U.S. patent application number 14/841368 was filed with the patent office on 2015-12-24 for membrane for continuous analyte sensors.
The applicant listed for this patent is DexCom, Inc.. Invention is credited to Robert J. Boock, Chris W. Dring, Jonathan Hughes, David Sze, Mark Wu, Huashi Zhang.
Application Number | 20150366494 14/841368 |
Document ID | / |
Family ID | 51530362 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150366494 |
Kind Code |
A1 |
Hughes; Jonathan ; et
al. |
December 24, 2015 |
MEMBRANE FOR CONTINUOUS ANALYTE SENSORS
Abstract
Devices are presented for measurement of an analyte
concentration. The devices comprise: a sensor configured to
generate a signal indicative of a concentration of an analyte; and
a sensing membrane located over the sensor. The sensing membrane
comprises an enzyme domain comprising an enzyme, a base polymer,
and a hydrophilic polymer which makes up from about 5 wt. % to
about 30 wt. % of the enzyme domain.
Inventors: |
Hughes; Jonathan; (Carlsbad,
CA) ; Boock; Robert J.; (Carlsbad, CA) ;
Dring; Chris W.; (Fremont, CA) ; Zhang; Huashi;
(San Diego, CA) ; Wu; Mark; (San Diego, CA)
; Sze; David; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DexCom, Inc. |
San Diego |
CA |
US |
|
|
Family ID: |
51530362 |
Appl. No.: |
14/841368 |
Filed: |
August 31, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13836530 |
Mar 15, 2013 |
|
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|
14841368 |
|
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Current U.S.
Class: |
600/347 ;
600/348 |
Current CPC
Class: |
A61B 5/14532 20130101;
A61B 5/14865 20130101; A61B 5/14546 20130101 |
International
Class: |
A61B 5/1486 20060101
A61B005/1486; A61B 5/145 20060101 A61B005/145 |
Claims
1-20. (canceled)
21. A device for measurement of an analyte concentration, the
device comprising: an in vivo sensor configured to generate a
signal indicative of a concentration of an analyte; and a membrane
located over the sensor, the membrane comprising: a first domain
that interfaces with tissue and/or a biological fluid; a second
domain, wherein the second domain comprises an enzyme and a
zwitterion.
22. The device of claim 21, wherein the zwitterion comprises a
betaine or a derivative thereof.
23. The device of claim 22, wherein the betaine or derivative
thereof is selected from the group consisting of cocamidopropyl
betaine, oleamidopropyl betaine, lauryl sulfobetaine, myristyl
sulfobetaine, betaine, octyl betaine, glycine betaine,
poly(carboxybetaine), poly(sulfobetaine), and derivatives
thereof.
24. The device of claim 21, wherein the second domain comprises a
base polymer.
25. The device of claim 25, where the base polymer is cross-linked
with another base polymer.
26. The device of claim 25, wherein the base polymer is selected
from the group consisting of epoxies, polyolefins, polysiloxanes,
polyethers, acrylics, polyesters, carbonates, and
polyurethanes.
27. The device of claim 25, wherein the polyurethanes comprise
polyurethane copolymers.
28. The device of claim 21, wherein the enzyme is selected from the
group consisting of glucose oxidase, glucose dehydrogenase,
galactose oxidase, cholesterol oxidase, amino acid oxidase, alcohol
oxidase, lactate oxidase, and uricase.
29. The device of claim 21, wherein the second domain is from about
0.05 micron to about 5 microns.
30. The device of claim 21, wherein the sensor comprises an
electrode.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[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. 13/836,530, filed Mar. 15, 2013. The
aforementioned application is incorporated by reference herein in
its entirety, and is hereby expressly made a part of this
specification.
FIELD OF THE INVENTION
[0002] Devices are presented for measurement of an analyte
concentration. The devices comprise: a sensor configured to
generate a signal indicative of a concentration of an analyte; and
a sensing membrane located over the sensor. The sensing membrane
comprises an enzyme domain comprising an enzyme, a base polymer,
and a hydrophilic polymer which makes up from about 5 wt. % to
about 30 wt. % of the enzyme domain.
BACKGROUND OF THE INVENTION
[0003] Electrochemical sensors are useful in chemistry and medicine
to determine the presence or concentration of a biological analyte.
Such sensors are useful, for example, to monitor glucose in
diabetic patients and lactate during critical care events. A
variety of intravascular, transcutaneous and implantable sensors
have been developed for continuously detecting and quantifying
blood analytes, such as blood glucose levels.
[0004] However, performance of enzymatic glucose sensors is
affected by the amount of oxygen present at the electrode surface.
For example, in enzymatic glucose sensors which rely on oxygen as
an electron mediator, sensor signal decreases under low oxygen
conditions (such as at about 0.25 mg/L or lower) for the same
glucose concentration. Unfortunately, these performance issues are
amplified in sensors with increased glucose sensitivity. As
enzymatic glucose sensors capable of increased glucose sensitivity
are developed, there is a desire for improved sensor performance
under low oxygen conditions.
[0005] Additionally, the enzymes used in enzymatic glucose sensors
are sensitive to temperature and pH degradation. Thus, the
processes for manufacturing enzymatic glucose sensors are required
to be conducted under pH and temperature conditions which preserve
enzymatic activity. For example, polymer membrane curing that
occurs after application of an enzyme layer is limited to
temperatures at which the enzyme does not degrade, which are
typically well below preferred curing temperatures for the polymer.
Curing polymer membranes at these restricted temperatures extends
curing time, increasing cost and limiting throughput. Accordingly,
there is a desire for enzyme stabilization in enzymatic glucose
sensors so as to provide enhancement in the tolerable pH range and
increased thermal stability in order to decrease manufacturing time
and cost.
SUMMARY OF THE INVENTION
[0006] In a first aspect, a device is provided for measurement of
an analyte concentration, the device comprising: a sensor
configured to generate a signal indicative of a concentration of an
analyte; and a sensing membrane located over the sensor, the
sensing membrane comprising an enzyme domain comprising an enzyme,
a base polymer, and a hydrophilic polymer. In devices of the first
aspect, the hydrophilic polymer comprises from about 5 wt. % to
about 30 wt. % of the enzyme domain, such as about 10 wt. % to
about 25 wt. % of the enzyme domain, such as from about 15 wt. % to
about 20 wt. % of the enzyme domain.
[0007] In some embodiments, the hydrophilic polymer is selected
from the group consisting of poly-N-vinylpyrrolidone (PVP),
poly(ethylene glycol) (PEG), polyacrylamide, acetates, polyethylene
oxide (PEO), polyethylacrylate (PEA),
poly-N-vinyl-3-ethyl-2-pyrrolidone,
poly-N-vinyl-4,5-dimethyl-2-pyrrolidone,
poly-N,N-dimethylacrylamide, polyvinyl alcohol, polyvinyl acetate,
polymers with pendent ionizable groups and copolymers or blends
thereof. In some embodiments, the hydrophilic polymer comprises
poly-N-vinylpyrrolidone (PVP).
[0008] In some embodiments, the enzyme is selected from the group
consisting of glucose oxidase, glucose dehydrogenase, galactose
oxidase, cholesterol oxidase, amino acid oxidase, alcohol oxidase,
lactate oxidase, and uricase. In some embodiments, the enzyme is
glucose oxidase.
[0009] In some embodiments, the base polymer comprises at least one
polymer selected from the group consisting of epoxies, polyolefins,
polysiloxanes, polyethers, acrylics, polyesters, carbonates, and
polyurethanes. In some related embodiments, the polyurethanes
comprise a polyurethane copolymer. In some embodiments, the base
polymer comprises polyurethane.
[0010] In some embodiments, the enzyme domain further comprises a
cross-linking agent in an amount sufficient to induce cross-linking
between polymer molecules. In some related embodiments, the
cross-linking agent comprises a cross-linking agent selected from
the group consisting of isocyanate, carbodiimide, gluteraldehyde or
other aldehydes, epoxy, acrylates, free-radical based agents,
ethylene glycol diglycidyl ether (EGDE), poly(ethylene glycol)
diglycidyl ether (PEGDE), and dicumyl peroxide (DCP). In other
related embodiments, the cross-linking agent comprises from about
0.1 wt. % to about 15 wt. % of the total dry weight of the enzyme,
cross-linking agent, and polymers.
[0011] In some embodiments, the thickness of the enzyme domain is
from about 0.05 micron to about 100 microns.
[0012] In some embodiments, the sensor comprises an electrode.
[0013] In some embodiments, the sensing membrane further comprises
a resistance domain configured to control a flux of said analyte
therethrough.
[0014] In some embodiments, the sensing membrane further comprises
an interference domain located more proximal to the sensor than the
enzyme domain, wherein the interference domain comprises at least
about 25% silicone by weight.
[0015] In some embodiments, the device is configured for continuous
measurement of an analyte concentration. In some embodiments, the
device is configured for in vivo intermittent or continuous
measurement of an analyte concentration.
[0016] In some embodiments, the device is configured for glucose
measurement, including continuous glucose measurement. In some
embodiments, the device is configured for in vivo intermittent or
continuous glucose measurement.
[0017] In a second aspect, a device is provided for measurement of
an analyte concentration, the device comprising: a sensor
configured to generate a signal indicative of a concentration of an
analyte; and a sensing membrane located over the sensor, the
sensing membrane comprising an enzyme domain comprising an enzyme,
a base polymer, and a dipolar enzyme stabilizing agent.
[0018] In some embodiments, the enzyme stabilizing agent reduces
thermal enzyme degradation, pH-related enzyme degradation, or
both.
[0019] In some embodiments, the dipolar enzyme stabilizing agent
comprises a zwitterionic enzyme stabilizing agent. In some related
embodiments, the zwitterionic enzyme stabilizing agent comprises a
zwitterionic compound, precursor, or derivative thereof. In further
related embodiments, the zwitterionic enzyme stabilizing agent
comprises a betaine compound or derivative thereof, such as a
carboxyl, sulfo, or phosphor betaine compound, precursor, or
derivative thereof. In some embodiments, the zwitterionic enzyme
stabilizing agent comprises one or more selected from the group
consisting of cocamidopropyl betaine, oleamidopropyl betaine,
lauryl sulfobetaine, myristyl sulfobetaine, betaine
(trimethylglycine), octyl betaine, phosphatidylcholine, glycine
betaine, poly(carboxybetaine) (pCB), poly(sulfobetaine) (pSB), and
precursors or derivatives thereof. In some embodiments, the one or
more the zwitterionic compounds or derivatives thereof comprise one
or more selected from the group consisting of poly(carboxybetaine)
(pCB), poly(sulfobetaine) (pSB), and precursors or derivatives
thereof.
[0020] In some embodiments, the zwitterionic enzyme stabilizing
agent comprises a betaine or derivative thereof, such as glycine
betaine. In some embodiments, the betaine or derivative thereof is
a hydrolyzable cationic betaine ester. In some related embodiments,
the hydrolyzable cationic betaine ester is a cationic
poly(carboxybetaine) (pCB) ester.
[0021] It will be appreciated that the above listing of
zwitterionic compounds is by no means complete, and is not intended
to be limiting. It is intended that other suitable zwitterionic
compounds may be recognized by those of skill in the art. By way of
further example, additional zwitterionic compounds, precursors, or
derivatives thereof may include one or more selected from the group
consisting of phosphorylcholine, phosphoryl ethanolamine,
phosphatidyl ethanolamine, phosphoethanolamine, phosphatidyl
serine, and precursors or derivatives thereof.
[0022] In some embodiments, the dipolar enzyme stabilizing agent
comprises a non-zwitterionic enzyme stabilizing agent. In some
related embodiments, the dipolar enzyme stabilizing agent comprises
an amine oxide.
[0023] In some embodiments, the dipolar enzyme stabilizing agent is
coated on the surface of said enzyme domain. In some embodiments,
the dipolar enzyme stabilizing agent is dispersed throughout the
enzyme domain. In some embodiments, the amount of dipolar enzyme
stabilizing agent is less than or equal to about the dry weight of
enzyme used in the enzyme domain, such as less than or equal 90%,
80%, 70%, 60%, 50%, 40%, 30%, 25%, 20%, 15%, 10%, 5%, 2%, or 1%, of
the dry weight of enzyme used in the enzyme domain.
[0024] In some embodiments, the enzyme is selected from the group
consisting of glucose oxidase, glucose dehydrogenase, galactose
oxidase, cholesterol oxidase, amino acid oxidase, alcohol oxidase,
lactate oxidase, and uricase. In some embodiments, the enzyme is
glucose oxidase.
[0025] In some embodiments, the base polymer comprises at least one
polymer selected from the group consisting of epoxies, polyolefins,
polysiloxanes, polyethers, acrylics, polyesters, carbonates, and
polyurethanes. In some related embodiments, the polyurethanes
comprise a polyurethane copolymer. In some embodiments, the base
polymer comprises a polyurethane.
[0026] In some embodiments, the enzyme domain further comprises a
cross-linking agent in an amount sufficient to induce cross-linking
between polymer molecules. In some related embodiments, the
cross-linking agent comprises a cross-linking agent selected from
the group consisting of isocyanate, carbodiimide, gluteraldehyde or
other aldehydes, epoxy, acrylates, free-radical based agents,
ethylene glycol diglycidyl ether (EGDE), poly(ethylene glycol)
diglycidyl ether (PEGDE), and dicumyl peroxide (DCP). In other
related embodiments, the cross-linking agent comprises from about
0.1 wt. % to about 15 wt. % of the total dry weight of the enzyme,
cross-linking agent, and polymers.
[0027] In some embodiments, the thickness of the enzyme domain is
from about 0.05 micron to about 100 microns.
[0028] In some embodiments, the sensor comprises an electrode.
[0029] In some embodiments, the sensing membrane further comprises
a resistance domain configured to control a flux of said analyte
therethrough.
[0030] In some embodiments, the sensing membrane further comprises
an interference domain located more proximal to the sensor than the
enzyme domain, wherein the interference domain comprises at least
about 25% silicone by weight.
[0031] In some embodiments, the device is configured for continuous
measurement of an analyte concentration. In some embodiments, the
device is configured for in vivo intermittent or continuous
measurement of an analyte concentration.
[0032] In some embodiments, the device is configured for glucose
measurement, including continuous glucose measurement. In some
embodiments, the device is configured for in vivo intermittent or
continuous glucose measurement.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1A is a side-view schematic illustrating an in vivo
portion of an analyte sensor, in one embodiment.
[0034] FIG. 1B is a perspective-view schematic illustrating an in
vivo portion of an analyte sensor, in one embodiment.
[0035] FIG. 1C is a side-view schematic illustrating an in vivo
portion of an analyte sensor, in another embodiment.
[0036] FIGS. 2A-2C are cross-sectional views through the sensor of
FIG. 1 on line 2-2, illustrating various embodiments of the
membrane system.
[0037] FIG. 3 is a graph illustrating the components of a signal
measured by a glucose sensor (after sensor break-in was complete),
in a non-diabetic volunteer host.
[0038] FIG. 4 shows comparative data demonstrating the relative
difference in signal in two specific embodiments of the instant
invention as compared to a prior art device in low oxygen (i.e., at
0.25 mg/L oxygen) and ambient oxygen environments at different
glucose sensitivities. Details are provided in Example 2.
[0039] FIG. 5 shows comparative data demonstrating the effect of
incremental increase of PVP content in the enzyme domain of a
glucose sensor of one embodiment on the signal difference from the
sensor in a low oxygen environment (i.e., at 0.25 mg/L oxygen)
relative to ambient oxygen conditions.
[0040] FIG. 6 shows comparative data demonstrating the effect of
increasing PVP content in the enzyme domain of a glucose sensor of
one embodiment on the signal difference from the sensor in a low
oxygen environment (i.e., at 0.25 mg/L oxygen) relative to ambient
oxygen conditions. Details are provided in Example 3.
DETAILED DESCRIPTION
[0041] The following description and examples describe in detail
some exemplary embodiments of devices and methods for providing
measurement of an analyte concentration. It should be appreciated
that there are numerous variations and modifications of the devices
and methods described herein that are encompassed by the present
invention. Accordingly, the description of a certain exemplary
embodiment should not be deemed to limit the scope of the present
invention.
DEFINITIONS
[0042] In order to facilitate an understanding of the devices and
methods described herein, a number of terms are defined below.
[0043] 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 fluid (for example, blood,
interstitial fluid, cerebral spinal fluid, lymph fluid, urine,
sweat, saliva, etc.) that can be analyzed. Analytes can include
naturally occurring substances, artificial substances, metabolites,
or reaction products. In some embodiments, the analyte for
measurement by the sensing regions, devices, and methods is
glucose. However, other analytes are contemplated as well,
including, but not limited to: 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; lead; lipoproteins ((a), B/A-1, .beta.); lysozyme;
mefloquine; netilmicin; phenobarbitone; phenyloin;
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 or endogenous, for example, a
metabolic product, a hormone, an antigen, an antibody, and the
like. Alternatively, the analyte can be introduced into the body or
exogenous, 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), and
5-hydroxyindoleacetic acid (FHIAA).
[0044] The phrase `continuous (or continual) analyte sensing` 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 period in which monitoring of analyte
concentration is continuously, continually, and or intermittently
(but regularly) performed, for example, about every 5 to 10
minutes.
[0045] The terms `operable connection`, `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 another component(s) in a manner 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 convert that information into a signal; the signal can
then be transmitted to a circuit. In this case, the electrode is
`operably linked` to the electronic circuitry.
[0046] 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 animals (e.g.
humans) and plants.
[0047] 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 the surface
of an electrode where an electrochemical reaction takes place. As
one example, in a working electrode, H.sub.2O.sub.2 (hydrogen
peroxide) produced by an enzyme-catalyzed reaction of an analyte
being detected reacts and thereby creates a measurable electric
current. For example, in the detection of glucose, glucose oxidase
produces H.sub.2O.sub.2 as a byproduct. The H.sub.2O.sub.2 reacts
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. In
the case of the counter electrode, a reducible species, for
example, O.sub.2 is reduced at the electrode surface in order to
balance the current being generated by the working electrode.
[0048] The terms `sensing region,` `sensor,` 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.
[0049] The terms `raw data stream` 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 directly related to the
measured glucose concentration from the glucose sensor. In one
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 a glucose concentration. The terms broadly
encompass a plurality of time spaced data points from a
substantially continuous glucose sensor, 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.
[0050] The term `counts` 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. In one example, a raw data stream
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 another example, counter
electrode voltage measured in counts is directly related to a
voltage.
[0051] 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.
[0052] The phrase `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. For example, some embodiments of a sensor
include a membrane system having a bioprotective domain and an
enzyme domain. If the sensor is deemed to be the point of reference
and the bioprotective domain is positioned farther from the sensor
than the enzyme domain, then the bioprotective domain is more
distal to the sensor than the enzyme domain.
[0053] The phrase `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. For example, some embodiments of a device
include a membrane system having a bioprotective domain and an
enzyme domain. If the sensor is deemed to be the point of reference
and the enzyme domain is positioned nearer to the sensor than the
bioprotective domain, then the enzyme domain is more proximal to
the sensor than the bioprotective domain.
[0054] 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.
[0055] 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.
[0056] The terms `sensing 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.
[0057] 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 the component
of an analyte sensor signal that is not related to the analyte
concentration. In one example of a glucose sensor, the baseline is
composed substantially of signal contribution due to factors other
than glucose (for example, interfering species,
non-reaction-related hydrogen peroxide, or other electroactive
species with an oxidation potential that overlaps with hydrogen
peroxide). In some embodiments wherein a calibration is defined by
solving for the equation y=mx+b, the value of b represents the
baseline of the signal.
[0058] 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.
[0059] The term `hydrophilic polymer" 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 customize meaning), and refer without limitation to
polymers which will absorb in atmospheric conditions more than
about 30% of its weight in water under common hydrophilicity test
conditions. A common measure of hydrophilicity of polymers is water
absorption by the bulk polymer within 24 hours or at equilibrium,
as detailed in ASTM D570 (standard method to measure water
absorption by polymers).
[0060] The term `dipole` or `dipolar compound` 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 refer without limitation
to compounds in which a neutral molecule of the compound has a
positive and negative electrical charge at different locations
within the molecule. The positive and negative electrical charges
within the molecule can be any non-zero charges up to and including
full unit charges.
[0061] The terms `zwitterion` and `zwitterionic compound` 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 is
not to be limited to a special or customized meaning), and refer
without limitation to compounds in which a neutral molecule of the
compound has a unit positive and unit negative electrical charge at
different locations within the molecule. Such compounds are a type
of dipolar compounds, and are also sometimes known as `inner
salts`.
[0062] A `zwitterion precursor` or `zwitterionic compound
precursor` is any compound that is not itself a zwitterion, but may
become a zwitterion in a final or transition state through chemical
reaction. In some embodiments described herein, devices comprise
zwitterion precursors that may be converted to zwitterions prior to
in vivo implantation of the device. Alternatively, in some
embodiments described herein, devices comprise zwitterion
precursors that may be converted to zwitterions by some chemical
reaction that occurs after in vivo implantation of the device.
[0063] A `zwitterion derivative` or `zwitterionic compound
derivative` is any compound that is not itself a zwitterion, but
rather is the product of a chemical reaction where a zwitterion is
converted to a non-zwitterion. Such reactions may be reversible,
such that under certain conditions zwitterion derivatives may act
as zwitterion precursors. For example, hydrolyzable betaine esters
formed from zwitterionic betaines are cationic zwitterion
derivatives that under the appropriate conditions are capable of
undergoing hydrolysis to revert to zwitterionic betaines.
[0064] The terms `non-zwitterionic dipole` and `non-zwitterionic
dipolar compound` 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 is not to be limited to a special or
customized meaning), and refer without limitation to compounds in
which a neutral molecule of the compound have a positive and
negative electrical charge at different locations within the
molecule. The positive and negative electrical charges within the
molecule can be any non-zero, but less than full unit, charges.
[0065] The term "polyampholytic polymer" 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
polymers comprising both cationic and anionic end groups. Such
polymers may be prepared to have about equal numbers of positive
and negative charges, and thus the surface of such polymers may be
about net neutrally charged. Alternatively, such polymers may be
prepared to have an excess of either positive or negative charges,
and thus the surface of such polymers may be net positively or
negatively charged, respectively.
[0066] 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); .mu.mol (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
[0067] Membrane systems of the various embodiments are suitable for
use with implantable devices in contact with a biological fluid.
For example, the membrane systems can be utilized with implantable
devices, such as devices for monitoring and determining analyte
levels in a biological fluid, for example, devices for monitoring
glucose levels for individuals having diabetes. In some
embodiments, the analyte-measuring device is a continuous device.
The analyte-measuring device can employ any suitable sensing
element to provide the raw signal, including but not limited to
those involving enzymatic, chemical, physical, electrochemical,
spectrophotometric, polarimetric, calorimetric, radiometric,
immunochemical, or like elements.
[0068] Although some of the description that follows is directed at
glucose-measuring devices, including the described membrane systems
and methods for their use, these membrane systems are not limited
to use in devices that measure or monitor glucose. These membrane
systems are suitable for use in any of a variety of devices,
including, for example, devices that detect and quantify other
analytes present in biological fluids (e.g., cholesterol, amino
acids, alcohol, galactose, and lactate), cell transplantation
devices (see, for example, U.S. Pat. No. 6,015,572, U.S. Pat. No.
5,964,745, and U.S. Pat. No. 6,083,523), drug delivery devices
(see, for example, U.S. Pat. No. 5,458,631, U.S. Pat. No.
5,820,589, and U.S. Pat. No. 5,972,369), and the like.
[0069] In one embodiment, the analyte sensor is an implantable
glucose sensor, such as described with reference to U.S. Pat. No.
6,001,067 and U.S. Patent Publication No. US-2005-0027463-A1. In
another embodiment, the analyte sensor is a glucose sensor, such as
described with reference to U.S. Patent Publication No.
US-2006-0020187-A1. In still other embodiments, the sensor is
configured to be implanted in a host vessel or extra-corporeally,
such as is described in U.S. Patent Publication No.
US-2007-0027385-A1, U.S. Patent Publication No. US-2008-0119703-A1,
U.S. Patent Publication No. US-2008-0108942-A1, and U.S. Patent
Publication No. US-2007-0197890-A1. In some embodiments, the sensor
is configured as a dual-electrode sensor, such as described in U.S.
Patent Publication No. US-2005-0143635-A1, U.S. Patent Publication
No. US-2007-0027385-A1, U.S. Patent Publication No.
US-2007-0213611-A1, and U.S. Patent Publication No.
US-2008-0083617-A1. In one alternative embodiment, the continuous
glucose sensor comprises a sensor such as described in U.S. Pat.
No. 6,565,509 to Say et al., for example. In another alternative
embodiment, the continuous glucose sensor comprises a subcutaneous
sensor such as described with reference to U.S. Pat. No. 6,579,690
to Bonnecaze et al. or U.S. Pat. No. 6,484,046 to Say et al., for
example. In another alternative embodiment, the continuous glucose
sensor comprises a refillable subcutaneous sensor such as described
with reference to U.S. Pat. No. 6,512,939 to Colvin et al., for
example. In yet another alternative embodiment, the continuous
glucose sensor comprises an intravascular sensor such as described
with reference to U.S. Pat. No. 6,477,395 to Schulman et al., for
example. In another alternative embodiment, the continuous glucose
sensor comprises an intravascular sensor such as described with
reference to U.S. Pat. No. 6,424,847 to Mastrototaro et al. In some
embodiments, the electrode system can be used with any of a variety
of known in vivo analyte sensors or monitors, such as U.S. Pat. No.
7,157,528 to Ward; U.S. Pat. No. 6,212,416 to Ward et al.; U.S.
Pat. No. 6,119,028 to Schulman et al.; U.S. Pat. No. 6,400,974 to
Lesho; U.S. Pat. No. 6,595,919 to Berner et al.; U.S. Pat. No.
6,141,573 to Kurnik et al.; U.S. Pat. No. 6,122,536 to Sun et al.;
European Patent Application EP 1153571 to Varall et al.; U.S. Pat.
No. 6,512,939 to Colvin et al.; U.S. Pat. No. 5,605,152 to Slate et
al.; U.S. Pat. No. 4,431,004 to Bessman et al.; U.S. Pat. No.
4,703,756 to Gough et al.; U.S. Pat. No. 6,514,718 to Heller et
al.; U.S. Pat. No. 5,985,129 to Gough et al.; WO patent application
Publication Ser. No. 04/021,877 to Caduff; U.S. Pat. No. 5,494,562
to Maley et al.; U.S. Pat. No. 6,120,676 to Heller et al.; and U.S.
Pat. No. 6,542,765 to Guy et al. In general, it is understood that
the disclosed embodiments are applicable to a variety of continuous
analyte measuring device configurations. In some embodiments, a
long term sensor (e.g., wholly implantable or intravascular) is
configured and arranged to function for a time period of from about
30 days or less to about one year or more (e.g., a sensor session).
In some embodiments, a short term sensor (e.g., one that is
transcutaneous or intravascular) is configured and arranged to
function for a time period of from about a few hours to about 30
days, including a time period of about 1, 2, 3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,
27, 28 or 29 days (e.g., a sensor session). As used herein, the
term `sensor session` is a broad term and refers without limitation
to the period of time the sensor is applied to (e.g., implanted in)
the host or is being used to obtain sensor values. For example, in
some embodiments, a sensor session extends from the time of sensor
implantation (e.g., including insertion of the sensor into
subcutaneous tissue and placing the sensor into fluid communication
with a host's circulatory system) to the time when the sensor is
removed.
Exemplary Glucose Sensor Configurations
[0070] FIGS. 1A-1C illustrate one exemplary embodiment 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.
[0071] 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-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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] As illustrated in FIGS. 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.
[0076] 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.
[0077] In some embodiments, the silver grain in the Ag/AgCl
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 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
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 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.
[0078] 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.
[0079] 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.
[0080] 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 resulting from
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.
[0081] 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.
[0082] As illustrated in FIG. 1C, the sensor also includes a
membrane 108 covering at least a portion of the working electrode.
Membranes are discussed in detail in greater detail elsewhere
herein, for example, with reference to FIGS. 2A-2C.
[0083] In embodiments wherein an outer insulator is disposed, a
portion of the coated assembly structure can be stripped or
otherwise removed, for example, by hand, excimer lasing, chemical
etching, laser ablation, grit-blasting, or the like, to expose the
electroactive surfaces. Alternatively, a portion of the electrode
can be masked prior to depositing the insulator in order to
maintain an exposed electroactive surface area.
[0084] In some embodiments, a radial window is formed through the
insulating material to expose a circumferential electroactive
surface of the working electrode. Additionally, sections of
electroactive surface of the reference electrode are 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. However, when the exposed electroactive surface is
distributed circumferentially about the sensor (e.g. 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.
[0085] In some alternative embodiments, additional electrodes can
be included within the assembly, for example, a three-electrode
system (working, reference, and counter electrodes) and 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. Pat. No. 7,081,195, U.S. Patent Publication No.
US-2005-0143635-A1 and U.S. Patent Publication No.
US-2007-0027385-A1, each of which are incorporated herein by
reference, describe some systems and methods for implementing and
using additional working, counter, and reference electrodes. In one
implementation wherein the sensor comprises two working electrodes,
the two working electrodes are juxtapositioned, 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 signals, and the additional working
electrode is configured to measure a baseline signal consisting of
the baseline signal only. In these embodiments, the second working
electrode may 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, i.e., 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. Patent Publication No.
US-2005-0143635-A1, U.S. Patent Publication No. US-2007-0027385-A1,
and U.S. Patent Publication No. US-2007-0213611-A1, and U.S. Patent
Publication No. US-2008-0083617-A1, which are incorporated herein
by reference in their entirety.
[0086] It has been found that in some electrode systems involving
two working electrodes, i.e., in some dual-electrode systems, the
working electrodes may sometimes be slightly different from each
other. For instance, two working electrodes, even when manufactured
from a single facility may slightly differ in thickness or
permeability because of the electrodes' high sensitivity to
environmental conditions (e.g. temperature, humidity) during
fabrication. Accordingly, the working electrodes of a
dual-electrode system may sometimes have varying diffusion,
membrane thickness, and diffusion characteristics. As a result, the
above-described difference signal (i.e., a glucose-only signal,
generated from subtracting the baseline signal from the first
signal) may not be completely accurate. To mitigate this, it is
contemplated that in some dual-electrode systems, both working
electrodes may be fabricated with one or more membranes that each
includes a bioprotective layer, which is described in more detail
elsewhere herein.
[0087] It is contemplated that the sensing region may include any
of a variety of electrode configurations. For example, in some
embodiments, in addition to one or more glucose-measuring working
electrodes, the sensing region may also include a reference
electrode or other electrodes associated with the working
electrode. In these particular embodiments, the sensing region may
also include a separate reference or counter electrode associated
with one or more optional auxiliary working electrodes. In other
embodiments, the sensing region may include a glucose-measuring
working electrode, an auxiliary working electrode, two counter
electrodes (one for each working electrode), and one shared
reference electrode. In yet other embodiments, the sensing region
may include a glucose-measuring working electrode, an auxiliary
working electrode, two reference electrodes, and one shared counter
electrode.
[0088] U.S. Patent Publication No. US-2008-0119703-A1 and U. S.
Patent Publication No. US-2005-0245799-A1 describe additional
configurations for using the continuous sensor in different body
locations. In some embodiments, the sensor is configured for
transcutaneous implantation in the host. In alternative
embodiments, the sensor is configured for insertion into the
circulatory system, such as a peripheral vein or artery. However,
in other embodiments, the sensor is configured for insertion into
the central circulatory system, such as but not limited to the vena
cava. In still other embodiments, the sensor can be placed in an
extracorporeal circulation system, such as but not limited to an
intravascular access device providing extracorporeal access to a
blood vessel, an intravenous fluid infusion system, an
extracorporeal blood chemistry analysis device, a dialysis machine,
a heart-lung machine (i.e., a device used to provide blood
circulation and oxygenation while the heart is stopped during heart
surgery), etc. In still other embodiments, the sensor can be
configured to be wholly implantable, as described in U.S. Pat. No.
6,001,067.
[0089] FIG. 2A is a cross-sectional view through the sensor of FIG.
1A on line 2-2, illustrating one embodiment of the membrane system
208. In this particular embodiment, the membrane system includes an
interference domain 242, an enzyme domain 244, and a diffusion
resistance domain 246 located around the working electrode 238, all
of which are described in more detail elsewhere herein.
[0090] As illustrated in FIG. 2B, in some embodiments, the membrane
system may include a bioprotective domain 248, also referred to as
a cell-impermeable domain or biointerface domain, comprising a
surface-modified base polymer as described in more detail elsewhere
herein. In some embodiments, a unitary diffusion resistance domain
and bioprotective domain may be included in the membrane system
(e.g., wherein the functionality of both domains is incorporated
into one domain, i.e., the bioprotective domain). In some
embodiments, the sensor is configured for short-term implantation
(e.g., from about 1 to 30 days). However, it is understood that the
membrane system 208 can be modified for use in other devices, for
example, by including only one or more of the domains, or
additional domains.
[0091] As illustrated in FIG. 2C, in some embodiments, the membrane
system may include an electrode domain 236. The electrode domain
236 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 may be
present to provide an environment between the surfaces of the
working electrode and the reference electrode, which facilitates an
electrochemical reaction between the electrodes.
[0092] A wide variety of configurations and combinations for the
various layers in the membrane system are encompassed by the
preferred embodiments. In various embodiments, any of the domains
illustrated in FIGS. 2A-2C may be omitted, altered, substituted
for, and/or incorporated together without departing from the spirit
of the preferred embodiments. It is to be understood that sensing
membranes modified for other sensors, for example, may include
fewer or additional layers. For example, in some embodiments, the
membrane system may comprise one electrode layer, one enzyme layer,
and two bioprotective layers, but in other embodiments, the
membrane system may comprise one electrode layer, two enzyme
layers, and one bioprotective layer. In some embodiments, the
bioprotective layer may be configured to function as the diffusion
resistance domain and control the flux of the analyte (e.g.,
glucose) to the underlying membrane layers.
[0093] In some embodiments, one or more domains of the sensing
membranes may 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.
[0094] In some embodiments, the sensing membrane 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 sensing membrane located over the working electrode does
not have to have the same structure as the sensing membrane located
over the reference electrode; for example, the enzyme domain
deposited over the working electrode does not necessarily need to
be deposited over the reference or counter electrodes.
[0095] Although the exemplary embodiments illustrated in FIGS.
2A-2C 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.
Sensor Electronics
[0096] In general, analyte sensor systems have electronics
associated therewith, also referred to as a `computer system` that
can include hardware, firmware, or software that enable measurement
and processing of data indicative of analyte levels in the host. In
one exemplary embodiment of an electrochemical sensor, the
electronics include a potentiostat, a power source for providing
power to the sensor, and other components useful for signal
processing. In additional embodiments, some or all of the
electronics can be in wired or wireless communication with the
sensor or other portions of the electronics. For example, a
potentiostat disposed on the device can be wired to the remaining
electronics (e.g. a processor, a recorder, a transmitter, a
receiver, etc.), which reside on the bedside. In another example,
some portion of the electronics is wirelessly connected to another
portion of the electronics (e.g., a receiver), such as by infrared
(IR) or RF. It is contemplated that other embodiments of
electronics may be useful for providing sensor data output, such as
those described in U.S. Patent Publication No. US-2005-0192557-A1,
U.S. Patent Publication No. US-2005-0245795-A1; U.S. Patent
Publication No. US-2005-0245795-A1, and U.S. Patent Publication No.
US-2005-0245795-A1, U.S. Patent Publication No. US-2008-0119703-A1,
and U.S. Patent Publication No. US-2008-0108942-A1, each of which
is incorporated herein by reference in their entirety.
[0097] In one preferred embodiment, a potentiostat is operably
connected to the electrode(s) (such as described elsewhere herein),
which biases the sensor to enable measurement of a current signal
indicative of the analyte concentration in the host (also referred
to as the analog portion). In some embodiments, the potentiostat
includes a resistor that translates the current into voltage. In
some alternative embodiments, a current to frequency converter is
provided that is configured to continuously integrate the measured
current, for example, using a charge counting device. In some
embodiments, the electronics include an A/D converter that
digitizes the analog signal into a digital signal, also referred to
as `counts` for processing. Accordingly, the resulting raw data
stream in counts, also referred to as raw sensor data, is directly
related to the current measured by the potentiostat.
[0098] In general, the electronics include a processor module that
includes the central control unit that controls the processing of
the sensor system. In some embodiments, the processor module
includes a microprocessor, however a computer system other than a
microprocessor can be used to process data as described herein, for
example an ASIC can be used for some or all of the sensor's central
processing. The processor typically provides semi-permanent storage
of data, for example, storing data such as sensor identifier (ID)
and programming to process data streams (for example, programming
for data smoothing or replacement of signal artifacts such as is
described in U.S. Patent Publication No. US-2005-0043598-A1). The
processor additionally can be used for the system's cache memory,
for example for temporarily storing recent sensor data. In some
embodiments, the processor module comprises memory storage
components such as ROM, RAM, dynamic-RAM, static-RAM, non-static
RAM, EEPROM, rewritable ROMs, flash memory, and the like.
[0099] In some embodiments, the processor module comprises a
digital filter, for example, an infinite impulse response (IIR) or
finite impulse response (FIR) filter, configured to smooth the raw
data stream. Generally, digital filters are programmed to filter
data sampled at a predetermined time interval (also referred to as
a sample rate). In some embodiments, wherein the potentiostat is
configured to measure the analyte at discrete time intervals, these
time intervals determine the sample rate of the digital filter. In
some alternative embodiments, wherein the potentiostat is
configured to continuously measure the analyte, for example, using
a current-to-frequency converter as described above, the processor
module can be programmed to request a digital value from the A/D
converter at a predetermined time interval, also referred to as the
acquisition time. In these alternative embodiments, the values
obtained by the processor are advantageously averaged over the
acquisition time due the continuity of the current measurement.
Accordingly, the acquisition time determines the sample rate of the
digital filter.
[0100] In some embodiments, the processor module is configured to
build the data packet for transmission to an outside source, for
example, an RF transmission to a receiver. Generally, the data
packet comprises a plurality of bits that can include a preamble, a
unique identifier identifying the electronics unit, the receiver,
or both, (e.g. sensor ID code), data (e.g. raw data, filtered data,
or an integrated value) or error detection or correction.
Preferably, the data (transmission) packet has a length of from
about 8 bits to about 128 bits, preferably about 48 bits; however,
larger or smaller packets can be desirable in certain embodiments.
The processor module can be configured to transmit any combination
of raw or filtered data. In one exemplary embodiment, the
transmission packet contains a fixed preamble, a unique ID of the
electronics unit, a single five-minute average (e.g. integrated)
sensor data value, and a cyclic redundancy code (CRC).
[0101] In some embodiments, the processor further performs the
processing, such as storing data, analyzing data streams,
calibrating analyte sensor data, estimating analyte values,
comparing estimated analyte values with time corresponding measured
analyte values, analyzing a variation of estimated analyte values,
downloading data, and controlling the user interface by providing
analyte values, prompts, messages, warnings, alarms, and the like.
In such cases, the processor includes hardware and software that
performs the processing described herein, for example flash memory
provides permanent or semi-permanent storage of data, storing data
such as sensor ID, receiver ID, and programming to process data
streams (for example, programming for performing estimation and
other algorithms described elsewhere herein) and random access
memory (RAM) stores the system's cache memory and is helpful in
data processing. Alternatively, some portion of the data processing
(such as described with reference to the processor elsewhere
herein) can be accomplished at another (e.g. remote) processor and
can be configured to be in wired or wireless connection
therewith.
[0102] In some embodiments, an output module, which is integral
with or operatively connected with the processor, includes
programming for generating output based on the data stream received
from the sensor system and it's processing incurred in the
processor. In some embodiments, output is generated via a user
interface.
Noise
[0103] Generally, implantable sensors measure a signal related to
an analyte of interest in a host. For example, an electrochemical
sensor can measure glucose, creatinine, or urea in a host, such as
an animal (e.g. a human). Generally, the signal is converted
mathematically to a numeric value indicative of analyte status,
such as analyte concentration, as described in more detail
elsewhere herein. In general, the signal generated by conventional
analyte sensors contains some noise. Noise is clinically important
because it can induce error and can reduce sensor performance, such
as by providing a signal that causes the analyte concentration to
appear higher or lower than the actual analyte concentration. For
example, upward or high noise (e.g. noise that causes the signal to
increase) can cause the reading of the host's glucose concentration
to appear higher than the actual value, which in turn can lead to
improper treatment decisions. Similarly, downward or low noise
(e.g. noise that causes the signal to decrease) can cause the
reading of the host's glucose concentration to appear lower than
its actual value, which in turn can also lead to improper treatment
decisions. Accordingly, noise reduction is desirable.
[0104] In general, the signal detected by the sensor can be broken
down into its component parts. For example, in an enzymatic
electrochemical analyte sensor, preferably after sensor break-in is
complete, the total signal can be divided into an `analyte
component,` which is representative of analyte (e.g., glucose)
concentration, and a `noise component,` which is caused by
non-analyte-related species that have a redox potential that
substantially overlaps with the redox potential of the analyte (or
measured species, e.g. H.sub.2O.sub.2) at an applied voltage. The
noise component can be further divided into its component parts,
e.g. constant and non-constant noise. It is not unusual for a
sensor to experience a certain level of noise. In general,
`constant noise` (sometimes referred to as constant background or
baseline) is caused by non-analyte-related factors that are
relatively stable over time, including but not limited to
electroactive species that arise from generally constant (e.g.
daily) metabolic processes. Constant noise can vary widely between
hosts. In contrast, `non-constant noise` (sometimes referred to as
non-constant background) is generally caused by non-constant,
non-analyte-related species (e.g. non-constant noise-causing
electroactive species) that may arise during transient events, such
as during host metabolic processes (e.g. wound healing or in
response to an illness), or due to ingestion of certain compounds
(e.g. certain drugs). In some circumstances, noise can be caused by
a variety of noise-causing electroactive species, which are
discussed in detail elsewhere herein.
[0105] FIG. 3 is a graph illustrating the components of a signal
measured by a transcutaneous glucose sensor (after sensor break-in
was complete), in a non-diabetic volunteer host. The Y-axis
indicates the signal amplitude (in counts) detected by the sensor.
The total signal collected by the sensor is represented by line
1000, which includes components related to glucose, constant noise,
and non-constant noise, which are described in more detail
elsewhere herein. In some embodiments, the total signal is a raw
data stream, which can include an averaged or integrated signal,
for example, using a charge-counting device.
[0106] The non-constant noise component of the total signal is
represented by line 1010. The non-constant noise component 1010 of
the total signal 1000 can be obtained by filtering the total signal
1000 to obtain a filtered signal 1020 using any of a variety of
known filtering techniques, and then subtracting the filtered
signal 1020 from the total signal 1000. In some embodiments, the
total signal can be filtered using linear regression analysis of
the n (e.g. 10) most recent sampled sensor values. In some
embodiments, the total signal can be filtered using non-linear
regression. In some embodiments, the total signal can be filtered
using a trimmed regression, which is a linear regression of a
trimmed mean (e.g., after rejecting wide excursions of any point
from the regression line). In this embodiment, after the sensor
records glucose measurements at a predetermined sampling rate
(e.g., every 30 seconds), the sensor calculates a trimmed mean
(e.g., removes highest and lowest measurements from a data set) and
then regresses the remaining measurements to estimate the glucose
value. In some embodiments, the total signal can be filtered using
a non-recursive filter, such as a finite impulse response (FIR)
filter. An FIR filter is a digital signal filter, in which every
sample of output is the weighted sum of past and current samples of
input, using only some finite number of past samples. In some
embodiments, the total signal can be filtered using a recursive
filter, such as an infinite impulse response (IIR) filter. An IIR
filter is a type of digital signal filter, in which every sample of
output is the weighted sum of past and current samples of input. In
some embodiments, the total signal can be filtered using a
maximum-average (max-average) filtering algorithm, which smoothes
data based on the discovery that the substantial majority of signal
artifacts observed after implantation of glucose sensors in humans,
for example, is not distributed evenly above and below the actual
blood glucose levels. It has been observed that many data sets are
actually characterized by extended periods in which the noise
appears to trend downwardly from maximum values with occasional
high spikes. To overcome these downward trending signal artifacts,
the max-average calculation tracks with the highest sensor values,
and discards the bulk of the lower values. Additionally, the
max-average method is designed to reduce the contamination of the
data with unphysiologically high data from the high spikes. The
max-average calculation smoothes data at a sampling interval (e.g.
every 30 seconds) for transmission to the receiver at a less
frequent transmission interval (e.g. every 5 minutes), to minimize
the effects of low non-physiological data. First, the
microprocessor finds and stores a maximum sensor counts value in a
first set of sampled data points (e.g. 5 consecutive, accepted,
thirty-second data points). A frame shift time window finds a
maximum sensor counts value for each set of sampled data (e.g. each
5-point cycle length) and stores each maximum value. The
microprocessor then computes a rolling average (e.g. 5-point
average) of these maxima for each sampling interval (e.g. every 30
seconds) and stores these data. Periodically (e.g. every 10.sup.th
interval), the sensor outputs to the receiver the current maximum
of the rolling average (e.g. over the last 10 thirty-second
intervals as a smoothed value for that time period (e.g. 5
minutes). In some embodiments, the total signal can be filtered
using a `Cone of Possibility Replacement Method,` which utilizes
physiological information along with glucose signal values in order
define a `cone` of physiologically feasible glucose signal values
within a human. Particularly, physiological information depends
upon the physiological parameters obtained from continuous studies
in the literature as well as our own observations. A first
physiological parameter uses a maximal sustained rate of change of
glucose in humans (e.g. about 4 to 6 mg/di/min) and a maximum
sustained acceleration of that rate of change (e.g. about 0.1 to
0.2 mg/min/min). A second physiological parameter uses the
knowledge that rate of change of glucose is lowest at the maxima
and minima, which are the areas of greatest risk in patient
treatment. A third physiological parameter uses the fact that the
best solution for the shape of the curve at any point along the
curve over a certain time period (e.g. about 20-25 minutes) is a
straight line. It is noted that the maximum rate of change can be
narrowed in some instances. Therefore, additional physiological
data can be used to modify the limits imposed upon the Cone of
Possibility Replacement Method for sensor glucose values. For
example, the maximum per minute rate of change can be lower when
the subject is lying down or sleeping; on the other hand, the
maximum per minute rate change can be higher when the subject is
exercising, for example. In some embodiments, the total signal can
be filtered using reference changes in electrode potential to
estimate glucose sensor data during positive detection of signal
artifacts from an electrochemical glucose sensor, the method
hereinafter referred to as reference drift replacement; in this
embodiment, the electrochemical glucose sensor comprises working,
counter, and reference electrodes. This method exploits the
function of the reference electrode as it drifts to compensate for
counter electrode limitations during oxygen deficits, pH changes,
or temperature changes. In alternative implementations of the
reference drift method, a variety of algorithms can therefore be
implemented based on the changes measured in the reference
electrode. Linear algorithms, and the like, are suitable for
interpreting the direct relationship between reference electrode
drift and the non-glucose rate limiting signal noise such that
appropriate conversion to signal noise compensation can be derived.
Additional description of signal filtering can be found in U.S.
Patent Publication No. US-2005-0043598-A1.
[0107] The constant noise signal component 1030 can be obtained by
calibrating the sensor signal using reference data, such as one or
more blood glucose values obtained from a hand-held blood glucose
meter, or the like, from which the baseline `b` of a regression can
be obtained, representing the constant noise signal component
1030.
[0108] The analyte signal component 1040 can be obtained by
subtracting the constant noise signal component 1030 from the
filtered signal 1020.
[0109] In general, non-constant noise is caused by interfering
species (non-constant noise-causing species), which can be
compounds, such as drugs that have been administered to the host,
or intermittently produced products of various host metabolic
processes. Exemplary interferents include but are not limited to a
variety of drugs (e.g. acetaminophen), H.sub.2O2 from exterior
sources (e.g. produced outside the sensor membrane system), and
reactive metabolic species (e.g. reactive oxygen and nitrogen
species, some hormones, etc.). Some known interfering species for a
glucose sensor include but are not limited to acetaminophen,
ascorbic acid, bilirubin, cholesterol, creatinine, dopamine,
ephedrine, ibuprofen, L-dopa, methyldopa, salicylate, tetracycline,
tolazamide, tolbutamide, triglycerides, and uric acid. It has also
been observed that non-constant noise may increase when a host is
intermittently sedentary, such as when sleeping or sitting for
extended periods. This noise may dissipate upon renewed activity by
the host. Additional description of this effect can be found in
U.S. Patent Publication No. US-2009-0247856-A1.
Interferents
[0110] Interferents are molecules or other species that may cause a
sensor to generate a false positive or negative analyte signal
(e.g. a non-analyte-related signal). Some interferents are known to
become reduced or oxidized at the electrochemically reactive
surfaces of the sensor, while other interferents are known to
interfere with the ability of the enzyme (e.g. glucose oxidase)
used to react with the analyte being measured. Yet other
interferents are known to react with the enzyme (e.g. glucose
oxidase) to produce a byproduct that is electrochemically active.
Interferents can exaggerate or mask the response signal, thereby
leading to false or misleading results. For example, a false
positive signal may cause the host's analyte concentration (e.g.,
glucose concentration) to appear higher than the true analyte
concentration. False-positive signals may pose a clinically
significant problem in some conventional sensors. For example in a
severe hypoglycemic situation, in which the host has ingested an
interferent (e.g. acetaminophen), the resulting artificially high
glucose signal can lead the host to believe that he is euglycemic
or hyperglycemic. In response, the host may make inappropriate
treatment decisions, such as by injecting himself with too much
insulin, or by taking no action, when the proper course of action
would be to begin eating. In turn, this inappropriate action or
inaction may lead to a dangerous hypoglycemic episode for the host.
Accordingly, certain embodiments contemplated herein include a
membrane system that substantially reduces or eliminates the
effects of interferents on analyte measurements. These membrane
systems may include one or more domains capable of blocking or
substantially reducing the flow of interferents onto the
electroactive surfaces of the electrode may reduce noise and
improve sensor accuracy as described in more detail in U.S. Patent
Publication No. US-2009-0247856-A1.
Membrane System with Improved Low Oxygen Performance
[0111] Performance of enzymatic glucose sensors which rely on
oxygen as an electron mediator is significantly affected by the
amount of oxygen present at the electrode surface. In these
sensors, oxygen acts as a co-substrate and reduction of oxygen to
hydrogen peroxide is measured as an indication of glucose levels.
However, in order to obtain an accurate measure of the glucose,
oxygen cannot be the limiting reactant. It has been observed that
under low oxygen conditions (such as at about 0.25 mg/L or lower)
signal drift becomes a significant problem, particularly for
sensors with high glucose sensitivity. For example, FIG. 4 shows
increasing signal drift (i.e., a drift toward less signal) with
increasing glucose sensitivity in the presence of about 0.25 mg/L
oxygen. As illustrated in FIG. 4, the low sensitivity sensors under
the above-described low oxygen conditions had negative changes in
signals (about -2.5% to about -4.1%) compared to signals under
ambient oxygen conditions. However, these negative changes in
signal in signal (about -2.5% to about -4.1%) for the low
sensitivity sensors were less severe than the negative changes in
signals (about -4.5% to about -7.4%) for high sensitivity
sensors.
[0112] Clearly, as enzymatic glucose sensors capable of increased
glucose sensitivity are developed, there is a desire for improved
sensor performance under low oxygen conditions.
[0113] As discussed in greater detail below, it has been found that
including one or more hydrophilic polymers in the polymer system of
the enzyme domain results in improved sensor performance (i.e.,
less signal drift) under low oxygen conditions.
Membrane Fabrication
[0114] Preferably, polymers of the preferred embodiments may be
processed by solution-based techniques, for example, dipping,
casting, electrospinning, vapor deposition, spin coating, coating,
and the like. Water-based polymer emulsions can be fabricated to
form membranes by methods similar to those used for solvent-based
materials. In both cases the evaporation of a volatile liquid (e.g.
organic solvent or water) leaves behind a film of the polymer.
Cross-linking of the deposited film may be performed through the
use of multi-functional reactive ingredients by a number of methods
well known to those skilled in the art. The liquid system may cure
by heat, moisture, high-energy radiation, ultraviolet light, or by
completing the reaction, which produces the final polymer in a mold
or on a substrate to be coated.
[0115] However, care must be taken in the fabrication process
because the enzymes used in enzymatic sensors are sensitive to
temperature and pH induced degradation. Thus, fabrication processes
must be conducted under pH and temperature conditions which
preserve enzymatic activity. For example, a process of heat curing
polymer membranes after application of an enzyme layer is limited
to temperatures below that which will denature the enzyme.
Unfortunately, such process temperatures are typically well below
curing temperatures tolerable by the polymer system. The
requirement to cure polymer membranes at these restricted
temperatures extends curing time, increasing cost and limiting
throughput in device fabrication. Accordingly, there is a desire
for enzyme stabilization in enzymatic glucose sensors so as provide
enhancement in the tolerable pH range and increased thermal
stability of the enzyme in order to decrease manufacturing time and
cost.
[0116] Domains that include at least two surface-active
group-containing polymers may be made using any of the methods of
forming polymer blends known in the art. In one exemplary
embodiment, a solution of a polyurethane containing silicone end
groups is mixed with a solution of a polyurethane containing
fluorine end groups (e.g. wherein the solutions include the polymer
dissolved in a suitable solvent such as acetone, ethyl alcohol,
DMAC, THF, 2-butanone, and the like). The mixture can then be drawn
into a film or applied to a surface using any method known in the
art (e.g. spraying, painting, dip coating, vapor depositing,
molding, 3-D printing, lithographic techniques (e.g.
photolithograph), micro- and nano-pipetting printing techniques,
etc.). The mixture can then be cured under high temperature (e.g.
50-150.degree. C.). Other suitable curing methods may include
ultraviolet or gamma radiation, for example.
[0117] Some amount of cross-linking agent can also be included in
the mixture to induce cross-linking between polymer molecules.
Non-limiting examples of suitable cross-linking agents include
isocyanate, carbodiimide, gluteraldehyde or other aldehydes, epoxy,
acrylates, free-radical based agents, ethylene glycol diglycidyl
ether (EGDE), poly(ethylene glycol) diglycidyl ether (PEGDE), or
dicumyl peroxide (DCP). In one embodiment, from about 0.1% to about
15% w/w of cross-linking agent is added relative to the total dry
weights of cross-linking agent and polymers added when blending the
ingredients (in one example, about 1% to about 10%). During the
curing process, substantially all of the cross-linking agent is
believed to react, leaving substantially no detectable unreacted
cross-linking agent in the final film.
Bioprotective Domain
[0118] The bioprotective domain is the domain or layer of an
implantable device configured to interface with (e.g. contact) a
biological fluid when implanted in a host or connected to the host
(e.g. via an intravascular access device providing extracorporeal
access to a blood vessel). The noise reducing capacity of certain
bioprotective domains is described in more detail in U.S. Patent
Publication No. US-2009-0247856-A1.
[0119] Some embodiments described herein may include membranes
which comprise a bioprotective domain 248 (see FIG. 2B), also
referred to as a bioprotective layer, including at least one
polymer containing a surface-active group. In some embodiments, the
surface-active group-containing polymer is a surface-active end
group-containing polymer. In some of these embodiments, the
surface-active end group-containing polymer is a polymer having
covalently bonded surface-active end groups. However, it is
contemplated that other surface-active group-containing polymers
may also be used and can be formed by modification of fully-reacted
base polymers via the grafting of side chain structures, surface
treatments or coatings applied after membrane fabrication (e.g.,
via surface-modifying additives), blending of a surface-modifying
additive to a base polymer before membrane fabrication,
immobilization of the surface-active-group-containing soft segments
by physical entrainment during synthesis, or the like. Certain
exemplary bioprotective domains which may be used in some
embodiments as described herein are described in more detail in
U.S. Patent Publication No. US-2009-0247856-A1.
[0120] Base polymers useful for certain embodiments may include any
linear or branched polymer on the backbone structure of the
polymer. Suitable base polymers may include, but are not limited
to, epoxies, polyolefins, polysiloxanes, polyethers, acrylics,
polyesters, carbonates, and polyurethanes, wherein polyurethanes
may include polyurethane copolymers such as
polyether-urethane-urea, polycarbonate-urethane,
polyether-urethane, silicone-polyether-urethane,
silicone-polycarbonate-urethane, polyester-urethane, and the like.
In some embodiments, base polymers may be selected for their bulk
properties, such as, but not limited to, tensile strength, flex
life, modulus, and the like. For example, polyurethanes are known
to be relatively strong and to provide numerous reactive pathways,
which properties may be advantageous as bulk properties for a
membrane domain of the continuous sensor.
[0121] In some embodiments, a base polymer synthesized to have
hydrophilic segments may be used to form the bioprotective layer.
For example, a linear base polymer including biocompatible
segmented block polyurethane copolymers comprising hard and soft
segments may be used. In some embodiments, the hard segment of the
copolymer may have a molecular weight of from about 160 daltons to
about 10,000 daltons, and sometimes from about 200 daltons to about
2,000 daltons. In some embodiments, the molecular weight of the
soft segment may be from about 200 daltons to about 10,000,000
daltons, and sometimes from about 500 daltons to about 5,000,000
daltons, and sometimes from about 500,00 daltons to about 2,000,000
daltons. It is contemplated that polyisocyanates used for the
preparation of the hard segments of the copolymer may be aromatic
or aliphatic diisocyanates. The soft segments used in the
preparation of the polyurethane may be a polyfunctional aliphatic
polyol, a polyfunctional aliphatic or aromatic amine, or the like
that may be useful for creating permeability of the analyte (e.g.
glucose) therethrough, and may include, for example, polyvinyl
acetate (PVA), poly(ethylene glycol) (PEG), polyacrylamide,
acetates, polyethylene oxide (PEO), polyethylacrylate (PEA),
polyvinylpyrrolidone (PVP), and variations thereof (e.g. PVP vinyl
acetate), and wherein PVP and variations thereof may be preferred
for their hydrolytic stability in some embodiments.
[0122] Alternatively, in some embodiments, the bioprotective layer
may comprise a combination of a base polymer (e.g. polyurethane)
and one or more hydrophilic polymers, such as, PVA, PEG,
polyacrylamide, acetates, PEO, PEA, PVP, and variations thereof
(e.g. PVP vinyl acetate), e.g. as a physical blend or admixture
wherein each polymer maintains its unique chemical nature. It is
contemplated that any of a variety of combination of polymers may
be used to yield a blend with desired glucose, oxygen, and
interference permeability properties. For example, in some
embodiments, the bioprotective layer may be formed from a blend of
a polycarbonate-urethane base polymer and PVP, but in other
embodiments, a blend of a polyurethane, or another base polymer,
and one or more hydrophilic polymers may be used instead. In some
of the embodiments involving use of PVP, the PVP portion of the
polymer blend may comprise from about 5% to about 50% by weight of
the polymer blend, sometimes from about 15% to 20%, and other times
from about 25% to 40%. It is contemplated that PVP of various
molecular weights may be used. For example, in some embodiments,
the molecular weight of the PVP used may be from about 25,000
daltons to about 5,000,000 daltons, sometimes from about 50,000
daltons to about 2,000,000 daltons, and other times from 6,000,000
daltons to about 10,000,000 daltons.
[0123] The term `surface-active group` and `surface-active end
group` as used herein are broad terms and are used in their
ordinary sense, including, without limitation, surface-active
oligomers or other surface-active moieties having surface-active
properties, such as alkyl groups, which preferentially migrate
towards a surface of a membrane formed there from. Surface-active
groups preferentially migrate toward air (e.g., driven by
thermodynamic properties during membrane formation). In some
embodiments, the surface-active groups are covalently bonded to the
base polymer during synthesis. In some preferred embodiments,
surface-active groups may include silicone, sulfonate, fluorine,
polyethylene oxide, hydrocarbon groups, and the like. The surface
activity (e.g., chemistry, properties) of a membrane domain
including a surface-active group-containing polymer reflects the
surface activity of the surface-active groups rather than that of
the base polymer. In other words, surface-active groups control the
chemistry at the surface (e.g., the biological contacting surface)
of the membrane without compromising the bulk properties of the
base polymer. The surface-active groups of the preferred
embodiments are selected for desirable surface properties, for
example, non-constant noise-blocking ability, break-in time
(reduced), ability to repel charged species, cationic or anionic
blocking, or the like. In some preferred embodiments, the
surface-active groups are located on one or more ends of the
polymer backbone, and referred to as surface-active end groups,
wherein the surface-active end groups are believed to more readily
migrate to the surface of the bioprotective domain/layer formed
from the surface-active group-containing polymer in some
circumstances.
[0124] In some embodiments, the bioprotective domain 248 is formed
from a polymer containing silicone as the surface-active group, for
example, a polyurethane containing silicone end group(s). Some
embodiments include a continuous analyte sensor configured for
insertion into a host, wherein the sensor has a membrane located
over the sensing mechanism, which includes a polyurethane
comprising silicone end groups configured to substantially block
the effect of non-constant noise-causing species on the sensor
signal, as described in more detail elsewhere herein. In some
embodiments, the polymer includes about 10%, 11%, 12%, 13%, 14%,
15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%,
28%, 29%, 30%, to about 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%,
39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%,
52%, 53%, 54% or 55% silicone by weight. In certain embodiments,
the silicone (e.g. a precursor such as PDMS) has a molecular weight
from about 500 to about 10,000 daltons, preferably at least about
200 daltons. In some embodiments, the base polymer includes at
least about 10% silicone by weight, and preferably from about 19%
to about 40% silicone by weight. These ranges are described in U.S.
Patent Publication No. US-2009-0247856-A1 as providing an
advantageous balance of noise-reducing functionality, while
maintaining sufficient glucose permeability in embodiments wherein
the sensor is a glucose sensor, for example.
[0125] In some embodiments, the bioprotective domain is formed from
a polymer containing fluorine as a surface-active group, for
example, a polyurethane that contains a fluorine end groups. In
preferred embodiments, the polymer includes from about 1% to about
25% fluorine by weight. Some embodiments include a continuous
analyte sensor configured for insertion into a host, wherein the
sensor has a membrane located over the sensing mechanism, wherein
the membrane includes a polyurethane containing fluorine
surface-active groups, and wherein the membrane is configured and
arranged to reduce a break-in time of a sensor as compared to a
membrane formed from a similar base polymer without the
surface-active group(s). For example, in preferred embodiments, a
glucose sensor having a bioprotective domain of the preferred
embodiments has a response time (e.g. t.sub.90) of less than 120
seconds, sometimes less than 60 seconds, and sometimes less than
about 45, 30, 20, or 10 seconds (across a physiological range of
glucose concentration).
[0126] In some embodiments, the bioprotective domain may be formed
from a polymer that contains sulfonate as a surface-active group,
for example, a polyurethane containing sulfonate end group(s). In
some embodiments, the continuous analyte sensor configured for
insertion into a host may include a membrane located over the
sensing mechanism, wherein the membrane includes a polymer that
contains sulfonate as a surface-active group, and is configured to
repel charged species, for example, due to the net negative charge
of the sulfonated groups.
[0127] In some embodiments, a blend of two or more (e.g. two,
three, four, five, or more) surface-active group-containing
polymers is used to form a bioprotective membrane domain. For
example, by blending a polyurethane with silicone end groups and a
polyurethane with fluorine end groups, and forming a bioprotective
membrane domain from that blend, a sensor can be configured to
substantially block non-constant noise-causing species and reduce
the sensor's t.sub.90, as described in more detail elsewhere
herein. Similarly, by blending a polyurethane containing silicone
end groups, a polyurethane containing fluorine end groups, and a
polyurethane containing sulfonate end groups, and forming a
bioprotective membrane domain from that blend, a sensor can be
configured to substantially block non-constant noise-causing
species, to reduce the sensor's break-in time and to repel charged
species, as described in more detail above. Although in some
embodiments, blending of two or more surface-active
group-containing polymers is used, in other embodiments, a single
component polymer can be formed by synthesizing two or more
surface-active groups with a base polymer to achieve similarly
advantageous surface properties; however, blending may be preferred
in some embodiments for ease of manufacture.
[0128] In some embodiments, the bioprotective domain 248 is
positioned most distally to the sensing region such that its outer
most domain contacts a biological fluid when inserted in vivo. In
some embodiments, the bioprotective domain is resistant to cellular
attachment, impermeable to cells, and may be composed of a
biostable material. While not wishing to be bound by theory, it is
believed that when the bioprotective domain 248 is resistant to
cellular attachment (for example, attachment by inflammatory cells,
such as macrophages, which are therefore kept a sufficient distance
from other domains, for example, the enzyme domain), hypochlorite
and other oxidizing species are short-lived chemical species in
vivo and biodegradation does not generally occur. Additionally, the
materials preferred for forming the bioprotective domain 248 may be
resistant to the effects of these oxidative species and have thus
been termed biodurable. In some embodiments, the bioprotective
domain controls the flux of oxygen and other analytes (for example,
glucose) to the underlying enzyme domain (e.g. wherein the
functionality of the diffusion resistance domain is built-into the
bioprotective domain such that a separate diffusion resistance
domain is not required).
[0129] In certain embodiments, the thickness of the bioprotective
domain may be from about 0.1, 0.5, 1, 2, 4, 6, 8 microns or less to
about 10, 15, 20, 30, 40, 50, 75, 100, 125, 150, 175, 200 or 250
microns or more. In some of these embodiments, the thickness of the
bioprotective domain may be sometimes from about 1 to about 5
microns, and sometimes from about 2 to about 7 microns. In other
embodiments, the bioprotective domain may be from about 20 or 25
microns to about 50, 55, or 60 microns thick. In some embodiments,
the glucose sensor may be configured for transcutaneous or
short-term subcutaneous implantation, and may have a thickness from
about 0.5 microns to about 8 microns, and sometimes from about 4
microns to about 6 microns. In one glucose sensor configured for
fluid communication with a host's circulatory system, the thickness
may be from about 1.5 microns to about 25 microns, and sometimes
from about 3 to about 15 microns. It is also contemplated that in
some embodiments, the bioprotective layer or any other layer of the
electrode may have a thickness that is consistent, but in other
embodiments, the thickness may vary. For example, in some
embodiments, the thickness of the bioprotective layer may vary
along the longitudinal axis of the electrode end.
Diffusion Resistance Domain
[0130] In some embodiments, a diffusion resistance domain 246, also
referred to as a diffusion resistance layer, may be used and is
situated more proximal to the implantable device relative to the
bioprotective domain. In some embodiments, the functionality of the
diffusion resistance domain may be built into the bioprotective
domain that comprises the surface-active group-containing base
polymer. Accordingly, it is to be noted that the description herein
of the diffusion resistance domain may also apply to the
bioprotective domain. The diffusion resistance domain serves to
control the flux of oxygen and other analytes (for example,
glucose) to the underlying enzyme domain. As described in more
detail elsewhere herein, there exists a molar excess of glucose
relative to the amount of oxygen in blood, i.e., 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 sensor
employing oxygen as cofactor is supplied with oxygen in
non-rate-limiting excess in order to respond linearly to changes in
glucose concentration, while not responding to changes in oxygen
tension. More 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 up
to about 40 mg/dL. However, in a clinical setting, a linear
response to glucose levels is desirable up to at least about 500
mg/dL.
[0131] The diffusion resistance domain 246 includes a semipermeable
membrane that controls the flux of oxygen and glucose to the
underlying enzyme domain 244, preferably rendering oxygen in
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 diffusion resistance domain. In some
embodiments, the diffusion resistance domain exhibits an
oxygen-to-glucose permeability ratio of approximately 200:1, but in
other embodiments the oxygen-to-glucose permeability ratio may be
approximately 100:1, 125:1, 130:1, 135:1, 150:1, 175:1, 225:1,
250:1, 275:1, 300:1, or 500:1. As a result of the high
oxygen-to-glucose permeability ratio, one-dimensional reactant
diffusion may provide sufficient excess oxygen at all reasonable
glucose and oxygen concentrations found in the subcutaneous matrix
(See Rhodes et al., Anal. Chem., 66:1520-1529 (1994)). In some
embodiments, a lower ratio of oxygen-to-glucose can be sufficient
to provide excess oxygen by using a high oxygen soluble domain (for
example, a silicone material) to enhance the supply/transport of
oxygen to the enzyme membrane or electroactive surfaces. By
enhancing the oxygen supply through the use of a silicone
composition, for example, glucose concentration can be less of a
limiting factor. In other words, if more oxygen is supplied to the
enzyme or electroactive surfaces, then more glucose can also be
supplied to the enzyme without creating an oxygen rate-limiting
excess.
[0132] In some embodiments, the diffusion resistance domain is
formed of a base polymer synthesized to include a polyurethane
membrane with both hydrophilic and hydrophobic regions to control
the diffusion of glucose and oxygen to an analyte sensor. A
suitable hydrophobic polymer component may be a polyurethane or
polyether urethane urea. Polyurethane is a polymer produced by the
condensation reaction of a diisocyanate and a difunctional
hydroxyl-containing material. A polyurea is a polymer produced by
the condensation reaction of a diisocyanate and a difunctional
amine-containing material. Preferred diisocyanates include
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 preferred embodiments. The material
that forms the basis of the hydrophobic matrix of the diffusion
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.
[0133] In one embodiment of a polyurethane-based resistance domain,
the hydrophilic polymer component is polyethylene oxide. For
example, one useful 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.
[0134] Alternatively, in some embodiments, the resistance domain
may comprise a combination of a base polymer (e.g. polyurethane)
and one or more hydrophilic polymers (e.g. PVA, PEG,
polyacrylamide, acetates, PEO, PEA, PVP, and variations thereof).
It is contemplated that any of a variety of combination of polymers
may be used to yield a blend with desired glucose, oxygen, and
interference permeability properties. For example, in some
embodiments, the resistance domain may be formed from a blend of a
silicone polycarbonate-urethane base polymer and a PVP hydrophilic
polymer, but in other embodiments, a blend of a polyurethane, or
another base polymer, and one or more hydrophilic polymers may be
used instead. In some of the embodiments involving the use of PVP,
the PVP portion of the polymer blend may comprise from about 5% to
about 50% by weight of the polymer blend, sometimes from about 15%
to 20%, and other times from about 25% to 40%. It is contemplated
that PVP of various molecular weights may be used. For example, in
some embodiments, the molecular weight of the PVP used may be from
about 25,000 daltons to about 5,000,000 daltons, sometimes from
about 50,000 daltons to about 2,000,000 daltons, and other times
from 6,000,000 daltons to about 10,000,000 daltons.
[0135] In some embodiments, the diffusion resistance domain 246 can
be formed as a unitary structure with the bioprotective domain 248;
that is, the inherent properties of the diffusion resistance domain
246 are incorporated into bioprotective domain 248 such that the
bioprotective domain 248 functions as a diffusion resistance domain
246.
[0136] In certain embodiments, the thickness of the resistance
domain may be from about 0.05 microns or less to about 200 microns
or more. In some of these embodiments, the thickness of the
resistance domain may be 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, 3.5, 4, 6, 8 microns
to about 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 19.5, 20, 30,
40, 50, 60, 70, 75, 80, 85, 90, 95, or 100 microns. In some
embodiments, the thickness of the resistance domain is from about
2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns in the case
of a transcutaneously implanted sensor or from about 20 or 25
microns to about 40 or 50 microns in the case of a wholly implanted
sensor.
Enzyme Domain for Improved Low Oxygen Performance
[0137] In some embodiments, an enzyme domain 244, also referred to
as the enzyme layer, may be used and is situated less distal from
the electrochemically reactive surfaces than the diffusion
resistance domain 246 and/or the bioprotective domain 248. The
enzyme domain comprises a catalyst configured to react with an
analyte. In one embodiment, the enzyme domain is an immobilized
enzyme domain 244 including glucose oxidase. In other embodiments,
the enzyme domain 244 can be impregnated with other oxidases, for
example, galactose oxidase, cholesterol oxidase, amino acid
oxidase, alcohol oxidase, lactate oxidase, or uricase. For example,
for an enzyme-based electrochemical glucose sensor to perform well,
the sensor's response should neither be limited by enzyme activity
nor cofactor concentration.
[0138] In some embodiments, the catalyst (enzyme) can be
impregnated or otherwise immobilized into the bioprotective and/or
diffusion resistance domain such that a separate enzyme domain 244
is not required (e.g. 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 244 is formed at least in part from polyurethane, for
example, aqueous dispersions of colloidal polyurethane polymers
including the enzyme.
[0139] Through experiments, it has been unexpectedly found that the
use of PVP blended with a base polymer, such as a polyurethane, may
provide the enzyme domain with the capability of substantially
improving sensor performance in low oxygen conditions (e.g., such
as at oxygen concentration of about 0.25 mg/L or less). This
increase in sensor performance allows for the achievement of
increased glucose sensitivity (e.g., from 5 to 200 pA/mg/dL) under
low oxygen conditions. Although PVP is described here to provide an
example of a hydrophilic polymer capable of providing this effect,
it is contemplated that any of a variety of other hydrophilic
polymers may also be used, such as, polyvinyl pyrrolidone-vinyl
acetate (PVP-VA), hydroxypropyl cellulose (HPC), hydroxypropyl
methylcellulose (HPMC), for example. Thus, in some embodiments, the
enzyme domain comprises a curable mixture of a urethane polymer and
a hydrophilic polymer.
[0140] In some embodiments, the hydrophilic portion of the polymer
blend (e.g., PVP) may comprise from about 5% to about 50% by weight
of the polymer blend, such as from about 5% to 30%, such as from
about 10% to 25%, such as from about 15% to 25%, such as about 15%.
In embodiments where PVP is the hydrophilic polymer, it is
contemplated that PVP of various molecular weights may be used. For
example, in some embodiments, the molecular weight of the PVP used
may be from about 25,000 daltons to about 5,000,000 daltons,
sometimes from about 50,000 daltons to about 2,000,000 daltons, and
other times from 6,000,000 daltons to about 10,000,000 daltons.
[0141] It is contemplated that in certain embodiments, the base
polymer and hydrophilic polymer blend may not be crosslinked, but
in other embodiments, crosslinking may be used and achieved by any
of a variety of methods, for example, by adding a crosslinking
agent. In some embodiments, a polyurethane polymer may be
crosslinked in the presence of PVP by preparing a premix of the
polymers and adding a cross-linking agent just prior to the
production of the membrane. Suitable cross-linking agents
contemplated include, but are not limited to, carbodiimides (e.g.
1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride, and
UCARLNK.RTM. XL-25 (Union Carbide)), epoxides and
melamine/formaldehyde resins. Alternatively, it is also
contemplated that crosslinking may be achieved 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.
[0142] In some embodiments, the thickness of the enzyme domain may
be from about 0.01, 0.05, 0.6, 0.7, or 0.8 microns to about 1, 1.2,
1.4, 1.5, 1.6, 1.8, 2, 2.1, 2.2, 2.5, 3, 4, 5, 10, 20, 30 40, 50,
60, 70, 80, 90, or 100 microns. In more preferred embodiments, the
thickness of the enzyme domain is between 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, 4, or 5
microns and 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
19.5, 20, 25, or 30 microns. In even more preferred embodiments,
the thickness of the enzyme domain is from about 2, 2.5, or 3
microns to about 3.5, 4, 4.5, or 5 microns in the case of a
transcutaneously implanted sensor or from about 6, 7, or 8 microns
to about 9, 10, 11, or 12 microns in the case of a wholly implanted
sensor.
Enzyme Domain with Improved Enzyme Stability
[0143] As described above, an enzyme domain 244 comprises a
catalyst configured to react with an analyte. In some embodiments,
catalysts included in an enzyme domain are susceptible to thermal
or pH induced degradation. In some related embodiments, the enzyme
domain may also comprise one or more enzyme stabilizing agents.
Such agents improve the enzyme's ability to resist thermal or pH
induced denaturing. Inclusion of enzyme stabilizing agents thus
facilitates device fabrication by allowing for use of fabrication
processes which would otherwise compromise the enzyme's activity.
Inclusion of these agents has the added benefits of extending
useable and shelf lives of the sensors.
[0144] Any material which improves thermal and/or pH stability of
the enzyme, without affecting analyte or oxygen permeability of the
enzyme domain to the point that the enzyme domain is no longer
suitable for use in a sensor, may be used as an enzyme stabilizing
agent. In some embodiments, an enzyme stabilizing agent may be
dipolar. Without wishing to be bound by theory, it is believed that
dipolar enzyme stabilizing agents stabilize the enzyme by orienting
around the enzyme in such a way as to provide a charged local
environment that stabilizes the enzyme's tertiary structure.
[0145] Dipolar enzyme stabilizing agents may be zwitterionic or
non-zwitterionic. That is, dipolar enzyme stabilizing agents are
neutral molecules with a positive and negative electrical charge at
different locations. In some embodiments, the positive and negative
electrical charges are full unit charges (i.e., the molecules are
zwitterionic). In other embodiments, the positive and negative
charges are less than full unit charges (i.e., the molecules are
dipolar, but non-zwitterionic).
[0146] In some embodiments, a zwitterionic enzyme stabilizing agent
may be a betaine, such as glycine betaine, poly(carboxybetaine)
(pCB), or poly(sulfobetaine) (pSB), or some other zwitterion, such
as ectoine or hydroxyectoine. In preferred embodiments, the
zwitterionic enzyme stabilizing agent is glycine betaine. In some
embodiments, a non-zwitterionic enzyme stabilizing agent may be an
amine oxide.
[0147] In embodiments where the enzyme domain comprises an enzyme
stabilizing reagent, the amount of enzyme stabilizing reagent
present in the enzyme domain is sufficient to provide an
improvement in the thermal and/or pH stability of the enzyme, while
not disrupting the permeability characteristics of the enzyme layer
so that the sensor retains high glucose sensitivity. The identity
and amount of enzyme stabilizing reagent used in the enzyme domain
may vary based on the particular enzyme used in the sensor;
however, the amount of enzyme stabilizing reagent is generally less
than about 50% wt. of the amount of the enzyme; such as less than
about 25% wt; such as less than about 10 wt. %. In a preferred
embodiment, the enzyme is glucose oxidase and the enzyme
stabilizing reagent is a betaine, such as glycine betaine.
[0148] As described above for enzyme domains with improved low
oxygen performance, the enzyme and enzyme stabilizing reagent can
be impregnated or otherwise immobilized into the bioprotective or
diffusion resistance domain such that a separate enzyme domain 244
is not required (e.g. 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 244 is formed from a polyurethane, for example,
aqueous dispersions of colloidal polyurethane polymers including
the enzyme and enzyme stabilizing reagent. Again, it is
contemplated that in some embodiments, the polymer system of the
enzyme domain may not be crosslinked, but in other embodiments,
crosslinking may be used and achieved by any of a variety of
methods, for example, by adding a crosslinking agent.
[0149] In some embodiments, the thickness of the enzyme domain may
be from about 0.01, 0.05, 0.6, 0.7, or 0.8 microns to about 1, 1.2,
1.4, 1.5, 1.6, 1.8, 2, 2.1, 2.2, 2.5, 3, 4, 5, 10, 20, 30 40, 50,
60, 70, 80, 90, or 100 microns. In more preferred embodiments, the
thickness of the enzyme domain is between 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, 4, or 5
microns and 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
19.5, 20, 25, or 30 microns. In even more preferred embodiments,
the thickness of the enzyme domain is from about 2, 2.5, or 3
microns to about 3.5, 4, 4.5, or 5 microns in the case of a
transcutaneously implanted sensor or from about 6, 7, or 8 microns
to about 9, 10, 11, or 12 microns in the case of a wholly implanted
sensor.
[0150] Finally, in some embodiments, enzyme stabilizing agents may
be included in the enzyme domains with improved low oxygen
performance described above. In other words, in some embodiments,
the enzyme domain comprises an enzyme, a base polymer, a
hydrophilic polymer, and an enzyme stabilizing agent.
Interference Domain
[0151] It is contemplated that in some embodiments, such as the
embodiment illustrated in FIGS. 2A-2B, an optional interference
domain 242, also referred to as the interference layer, may be
provided, in addition to the bioprotective domain and the enzyme
domain. The interference domain 242 may substantially reduce the
permeation of one or more interferents into the electrochemically
reactive surfaces. Preferably, the interference domain 242f is
configured to be much less permeable to one or more of the
interferents than to the measured species. It is also contemplated
that in some embodiments, where interferent blocking may be
provided by the bioprotective domain (e.g. via a surface-active
group-containing polymer of the bioprotective domain), a separate
interference domain may not be used.
[0152] In some embodiments, the interference domain is formed from
a silicone-containing polymer, such as a polyurethane containing
silicone, or a silicone polymer. While not wishing to be bound by
theory, it is believed that, in order for an enzyme-based glucose
sensor to function properly, glucose would not have to permeate the
interference layer, where the interference domain is located more
proximal to the electroactive surfaces than the enzyme domain.
Accordingly, in some embodiments, a silicone-containing
interference domain, comprising a greater percentage of silicone by
weight than the bioprotective domain, may be used without
substantially affecting glucose concentration measurements. For
example, in some embodiments, the silicone-containing interference
domain may comprise a polymer with a high percentage of silicone
(e.g. from about 25%, 30%, 35%, 40%, 45%, or 50% to about 60%, 70%,
80%, 90% or 95%).
[0153] In one embodiment, the interference domain may include ionic
components incorporated into a polymeric matrix to reduce the
permeability of the interference domain to ionic interferents
having the same charge as the ionic components. In another
embodiment, the interference domain may include a catalyst (for
example, peroxidase) for catalyzing a reaction that removes
interferents. U.S. Pat. No. 6,413,396 and U.S. Pat. No. 6,565,509
disclose methods and materials for eliminating interfering
species.
[0154] In certain embodiments, the interference domain may include
a thin membrane that is designed to limit diffusion of certain
species, for example, those greater than 34 kD in molecular weight.
In these embodiments, the interference domain permits certain
substances (for example, hydrogen peroxide) that are to be measured
by the electrodes to pass through, and prevents passage of other
substances, such as potentially interfering substances. In one
embodiment, the interference domain is constructed of polyurethane.
In an alternative embodiment, the interference domain comprises a
high oxygen soluble polymer, such as silicone.
[0155] In some embodiments, the interference domain is formed from
one or more cellulosic derivatives. In general, cellulosic
derivatives may include polymers such as cellulose acetate,
cellulose acetate butyrate, 2-hydroxyethyl cellulose, cellulose
acetate phthalate, cellulose acetate propionate, cellulose acetate
trimellitate, or blends and combinations thereof.
[0156] 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 low 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 of the preferred
embodiments are described in U.S. Pat. No. 7,074,307, U.S. Patent
Publication No. US-2005-0176136-A1, U.S. Pat. No. 7,081,195, and
U.S. Patent Publication No. US-2005-0143635-A1, each of which is
incorporated by reference herein in its entirety.
[0157] It is contemplated that in some embodiments, the thickness
of the interference domain may be from about 0.01 microns or less
to about 20 microns or more. In some of these embodiments, the
thickness of the interference domain may be between about 0.01,
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 and about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
14, 15, 16, 17, 18, 19, or 19.5 microns. In some of these
embodiments, the thickness of the interference domain may be from
about 0.2, 0.4, 0.5, or 0.6, microns to about 0.8, 0.9, 1, 1.5, 2,
3, or 4 microns.
[0158] In general, the membrane systems described herein may be
formed 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.
In some embodiments, the interference domain may be deposited by
spray or dip coating. In one exemplary embodiment, 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, and sometimes about 1 inch/min; a dwell
time of from about 0.01 minutes to about 2 minutes, and sometimes
about 1 minute; and a withdrawal rate of from about 0.5 inch/minute
to about 60 inches/minute, and sometimes about 1 inch/minute; and
curing (drying) the domain from about 1 minute to about 14 hours,
and sometimes 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 a cellulose acetate
butyrate interference domain, a 3-minute cure (i.e., dry) time is
used between each layer applied. In another exemplary embodiment
employing a cellulose acetate interference domain, a 15 minute cure
time is used between each layer applied.
[0159] 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 preferred. The preferred number of repeated dip
processes may depend 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 one embodiment,
an interference domain is formed from three layers of cellulose
acetate butyrate. In another embodiment, an interference domain is
formed from 10 layers of cellulose acetate. In yet 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.
Electrode Domain
[0160] It is contemplated that in some embodiments, such as the
embodiment illustrated in FIG. 2C, an optional electrode domain 36,
also referred to as the electrode layer, may be provided, in
addition to the bioprotective domain and the enzyme domain;
however, in other embodiments, the functionality of the electrode
domain may be incorporated into the bioprotective domain so as to
provide a unitary domain that includes the functionality of the
bioprotective domain, diffusion resistance domain, enzyme domain,
and electrode domain.
[0161] In some embodiments, the electrode domain is located most
proximal to the electrochemically reactive surfaces. To facilitate
electrochemical reaction, the electrode domain may include a
semipermeable coating that maintains hydrophilicity at the
electrochemically reactive surfaces of the sensor interface. The
electrode domain can enhance the stability of an adjacent domain by
protecting and supporting the material that makes up the adjacent
domain. The electrode domain may also facilitate in stabilizing the
operation of the device by overcoming electrode start-up problems
and drifting problems caused by inadequate electrolyte. The
buffered electrolyte solution contained in the electrode domain may
also protect against pH-mediated damage that can result from the
formation of a large pH gradient between the substantially
hydrophobic interference domain and the electrodes due to the
electrochemical activity of the electrodes.
[0162] In some embodiments, the electrode domain includes a
flexible, water-swellable, substantially solid gel-like film (e.g.
a hydrogel) having a `dry film` thickness of from about 0.05
microns to about 100 microns, and sometimes from about 0.05, 0.1,
0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1 microns to about 1.5,
2, 2.5, 3, or 3.5, 4, 4.5, 5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10,
10.5, 11, 11.5, 12, 13, 14, 15, 16, 17, 18, 19, 19.5, 20, 30, 40,
50, 60, 70, 80, 90, or 100 microns. In some embodiments, the
thickness of the electrode domain may be from about 2, 2.5 or 3
microns to about 3.5, 4, 4.5, or 5 microns in the case of a
transcutaneously implanted sensor, or from about 6, 7, or 8 microns
to about 9, 10, 11, or 12 microns in the case of a wholly implanted
sensor. The term `dry film thickness` 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 thickness of a cured film cast from a coating formulation onto
the surface of the membrane by standard coating techniques. The
coating formulation may comprise a premix of film-forming polymers
and a crosslinking agent and may be curable upon the application of
moderate heat.
[0163] In certain embodiments, the electrode domain may be formed
of a curable mixture of a urethane polymer and a hydrophilic
polymer. In some of these embodiments, coatings are formed of a
polyurethane polymer having anionic carboxylate functional groups
and non-ionic hydrophilic polyether segments, which are crosslinked
in the presence of polyvinylpyrrolidone and cured at a moderate
temperature of about 50.degree. C.
[0164] Particularly suitable for this purpose are aqueous
dispersions of fully-reacted colloidal polyurethane polymers having
cross-linkable carboxyl functionality (e.g., BAYBOND.RTM.; Mobay
Corporation). These polymers are supplied in dispersion grades
having a polycarbonate-polyurethane backbone containing carboxylate
groups identified as XW-121 and XW-123; and a
polyester-polyurethane backbone containing carboxylate groups,
identified as XW-110-2. In some embodiments, BAYBOND.RTM. 123, an
aqueous anionic dispersion of an aliphate polycarbonate urethane
polymer sold as a 35 weight percent solution in water and
co-solvent N-methyl-2-pyrrolidone, may be used.
[0165] In some embodiments, the electrode domain is formed from a
hydrophilic polymer that renders the electrode domain equally or
more hydrophilic than an overlying domain (e.g., interference
domain, enzyme domain). Such hydrophilic polymers may include, a
polyamide, a polylactone, a polyimide, a polylactam, a
functionalized polyamide, a functionalized polylactone, a
functionalized polyimide, a functionalized polylactam or
combinations thereof, for example.
[0166] In some embodiments, the electrode domain is formed
primarily from a hydrophilic polymer, and in some of these
embodiments, the electrode domain is formed substantially from PVP.
PVP is a hydrophilic water-soluble polymer and is available
commercially in a range of viscosity grades and average molecular
weights ranging from about 18,000 to about 500,000, under the PVP
K.RTM. homopolymer series by BASF Wyandotte and by GAF Corporation.
In certain embodiments, a PVP homopolymer having an average
molecular weight of about 360,000 identified as PVP-K90 (BASF
Wyandotte) may be used to form the electrode domain. Also suitable
are hydrophilic, film-forming copolymers of N-vinylpyrrolidone,
such as a copolymer of N-vinylpyrrolidone and vinyl acetate, a
copolymer of N-vinylpyrrolidone, ethylmethacrylate and methacrylic
acid monomers, and the like.
[0167] In certain embodiments, the electrode domain is formed
entirely from a hydrophilic polymer. Useful hydrophilic polymers
contemplated include, but are not limited to,
poly-N-vinylpyrrolidone, 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 preferred in some embodiments.
[0168] It is contemplated that in certain embodiments, the
hydrophilic polymer used may not be crosslinked, but in other
embodiments, crosslinking may be used and achieved by any of a
variety of methods, for example, by adding a crosslinking agent. In
some embodiments, a polyurethane polymer may be crosslinked in the
presence of PVP by preparing a premix of the polymers and adding a
cross-linking agent just prior to the production of the membrane.
Suitable cross-linking agents contemplated include, but are not
limited to, carbodiimides (e.g. 1-ethyl-3-(3-dimethylaminopropyl)
carbodiimide hydrochloride, and UCARLNK.RTM. XL-25 (Union
Carbide)), epoxides and melamine/formaldehyde resins.
Alternatively, it is also contemplated that crosslinking may be
achieved 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.
[0169] The flexibility and hardness of the coating can be varied as
desired by varying the dry weight solids of the components in the
coating formulation. The term `dry weight solids` 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 dry weight percent based on the total coating composition
after the time the crosslinker is included. In one embodiment, a
coating formulation can contain about 6 to about 20 dry weight
percent, preferably about 8 dry weight percent, PVP; about 3 to
about 10 dry weight percent, sometimes about 5 dry weight percent
cross-linking agent; and about 70 to about 91 weight percent,
sometimes about 87 weight percent of a polyurethane polymer, such
as a polycarbonate-polyurethane polymer, for example. The reaction
product of such a coating formulation is referred to herein as a
water-swellable cross-linked matrix of polyurethane and PVP.
[0170] In some embodiments, underlying the electrode domain is an
electrolyte phase that when hydrated is a free-fluid phase
including a solution containing at least one compound, typically a
soluble chloride salt, which conducts electric current. In one
embodiment wherein the membrane system is used with a glucose
sensor such as is described herein, the electrolyte phase flows
over the electrodes and is in contact with the electrode domain. It
is contemplated that certain embodiments may use any suitable
electrolyte solution, including standard, commercially available
solutions. Generally, the electrolyte phase can have the same
osmotic pressure or a lower osmotic pressure than the sample being
analyzed. In preferred embodiments, the electrolyte phase comprises
normal saline.
Bioactive Agents
[0171] It is contemplated that any of a variety of bioactive
(therapeutic) agents can be used with the analyte sensor systems
described herein, such as the analyte sensor system shown in FIG.
1. In some embodiments, the bioactive agent is an anticoagulant.
The term `anticoagulant` 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
the prevents coagulation (e.g. minimizes, reduces, or stops
clotting of blood). In these embodiments, the anticoagulant
included in the analyte sensor system may prevent coagulation
within or on the sensor. Suitable anticoagulants for incorporation
into the sensor system include, but are not limited to, vitamin K
antagonists (e.g. Acenocoumarol, Clorindione, Dicumarol
(Dicoumarol), Diphenadione, Ethyl biscoumacetate, Phenprocoumon,
Phenindione, Tioclomarol, or Warfarin), heparin group
anticoagulants (e.g. Platelet aggregation inhibitors: Antithrombin
III, Bemiparin, Dalteparin, Danaparoid, Enoxaparin, Heparin,
Nadroparin, Parnaparin, Reviparin, Sulodexide, Tinzaparin), other
platelet aggregation inhibitors (e.g. Abciximab, Acetylsalicylic
acid (Aspirin), Aloxiprin, Beraprost, Ditazole, Carbasalate
calcium, Cloricromen, Clopidogrel, Dipyridamole, Epoprostenol,
Eptifibatide, Indobufen, Iloprost, Picotamide, Ticlopidine,
Tirofiban, Treprostinil, Triflusal), enzymes (e.g., Alteplase,
Ancrod, Anistreplase, Brinase, Drotrecogin alfa, Fibrinolysin,
Protein C, Reteplase, Saruplase, Streptokinase, Tenecteplase,
Urokinase), direct thrombin inhibitors (e.g., Argatroban,
Bivalirudin, Desirudin, Lepirudin, Melagatran, Ximelagatran, other
antithrombotics (e.g., Dabigatran, Defibrotide, Dermatan sulfate,
Fondaparinux, Rivaroxaban), and the like.
[0172] In one embodiment, heparin is incorporated into the analyte
sensor system, for example by dipping or spraying. While not
wishing to be bound by theory, it is believed that heparin coated
on the catheter or sensor may prevent aggregation and clotting of
blood on the analyte sensor system, thereby preventing
thromboembolization (e.g., prevention of blood flow by the thrombus
or clot) or subsequent complications. In another embodiment, an
antimicrobial is coated on the catheter (inner or outer diameter)
or sensor.
[0173] In some embodiments, an antimicrobial agent may be
incorporated into the analyte sensor system. The antimicrobial
agents contemplated may include, but are not limited to,
antibiotics, antiseptics, disinfectants and synthetic moieties, and
combinations thereof, and other agents that are soluble in organic
solvents such as alcohols, ketones, ethers, aldehydes,
acetonitrile, acetic acid, methylene chloride and chloroform. The
amount of each antimicrobial agent used to impregnate the medical
device varies to some extent, but is at least of an effective
concentration to inhibit the growth of bacterial and fungal
organisms, such as staphylococci, gram-positive bacteria,
gram-negative bacilli and Candida.
[0174] In some embodiments, an antibiotic may be incorporated into
the analyte sensor system. Classes of antibiotics that can be used
include tetracyclines (e.g., minocycline), rifamycins (e.g.,
rifampin), macrolides (e.g., erythromycin), penicillins (e.g.,
nafeillin), cephalosporins (e.g., cefazolin), other beta-lactam
antibiotics (e.g., imipenem, aztreonam), aminoglycosides (e.g.,
gentamicin), chloramphenicol, sulfonamides (e.g.,
sulfamethoxazole), glycopeptides (e.g., vancomycin), quinolones
(e.g., ciprofloxacin), fusidic acid, trimethoprim, metronidazole,
clindamycin, mupirocin, polyenes (e.g., amphotericin B), azoles
(e.g., fluconazole), and beta-lactam inhibitors (e.g.,
sulbactam).
[0175] Examples of specific antibiotics that can be used include
minocycline, rifampin, erythromycin, nafcillin, cefazolin,
imipenem, aztreonam, gentamicin, sulfamethoxazole, vancomycin,
ciprofloxacin, trimethoprim, metronidazole, clindamycin,
teicoplanin, mupirocin, azithromycin, clarithromycin, ofloxacin,
lomefloxacin, norfloxacin, nalidixic acid, sparfloxacin,
pefloxacin, amifloxacin, enoxacin, fleroxacin, temafloxacin,
tosufloxacin, clinafloxacin, sulbactam, clavulanic acid,
amphotericin B, fluconazole, itraconazole, ketoconazole, and
nystatin.
[0176] In some embodiments, an antiseptic or disinfectant may be
incorporated into the analyte sensor system. Examples of
antiseptics and disinfectants are hexachlorophene, cationic
bisiguanides (e.g. chlorhexidine, cyclohexidine) iodine and
iodophores (e.g. povidoneiodine), para-chloro-meta-xylenol,
triclosan, furan medical preparations (e.g. nitrofurantoin,
nitrofurazone), methenamine, aldehydes (glutaraldehyde,
formaldehyde) and alcohols. Other examples of antiseptics and
disinfectants will readily suggest themselves to those of ordinary
skill in the art.
[0177] In some embodiments, an anti-barrier cell agent may be
incorporated into the analyte sensor system. Anti-barrier cell
agents may include compounds exhibiting effects on macrophages and
foreign body giant cells (FBGCs). It is believed that anti-barrier
cell agents prevent closure of the barrier to solute transport
presented by macrophages and FBGCs at the device-tissue interface
during FBC maturation. Anti-barrier cell agents may provide
anti-inflammatory or immunosuppressive mechanisms that affect the
wound healing process, for example, healing of the wound created by
the incision into which an implantable device is inserted.
Cyclosporine, which stimulates very high levels of
neovascularization around biomaterials, can be incorporated into a
bioprotective membrane of a preferred embodiment (see U.S. Pat. No.
5,569,462 to Martinson et al.). Alternatively, Dexamethasone, which
abates the intensity of the FBC response at the tissue-device
interface, can be incorporated into a bioprotective membrane of a
preferred embodiment. Alternatively, Rapamycin, which is a potent
specific inhibitor of some macrophage inflammatory functions, can
be incorporated into a bioprotective membrane of a preferred
embodiment.
[0178] In some embodiments, an, anti-inflammatory agent may be
incorporated into the analyte sensor system to reduce acute or
chronic inflammation adjacent to the implant or to decrease the
formation of a FBC capsule to reduce or prevent barrier cell layer
formation, for example. Suitable anti-inflammatory agents include
but are not limited to, for example, nonsteroidal anti-inflammatory
drugs (NSAIDS) such as acetometaphen, aminosalicylic acid, aspirin,
celecoxib, choline magnesium trisalicylate, diclofenac potassium,
diclofenac sodium, diflunisal, etodolac, fenoprofen, flurbiprofen,
ibuprofen, indomethacin, interleukin (IL)-10, IL-6 mutein,
anti-IL-6 iNOS inhibitors (for example, L-NAME or L-NMDA),
Interferon, ketoprofen, ketorolac, leflunomide, melenamic acid,
mycophenolic acid, mizoribine, nabumetone, naproxen, naproxen
sodium, oxaprozin, piroxicam, rofecoxib, salsalate, sulindac, and
tolmetin; and corticosteroids such as cortisone, hydrocortisone,
methylprednisolone, prednisone, prednisolone, betamethesone,
beclomethasone dipropionate, budesonide, dexamethasone sodium
phosphate, flunisolide, fluticasone propionate, paclitaxel,
tacrolimus, tranilast, triamcinolone acetonide, betamethasone,
fluocinolone, fluocinonide, betamethasone dipropionate,
betamethasone valerate, desonide, desoximetasone, fluocinolone,
triamcinolone, triamcinolone acetonide, clobetasol propionate, and
dexamethasone.
[0179] In some embodiments, an immunosuppressive or
immunomodulatory agent may be incorporated into the analyte sensor
system in order to interfere directly with several key mechanisms
necessary for involvement of different cellular elements in the
inflammatory response. Suitable immunosuppressive and
immunomodulatory agents include, but are not limited to,
anti-proliferative, cell-cycle inhibitors, (for example,
paclitaxel, cytochalasin D, infiximab), taxol, actinomycin,
mitomycin, thospromote VEGF, estradiols, NO donors, QP-2,
tacrolimus, tranilast, actinomycin, everolimus, methothrexate,
mycophenolic acid, angiopeptin, vincristing, mitomycine, statins, C
MYC antisense, sirolimus (and analogs), RestenASE,
2-chloro-deoxyadenosine, PCNA Ribozyme, batimstat, prolyl
hydroxylase inhibitors, PPAR.gamma. ligands (for example
troglitazone, rosiglitazone, pioglitazone), halofuginone,
C-proteinase inhibitors, probucol, BCP671, EPC antibodies,
catchins, glycating agents, endothelin inhibitors (for example,
Ambrisentan, Tesosentan, Bosentan), Statins (for example,
Cerivasttin), E. coli heat-labile enterotoxin, and advanced
coatings.
[0180] In some embodiments, an anti-infective agent may be
incorporated into the analyte sensor system. In general,
anti-infective agents are substances capable of acting against
infection by inhibiting the spread of an infectious agent or by
killing the infectious agent outright, which can serve to reduce an
immuno-response without an inflammatory response at the implant
site, for example. Anti-infective agents include, but are not
limited to, anthelmintics (e.g. mebendazole), antibiotics (e.g.
aminoclycosides, gentamicin, neomycin, tobramycin), antifungal
antibiotics (e.g. amphotericin b, fluconazole, griseofulvin,
itraconazole, ketoconazole, nystatin, micatin, tolnaftate),
cephalosporins (e.g. cefaclor, cefazolin, cefotaxime, ceftazidime,
ceftriaxone, cefuroxime, cephalexin), beta-lactam antibiotics (e.g.
cefotetan, meropenem), chloramphenicol, macrolides (e.g.
azithromycin, clarithromycin, erythromycin), penicillins (e.g.
penicillin G sodium salt, amoxicillin, ampicillin, dicloxacillin,
nafcillin, piperacillin, ticarcillin), tetracyclines (e.g.
doxycycline, minocycline, tetracycline), bacitracin, clindamycin,
colistimethate sodium, polymyxin b sulfate, vancomycin, antivirals
(e.g. acyclovir, amantadine, didanosine, efavirenz, foscarnet,
ganciclovir, indinavir, lamivudine, nelfinavir, ritonavir,
saquinavir, silver, stavudine, valacyclovir, valganciclovir,
zidovudine), quinolones (e.g. ciprofloxacin, levofloxacin);
sulfonamides (e.g. sulfadiazine, sulfisoxazole), sulfones (e.g.
dapsone), furazolidone, metronidazole, pentamidine, sulfanilamidum
crystallinum, gatifloxacin, and sulfamethoxazole/trimethoprim.
[0181] In some embodiments, a vascularization agent may be
incorporated into the analyte sensor system. Vascularization agents
generally may include substances with direct or indirect angiogenic
properties. In some cases, vascularization agents may additionally
affect formation of barrier cells in vivo. By indirect
angiogenesis, it is meant that the angiogenesis can be mediated
through inflammatory or immune stimulatory pathways. It is not
fully known how agents that induce local vascularization indirectly
inhibit barrier-cell formation; however, while not wishing to be
bound by theory, it is believed that some barrier-cell effects can
result indirectly from the effects of vascularization agents.
[0182] Vascularization agents may provide mechanisms that promote
neovascularization and accelerate wound healing around the membrane
or minimize periods of ischemia by increasing vascularization close
to the tissue-device interface. Sphingosine-1-Phosphate (S1P), a
phospholipid possessing potent angiogenic activity, may be
incorporated into the bioprotective membrane. Monobutyrin, a
vasodilator and angiogenic lipid product of adipocytes, may also be
incorporated into the bioprotective membrane. In another
embodiment, an anti-sense molecule (for example, thrombospondin-2
anti-sense), which may increase vascularization, is incorporated
into a bioprotective membrane.
[0183] Vascularization agents may provide mechanisms that promote
inflammation, which is believed to cause accelerated
neovascularization and wound healing in vivo. In one embodiment, a
xenogenic carrier, for example, bovine collagen, which by its
foreign nature invokes an immune response, stimulates
neovascularization, and is incorporated into a bioprotective
membrane of some embodiments. In another embodiment,
Lipopolysaccharide, an immunostimulant, may be incorporated into a
bioprotective membrane. In another embodiment, a protein, for
example, a bone morphogenetic protein (BMP), which is known to
modulate bone healing in tissue, may be incorporated into the
bioprotective membrane.
[0184] In some embodiments, an angiogenic agent may be incorporated
into the analyte sensor system. Angiogenic agents are substances
capable of stimulating neovascularization, which can accelerate and
sustain the development of a vascularized tissue bed at the
tissue-device interface, for example. Angiogenic agents include,
but are not limited to, Basic Fibroblast Growth Factor (bFGF),
(also known as Heparin Binding Growth Factor-II and Fibroblast
Growth Factor II), Acidic Fibroblast Growth Factor (aFGF), (also
known as Heparin Binding Growth Factor-I and Fibroblast Growth
Factor-I), Vascular Endothelial Growth Factor (VEGF), Platelet
Derived Endothelial Cell Growth Factor BB (PDEGF-BB),
Angiopoietin-1, Transforming Growth Factor Beta (TGF-.beta.),
Transforming Growth Factor Alpha (TGF-Alpha), Hepatocyte Growth
Factor, Tumor Necrosis Factor-Alpha (TNF.alpha.), Placental Growth
Factor (PLGF), Angiogenin, Interleukin-8 (IL-8), Hypoxia Inducible
Factor-I (HIF-1), Angiotensin-Converting Enzyme (ACE) Inhibitor
Quinaprilat, Angiotropin, Thrombospondin, Peptide KGHK, Low Oxygen
Tension, Lactic Acid, Insulin, Copper Sulphate, Estradiol,
prostaglandins, cox inhibitors, endothelial cell binding agents
(for example, decorin or vimentin), glenipin, hydrogen peroxide,
nicotine, and Growth Hormone.
[0185] In some embodiments, a pro-inflammatory agent may be
incorporated into the analyte sensor system. Pro-inflammatory
agents are generally substances capable of stimulating an immune
response in host tissue, which can accelerate or sustain formation
of a mature vascularized tissue bed. For example, pro-inflammatory
agents are generally irritants or other substances that induce
chronic inflammation and chronic granular response at the
wound-site. While not wishing to be bound by theory, it is believed
that formation of high tissue granulation induces blood vessels,
which supply an adequate or rich supply of analytes to the
device-tissue interface. Pro-inflammatory agents include, but are
not limited to, xenogenic carriers, Lipopolysaccharides, S. aureus
peptidoglycan, and proteins.
[0186] These bioactive agents can be used alone or in combination.
The bioactive agents can be dispersed throughout the material of
the sensor, for example, incorporated into at least a portion of
the membrane system, or incorporated into the device (e.g.,
housing) and adapted to diffuse through the membrane.
[0187] There are a variety of systems and methods by which a
bioactive agent may be incorporated into the sensor membrane. In
some embodiments, the bioactive agent may be incorporated at the
time of manufacture of the membrane system. For example, the
bioactive 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 in some embodiments the
bioactive agent is incorporated into the membrane system, in other
embodiments the bioactive agent can be administered concurrently
with, prior to, or after insertion of the device in vivo, for
example, by oral administration, or locally, by subcutaneous
injection near the implantation site. A combination of bioactive
agent incorporated in the membrane system and bioactive agent
administration locally or systemically can be preferred in certain
embodiments.
[0188] In general, a bioactive agent can be incorporated into the
membrane system, or incorporated into the device and adapted to
diffuse therefrom, in order to modify the in vivo response of the
host to the membrane. In some embodiments, the bioactive agent may
be incorporated only into a portion of the membrane system adjacent
to the sensing region of the device, over the entire surface of the
device except over the sensing region, or any combination thereof,
which can be helpful in controlling different mechanisms or stages
of in vivo response (e.g., thrombus formation). In some alternative
embodiments however, the bioactive agent may be incorporated into
the device proximal to the membrane system, such that the bioactive
agent diffuses through the membrane system to the host circulatory
system.
[0189] The bioactive agent can include a carrier matrix, wherein
the matrix includes one or more of collagen, a particulate matrix,
a resorbable or non-resorbable matrix, a controlled-release matrix,
or a gel. In some embodiments, the carrier matrix includes a
reservoir, wherein a bioactive agent is encapsulated within a
microcapsule. The carrier matrix can include a system in which a
bioactive agent is physically entrapped within a polymer network.
In some embodiments, the bioactive agent is cross-linked with the
membrane system, while in others the bioactive agent is sorbed into
the membrane system, for example, by adsorption, absorption, or
imbibing. The bioactive agent can be deposited in or on the
membrane system, for example, by coating, filling, or solvent
casting. In certain embodiments, ionic and nonionic surfactants,
detergents, micelles, emulsifiers, demulsifiers, stabilizers,
aqueous and oleaginous carriers, solvents, preservatives,
antioxidants, or buffering agents are used to incorporate the
bioactive agent into the membrane system. The bioactive agent can
be incorporated into a polymer using techniques such as described
above, and the polymer can be used to form the membrane system,
coatings on the membrane system, portions of the membrane system,
or any portion of the sensor system.
[0190] The membrane system can be manufactured using techniques
known in the art. The bioactive agent can be sorbed into the
membrane system, for example, by soaking the membrane system for a
length of time (for example, from about an hour or less to about a
week, or more preferably from about 4, 8, 12, 16, or 20 hours to
about 1, 2, 3, 4, 5, or 7 days).
[0191] The bioactive agent can be blended into uncured polymer
prior to forming the membrane system. The membrane system is then
cured and the bioactive agent thereby cross-linked or encapsulated
within the polymer that forms the membrane system.
[0192] In yet another embodiment, microspheres are used to
encapsulate the bioactive agent. The microspheres can be formed of
biodegradable polymers, most preferably synthetic polymers or
natural polymers such as proteins and polysaccharides. As used
herein, the term polymer is used to refer to both to synthetic
polymers and proteins. U.S. Pat. No. 6,281,015, discloses some
systems and methods that can be used in conjunction with the
preferred embodiments. In general, bioactive agents can be
incorporated in (1) the polymer matrix forming the microspheres,
(2) microparticle(s) surrounded by the polymer which forms the
microspheres, (3) a polymer core within a protein microsphere, (4)
a polymer coating around a polymer microsphere, (5) mixed in with
microspheres aggregated into a larger form, or (6) a combination
thereof. Bioactive agents can be incorporated as particulates or by
co-dissolving the factors with the polymer. Stabilizers can be
incorporated by addition of the stabilizers to the factor solution
prior to formation of the microspheres.
[0193] The bioactive agent can be incorporated into a hydrogel and
coated or otherwise deposited in or on the membrane system. Some
hydrogels suitable for use in the preferred embodiments include
cross-linked, hydrophilic, three-dimensional polymer networks that
are highly permeable to the bioactive agent and are triggered to
release the bioactive agent based on a stimulus.
[0194] The bioactive agent can be incorporated into the membrane
system by solvent casting, wherein a solution including dissolved
bioactive agent is disposed on the surface of the membrane system,
after which the solvent is removed to form a coating on the
membrane surface.
[0195] The bioactive agent can be compounded into a plug of
material, which is placed within the device, such as is described
in U.S. Pat. No. 4,506,680 and U.S. Pat. No. 5,282,844. In some
embodiments, it is preferred to dispose the plug beneath a membrane
system; in this way, the bioactive agent is controlled by diffusion
through the membrane, which provides a mechanism for
sustained-release of the bioactive agent in the host.
Release of Bioactive Agents
[0196] Numerous variables can affect the pharmacokinetics of
bioactive agent release. The bioactive agents of the preferred
embodiments can be optimized for short- or long-term release. In
some embodiments, the bioactive agents of the preferred embodiments
are designed to aid or overcome factors associated with short-term
effects (e.g. acute inflammation or thrombosis) of sensor
insertion. In some embodiments, the bioactive agents of the
preferred embodiments are designed to aid or overcome factors
associated with long-term effects, for example, chronic
inflammation or build-up of fibrotic tissue or plaque material. In
some embodiments, the bioactive agents of the preferred embodiments
combine short- and long-term release to exploit the benefits of
both.
[0197] As used herein, `controlled`, `sustained` or `extended`
release of the factors can be continuous or discontinuous, linear
or non-linear. This can be accomplished using one or more types of
polymer compositions, drug loadings, selections of excipients or
degradation enhancers, or other modifications, administered alone,
in combination or sequentially to produce the desired effect.
[0198] Short-term release of the bioactive agent in the preferred
embodiments generally refers to release over a period of from about
a few minutes or hours to about 2, 3, 4, 5, 6, or 7 days or
more.
Loading of Bioactive Agents
[0199] The amount of loading of the bioactive agent into the
membrane system can depend upon several factors. For example, the
bioactive agent dosage and duration can vary with the intended use
of the membrane system, for example, the intended length of use of
the device and the like; differences among patients in the
effective dose of bioactive agent; location and methods of loading
the bioactive agent; and release rates associated with bioactive
agents and optionally their carrier matrix. Therefore, one skilled
in the art will appreciate the variability in the levels of loading
the bioactive agent, for the reasons described above.
[0200] In some embodiments, in which the bioactive agent is
incorporated into the membrane system without a carrier matrix, the
preferred level of loading of the bioactive agent into the membrane
system can vary depending upon the nature of the bioactive agent.
The level of loading of the bioactive agent is preferably
sufficiently high such that a biological effect (e.g., thrombosis
prevention) is observed. Above this threshold, the bioactive agent
can be loaded into the membrane system so as to imbibe up to 100%
of the solid portions, cover all accessible surfaces of the
membrane, or fill up to 100% of the accessible cavity space.
Typically, the level of loading (based on the weight of bioactive
agent(s), membrane system, and other substances present) is from
about 1 ppm or less to about 1000 ppm or more, preferably from
about 2, 3, 4, or 5 ppm up to about 10, 25, 50, 75, 100, 200, 300,
400, 500, 600, 700, 800, or 900 ppm. In certain embodiments, the
level of loading can be 1 wt. % or less up to about 50 wt. % or
more, preferably from about 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, or 20
wt. % up to about 25, 30, 35, 40, or 45 wt. %.
[0201] When the bioactive agent is incorporated into the membrane
system with a carrier matrix, such as a gel, the gel concentration
can be optimized, for example, loaded with one or more test
loadings of the bioactive agent. It is generally preferred that the
gel contain from about 0.1 or less to about 50 wt. % or more of the
bioactive agent(s), preferably from about 0.2, 0.3, 0.4, 0.5, 0.6,
0.7, 0.8, or 0.9 wt. % to about 6, 7, 8, 9, 10, 15, 20, 25, 30, 35,
40, or 45 wt. % or more bioactive agent(s), more preferably from
about 1, 2, or 3 wt. % to about 4 or 5 wt. % of the bioactive
agent(s). Substances that are not bioactive can also be
incorporated into the matrix.
[0202] Referring now to microencapsulated bioactive agents, the
release of the agents from these polymeric systems generally occurs
by two different mechanisms. The bioactive agent can be released by
diffusion through aqueous filled channels generated in the dosage
form by the dissolution of the agent or by voids created by the
removal of the polymer solvent or a pore forming agent during the
original micro-encapsulation. Alternatively, release can be
enhanced due to the degradation of the encapsulating polymer. With
time, the polymer erodes and generates increased porosity and
microstructure within the device. This creates additional pathways
for release of the bioactive agent.
[0203] In some embodiments, the sensor is designed to be bioinert,
e.g. by the use of bioinert materials. Bioinert materials do not
substantially cause any response from the host. As a result, cells
can live adjacent to the material but do not form a bond with it.
Bioinert materials include but are not limited to alumina,
zirconia, titanium oxide or other bioinert materials generally used
in the `catheter/catheterization` art. While not wishing to be
bound by theory, it is believed that inclusion of a bioinert
material in or on the sensor can reduce attachment of blood cells
or proteins to the sensor, thrombosis or other host reactions to
the sensor.
EXAMPLES
Example 1
[0204] Transcutaneous sensors, with electrode, enzyme, and
bioprotective/diffusion resistance domains, were built and tested.
Test sensors were built as described in the section entitled
`Exemplary Glucose Sensor Configuration,` and included an enzyme
domain comprising a Hauthane polyurethane from Hauthaway), PVP
(about 18% by weight), and glucose oxidase. Control sensors with no
PVP in the enzyme domain were also built using Hauthane
polyurethane in the enzyme domain.
[0205] The sensors comprising PVP in the enzyme domain were
confirmed to be operable and capable of measuring glucose with a
sensitivity of between about 30 and 80 pA/mg/dL.
Example 2
[0206] Sensors built as described in Example 1 were further
evaluated to determine the effect of PVP in the enzyme domain on
the difference in sensor performance in low versus high oxygen
conditions. The difference in signal, which is indicative of sensor
drift, was determined by measuring the signals produced by control
and test sensors under ambient oxygen conditions and under low
oxygen conditions (i.e., with oxygen concentrations of 0.25 mg/L).
Differences in signal strength relative to glucose sensitivities
for the three sensors are seen in FIG. 4.
[0207] Control sensors (i.e., sensors without PVP in the enzyme
domain) operated with a glucose sensitivity at between about 50-65
pA/mg/dL, and demonstrated about 6 to 11% signal loss when
operating in low oxygen conditions relative to their performance
under ambient conditions. Conversely, test sensors comprising PVP
in the enzyme layers demonstrated increased signal strength when
operating in low oxygen conditions relative to their performance
under ambient conditions. In fact, the test sensors exhibited up to
about 5% signal increase at an even higher glucose sensitivity
(about 60-85 pA/mg/dL).
[0208] Accordingly, glucose sensors with enzyme domains comprising
polyurethane and PVP demonstrated improved performance under low
oxygen conditions compared to sensors without PVP in the enzyme
domain.
Example 3
[0209] Additional testing was conducted to further evaluate
relative sensor performance under high and low oxygen conditions as
a function of the amount of PVP in the enzyme domain.
[0210] A series of sensors with varying amounts of PVP in the
enzyme layer (0, 5, 10, 15, 20, and 25 dry wt. % PVP) were
prepared. Measurements were generated with each of these sensors
for samples containing about 400 mg/L glucose. The sensors all
operated with a sensitivity of about 60 pA/mg/dL, with measurements
taken under ambient oxygen conditions (i.e., about 5-10 mg/L
oxygen) and under low oxygen conditions (i.e., about 0.25 mg/L). As
illustrated in FIG. 5, all the test sensors with PVP in the enzyme
domain showed reduced change in current response/signal. In fact,
even the test sensor with 5% PVP showed substantial improvement
over the control sensor (with no PVP) with respect to loss of
signal attributed to being present in a low oxygen environment.
[0211] As illustrated in FIG. 6, the control sensor (with 0% PVP in
the enzyme layer) exhibited about a 56% decrease in signal in the
low oxygen conditions relative to the ambient oxygen conditions. By
comparison, all sensors with PVP-containing enzyme layers exhibited
greatly improved low oxygen performance.
Example 4
[0212] Sensors are built as described in the section entitled
`Exemplary Glucose Sensor Configuration,` and included an enzyme
domain comprising a polyurethane, glucose oxidase, and glycine
betaine at an amount sufficient to provide some degree of enzyme
stabilization against thermal, pH, or other chemical
degradation.
[0213] As enzymes are denatured at elevated temperatures, the
manufacture of polymer-based enzymatic glucose sensors requires
curing the polymer membrane system at temperatures at or below
which enzyme activity is preserved. The inclusion of glycine
betaine in the enzyme domain enhances the thermal stability of
glucose oxidase to allow for curing the polymer membrane system at
temperatures that are 5.degree. C., 10.degree. C., 15.degree. C.,
or even 20.degree. C. or higher than temperatures possible in the
absence of glycine betaine. Because of the increased curing
temperature, sensors with glycine betaine in the enzyme domain
require less curing time, resulting in a faster production rate and
decreased cost.
[0214] Further, glycine betaine in the enzyme domain serves to
extend the shelf life of enzymatic glucose sensors. For example,
sensors comprising glycine betaine in the enzyme domain have a
shelf life that is 2 weeks, 1 month, 6 months, or longer than the
shelf life of sensors without glycine betaine in the enzyme
domain.
[0215] Methods and devices that are suitable for use in conjunction
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[0216] Methods and devices that are suitable for use in conjunction
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[0217] 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"; 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"; U.S. application Ser. No. 13/779,607 filed on Feb. 27,
2013 and entitled "ZWITTERION SURFACE MODIFICATIONS FOR CONTINUOUS
SENSORS"; U.S. application Ser. No. 13/780,808 filed on Feb. 28,
2013 and entitled "SENSORS FOR CONTINUOUS ANALYTE MONITORING, AND
RELATED METHODS"; U.S. application Ser. No. 13/784,523 filed on
Mar. 4, 2013 and entitled "ANALYTE SENSOR WITH INCREASED REFERENCE
CAPACITY"; U.S. application Ser. No. 13/789,371 filed on Mar. 7,
2013 and entitled "MULTIPLE ELECTRODE SYSTEM FOR A CONTINUOUS
ANALYTE SENSOR, AND RELATED METHODS"; U.S. application Ser. No.
13/789,279 filed on Mar. 7, 2013 and entitled "USE OF SENSOR
REDUNDANCY TO DETECT SENSOR FAILURES"; U.S. application Ser. No.
13/789,339 filed on Mar. 7, 2013 and entitled "DYNAMIC REPORT
BUILDING"; U.S. application Ser. No. 13/789,341 filed on Mar. 7,
2013 and entitled "REPORTING MODULES"; U.S. application Ser. No.
13/790,281 filed on Mar. 8, 2013 and entitled "SYSTEMS AND METHODS
FOR MANAGING GLYCEMIC VARIABILITY"; U.S. application Ser. No.
13/796,185 filed on Mar. 12, 2013 and entitled "SYSTEMS AND METHODS
FOR PROCESSING ANALYTE SENSOR DATA"; U.S. application Ser. No.
13/796,642 filed on Mar. 12, 2013 and entitled "SYSTEMS AND METHODS
FOR PROCESSING ANALYTE SENSOR DATA"; U.S. application Ser. No.
13/801,445 filed on Mar. 13, 2013 and entitled "SYSTEMS AND METHODS
FOR LEVERAGING SMARTPHONE FEATURES IN CONTINUOUS GLUCOSE
MONITORING"; U.S. application Ser. No. 13/802,424 filed on Mar. 13,
2013 and entitled "SYSTEMS AND METHODS FOR LEVERAGING SMARTPHONE
FEATURES IN CONTINUOUS GLUCOSE MONITORING"; U.S. application Ser.
No. 13/802,237 filed on Mar. 13, 2013 and entitled "SYSTEMS AND
METHODS FOR LEVERAGING SMARTPHONE FEATURES IN CONTINUOUS GLUCOSE
MONITORING"; and U.S. application Ser. No. 13/802,317 filed on Mar.
13, 2013 and entitled "SYSTEMS AND METHODS FOR LEVERAGING
SMARTPHONE FEATURES IN CONTINUOUS GLUCOSE MONITORING".
[0218] All references cited herein, including but not limited to
published and unpublished applications, patents, and literature
references, are incorporated herein by reference in their entirety
and are hereby made a part of this specification. 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.
[0219] Terms and phrases used in this document, and variations
thereof, 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` 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 `example` is used to
provide exemplary instances of the item in discussion, not an
exhaustive or limiting list thereof; and 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.
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. In addition, as used in
this application, the articles `a` and `an` should be construed as
referring to one or more than one (i.e., to at least one) of the
grammatical objects of the article. By way of example, `an element`
means one element or more than one element.
[0220] The presence in some instances of broadening words and
phrases such as `one or more,` `at least,` `but not limited to,` or
other like phrases shall not be read to mean that the narrower case
is intended or required in instances where such broadening phrases
may be absent.
[0221] 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.
[0222] 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.
* * * * *