Title: Electrochemical test device and related methods
Abstract: An electrochemical test device is provided for determining the presence or concentration of an analyte in an aqueous fluid sample. The electrochemical test device includes a working electrode and a counter electrode made of an amorphous semiconductor material. The working electrode is overlaid with a reagent capable of reacting with an analyte to produce a measurable change in potential which can be correlated to the concentration of the analyte in the fluid sample. The test device optionally contains a reference electrode made of an amorphous semiconductor material having a reference material on the reference electrode. The test device electrodes can be constructed on a flexible film substrate, such as a polymeric film or a metal foil coated with a non-conductive coating.
Patent Number: 7,018,848 Issued on 03/28/2006 to Douglas, et al.
| Inventors:
|
Douglas; Joel S. (Santa Clara, CA);
Roe; Jeffrey N. (San Ramon, CA);
Priest; John H. (Everett, WA)
|
| Assignee:
|
Roche Diagnostic Operations, Inc. (Indianapolis, IN)
|
| Appl. No.:
|
224875 |
| Filed:
|
August 21, 2002 |
| Current U.S. Class: |
436/524; 422/82.05; 422/100; 204/400; 204/403; 435/287.1; 435/287.2; 435/289; 435/291; 435/817; 436/169; 436/170; 436/518; 436/525; 436/805 |
| Current Intern'l Class: |
G01N 33/55.1 (20060101) |
| Field of Search: |
422/8205,100
204/400,403
435/287.1,287.2,289,291,817
436/169,170,518,524,525,805
|
References Cited [Referenced By]
U.S. Patent Documents
| 3271591 | Sep., 1966 | Ovshinsky.
| |
| 3710205 | Jan., 1973 | Swanson.
| |
| 3983076 | Sep., 1976 | Rockstad et al.
| |
| 4178415 | Dec., 1979 | Ovshinsky et al.
| |
| 4217374 | Aug., 1980 | Ovshinsky et al.
| |
| 4226898 | Oct., 1980 | Ovshinsky et al.
| |
| 4634514 | Jan., 1987 | Nishizawa et al.
| |
| 4994167 | Feb., 1991 | Shults et al.
| |
| 5126034 | Jun., 1992 | Carter et al.
| |
| 5137827 | Aug., 1992 | Mroczkowski et al.
| |
| 5143694 | Sep., 1992 | Schafer et al.
| |
| 5174963 | Dec., 1992 | Fuller et al.
| |
| 5277870 | Jan., 1994 | Fuller et al.
| |
| 5296194 | Mar., 1994 | Igarashi.
| |
| 5312762 | May., 1994 | Guiseppi-Elie.
| |
| 5344754 | Sep., 1994 | Zweig.
| |
| 5391250 | Feb., 1995 | Cheney, II et al.
| |
| 5395504 | Mar., 1995 | Saurer et al.
| |
| 5413690 | May., 1995 | Kost.
| |
| 5437999 | Aug., 1995 | Diebold.
| |
| 5468606 | Nov., 1995 | Bogart et al.
| |
| 5512882 | Apr., 1996 | Stetter et al.
| |
| 5580794 | Dec., 1996 | Allen.
| |
| 5708247 | Jan., 1998 | McAleer et al.
| |
| 5820551 | Oct., 1998 | Hill et al.
| |
| Foreign Patent Documents |
| 0 255 291 | Feb., 1988 | EP.
| |
| 0 351 891 | Jan., 1990 | EP.
| |
| 0 470 649 | Feb., 1992 | EP.
| |
| 0 213 825 | Nov., 1992 | EP.
| |
| 0 593 096 | Apr., 1994 | EP.
| |
| 91/08404 | Jun., 1991 | WO.
| |
Other References
Yokoyama, K. et al., "Amperometric Glucose Sensor Using Silicon Oxide Deposited
Gold Electrodes", Electroanalysis, May 1991.
|
Primary Examiner: Chin; Christopher L.
Attorney, Agent or Firm: Woodard, Emhardt, Moriarty, McNett & Henry LLP
Parent Case Text
REFERENCE TO RELATED APPLICATION
This application is a continuation and claims priority to U.S. patent application
Ser. No. 08/876,812 filed Jun. 16, 1997 now U.S. Pat. No. 6,638,772 and entitled:
"Electrochemical Test Device and Related Methods" and to U.S. Provisional Application
Ser. No. 60/019,864 filed Jun. 17, 1996 now abandoned, which are incorporated by
reference herein in their entirety.
Claims
What is claimed is:
1. A method for determining the presence or concentration of an analyte in an
aqueous fluid sample, said method comprising:
(a) providing an electrochemical test device comprising:
(i) a single substrate, the single substrate comprising a non-conductive coating
affixed to one side of a flexible material;
(ii) a working electrode comprising an amorphous semiconductor material affixed
to the non-conductive coating, said working electrode having an first electrode
area, a first lead and a first contact pad;
(iii) a counter electrode comprising an amorphous semiconductor material affixed
to the non-conductive coating, said counter electrode having a second electrode
area, a second lead, and a second contact pad; and
(iv) a reagent capable of reacting with the analyte to produce a measurable change
in potential which can be correlated to the presence or concentration of the analyte
in the fluid sample, said reagent overlaying at least of portion of the first electrode
area of the working electrode;
(b) inserting the electrochemical test device into a meter device;
(c) applying a sample of art aqueous fluid to the first electrode area of the
working electrode; and
(d) reading the meter device to determine the presence or concentration of the
analyte in the fluid sample.
2. The method of claim 1, wherein the electrochemical test device further comprises
a reference electrode comprising an amorphous semiconductor material affixed to
the non-conductive coating, said reference electrode having a third electrode area,
a third lead, and a third contact pad, and wherein at least a portion of the third
electrode area is overlaid with a reference material.
3. The method of claim 2 wherein the reference material is silver/silver chloride.
4. The method of claim 1 wherein the flexible material is metallic.
5. The method of claim 1 wherein the flexible material is a polymeric sheet material.
6. The method of claim 5 wherein the polymeric sheet material is selected from
the group consisting of polyesters, polycarbonates and polyimides.
7. The method of claim 1 wherein the non-conductive coating is an epoxy resin.
8. The method of claim 1 wherein the amorphous semiconductor material is amorphous
silicon oxide.
9. The method of claim 8 wherein the amorphous silicon oxide is doped with an
ion to increase conductivity.
10. The method of claim 9 wherein the amorphous silicon oxide is doped with lithium.
11. The method of claim 1 where the amorphous semiconductor material is gold.
12. The method of claim 1 where the amorphous semiconductor material is silver.
13. The method of claim 1 wherein the reagent comprises an enzyme and a redox mediator.
14. The method of claim 13 wherein the enzyme is glucose oxidase.
15. The method of claim 13 wherein the redox mediator is potassium ferricyanide.
16. The method of claim 1 wherein the electrochemical test device further comprises
a blood separating membrane.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrochemical test device suitable for
determining the presence or concentration of chemical and biochemical components
(analytes) in aqueous fluid samples and body fluids such as whole blood. Additionally,
this invention relates to a method of using such test devices for determining the
presence or concentration of an analyte and to processes for preparing such a test devices.
2. State of the Art
Medical studies have demonstrated that the incidence of serious complications
resulting from diabetes, such as vision loss and kidney malfunction, can be significantly
reduced by careful control of blood glucose levels. As a result, millions of diabetics
use glucose testing devices daily to monitor their blood glucose concentrations.
Additionally, a wide variety of other blood testing devices are used to determine
the presence or concentration of other analytes, such as alcohol or cholesterol,
in aqueous samples, such as blood.
Such blood testing devices typically employ either a dry chemistry reagent system
or an electrochemical method to test for the analyte in the fluid sample. In recent
years, electrochemical testing systems have become increasingly popular due to
their small size and ease of use. Such electrochemical testing systems typically
use electrochemistry to create an electrical signal which correlates to the concentration
of the analyte in the aqueous sample.
Numerous electrochemical testing systems and related methods are known in
the art. For example, European Patent Publication No. 0 255 291 B1, to Birch et
al., describes methods and an apparatus for making electrochemical measurements,
in particular but not exclusively for the purpose of carrying out microchemical
testing on small liquid samples of biological, e.g. of clinical, origin.
European Patent Publication No. 0 351 891 B1, to Hill et al., teaches a
method of making an electrochemical sensor by printing. The sensor is used to detect,
measure or monitor a given dissolved substrate in a mixture of dissolved substrates,
most specifically glucose in body fluid.
U.S. Pat. No. 5,391,250, to Cheney II et al., teaches a method of fabricating
thin film electrochemical sensors for use in measuring subcutaneous or transdermal
glucose. Fabrication of the sensors comprises placing a thin film base layer of
insulating material onto a rigid substrate. Conductor elements for the sensors
are formed on the base layer using contact mask photolithography and a thin film
cover layer.
U.S. Pat. No. 5,437,999, to Diebold et al., teaches a method of fabricating
thin film electrochemical devices which are suitable for biological applications
using photolithography to define the electrode areas. The disclosures of each of
the above patent specifications are incorporated herein by reference in their entirety.
An excellent reference on materials and process for fabricating electronic components
is Charles A. Harper, Handbook of Materials and Processes for Electronics, 1984,
Library of Congress card number 76-95803. It provides detail process information
on thick film, thin film and photoresist processes.
Existing electrochemical testing systems, however, have certain limitations
from the perspective of the end user or the manufacturer. For example, some electrochemical
testing systems are difficult or costly to manufacture. As a result, such devices
are too expensive to be used on a daily basis by, for example, diabetics. Other
electrochemical testing systems are not sufficiently accurate to detect certain
analytes at very low concentrations or to give reliable measurements of the analyte's
concentration. Additionally, many electrochemical devices are too large to be easily
carried by those needing to test their blood on a regular basis throughout the
day. Thus, a need exists for improved electrochemical test devices.
SUMMARY OF THE INVENTION
The present invention utilizes amorphous semiconductor materials and semiconductor
and printed circuit board (PCB) manufacturing techniques to provide an electrochemical
test device suitable for determining the presence or concentration of analytes
in aqueous fluid samples. By using amorphous semiconductor materials and PCB manufacturing
techniques, uniform electrochemical test devices having well-defined reproducible
electrode areas can be manufactured economically.
In particular, the test devices of this invention have very uniform surface areas
which reduces the variability of the electrochemical test. In this regard, it has
been found that the surface area of the electrodes and the accuracy of applying
the reagent are critical to producing an accurate test. If the surface area is
not consistent from test to test then each of the test devices must be individually
calibrated to insure accurate readings. The test devices of the present invention
permit highly accurate electrochemical analyte measurements to be performed on
very small aqueous fluid samples without the need for individual calibration of
each test device. The present inventions provide for the accurate reproduction
of the test devices by using controlled deposition methods, such as sputtering,
and chemical machining methods to accurately form the geometries of consistent
size and shape from device to device in continuous production. These devices can
also be readily manufactured due to the lower cost and the flexible nature of the
amorphous semiconductor materials which facilitates production by continuous roll
processing versus the step and repeat printing methods currently employed. The
ability to use continuous processing to fabricate the device, such as continuous
processes utilizing continuous roll coating, continuous roll sputtering, continuous
photolithography systems utilizing contact masks and flow through baths, results
in high volume manufacturing capability an substantial cost reductions over the
step and repeat processes. Additionally, the amorphous nature of the conductors
electrodes and constructed and used according to this invention eliminates problems
found in prior test devices which utilize conventional conductor and semiconductor
materials, which are crystalline in nature or are noble metals and, as a result,
require flat and rigid substrates to prevent cracking during manufacture, distribution
or use.
Dry electrochemical test devices fall into two primary configurations. The first
configuration utilizes two electrodes, i.e., a working electrode and a counter
electrode. The second configuration utilizes three electrodes, i.e., a working
electrode, a counter electrode and a reference electrode. The use of the reference
electrode and a reference material provides a fixed reference for the test. The
test devices of the present invention can be of either configuration.
Accordingly, in one of its aspects, the present invention provides an
electrochemical test device for determining the presence or concentration of an
analyte in an aqueous fluid sample, said electrochemical test device comprising:
(a) a non-conductive surface;
(b) a working electrode comprising an amorphous semiconductor material affixed
to the non-conductive surface, said working electrode having an first electrode
area, a first lead and a first contact pad;
(c) a counter electrode comprising an amorphous semiconductor material affixed
to the non-conductive surface, said counter electrode having an second electrode
area, a second lead and a second contact pad; and
(d) a reagent capable of reacting with the analyte to produce a measurable change
in potential which can be correlated to the concentration of the analyte in the
fluid sample, said reagent overlaying at least of portion of the first electrode
area of the working electrode.
In another embodiment of this invention, the test device further comprises a
reference
electrode comprising an amorphous semiconductor material affixed to the non-conductive
surface, said reference electrode having a third electrode area, a third lead,
and a third contact pad, and wherein at least a portion of the third electrode
area is overlaid with a reference material. Preferably, the reference material
is a silver/silver chloride layer, a mercury/mercury chloride layer or a platinum/hydrogen material.
The non-conductive surface used in the test device of this invention can be any
rigid or flexible material which has appropriate insulating and dielectric properties
such as ceramics, polymeric board materials, flexible polymer sheets and the like.
Preferably, the non-conductive surface comprises a non-conductive coating
affixed to one side of a flexible substrate comprising a metallic sheet material
or a polymeric sheet material, such as polyester, polycarbonate and polyimide sheets
or films. Preferred metallic sheets are metal foils which include aluminum, copper
and stainless steel foil. Aluminum and stainless steel foil are particularly preferred.
The non-conductive coating used in the electrochemical test device is preferably
an epoxy resin. The purpose of this coating is to provide a non-conductive barrier
between the base material and the conductive layer and to improve the flatness
of the surface morphology of the non-conductive surface on which the amorphous
semiconductor electrodes are formed according to this invention. Better surface
morphology of the non-conductive layer and the amorphous semiconductor electrodes
provides improved accuracy of test results and consistency of performance.
Preferably, the amorphous semiconductor material used in this invention
is amorphous silicon oxide. More preferably, the amorphous silicon oxide is doped
with lithium, potassium, or a similar conducting ion to increased conductivity.
Doping with lithium is particularly preferred. However, amorphous carbon, gold,
silver or other conductor materials which do not interfere with the electrochemistry
of the reagent system are also suitable. The amorphous semiconductor material can
be applied by using various techniques including sputtering, evaporation, vapor
phase deposition or other thin film deposition technique to form a conductive layer
on the non-conductive surface, and technology can be used to form the electrodes.
Thick film technologies can also be employed when using processes which control
the application of the material and provide for uniform surface morphology. The
surface texture of the amorphous semiconductor material is preferably less than
13 microinches or 0.33 microns. However, rougher textures can be used depending
on the accuracy of the desired test device.
The reagent employed in the electrochemical test device is typically selected
based on the analyte to be tested and the desired detection limits. The reagent
preferably comprises an enzyme and a redox mediator. When the analyte to be detected
or measured is glucose, the enzyme is preferably glucose oxidase and the redox
mediator is potassium ferrocyanide.
In a preferred embodiment of this invention, the test device further comprises
a blood separating membrane. The blood separating membrane separates whole blood
samples into highly colored and relatively clear fluid portions before analysis.
The blood separating membrane effectively blocks or filters red blood cells and
allows essentially clear fluid to pass to the test electrodes. As a result, the
analyte is measured in the clear fluid portion of the sample contacting the electrodes
thereby substantially eliminating the red blood cells from the reaction and minimizing
interference from the cells present in blood. This embodiment has the additional
benefit of keeping the test site from drying out and thereby improves the performance
of test devices designed for small sample sizes, such as in the 1 to 5 microlite range.
The electrochemical test device of the present invention is used to determine
the presence or concentration of an analyte in an aqueous fluid sample. Accordingly,
in one of its method aspects, the present invention provides a method for determining
the presence or concentration of an analyte in an aqueous fluid sample, said method comprising:
(a) providing an electrochemical test device comprising: (i) a non-conductive
surface; (ii) a working electrode comprising an amorphous semiconductor material
affixed to the non-conductive surface, said working electrode having an first electrode
area, a first lead and a first contact pad; (iii) a counter electrode comprising
an amorphous semiconductor material affixed to the non-conductive surface, said
counter electrode having a second electrode area, a second lead, and a second contact
pad; and (iv) a reagent capable of reacting with the analyte to produce a measurable
change in potential which can be correlated to the concentration of the analyte
in the fluid sample, said reagent overlaying at least of portion of the first electrode
area of the working electrode;
(b) inserting the electrochemical test device into a meter device;
(c) applying a sample of an aqueous fluid to the first electrode area of the
working electrode;
(d) reading the meter device to determine the presence or concentration of the
analyte in the fluid sample.
In another embodiment, the test device employed in this method further comprises
a reference electrode comprising an amorphous semiconductor material affixed to
the non-conductive surface, said reference electrode having a third electrode area,
a third lead, and a third contact pad, and wherein at least a portion of the third
electrode area is overlaid with a reference material. Preferably, the reference
material is a silver/silver chloride layer, a mercury/mercury chloride layer or
a platinum/hydrogen material. Silver/silver chloride is a particularly preferred
reference material.
The non-conductive surface may be any rigid or flexible material having appropriate
insulating and dielectric properties, as mentioned above. Preferably, the non-conductive
surface is provided by affixing a non-conductive coating to one side of a substrate,
which substrate is preferably a flexible metallic sheet material or a polymeric
sheet material.
As discussed above, the present invention utilizes amorphous semiconductor materials
and PCB manufacturing techniques to provide electrochemical test devices. This
film or thick film methods and technologies can be used to create the amorphous
semiconductor material conductive layers and electrodes according to this invention.
Accordingly, in one of its process aspects, the present invention provides a process
for preparing an electrochemical test device suitable for determining the presence
or concentration of an analyte in an aqueous fluid sample, said process comprising
the steps of:
(a) providing a non-conductive surface;
(b) depositing an amorphous semiconductor material on said surface to form a
conductive layer;
(c) chemically machining the conductive layer to form a working electrode comprising
a first electrode having a first electrode area, a first lead and a first contact
pad, and to form a counter electrode comprising a second electrode having a second
electrode area, a second lead and a second contact pad;
(d) applying a reagent to at least a portion of the first electrode area of the
working electrode, said reagent being capable of reacting with an analyte in an
aqueous fluid sample to produce a measurable change in potential which can be correlated
to the concentration of the analyte in the fluid sample.
In another embodiment, step (c) of this process further comprises forming a reference
electrode comprising a third electrode having a third electrode area, a third lead
and a third contact pad.
Preferably, the non-conductive surface is provided by affixing a non-conductive
coating to one side of a substrate, which substrate is preferably a flexible metallic
sheet material or a polymeric sheet material. Accordingly, in a preferred embodiment,
step (a) above comprises the steps of:
(f) providing a flexible substrate; and
(g) applying a non-conductive coating to the substrate to form a non-conductive surface.
In another preferred embodiment, step (c) above comprises the steps of:
(h) applying a photoresist to the conductive layer to form a first photoresist layer;
(i) positioning a first developer mask on the first photoresist layer;
(j) exposing the unmasked first photoresist layer to ultraviolet light to form
a first patterned photoresist area;
(k) removing the first developer mask;
(l) removing the first photoresist layer not exposed to ultraviolet light with
a developer to form a first exposed conductive layer;
(m) contacting the first exposed conductive layer with a chemical etchant to
remove the first exposed conductive layer; and
(n) removing the first patterned photoresist area with a solvent to form a second
exposed conductive layer, said second exposed conductive area comprising (i) a
working electrode comprising a first electrode having a first electrode area, a
first lead and a first contact pad, (ii) a counter electrode comprising a second
electrode having a second electrode area, a second lead and a second contact pad,
and optionally (iii) a reference electrode comprising a third electrode having
a third electrode area, a third lead and a third contact pad.
In further preferred embodiment, step (c) above further comprises the steps of:
(o) applying a photoresist to the second exposed conductive layer to form a second
photoresist layer;
(p) positioning a second developer mask on the second photoresist so that the
second photoresist layer covering the third electrode area is masked;
(q) exposing the unmasked second photoresist layer to ultraviolet light to form
a second patterned photoresist layer;
(r) removing the second developer mask;
(s) removing the second photoresist layer not exposed to ultraviolet light with
a developer to expose the third electrode area;
(t) applying a reference material the third electrode area;
(u) removing the second patterned photoresist layer with a solvent.
Preferably, the process employed to prepare the test devices of this
invention is a continuous process. The ability to use continuous processing to
fabricate the test devices, such as a continuous process utilizing continuous roll
coating, continuous roll sputtering, continuous photolithography systems utilizing
contact masks and flow through baths, results in high volume manufacturing capability
and substantial cost reductions over the step and repeat processes.
BRIEF DESCRIPTION OF DRAWINGS
FIGS. 1A, 1B, 1C and 1D illustrate the manufacturing process
from substrate or non-conductive surface to photoresist.
FIGS. 2A, 2B, 2C and 2D illustrate the manufacturing process
from photoresist to solvent application resulting in a finished working or counter electrode.
FIG. 3 illustrates finished reference electrode.
FIG. 4 illustrates a plan view of a test device having working, counter and
reference electrode.
FIGS. 5A, 5B and 5C illustrate the device of FIG. 4 with a blood
separating membrane.
DETAILED DESCRIPTION OF THE INVENTION
The present invention utilizes amorphous semiconductor materials and semiconductor
and printed circuit board (PCB) manufacturing techniques to provide an electrochemical
test device suitable for determining the presence or concentration of analytes
in aqueous fluid samples. By using amorphous semiconductors and PCB manufacturing
techniques, uniform electrochemical test devices having well-defined reproducible
electrode areas can be manufactured economically. These areas have very uniform
surface morphology which reduces the variability of the electrochemical test. The
surface morphology is directly related to the surface area of the electrodes. The
amount and concentration of applied reagents are also critical to producing an
accurate test. If the surface area is not consistent from device to device then
the individual devices manufactured have to be individually calibrated to insure
accurate readings in a meter. The test devices of the present invention permit
highly accurate electrochemical analyte measurements to be performed on very small
aqueous fluid samples due to the accurate reproduction of the test devices using
controlled deposition methods such as sputtering, vapor phase deposition, glow
discharge methods or evaporation and PCB chemical machining methods to form the
geometries. These devices can also be readily manufactured due to the lower cost
and flexible nature of the amorphous semiconductor materials which allow the use
of volume production in continuous processing manufacturing methods.
Thin film technologies are preferred for forming the amorphous semiconductor
material layer or coating, such as sputtering, vapor phase deposition, glow discharge
methods or evaporation. Such processes are capable of producing coatings up to
a thickness of a few microns and of applying the coatings uniformly over the entire
surface. If desired, thick film technologies are capable of producing much thicker
layers in the range of less than 0.005 inches. While both methods can produce uniform
surfaces, thin film surface morphology is dependent on the surface roughness of
the underlying substrate, whereas the thick film technologies are capable of changing
the final surface morphology of the device due to their thicker structure.
The electrochemical test device of this invention employs at least two types
of electrodes. The first type of electrode is referred to as the "working or indicating
electrode" and the second type is referred to as the "counter or opposing electrode".
These electrodes permit changes in potential between the electrodes to be accurately
measured when the analyte is applied across the electrodes, where one of the electrodes
has on its surface a reagent reactive with the analyte. Optionally, the test device
may also contain a reference electrode. The reference electrode allows accurate
measurements of potential to be made with respect to a fixed reference point. Any
suitable reference electrode may be used, include silver/silver chloride, mercury/mercury
chloride and platinum/hydrogen reference electrodes. Preferably, the reference
electrode is a silver/silver chloride reference electrode and the working electrode
has a potential ranging from about +1 to -1 volts versus the reference electrode.
Reference electrodes suitable for use in this invention are further described in
R. S. C. Cobbold,
Transducers for Biomedical Measurements: Principles and Applications,
pp. 343-349, John Wiley & Sons (New York).
The electrodes in the test device of this invention are preferably supported
on a flexible support material or substrate. Any suitable dielectric material may
be used as the substrate. For example, the substrate may be selected from a polymeric
sheet, film or foil material, such as a polyester, a polycarbonate, or a polyimide
support material. Such polymeric materials are well known in the art and are commercially
available. Typically, such polymeric support materials are annealed at a temperature
ranging from about 200 to about 220° F. (about 90 to 105° C.) prior to
use. Such an annealing process preshrinks the substrate thereby reducing the effects
of shrinkage during subsequent processing. Alternatively, the substrate may be
a metal foil having a thickness ranging from about 0.0005 to about 0.005 inches.
Flexible metal sheet materials can be used, but must be coated with a dielectric
coating. Preferred metal sheets include, foils such as aluminum, copper and stainless
steel foils. Aluminum and stainless steel foils are particularly preferred.
The substrate employed in this invention, whether ceramic, polymeric or metallic,
is preferably coated with a surface coating material prior to further use to improve
the surface morphology of the substrate. Suitable surface coating materials include,
by way of example, ink, paint, clay base coating materials and epoxy resins. When
metal foil is used as the substrate, the surface coating is preferably an epoxy
coating. Preferred epoxy resins have a high temperature resistance and include,
by way of example, epoxy anhydrides (Dicy), phenolic epoxy resins, and polyamide
epoxy resins (such as VERSAMIDES), available from Conat, Inc.
The surface coating material may be applied to the substrate by any conventional
method, such as roll coating, knife over roll, slot coating, reverse roll, lip
coating, spin coating or any another coating method or technique which applies
a thin layer of surface coating material sufficient to fill the surface valleys
of the substrate. Preferably, the surface coating material is applied to the substrate
such that the surface coating is less than about 0.005 inches in thickness. In
addition to providing an improved surface flatness by reducing the variation on
surface morphology, the surface coating increases the bonding of the conductive
layer to the substrate and provides a non-conductive barrier between the substrate
and the electrodes formed on the surface coating material.
The electrodes are then prepared on the coated substrate using semiconductor
and printed circuit board manufacturing techniques. Typically, a conductive layer
is first deposited on or applied to the coated substrate. The conductive layer
comprises an amorphous semiconductor material, such as amorphous silicon oxide,
amorphous carbon, gold or silver and the like. Amorphous metals can be formed by
permitting a small amount of helium gas to escape in the sputtering chamber during
sputtering. Such materials are well known in the art. For example, formation of
suitable amorphous semiconductor films are described in U.S. Pat. No. 4,226,898,
to Ovshinsky, the disclosure of which is incorporated herein by reference in its entirety.
Preferably, the amorphous semiconductor material employed in this invention
is doped with lithium, potassium, or a similar conducting ion to increased conductivity.
U.S. Pat. Nos. 3,983,076, and 4,178,415, to Ovshinsky, describe suitable methods
for adding various compounds to amorphous semiconductor material to increase its
conductivity. The disclosures of these patents are incorporated herein by reference
in their entirety.
The amorphous semiconductor material is preferably deposited on the non-conducting
coating or substrate to form a conductive layer having a thickness of less than
about 0.005 inches, more preferably a thickness ranging from about 1 to about 5
microns. The amorphous semiconductor material may be applied to the non-conducting
coating or substrate using any art recognized procedure to form a conductive layer,
such as sputtering, vapor phase deposition, glow discharge deposition, evaporation
and the like. Alternatively, other deposition techniques such as electroplating,
thick film laminating, roll transfer and the like may be employed. Thinner films
are best achieved by sputtering, evaporation, vapor phase deposition, glow discharge
methods or other thin film deposition method whereas thicker films are best formed
using thick film deposition techniques such as electroplating, thick film laminating,
roll transfer and the like. Charles A. Harper,
Handbook of Materials and Processes
for Electronics, 1984, (Library of Congress card number 76-95803) discusses
various methods of forming thin films in Chapter 11, describes various thick film
process in Chapter 12 and is devoted to photoresist processing in Chapter 14.
Preferably, the amorphous semiconductor material is deposited using vapor
phase deposition or sputtering techniques. In a particularly preferred embodiment,
the amorphous semiconductor material is formed by codeposition of the amorphous
semiconductor material and the doping material such as by co-vacuum deposition
or cosputtering. Cosputtering can be accomplished in a conventional r.f. sputtering
system, such as available from ULVAX Vacuum Systems. In this process, a cathode
or target is bonded to a standard aluminum backing plate and is made of the semiconductor
material to be deposited. Also, pieces of the doping material are secured to the
cathode or target. Substrates upon which the amorphous semiconductor material and
the doping material are to be deposited are carried by a holder located from the
target by a distance of about 3.5 cm for a 3 ⅓" diameter cathode or passed
continuously by the target.
The sputtering machine is first evacuated to a vacuum pressure somewhat less
than about 6×10
-6 torr to provide a background vacuum pressure.
Argon is bled into the machine to provide an operating pressure of about 5×10
-3
torr as determined by a reading on a Pirani vacuum gauge, giving an actual
vacuum pressure of about 7×10
-3 torr in the machine. The surface
of the cathode or target and pieces of doping material are first cleaned by sputtering
against the shutter of the machine adjacent to substrates for about 30 minutes.
Thereafter, the shutter is opened and the semiconductor material of the target
and the pieces of doping material on the target are cosputtered onto the non-conductive
coating or substrate. The cathode or target and the holder for the substrates are
both water cooled so that their temperature is low during the sputtering operation,
as for example, approximately 50° C. The r.f. power of the machine may have
a frequency of about 13.56 MegaHertz, and about 1000 Volts of forward power, about
50-70 Watts being utilized for 3.5" diameter cathode or target.
The deposition rates depend upon the materials being sputtered, and the time
of deposition is varied to obtain desired thicknesses of the deposit. The thicknesses
of the simultaneously deposited amorphous semiconductor film having the doping
material therein may vary from a few 100 Å to about 5μ, depending
upon the uses to which the amorphous semiconductor film is to be put. The amorphous
semiconductor material is deposited on the non-conductive coatings or substrates
in amorphous form.
The amount of doping material homogeneously incorporated in the amorphous semiconductor
material is generally determined by the area of the pieces of the doping material
applied to the semiconductor material of the cathode or target. Desired percentage
of modifier material added to the amorphous semiconductor material may accordingly
be conveniently controlled. By utilizing cosputtering as generally here described,
the doping material is substantially homogeneously incorporated in the semiconductor
material to provide the beneficial features of this invention.
Preferably, the amorphous semiconductor material is applied continuously
to the non-conductive coating or substrate using art recognized continuous processes.
Such continuous processes are described, for example, in U.S. Pat. No. 3,983,076,
to Rockstad, and in U.S. Pat. Nos. 3,271,591 and 4,178,415, to Ovshinsky, the disclosures
of which are incorporated herein by reference in their entirety. The ability to
use continuous processing to fabricate the test devices of this invention, such
as a continuous process utilizing continuous roll coating, continuous roll sputtering,
continuous photolithography systems utilizing contact masks and flow through baths,
results in high volume manufacturing capability an substantial cost reductions
over the step and repeat processes currently used. Suitable machines for such continuous
processes are made by Ulvac Vacuum Systems of Japan.
After the amorphous semiconductor material has been deposited on the non-conductive
coating or substrate, conventional photolithography techniques or other electronic
circuit fabrication technologies are used to form the electrodes. In the first
step of a typical process, a photoresist material is applied to the conductive
layer and dried. Any suitable photoresist material may be employed, including both
negative and positive photoresist materials. A preferred material is the negative
semi-aqueous resist available from Dupont under the tradename "Resiston".
A developer mask is then positioned over the photoresist layer. The mask can
be
either a contact or non-contact type. The patterning and masking methods that can
be employed to form the electrode shapes, conductive lines, contact pads, etc.,
according to this invention can include mechanical masks, contact masks and the
like, as well as other methods useful herein. for example, Chapter 14 of the above
metnioned harper, Handbood of Materials and Processes for Electronics, can be referred
to for such methods. The developer mask, which has cutout portions in the shape
of the electrodes, only covers a portion of the photoresist layer leaving a portion
of photoresist layer exposed. The uncovered or exposed photoresist layer is then
irradiated with ultraviolet (UV) light. Upon exposure to ultraviolet light, the
photoresist material becomes insoluble in the developer solvent. The UV-exposed,
insoluble photoresist material is termed "patterned photoresist". The developer
mask is then removed and the photoresist layer is contacted with developer to remove
the photoresist material previously covered by the developer mask. The developer
used in this step will vary depending on the particular photoresist material employed.
Typically, the proper developer for use with a particular photoresist will be specified
by the manufacturer of the resist material. When "Resiston" is used as the photoresist,
the developer/solvent recommended by Dupont should be employed and careful attention
paid to recommended procedures. If an alternate photoresist is selected, such as
Shipley "AZ-11", then an alternate developer would be used to remove the unexposed photoresist.
A chemical etchant is then used to remove the conductive layer no longer protected
by the photoresist material. The chemical etchant does not remove the conductive
material still protected by the remaining exposed, insoluble photoresist layer.
Suitable chemical etchants include hydrofluoric acid (HF) or ammonium fluoride/hydrofluoric
acid (NH
4F—HF) mixtures. A solvent is then applied to the patterned
photoresist areas defining the electrodes to remove the patterned photoresist layer.
Suitable solvents for removing the photoresist layer include, by way of example,
sulfuric acid/dichromate or ammonia/hydrogen peroxide. Treatment with the solvent
exposes the surface of the amorphous semiconductor electrodes. Each electrode comprises
three areas: a contact pad, a lead and an electrode area. Preferably, after exposure
of the electrodes, the leads of each electrode are insulated by applying an epoxy
resin material to the leads.
Optionally, the third electrode, if present, is then converted into a
reference electrode by applying a suitable reference material. Suitable reference
materials include silver/silver chloride, a mercury/mercury chloride and platinum/hydrogen
materials. Such materials can be applied to the third electrode area of the reference
electrode by any thin film deposition method described above.
A particularly preferred reference material is silver/silver chloride. In order
to obtain consistent results from device to device, the silver/silver chloride
layer must be applied such that the silver/silver chloride layer covers essentially
the same surface area in each device manufactured. Preferably, this is accomplished
using photolithography techniques as described above to expose a precise area of
the electrode after developing. A thin layer of silver is then deposited on the
exposed area, preferably by sputtering or other consistent film application method.
The silver layer is preferably less than about 0.001 inches in thickness, more
preferably about 1 micron to about 5 microns in thickness. Alternatively, a silver
layer can be applied to the electrode area of the reference electrode using pad
printing, inkjet methods, transfer roll printing or similar techniques. The silver
layer is then contacted with FeCl
3 solution to convert the silver surface
to silver chloride thereby forming a silver/silver chloride layer. Subsequent removal
of the photoresist using a solvent completes the formation of the reference electrode.
The electrochemical test device is then completed by applying an appropriate
reagent to the working electrode. Suitable reagents for determining the presence
or concentration of various analytes are well known in the art and are described
in further detail herein below.
Preferred Process for Preparing the Electrodes
As described above, the electrodes for the electrochemical test device of this
invention are prepared using amorphous semiconductor materials and semiconductor
and printed circuit board manufacturing techniques. Preferably, such electrodes
are formed in a continuous manner. A preferred process for the preparation of electrodes
suitable for use in this invention is illustrated in FIGS. 1-5.
As illustrated in cross section FIG. 1A, a thin support material
1 of
aluminum
foil 0.005 inches thick is employed.
Cross section FIG. 1B shows the substrate after being coated with epoxy
2
using a lip coater available from Hirano. The thickness of the epoxy
2 is
0.001 to 0.005 inches. The epoxy is thoroughly dried prior to the next step of
the process.
Cross section FIG. 1C shows the substrate formed from support material
1
and epoxy
2 in a sputtering apparatus where an amorphous silicon oxide layer
5 is applied to the epoxy
2. The target
3 and plasma
4
create an amorphous silicon layer
5 with a thickness of less than about
0.005 inches and preferable in the range of 1 to 5 microns.
As shown in cross section FIG. 1D after applying the amorphous silicon layer
5,
a photoresist layer
6 is applied to the composite. A suitable negative semi-aqueous
resist from Dupont, "Resiston" is appropriate for use in this invention. The resist
is applied by a hot roll lamination process.
As illustrated in cross section FIG. 2A, a photomask
7 having a selected
cutout pattern for the desired electrode and circuit lines and ultraviolet light
8 are then applied to the composite which is baked as specified by the Dupont
product directions. The photoresist that was exposed to the ultra violet light
is now insoluble to the developer.
The composite shown in cross section FIG. 2B is after processing in the developer
to expose portions of the amorphous silicon oxide layer
5. The entire composite
is then placed in a chemical etchant bath and the exposed areas of the amorphous
silicon layer
5 shown in FIG. 2B are removed.
As shown in cross section FIG. 2C, the chemical etchant bath removes unwanted
amorphous silicon oxide layer
5 leaving a composite of support material
1, epoxy
2, selective areas of amorphous silicon oxide
5 and
photoresist
6. The etchant is unable to dissolve the area of amorphous silicon
layer
5 covered by the photoresist layer
6.
As shown in cross section FIG. 2D, the activated or patterned photoresist layer
6 is then removed by applying a solvent which exposes the amorphous silicon
layer
5, leaving the desired electrode and conductive lines on epoxy
2.
Cross section FIG. 3 shows the reference electrode
21 after a silver/silver
chloride material layer
9 has been applied. Precise application of the silver/silver
chloride layer is extremely important and needs to be controlled so that the surface
area is constant and consistent from device to device. This is accomplished by
applying a photoresist similarly to previous steps and processing the electrodes
so that only the reference electrode
21 as shown in FIG. 4 is exposed after
developing. The part is then sputtered with a thin layer of silver less than 0.001
inches in thickness over the reference electrode
21 formed from amorphous
silicon oxide
5. The preferred thickness of the silver is between 1 and
5 microns of silver. Thinner conductors can be used as long as the system is controlled
and surface morphology is consistent. The part is then treated with FeCl
3
to convert the silver surface to silver chloride
9.
Plan view of FIG. 4 shows the exposed amorphous silicon layer
5 which
defines the electrodes, including the contact pads
15, leads
18 and
working electrode
20, reference electrode
21 and counter electrode
23. The lead lines
18 are then insulated by applying an epoxy
22.
Reagent
10 is applied to working electrode
20 after the silver/silver
chloride layer is applied to reference electrode
21.
Plan view FIG. 5A shows the preferred embodiment using a blood separation membrane
30 of this invention. Membrane
30 preferably has a skin side
31
adapted to block or filter red blood cells and a non-skin or opposite side
37.
Cross section view FIG. 5B shows that the skin side
31 of the membrane
30
is applied so that the skin side can receive the blood sample
32. Perspective
view FIG. 5C shows the membrane
30 has been embossed so it is forced into
hole
34 in handle or cover
36. Handle
36 is made of any suitable
dielectric material such as PET, polystyrene, or acrylic and must be as thick as
the membrane. The membrane is glued to the handle
36 from the underside
of handle
36 and to epoxy layer
22 which insulates the leads
18
using adhesive
38 as shown in FIG. 5B. A separating agent is imbibed and
dried to the membrane
30 prior to applying the membrane as shown in FIG.
5C. In this embodiment the side
37 opposite the skin side
31 of the
membrane
30 is in communication with electrodes
20,
21 and
23 as shown in FIG. 5A. To insure communication between the two layers,
the assembly process during manufacture includes applying mechanical pressure to
the skin side
31 of the membrane
30 to force the two layers together
so that they contact each other as shown in FIG. 5B.
Reagents
Various types of analytical or electrochemical sensor reagents may be applied
to the electrodes. To create a functional electrochemical test device, a reagent
chemistry must be selected based on the analyte to be tested and the desired detection
limits. Preferably, the reagent is deposited on the specific electrodes such that
a uniform amount is applied from sensor to sensor and the reagent is applied uniformly
over the appropriate electrodes. The reagent may be applied using any conventional
procedure, such as screen printing, inkjet printing, or discrete application using
IVEK pumps or any other drop on demand system capable of delivering consistent
and uniform volume of reagent.
The specific electrodes coated will depend on the specific reagent(s) employed.
Typically, the reagent is applied to the working electrode, but may in some cases
also be applied to the other electrodes. After the reagent has been placed on the
appropriate electrodes, it is typically dried. Subsequently, when the test device
is used, the test sample of aqueous fluid, such as blood, rehydrates the reagent
and a potential is applied to the electrodes from which a current measurement may
be taken by a meter.
An example reagent formulation suitable for use in the present invention is described
below. This reagent may be used to determine the presence or concentration of glucose
in an aqueous fluid sample. Preferably, this reagent formulation is used with an
electrochemical sensor having an counter electrode
20, working electrode
23 and reference electrode
21.
| |
Material |
Amount/Concentration |
| |
|
| |
2-(N-morpholino)ethanesulfonic acid |
100 millimolar (mM) |
| |
(MES buffer) |
| |
Triton X-100 |
0.08% wt/wt |
| |
Polyvinyl alcohol (PVA) mol. wt. 10 K |
1.00% wt/wt |
| |
88% hydrolized |
| |
Imidazole osmium mediator, reduced, |
6.2 mM |
| |
as defined in U.S. Pat. No. 5,437,999 |
| |
Glucose Oxidase |
6000 units/mL |
| |
|
The above reagent formulation may be prepared using the following procedures:
(a) 1.952 grams of MES buffer is added to 85 mL of nanograde water. The mixture
is stirred until the components dissolve. The pH of the solution is adjusted to
5.5 with NaOH. The volume of the solution is then brought to 100 mL of final buffer solution.
(b) 0.08 grams of Triton X-100 and 1 gram of PVA is added to a beaker capable
of holding all the components (e.g., a 200 mL beaker). The buffer solution is added
to bring the total solution weight to 100 grams. The mixture is heated to boiling
and stirred to dissolve the PVA.
(c) 4.0 mg of the reduced osmium mediator is added to 1 mL of the solution from
step (b) above. The mixture is stirred to dissolve the mediator.
(d) The mixture is left to cool to room temperature.
(e) 6000 units of glucose oxidase are added and the mixture is mixed until the
enzyme is dissolved.
The above reagent formulation may be used to determine the presence or concentration
of glucose in an aqueous fluid sample. As will be apparent to those skilled in
the art, other reagent formulations may be employed to assay different analytes.
Such reagent formulations are well known in the art. Typically, such reagent formulations
are designed to react specifically with the desired analyte to form a measurable
electrochemical signal.
Without being limited to theory, it is believed that in the example reagent
formulation described above, glucose is anaerobically oxidized or reduced with
the involvement of the enzyme and the redox mediator. Such a system is sometimes
referred to as an amperometric biosensor. Amperometry refers to a current measurement
at constant applied voltage on the working electrode. In such a system, the current
flowing is limited by mass transport. Therefore, the current is proportional to
the bulk glucose concentration. The analyte, enzyme and mediator participate in
a reaction where th
