Patent Grant
8940153
The present invention relates generally to reagents used in test sensors and, more particularly, to reagents having cellulose polymers for improving test sensor stability and reducing total assay time.
The quantitative determination of analytes in body fluids is of great importance in the diagnoses and maintenance of certain physiological abnormalities. For example, lactate, cholesterol and bilirubin should be monitored in certain individuals. In particular, determining glucose in body fluids is important to diabetic individuals who must frequently check the glucose level in their body fluids to regulate the glucose intake in their diets. The results of such tests can be used to determine what, if any, insulin or other medication needs to be administered. In one type of blood glucose testing system, test sensors are used to test a fluid such as a sample of blood.
A test sensor contains biosensing or reagent material that will react with the analyte of interest, such as blood glucose. The testing end of the test sensor is adapted to be placed into the fluid being tested, for example, blood that has accumulated on a person's finger after the finger has been pricked. The fluid is drawn into a capillary channel that extends in the test sensor from the testing end to the reagent material by capillary action so that a sufficient amount of fluid to be tested is drawn into the test sensor. In some test sensors, the fluid then chemically reacts with the reagent material in the test sensor resulting in an electrical signal indicative of the glucose level in the fluid being tested.
One problem with current test sensors is that the reagents may contain components that interfere with sensor stability. In particular, some components, such as polyethylene oxide (“PEO”), may be incompatible with other components, such as the enzyme and the electron transfer mediator, which are important for test sensors. Test sensors having reagents that are formulated with components that are, for example, incompatible with the enzyme and the electron transfer mediator may exhibit poor test sensor stability over time. This instability is especially apparent when the total assay time is less than about 35 seconds. Thus, it would be desirable to have a test sensor reagent having components that improve test sensor stability.
According to one embodiment of the invention, a test sensor reagent composition is adapted to assist in determining an analyte concentration of a fluid sample. The reagent comprises an enzyme, an electron transfer mediator, a cellulose polymer and a rheological additive.
According to another embodiment of the invention, a method of determining an analyte concentration of a fluid sample comprises the acts of providing an electrochemical test sensor that is adapted to assist in determining the analyte concentration. The electrochemical test sensor comprises a plurality of electrodes including a counter electrode and a working electrode, a fluid receiving area, and a test sensor reagent including a cellulose polymer. The method also includes the acts of determining the analyte concentration in an assay time of less than about 35 seconds.
According to another embodiment of the invention, a method of determining an analyte concentration of a fluid sample comprises the acts of pricking a finger of a test subject to produce the fluid sample, placing the fluid sample having at least one analyte within a test sensor, contacting the fluid sample with a reagent comprising a cellulose polymer which assists in stabilizing the test sensor, providing an electrical signal indicative of the analyte in the fluid sample, and determining the analyte using the electrical signal.
According to another embodiment of the invention, a cartridge for use in a test sensor comprises a plurality of test sensors and a housing adapted to store the plurality of test sensors. Each test sensor includes a reagent comprising a cellulose polymer that is adapted to stabilize the test sensor and reduce a total assay time to less than about 35 seconds.
According to another embodiment of the invention, a method of determining an analyte concentration of a fluid sample comprises the acts of pricking a finger of a test subject to produce the fluid sample, placing the fluid sample having at least one analyte within a test sensor, contacting the fluid sample with a reagent comprising a cellulose polymer which assists in stabilizing the test sensor and determining the analyte concentration of the fluid sample.
According to a further embodiment of the invention, a method of screen printing on a substrate comprises the acts of providing a screen that includes a first portion with a photosensitive emulsion and a second portion formed in the absence of a photosensitive emulsion, supplying a reagent comprising a solvent, a cellulose polymer and an enzyme to assist in determining an analyte concentration of a fluid sample on the screen, and contacting the reagent onto the substrate via the second portion of the screen.
The above summary of the present invention is not intended to represent each embodiment, or every aspect, of the present invention. Additional features and benefits of the present invention will become apparent from the detailed description, figures, and claims set forth below.
a is a front view of a disposable cartridge with a plurality of stacked test sensors according to one embodiment.
b is a front view of a sensor-dispensing instrument according to one embodiment that is adapted to receive the cartridge of
a-7b and 7c-7d are a series of graphs comparing the assay bias and assay % bias based on 10-second assays and 30-second assays, respectively, for test sensors having an HEC-based reagent and PEO-based reagent according to one embodiment of the present invention.
The present invention is directed to a reagent to be used in a single sensor instrument or a sensor-dispensing instrument that contains a plurality of electrochemical or optical test sensors. The electrochemical or optical test sensors are used to determine concentrations of at least one analyte in a fluid. Analytes that may be determined using the reagent of the present invention include glucose, lipid profiles (e.g., cholesterol, triglycerides, LDL and HDL), hemoglobin A1C, fructose, lactate, or bilirubin. The present invention is not limited, however, to determining these specific analytes and it is contemplated that other analyte concentrations may be determined. The analytes may be in, for example, a whole blood sample, a blood serum sample, a blood plasma sample, or other body fluids like ISF (interstitial fluid) and urine.
The plurality of test sensors is typically stored in a disposable cartridge or container. In one embodiment, the plurality of test sensors may be stored in a sensor pack where the test sensors are individually packaged in sensor cavities (e.g., a blister-type pack). An example of a disposable cartridge 10 being placed in a sensor-dispensing instrument 20 is depicted in
In an alternative embodiment, the plurality of test sensors may be stacked in a disposable cartridge such as shown in
The cartridges 10, 50 of
In some embodiments, the test sensors to be used in the cartridges or containers are typically provided with a capillary channel that extends from the front or testing end of the test sensor to the biosensing or reagent material disposed in the test sensor. When the testing end of the test sensor is placed into fluid (e.g., blood that is accumulated on a person's finger after the finger has been pricked), a portion of the fluid is drawn into the capillary channel by capillary action. The fluid then chemically reacts with the reagent in the test sensor so that an electrical signal indicative of the analyte (e.g., glucose) level in the fluid being tested is supplied and subsequently transmitted to an electrical assembly.
In some test sensors, the reagent is applied to a substrate via a screen printing process. The screen printing process allows a thin layer of the reagent to be applied to a small, flat test sensor, such as the test sensor shown in
The composition of the reagent that is applied to the test sensor may influence such items as the length of time needed to perform the testing to determine the analyte concentration (i.e., the assay time), the stability of the test sensor and the ease of the application of the reagent via the screen printing process. The composition of the reagent of the present invention includes ingredients that provide desirable test sensor characteristics, such as increased stability of the test sensor, reduced total assay time and improved adherence of the reagent to the substrate.
One embodiment of the present invention that provides such desirable characteristics includes a reagent having cellulose polymers. The cellulose polymers serve as a binder for the components of the reagent layer and help to increase the viscosity of the reagent. It has also been found that the use of cellulose polymers in the reagent improves the stability of the test sensor. A particularly desirable cellulose polymer includes hydroxyethyl cellulose (“HEC”) polymer. HEC is desirable due to its stabilizing properties. Specifically, when HEC is used in place of other polymer materials, the degradation of glucose oxidase is reduced, as well as the occurrence of the reduction of the mediator. The reduction in these reactions leads to improved test sensor stability by reducing the background current of the test sensor. Other suitable polymers that may be used in the reagent formulation include carboxymethyl cellulose, cellulose acetate, ethylcellulose, or hydroxypropyl methylcellulose, polyvinyl pyrrolidone, polyvinyl alcohol, or combinations thereof.
According to one embodiment of the present invention, the reagent generally comprises about 1 wt. % to about 10 wt. % cellulose polymers of molecular weights between about 25,000 and about 2,000,000, and desirably from about 3 wt. % to about 6 wt. % cellulose polymers of molecular weights between about 300,000 to about 1,000,000. Cellulose polymers are commercially available from various suppliers. For example, Natrasol®, an HEC polymer, is available from Hercules Inc. in Wilmington, Del.
According to another embodiment of the present invention, in addition to the cellulose polymers described above, the reagent includes additional components such as an enzyme, an electron transfer mediator and Theological additives.
For testing blood glucose levels, a glucose oxidase enzyme may be used. Glucose oxidase enzyme reacts with glucose in the blood sample and produces an electrical signal that indicates the glucose concentration. The enzyme activities may be measured in terms of the activity unit (U) which is defined as the amount of enzyme that will catalyze the transformation of one micromole of a substrate per minute under standard conditions. The reagent may comprise about 0.5 wt. % up to about 5 wt. % glucose oxidase enzyme, and desirably from about 1.0 wt. % up to about 4.0 wt. %.
Glucose oxidase enzyme can be obtained commercially from companies such as Biozyme Laboratories International Ltd. in San Diego, Calif., Genzyme Corporation in Cambridge, Mass. and Amano Enzyme Inc. in Elgin, Ill. Depending on the analyte being tested, the reagent may contain other enzymes, such as glucose dehydrogenase, cholesterol oxidase, cholesterol dehydrogenase, lactate oxidase, etc., to detect other analytes in the blood sample in addition to glucose.
As described above, the reagent may also include an electron transfer mediator. Examples of mediators that may be used with the present invention include potassium ferricyanide, potassium ferrocyanide, ferrocene or its derivatives, quinone or its derivatives, organic conducting salts or viologen, in addition to other mediators. Preferably, the electron transfer mediator is a mixed-valence compound capable of forming redox couples. Depending on the electron transfer mediator that is used, the reagent generally comprises from about 1 wt. % to about 20 wt. % electron transfer mediator, and desirably from about 15 wt. % to about 20 wt. % electron transfer mediator. In one embodiment of the present invention, the electron transfer mediator is a ferricyanide mediator. Ferricyanide mediators, as well as other electron transfer mediators, are commercially available from various suppliers such as Sigma-Aldrich Co. It is contemplated that other electron transfer mediators, in addition to ferricyanide mediators, may be used in the present invention.
Rheological additives that are included in the reagent may include smectite clays such as montmorillonite, hectorite or bentonite clays or other suitable natural or synthetic materials. Hectorite is composed of clay minerals and is commercially available as Bentone® from Elementis Specialities Inc. in Hightstown, N.J. or as OPTIGEL® SH Synthetic Hectorite which is commercially available from Sud-Chemie Inc. in Louisville, Ky. Other rheological additives that may be used with the present invention may include other beneficiated clays, xanthum gum, fumed silica and Acti-Gel™ 208, a magnesium alumino silicate which is commercially available from Active Minerals Company LLC.
The rheological additives that may be used in the reagent of the present invention are desirably thixotropic or viscosity-modifying materials. Such materials improve the screen printing properties of the reagent. Specifically, the thixotropic additives of the present invention include materials that exhibit a decrease in viscosity over time. Additionally, the viscosity of the thixotropic additives of the present invention also decreases the longer the additives undergo shear. The rheological additives in the reagent may also serve as binder or filler materials.
In one embodiment, the reagent may comprise about 0.1 wt. % to about 3 wt. % smectite clay or other suitable rheological additive, and desirably from about 0.2 wt. % to about 1.6 wt. %. It is contemplated that other rheological additives having the properties described above may be used in the reagent. The amount and type of rheological additive that is used may vary depending on the polymer that is used in the reagent, as well as whether the reagent is aqueous- or organic-based.
In yet other embodiments of the present invention, the reagent may include additional components, such as a buffer and a wetting agent. Examples of buffers that may be used include citric acid, sodium citrate and other suitable buffers, such as phosphate buffers. The reagent may comprise about 10 mMolar to about 500 mMolar of the buffer and desirably from about 25 mMolar to about 200 mMolar. Other suitable buffers may include sodium acetate, Hepes buffer, etc. The buffer that is used in the reagent may be selected based on the electron transfer mediator that is used. For example, if a ferricyanide mediator is included in the reagent, a buffer that will maintain a lower pH level and that will not react with the ferricyanide mediator is desirable.
Suitable wetting agents may include fluorocarbon- or hydrocarbon-based surfactants. Some examples of surfactants that may be used with the present invention include Triton™ surfactants from The Dow Chemical Company in Midland, Mich. and Surfynol® additives from Air Products and Chemicals, Inc. in Allentown, Pa. The reagent may comprise about 0.01 wt. % to about 0.3 wt. % of a fluorocarbon-based surfactant, and desirably from about 0.02 wt. % to about 0.06 wt. %. Additionally, or alternatively, the reagent may comprise about 0.1 wt. % to about 5.0 wt. % of a hydrocarbon-based surfactant, and desirably from about 1.0 wt. % to about 3.0 wt. %.
The remainder of the formulation may contain water or other suitable solvents which may vary depending on the enzyme and electron transfer mediator chosen. The solvent should be inert to the enzyme and the electron transfer mediator.
The reaction layer 124 includes the reagent that converts the analyte of interest (e.g., glucose) into a chemical species that is electrochemically measurable, in terms of the electrical current it produces, by the components of the electrode pattern 116, 118. The reaction layer 124 is disposed over the opening 122 and channel 125 in the insulating layer 120. Thus, the portion of the reaction layer 124 exposed to the electrode pattern 116,118 is defined by the opening 122 and the channel 125 in the insulating layer 120. The working electrode 116 is electrically coupled to the first lead 114a, and the counter electrode 118 is electrically coupled to the second lead 114b. A trigger counter electrode subunit 119 is electrically coupled to the counter electrode 118 and serves as an underfill detection electrode in a two electrode system.
The test sensor 110 includes a lid 130 having a concave portion 132 that forms a capillary channel when mated with the insulating layer 120 for moving the liquid sample from an inlet 134 into the test sensor 110. The downstream end of the capillary channel includes one or more openings 136 for venting the capillary channel—the fluid sample flows from the inlet 134 into the test sensor 110 toward the opening 136. In use, the test sensor 110 collects a fluid sample (e.g., a blood sample from a patient's finger) by bringing the capillary channel inlet 134 into contact with the fluid sample.
The reagent of the present invention described herein may be used in a variety of test sensors. Some example of test sensors that may use the reagent formulation are the Ascensia™ Autodisc™ and Glucodisc Blood Glucose Test Strips that are designed to be used by the Ascensia™ BREEZE™ Blood Glucose Meter and the Ascensia™ DEX® 2/DEX® Blood Glucose Meter from Bayer Healthcare LLC of Tarrytown, N.Y.
As mentioned above, test sensor stability is improved by using reagents having cellulose-based polymers. This is particularly true for assay tests less than about 35 seconds, and especially desirable for assay tests less than about 25 seconds. The improved stability of the test sensor leads to longer shelf-life and use-life of the test sensor.
To compare test sensor stability, changes in reagent background as a function of time and temperature and thermal stability, a group of test sensors having an HEC-based reagent was provided as described below in Example 1. Another group of test sensors having a PEO-based reagent was provided as described below in Example 2. The results of the testing are described in Examples 3, 4 and 5 and are depicted in
To assess test sensor stability, a test was performed on two lots of test sensors to determine the percentage of glucose oxidase recovery that occurred after the test sensors were stored at −20 degrees C. and 50 degrees C. for two and four weeks. One lot of test sensors having an HEC-based reagent was compared with a second lot of test sensors having a PEO-based reagent. At the end of the test sensor storage period, the test sensors were extracted with a buffer and the glucose oxidase activity in the test sensor extracts was analyzed using standard enzyme activity analysis methods.
The results of the testing are shown in
a, 7b, 7c and 7d show a comparison of the assay bias for stressed test sensors formulated with HEC-based and PEO-based reagents. To assess the thermal stability of the test sensors, the test sensors were stored at −20 degrees C. and 50 degrees C. for two and four weeks. At the end of the test sensor storage periods, the test sensors were evaluated with 40% hematocrit whole blood at 50, 100, and 400 mg/dL glucose concentrations. Twenty replicates per samples were collected using 30 second and 10 second assay protocols. The difference in glucose assays results between the 50 degree C. stressed test sensors and the −20 degree C. stressed test sensors was calculated. For samples with 50 mg/dL glucose, the difference in assay results was expressed as “assay bias” (see
Improvement in stability of the test sensors was most notable at the lower glucose levels due to lower test sensor background drift. The results showed that the differences in the assay bias and assay % bias were more notable when the total assay time was changed from 30 seconds to 10 seconds. The HEC-based reagent dramatically reduced the assay bias between the 50 degree C. and the −20 degree C. test sensors. This was observed for both the 30 second and 10 second assays.
While the test sensor reagent of the present invention has been described for use primarily with an electrochemical test sensor, it is contemplated that the test sensor reagent of the present invention may also be adapted for use with other test sensors, such as optical test sensors.
A test sensor reagent composition adapted to assist in determining an analyte concentration of a fluid sample, the reagent comprising an enzyme, an electron transfer mediator, a cellulose polymer and a rheological additive.
The composition according to Alternative Embodiment A, wherein the reagent comprises from about 3.6 wt. % to about 6.0 wt. % of the cellulose polymer.
The composition according to Alternative Embodiment A, wherein the reagent comprises from about 1 wt. % to about 4 wt. % of the enzyme.
The composition according to Alternative Embodiment A, wherein the reagent comprises from about 15 wt. % to about 20 wt. % of the electron transfer mediator.
The composition according to Alternative Embodiment A, wherein the reagent comprises from about 0.2 wt. % to about 1.6 wt. % of the Theological additive.
The composition according to Alternative Embodiment A, wherein the reagent comprises from about 3.6 wt. % to about 6.0 wt. % of a hydroxyethyl cellulose polymer, from about 1 wt. % to about 4 wt. % of a glucose oxidase enzyme, from about 15 wt. % to about 20 wt. % of a ferricyanide mediator and from about 0.2 wt. % to about 1.6 wt. % of a smectite clay.
The composition according to Alternative Embodiment F, wherein the smectite clay includes bentonite, hectorite, montmorillonite, or a combination thereof.
The composition according to Alternative Embodiment A, wherein the reagent further comprises about 10 mMolar to about 500 mMolar of a citrate buffer.
The composition according to Alternative Embodiment H, wherein the citrate buffer comprises citric acid, sodium citrate, or a combination thereof.
The composition according to Alternative Embodiment A, wherein the reagent further comprises from about 0.02 wt. % to about 0.1 wt. % of a fluorocarbon surfactant.
The composition according to Alternative Embodiment A, wherein the reagent further comprises from about 1.0 wt. % to about 3.0 wt. % of a hydrocarbon surfactant.
The composition according to Alternative Embodiment A, where a total assay time of a test sensor including the test sensor reagent composition is less than about 35 seconds.
A method of determining an analyte concentration of a fluid sample comprising the acts of:
providing an electrochemical test sensor adapted to assist in determining the analyte concentration, the electrochemical test sensor comprising a plurality of electrodes including a counter electrode and a working electrode, a fluid receiving area and a test sensor reagent, the test sensor reagent including a cellulose polymer; and
determining the analyte concentration in an assay time of less than about 35 seconds.
The electrochemical test sensor according to Alternative Process M, wherein the cellulose polymer comprises hydroxyethyl cellulose.
The electrochemical test sensor according to Alternative Process M, wherein the test sensor reagent further comprises an enzyme, an electron transfer mediator and a rheological additive.
The electrochemical test sensor according to Alternative Embodiment O, wherein the electron transfer mediator comprises a ferricyanide mediator.
The electrochemical test sensor according to Alternative Embodiment O, wherein the rheological additive comprises a smectite clay.
The electrochemical test sensor according to Alternative Embodiment Q, wherein the smectite clay comprises bentonite, hectorite, montrnorillonite, or combinations thereof.
The electrochemical test sensor according to Alternative Embodiment O, wherein the enzyme comprises a glucose oxidase enzyme.
The electrochemical test sensor according to Alternative Process M, wherein the total assay time is reduced to less than about 25 seconds.
A method of determining an analyte concentration of a fluid sample, the method comprising the acts of:
pricking a finger of a test subject to produce the fluid sample;
placing the fluid sample within a test sensor, the fluid sample having at least one analyte;
contacting the fluid sample with a reagent comprising a cellulose polymer, the cellulose polymer assisting in stabilizing the test sensor;
providing an electrical signal indicative of the analyte in the fluid sample; and
determining the analyte using the electrical signal.
The method according to Alternative Process U, wherein the analyte concentration is determined in less than about 35 seconds.
The method according to Alternative Process U, wherein the reagent comprises hydroxyethyl cellulose.
The method according to Alternative Process W, wherein the reagent further comprises a glucose oxidase enzyme, a ferricyanide mediator and a smectite clay.
A method of determining an analyte concentration of a fluid sample, the method comprising the acts of:
pricking a finger of a test subject to produce the fluid sample;
placing the fluid sample within a test sensor, the fluid sample having at least one analyte;
contacting the fluid sample with a reagent comprising a cellulose polymer, the cellulose polymer assisting in stabilizing the test sensor; and
determining the analyte concentration of the fluid sample.
The method according to Alternative Process Y, wherein the test sensor is an optical test sensor.
A method of screen printing on a substrate, the method comprising the acts of:
providing a screen that includes a first portion with a photosensitive emulsion and a second portion formed in the absence of a photosensitive emulsion;
supplying a reagent on the screen, the reagent comprising a solvent, a cellulose polymer and an enzyme to assist in determining an analyte concentration of a fluid sample; and
contacting the reagent onto the substrate via the second portion of the screen.
The method according to Alternative Process AA, wherein the reagent further comprises a Theological additive.
The method according to Alternative Process BB, wherein the rheological additive is a smectite clay.
The method according to Alternative Process CC, wherein the smectite clay includes hectorite, bentonite, montmorillonite, or combinations thereof.
The method according to Alternative Process BB, wherein the rheological additive is a thixotropic material.
The method according to Alternative Process AA, wherein the reagent further comprises an electron transfer mediator.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are described in detail herein. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, to the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
This application claims priority to Application Ser. No. 60/736,537 filed on Nov. 14, 2005, which is incorporated by reference in its entirety.
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