a combination of hydrophobic
and hydrophilic components to ensure favorable bonding properties
and permeability to the underlying structure.
[0277] FIGS. 13A-13C illustrates another embodiment of a
multi-electrode sensor system and the process for manufacturing it.
As illustrated in FIG. 13A, an elongated body in the form of a
carrier 1350 is provided which includes grooves 1360 that extend
along the longitudinal axis of the carrier 1350. The grooves 1360
are configured to hold a conductive paste, such as, for example, a
platinum paste to form a working electrode, or a silver/silver
chloride paste to form a reference electrode. Although in this
particular embodiment, the carrier 1350 includes three grooves, in
other embodiments, the carrier may include more (e.g., 4, 5, 6, 7,
10, 15, 20) or less (e.g., 2) grooves to form a sensor system with
any number of working and reference electrodes. In certain
embodiments, the carrier 1350 is formed of a non-conductive
material to prevent an electrical connection between the different
electrodes. As shown in FIG. 13B, two of the grooves 1362, 1364 are
then filled with a conductive material (e.g., a conductive metal
paste such as a platinum filled screen print ink) to form a working
electrode, and one groove 1366 is filled with a different
conductive material (e.g., a silver/silver chloride paste) to form
a reference electrode. To fill the grooves, a die can be used
whereby channels are provided to guide paste to the selected
groove. To prevent cross-contamination of the materials in between
the grooves, a scraper can be provided at the exit end of the die
that scrapes conductive material to be flush with the carrier. As
illustrated in FIG. 13C, a membrane system 1310 can then applied to
the assembly using any of the processes described elsewhere
herein.
[0278] FIGS. 13D-13F illustrates another embodiment similar to the
embodiment illustrated in FIGS. 13A-13C. As illustrated in FIG.
13D, in this embodiment, a carrier 1350 is provided that includes
two grooves 1368, 1370. As shown in FIG. 13E, the two grooves 1368,
1370 are then filled with a conductive material to form two working
electrodes. Next, an insulating layer 1310 is deposited onto the
assembly, followed by the deposition of a conductive layer 1320
which forms a reference electrode, as shown in FIGS. 13F-13G.
Subsequently, the assembly undergoes an etching process (e.g.,
laser ablation, chemical etching, grit blasting) to remove certain
portions of the insulating layer 1310 and conductive layer 1320,
thereby forming working electrodes.
[0279] Referring to FIG. 1A, in some embodiments, the sensor 100
comprises a membrane 108 in contact with the electroactive surface
(e.g., at least the working electrode). As described elsewhere
herein, an implanted sensor can be subjected to repeated bending
and/or flexing along a plurality of axes. In order to withstand
this harsh treatment and to provide analyte data over 1, 2, 3 or
more days, the sensor membrane is configured to withstand repeated
compression, flexing and pulling while substantially maintaining
membrane function (e.g., without ripping, buckling or tearing).
Accordingly, the sensor membrane may be strong yet elastic. Shore
hardness is a measure of a material's elasticity. In some
embodiments, the membrane comprises a polymer having a Shore
hardness of at least about 65A, 70A, 75A, 80A, 85A, 90A, 95A, 30D,
35D, 40D, 45D, 50D, 55D, 60D, or more. In some embodiments, the
polymer has a Shore hardness of from about 70A to about 55D.
Polymers having a Shore hardness within the range of from about 70A
to about 55D include but are not limited to polyurethanes,
polyimides, silicones, and the like, such as described elsewhere
herein.
[0280] In some embodiments, the entire membrane is formed of one or
more polymers having a Shore hardness of from about 70A to about
55D. However, in other embodiments, only a portion of the membrane,
such as a membrane layer or domain, is formed of one of these
polymers. In one exemplary embodiment, an outer layer of the
membrane is formed of a polymer having a Shore hardness of from
about 70A to about 55D. In some embodiments, approximately the
outer 5%, 10%, 15%, 20%, 25%, 30%, 35%, 35%, 40%, 45%, or 50% of
the membrane is formed of a polymer having a Shore hardness of from
about 70A to about 55D. In some embodiments, the resistance domain
is formed from a polymer having a Shore hardness of from about 70A
to about 55D. Additional membrane domains are also formed of a
polymer having a Shore hardness within this range, in other
embodiments. In one embodiment, at least a portion of the membrane
is formed of a polymer having a Shore hardness of from about 70A to
about 55D and an enzyme is disposed in the polymer.
[0281] As discussed herein, membranes can be formed by any suitable
method. It can be desirable to form membranes on the various
exposed electrodes of the sensors of some embodiments by dipping
the sensor at different lengths, by masking, controlled UV curing,
and similar methods.
[0282] FIG. 6A is a cross section of the sensor shown in FIG. 1A,
taken at line 6-6. A membrane system (see FIG. 6A) is deposited
over the electroactive surfaces of the sensor and includes a
plurality of domains or layers, such as described in more detail
below. In some embodiments, the membrane comprises a single layer.
In some embodiments, the single layer includes one or more
functional domains (e.g., portions or areas). In other embodiments,
the membrane comprises two or more layers. In some embodiments,
each of the layers performs a different function. Alternative,
multiple layers can perform the same function. The membrane system
can be deposited on the exposed electroactive surfaces using known
thin film techniques (for example, spraying, electro-depositing,
dipping, and the like). In one exemplary embodiment, each domain is
deposited by dipping the sensor into a solution and drawing out the
sensor at a speed that provides the appropriate domain thickness.
In another exemplary embodiment, each domain is deposited by
spraying the solution onto the sensor for a period of time that
provides the appropriate domain thickness. In general, the membrane
system can be disposed over (deposited on) the electroactive
surfaces using methods appreciated by one skilled in the art.
[0283] In general, the membrane system includes a plurality of
domains, for example, an electrode domain 602, an interference
domain 304, an enzyme domain 606 (for example, including glucose
oxidase), and/or a resistance domain 608, as shown in FIG. 6A, and
can include a high oxygen solubility domain, and/or a bioprotective
domain (not shown), such as is described in more detail in U.S.
Patent Publication No. 2005-0245799-A1, and such as is described in
more detail below. The membrane system can be deposited on the
exposed electroactive surfaces using known thin film techniques
(for example, vapor deposition, spraying, electro-depositing,
dipping, and the like). In alternative embodiments, however, other
vapor deposition processes (e.g., physical and/or chemical vapor
deposition processes) can be useful for providing one or more of
the insulating and/or membrane layers, including ultrasonic vapor
deposition, electrostatic deposition, evaporative deposition,
deposition by sputtering, pulsed laser deposition, high velocity
oxygen fuel deposition, thermal evaporator deposition, electron
beam evaporator deposition, deposition by reactive sputtering
molecular beam epitaxy, atmospheric pressure chemical vapor
deposition (CVD), atomic layer CVD, hot wire CVD, low-pressure CVD,
microwave plasma-assisted CVD, plasma-enhanced CVD, rapid thermal
CVD, remote plasma-enhanced CVD, and ultra-high vacuum CVD, for
example. However, the membrane system can be disposed over (or
deposited on) the electroactive surfaces using any known method, as
will be appreciated by one skilled in the art. When enzymes are
employed, e.g., in certain dual working electrode glucose sensor
configurations, for ease of fabrication each of the electrodes can
be dipped with an enzyme domain including enzyme. Enzyme over one
of the working electrodes (e.g., the second working electrode) can
then be denatured/killed, e.g., by exposure to UV or other
irradiation, or by exposure to other agents or treatment methods as
known in the art for denaturing enzyme. In some embodiments, one or
more domains of the membrane systems are formed from materials such
as silicone, polytetrafluoroethylene,
polyethylene-co-tetrafluoroethylene, polyolefin, polyester,
polycarbonate, biostable polytetrafluoroethylene, homopolymers,
copolymers, terpolymers of polyurethanes, polypropylene (PP),
polyvinylchloride (PVC), polyvinylidene fluoride (PVDF),
polybutylene terephthalate (PBT), polymethylmethacrylate (PMMA),
polyether ether ketone (PEEK), polyurethanes, cellulosic polymers,
polysulfones and block copolymers thereof including, for example,
di-block, tri-block, alternating, random and graft copolymers. U.S.
Patent Publication No. 2005-0245799-A1 describes biointerface and
membrane system configurations and materials that may be
applied.
[0284] In selected embodiments, the membrane system comprises an
electrode domain. The electrode domain 602 is provided to ensure
that an electrochemical reaction occurs between the electroactive
surfaces of the working electrode and the reference electrode, and
thus the electrode domain may be situated more proximal to the
electroactive surfaces than the interference and/or enzyme domain.
The electrode domain may include a coating that maintains a layer
of water at the electrochemically reactive surfaces of the sensor.
In other words, the electrode domain is present to provide an
environment between the surfaces of the working electrode and the
reference electrode, which facilitates an electrochemical reaction
between the electrodes. For example, a humectant in a binder
material can be employed as an electrode domain; this allows for
the full transport of ions in the aqueous environment. The
electrode domain can also assist in stabilizing the operation of
the sensor by accelerating electrode start-up and drifting problems
caused by inadequate electrolyte. The material that forms the
electrode domain can also provide an environment that protects
against pH-mediated damage that can result from the formation of a
large pH gradient due to the electrochemical activity of the
electrodes.
[0285] In one embodiment, the electrode domain includes hydrophilic
polymer film (e.g., a flexible, water-swellable, hydrogel) having a
"dry film" thickness of from about 0.05 microns or less to about 20
microns or more, or from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3,
0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4,
5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5
microns, or from about 3, 2.5, 2, or 1 microns, or less, to about
3.5, 4, 4.5, or 5 microns or more. "Dry film" thickness refers to
the thickness of a cured film cast from a coating formulation by
standard coating techniques.
[0286] In certain embodiments, the electrode domain is formed of a
curable mixture of a urethane polymer and a hydrophilic polymer. In
some embodiments, the coatings are formed of a polyurethane polymer
having carboxylate or hydroxyl functional groups and non-ionic
hydrophilic polyether segments, wherein the polyurethane polymer is
crosslinked with a water-soluble carbodiimide (e.g.,
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)) in the
presence of polyvinylpyrrolidone and cured at a moderate
temperature of about 50.degree. C.
[0287] In some embodiments, the electrode domain is formed from a
hydrophilic polymer (e.g., a polyamide, a polylactone, a polyimide,
a polylactam, a functionalized polyamide, a functionalized
polylactone, a functionalized polyimide, a functionalized
polylactam or a combination thereof) that renders the electrode
domain substantially more hydrophilic than an overlying domain,
(e.g., interference domain, enzyme domain). In some embodiments,
the electrode domain is formed substantially entirely and/or
primarily from a hydrophilic polymer. In some embodiments, the
electrode domain is formed substantially entirely from PVP. In some
embodiments, the electrode domain is formed entirely from a
hydrophilic polymer. Useful hydrophilic polymers include but are
not limited to poly-N-vinylpyrrolidone (PVP),
poly-N-vinyl-2-piperidone, poly-N-vinyl-2-caprolactam,
poly-N-vinyl-3-methyl-2-caprolactam,
poly-N-vinyl-3-methyl-2-piperidone,
poly-N-vinyl-4-methyl-2-piperidone,
poly-N-vinyl-4-methyl-2-caprolactam,
poly-N-vinyl-3-ethyl-2-pyrrolidone,
poly-N-vinyl-4,5-dimethyl-2-pyrrolidone, polyvinylimidazole,
poly-N,N-dimethylacrylamide, polyvinyl alcohol, polyacrylic acid,
polyethylene oxide, poly-2-ethyl-oxazoline, copolymers thereof and
mixtures thereof. A blend of two or more hydrophilic polymers may
be used in some embodiments. In some embodiments, the hydrophilic
polymer(s) is not crosslinked. In alternative embodiments,
crosslinking may be performed, such as by adding a crosslinking
agent, such as but not limited to EDC, or by irradiation at a
wavelength sufficient to promote crosslinking between the
hydrophilic polymer molecules, which is believed to create a more
tortuous diffusion path through the domain.
[0288] An electrode domain formed from a hydrophilic polymer (e.g.,
PVP) has been shown to substantially reduce break-in time of
analyte sensors; for example, a glucose sensor utilizing a
cellulosic-based interference domain such as described in more
detail elsewhere herein. In some embodiments, a uni-component
electrode domain formed from a single hydrophilic polymer (e.g.,
PVP) has been shown to substantially reduce break-in time of a
glucose sensor to less than about 2 hours, less than about 1 hour,
less than about 20 minutes and/or substantially immediately, such
as exemplified in Examples 9 through 11 and 13. Generally, sensor
break-in is the amount of time required (after implantation) for
the sensor signal to become substantially representative of the
analyte concentration. Sensor break-in includes both membrane
break-in and electrochemical break-in, which are described in more
detail elsewhere herein. In some embodiments, break-in time is less
than about 2 hours. In other embodiments, break-in time is less
than about 1 hour. In still other embodiments, break-in time is
less than about 30 minutes, less than about 20 minutes, less than
about 15 minutes, less than about 10 minutes, or less. In one
embodiment, sensor break-in occurs substantially immediately.
Advantageously, in embodiments wherein the break-in time is about 0
minutes (substantially immediately), the sensor can be inserted and
begin providing substantially accurate analyte (e.g., glucose)
concentrations almost immediately post-insertion, for example,
wherein membrane break-in does not limit start-up time.
[0289] While not wishing to be bound by theory, it is believed that
providing an electrode domain that is substantially more
hydrophilic than the next more distal membrane layer or domain
(e.g., the overlaying domain; the layer more distal to the
electroactive surface than the electrode domain, such as an
interference domain or an enzyme domain) reduces the break-in time
of an implanted sensor, by increasing the rate at which the
membrane system is hydrated by the surrounding host tissue. While
not wishing to be bound by theory, it is believed that, in general,
increasing the amount of hydrophilicity of the electrode domain
relative to the overlaying layer (e.g., the distal layer in contact
with electrode domain, such as the interference domain, enzyme
domain, etc.), increases the rate of water absorption, resulting in
reduced sensor break-in time. The hydrophilicity of the electrode
domain can be substantially increased by the proper selection of
hydrophilic polymers, based on their hydrophilicity relative to
each other and relative to the overlaying layer (e.g.,
cellulosic-based interference domain), with certain polymers being
substantially more hydrophilic than the overlaying layer. In one
exemplary embodiment, PVP forms the electrode domain, the
interference domain is formed from a blend of cellulosic
derivatives, such as but not limited to cellulose acetate butyrate
and cellulose acetate; it is believed that since PVP is
substantially more hydrophilic than the cellulosic-based
interference domain, the PVP rapidly draws water into the membrane
to the electrode domain, and enables the sensor to function with a
desired sensitivity and accuracy and starting within a
substantially reduced time period after implantation. Reductions in
sensor break-in time reduce the amount of time a host has to wait
to obtain sensor readings, which is particularly advantageous not
only in ambulatory applications, but particularly in hospital
settings where time is critical.
[0290] While not wishing to be bound by theory, it is believed that
when the water absorption of the overlying domain (e.g., the domain
overlying the electrode domain) is less than the water absorption
of the electrode domain (e.g., during membrane equilibration), then
the difference in water absorption between the two domains will
drive membrane equilibration and thus membrane break-in. Namely,
increasing the difference in hydrophilicity (e.g., between the two
domains) results in an increase in the rate of water absorption,
which, in turn, results in a decrease in membrane break-in time
and/or sensor break-in time. As discussed elsewhere herein, the
relative hydrophilicity of the electrode domain as compared to the
overlying domain can be modulated by a selection of more
hydrophilic materials for formation of the electrode domain (and/or
more hydrophobic materials for the overlying domain(s)). For
example, an electrode domain with hydrophilic polymer capable of
absorbing larger amounts of water can be selected instead of a
second hydrophilic polymer that is capable of absorbing less water
than the first hydrophilic polymer. In some embodiments, the water
content difference between the electrode domain and the overlying
domain (e.g., during or after membrane equilibration) is from about
1% or less to about 90% or more. In other embodiments, the water
content difference between the electrode domain and the overlying
domain is from about 10% or less to about 80% or more. In still
other embodiments, the water content difference between the
electrode domain and the overlying domain is from about 30% or less
to about 60% or more. In some embodiments, the electrode domain
absorbs 5 wt. % or less to 95 wt. % or more water, or from about 5,
10, 15, 20, 25, 30, 35, 40, 45, or 50 wt. % to about 55, 60, 65,
70, 75, 80, 85, 90 or 95 wt. % water than the adjacent (overlying)
domain (e.g., the domain that is more distal to the electroactive
surface than the electrode domain).
[0291] In another example, the rate of water absorption by a
polymer can be affected by other factors, such as but not limited
to the polymer's molecular weight. For example, the rate of water
absorption by PVP is dependent upon its molecular weight, which is
typically from about 40 kDa or less to about 360 kDa or more; with
a lower molecular weight PVP (e.g., 40 kDa) absorbing water faster
than a higher molecular weight PVP. Accordingly, modulating
factors, such as molecular weight, that affect the rate of water
absorption by a polymer, can promote the proper selection of
materials for electrode domain fabrication. In one embodiment, a
lower molecular weight PVP is selected, to reduce break-in
time.
[0292] The electrode domain may be deposited by known thin film
deposition techniques (e.g., spray coating or dip-coating the
electroactive surfaces of the sensor). In some embodiments, the
electrode domain is formed by dip-coating the electroactive
surfaces in an electrode domain solution (e.g., 5, 10, 15, 20, 25
or 30% or more PVP in deionized water) and curing the domain for a
time of from about 15 minutes to about 30 minutes at a temperature
of from about 40.degree. C. to about 55.degree. C. (and can be
accomplished under vacuum (e.g., 20 to 30 mmHg)). In embodiments
wherein dip-coating is used to deposit the electrode domain, an
insertion rate of from about 1 to about 3 inches per minute into
the electrode domain solution may be used, with a dwell time of
from about 0.5 to about 2 minutes in the electrode domain solution,
and a withdrawal rate of from about 0.25 to about 2 inches per
minute from the electrode domain solution provide a functional
coating. However, values outside of those set forth above can be
acceptable or even desirable in certain embodiments, for example,
depending upon solution viscosity and solution surface tension, as
is appreciated by one skilled in the art. In one embodiment, the
electroactive surfaces of the electrode system are dip-coated one
time (one layer) and cured at 50.degree. C. under vacuum for 20
minutes. In another embodiment, the electroactive surfaces of the
electrode system is dip-coated and cured at 50.degree. C. under
vacuum for 20 minutes a first time, followed by dip coating and
curing at 50.degree. C. under vacuum for 20 minutes a second time
(two layers). In still other embodiments, the electroactive
surfaces can be dip-coated three or more times (three or more
layers). In other embodiments, the 1, 2, 3 or more layers of PVP
are applied to the electroactive surfaces by spray coating or vapor
deposition. In some embodiments, a crosslinking agent (e.g., EDC)
can be added to the electrode domain casting solution to promote
crosslinking within the domain (e.g., between electrode domain
polymer components, latex, etc.). In some alternative embodiments
however, no crosslinking agent is used and the electrode domain is
not substantially crosslinked.
[0293] In some embodiments, the deposited PVP electrode domain has
a "dry film" thickness of from about 0.05 microns or less to about
20 microns, or from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35,
0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6,
7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns,
or from about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5
microns. Although an independent electrode domain is described
herein, in some embodiments sufficient hydrophilicity can be
provided in the interference domain and/or enzyme domain (the
domain adjacent to the electroactive surfaces) so as to provide for
the full transport of ions in the aqueous environment (e.g. without
a distinct electrode domain). In these embodiments, an electrode
domain is not necessary.
[0294] Interferents are molecules or other species that are reduced
or oxidized at the electrochemically reactive surfaces of the
sensor, either directly or via an electron transfer agent, to
produce a false positive analyte signal (e.g., a
non-analyte-related signal). This false positive signal causes the
host's analyte concentration (e.g., glucose concentration) to
appear higher than the true analyte concentration. False-positive
signal is a clinically significant problem in some conventional
sensors. For example in a case of a dangerously hypoglycemic
situation, wherein the host has ingested an interferent (e.g.,
acetaminophen), the artificially high glucose signal can lead the
host to believe that he is euglycemic (or, in some cases,
hyperglycemic). As a result, the host can make inappropriate
treatment decisions, such as taking no action, when the proper
course of action is to begin eating. In another example, in the
case of a euglycemic or hyperglycemic situation, wherein a host has
consumed acetaminophen, an artificially high glucose signal caused
by the acetaminophen can lead the host to believe that his glucose
concentration is much higher than it truly is. Again, as a result
of the artificially high glucose signal, the host can make
inappropriate treatment decisions, such as giving himself too much
insulin, which in turn can lead to a dangerous hypoglycemic
episode.
[0295] In some embodiments, an interference domain 604 is provided
that substantially restricts or blocks the flow of one or more
interfering species therethrough; thereby substantially preventing
artificial signal increases. Some known interfering species for a
glucose sensor, as described in more detail herein, include
acetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine,
dopamine, ephedrine, ibuprofen, L-dopa, methyl dopa, salicylate,
tetracycline, tolazamide, tolbutamide, triglycerides, and uric
acid. In general, the interference domain of some embodiments is
less permeable to one or more of the interfering species than to
the measured species, e.g., the product of an enzymatic reaction
that is measured at the electroactive surface(s), such as but not
limited to H.sub.2O.sub.2.
[0296] In one embodiment, the interference domain is formed from
one or more cellulosic derivatives. Cellulosic derivatives can
include, but are not limited to, cellulose esters and cellulose
ethers. In general, cellulosic derivatives include polymers such as
cellulose acetate, cellulose acetate butyrate, 2-hydroxyethyl
cellulose, cellulose acetate phthalate, cellulose acetate
propionate, cellulose acetate trimellitate, and the like, as well
as their copolymers and terpolymers with other cellulosic or
non-cellulosic monomers. Cellulose is a polysaccharide polymer of
.beta.-D-glucose. While cellulosic derivatives may be used in some
embodiments, other polymeric polysaccharides having similar
properties to cellulosic derivatives can also be employed in other
embodiments. Descriptions of cellulosic interference domains can be
found in U.S. Patent Publication No. 2006-0229512-A1, U.S. Patent
Publication No. 2007-0173709-A1, U.S. Patent Publication No.
2006-0253012-A1, and U.S. Patent Publication No.
2007-0213611-A1.
[0297] In some embodiments, the interferent's equivalent glucose
signal response (measured by the sensor) is 50 mg/dL or less. In
certain embodiments, the interferent produces an equivalent glucose
signal response of 40 mg/dL or less. In some embodiments, the
interferent produces an equivalent glucose signal response of less
than about 30, 20 or 10 mg/dL. In one exemplary embodiment, the
interference domain is configured to substantially block
acetaminophen passage therethrough, wherein the equivalent glucose
signal response of the acetaminophen is less than about 30
mg/dL.
[0298] In alternative embodiments, the interference domain is
configured to substantially block a therapeutic dose of
acetaminophen. The term "therapeutic dose" as used herein is a
broad term, and is to be given its ordinary and customary meaning
to a person of ordinary skill in the art (and is not to be limited
to a special or customized meaning), and refers without limitation
to the quantity of any substance required to effect the cure of a
disease, to relieve pain, or that will correct the manifestations
of a deficiency of a particular factor in the diet, such as the
effective dose used with therapeutically applied compounds, such as
drugs. For example, a therapeutic dose of acetaminophen can be an
amount of acetaminophen required to relieve headache pain or reduce
a fever. As a further example, 1,000 mg of acetaminophen taken
orally, such as by swallowing two 500 mg tablets of acetaminophen,
is the therapeutic dose frequently taken for headaches. In some
embodiments, the interference membrane is configured to block a
therapeutic dose of acetaminophen, wherein the equivalent glucose
signal response of the acetaminophen is less than about 60 mg/dL.
In one embodiment, the interference membrane is configured to block
a therapeutic dose of acetaminophen, wherein the equivalent glucose
signal response of the acetaminophen is less than about 40 mg/dL.
In another embodiment, the interference membrane is configured to
block a therapeutic dose of acetaminophen, wherein the equivalent
glucose signal response of the acetaminophen is less than about 30
mg/dL.
[0299] In some alternative embodiments, additional polymers, such
as NAFION.RTM., can be used in combination with cellulosic
derivatives to provide equivalent and/or enhanced function of the
interference domain. As one example, a layer of a 5 wt. %
NAFION.RTM. casting solution was applied over a previously applied
(e.g., and cured) layer of 8 wt. % cellulose acetate, e.g., by dip
coating at least one layer of cellulose acetate and subsequently
dip coating at least one layer NAFION.RTM. onto a needle-type
sensor. Any number of coatings or layers formed in any order may be
suitable for forming the interference domain.
[0300] In some alternative embodiments, other polymer types that
can be utilized as a base material for the interference domain
include polyurethanes, polymers having pendant ionic groups, and
polymers having controlled pore size, for example. In one such
alternative embodiment, the interference domain includes a thin,
hydrophobic membrane that is non-swellable and restricts diffusion
of high molecular weight species. The interference domain is
permeable to relatively low molecular weight substances, such as
hydrogen peroxide, but restricts the passage of higher molecular
weight substances, including glucose and ascorbic acid. Other
systems and methods for reducing or eliminating interference
species that can be applied to the membrane system are described in
U.S. Pat. No. 7,074,307, U.S. Patent Publication No.
2005-0176136-A1, U.S. Pat. No. 7,081,195, and U.S. Patent
Publication No. 2005-0143635-A1. In some alternative embodiments, a
distinct interference domain is not included.
[0301] In some embodiments, the interference domain is deposited
either directly onto the electroactive surfaces of the sensor or
onto the distal surface of the electrode domain, for a domain
thickness of from about 0.05 microns to about 20 microns, or from
about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1,
1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11,
12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, or from about 1,
1.5 or 2 microns to about 2.5 or 3 microns. Thicker membranes can
also be desirable in certain embodiments, but thinner membranes may
be used because they have a lower impact on the rate of diffusion
of hydrogen peroxide from the enzyme membrane to the electroactive
surface(s).
[0302] In general, the membrane systems of some embodiments can be
formed and/or deposited on the exposed electroactive surfaces
(e.g., one or more of the working and reference electrodes) using
known thin film techniques (for example, casting, spray coating,
drawing down, electro-depositing, dip coating, and the like),
however casting or other known application techniques can also be
utilized. The interference domain may be deposited by spray or dip
coating. In one exemplary embodiment of a needle-type
(transcutaneous) sensor such as described herein, the interference
domain is formed by dip coating the sensor into an interference
domain solution using an insertion rate of from about 0.5 inch/min
to about 60 inches/min, or about 1 inch/min, a dwell time of from
about 0 minute to about 2 minutes, or about 1 minute, and a
withdrawal rate of from about 0.5 inch/minute to about 60
inches/minute, or about 1 inch/minute, and curing (drying) the
domain from about 1 minute to about 30 minutes, or from about 3
minutes to about 15 minutes (and can be accomplished at room
temperature or under vacuum (e.g., 20 to 30 mmHg)). In one
exemplary embodiment including cellulose acetate butyrate
interference domain, a 3-minute cure (i.e., dry) time is sometimes
between each layer applied. In another exemplary embodiment
employing a cellulose acetate interference domain, a 15 minute cure
(i.e., dry) time is used between each layer applied.
[0303] In some embodiments, the dip process can be repeated at
least one time and up to 10 times or more. In other embodiments,
only one dip is performed. The number of repeated dip processes
used depends upon the cellulosic derivative(s) used, their
concentration, conditions during deposition (e.g., dipping) and the
desired thickness (e.g., sufficient thickness to provide functional
blocking of certain interferents), and the like. In some
embodiments, 1 to 3 microns may be used for the interference domain
thickness; however, values outside of these can be acceptable or
even desirable in certain embodiments, for example, depending upon
viscosity and surface tension, as is appreciated by one skilled in
the art. In one exemplary embodiment, an interference domain is
formed from three layers of cellulose acetate butyrate. In another
exemplary embodiment, an interference domain is formed from 10
layers of cellulose acetate. In another embodiment, an interference
domain is formed from 1 layer of a blend of cellulose acetate and
cellulose acetate butyrate. In alternative embodiments, the
interference domain can be formed using any known method and
combination of cellulose acetate and cellulose acetate butyrate, as
will be appreciated by one skilled in the art. In some embodiments,
the electroactive surface can be cleaned, smoothed or otherwise
treated prior to application of the interference domain. In some
embodiments, the interference domain of some embodiments can be
useful as a bioprotective or biocompatible domain, namely, a domain
that interfaces with host tissue when implanted in an animal (e.g.,
a human) due to its stability and biocompatibility. In still other
embodiments, other portions of the membrane system, such as but not
limited to the enzyme domain and/or the resistance domain can be
configured for interference blocking. In still other embodiments,
an interference domain can be located either more distally or
proximally to the electroactive surface than other membrane
domains. For example, an interference domain can be located more
distal to the electroactive surface than an enzyme domain or a
resistance domain, in some embodiments.
[0304] In certain embodiments, the membrane system further includes
an enzyme domain 606 disposed more distally from the electroactive
surfaces than the interference domain; however other configurations
can be desirable. In certain embodiments, the enzyme domain
provides an enzyme to catalyze the reaction of the analyte and its
co-reactant, as described in more detail below. In the some
embodiments of a glucose sensor, the enzyme domain includes glucose
oxidase (GOX); however other oxidases, for example, galactose
oxidase or uricase oxidase, can also be used. In some embodiments,
the enzyme domain is configured and arranged for detection of at
least one of albumin, alkaline phosphatase, alanine transaminase,
aspartate aminotransferase, bilirubin, blood urea nitrogen,
calcium, CO.sub.2, chloride, creatinine, glucose, gamma-glutamyl
transpeptidase, hematocrit, lactate, lactate dehydrogenase,
magnesium, oxygen, pH, phosphorus, potassium, sodium, total
protein, uric acid, a metabolic marker, a drug, various minerals,
various metabolites, and/or the like. In a further embodiment, the
sensor is configured and arranged to detect two or more of albumin,
alkaline phosphatase, alanine transaminase, aspartate
aminotransferase, bilirubin, blood urea nitrogen, calcium,
CO.sub.2, chloride, creatinine, glucose, gamma-glutamyl
transpeptidase, hematocrit, lactate, lactate dehydrogenase,
magnesium, oxygen, pH, phosphorus, potassium, sodium, total
protein, uric acid, a metabolic marker, a drug, various minerals,
various metabolites, and/or the like.
[0305] For an enzyme-based electrochemical glucose sensor to
perform well, the sensor's response may be limited by neither
enzyme activity nor co-reactant concentration. Because enzymes,
including glucose oxidase, are subject to deactivation as a
function of time even in ambient conditions, this behavior is
compensated for in forming the enzyme domain. In some embodiments,
the enzyme domain is constructed of aqueous dispersions of
colloidal polyurethane polymers including the enzyme. However, in
alternative embodiments the enzyme domain is constructed from an
oxygen enhancing material, for example, silicone, or fluorocarbon,
in order to provide a supply of excess oxygen during transient
ischemia. The enzyme may be immobilized within the domain. See,
e.g., U.S. Patent Publication No. 2005-0054909-A1. In some
embodiments, the enzyme domain is deposited onto the interference
domain for a domain thickness of from about 0.05 micron to about 20
microns, or from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4,
0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7,
8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, or
from about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns.
However in some embodiments, the enzyme domain can be deposited
directly onto the electroactive surfaces. The enzyme domain may be
deposited by spray or dip coating. In one embodiment of needle-type
(transcutaneous) sensor such as described herein, the enzyme domain
is formed by dip coating the interference domain coated sensor into
an enzyme domain solution and curing the domain for from about 15
to about 30 minutes at a temperature of from about 40.degree. C. to
about 55.degree. C. (and can be accomplished under vacuum (e.g., 20
to 30 mmHg)). In embodiments wherein dip coating is used to deposit
the enzyme domain at room temperature to provide a functional
coating, the insertion rate used may be from about 0.25 inch per
minute to about 3 inches per minute, with a dwell time of from
about 0.5 minutes to about 2 minutes, and a withdrawal rate of from
about 0.25 inch per minute to about 2 inches per minute. However,
values outside of those set forth above can be acceptable or even
desirable in certain embodiments, for example, depending upon
viscosity and surface tension, as is appreciated by one skilled in
the art. In one embodiment, the enzyme domain is formed by dip
coating two times (namely, forming two layers) in an enzyme domain
solution and curing at 50.degree. C. under vacuum for 20 minutes.
However, in some embodiments, the enzyme domain can be formed by
dip coating and/or spray coating one or more layers at a
predetermined concentration of the coating solution, insertion
rate, dwell time, withdrawal rate, and/or desired thickness.
[0306] Enzymatic analyte sensors are dependent upon the kinetics of
the enzymes that they comprise. As is understood by one skilled in
the art, in order to calculate the concentration of one
reactant/analyte (e.g., using a defined amount of enzyme) all other
reactants/co-reactants are present in excess. However, in the body,
this is often not the case; sometimes the analyte is in excess.
Accordingly, in some embodiments, the sensor membrane is configured
and arranged to restrict diffusion of the analyte to the enzyme
domain, such that the analyte can be measured accurately. In some
embodiments, the membrane system includes a resistance domain 608
disposed more distal from the electroactive surfaces than the
enzyme domain. Although the following description is directed to a
resistance domain for a glucose sensor, the resistance domain can
be modified for other analytes and co-reactants as well.
[0307] With respect to glucose sensors, there exists a molar excess
of glucose relative to the amount of oxygen in blood; that is, for
every free oxygen molecule in extracellular fluid, there are
typically more than 100 glucose molecules present (see Updike et
al., Diabetes Care 5:207-21(1982)). However, an immobilized
enzyme-based glucose sensor employing oxygen as co-reactant may be
supplied with oxygen in non-rate-limiting excess in order for the
sensor to respond linearly to changes in glucose concentration,
while not responding to changes in oxygen concentration.
Specifically, when a glucose-monitoring reaction is oxygen limited,
linearity is not achieved above minimal concentrations of glucose.
Without a semipermeable membrane situated over the enzyme domain to
control the flux of glucose and oxygen, a linear response to
glucose levels can be obtained only for glucose concentrations of
up to about 40 mg/dL. However, in a clinical setting, a linear
response to glucose levels is desirable up to at least about 400
mg/dL.
[0308] The resistance domain includes a semipermeable membrane that
controls the flux of oxygen and glucose to the underlying enzyme
domain, thereby rendering oxygen in a non-rate-limiting excess. As
a result, the upper limit of linearity of glucose measurement is
extended to a much higher value than that which is achieved without
the resistance domain. In one embodiment, the resistance domain
exhibits an oxygen to glucose permeability ratio of from about 50:1
or less to about 400:1 or more, or about 200:1. As a result,
one-dimensional reactant diffusion is adequate to provide excess
oxygen at all reasonable glucose and oxygen concentrations found in
the subcutaneous matrix (See Rhodes et al., Anal. Chem.,
66:1520-1529 (1994)).
[0309] In alternative embodiments, a lower ratio of
oxygen-to-glucose can be sufficient to provide excess oxygen by
using a high oxygen solubility domain (for example, a silicone or
fluorocarbon-based material or domain) to enhance the
supply/transport of oxygen to the enzyme domain. If more oxygen is
supplied to the enzyme, then more glucose can also be supplied to
the enzyme without creating an oxygen rate-limiting excess. In
alternative embodiments, the resistance domain is formed from a
silicone composition, such as is described in U.S. Patent
Publication No. 2005-0090607-A1.
[0310] In one embodiment, the resistance domain includes a
polyurethane membrane with both hydrophilic and hydrophobic regions
to control the diffusion of glucose and oxygen to an analyte
sensor, the membrane being fabricated easily and reproducibly from
commercially available materials. A suitable hydrophobic polymer
component is a polyurethane, or polyetherurethaneurea. Polyurethane
is a polymer produced by the condensation reaction of a
diisocyanate and a difunctional hydroxyl-containing material. A
polyurethaneurea is a polymer produced by the condensation reaction
of a diisocyanate and a difunctional amine-containing material.
Diisocyanates that may be used include, but are not limited to,
aliphatic diisocyanates containing from about 4 to about 8
methylene units. Diisocyanates containing cycloaliphatic moieties
can also be useful in the preparation of the polymer and copolymer
components of the membranes of some embodiments. The material that
forms the basis of the hydrophobic matrix of the resistance domain
can be any of those known in the art as appropriate for use as
membranes in sensor devices and as having sufficient permeability
to allow relevant compounds to pass through it, for example, to
allow an oxygen molecule to pass through the membrane from the
sample under examination in order to reach the active enzyme or
electrochemical electrodes. Examples of materials which can be used
to make non-polyurethane type membranes include vinyl polymers,
polyethers, polyesters, polyamides, inorganic polymers such as
polysiloxanes and polycarbosiloxanes, natural polymers such as
cellulosic and protein based materials, and mixtures or
combinations thereof.
[0311] In one embodiment, the hydrophilic polymer component is
polyethylene oxide. For example, one useful hydrophobic-hydrophilic
copolymer component is a polyurethane polymer that includes about
20% hydrophilic polyethylene oxide. The polyethylene oxide portions
of the copolymer are thermodynamically driven to separate from the
hydrophobic portions of the copolymer and the hydrophobic polymer
component. The 20% polyethylene oxide-based soft segment portion of
the copolymer used to form the final blend affects the water
pick-up and subsequent glucose permeability of the membrane.
[0312] In some embodiments, the resistance domain is formed from a
silicone polymer modified to allow analyte (e.g., glucose)
transport.
[0313] In some embodiments, the resistance domain is formed from a
silicone polymer/hydrophobic-hydrophilic polymer blend. In one
embodiment, The hydrophobic-hydrophilic polymer for use in the
blend may be any suitable hydrophobic-hydrophilic polymer,
including but not limited to components such as
polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate,
polyvinylalcohol, polyacrylic acid, polyethers such as polyethylene
glycol or polypropylene oxide, and copolymers thereof, including,
for example, di-block, tri-block, alternating, random, comb, star,
dendritic, and graft copolymers (block copolymers are discussed in
U.S. Pat. Nos. 4,803,243 and 4,686,044). In one embodiment, the
hydrophobic-hydrophilic polymer is a copolymer of poly(ethylene
oxide) (PEO) and poly(propylene oxide) (PPO). Suitable such
polymers include, but are not limited to, PEO-PPO diblock
copolymers, PPO-PEO-PPO triblock copolymers, PEO--PPO-PEO triblock
copolymers, alternating block copolymers of PEO-PPO, random
copolymers of ethylene oxide and propylene oxide, and blends
thereof. In some embodiments, the copolymers may be optionally
substituted with hydroxy substituents. Commercially available
examples of PEO and PPO copolymers include the PLURONIC.RTM. brand
of polymers available from BASF.RTM.. In one embodiment,
PLURONIC.RTM. F-127 is used. Other PLURONIC.RTM. polymers include
PPO-PEO-PPO triblock copolymers (e.g., PLURONIC.RTM. R products).
Other suitable commercial polymers include, but are not limited to,
SYNPERONICS.RTM. products available from UNIQEMA.RTM.. U.S. Patent
Publication No. 2007-0244379-A1 describes systems and methods
suitable for the resistance and/or other domains of the membrane
system.
[0314] In some embodiments, the resistance domain is deposited onto
the enzyme domain to yield a domain thickness of from about 0.05
microns to about 20 microns, or from about 0.05, 0.1, 0.15, 0.2,
0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns
to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
or 19.5 microns, or from about 2, 2.5 or 3 microns to about 3.5, 4,
4.5, or 5 microns. The resistance domain may be deposited onto the
enzyme domain by vapor deposition, spray coating, or dip coating.
In one embodiment, spray coating is the preferred deposition
technique. The spraying process atomizes and mists the solution,
and therefore most or all of the solvent is evaporated prior to the
coating material settling on the underlying domain, thereby
minimizing contact of the solvent with the enzyme.
[0315] In another embodiment, physical vapor deposition (e.g.,
ultrasonic vapor deposition) is used for coating one or more of the
membrane domain(s) onto the electrodes, wherein the vapor
deposition apparatus and process include an ultrasonic nozzle that
produces a mist of micro-droplets in a vacuum chamber. In these
embodiments, the micro-droplets move turbulently within the vacuum
chamber, isotropically impacting and adhering to the surface of the
substrate. Advantageously, vapor deposition as described above can
be implemented to provide high production throughput of membrane
deposition processes (e.g., at least about 20 to about 200 or more
electrodes per chamber), greater consistency of the membrane on
each sensor, and increased uniformity of sensor performance, for
example, as described below.
[0316] In some embodiments, depositing the resistance domain (for
example, using one of the techniques described above) includes
formation of a membrane system that substantially blocks or resists
ascorbate (a known electrochemical interferent in hydrogen
peroxide-measuring glucose sensors). While not wishing to be bound
by theory, it is believed that during the process of depositing the
resistance domain, a structural morphology is formed that is
characterized in that ascorbate does not substantially permeate
therethrough.
[0317] In one embodiment, the resistance domain is deposited on the
enzyme domain by spray coating a solution of from about 1 wt. % to
about 5 wt. % polymer and from about 95 wt. % to about 99 wt. %
solvent. In spraying a solution of resistance domain material,
including a solvent, onto the enzyme domain, it is desirable to
mitigate or substantially reduce any contact with enzyme of any
solvent in the spray solution that can deactivate the underlying
enzyme of the enzyme domain. Tetrahydrofuran (THF) is one solvent
that minimally or negligibly affects the enzyme of the enzyme
domain upon spraying. Other solvents can also be suitable for use,
as is appreciated by one skilled in the art.
[0318] Although a variety of spraying or deposition techniques can
be used, spraying the resistance domain material and rotating the
sensor at least one time by 180.degree. can typically provide
adequate coverage by the resistance domain. Spraying the resistance
domain material and rotating the sensor at least two times by
120.degree. provides even greater coverage (one layer of
360.degree. coverage), thereby ensuring resistivity to glucose,
such as is described in more detail above.
[0319] In some embodiment, the resistance domain is spray coated
and subsequently cured for a time of from about 15 minutes to about
90 minutes at a temperature of from about 40.degree. C. to about
60.degree. C. (and can be accomplished under vacuum (e.g., from 20
to 30 mmHg)). A cure time of up to about 90 minutes or more can be
advantageous to ensure complete drying of the resistance
domain.
[0320] In one embodiment, the resistance domain is formed by spray
coating at least six layers (namely, rotating the sensor seventeen
times by 120.degree. for at least six layers of 360.degree.
coverage) and curing at 50.degree. C. under vacuum for 60 minutes.
However, the resistance domain can be formed by dip coating or
spray coating any layer or plurality of layers, depending upon the
concentration of the solution, insertion rate, dwell time,
withdrawal rate, and/or the desired thickness of the resulting
film. Additionally, curing in a convention oven can also be
employed.
[0321] In certain embodiments, a variable frequency microwave oven
can be used to cure the membrane domains/layers. In general,
microwave ovens directly excite the rotational mode of solvents.
Consequently, microwave ovens cure coatings from the inside out
rather than from the outside in as with conventional convection
ovens. This direct rotational mode excitation is responsible for
the typically observed "fast" curing within a microwave oven. In
contrast to conventional microwave ovens, which rely upon a fixed
frequency of emission that can cause arcing of dielectric
(metallic) substrates if placed within a conventional microwave
oven, Variable Frequency Microwave (VFM) ovens emit thousands of
frequencies within 100 milliseconds, which substantially eliminates
arcing of dielectric substrates. Consequently, the membrane
domains/layers can be cured even after deposition on metallic
electrodes as described herein. While not wishing to be bound by
theory, it is believe that VFM curing can increase the rate and
completeness of solvent evaporation from a liquid membrane solution
applied to a sensor, as compared to the rate and completeness of
solvent evaporation observed for curing in conventional convection
ovens. In certain embodiments, VFM is can be used together with
convection oven curing to further accelerate cure time. In some
sensor applications wherein the membrane is cured prior to
application on the electrode (see, for example, U.S. Patent
Publication No. 2005-0245799-A1, conventional microwave ovens
(e.g., fixed frequency microwave ovens) can be used to cure the
membrane layer.
[0322] A variety of therapeutic (bioactive) agents can be used with
the analyte sensor system of some embodiments. In some embodiments,
the therapeutic agent is an anticoagulant. In some embodiments, an
anticoagulant is included in the analyte sensor system to prevent
coagulation within or on the sensor (e.g., within or on the
catheter or within or on the sensor). In some embodiments, the
therapeutic agent is an antimicrobial, such as but not limited to
an antibiotic or antifungal compound. In some embodiments, the
therapeutic agent is an antiseptic and/or disinfectant. Therapeutic
agents can be used alone or in combination of two or more of them.
The therapeutic agents can be dispersed throughout the material of
the sensor (and/or catheter). In some embodiments, the membrane
system of some embodiments includes a therapeutic agent, which is
incorporated into at least a portion of the membrane system, or
which is incorporated into the device and adapted to diffuse
through the membrane. There are a variety of systems and methods by
which the therapeutic agent is incorporated into the membrane. In
some embodiments, the therapeutic agent is incorporated at the time
of manufacture of the membrane system. For example, the therapeutic
agent can be blended prior to curing the membrane system, or
subsequent to membrane system manufacture, for example, by coating,
imbibing, solvent-casting, or sorption of the bioactive agent into
the membrane system. Although the therapeutic agent may be
incorporated into the membrane system, in some embodiments the
therapeutic agent can be administered concurrently with, prior to,
or after insertion of the device intravascularly, for example, by
oral administration, or locally, for example, by subcutaneous
injection near the implantation site. A combination of therapeutic
agent incorporated in the membrane system and therapeutic agent
administration locally and/or systemically can be used in certain
embodiments.
[0323] As a non-limiting example, in some embodiments, the analyte
sensor 100 is a continuous electrochemical analyte sensor
configured to provide at least one working electrode and at least
one reference electrode, which are configured to measure a signal
associated with a concentration of the analyte in the host, such as
described in more detail below. The output signal is typically a
raw data stream that is used to provide a useful value of the
measured analyte concentration in a host to the patient or doctor,
for example. However, the analyte sensors of some embodiments
comprise at least one additional working electrode configured to
measure at least one additional signal, as discussed elsewhere
herein. For example, in some embodiments, the additional signal is
associated with the baseline and/or sensitivity of the analyte
sensor, thereby enabling monitoring of baseline and/or sensitivity
changes that may occur over time.
[0324] In general, electrochemical continuous analyte sensors
define a relationship between sensor-generated measurements (for
example, current in pA, nA, or digital counts after A/D conversion)
and a reference measurement (for example, glucose concentration
mg/dL or mmol/L) that are meaningful to a user (for example,
patient or doctor). For example, in the case of an implantable
diffusion-based glucose oxidase electrochemical glucose sensor, the
sensing mechanism generally depends on phenomena that are linear
with glucose concentration, for example: (1) diffusion of glucose
through a membrane system (for example, biointerface membrane and
membrane system) situated between implantation site and/or the
electrode surface, (2) an enzymatic reaction within the membrane
system, and (3) diffusion of the H.sub.2O.sub.2 to the sensor.
Because of this linearity, calibration of the sensor can be
understood by solving an equation:
y=mx+b
wherein y represents the sensor signal (e.g., counts), x represents
the estimated glucose concentration (e.g., mg/dL), m represents the
sensor sensitivity to glucose (e.g., counts/mg/dL), and b
represents the baseline signal (e.g., counts). When both
sensitivity m and baseline (background) b change over time in vivo,
calibration has generally requires at least two independent,
matched data pairs (x.sub.1, y.sub.1; x.sub.2, y.sub.2) to solve
for m and b and thus allow glucose estimation when only the sensor
signal, y is available. Matched data pairs can be created by
matching reference data (for example, one or more reference glucose
data points from a blood glucose meter, or the like) with
substantially time corresponding sensor data (for example, one or
more glucose sensor data points) to provide one or more matched
data pairs, such as described in co-pending U.S. Patent Publication
No. 2005-0027463-A1. In some implantable glucose sensors, such as
described in more detail in U.S. Pat. No. 6,329,161 to Heller et
al., the sensing layer utilizes immobilized mediators (e.g., redox
compounds) to electrically connect the enzyme to the working
electrode, rather than using a diffusional mediator. In some
implantable glucose sensors, such as described in more detail in
U.S. Pat. No. 4,703,756, the system has two oxygen sensors situated
in an oxygen-permeable housing, one sensor being unaltered and the
other contacting glucose oxidase allowing for differential
measurement of oxygen content in body fluids or tissues indicative
of glucose levels. A variety of systems and methods of measuring
glucose in a host are known, all of which may benefit from some of
all of the embodiments to provide a sensor having a signal-to-noise
ratio that is not substantially affected by non-constant noise.
[0325] Advantageously, continuous analyte monitoring is enabled.
For example, when the analyte is glucose, continuous glucose
monitoring enables tight glucose control, which can lead to reduced
morbidity and mortality among diabetic hosts. In some embodiments,
the medical staff is not unduly burdened by additional patient
interaction requirements. Advantageously, there is no net sample
(e.g., blood) loss for the host, which is a critical feature in
some clinical settings. For example, in a neonatal intensive care
unit, the host is extremely small and loss of even a few
milliliters of blood can be life threatening. Furthermore,
returning the body fluid sample to the host, instead of delivering
to a waste container greatly reduces the accumulation of
biohazardous waste that requires special disposal procedures. The
integrated sensor system components, as well as their use in
conjunction with an indwelling analyte sensor, are discussed in
greater detail below.
[0326] A variety of known sensor configurations can be employed
with the sensor systems described herein, such as U.S. Pat. No.
5,711,861 to Ward et al., U.S. Pat. No. 6,642,015 to Vachon et al.,
U.S. Pat. No. 6,654,625 to Say et al., U.S. Pat. No. 6,565,509 to
Say et al., U.S. Pat. No. 6,514,718 to Heller, U.S. Pat. No.
6,465,066 to Essenpreis et al., U.S. Pat. No. 6,214,185 to
Offenbacher et al., U.S. Pat. No. 5,310,469 to Cunningham et al.,
and U.S. Pat. No. 5,683,562 to Shaffer et al., U.S. Pat. No.
6,579,690 to Bonnecaze et al., U.S. Pat. No. 6,484,046 to Say et
al., U.S. Pat. No. 6,512,939 to Colvin et al., U.S. Pat. No.
6,424,847 to Mastrototaro et al., U.S. Pat. No. 6,424,847 to
Mastrototaro et al, U.S. Patent Publication No. 2006-0020187-A1 to
Brister et al., U.S. Patent Publication No. 2007-0027370-A1 to
Brauker et al., U.S. Patent Publication No. 2005-0143635-A1 to
Kamath et al., U.S. Patent Publication No. 2007-0027385-A1 to
Brister et al., U.S. Patent Publication No. 2007-0213611-A1 to
Simpson et al., U.S. Patent Publication No. 2008-0083617-A1 to
Simpson et al., U.S. Patent Publication No. 2008-0119703-A1 to
Brister et al., U.S. Patent Publication No. 2008-0108942-A1 to
Brister et al., and U.S. Patent Publication No. 2009-0018424-A1
Kamath et al., for example. The above-referenced patents and
publications are not inclusive of all applicable analyte sensors;
in general, it should be understood that the disclosed embodiments
are applicable to a variety of analyte sensor configurations.
[0327] In some embodiments, the dual-electrode sensor includes
electronics (e.g., a processor module, processing memory) that are
operably connected to the first and second working electrodes and
are configured to provide the first and second signals to generate
analyte concentration data substantially without signal
contribution due to non-analyte-related noise. For example, the
sensor electronics process and/or analyze the signals from the
first and second working electrodes and calculate the portion of
the first electrode signal that is due to analyte concentration
only. The portion of the first electrode signal that is not due to
the analyte concentration can be considered to be background, such
as but not limited to noise. Accordingly, in one embodiment of a
dual-electrode sensor system configured for fluid communication
with a host's circulatory system (e.g., via a vascular access
device) the system comprising electronics operably connected to the
first and second working electrodes; the electronics are configured
to process the first and second signals to generate analyte
concentration data substantially without signal contribution due to
noise.
[0328] In certain embodiments, the sensor electronics (e.g.,
electronic components) are operably connected to the first and
second working electrodes. The electronics are configured to
calculate at least one analyte sensor data point. For example, the
electronics can include a potentiostat, A/D converter, RAM, ROM,
transmitter, and the like. In some embodiments, the potentiostat
converts the raw data (e.g., raw counts) collected from the sensor
to a value familiar to the host and/or medical personnel. For
example, the raw counts from a glucose sensor can be converted to
milligrams of glucose per deciliter of glucose (e.g., mg/dL). In
some embodiments, the electronics are operably connected to the
first and second working electrodes and are configured to process
the first and second signals to generate a glucose concentration
substantially without signal contribution due to non-glucose noise
artifacts. The sensor electronics determine the signals from
glucose and non-glucose related signal with an overlapping
measuring potential (e.g., from a first working electrode) and then
non-glucose related signal with an overlapping measuring potential
(e.g., from a second electrode). The sensor electronics then use
these data to determine a substantially glucose-only concentration,
such as but not limited to subtracting the second electrode's
signal from the first electrode's signal, to give a signal (e.g.,
data) representative of substantially glucose-only concentration,
for example. In general, the sensor electronics may perform
additional operations, such as but not limited to data smoothing
and noise analysis.
[0329] In certain embodiments, the dual-electrode sensor includes
electronics (e.g., a processor module, processing memory) that are
operably connected to the first and second working electrodes and
are configured to provide the first and second signals to generate
an analyte concentration data substantially without signal
contribution due to non-analyte-related noise. For example, the
sensor electronics process and/or analyze the signals from the
first and second working electrodes and calculate the portion of
the first electrode signal that is due to analyte concentration
only. The portion of the first electrode signal that is not due to
the analyte concentration can be considered to be background, such
as but not limited to noise. Accordingly, in one embodiment of a
dual-electrode sensor system configured for fluid communication
with a host's circulatory system (e.g., via a vascular access
device) the system comprising electronics operably connected to the
first and second working electrodes; the electronics are configured
to process the first and second signals to generate analyte
concentration data substantially without signal contribution due to
noise.
[0330] In some embodiments, the dual-electrode analyte sensor
includes a reference sensor/system, as described elsewhere therein,
whereby reference data can be provided for calibration (e.g.,
internal to the system), without the use of an external (e.g.,
separate from the system) analyte-measuring device. In an exemplary
embodiment, the dual-electrode sensor is a glucose sensor and
external glucose data points (e.g., from a hand-held glucose meter
or a YSI device) are not required for calibration of a
dual-electrode glucose sensor system that includes a reference
sensor. In some embodiments, the reference sensor is configured to
be disposed within the same local environment as the dual-electrode
analyte sensor, such that the reference sensor and the
dual-electrode analyte sensor can be simultaneously exposed to a
sample. In some embodiments, the reference sensor/system can be
disposed remotely from the dual-electrode sensor. In these
embodiments, the electronics module is configured to process the
reference data with the first and second signals to generate
analyte concentration data substantially without signal
contribution due to noise. In some embodiments, the electronics
module is configured to calibrate the dual-electrode analyte sensor
data using the reference sensor data, as described elsewhere
herein.
[0331] In some embodiments, the electronics module is configured to
determine a scaling factor (k) as described in the section entitled
"Calibration Systems and Methods." Briefly, a scaling factor
defines a relationship between the enzymatic portion of the
membrane and the non-enzymatic portion of the membrane.
Accordingly, in some embodiments, the electronics module, also
referred to as the processor module herein, is configured to
calibrate the analyte sensor data using the scaling factor, such
that the calibrated sensor data does not include inaccuracies that
can arise due to small differences between the plus- and
minus-enzyme portions of the membrane at the first and second
working electrodes, respectively.
[0332] In some embodiments, the system is configured to calibrate
the continuous dual-electrode analyte sensor using a reference
fluid (e.g., 602a), as described in the section entitled
"integrated sensor system." In some embodiments, the system is
configured to calibrate the sensor using single-point calibration,
in other embodiments, the system is configured to calibrate the
sensor without a reference data point provided by an external
analyte monitor (e.g., SMBG, YSI), as described elsewhere herein.
In some embodiments, the system includes a reference sensor
configured to generate a signal associated with a reference analyte
in the sample (e.g., internal to the system), wherein the
continuous analyte sensor is further configured to generate a third
signal associated with the reference analyte, and wherein the
system is configured to calibrate the continuous analyte sensor
using the reference signal and the third signal. In some
embodiments, the reference sensor comprises an optical sensing
apparatus, such as but not limited to an optical O.sub.2 sensor. In
some embodiments, the continuous analyte sensor is a glucose
sensor. In other embodiments, a substantial portion of the
continuous analyte sensor has a diameter of less than about 0.008
inches, as is described elsewhere herein.
[0333] In some further embodiments, the continuous analyte sensor
further comprises a bioinert material or a bioactive agent
incorporated therein or thereon. Applicable bioactive agent include
but are not limited to vitamin K antagonists, heparin group
anticoagulants, platelet aggregation inhibitors, enzymes, direct
thrombin inhibitors, Dabigatran, Defibrotide, Dermatan sulfate,
Fondaparinux, and Rivaroxaban.
[0334] As a non-limiting example, in some embodiments, a method for
continuously detecting an analyte in the host in vivo using a
dual-electrode analyte sensor is provided. In some embodiments, a
vascular access device (e.g., a catheter) is inserted into the
host's circulatory system, such as into a vein or artery. The
sensor is contacted with a sample of the circulatory system, such
as a sample of blood withdrawn into the catheter. A first signal is
generated by the sensor, wherein the first signal is associated
with associated with the analyte and non-analyte related
electroactive compounds having a first oxidation/reduction
potential in a sample of the circulatory system of the host. In
certain embodiments, the analyte sensor is configured to detect
glucose. A second signal is also generated, wherein the second
signal is associated with noise of the analyte sensor, wherein the
noise comprises signal contribution due to non-analyte related
electroactive species with an oxidation/reduction potential that
substantially overlaps with the first oxidation/reduction potential
in the sample. The first and second signals are processed to
provide a processed signal substantially without a signal component
associated with noise. In some embodiments, the first and second
signals are processed to provide a scaling factor, which can then
be used to calibrate the first signal. In some embodiments, a
reference sensor is also contacted with the sample, and a third
signal associated with a reference analyte generated. In some
embodiments, the reference sensor is an optical detection
apparatus, such as but not limited to an optical O.sub.2 sensor. In
this embodiment, the first and second signals can be calibrated
using the third and/or reference signal. In some embodiments, the
processing step comprises evaluating steady-state information and
transient information, wherein the first and second signals each
comprise steady-state and transient information. In some further
embodiments, the evaluating step includes evaluating at least one
of sensitivity information and baseline information, wherein the
steady-state information comprises the sensitivity and baseline
information.
Example
[0335] Test sensors were designed and fabricated with selection of
certain materials, processing, and structure that improve fatigue
life, and yet also improve comfort for the user. During the
fabrication process, an elongated conductive body is first built by
inserting/slip fitting a rod or wire into a tube formed of a
conductive material, the combination of which forms an initial
structure of an elongated conductive body. The resulting elongated
conductive body is then passed through a series of dies to draw
down the diameter of the elongated conductive body from a large
diameter to a small diameter. With each pass through the die, the
cross-sectional profile of the elongated conductive body is
compressed, and the diameter associated therewith is reduced.
Between passes through the die, an annealing step is performed to
cause changes in the mechanical and structural properties of the
elongated conductive body, and to relieve internal stresses, refine
the structure by making it more homogeneous, and improve general
cold working properties. It has been found that drawing down the
diameter of the elongated conductive body through large numbers of
dies in small incremental steps, instead of through one or a few
number of large incremental step(s), can result in certain
mechanical and structural properties that improve fatigue life. It
has also been found that performing the annealing on the elongated
conductive body in between drawing passes also improves fatigue
life and provided additional comfort for the user. Afterwards, a
polyurethane and a silver/silver chloride layer are coated onto the
elongated conductive body, followed by deposition of a membrane
(with an electrode, enzyme, and resistance layer).
[0336] The fabricated test sensors were then tested with a fatigue
measurement device 1410 to determine test sensor fatigue lives
under conditions that better models subcutaneous conditions than
other conventional models. As illustrated in FIG. 14, the fatigue
life measurement device 1410 includes clamp members 1420, holding
members 1440, and fixtures 1430 used to hold and support the test
sensor 1450. The fatigue life measurement device 1410 also includes
a rotator 1460 that rotates about 45 degrees in each direction for
a total of about 90 degrees per cycle. The radius of the holding
members 1440 are about 0.031 inches. During testing, the rotator
1460 rotates back and forth at a constant rate, thereby causing the
sensor 1450 to bend back and forth at a degree corresponding to the
radius of the holding members 1440.
[0337] FIG. 15 illustrates a table summarizing the results of the
performance of test sensors with conventional sensors, with respect
to fatigue life. As shown, the test sensors using the fabrication
techniques described above were able to achieve substantially
longer fatigue lives than conventional sensors. Indeed the average
fatigue life of the test sensors was about 61 cycles, as compared
to the average fatigue life of the conventional sensors which was
about 13.87 cycles. Accordingly, the fabrication methods described
herein were able to extend sensor fatigue life by a factor over
four. Indeed, sensors built in accordance with the embodiments
described herein can achieve a fatigue life of greater than 20
cycles, greater than 40 cycles, greater than 60 cycles, or greater
than 65 cycles.
Electrode Regeneration
[0338] Devices, systems, and methods for increasing reference
electrode capacity of an analyte sensor are provided. A typical
analyte sensor can include a working electrode and a reference
electrode, and optionally one or more additional working
electrodes. Silver/silver chloride reference electrodes have been
used in continuous analyte sensors, such as transcutaneous and
subcutaneous electrochemical glucose sensors. However, due to
significant depletion of silver chloride during typical periods of
usage, e.g., days, weeks, or more, this reduction in electrode
capacity can substantially limit the effectiveness of the reference
electrode over time. During use, the silver-chloride component of
the reference electrode is reduced to silver and chloride according
to the following reaction:
AgCl+e-.fwdarw.Ag+Cl-
[0339] When the silver chloride becomes completely or substantially
depleted, the reference electrode potential loses stability, such
that the sensor response to the analyte becomes non-linear. For
proper functioning of the sensor, however, the reference electrode
potential should be substantially stable for the duration of the
sensor life at (or above) the plateau potential for the analyte. An
advantage of certain systems and methods of various embodiments is
the ability to increase the reference capacity of the reference
electrode by preventing premature depletion of silver chloride,
such that the useful life of the sensor is extended.
[0340] It has been found that in certain embodiments, the reference
capacity of the reference electrode can be increased, if at least a
portion of the reference electrode is covered with an enzyme layer.
The enzyme selected for the enzyme layer can be specific to a
particular analyte, or to various other component(s) of the in vivo
environment. For example, the enzyme layer can advantageously be
provided to interact with the analyte to be measured. When the
analyte being measured is glucose, a layer containing glucose
oxidase can advantageously be provided. Glucose oxidase catalyzes
the conversion of oxygen and glucose to hydrogen peroxide and
gluconate according to the following reaction:
Glucose+O.sub.2.fwdarw.Gluconate+H.sub.2O.sub.2
[0341] Hydrogen peroxide produced from the catalyzed conversion of
oxygen and glucose can diffuse to the surface of the reference
electrode. The hydrogen peroxide can then react with silver
produced from the depletion of silver chloride in order to
regenerate silver ion at the reference electrode surface, with the
silver ion associating with in situ Cl- ion.
H.sub.2O.sub.2+2Ag+2Cl.sup.-.fwdarw.2AgCl+H.sub.2O+1/2O.sub.2+2e.sup.-
[0342] By regenerating AgCl at the reference electrode, the
reference capacity of the reference electrode can be increased,
thereby effectively increasing the useful life of the sensor.
Advantageously, the amount of enzyme coated over a portion of the
reference electrode can be carefully selected and controlled in
order to provide for a predicted/controlled reference capacity. The
predicted/controlled reference capacity can be employed to impart a
longer sensor life than is observed for conventional sensors, or
can be employed to impart a preselected (limited) duration of use
of the sensor in vivo.
[0343] Working Electrode
[0344] In the case of certain glucose-oxidase-based glucose
sensors, the species being measured at the working electrode is
H.sub.2O.sub.2. Glucose oxidase catalyzes the conversion of oxygen
and glucose to hydrogen peroxide and gluconate. The change in
H.sub.2O.sub.2 levels can be monitored to determine glucose
concentration because for each glucose molecule metabolized, there
is a proportional change in the product H.sub.2O.sub.2. Oxidation
of H.sub.2O.sub.2 by the working electrode is balanced by reduction
of ambient oxygen, enzyme generated H.sub.2O.sub.2, or other
reducible species at the silver/silver chloride electrode. The
H.sub.2O.sub.2 produced from the glucose oxidase reaction further
reacts at the surface of working electrode and produces two protons
(2H.sup.+), two electrons (2e.sup.-), and one oxygen molecule
(O.sub.2) (See, e.g., Fraser, D. M. "An Introduction to In vivo
Biosensing: Progress and problems." In "Biosensors and the Body,"
D. M. Fraser, ed., 1997, pp. 1-56 John Wiley and Sons, New
York).
[0345] A potentiostat can be used to measure the electrochemical
reaction(s) at the electrode(s). The potentiostat applies a
constant potential between the working and reference electrodes to
produce a current value. The current that is produced at the
working electrode is proportional to the diffusional flux of
H.sub.2O.sub.2. Accordingly, a raw signal may be produced that is
representative of the concentration of glucose in the user's body,
and therefore may be utilized to estimate a meaningful glucose
value, such as described elsewhere herein. As discussed elsewhere
herein, in some embodiments, the working and reference electrode
may be configured to substantially prevent any hydrogen peroxide
generated at the silver/silver chloride reference electrode from
migrating to the working electrode where it is oxidized.
[0346] Reference Electrode
[0347] In some embodiments, the reference electrode comprises a
silver-containing material. The silver-containing material may
include any of a variety of materials and be in various forms, such
as, silver/silver chloride polymer pastes, paints, polymer-based
conducting mixture, and/or inks that are commercially available,
for example. The third layer can be processed using a
pasting/dipping/coating step, for example, using a die-metered dip
coating process. In one exemplary embodiment, a silver/silver
chloride polymer paste is applied to an elongated body by
dip-coating the body (for example, using a meniscus coating
technique) and then drawing the body through a die to meter the
coating to a precise thickness. Multiple coating steps can be used
to build up the coating to a predetermined thickness. Such a
drawing method can be utilized for forming one or more of the
electrodes in the device depicted in FIG. 1B.
[0348] In some embodiments, silver/silver chloride particles are
mixed into a polymer, such as polyurethane, polyimide, or the like,
to form the silver-containing material for the reference electrode.
In some embodiments, the material used to form the reference
electrode is cured, for example, by using an oven or other curing
process. A covering of fluid-permeable polymer with conductive
particles (such as, for example, carbon particles) therein can then
be applied over the reference electrode.
[0349] One challenge presented by employing silver/silver chloride
reference electrodes, such as described elsewhere herein, with
glucose sensors is that as current flows through the reference
electrode, the reference capacity decreases as the silver chloride
component of the reference electrode is reduced to silver and
chloride according to the following reaction:
AgCl+e.sup.-.fwdarw.Ag+Cl.sup.-
[0350] Over time, as the silver chloride becomes completely
depleted, the reference electrode potential will shift and the
sensor response to glucose becomes less linear. For proper
functioning of the sensor, however, the reference electrode
potential should be stable throughout the duration of the targeted
sensor life. Sensor life can therefore be limited as the silver
chloride depletes and the reference potential shifts away from the
plateau potential for the analyte.
[0351] In order to extend the useful life of the sensor, a
reference electrode can be configured to have an increased or
prolonged reference capacity. For example, with a silver/silver
chloride reference electrode, the reference electrode can be
configured such that the silver chloride withstands depletion or
otherwise becomes depleted more slowly (that is, over a greater
period of time). Thus, by slowing the overall rate of depletion of
silver chloride from the reference electrode (for example, by
providing a mechanism for regenerating silver chloride), the
reference capacity can remain stable (that is, linear) for a longer
period of time. Effectively, an increase in time of the period of
stability of the reference electrode can increase the useful life
of the sensor.
[0352] Advantageously, covering at least a portion of the
silver/silver chloride reference electrode with an enzyme layer can
increase a reference capacity of a silver/silver chloride reference
electrode. For example, in a silver/silver chloride reference
electrode covered at least in part with an enzyme that comprises
glucose oxidase, after the glucose oxidase catalyzes the reaction
of glucose and oxygen to generate gluconate and hydrogen peroxide,
the hydrogen peroxide can diffuse to the reference electrode
surface where it oxidizes silver to silver ion to regenerate silver
chloride at the reference electrode surface.
[0353] As illustrated in FIGS. 17A-17D, the advantages of covering
at least a portion of a reference electrode with an enzyme layer
can be seen as compared to a reference electrode without an enzyme
layer. With reference to FIGS. 17A-17D, three sets of sensors were
built with substantially identical specifications, with the only
difference being the surface area of the enzyme layer covering the
reference electrode. The sensors were of a design corresponding to
that illustrated in FIG. 1C. With this design, the third layer 114
of the sensor comprised a conductive material in the form of a
silver/silver chloride material that formed the reference
electrode. As illustrated, the third layer 114 was applied onto the
second layer 104, which was an insulator. After the sensors were
built, a membrane 108 was deposited onto the reference electrode
114 of two sets of sensors The membrane 108 included an enzyme
layer containing glucose oxidase, which catalyzes a reaction of
glucose and oxygen to produce gluconate and hydrogen peroxide, as
described elsewhere herein. For a first set of four test sensors,
no membrane (and thus no enzyme layer) was applied onto the
reference electrodes. After all post-processing steps were
completed, the sensors were placed and then remained over time in a
solution having a glucose concentration of about 500 mg/dL. At this
concentration, the signal current continuously generated by the
sensor was measured to be about 20 nA. The potential of the
reference electrodes of these four test sensors were then measured
over time. Because of a break-in time of about two hours necessary
to obtain reference potential stability, measurements of reference
potential over time were compared to the reference potential at the
two hour mark. As illustrated in FIG. 17A, which plots the change
in reference electrode potential (compared to the potential at the
two hour mark) of the four above-described test sensors as a
function of time, the reference potential of these
no-enzyme-covering reference electrodes dropped fairly quickly,
beginning on the first day.
[0354] With reference to FIG. 17B, a second set of four test
sensors were built having substantially identical specifications as
the tests sensors corresponding to FIG. 17A, except that the
reference electrodes of these four test sensors were covered with
an enzyme layer. The enzyme layer coverage (i.e., in terms of
surface area) of the reference electrodes for these four test
sensors was about 0.00225 in.sup.2. As illustrated in FIG. 17B, the
reference potential of these enzyme-covering reference electrodes
did not noticeably drop until about the eighth day after the sensor
was placed in the solution.
[0355] With reference to FIG. 17C, a third set of four test sensors
were built having substantially identical specifications as the
tests sensors corresponding to FIG. 17B, except that the reference
electrodes of these four test sensors were covered with an enzyme
layer with a surface area of about 0.00633 in.sup.2. As illustrated
in FIG. 17C, the reference potential of these enzyme-covering
reference electrodes did not noticeably drop until about the 17th
day after the sensor was placed in the solution.
[0356] With reference to FIG. 17D, the reference potentials of each
of the above-described sensor set were then averaged (i.e.
arithmetically averaging of four sensors in each set), and then
plotted as a function of time. As illustrated, with the reference
electrodes covered at least in part with 0.00633 in.sup.2 of enzyme
layer, the reference potential remained substantially constant for
a longer period of time. In fact, the reference potential of these
reference electrodes did not drop until at around 16 days. With
reference electrodes covered at least in part with 0.00225 in.sup.2
of enzyme layer, the reference potential remained substantially
constant for a shorter period of time. The reference potential of
these reference electrodes did not drop until at around 8 days. In
comparison, with reference electrodes that included no enzyme layer
coverage, the reference potential dropped much more quickly. As
illustrated, the reference potential of these reference electrodes
began to drop at around the first day. Accordingly, by covering at
least a portion of a reference electrode with a hydrogen-peroxide
generating enzyme layer, the reference capacity of the reference
electrode can be greatly increased.
[0357] In certain embodiments, the enzyme layer is configured to
cover at least a portion of a length of a reference electrode. As
illustrated in FIG. 17D, generally, the larger the area of the
reference electrode that is covered with an enzyme layer (that is,
the longer the enzyme layer covering the reference electrode while
holding all other enzyme-layer dimensions (e.g., width or depth)
constant), the more the reference capacity of the reference
electrode can be increased.
[0358] In order to increase the useful life of the sensor, the
length of the enzyme layer (and by extension the area of the enzyme
layer) can be increased over the length of the covering used in the
standard sensor configuration. For example, while holding all other
enzyme-layer dimensions constant, the length of the enzyme layer
can be more than doubled as compared to the length of the enzyme
layer on the standard sensor, from a length of about 0.101 inches
to a length of about 0.284 inches, resulting in a coverage of about
60-100% of the electroactive surface area of the reference
electrode. As a result, the reference potential remains
substantially constant for a substantially longer period of
time.
[0359] Accordingly, the percent coverage of the electroactive
surface area of the reference electrode by the enzyme layer can be
up to about 10%, about 10% or more, about 20% or more, about 30%,
about 40% or more, about 50% or more, about 60% or more, about 70%
or more, about 80% or more, about 90% or more, or about 100%.
Additional coverage, e.g., of regions proximal to the reference
electrode, can also be provided, and can also generate hydrogen
peroxide that can migrate to the surface of the reference
electrode. In some embodiments, e.g., wherein a dip coating method
is employed to deposit the enzyme layer, the length of the enzyme
layer can be about two times, about three times, about four times,
about five times, about six times, about seven times, about eight
times, about nine times, about ten times, or more than about ten
times the length of the enzyme layer employed in a working or other
sensing electrode as described herein, e.g., up to or exceeding the
length of the in vivo portion of the analyte sensor.
[0360] The thickness of the enzyme layer, e.g., on the
silver/silver chloride of the reference electrode, can, independent
of any other enzyme layer, be from about 0.01, about 0.05, about
0.6, about 0.7, or about 0.8 microns to about 1, about 1.2, about
1.4, about 1.5, about 1.6, about 1.8, about 2, about 2.1, about
2.2, about 2.5, about 3, about 4, about 5, about 10, about 20,
about 30 about 40, about 50, about 60, about 70, about 80, about
90, or about 100 microns. Preferably, the thickness of the enzyme
layer can be from about 0.05, about 0.1, about 0.15, about 0.2,
about 0.25, about 0.3, about 0.35, about 0.4, about 0.45, about
0.5, about 1, about 1.5, about 2, about 2.5, about 3, about 4, or
about 5 microns to about 6, about 7, about 8, about 9, about 10,
about 11, about 12, about 13, about 14, about 15, about 16, about
17, about 18, about 19, about 19.5, about 20, about 25, or about 30
microns or more. In some embodiments, the thickness of the enzyme
layer can be from about 2, about 2.5, or about 3 microns to about
3.5, about 4, about 4.5, or about 5 microns in the case of a
transcutaneously implanted sensor or from about 6, about 7, or
about 8 microns to about 9, about 10, about 11, or about 12 microns
in the case of a wholly implanted sensor. While uniform coverage
may be preferred in some embodiments, in other embodiments portions
of the surface can remain uncovered, or the thickness of the enzyme
layer can be irregular.
[0361] In some embodiments, the enzyme layer is configured to bring
about a rate of silver chloride regeneration that substantially
correlates to the rate of silver chloride depletion during a time
period over different levels of glucose concentration. In certain
embodiments that use hydrogen peroxide to regenerate the reference
electrode, covering at least a portion of a reference electrode
with an enzyme layer that can react with a hydrogen
peroxide-generating analyte (e.g., glucose) having a fluctuating
concentration over any period of time, can result in silver
chloride being efficiently regenerated at the surface of the
reference electrode at a rate that correlates with the rate of
depletion of silver chloride. That is, at high analyte
concentrations (e.g., glucose concentrations), the rate of silver
chloride depletion increases as more hydrogen peroxide is produced,
thereby producing more electrons as the hydrogen peroxide reacts at
the electroactive surface of working electrode. In turn, these
electrons decrease the silver chloride component of the reference
electrode in accordance with the reaction:
AgCl+e.sup.-.fwdarw.Ag+Cl.sup.-
Simultaneously, with the use of an enzyme-layer-containing membrane
that covers the reference electrode, at these same high analyte
concentrations, the hydrogen peroxide regenerates silver chloride
at an increased rate, because of the higher concentration of
hydrogen peroxide. Accordingly, a sense of balance or equilibrium
is created in which as the rate of silver chloride depletion
increases (e.g., at high analyte concentrations) at the Ag/AgCl
reference electrode, the rate of silver chloride regeneration
simultaneously also increases. As long as hydrogen peroxide is
continuously generated, silver is continuously regenerated to
silver ion (and thus to silver chloride, the chloride ion being a
naturally occurring constituent of the in vivo environment). In
these embodiments, the reference electrode is continuously
regenerated, with the rate of silver chloride regeneration
positively correlating with the rate of silver chloride
depletion.
[0362] As discussed elsewhere herein, the enzyme layer can be
configured in vivo to generate hydrogen peroxide upon exposure to a
particular analyte. For example, the enzyme layer can include
glucose oxidase, which can catalyze a reaction between glucose and
oxygen to generate gluconate and hydrogen peroxide. Alternative or
additional oxidases can be included in the enzyme layer, including
any one or more of galactose oxidase, cholesterol oxidase, amino
acid oxidase, alcohol oxidase, lactate oxidase, or uricase, for
example. Moreover, alternative or additional analytes other than
glucose (e.g., urate, ascorbate, citrate, L-lactate, succinate,
D-glucose, ethanol, etc.) can react with the enzyme layer to
generate hydrogen peroxide.
[0363] In certain embodiments, the sensor membrane includes an
enzyme layer configured to react with a non-measured species (i.e.,
one that is not being measured, such as, for example, uric acid)
that has a concentration that is not subject to substantial
fluctuation over time, so as to ensure uninterrupted generation of
a reference electrode regenerating species. For example, a species
present in a host can be selected that does not substantially
fluctuate in concentration throughout the morning, the afternoon,
the evening, and/or the night, or does not substantially fluctuate
over about a 6-hour period, over about a 12-hour period, over about
an 18-hour period, over about a 24-hour period, over about a
36-hour period, over about a 48-hour period, or over about a
72-hour period or more. In certain embodiments, the concentration
of the substantially constant species fluctuates less than about
100%, less than about 90%, less than about 80%, less than about
70%, less than about 60%, less than about 50%, less than about 40%,
less than about 30%, less than about 20%, less than about 10%, or
less than about 5% from a maximum concentration to a minimum
concentration and vice versa throughout a given period of time.
Preferably, an enzyme layer is selected to react with a particular
species having a substantially constant concentration throughout a
particular period of time, for example, the analyte concentration
fluctuates less than about 25%, less than about 20%, less than
about 15%, less than about 10%, less than about 5%, or less than
about 1%. In some embodiments, the enzyme layer comprises at least
two different enzymes, with a first enzyme (e.g., glucose oxidase)
that catalyzes a reaction involving an analyte (e.g., glucose)
being a reactant and hydrogen peroxide being the product. The
second enzyme (e.g., uricase) catalyzes a reaction involving a
non-measured species (e.g., uric acid), which is not subject to
substantial fluctuation over time, being the reactant and hydrogen
peroxide being the product. In these embodiments, the rate of
silver chloride regeneration may be more stable than one that
entirely relies on analyte concentration. In further embodiments,
the enzyme layer may comprise a third enzyme, in which the third
enzyme catalyzes a reaction involving another different
non-measured species, which is not subject to substantial
fluctuation over time, being the reactant and hydrogen peroxide
being the product. It is contemplated that such an embodiment may
further stabilize the rate of silver chloride regeneration, and
make the rate less dependent on glucose concentration. While in
some embodiments, the reference electrode is substantially and
continuously regenerated, in other embodiments, the reference
electrode may be periodically or intermittently regenerated.
[0364] Similarly, the concentration of enzyme in the enzyme layer
can be any amount suitable for generating a desired amount of
hydrogen peroxide, e.g., sufficient to regenerate silver chloride,
but not excess amounts that may interfere with accurate
measurements at the working electrode. Advantageously, the amount
of enzyme (for example, determined by the length and thickness of
the layer and/or the concentration of enzyme in the layer) covering
the silver/silver chloride reference electrode, can be controlled
to provide for a sensor having a useful life of a known duration.
For example, the increase in sensor useful life as a function of
various amounts of enzyme layer covering on the silver/silver
chloride reference electrode can be studied to determine the
corresponding relationship. Thus, when a known quantity of enzyme
layer is applied to (for example, covers, coats) at least a portion
of the silver/silver reference electrode, a desired useful life of
the sensor, as limited by the reference capacity of the reference
electrode, can be selected. Accordingly, sensor electronics can be
programmed with a failure mode that can instruct a user to change
the sensor after a pre-determined amount of time has passed,
corresponding to the predicted useful life of the sensor as based
on the amount of enzyme applied to the reference electrode.
[0365] In some embodiments, the material used to form the reference
electrode may be designed or selected to affect and/or control the
rate of reference electrode regeneration and/or the reference
electrode capacity. For example, the concentration of silver and
silver chloride in the silver/silver chloride material (e.g.,
paste) can be increased in certain embodiments to increase the
reference electrode capacity and/or the rate of reference electrode
regeneration. In some embodiments, the silver/silver chloride
component can form from about 10% to about 65% by weight of the
total material that forms the reference electrode (i.e., including
the carrier, such as polyurethane), or from about 20% to about 50%,
or from about 23% to about 37%.
[0366] In addition, silver and silver chloride grains having
certain particle sizes can be selectively chosen to affect and/or
control the rate of reference electrode regeneration. In certain
embodiments, the silver grain in the silver/silver chloride
solution or paste has an average particle size corresponding to a
maximum particle dimension that is less than about 100 microns, or
less than about 50 microns, or less than about 30 microns, or less
than about 20 microns, or less than about 10 microns, or less than
about 5 microns. The silver chloride grain in the silver/silver
chloride solution or paste can have an average particle size
corresponding to a maximum particle dimension that is less than
about 100 microns, or less than about 80 microns, or less than
about 60 microns, or less than about 50 microns, or less than about
20 microns, or less than about 10 microns. The silver grain and the
silver chloride grain may be incorporated at a ratio of the silver
chloride grain:silver grain of from about 0.01:1 to 2:1 by weight,
or from about 0.1:1 to 1:1. The silver grains and the silver
chloride grains can then be mixed with a carrier (for example, a
polyurethane) to form a solution or paste. The silver/silver
chloride solution or paste can have a viscosity (under ambient
conditions) that can be from about 1 to about 500 centipoise, or
from about 10 to about 300 centipoise, of from about 50 to about
150 centipoise.
[0367] In some embodiments, the surface areas of the silver and
silver chloride grains in the conductive material used to form the
reference electrode may be selected to affect and/or control the
rate of reference electrode regeneration and/or the reference
electrode capacity. Typically, the more finely divided a reactant
particle is, the faster a reaction occurs. A finely powdered solid
with numerous particles will generally produce a faster reaction
than if the same mass is present as a single large solid, all else
being equal. The powdered solid has a greater surface area than the
single solid. Accordingly, as the surface area of the silver and
silver chloride grains in the silver/silver chloride conductive
material is increased, the rate of silver chloride regeneration in
the reference electrode may also be increased, all else being
equal. In some embodiments, the average surface area per weight (or
unit volume) of the silver grains in the reference electrode prior
to use is from about 0.5 m.sup.2/gm to about 5 m.sup.2/gm, from
about 0.7 m.sup.2/gm to about 3.5 m.sup.2/gm, from 0.75 m.sup.2/gm
to about 2.5 m.sup.2/gm, or from about 1 m.sup.2/gm to about 1.5
m.sup.2/gm. In certain embodiments, the average surface area per
unit volume of the silver chloride grains in the reference
electrode prior to use is from about 0.1 m.sup.2/gm to about 0.9
m.sup.2/gm, from about 0.2 m.sup.2/gm to about 0.7 m.sup.2/gm, from
about 0.3 m.sup.2/gm to about 0.6 m.sup.2/gm, or from about 0.4
m.sup.2/gm to about 0.6 m.sup.2/gm.
[0368] Exemplary Analyte Sensors, Electrode Configurations, and
Methods of Manufacturing Same
[0369] FIG. 2C is an expanded view of an exemplary embodiment of a
continuous analyte sensor, also referred to as an analyte sensor,
illustrating the sensing mechanism. In some embodiments, the
sensing mechanism is adapted for insertion under the host's skin,
and the remaining body of the sensor (including, for example,
electronics) can reside ex vivo. In the illustrated embodiment, the
analyte sensor includes two electrodes, such as, for example, a
working electrode 112 and at least one additional electrode 114.
The additional electrode may be a silver/silver chloride reference
electrode; however it is contemplated that other types of reference
electrodes can be employed.
[0370] It is contemplated that the electrode(s) can be formed to
have any of a variety of cross-sectional shapes. For example, in
some embodiments, the electrode may be formed to have a circular or
substantially circular cross-sectional shape, for example, as
illustrated in FIGS. 2A-2C. Alternatively, the electrode may be
formed to have a cross-sectional shape that resembles an ellipse, a
polygon (such as, for example, triangle, square, rectangle,
parallelogram, trapezoid, pentagon, hexagon, or octagon), or the
like. The cross-sectional shape of the electrode can be
symmetrical. Alternatively, the cross-sectional shape can be
asymmetrical. In some embodiments, each electrode can be formed
from a fine wire with a diameter of from about 0.001 inches or less
to about 0.05 inches or more. Each electrode can be formed from,
for example, a plated insulator, a plated wire, or bulk
electrically conductive material. In some embodiments, the wire
used to form a working electrode may be about 0.002 inches, about
0.003 inches, about 0.004 inches, about 0.005 inches, about 0.006
inches, about 0.007 inches, about 0.008 inches, about 0.009 inches,
about 0.01 inches, about 0.015 inches, about 0.02 inches, about
0.025 inches, about 0.03 inches, about 0.035 inches, about 0.04
inches, or about 0.045 inches in diameter. In some embodiments, the
working electrode can comprise a wire formed from a conductive
material, such as platinum, platinum-black, platinum-iridium,
palladium, graphite, gold, carbon, conductive polymer, alloys, or
the like. Any alternate sensor configurations can be employed with
the analyte sensor system as described herein.
[0371] The working electrode 112 can be configured to measure the
concentration of an analyte, such as, but not limited, to glucose,
uric acid, cholesterol, lactate, and the like. In an enzymatic
electrochemical sensor for detecting glucose, for example, the
working electrode can measure the hydrogen peroxide produced by an
enzyme-catalyzed reaction of the analyte being detected, which can
create a measurable electric current. For example, in the detection
of glucose, glucose oxidase (GOx) produces, inter alia, hydrogen
peroxide (H.sub.2O.sub.2) as a byproduct. The H.sub.2O.sub.2 can
react with the surface of the working electrode to produce two
protons (2H.sup.+), two electrons (2e.sup.-) and one molecule of
oxygen (O.sub.2), which produces the electric current being
detected.
[0372] Moreover, an insulator can be provided to electrically
insulate the working and reference electrodes. For example, the
working electrode 112 can be covered with an insulating material,
such as a non-conductive polymer. Dip-coating, spray-coating,
vapor-deposition, or other coating or deposition techniques can be
used to deposit the insulating material on the working electrode.
In some embodiments, the insulating material comprises parylene.
Parylene can be an advantageous polymer coating because of its
strength, lubricity, and electrical insulation properties.
Generally, parylene is produced by vapor deposition and
polymerization of para-xylylene (or its substituted derivatives).
Any suitable insulating material can be used, including, for
example, fluorinated polymers, polyethyleneterephthalate,
polyurethane, polyimide, other nonconducting polymers, or the like.
Glass or ceramic materials can also be employed. Other materials
suitable for use include surface energy modified coating systems
such as those marketed under the trade names AMC18, AMC148, AMC141,
and AMC321 by Advanced Materials Components Express of Bellafonte,
Pa. In some embodiments, the working electrode may not require a
coating of insulator.
[0373] In some embodiments, the additional electrode 114 is
configured to function as a reference electrode alone. When
employed as a reference electrode, the additional electrode 114 can
be a silver/silver chloride electrode. In some embodiments, the
electrodes are juxtapositioned or twisted with or around each
other. Other configurations can also be used, as described
elsewhere herein. In some embodiments, the reference electrode 114
is helically wound around the working electrode 112. The assembly
of wires can then be optionally coated together with an insulating
material, similar to that described above, in order to provide an
insulating attachment (for example, securing together of the
working and reference electrodes).
[0374] In some embodiments, a radial window is formed through the
insulating material to expose a circumferential electroactive
surface of the working electrode 112. Additionally, sections of
electroactive surface of the reference electrode can be exposed.
For example, the sections of electroactive surface can be masked
during deposition of an outer insulating layer or etched after
deposition of an outer insulating layer. In some applications,
cellular attack or migration of cells to the sensor can cause
reduced sensitivity or function of the device, particularly after
the first day of implantation. When the exposed electroactive
surface is distributed circumferentially about the sensor (for
example, as in a radial window), the available surface area for
reaction can be sufficiently distributed so as to minimize the
effect of local cellular invasion of the sensor on the sensor
signal. Alternatively, a tangential exposed electroactive window
can be formed, for example, by stripping only one side of the
coated assembly structure. In other alternative embodiments, the
window can be provided at the tip of the coated assembly structure
such that the electroactive surfaces are exposed at the tip of the
sensor. Other methods and configurations for exposing electroactive
surfaces can also be employed.
[0375] In some embodiments, wherein the sensor comprises two
working electrodes, the two working electrodes can be
juxtapositioned, around which the reference electrode can be
disposed (for example, helically wound). In some embodiments with
two or more working electrodes, the working electrodes can be
formed in a double-, triple-, quad-, etc. helix configuration along
the length of the sensor (for example, surrounding a reference
electrode, insulated rod, or other support structure). The
resulting electrode system can be configured with an appropriate
membrane system, wherein the first working electrode can be
configured to measure a first signal comprising glucose and
baseline signals, and the additional working electrode can be
configured to measure a baseline signal consisting of the baseline
signal only. In these embodiments, the second working electrode can
be configured to be substantially similar to the first working
electrode, but without an enzyme disposed thereon. In this way, the
baseline signal can be determined and subtracted from the first
signal to generate a difference signal, that is, a glucose-only
signal that is substantially not subject to fluctuations in the
baseline or interfering species on the signal, such as described in
U.S. Pat. Nos. 7,715,893; 7,460,898; 7,761,130; and U.S. Patent
Publication No. 2008-0083617-A1, each of which are incorporated
herein by reference in their entirety.
[0376] In some embodiments, the sensor is configured and arranged
for implantation in a host and for generating in vivo a signal
associated with an analyte in a sample of the host during a sensor
session. For example, the sensor can be configured to generate in
vivo a signal associated with a glucose concentration of the host
during a sensor session. The length of time of the sensor session
can be from less than about 10 minutes, to about 10 minutes or
more, about 20 minutes or more, about 30 minutes or more, about 40
minutes or more, or about 50 minutes or more, about 1 hour or more,
about 2 hours or more, about 3 hours or more, about 4 hours or
more, or about 5 hours or more. In some embodiments, the time
length of the sensor session can be from about 1 hour or more to
about 2 hours or more, about 3 hours or more, about 4 hours or
more, about 5 hours or more, about 6 hours or more, about 7 hours
or more, about 8 hours or more, about 9 hours or more, about 10
hours or more, about 11 hours or more, about 12 hours or more,
about 13 hours or more, about 14 hours or more, about 15 hours or
more, about 16 hours or more, about 17 hours or more, about 18
hours or more, about 19 hours or more, about 20 hours or more,
about 21 hours or more, about 22 hours or more, about 23 hours or
more, or about 24 hours or more. In some embodiments, the time
length of the sensor session can be from less than about 0.25 days
to about 0.25 days or more, about 0.5 days or more about 0.75 days
or more, about 1 day or more, about 2 days or more, about 3 days or
more, about 4 days or more, about 5 days or more, about 6 days or
more, about 7 days or more, about 8 days or more, about 9 days or
more, about 10 days or more, about 20 days or more, about 30 days
or more, about 40 days or more, or about 50 days or more.
[0377] The analyte sensor can be configured for any type of
implantation, such as transcutaneous implantation, subcutaneous
implantation, or implantation into the host's circulatory system
(for example, into a vessel, such as a vein or artery). In
addition, the sensor may be configured to be wholly implantable or
extracorporeally implantable (for example, into an extracorporeal
blood circulatory device, such as a heart-bypass machine or a blood
dialysis machine). U.S. Pat. No. 7,497,827 describes an exemplary
continuous analyte sensor that can be used for transcutaneous
implantation by insertion into the abdominal tissue of a host. U.S.
Patent Publication No. 2008-0119703-A1 describes an exemplary
embodiment of a continuous analyte sensor that can be used for
insertion into a host's vein (for example, via a catheter). In some
embodiments, the sensor can be configured and arranged for in vitro
use.
[0378] FIG. 6A is a cross-sectional view through the sensor of FIG.
2C on line 6-6, illustrating one embodiment of the membrane system
612. As illustrated, the membrane system can include an enzyme
domain 602, a diffusion resistance domain 604, and a bioprotective
domain 606 located around the working electrode 602. In some
embodiments, a unitary diffusion resistance domain and
bioprotective domain can be included in the membrane system (for
example, wherein the functionality of both domains is incorporated
into one domain, that is, the bioprotective domain). In some
embodiments, the sensor is configured for short-term implantation
(for example, from about 1 to about 30 days). However, it is
understood that the membrane system 612 can be modified for use in
other devices, for example, by including only one or more of the
domains, or additional domains.
[0379] In some embodiments, the membrane system can include a
bioprotective domain, also referred to as a cell-impermeable domain
or biointerface domain, comprising a surface-modified base polymer
as described in more detail elsewhere herein. The membrane systems
612 of some embodiments can also include a plurality of domains or
layers including, for example, an electrode domain 610 (for
example, as illustrated in FIG. 6C), an interference domain 608
(for example, as illustrated in FIG. 6B), or a cell disruptive
domain (not shown), such as described in more detail elsewhere
herein and in U.S. Pat. No. 7,494,465, which is incorporated herein
by reference in its entirety.
[0380] Membranes modified for other sensors or electrodes, for
example, may include fewer or additional layers. For example, the
membrane system can comprise one electrode layer, one enzyme layer,
and two bioprotective layers. Alternatively, the membrane system
can comprise one electrode layer, two enzyme layers, and one
bioprotective layer. Furthermore, the bioprotective layer can be
configured to function as the diffusion resistance domain and
control the flux of the analyte (such as, for example, glucose) to
the underlying membrane layers.
[0381] In the case of a silver/silver chloride reference electrode,
one or more enzyme layers can be employed. It is generally
preferred to avoid additional layers that would block reactants
that generate hydrogen peroxide upon contact with the enzyme layer;
however, such layers may be present, e.g., for ease of fabrication,
provided that sufficient reactant reaches the enzyme layer so as to
generate hydrogen peroxide to regenerate silver chloride.
[0382] In some embodiments, one or more domains of the sensing
membranes can be formed from materials such as silicone,
polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene,
polyolefin, polyester, polycarbonate, biostable
polytetrafluoroethylene, homopolymers, copolymers, terpolymers of
polyurethanes, polypropylene (PP), polyvinylchloride (PVC),
polyvinylidene fluoride (PVDF), polybutylene terephthalate (PBT),
polymethylmethacrylate (PMMA), polyether ether ketone (PEEK),
polyurethanes, cellulosic polymers, poly(ethylene oxide),
poly(propylene oxide) and copolymers and blends thereof,
polysulfones and block copolymers thereof including, for example,
di-block, tri-block, alternating, random and graft copolymers.
[0383] In some embodiments, various domains or layers, including
the enzyme layer, can be deposited on the electroactive surfaces of
the electrode material using known thin or thick film techniques
(for example, spraying, electro-depositing, dipping, or the like).
It should be appreciated that the enzyme layer located over the
working electrode does not have to have the same structure or
composition as the enzyme layer located over, e.g., a silver/silver
chloride reference electrode. As described in greater detail
elsewhere herein, however, a membrane or other layer including an
enzyme domain deposited over the working electrode can
advantageously be deposited over the silver/silver chloride
reference electrode as well, to increase or control reference
capacity via action of the enzyme therein as described above.
[0384] Although the exemplary embodiments illustrated in FIGS.
6A-6C involve circumferentially extending membrane systems, the
membranes described herein may be applied to any planar or
non-planar surface, for example, the substrate-based sensor
structure of U.S. Pat. No. 6,565,509 to Say et al.
[0385] In some embodiments, an enzyme domain, also referred to as
the enzyme layer, can be used and can be situated less distal from
the electrochemically reactive surfaces than the diffusion
resistance domain. The enzyme domain can comprise a catalyst or
enzyme configured to react with an analyte. For example, the enzyme
domain can be an immobilized enzyme domain including glucose
oxidase. In other embodiments, the enzyme domain can be impregnated
with other oxidases, including, for example, galactose oxidase,
cholesterol oxidase, amino acid oxidase, alcohol oxidase, lactate
oxidase, or uricase. For an enzyme-based electrochemical glucose
sensor to perform well, the sensor's response should not be limited
by either enzyme activity or cofactor concentration.
[0386] In some embodiments, the catalyst or enzyme can be
impregnated or otherwise immobilized into the bioprotective or
diffusion resistance domain such that a separate enzyme domain is
not required (for example, wherein a unitary domain is provided
including the functionality of the bioprotective domain, diffusion
resistance domain, and enzyme domain). In some embodiments, the
enzyme domain is formed from a polyurethane, for example, aqueous
dispersions of colloidal polyurethane polymers including the
enzyme.
[0387] FIGS. 1A-1C illustrate alternative embodiments of the in
vivo portion of a continuous analyte sensor 100, which includes an
elongated conductive body 102. The elongated conductive body 102
includes a core 110 (see FIG. 1B) and a first layer 112 at least
partially surrounding the core. The first layer includes a working
electrode (for example, located in window 106) and a membrane 108
located over the working electrode. In some embodiments, the core
and first layer can be of a single material (such as, for example,
platinum). In some embodiments, the elongated conductive body is a
composite of at least two materials, such as a composite of two
conductive materials, or a composite of at least one conductive
material and at least one non-conductive material. In some
embodiments, the elongated conductive body comprises a plurality of
layers. In certain embodiments, there are at least two concentric
or annular layers, such as a core formed of a first material and a
first layer formed of a second material. However, additional layers
can be included in some embodiments. In some embodiments, the
layers are coaxial.
[0388] The elongated conductive body may be long and thin, yet
flexible and strong. For example, in some embodiments, the smallest
dimension of the elongated conductive body is less than about 0.1
inches, 0.075 inches, 0.05 inches, 0.025 inches, 0.01 inches, 0.004
inches, or 0.002 inches. While the elongated conductive body is
illustrated in FIGS. 1A through 1C as having a circular
cross-section, in other embodiments the cross-section of the
elongated conductive body can be ovoid, rectangular, triangular,
polyhedral, star-shaped, C-shaped, T-shaped, X-shaped, Y-Shaped,
irregular, or the like. In one embodiment, a conductive wire
electrode is employed as a core. To such a clad electrode, two
additional conducting layers may be added (e.g., with intervening
insulating layers provided for electrical isolation). The
conductive layers can be comprised of any suitable material. In
certain embodiments, it can be desirable to employ a conductive
layer comprising conductive particles (i.e., particles of a
conductive material) in a polymer or other binder.
[0389] The materials used to form the elongated conductive body
(such as, for example, stainless steel, titanium, tantalum,
platinum, platinum-iridium, iridium, certain polymers, and/or the
like) can be strong and hard, and therefore are resistant to
breakage. In some embodiments, the sensor's small diameter provides
flexibility to these materials, and therefore to the sensor as a
whole. Thus, the sensor can withstand repeated forces applied to it
by surrounding tissue.
[0390] In addition to providing structural support, resiliency and
flexibility, in some embodiments, the core 110, or a component
thereof, provides electrical conduction for an electrical signal
from the working electrode to sensor electronics (not shown). In
some embodiments, the core 110 comprises a conductive material,
such as stainless steel, titanium, tantalum, a conductive polymer,
and/or the like. However, in other embodiments, the core is formed
from a non-conductive material, such as a non-conductive polymer.
In yet other embodiments, the core comprises a plurality of layers
of materials. For example, in one embodiment the core includes an
inner core and an outer core. In a further embodiment, the inner
core is formed of a first conductive material and the outer core is
formed of a second conductive material. For example, in some
embodiments, the first conductive material is stainless steel,
titanium, tantalum, a conductive polymer, an alloy, and/or the
like, and the second conductive material is a conductive material
selected to provide electrical conduction between the core and the
first layer, and/or to attach the first layer to the core (that is,
if the first layer is formed of a material that does not attach
well to the core material). In another embodiment, the core is
formed of a non-conductive material (such as, for example, a
non-conductive metal and/or a non-conductive polymer) and the first
layer is formed of a conductive material, such as stainless steel,
titanium, tantalum, a conductive polymer, and/or the like. The core
and the first layer can be of a single (or same) material, such as
platinum. One skilled in the art appreciates that additional
configurations are possible.
[0391] Referring again to FIGS. 1A-1C, the first layer 112 can be
formed of a conductive material and the working electrode can be an
exposed portion of the surface of the first layer 112. Accordingly,
the first layer 112 can be formed of a material configured to
provide a suitable electroactive surface for the working electrode,
a material such as, but not limited to, platinum, platinum-iridium,
gold, palladium, iridium, graphite, carbon, a conductive polymer,
an alloy and/or the like.
[0392] As illustrated in FIGS. 1B-1C, a second layer 104 surrounds
at least a portion of the first layer 112, thereby defining the
boundaries of the working electrode. In some embodiments, the
second layer 104 serves as an insulator and is formed of an
insulating material, such as polyimide, polyurethane, parylene, or
any other known insulating materials. For example, in one
embodiment the second layer is disposed on the first layer and
configured such that the working electrode is exposed via window
106. In some embodiments, an elongated conductive body, including
the core, the first layer and the second layer, is provided. A
portion of the second layer can be removed to form a window 106,
through which the electroactive surface of the working electrode
(that is, the exposed surface of the first layer 112) is exposed.
In some embodiments, a portion of the second and (optionally) third
layers can be removed to form the window 106, thus exposing the
working electrode. Removal of coating materials from one or more
layers of the elongated conductive body (for example, to expose the
electroactive surface of the working electrode) can be performed by
hand, excimer lasing, chemical etching, laser ablation,
grit-blasting, or the like.
[0393] The sensor can further comprise a third layer 114 comprising
a conductive material. For example, the third layer 114 may
comprise a reference electrode, which may be formed of a
silver-containing material that is applied onto the second layer
104 (that is, the insulator). A more detailed description of the
various embodiments of the reference electrode is described
elsewhere herein.
[0394] The elongated conductive body 102 can further comprise one
or more intermediate layers (not shown) located between the core
110 and the first layer 112. For example, the intermediate layer
can be one or more of an insulator, a conductor, a polymer, and/or
an adhesive.
[0395] It is contemplated that the ratio between the thickness of
the silver/silver chloride layer and the thickness of an insulator
(such as, for example, polyurethane or polyimide) layer can be
controlled, so as to allow for a certain error margin (that is, an
error margin associated with the etching process) that would not
result in a defective sensor (for example, due to a defect
resulting from an etching process that cuts into a depth more than
intended, thereby unintentionally exposing an electroactive
surface). This ratio may be different depending on the type of
etching process used, whether it is laser ablation, grit blasting,
chemical etching, or some other etching method. In one embodiment
in which laser ablation is performed to remove a silver/silver
chloride layer and a polyurethane layer, the ratio of the thickness
of the silver/silver chloride layer and the thickness of the
polyurethane layer can be from about 1:5 to about 1:1, or from
about 1:3 to about 1:2.
[0396] In some embodiments, the core 110 comprises a non-conductive
polymer and the first layer 112 comprises a conductive material.
Such a sensor configuration can advantageously provide reduced
material costs, in that it replaces a typically expensive material
with an inexpensive material. For example, the core 110 can be
formed of a non-conductive polymer, such as, a nylon or polyester
filament, string or cord, which can be coated and/or plated with a
conductive material, such as platinum, platinum-iridium, gold,
palladium, iridium, graphite, carbon, a conductive polymer, and
allows or combinations thereof.
[0397] As illustrated in FIGS. 1C-1D, the sensor can also include a
membrane 108, such as those discussed elsewhere herein, for
example, with reference to FIGS. 6A-6C. The membrane 108 can
include an enzyme layer (not shown), as described elsewhere herein.
For example, the enzyme layer can include a catalyst or enzyme
configured to react with an analyte. For example, the enzyme layer
can be an immobilized enzyme layer including glucose oxidase. In
other embodiments, the enzyme layer can be impregnated with other
oxidases, including, for example, galactose oxidase, cholesterol
oxidase, amino acid oxidase, alcohol oxidase, lactate oxidase, or
uricase.
[0398] The enzyme reacts with the analyte and/or another species to
generate hydrogen peroxide at the reference electrode surface. For
example, an enzyme layer including glucose oxidase can catalyze the
reaction of glucose and oxygen to generate gluconate and hydrogen
peroxide as described elsewhere herein, or a uricase oxidase can be
employed to generate hydrogen peroxide by reaction with urea. The
hydrogen peroxide thus generated can diffuse to the surface of the
reference electrode to regenerate silver chloride, thereby
increasing the reference capacity of the reference electrode.
[0399] The membrane 108, including the enzyme layer, can cover at
least a portion of the working and reference electrodes. By
covering at least a portion of the working electrode with the
membrane 108, for example, the enzyme layer can react with the
analyte to be measured. Advantageously, the enzyme can be very
specific to a particular analyte. Additionally, when the analyte
itself is not sufficiently electro-active, the enzyme can be used
to interact with the analyte to generate another species which is
electro-active and to which the sensor can produce a desired
output. For example, when the analyte being measured is glucose, a
glucose oxidase enzyme layer can be provided in the membrane 108 to
catalyze the conversion of glucose and oxygen into gluconate and
hydrogen peroxide. Hydrogen peroxide can then be qualified or
quantified at the working electrode to determine a concentration of
glucose at the working electrode.
[0400] Moreover, by covering at least a portion of the reference
electrode with the membrane 108, including the enzyme layer, silver
chloride at the surface of the reference electrode can be
regenerated to advantageously increase reference capacity, and
thereby increase the useful life of the sensor. For example, an
enzyme can be included in the enzyme layer that will catalyze the
conversion of oxygen and an analyte to generate at least hydrogen
peroxide. In some embodiments, glucose oxidase can be included in
the enzyme layer to convert glucose and oxygen into hydrogen
peroxide and gluconate. Hydrogen peroxide can then reduce silver
ion to silver metal, which can react with chloride ions to
regenerate silver chloride at the reference electrode surface.
Advantageously, because the enzyme layer covers at least a portion
of the reference electrode, by means of the membrane 108, hydrogen
peroxide can more efficiently regenerate silver chloride at the
surface of the reference electrode. For example, by covering at
least a portion of the reference electrode with the membrane 108,
and thus the enzyme layer, hydrogen peroxide will be generated in
the vicinity of the reference electrode, thus reducing the risk
that the hydrogen peroxide will either not diffuse towards the
reference electrode, or will diffuse away from the reference
electrode. That is, generating hydrogen peroxide in the vicinity of
the reference electrode can ensure that at least some hydrogen
peroxide will be available to reduce silver ion to silver metal in
order to regenerate silver chloride at the reference electrode
surface.
[0401] FIG. 1B is a schematic illustrating an embodiment of an
elongated conductive body 102, or elongated body, wherein the
elongated conductive body is formed from at least two materials
and/or layers of conductive material, as described in greater
detail elsewhere herein. The term "electrode" can be used herein to
refer to the elongated conductive body, which includes the
electroactive surface that detects the analyte. In some
embodiments, the elongated conductive body provides an electrical
connection between the electroactive surface (that is, the working
electrode) and the sensor electronics (not shown). In certain
embodiments, each electrode (that is, the elongated conductive body
on which the electroactive surface is located) is formed from a
fine wire with a diameter of from about 0.001 inches or less to
about 0.01 inches or more. Each electrode can be formed from, for
example, a plated insulator, a plated wire, or bulk electrically
conductive material. For example, in some embodiments, the wire
and/or elongated conductive body used to form a working electrode
is about 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009,
0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04 or 0.045 inches in
diameter.
[0402] Furthermore, the first layer can comprise an electroactive
surface (that is, the portion exposed through the window 106). The
exposed electroactive surface can be the working electrode. For
example, if the sensor is an enzymatic electrochemical analyte
sensor, the analyte enzymatically reacts with an enzyme in the
membrane covering at least a portion of the electroactive surface.
The reaction can generate electrons (e.sup.-) that are detected at
the electroactive surface as a measurable electronic current. For
example, in the detection of glucose wherein glucose oxidase
produces hydrogen peroxide as a byproduct, hydrogen peroxide reacts
with the surface of the working electrode producing two protons
(2H.sup.+), two electrons (2e.sup.-) and one molecule of oxygen
(O.sub.2), which produces the electronic current being
detected.
[0403] As previously described with reference to FIG. 1A and as
illustrated in FIG. 1C, an insulator 104 is disposed on at least a
portion of the elongated conductive body 102. In some embodiments,
the sensor is configured and arranged such that the elongated body
includes a core 110 and a first layer 112, and a portion of the
first layer 112 is exposed via window 106 in the insulator 104. In
other embodiments, the sensor is configured and arranged such that
the elongated body 102 includes a core 110 embedded in an insulator
104, and a portion of the core 110 is exposed via the window 106 in
the insulator 104. For example, the insulating material can be
applied to the elongated body 102 (by, for example, screen-,
ink-jet and/or block-print) in a configuration designed to leave at
least a portion of the first layer's 112 surface (or the core's 110
surface) exposed. For example, the insulating material can be
printed in a pattern that does not cover a portion of the elongated
body 102. Alternatively, a portion of the elongated body 102 can be
masked prior to application of the insulating material. Removal of
the mask, after insulating material application, can expose the
portion of the elongated body 102.
[0404] In some embodiments, the insulating material 104 comprises a
polymer, for example, a non-conductive (that is, dielectric)
polymer. Dip-coating, spray-coating, vapor-deposition, printing
and/or other thin film and/or thick film coating or deposition
techniques can be used to deposit the insulating material on the
elongated body 102 and/or core 110. For example, in some
embodiments, the insulating material is applied as a layer of from
about less than 5 microns, or from 5, 10 or 15-microns to about 20,
25, 30 or 35-microns or more in thickness. The insulator can be
applied as a single layer of material, or as two or more layers,
which are comprised of either the same or different materials, as
described elsewhere herein. Alternatively, the conductive core may
not require a coating of insulator. In some embodiments, the
insulating material defines an electroactive surface of the analyte
sensor (that is, the working electrode). For example, a surface of
the conductive core (such as, for example, a portion of the first
layer 112) can either remain exposed during the insulator
application, or a portion of applied insulator can be removed to
expose a portion of the conductive core's surface, as described
above.
[0405] In some embodiments, in which the sensor has an insulated
elongated body or an insulator disposed upon a conductive
structure, a portion of the insulating material can be stripped or
otherwise removed, for example, by hand, excimer lasing, chemical
etching, laser ablation, grit-blasting (such as, for example, with
sodium bicarbonate or other suitable grit), or the like, to expose
the electroactive surfaces. In one exemplary embodiment, grit
blasting is implemented to expose the electroactive surface(s), for
example, by utilizing a grit material that is sufficiently hard to
ablate the polymer material yet also sufficiently soft so as to
minimize or avoid damage to the underlying metal electrode (for
example, a platinum electrode). Although a variety of "grit"
materials can be used (such as, for example, sand, talc, walnut
shell, ground plastic, sea salt, and the like), in some
embodiments, sodium bicarbonate is an advantageous grit-material
because it is sufficiently hard to ablate, e.g., a parylene coating
without damaging, e.g., an underlying platinum conductor. An
additional advantage of sodium bicarbonate blasting includes its
polishing action on the metal as it strips the polymer layer,
thereby eliminating a cleaning step that might otherwise be
necessary. Alternatively, a portion of an electrode or other
conductive body can be masked prior to depositing the insulator in
order to maintain an exposed electroactive surface area.
[0406] The electroactive surface of the working electrode can be
exposed by formation of a window 106 in the insulator 104. The
electroactive window 106 of the working electrode can be configured
to measure the concentration of an analyte.
[0407] The reference electrode can be juxtapositioned and/or
twisted with or around at least a portion of the sensor, and can
then optionally be coated or adhered together with an insulating
material, similar to that described above, so as to provide an
insulating attachment.
[0408] In some embodiments, a silver wire is formed onto and/or
fabricated into the sensor and subsequently chloridized to form a
silver/silver chloride reference electrode. Advantageously,
chloridizing the silver wire as described herein enables the
manufacture of a reference electrode with good in vivo performance.
By controlling the quantity and amount of chloridization of the
silver to form silver/silver chloride, improved break-in time,
stability of the reference electrode and extended life can be
obtained in some embodiments. Additionally, use of silver chloride
as described above allows for relatively inexpensive and simple
manufacture of the reference electrode.
[0409] Referring to FIGS. 1B-1C, the reference electrode 114 can
comprise a silver-containing material (e.g., silver/silver
chloride) applied over at least a portion of the insulating
material 104, as discussed in greater detail elsewhere herein. For
example, the silver-containing material can be applied using thin
film and/or thick film techniques, such as but not limited to
dipping, spraying, printing, electro-depositing, vapor deposition,
spin coating, and sputter deposition, as described elsewhere
herein. For example, a silver or silver chloride-containing paint
(or similar formulation) can be applied to a reel of the insulated
conductive core. Alternatively, the reel of insulated elongated
body (or core) is cut into single unit pieces (that is,
"singularized"), and silver-containing ink is pad printed thereon.
In still other embodiments, the silver-containing material is
applied as a silver foil. For example, an adhesive can be applied
to an insulated elongated body, around which the silver foil can
then be wrapped in. Alternatively, the sensor can be rolled in
Ag/AgCl particles, such that a sufficient amount of silver sticks
to and/or embeds into and/or otherwise adheres to the adhesive for
the particles to function as the reference electrode. In some
embodiments, the sensor's reference electrode includes a sufficient
amount of chloridized silver that the sensor measures and/or
detects the analyte for at least three days.
[0410] FIG. 5A is a perspective view of the in vivo portion a
dual-electrode analyte sensor, in another embodiment. As
illustrated, the sensor cab comprises first and second bundled
elongated bodies (that is, conductive cores) E1, E2, wherein a
working electrode comprises an exposed electroactive surface of the
elongated body, and a reference electrode 114, wherein each working
electrode comprises a conductive core. For example, the first
working electrode comprises an exposed portion of the surface of a
first elongated body 102A having an insulating material 104A
disposed thereon, such that the portion of the surface of the
elongated body (that is, the working electrode) is exposed via a
radial window 106A in the insulator. In some embodiments, the
elongated body comprises a core and a first layer, wherein an
exposed surface (e.g., electroactive) of the first layer is the
first working electrode. The second working electrode comprises an
exposed surface of a second core 102B having an insulator 104B
disposed thereon, such that a portion of the surface of the core is
exposed via a radial window 106B in the insulator. A first layer
(not shown) can be applied to the exposed surface of the second
core to form the second working electrode. In this embodiment, the
radial windows are spaced such that the working electrodes (that
is, electroactive surfaces) are substantially overlapping along the
length of the sensor. Alternatively, the working electrodes can be
spaced such that they are not substantially overlapping along the
length of the sensor. In this embodiment, the reference electrode
comprises a wire (such as, for example, Ag/AgCl wire) wrapped
around the bundled conductive cores. However, in some embodiments,
the reference electrode comprises a layer of silver-containing
material applied to at least one of the insulating materials 104A,
104B.
[0411] FIG. 5B is a perspective view of the in vivo portion of a
dual-electrode analyte sensor, in another embodiment. As
illustrated, the first and second elongated bodies E1, E2 can be
twisted into a twisted pair, such as a helix. In the embodiment
shown in FIG. 5B, the reference electrode 114 can then be wrapped
around the twisted pair. However, in some embodiments, the
reference electrode comprises a layer of silver-containing material
applied to at least one of the insulating materials 104A, 104B.
[0412] FIGS. 5C and 5D include views of the in vivo portion of a
dual-electrode analyte sensor, in additional embodiments. As
illustrated, the first and second elongated bodies E1, E2 can be
bundled together with the reference electrode 114. Connectors 502
can be configured and arranged to hold the conductive cores and
reference electrode together. Alternatively, instead of connectors
502, a tube 530 or heat shrink material can be employed as a
connector and/or supporting member. The tubing or heat shrink
material may include an adhesive inside the tube so as to provide
enhanced adhesion to the components secured within (that is,
wire(s), core, layer materials, etc.). In such a configuration, the
heat-shrink material functions not only as an insulator, but also
to hold the proximal ends of the sensor together so as to prevent
or reduce fatigue and/or to maintain the electrodes together in the
event of a fatigue failure. In the embodiment depicted in FIG. 5C,
the wires need not be a core and a layer, but can instead comprise
bulk materials. The distal ends of the sensor can be loose and
finger-like, as depicted in FIG. 5C, or can be held together with
an end cap. A reference electrode can be placed on one or more of
the first and second elongated bodies instead of being provided as
a separate electrode, and the first and second elongated bodies
including at least one reference electrode thereof can be bundled
together. Heat shrink tubing, crimp wrapping, dipping, or the like
can be employed to bundle one or more elongated bodies together. In
some embodiments, the reference electrode is a wire, such as
described elsewhere herein. In other embodiments, the reference
electrode comprises a foil. In an embodiment of a dual-electrode
analyte sensor, the first and second elongated bodies can be
present as or formed into a twisted pair, which is subsequently
bundled with a wire or foil reference electrode. Connectors, which
can also function as supporting members, can be configured and
arranged to hold the conductive cores and reference electrode
together. Although in the embodiment shown in FIG. 5C, the
reference electrode 114 comprise an elongated body that is bundled
together with first and second elongated bodies E1, E2, in other
embodiments, the reference electrode may be formed of a layer of
silver-containing material applied to at least one of the
insulating materials 504A, 504B.
[0413] A membrane (not shown), as described more fully elsewhere
herein, can also be included with the sensors shown in FIGS. 5A-5D.
The membrane can include an enzyme layer as described elsewhere
herein. For example, the enzyme layer can include a catalyst or
enzyme configured to react with an analyte. For example, the enzyme
layer can be an immobilized enzyme layer including glucose oxidase.
In other embodiments, the enzyme layer can be impregnated with
other oxidases, including, for example, galactose oxidase,
cholesterol oxidase, amino acid oxidase, alcohol oxidase, lactate
oxidase, or uricase.
[0414] FIGS. 8A-8C provide views of the in vivo portion of another
embodiment of a multi-electrode sensor system 800 comprising two
working electrodes and at least one reference electrode. The sensor
system 800 can comprise first and second elongated bodies E1, E2,
each formed of a conductive core or of a core with a conductive
layer deposited thereon. As illustrated, an insulating layer 810, a
conductive layer 820, and a membrane layer (not shown) can be
deposited on top of the elongated bodies E1, E2. The insulating
layer 810 can separate the conductive layer 820 from the elongated
body. The materials selected to form the insulating layer 810 can
include any of the insulating materials described elsewhere herein,
including, for example, polyurethane and polyimide. The materials
selected to form the conductive layer 820 can include any of the
conductive materials described elsewhere herein, including, for
example, silver/silver chloride, platinum, gold, can the like.
Working electrodes 802', 802'' can be formed by removing portions
of the conductive layer 820 and the insulating layer 810, thereby
exposing an electroactive surface of the elongated bodies E1, E2,
respectively. By depositing a membrane layer comprising an enzyme
on top of the elongated bodies E1, E2, the enzyme can cover at
least a portion of the working and reference electrodes. Although
not shown in FIGS. 6A-6C, the distal ends 830', 830'' of the core
portions of the elongated bodies E1, E2, respectively, can be
covered with an insulating material (such as, for example,
polyurethane or polyimide). Alternatively, the exposed core
portions 830', 830'' can be covered with a membrane system and
serve as additional working electrode surface area. Contacts 804',
804'' are used to provide electrical connection between the working
electrodes and other components of the sensor system may be formed
in a similar manner. As shown, contacts 804' and 804'' are
separated from each other to prevent an electrical connection
therebetween. Because the layer removal process is performed on
each individual elongated body E1, E2, instead of a single
geometrically complicated elongated body, this particular sensor
design (i.e., two elongated bodies placed side by side) may provide
ease of manufacturing, as compared to the manufacturing processes
involved with other multi-electrode systems having other
geometries.
[0415] The two elongated bodies illustrated in FIG. 8A can be
fabricated to have substantially the same shape and dimensions. For
example, the working electrodes can be fabricated to have the same
properties, thereby providing a sensor system capable of providing
redundancy of signal measurements. In other embodiments, the
working electrodes, associated with the elongated bodies E1, E2,
can each have one or more characteristics that distinguish each
working electrode from the other. For example, in one embodiment,
each of the elongated bodies E1, E2 can be covered with a different
membrane so that each working electrode can have a different
membrane property than the other working electrode. For example,
one of the working electrodes can have a membrane comprising an
enzyme layer and the other working electrode can have a membrane
comprising a layer having either an inactivated form of the enzyme
or no enzyme. Additional sensor system configurations that are
possible with a plurality of working electrodes (that is, sensor
elements) are described in U.S. Patent Publication No. 2011-0024307
A1, which is incorporated by reference herein in its entirety.
[0416] FIGS. 9A-9C provide views of the in vivo portion of another
embodiment of a multi-electrode sensor system 900 comprising two
working electrodes and one reference electrode. As illustrated, the
three electrodes are integrated into one piece. The sensor system
900 comprises first, second, and third elongated bodies E1, E2, E3,
each formed of a conductive core or of a core with a conductive
layer deposited thereon. As illustrated in FIG. 9C, an insulating
domain 910 and a membrane layer (not shown) can be deposited on top
of an assembly comprising the elongated bodies E1, E2, E3. The
insulating domain 910 can bind the three elongated bodies E1, E2,
E3 in close proximity of each other, while also separating them
from direct contact with each other. The materials selected to form
the insulating domain 910 may include any of the insulating
materials described elsewhere herein, including polyurethane and
polyimide, for example. Working electrode 904 on elongated body E1
and another working electrode (not shown) on elongated body E2, can
be formed by removing portions of the insulating domain 910,
thereby exposing electroactive surface of the elongated bodies E1,
E2. Similarly, the reference electrode 906 on elongated body E3 can
also be formed by removing portions of the insulating domain 910,
thereby exposing electroactive surface of the elongated body E3. By
depositing a membrane layer comprising an enzyme on top of the
assembly comprising the elongated bodies E1, E2, E3, the enzyme can
cover at least a portion of the working and reference electrodes.
Although not shown in FIG. 9A and FIG. 9B, the distal ends 930',
930'', 930''' of the core portions of the elongated bodies E1, E2,
E3, respectively, can be covered with an insulating material (such
as, for example, polyurethane or polyimide). Alternatively, the
exposed core portions 930', 930'', 930''' can be covered with a
membrane system and serve as additional working electrode surface
area.
[0417] As described elsewhere herein, the working electrodes,
associated with the elongated bodies can each have one or more
characteristics that distinguish each working electrode from the
other. For example, one of the working electrodes can have a
membrane comprising an enzyme layer and the other working electrode
can have a membrane comprising a layer having either an inactivated
form of the enzyme or no enzyme. Additional sensor system
configurations that are possible with a plurality of working
electrodes (that is, sensor elements) are described in U.S. Patent
Publication No. 2011-0024307-A1, which is incorporated by reference
herein in its entirety. In other embodiments, the working
electrodes can be fabricated to have the same properties, thereby
providing a sensor system capable of providing redundancy of signal
measurements.
[0418] FIGS. 10A-10C provide views of the in vivo portion of
another embodiment of a multi-electrode sensor system 1000
comprising two working electrodes and at least one reference
electrode. The sensor system 1000 comprises first and second
elongated bodies E1, E2, each formed of a conductive core or of a
core with a conductive layer deposited thereon. An insulating layer
1010 can be deposited onto each elongated body E1, E2. Furthermore,
a conductive domain 1020 and a membrane layer (not shown) can be
deposited on top of an assembly comprising the elongated bodies E1,
E2 and the insulating layer 1010. The conductive domain 1020 can
bind the two elongated bodies E1, E2 into one elongated body. The
insulating layers 1010 surrounding each elongated body E1, E2 can
prevent electrical contact between the two elongated bodies E1, E2.
The materials selected to form the insulating layer 1010 can
include any of the insulating materials described elsewhere herein,
including polyurethane and polyimide, for example. The materials
selected to form the conductive domain 1020 can include any of the
conductive materials described elsewhere herein, including
silver/silver chloride and platinum, for example. Working electrode
1002' on elongated body E1 and another working electrode (not
shown) on elongated body E2, can be formed by removing portions of
the conductive domain 1020 and portions of the insulating layer
810, thereby exposing electroactive surfaces of elongated bodies
E1, E2. The portion of the conductive domain 1020 not removed forms
the reference electrode. By depositing a membrane layer comprising
an enzyme on top of the assembly comprising the elongated bodies
E1, E2, the enzyme can cover at least a portion of the working and
reference electrodes. With this particular sensor design, because
the conductive domain 1020 is disposed between the contact point
between the two elongated bodies E1, E2, the sensor system's
largest cross-sectional dimension is minimized, as compared to a
design in which both of the elongated bodies were each individually
covered with a conductive layer. Contacts 1004', 1004'' are used to
provide electrical connection between the working electrodes and
other components of the sensor system may be formed in a similar
manner. As shown, contacts 1004' and 1004'' are separated from each
other to prevent an electrical connection therebetween. Although
not shown in FIG. 10B, the distal ends 1030', 1030'' of the core
portions of the elongated bodies E1, E2, respectively, can be
covered with an insulating material (such as, for example,
polyurethane or polyimide). Alternatively, the exposed core
portions 1030', 1030'' can be covered with a membrane system and
serve as additional working electrode surface area.
[0419] FIGS. 11A-11C provide views of the in vivo portion of
another embodiment of a multi-electrode sensor system 1100
comprising two working electrodes and one reference electrode. The
sensor system can comprise a first, second, and third elongated
bodies, each formed of a conductive core or of a core with a
conductive layer deposited thereon. For example, an insulating
layer 1110 and a membrane layer (not shown) can be deposited on top
of the elongated bodies. The insulating layer 1110 separates the
elongated bodies from each other. The materials selected to form
the insulating layer 1110 can include any of the insulating
materials described elsewhere herein, including, for example,
polyurethane and polyimide. Working electrodes 1102', 1102'' and
reference electrode 1106 can be formed by removing portions of the
insulating layer 1110, thereby exposing electroactive surface of
the elongated bodies respectively. Contacts 1104', 1104'' are used
to provide electrical connection between the working electrodes and
other components of the sensor system may be formed in a similar
manner. As shown, contacts 1104' and 1104'' are separated from each
other to prevent an electrical connection therebetween. Although
not shown in FIG. 11A, the distal ends 1130', 1130'' of the core
portions can be covered with an insulating material (such as, for
example, polyurethane or polyimide). Alternatively, the exposed
core portions 1130', 1130'' can be covered with a membrane system
and serve as additional working electrode surface area.
[0420] To fabricate the sensor systems illustrated, e.g., in FIGS.
8A-8C, 9A-9C, 10A-10C, and 11A-11C, the requisite number of
elongated bodies can be provided. As described above, the elongated
bodies can be formed as an elongated conductive core, or
alternatively as a core (conductive or non-conductive) having at
least one conductive material deposited thereon. The elongated
bodies that correspond to working electrodes may comprise an
elongated core with a conductive material typically used with
working electrodes (such as, for example, a core formed of a
conductive material like platinum, or a core plated, coated, or
cladded with a conductive material like platinum). The elongated
body that corresponds to a reference electrode may comprise an
elongated core plated, coated, or cladded with a silver/silver
chloride conductive material.
[0421] Next, an insulating layer can be deposited onto each of the
elongated bodies. In some embodiments, the insulating or deposited
layer can be formed of a thermoplastic material, thereby allowing
the elongated bodies to be attached together by a heating process
that permits the insulating layers of the elongated bodies to
adhere together. In other arrangements, the elongated bodies can be
formed as an elongated conductive core, or alternatively, as a core
(conductive or non-conductive) having at least one conductive
material deposited thereon. Next, an insulating layer can be
deposited onto each of the elongated bodies. Thereafter, a
conductive layer can be deposited over the insulating layer. In
some embodiments, the elongated bodies can be coated with a
thermoplastic material and fed through an aligning die. Afterwards,
a conductive domain can be deposited over this single elongated
body. The coated domain is then allowed to dry or be cured, after
which the one unitary elongated body is formed, in which the two
elongated bodies are encased and held together by conductive
domain.
[0422] Thereafter, a layer removal process can be performed to
remove portions of the insulating or deposited layer. Any of the
techniques described herein (such as, for example, laser ablation,
chemical etching, grit blasting) can be used. The insulating layer
can be removed to form the working electrode(s) and reference
electrode. Contacts can be used to provide electrical connection
between the working electrodes and other components of the sensor
system may be formed in a similar manner. Contacts are separated
from each other to prevent an electrical connection therebetween.
The layer removal process can be performed on each individual
elongated body, or a single geometrically complicated elongated
body.
[0423] After the conductive and insulating layers have been
deposited onto the elongated body, and after selected portions of
the deposited layers have been removed, a membrane or other
layer(s) can be applied onto at least a portion of the elongated
bodies. In certain embodiments, the membrane or other layer(s) are
applied only to the working electrodes, but in other embodiments
the membrane or other layer(s) can be applied to the entire
elongated body. In one embodiment, a membrane system is deposited
onto the two working electrodes simultaneously while they are
placed together (such as, by bundling). Alternatively, a membrane
can be deposited onto each individual working electrode, and the
two working electrodes can then be placed together.
[0424] In one exemplary embodiment, two or more elongated bodies
can be bundled together first (such as, for example, by providing
adherence between the insulating layers of the working electrodes)
to form a subassembly. An uncoated elongated conductive body can
then be adhered to the subassembly to form an assembly including
all elongated bodies. Subsequently, a silver-comprising elongated
conductive body can be chloridized to form a silver/silver chloride
reference electrode.
[0425] In certain embodiments, the distal ends of the core portions
of the elongated bodies can be covered with an insulating material
(such as, for example, polyurethane or polyimide). In alternative
embodiments, the exposed core portions can be covered with a
membrane system or other layer and serve as, e.g., additional
working or electrode surface area.
[0426] It should be understood that with any of the embodiments
described herein involving multiple working electrodes, one or more
working electrodes may be designed to serve as an enzymatic
electrode and one or more working electrodes may be designed to
serve as a "blank" working electrode configured to measure
baseline. This configuration allows for subtraction of a signal
associated with the "blank" working electrode (that is, the
baseline non-analyte related signal) from the signal associated
with the enzymatic-working electrode. The subtraction, in turn,
results in a signal that contains substantially reduced (or no)
non-analyte-related signal contribution (such as, for example,
contribution from interferents).
[0427] Prevention of Cross Talk and Removal of Noise
[0428] In some circumstances, cross talk can interfere with
analyte/noise detection. In general, cross talk can occur when
signal (for example, in the form of a detectable species such as
H.sub.2O.sub.2) is transferred from one electrode (for example, the
enzyme coated silver/silver chloride reference electrode) to
another (for example, the working electrode), and detected as a
signal by the other electrode. To prevent cross talk, in certain
embodiments, the silver/silver chloride reference electrode and the
working electrode can be spaced apart or can be separated by a
diffusion barrier (such as an insulator, a non-conductive material,
a resistance layer and/or the like). The diffusion barrier can be a
physical diffusion barrier, a spatial diffusion barrier, or a
temporal diffusion barrier, as discussed in more detail elsewhere
herein.
[0429] In an electrochemical sensor system, noise can be recognized
and substantially reduced and/or eliminated by a variety of sensor
configurations and/or methods. Noise can be reduced and/or
eliminated by using, for example: 1) sensor configurations that can
block and/or remove the interferent, or that can specifically
detect the noise; and 2) mathematical algorithms that can recognize
and/or remove the signal noise component. Devices and methods for
reducing and/or eliminating noise can be provided, such as blocking
interferent passage to the sensor's electroactive surfaces,
diluting and/or removing interferents around the sensor, and
mathematically determining and eliminating the noise signal
component. Various sensor structures (for example, multiple working
electrodes, membrane interference domains, etc.), bioactive agents,
algorithms and the like, disclosed elsewhere herein, can be
employed in a plurality of combinations, depending upon the desired
effect and the noise reduction strategy selected.
[0430] In one embodiment, the sensor system comprises a
silver/silver chloride reference electrode covered at least in part
with an enzyme layer, a working electrode with enzyme over its
electroactive surface, and an interference domain configured to
substantially block interferent passage therethrough, such that at
least some interferent no longer has a substantial effect on sensor
measurements (for example, at the working electrode). The
interference domain can be a component of the membrane system, such
as illustrated in FIGS. 6A-6C, and can be disposed at any level
(that is, layer or domain) of the membrane system (for example,
more proximal or more distal to the electroactive surfaces than as
illustrated in FIG. 6A-6C). In some embodiments, the interference
domain is combined with an additional membrane domain, such as the
resistance domain or the enzyme domain.
[0431] In another aspect, the sensor can be configured to reduce
noise, including non-constant non-analyte related noise with an
overlapping measuring potential with the analyte. A variety of
noise can occur when a sensor has been implanted in a host.
Generally, implantable sensors measure a signal (that is, counts)
that generally comprises at least two components, the background
signal (that is, background noise) and the analyte signal. The
background signal can be composed substantially of signal
contribution due to factors other than glucose (for example,
non-reaction-related hydrogen peroxide generated in connection with
regeneration of the reference electrode). The analyte signal (for
example, glucose) can be composed substantially of signal
contribution due to the analyte. Consequently, because the signal
includes these two components, a calibration can be performed in
order to determine the analyte (for example, glucose) concentration
by solving for the equation y=mx+b, where the value of b represents
the background of the signal.
[0432] In some circumstances, the background is comprised of both
constant (for example, baseline) and non-constant (for example,
noise) factors. Generally, it is desirable to remove the background
signal to provide a more accurate analyte concentration to the host
and/or health care professional.
[0433] Baseline does not significantly adversely affect the
accuracy of the calibration of the analyte concentration because
baseline can be relatively constantly eliminated using the equation
y=mx+b. In contrast, noise can be difficult to remove from the
sensor signal by calibration using standard calibration equations,
for example, because the background of the signal does not remain
constant. Noise can significantly adversely affect the accuracy of
the calibration of the analyte signal. Additionally noise can occur
in the signal of conventional sensors with electrode configurations
that are not particularly designed to measure noise substantially
equally at both active and in-active electrodes (for example,
wherein the electrodes are spaced and/or non-symmetrical, noise may
not be equally measured and therefore not easily removed using
conventional dual-electrode designs).
[0434] There are a variety of ways noise can be recognized and/or
analyzed. For example, the sensor data stream can be monitored,
signal artifacts can be detected, and data processing can be based
at least in part on whether or not a signal artifact has been
detected, such as described in U.S. Pat. No. 8,101,174 and U.S.
Patent Publication No. 2007-0027370-A1, herein incorporated by
reference in their entirety.
[0435] Accordingly, if a sensor is designed such that the signal
contribution due to baseline and noise can be removed, then more
accurate analyte concentration data can be provided to the host
and/or a healthcare professional.
[0436] In some embodiments, an analyte sensor (for example, glucose
sensor) is configured for insertion into a host for measuring an
analyte (for example, glucose) in the host. The sensor includes a
silver/silver chloride reference electrode disposed at least
partially beneath an active enzymatic portion of a membrane on the
sensor, a first working electrode disposed beneath an active
enzymatic portion of a membrane on the sensor, a second working
electrode disposed beneath an inactive- or non-enzymatic portion of
the membrane on the sensor, and electronics operably connected to
the first and second working electrode and configured to process
the first and second signals to generate an analyte (for example,
glucose) concentration substantially without signal contribution
due to non-glucose related noise artifacts. Noise due to
biochemical/chemical factors, e.g., hydrogen peroxide generated by
the enzyme-containing membrane covering the reference electrode,
can impinge upon the two working electrodes of certain embodiments
(for example, on with and one without active enzyme) to about the
same extent because of the sensor's small size and symmetrical
configuration. Accordingly, the sensor electronics can use these
data to calculate the glucose-only signal, as described elsewhere
herein. The signal corresponding to the hydrogen peroxide generated
by the enzyme-containing membrane covering the reference electrode
can then be canceled out by use of the signal from the second
working electrode.
[0437] In one exemplary embodiment, the analyte sensor is a glucose
sensor that can measure a first signal associated with both glucose
and non-glucose related electroactive compounds having a first
oxidation/reduction potential. The glucose sensor can also measure
a second signal, which can be associated with background noise of
the glucose sensor. The background noise can be composed of signal
contribution due to non-reaction-related hydrogen peroxide. The
first and second working electrodes can integrally form at least a
portion of the sensor, such as but not limited to the in vivo
portion of the sensor, as discussed elsewhere herein, and employed
in combination with a silver/silver chloride reference electrode
covered at least in part with an enzyme layer.
[0438] Furthermore, the sensor can have a diffusion barrier that
can substantially block (for example, attenuate) diffusion of
glucose or H.sub.2O.sub.2 between the first and second working
electrodes. Advantageously, the sensor can also have a diffusion
barrier that can substantially block (for example, attenuate)
diffusion of glucose or H.sub.2O.sub.2 between the silver/silver
chloride reference electrode and the first working electrode. In
various embodiments, the sensor includes a diffusion barrier
configured to be physical, spatial, and/or temporal.
[0439] FIG. 16 schematically illustrates one embodiment of a sensor
(for example, a portion of an in vivo portion of a sensor) having
one or more components that can act as a diffusion barrier (for
example, prevent diffusion of electroactive species from one
electrode to another). A silver/silver chloride reference electrode
E1 can be covered at least in part by a membrane layer 10000
comprising active enzyme. For example, in a glucose sensor, the
silver/silver chloride reference electrode E1 can be coated with
glucose oxidase enzyme (GOx). Although not illustrated, a working
electrode E2 can also be disposed beneath a membrane layer 10000.
The working electrode E2 can be separated from the silver/silver
chloride reference electrode E1 by a diffusion barrier D, such as,
but not limited, to a physical diffusion barrier (for example, a
layer of non-conductive material/insulator). The diffusion barrier
D can also be spatial or temporal, as discussed elsewhere
herein.
[0440] Glucose and oxygen can diffuse into the membrane layer
10000, where they can react with GOx to produce gluconate and
H.sub.2O.sub.2. At least a portion of the H.sub.2O.sub.2 can
diffuse to the silver/silver chloride reference electrode E1 where
it can reduce silver ion to silver metal. Advantageously, silver
metal can react with chloride to regenerate silver chloride at the
surface of the silver/silver chloride reference electrode E1. The
remaining H.sub.2O.sub.2 can diffuse to other locations in the
membrane layer 10000 or out of the membrane layer 10000
(illustrated by the wavy arrows). Without a diffusion barrier D, a
portion of the H.sub.2O.sub.2 can diffuse to the working electrode
E2, where it can be electrochemically oxidized to oxygen and
transfer two electrons (2e.sup.-) to the working electrode E2,
which can result in a glucose signal that is recorded by the sensor
electronics (not shown). As a result, an aberrant signal can be
recorded by the sensor electronics that can be interpreted as an
artificially high glucose concentration (for example, cross talk or
noise).
[0441] A substantial diffusion barrier D between the silver/silver
chloride reference electrode E1 and the working electrode E2 can be
provided, such that the H.sub.2O.sub.2 cannot substantially diffuse
from the silver/silver chloride reference electrode E1 to the
working electrode E2. Accordingly, the possibility of an aberrant
signal produced by H.sub.2O.sub.2 from the silver/silver chloride
reference electrode E1 at the working electrode E2 is reduced or
avoided.
[0442] A variety of diffusion barriers can be employed to prevent
cross talk or noise. For example, the diffusion barrier D can be a
physical diffusion barrier, such as a structure between the
silver/silver chloride reference electrode E1 and the working
electrode E2, that can block glucose and H.sub.2O.sub.2 from
diffusing from the silver/silver chloride reference electrode E1 to
the working electrode E2. In some embodiments, a physical diffusion
barrier is formed of one or more membrane materials, such as those
used in formation of an interference domain and/or a resistance
domain. Such materials can include, but are not limited to,
silicones, polyurethanes, cellulose derivatives (cellulose
butyrates and cellulose acetates, and the like) and combinations
thereof, as described elsewhere herein. In some embodiments, the
physical diffusion barrier includes one or more membrane domains.
For example, the physical diffusion barrier can be a discontinuous
portion of the membrane (for example, separate, distinct or
discontinuous membrane structures) disposed between the
silver/silver chloride reference electrode and the working
electrode, and can include one or more membrane portion(s) (for
example, interference and/or resistance domains). In some
embodiments, the physical diffusion barrier includes first and
second barrier layers formed independently on the silver/silver
chloride reference electrode and the working electrode. In some
embodiments the barrier layer is the resistance domain. In still
other embodiments, the physical diffusion barrier can be a
continuous membrane (and/or membrane domain(s)) disposed between
the silver/silver chloride reference electrode and the working
electrode. In some embodiments, the physical diffusion barrier
attenuates (for example, suppresses, blocks, prevents) diffusion of
the H.sub.2O.sub.2 (for example, cross talk) by at least 2-fold, or
at least 5-fold, or even at least 10-fold. In some embodiments, the
physical diffusion barrier attenuates crosstalk at least about 50%,
at least about 75%, at least about 80%, at least about 90%, at
least about 95%, or at least about 99%.
[0443] Alternatively, the diffusion barrier D can be a spatial
diffusion barrier, such as a distance between the silver/silver
chloride reference electrode E1 and the working electrode E2 that
can block glucose and H.sub.2O.sub.2 from diffusing from the first
silver/silver chloride reference electrode E1 to the working
electrode E2. For example, a spatial diffusion barrier can be
created by separating the silver/silver chloride reference
electrode and the working electrode by a distance that is too great
for the H.sub.2O.sub.2 to substantially diffuse therebetween. In
some embodiments, the spatial diffusion barrier is about 0.01,
about 0.02, about 0.03, about 0.04, about 0.05, about 0.06, about
0.07, or about 0.08 inches to about 0.09, about 0.10, about 0.11,
or about 0.120 inches. In other embodiments, the spatial diffusion
barrier is about 0.020 inches to about 0.050 inches. Still in other
embodiments, the spatial diffusion barrier is about 0.055 inches to
about 0.095 inches. A non-conductive material (for example, a
polymer structure or coating such as Parylene) can be configured to
act as a spatial diffusion barrier.
[0444] In other embodiments, the diffusion barrier D can be a
temporal diffusion barrier, such as a period of time between the
activity of the silver/silver chloride reference electrode E1 and
the working electrode E2 such that if glucose or H.sub.2O.sub.2
were to diffuse from the silver/silver chloride reference electrode
E1 to the working electrode E2, the working electrode E2 would not
substantially be influenced by the H.sub.2O.sub.2 from the
silver/silver chloride reference electrode E1.
[0445] In some embodiments, the dual-electrode sensor can comprise
an insulator, such as an electrical insulator, located between the
silver/silver chloride reference electrode and the working
electrode, wherein the insulator can comprise a physical diffusion
barrier. The physical diffusion barrier can be configured to
structurally block a substantial amount of diffusion of at least
one of an analyte (for example, glucose) and a co-analyte (for
example, H.sub.2O.sub.2) between the silver/silver chloride
reference electrode and the working electrode. The diffusion
barrier can comprise a structure that protrudes from a plane that
intersects both the silver/silver chloride reference electrode and
the working electrode. Moreover, the structure that protrudes can
comprise an electrical insulator and/or an electrode.
[0446] In some embodiments, the sensor can comprise an insulator
located between the silver/silver chloride reference electrode and
the working electrode. The insulator can comprise a diffusion
barrier configured to substantially block diffusion of at least one
of an analyte (for example, glucose) and a co-analyte (for example,
H.sub.2O.sub.2) between the silver/silver chloride reference
electrode and the working electrode. The diffusion barrier can be a
temporal diffusion barrier configured to block or avoid a
substantial amount of diffusion or reaction of at least one of the
analyte (for example, glucose) and the co-analyte (for example,
H.sub.2O.sub.2) between the silver/silver chloride reference
electrode and the working electrode.
[0447] In other embodiments, the electrochemical sensor can
comprise a sensor membrane configured to substantially block
diffusion of at least one of an analyte (for example, glucose) and
a co-analyte (for example, H.sub.2O.sub.2) between the
silver/silver chloride reference electrode and the working
electrode by a discontinuity of the sensor membrane between the
silver/silver chloride reference electrode and the working
electrode. A discontinuity of the sensor membrane can be a type of
physical diffusion barrier formed by a portion of the membrane
between the silver/silver chloride reference electrode and the
working electrode, for example, wherein a discontinuity in the
membrane structure blocks diffusion of H.sub.2O.sub.2 between the
silver/silver chloride reference electrode and the working
electrode.
[0448] In some embodiments, the sensor is an indwelling sensor,
such as configured for insertion into the host's circulatory system
via a vein or an artery. In some exemplary embodiments, an
indwelling sensor includes at least two working electrodes that are
inserted into the host's blood stream through a catheter. The
sensor includes at least a silver/silver chloride reference
electrode that can be disposed either with the working electrodes
or remotely from the working electrodes. The sensor includes a
spatial, a physical, or a temporal diffusion barrier. A spatial
diffusion barrier can be configured as described in U.S. Patent
Publication No. 2008-0083617-A1, which is incorporated by reference
herein in its entirety.
[0449] To configure a spatial diffusion barrier between the
silver/silver chloride reference electrode and the working
electrode, the location of the silver/silver chloride reference
electrode and the working electrode can be dependent upon the
orientation of the sensor after insertion into the host's artery or
vein. For example, in an embodiment configured for insertion in the
host's blood flow (for example, in an artery or vein), the
silver/silver chloride reference electrode can be downstream from
the working electrode (for example, relative to the direction of
blood flow). Due to this configuration, H.sub.2O.sub.2 produced the
silver/silver chloride reference electrode would be carrier
downstream (e.g., away from the working electrode) and thus not
affect the working electrode.
[0450] Methods and devices that are suitable for use in conjunction
with aspects of the preferred embodiments are disclosed in U.S.
Pat. Nos. 4,757,022; 4,994,167; 6,001,067; 6,558,321; 6,702,857;
6,741,877; 6,862,465; 6,931,327; 7,074,307; 7,081,195; 7,108,778;
7,110,803; 7,134,999; 7,136,689; 7,192,450; 7,226,978; 7,276,029;
7,310,544; 7,364,592; 7,366,556; 7,379,765; 7,424,318; 7,460,898;
7,467,003; 7,471,972; 7,494,465; 7,497,827; 7,519,408; 7,583,990;
7,591,801; 7,599,726; 7,613,491; 7,615,007; 7,632,228; 7,637,868;
7,640,048; 7,651,596; 7,654,956; 7,657,297; 7,711,402; 7,713,574;
7,715,893; 7,761,130; 7,771,352; 7,774,145; 7,775,975; 7,778,680;
7,783,333; 7,792,562; 7,797,028; 7,826,981; 7,828,728; 7,831,287;
7,835,777; 7,857,760; 7,860,545; 7,875,293; 7,881,763; 7,885,697;
7,896,809; 7,899,511; 7,901,354; 7,905,833; 7,914,450; 7,917,186;
7,920,906; 7,925,321; 7,927,274; 7,933,639; 7,935,057; 7,946,984;
7,949,381; 7,955,261; 7,959,569; 7,970,448; 7,974,672; 7,976,492;
7,979,104; 7,986,986; 7,998,071; 8,000,901; 8,005,524; 8,005,525;
8,010,174; 8,027,708; 8,050,731; 8,052,601; 8,053,018; 8,060,173;
8,060,174; 8,064,977; 8,073,519; 8,073,520; 8,118,877; 8,128,562;
8,133,178; 8,150,488; 8,155,723; 8,160,669; 8,160,671; 8,167,801;
8,170,803; 8,195,265; 8,206,297; 8,216,139; 8,229,534; 8,229,535;
8,229,536; 8,231,531; 8,233,958; 8,233,959; 8,249,684; 8,251,906;
8,255,030; 8,255,032; 8,255,033; 8,257,259; 8,260,393; 8,265,725;
8,275,437; 8,275,438; 8,277,713; 8,280,475; 8,282,549; 8,282,550;
8,285,354; 8,287,453; 8,290,559; 8,290,560; 8,290,561; 8,290,562;
8,292,810; 8,298,142; 8,311,749; 8,313,434; 8,321,149; 8,332,008;
8,346,338; 8,364,229; 8,369,919; 8,374,667; 8,386,004; and
8,394,021.
[0451] Methods and devices that are suitable for use in conjunction
with aspects of the preferred embodiments are disclosed in U.S.
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[0452] Methods and devices that are suitable for use in conjunction
with aspects of the preferred embodiments are disclosed in U.S.
application Ser. No. 09/447,227 filed on Nov. 22, 1999 and entitled
"DEVICE AND METHOD FOR DETERMINING ANALYTE LEVELS"; U.S.
application Ser. No. 12/828,967 filed on Jul. 1, 2010 and entitled
"HOUSING FOR AN INTRAVASCULAR SENSOR"; U.S. application Ser. No.
13/461,625 filed on May 1, 2012 and entitled "DUAL ELECTRODE SYSTEM
FOR A CONTINUOUS ANALYTE SENSOR"; U.S. application Ser. No.
13/594,602 filed on Aug. 24, 2012 and entitled "POLYMER MEMBRANES
FOR CONTINUOUS ANALYTE SENSORS"; U.S. application Ser. No.
13/594,734 filed on Aug. 24, 2012 and entitled "POLYMER MEMBRANES
FOR CONTINUOUS ANALYTE SENSORS"; U.S. application Ser. No.
13/607,162 filed on Sep. 7, 2012 and entitled "SYSTEM AND METHODS
FOR PROCESSING ANALYTE SENSOR DATA FOR SENSOR CALIBRATION"; U.S.
application Ser. No. 13/624,727 filed on Sep. 21, 2012 and entitled
"SYSTEMS AND METHODS FOR PROCESSING AND TRANSMITTING SENSOR DATA";
U.S. application Ser. No. 13/624,808 filed on Sep. 21, 2012 and
entitled "SYSTEMS AND METHODS FOR PROCESSING AND TRANSMITTING
SENSOR DATA"; U.S. application Ser. No. 13/624,812 filed on Sep.
21, 2012 and entitled "SYSTEMS AND METHODS FOR PROCESSING AND
TRANSMITTING SENSOR DATA"; U.S. application Ser. No. 13/732,848
filed on Jan. 2, 2013 and entitled "ANALYTE SENSORS HAVING A
SIGNAL-TO-NOISE RATIO SUBSTANTIALLY UNAFFECTED BY NON-CONSTANT
NOISE"; U.S. application Ser. No. 13/733,742 filed on Jan. 3, 2013
and entitled "END OF LIFE DETECTION FOR ANALYTE SENSORS"; U.S.
application Ser. No. 13/733,810 filed on Jan. 3, 2013 and entitled
"OUTLIER DETECTION FOR ANALYTE SENSORS"; U.S. application Ser. No.
13/742,178 filed on Jan. 15, 2013 and entitled "SYSTEMS AND METHODS
FOR PROCESSING SENSOR DATA"; U.S. application Ser. No. 13/742,694
filed on Jan. 16, 2013 and entitled "SYSTEMS AND METHODS FOR
PROVIDING SENSITIVE AND SPECIFIC ALARMS"; U.S. application Ser. No.
13/742,841 filed on Jan. 16, 2013 and entitled "SYSTEMS AND METHODS
FOR DYNAMICALLY AND INTELLIGENTLY MONITORING A HOST'S GLYCEMIC
CONDITION AFTER AN ALERT IS TRIGGERED"; and U.S. application Ser.
No. 13/747,746 filed on Jan. 23, 2013 and entitled "DEVICES,
SYSTEMS, AND METHODS TO COMPENSATE FOR EFFECTS OF TEMPERATURE ON
IMPLANTABLE SENSORS".
[0453] The above description presents the best mode contemplated
for carrying out the present invention, and of the manner and
process of making and using it, in such full, clear, concise, and
exact terms as to enable any person skilled in the art to which it
pertains to make and use this invention. This invention is,
however, susceptible to modifications and alternate constructions
from that discussed above that are fully equivalent. Consequently,
this invention is not limited to the particular embodiments
disclosed. On the contrary, this invention covers all modifications
and alternate constructions coming within the spirit and scope of
the invention as generally expressed by the following claims, which
particularly point out and distinctly claim the subject matter of
the invention. While the disclosure has been illustrated and
described in detail in the drawings and foregoing description, such
illustration and description are to be considered illustrative or
exemplary and not restrictive.
[0454] All references cited herein are incorporated herein by
reference in their entirety. To the extent publications and patents
or patent applications incorporated by reference contradict the
disclosure contained in the specification, the specification is
intended to supersede and/or take precedence over any such
contradictory material.
[0455] Unless otherwise defined, all terms (including technical and
scientific terms) are to be given their ordinary and customary
meaning to a person of ordinary skill in the art, and are not to be
limited to a special or customized meaning unless expressly so
defined herein. It should be noted that the use of particular
terminology when describing certain features or aspects of the
disclosure should not be taken to imply that the terminology is
being re-defined herein to be restricted to include any specific
characteristics of the features or aspects of the disclosure with
which that terminology is associated. Terms and phrases used in
this application, and variations thereof, especially in the
appended claims, unless otherwise expressly stated, should be
construed as open ended as opposed to limiting. As examples of the
foregoing, the term `including` should be read to mean `including,
without limitation,` `including but not limited to,` or the like;
the term `comprising` as used herein is synonymous with
`including,` `containing,` or `characterized by,` and is inclusive
or open-ended and does not exclude additional, unrecited elements
or method steps; the term `having` should be interpreted as `having
at least;` the term `includes` should be interpreted as `includes
but is not limited to;` the term `example` is used to provide
exemplary instances of the item in discussion, not an exhaustive or
limiting list thereof; adjectives such as `known`, `normal`,
`standard`, and terms of similar meaning should not be construed as
limiting the item described to a given time period or to an item
available as of a given time, but instead should be read to
encompass known, normal, or standard technologies that may be
available or known now or at any time in the future; and use of
terms like `preferably,` `preferred,` `desired,` or `desirable,`
and words of similar meaning should not be understood as implying
that certain features are critical, essential, or even important to
the structure or function of the invention, but instead as merely
intended to highlight alternative or additional features that may
or may not be utilized in a particular embodiment of the invention.
Likewise, a group of items linked with the conjunction `and` should
not be read as requiring that each and every one of those items be
present in the grouping, but rather should be read as `and/or`
unless expressly stated otherwise. Similarly, a group of items
linked with the conjunction `or` should not be read as requiring
mutual exclusivity among that group, but rather should be read as
`and/or` unless expressly stated otherwise.
[0456] Where a range of values is provided, it is understood that
the upper and lower limit, and each intervening value between the
upper and lower limit of the range is encompassed within the
embodiments.
[0457] With respect to the use of substantially any plural and/or
singular terms herein, those having skill in the art can translate
from the plural to the singular and/or from the singular to the
plural as is appropriate to the context and/or application. The
various singular/plural permutations may be expressly set forth
herein for sake of clarity. The indefinite article `a` or `an` does
not exclude a plurality. A single processor or other unit may
fulfill the functions of several items recited in the claims. The
mere fact that certain measures are recited in mutually different
dependent claims does not indicate that a combination of these
measures cannot be used to advantage. Any reference signs in the
claims should not be construed as limiting the scope.
[0458] It will be further understood by those within the art that
if a specific number of an introduced claim recitation is intended,
such an intent will be explicitly recited in the claim, and in the
absence of such recitation no such intent is present. For example,
as an aid to understanding, the following appended claims may
contain usage of the introductory phrases `at least one` and `one
or more` to introduce claim recitations. However, the use of such
phrases should not be construed to imply that the introduction of a
claim recitation by the indefinite articles `a` or `an` limits any
particular claim containing such introduced claim recitation to
embodiments containing only one such recitation, even when the same
claim includes the introductory phrases `one or more` or `at least
one` and indefinite articles such as `a` or `an` (e.g., `a` and/or
`an` should typically be interpreted to mean `at least one` or `one
or more`); the same holds true for the use of definite articles
used to introduce claim recitations. In addition, even if a
specific number of an introduced claim recitation is explicitly
recited, those skilled in the art will recognize that such
recitation should typically be interpreted to mean at least the
recited number (e.g., the bare recitation of `two recitations,`
without other modifiers, typically means at least two recitations,
or two or more recitations). Furthermore, in those instances where
a convention analogous to `at least one of A, B, and C, etc.` is
used, in general such a construction is intended in the sense one
having skill in the art would understand the convention (e.g., `a
system having at least one of A, B, and C` would include but not be
limited to systems that have A alone, B alone, C alone, A and B
together, A and C together, B and C together, and/or A, B, and C
together, etc.). In those instances where a convention analogous to
`at least one of A, B, or C, etc.` is used, in general such a
construction is intended in the sense one having skill in the art
would understand the convention (e.g., `a system having at least
one of A, B, or C` would include but not be limited to systems that
have A alone, B alone, C alone, A and B together, A and C together,
B and C together, and/or A, B, and C together, etc.). It will be
further understood by those within the art that virtually any
disjunctive word and/or phrase presenting two or more alternative
terms, whether in the description, claims, or drawings, should be
understood to contemplate the possibilities of including one of the
terms, either of the terms, or both terms. For example, the phrase
`A or B` will be understood to include the possibilities of `A` or
`B` or `A and B.`
[0459] All numbers expressing quantities of ingredients, reaction
conditions, and so forth used in the specification are to be
understood as being modified in all instances by the term `about.`
Accordingly, unless indicated to the contrary, the numerical
parameters set forth herein are approximations that may vary
depending upon the desired properties sought to be obtained. At the
very least, and not as an attempt to limit the application of the
doctrine of equivalents to the scope of any claims in any
application claiming priority to the present application, each
numerical parameter should be construed in light of the number of
significant digits and ordinary rounding approaches.
[0460] Furthermore, although the foregoing has been described in
some detail by way of illustrations and examples for purposes of
clarity and understanding, it is apparent to those skilled in the
art that certain changes and modifications may be practiced.
Therefore, the description and examples should not be construed as
limiting the scope of the invention to the specific embodiments and
examples described herein, but rather to also cover all
modification and alternatives coming with the true scope and spirit
of the invention.
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