U.S. patent application number 10/382170 was filed with the patent office on 2004-01-15 for electrical sensor.
This patent application is currently assigned to Bayer Healthcare, LLC. Invention is credited to Edelbrock, Andrew J., Musho, Matthew K., Vreeke, Mark S..
Application Number | 20040007461 10/382170 |
Document ID | / |
Family ID | 27757795 |
Filed Date | 2004-01-15 |
United States Patent
Application |
20040007461 |
Kind Code |
A1 |
Edelbrock, Andrew J. ; et
al. |
January 15, 2004 |
Electrical sensor
Abstract
Disclosed is an electrochemical sensor for the determination of
analytes in body fluids, e.g. glucose in blood. The sensor involves
a non-conductive base which provides a flow path for the body fluid
with the base having a working and counter electrode on its surface
which are in electrical communication with a detector of current.
The base and a cover therefore provide a capillary space containing
the electrodes into which the body fluid is drawn by capillary
action. The counter electrode has a sub-element which contains an
electroactive material and is configured in the system (sensor and
meter) to provide an error signal when insufficient body fluid is
drawn into the capillary.
Inventors: |
Edelbrock, Andrew J.;
(Granger, IN) ; Musho, Matthew K.; (York, PA)
; Vreeke, Mark S.; (Houston, TX) |
Correspondence
Address: |
Jerome L. Jeffers, Esq.
Bayer Healthcare, LLC
P.O. Box 40
Elkhart
IN
46515-0040
US
|
Assignee: |
Bayer Healthcare, LLC
|
Family ID: |
27757795 |
Appl. No.: |
10/382170 |
Filed: |
March 4, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60363380 |
Mar 7, 2002 |
|
|
|
Current U.S.
Class: |
204/403.11 ;
204/403.14 |
Current CPC
Class: |
C12Q 1/004 20130101;
G01N 27/3274 20130101; C12Q 1/001 20130101; A61M 2230/201
20130101 |
Class at
Publication: |
204/403.11 ;
204/403.14 |
International
Class: |
G01N 027/26 |
Claims
What is claimed is:
1. In an electrochemical sensor for determining the concentration
of an analyte in a fluid test sample comprising 1) a non-conductive
base which provides a flow path for the fluid test sample and has a
counter electrode and a working electrode on its upper surface
which are in electrical communication with a detector of electrical
current; 2) a reaction layer on the upper surface of the working
electrode comprising an enzyme and mediator which reacts with the
analyte to cause electrons to be transferred between the analyte
and the working electrode; and 3) a cover which when mated with the
base forms a capillary space with an opening for the introduction
of fluid test sample thereto which space contains the flow path for
the fluid test sample in which the working and counter electrodes
are situated so that the major portion of the counter electrode is
located downstream of the opening from the working electrode, with
a sub-element of the counter electrode being upstream from the
working electrode, so that when electrical communication between
only the sub-element of the counter electrode and the working
electrode takes place, there is insufficient flow of electrical
current through the detector to constitute a valid test upon the
application of between the working electrode and the sub-element,
the improvement which comprises employing a sub-element of the
counter electrode which comprises an electroactive material capable
of allowing oxidation to occur at the surface of the working
electrode under the applied potential and is present in an amount
which is insufficient to maintain the flow of electric current
through the detector to constitute a valid determination of analyte
concentration.
2. The sensor of claim 1 which comprises an insulating base, an
electrical conductor pattern, an electrode pattern comprising a
working electrode and a counter electrode, a sub-element of the
counter electrode, an insulating layer partially covering the
electrodes, and a reaction layer.
3. The sensor of claim 2 wherein the reaction layer comprises
poly(ethylene oxide), an enzyme and a ferricyanide salt.
4. The sensor of claim 3 wherein the enzyme is glucose oxidase.
5. The sensor of claim 1 wherein the electroactive material is an
oxide of Ag, Cu, Mn, Pb, Hg, Ni, Co, Bi, Re, or Te.
6. The sensor of claim 1 wherein the electroactive material is
AgO.
7. The sensor of claim 1 wherein silver has been deposited on the
surface of the sub-element of the counter electrode and oxidized in
place to form silver oxide.
8. The sensor of claim 1 wherein the electroactive material is a
metal halide.
9. The sensor of claim 1 wherein the sub-element of the counter
electrode is physically disconnected from the main body of the
counter electrode and provided with its own connection to the
detector through the main body of the counter electrode.
10. The sensor of claim 2 wherein there is no reagent in contact
with the sub-element of the counter electrode.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an electrochemical
biosensor which can be used for the determination of analytes such
as glucose in blood. Electrochemical biosensors of the type under
consideration are disclosed in U.S. Pat. Nos. 5,120,420 and
5,798,031. These devices have an insulating base upon which carbon
electrodes are printed and are then covered with a reagent layer
comprising a hydrophilic material in combination with an
oxidoreductase specific for the analyte. These devices typically
involve a base and a cover which are separated by a generally
U-shaped piece as a spacer element or, in the case of the '031
patent, use an embossed cover, so that when the base and cover are
mated there is created a capillary space containing the electrodes
and the reagent layer. A hydrophilic polymer, e.g. carboxymethyl
cellulose or poly(ethylene oxide) is used to facilitate the drawing
of the aqueous test fluid into the capillary space.
[0002] In either embodiment, working and counter electrodes are
screen printed onto the base so that an electrochemically created
current can flow when these electrodes are electrically connected
and a potential is created between them. Touching the opening in
the end of the sensor to a drop of test fluid such as blood results
in the fluid being drawn into the capillary space, so that it
covers the reaction layer on the surface of the electrode. An
enzymatic reaction between the oxidoreductase and the analyte
creates a flow of electrons which are carried to the working
electrode by a mediator such as ferricyanide and flow through the
working electrode to a meter which measures the magnitude of the
flow. The counter electrode serves dual purposes. First, it
provides a fixed potential against which the working electrode is
controlled. Second, for a two electrode system, such as that
depicted in the drawings, the counter electrode is used to complete
the electrical circuit. In this mode, each electron that is
transferred to the working electrode is returned to the test fluid
at the counter electrode side of the cell. The device's software is
programmed to correlate the magnitude of this flow with the
concentration of analyte in the test sample. In order for this
current to flow, a complete circuit is formed by covering both
electrodes with the conductive fluid test sample and applying a
potential therebetween.
[0003] A problem which is sometimes associated with this type of
sensor occurs when an insufficient amount of blood is applied to
the opening, so that the working and counter electrodes are not
completely covered with the sample. This results in an incomplete
current flowing across the electrodes, and, since the amount of
analyte detected is directly proportional to the current flowing
through the detection meter, failure to completely cover the
sensor's electrodes can result in an artificially low reading of
the sample's analyte concentration. One technique for dealing with
this under filling problem is discussed in U.S. Pat. No. 5,628,890
which involves a mechanism for preventing any response from being
detected when the sample volume is too low to provide an accurate
reading.
[0004] In co-pending application U.S. Ser. No. 09/731,943 there is
disclosed an electrochemical sensor of the type described above in
which a small sub-element of the non-working electrode is
positioned upstream from the working electrode, so that when there
is insufficient flow of electrical current through the detector to
constitute a valid test for the concentration of analyte in the
fluid test sample, the pre-programmed detector causes the emission
of an error signal to alert the user of the device that the test
result should be disregarded. This is achievable because there is
generated an altered current profile in the event that the
capillary space of the sensor is under-filled. However, in this
device, the tripping current carried by the straight carbon
sub-element requires some time to reach the necessary potential
thereby increasing the duration of the test.
SUMMARY OF THE INVENTION
[0005] The present invention is an improvement to an
electrochemical sensor for determining the concentration of an
analyte in a fluid test sample, e.g. glucose in blood. The sensor
comprises:
[0006] 1) a non-conductive base which provides a flow path for the
fluid test sample and has a counter electrode and a working
electrode on its upper surface which are in electrical
communication with a detector of electrical current;
[0007] 2) a reaction layer on at least the upper surface of the
working electrode comprising an enzyme and a mediator which reacts
with the analyte to cause electrons to be transferred between the
analyte and the working electrode; and
[0008] 3) a cover which when mated with the base forms a capillary
space with an opening for the introduction of fluid test sample in
which the working and counter electrodes are situated within the
capillary space so that the major portion of the counter electrode
is located downstream of the opening from the working electrode,
with a sub-element of the counter electrode being upstream from the
working electrode, so that when electrical communication between
only the sub-element of the counter electrode and the working
electrode takes place, there is insufficient flow of electrical
current through the detector to constitute a valid test.
[0009] The improvement to the sensor involves the use of a
sub-element of the counter electrode which comprises an
electroactive material. The electroactive material is sufficiently
(electro) positive to allow oxidation to occur at the working
electrode under the applied potential and is present in an amount
which is insufficient to maintain the flow of electric current
through the detector to constitute a valid determination of analyte
concentration.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 represents an exploded view of the sensor of the
present invention.
[0011] FIG. 2 is a graphical representation of the functionality of
a sensor made according to the present invention and one made
according to standard techniques.
DESCRIPTION OF THE INVENTION
[0012] The construction of the electrochemical sensor with which
the present invention is concerned is illustrated by FIG. 1.
Referring to FIG. 1, the sensor of the present invention is made by
several printing passes using inks of various compositions. The
sensor 34 is made up of insulating base 36 upon which is printed in
sequence (typically by screen printing techniques) and counter
electrode subunit 40a, an electrical conductive pattern consisting
of the conductive leads (38) and the counter electrode subelement
40A.
[0013] In the drawing, this sub-element 40a is a discrete unit, but
it could form a continuous trace to the conductive leads (38). The
electrode pattern to which 39 is the working electrode and 40 is
the counter electrode is produced next. The electrodes are covered
with an insulating (dielectric) pattern 42 with a slot 41 to expose
the sub-element to the reaction layer 44. The function of the
reaction layer is to convert glucose, or other analyte in the fluid
test sample, stoichiometrically into a chemical species which is
electrochemically measurable in terms of an electrode pattern
produced by the components of the electrode pattern. The reaction
layer typically contains an enzyme which reacts with the analyte to
produce mobile electrons, and an electron acceptor (mediator) such
as ferricyanide to carry the mobile electrons to the surface of the
working electrode. The enzyme in the reaction layer can be combined
with a hydrophilic polymer such as carboxymethylcellulose or
poly(ethylene oxide). The two parts, 39 and 40, of the electrode
print provide the working 39 and counter 40 electrodes necessary
for the electrochemical determination of the analyte which is the
crux of the present invention. The working and counter electrodes
are configured in a manner such that the major portion of the
counter electrode is located downstream (in terms of the direction
of the fluid flow along the flow path) from the forward portion of
the working electrode 40. This configuration offers the advantage
of allowing the test fluid to completely cover the exposed portion
of the working electrode for all cases in which an undetected
partial fill has occurred. However, sub-element 40a of the counter
electrode is positioned upstream from the forward element of the
working electrode 39, so that when an inadequate amount of fluid
test sample to completely cover the working electrode enters the
capillary space, there will be formed an electrical connection
between counter electrode sub-element 40a and the exposed portion
of the upper part of the working electrode due to the conductivity
of the fluid sample, e.g. blood. By programming the current
detector to give an error signal when the current profile it
receives is below certain pre-determined levels, the sensor system
can be designed to actively advise the user that insufficient blood
has entered the sensor's cavity and that another test for analyte
concentration should be conducted. The system is designed to give
an error signal in the case of a short fill by generating a current
profile when the capillary space is underfilled which is different
from that, which is obtained when there is complete filling of the
capillary space. However, it was found that this design requires
more time than is desirable to activate the assay, which delay may
be caused by a lower initial current being generated by the
electrodes. It has now been discovered that this delay can be
shortened or eliminated by printing the sub-element of the counter
electrode with an ink which comprises an electroactive material
with the electro-active material being present in an amount which
is insufficient to maintain the flow of electrons through the
detector to constitute a valid determination of analyte
concentration. Conversely, the sub-element is designed to allow a
sufficient flow of electrons through the detector to start the
timing sequence of the instrument.
[0014] The term electroactive material is intended to mean material
which is sufficiently positive to allow oxidation to occur at the
working electrode under the applied potential. Suitable
electroactive materials include AgO or other electropositive metal
oxide such as, for example, the oxide of Cu, Mn, Pb, Hg, Ni, Co,
Bi, Re or Te or compounds made from these metals. Other classes of
electropositive materials which can be used are oxides of metals
including those mentioned above and halides of such metals. A metal
capable of forming an electropositive metal oxide such as by auto
oxidation can be applied to the sub-element of the counter
electrode and then oxidized in place to form the metal oxide.
[0015] The only requirement of the electropositive material is as
stated above. Accordingly, compounds of an electropositive metal
such as AgCl can be used since the Ag can be converted to the AgCl
species upon contact of the sub-element with a fluid test sample
such as plasma or whole blood which contains chloride. This
embodiment offers the advantage of not requiring a secondary
operation to form the silver oxide layer on the electrode's
surface.
[0016] The sensor of the present invention is constructed using
several screen printing passes using inks of various compositions.
The ink for the first pass comprises a material which is of
sufficiently low resistance to serve as the conductive leads (38)
and contains the electropositive material that forms the
sub-element of the counter electrode (40a) as shown in FIG. 1. The
second pass provides the electrodes 39 and 40 which typically
comprise a material incorporating carbon, graphite, palladium or
platinum. The electropositive material which can be an oxidizable
metal such as one of the metals set out above. As an example, when
silver is used, the silver at the surface of the sub-element is
oxidized through the curing of the ink or by a secondary oxidizing
step. This silver oxide, when contacted with the blood sample which
also reaches the working electrode, provides the necessary positive
potential to produce the tripping current to start the meter's
timing sequence. However, the sub-element (trigger) cannot support
the full current generated from the reaction layer which results in
the detector determining that the capillary space has not received
sufficient test fluid to constitute a valid test.
[0017] While the particular dimensions of the electrodes are not
critical, the area of the sub-element of the counter electrode is
typically less than that of the working electrode. This element is
made as small as possible in view of the restraints of the screen
printing process. The area which is exposed to the fluid test
sample can be made even smaller by printing the dielectric layer
42, so that only a very small portion (2% to 7% of the area of the
working electrode) is exposed to provide the sub-element of the
counter electrode. In one embodiment, reaction layer 44 can be
denied contact with the sub-element 40a of the counter electrode by
providing a screen that does not allow printing of reagent ink over
the counter electrode sub-element 40a and serves the purpose of
starving the sub-element for reagent thereby not allowing it to
function as a proper counter electrode. This is preferred, so that
an error condition is achieved in the case of failure of the test
fluid to contact the bulk of the counter electrode 40.
[0018] While sub-element 40a is depicted as being physically
disconnected from the rest of the conductive leads (38) in the
drawing, it can be physically connected to them forming one
continuous path on the counter electrode side. In the embodiment
where the sub-element is physically disconnected from the main body
of the counter electrode, it is provided with its own connection to
the detector through the main body of the counter electrode.
[0019] The two parts 39 and 40 of the printed electrodes provide
the working and counter electrodes necessary for the
electrochemical determination of the analyte. The electrode ink,
which is about 14 .mu.m thick, typically contains electrochemically
active carbon. Components of the conductor ink are preferably a
mixture of carbon and silver which is chosen to provide a path of
low electrical resistance between the electrodes and the detector
with which they are in operative connection via contact with the
conductive pattern at the fish-tail end 45 of the type of sensor
depicted in the drawing. The counter electrode can be comprised of
silver/silver chloride in which case it will function more like a
reference electrode. The function of the dielectric pattern 42 is
to insulate the electrodes from the fluid test sample except in a
defined area near the center of the electrode patterns to enhance
the reproducibility of the detector reading. A defined area is
important in this type of electrochemical determination because the
measured current is dependent both on the concentration of the
analyte and the area of the reaction layer which is exposed to the
analyte containing test sample. A typical dielectric layer 42
comprises a UV cured acrylic modified polymethane which is about 10
.mu. thick.
[0020] In FIG. 1, the capillary space is formed by mating the
embossed lid 46 with base 36 after the various layers have been
printed. Outlet 50 serves as a vent for air so that the test fluid
can be drawn into the capillary.
[0021] The present invention is further illustrated by the
following examples:
EXAMPLE I
[0022] Metal Converted to Metal Oxide/Metal Halide Construction
[0023] The sensor is constructed using various layers of polymer
thick film (PTF) to form the working sensor. The conductive leads
and trigger subunit is printed from any standard conductive PTF
utilizing a metal pigment for high conductivity. In this case, a
thermoplastic silver/graphite PTF was used for its cost advantage
and cured in an air furnace. The working and counter electrodes
were printed using a standard carbon/graphite PTF and the area can
be defined using a standard UV or conventional dielectric PTF. The
curing of the PTF converts the silver to silver oxide on the PTF
surface. Any remaining silver on the surface of the trigger
sub-electrode is further converted using an oxygen plasma process
that is currently used to improve the performance of the working
electrode surface as disclosed in U.S. Pat. No. 5,429,735. This
plasma process could also use a halide gas that can react with the
metal to form a metal halide at the surface. Alternatively, any wet
chemical reaction capable of oxidizing the silver could be used as
long as it does not adversely affect the performance of the
carbon/graphite electrode. The sensor is then printed with a
proprietary reagent PTF to form the working base. The lid is then
attached to form the working cell of the sensor.
EXAMPLE II
[0024] Metal Oxide Construction
[0025] The sensor can be constructed using various layers of PTF's
where the trigger PTF has incorporated in it a metal oxide or metal
halide. The advantage of this system is that it does not need to be
converted to form the necessary electropositive material. In this
example, a specialty PTF ink is made using AgO and graphite to form
the conductive leads. This ink is screen printed as previously
described except that the AgO is not formed during the subsequent
air curing or O.sub.2 plasma treatment. The electrode and
dielectric PTF is printed and cured as in Example I.
[0026] Other metal oxides can be used in the manufacture of the
PTF, but depending on the conductivity of the oxide it may not
function as well for the conducting leads, so that a separate print
from the conductive leads would be required. This would increase
the cost of manufacturing the sensor due to the need for an
additional printing layer.
EXAMPLE III
[0027] A base stock, typically of polycarbonate, is printed with
various inks to form the electrodes 39 and 40 and then overcoated
with a dielectric layer 42 in a predetermined pattern designed to
leave a desired surface of the electrode exposed to contact by the
fluid test sample as it enters the space formed by mating the lid
46 and the base 36. The particular configuration of the dielectric
layer 42 is as depicted in FIG. 1 in which opening 43 leaves the
reagent layer in electrical communication with the electrodes 39
and 40 is designed to define the extent to which all of the
conductive elements (working, reference and sub-element electrodes)
are exposed to the test fluid. Along with the printed conductive
features, the dielectric layer defines the size of each of these
elements. The electrodes are preferably printed so that the
conductive and dielectric layers are close to 90 degrees to each
other. This helps in the tolerance stackup for building the sensor
because it reduces the registration issues since as either printing
shifts around the element, definition remains constant.
[0028] Two sensors having trigger electrodes as in FIG. 1, one with
a trigger electrode made of carbon and the other comprising Ag/AgO,
were prepared as follows:
[0029] Carbon Trigger Electrode
[0030] A polycarbonate base was printed using DuPont 5085
Ag/graphite conductive PTF ink and cured using either a standard
forced air tunnel or an IR tunnel oven to form the conductive leads
38. The electrodes 39, 40 and 40a were then printed using DuPont
7102T PTF ink. This ink can be cured using either the forced air
tunnel or an IR tunnel oven. A dielectric ink was applied to define
the working, reference and trigger electrode area. The sensor was
then subjected to an O.sub.2 plasma treatment to enhance the
electrode surfaces. Ag/AgO Trigger
[0031] In this embodiment, the sensor was produced in the same
manner as that with the carbon trigger except that the trigger
electrode 40a was printed with the Ag/graphite PTF ink- along with
the conductive leads 38.
[0032] The air curing of the DuPont PTF inks (5085 and 7102T)
converted the Ag at the surface of the trigger electrode to AgO.
The O.sub.2 plasma used to enhance the electrode efficiency further
oxidized any residual Ag at the surface of the trigger
electrode.
[0033] These sensors were tested by determining the time required
for the counter electrode subunit to reach the tripping current.
The meter required that a minimum threshold current be obtained in
order to start the timing sequence of the test. In a complete fill
situation, the mediator is on the surface of the counter electrode
and conversion of the glucose begins immediately to produce a
current exceeding the threshold current. In an underfill condition,
the mediator is situated several thousandths of an inch away from
the trigger electrode. As the conversion of glucose occurs, the
mediator must diffuse to the trigger electrode to produce the
threshold current needed to start the meter timing sequence. The
AgO or other electropositive species has a higher potential than
the carbon and, therefore, the time required to render the
threshold current is reduced. The trigger electrode is not large
enough to carry the full burden of the reaction and is, therefore,
not suitable as the counter electrode. The meter is preprogrammed
with algorithms which monitor the current profile to detect an
underfill condition and give the user an error signal. The result,
termed countdown efficiency, was calculated inoculating the sensor
with a known volume of blood (1.0 and 1.5 .mu.L in this case) to
produce an underfill condition. These volumes, through
experimentation, have been determined to partially or fully cover
the working electrode. The time required from sample inoculation
until the meter starts its countdown was measured as countdown
efficiency.
[0034] From the data presented in FIG. 2, it can be determined that
the carbon trigger is effective at determining an underfill
condition, but the user would have to wait anywhere from 1 second
to 3 minutes before the meter would report an error. The present
invention was found to decrease the time required for error
detection to between 1 second and less than 1 minute depending on
the degree of underfill.
[0035] For Ag/AgO at 1.0 .mu.L, 70% of the tested sensors tripped
immediately and another 30% tripped in less than 1 minute. At 1.5
.mu.L, 100% of the sensors tripped immediately.
[0036] For carbon at 1.0 .mu.L, 84% tripped in less than 1 minute
and 16% never tripped until the meter errored out at 3 minutes. At
1.5 .mu.L, 100% tripped within 1 minute.
* * * * *