Techniques to improve polyurethane membranes for implantable glucose sensors

Abstract

The invention provides an implantable membrane for regulating the transport of analytes therethrough that includes a matrix including a first polymer; and a second polymer dispersed throughout the matrix, wherein the second polymer forms a network of microdomains which when hydrated are not observable using photomicroscopy at 400× magnification or less. In one aspect, the homogeneous membrane of the present invention has hydrophilic domains dispersed substantially throughout a hydrophobic matrix to provide an optimum balance between oxygen and glucose transport to an electrochemical glucose sensor.

Claims

1. An implantable device for measuring an analyte in a biological fluid, comprising: an implantable sensor comprising an electrochemically reactive surface; and a membrane disposed over at least a portion of the electrochemically reactive surface, wherein the membrane comprises a matrix comprising a first polymer and a second polymer dispersed throughout the matrix, wherein the first polymer is polyurethane urea, wherein the second polymer comprises at least one hydrophilic component, wherein the hydrophilic component of the second polymer forms in the membrane hydrophilic segments along which the analyte diffuses.

2. The implantable device of claim 1, wherein the second polymer comprises polyethylene oxide.

3. The implantable device of claim 2, wherein polyethylene oxide comprises about 20% by weight of the second polymer.

4. The implantable device of claim 2, wherein the polyethylene oxide has an average molecular weight from 200 to 3,000 Daltons.

5. The implantable device of claim 1, wherein the hydrophilic segments comprise a network of microdomains.

6. The implantable device of claim 5, wherein the microdomains are not photomicroscopically observable when hydrated at 400× magnification or less.

7. The implantable device of claim 5, wherein the network of are substantially dispersed throughout the matrix.

8. The implantable device of claim 1, wherein the membrane has a thickness from 5 to 100 microns.

9. The implantable device of claim 1, wherein the membrane is configured to yield calibration factors that are stable over the life of the sensor.

10. The implantable device of claim 1, wherein the analyte is glucose.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a photomicrograph of a cross-section of prior art membrane at 200× magnification following hydration with water for two hours.

(2) FIG. 2A is a schematic representation of a cross-section of a prior art membrane having large hydrated structures dispersed substantially throughout a hydrophobic matrix, the hydrated structures being photomicroscopically observable at 400× magnification or less. The figure illustrates the positioning of a working electrode relative to a glucose diffusion pathway.

(3) FIG. 2B is another schematic representation of a cross-section of the prior art membrane of FIG. 2A, where the working electrode is placed in association with a locally high concentration of the hydrophobic matrix.

(4) FIG. 2C is yet another schematic representation of a cross-section of the prior art membrane of FIG. 2A where glucose diffusion is variable across the dimension adjacent the electrode surface.

(5) FIG. 3 is a photomicrograph of a cross-section of a membrane of the present invention at 200× magnification following hydration with water for two hours.

(6) FIG. 4 is a schematic representation of a cross-section illustrating one particular form of the membrane of the present invention that shows a network of microdomains which are not photomicroscopically observable at 400× or less magnification dispersed through a hydrophobic matrix, where the membrane is positioned in association with a sensor that includes a working electrode.

(7) FIG. 5 is a schematic representation of a cross-section of the membrane of FIG. 3 in combination with an enzyme containing layer positioned more adjacent to a sensor 50.

(8) FIG. 6 is a graph showing sensor output versus the percent of the hydrophobic-hydrophilic copolymer component in the coating blend.

(9) FIG. 7 is a graph showing the percent standard deviation of the sensor current versus the percent of the hydrophobic-hydrophilic copolymer component in the coating blend.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(10) In order to facilitate understanding of the present invention, a number of terms are defined below.

(11) The term “analyte” refers to a substance or chemical constituent in a biological fluid (e.g. blood or urine) that is intended to be analyzed. A preferred analyte for measurement by analyte detecting devices including the membrane of the present invention is glucose.

(12) The term “sensor” refers to the component or region of a device by which an analyte can be evaluated.

(13) By the terms “evaluated”, “monitored”, “analyzed”, and the like, it is meant that an analyte may be detected and/or measured.

(14) The phrase “continuous glucose sensing” refers to the period in which monitoring of plasma glucose concentration is repeatedly performed over short periods of time, for example, 10 seconds to about every 15 minutes.

(15) The term “domain” refers to regions of the membrane of the present invention that may be layers, uniform or non-uniform gradients (e.g. anisotropic) or provided as portions of the membrane. Furthermore, the region possesses physical properties distinctly different from other portions of the membrane.

(16) The terms “accurate” and “accurately” means, for example, 85% of measured glucose values are within the “A” and “B” region of a standard Clarke Error Grid when the sensor measurements are compared to a standard reference measurement. It is understood that like any analytical device, calibration, calibration validation and recalibration are required for the most accurate operation of the device.

(17) The term “host” refers to humans and other animals.

(18) In the disclosure that follows, the invention will primarily be referred to in terms of assay of glucose and solutions such as blood that tend to contain a large excess of glucose over oxygen. However, it is well within the contemplation of the present invention that the membrane is not limited solely to the assay of glucose in a biological fluid, but may be used for the assay of other compounds. In addition, the sensor primarily referred to is an electrochemical sensor that directly measures hydrogen peroxide. However, it is well within the contemplation of the present invention that non-electrochemical based sensors that use optical detectors or other suitable detectors may be used to evaluate an analyte.

(19) Membranes of the prior art have generally been unreliable at limiting the passage of glucose to implantable glucose sensors. This has presented a problem in the past in that the amount of glucose coming into contact with the immobilized enzyme exceeds the amount of oxygen available. As a result, the oxygen concentration is the rate-limiting component of the reaction, rather than the glucose concentration, such that the accuracy of the glucose measurement in the body fluid is compromised.

(20) As described above, in contrast to the present invention, a disadvantage of prior art membranes for regulating analyte transport therethrough has been their tendency to form large undesirable structures (see FIG. 1) that are observable when the membrane is hydrated. In particular, these hydrated structures can be detected by photomicroscopy under magnifications in the range of between 200×-400×, for example. They have been shown by the present inventors to be non-uniform in their dimensions through the membrane, with some being of the same size and same order of dimensions as the electrode size. These large structures have been found to be problematic in that they can result in a locally high concentration of either hydrophobic or hydrophilic material in association with the working electrode, which can lead to inaccurate glucose readings. Moreover, they can greatly reduce the number of glucose diffusion paths available.

(21) The membrane of the present invention seeks to circumvent these problems associated with prior art membranes by providing a reliable homogeneous membrane that regulates the transport of glucose or other analytes therethrough, the membrane having (a) a matrix including a first polymer; and (b) a second polymer dispersed throughout the matrix, wherein the second polymer forms a network of microdomains which when hydrated are not observable using photomicroscopy at 400× magnification or less. In one embodiment of the invention, the membrane is substantially free of observable domains.

(22) We refer now to FIG. 3, which shows a photomicrograph of a cross-section of a membrane 5 according to the present invention following hydration at two hours. As shown in FIG. 3, the membrane is devoid of any undesirable, large elliptical or spherical structures, such as were observable in hydrated prior art membranes at similar magnifications. It is noted that particles 6 in membrane 5 are dust particles.

(23) For purposes of the present invention, it is likely that glucose permeability and diffusion is related to the ratio of hydrophobic to hydrophilic constituents and their distribution throughout the membrane, with diffusion occurring substantially along assembled hydrophilic segments from the side of the membrane in contact with the host to the sensing side.

(24) Referring now to FIG. 4, membrane 42 of the present invention, in accordance with a particular arrangement, is schematically shown having hydrophilic segments 44 dispersed substantially throughout a hydrophobic matrix 46 and presenting a surface 48 to a hydrophilic body fluid. The hydrophilic body fluid contains the sample to be assayed. In one embodiment, the body fluid contains both glucose and oxygen. Membrane 42 restricts the rate at which glucose enters and passes through the membrane and/or may increase the rate at which oxygen enters and passes through membrane 42.

(25) While not wishing to be bound by any one theory, it is likely that glucose diffuses substantially along hydrophilic segments 44, but is generally excluded from the hydrophobic matrix 46. It is noted that while the hydrophilic segments 44 are shown as comprising discrete microdomains in FIG. 4, small amounts of hydrophobic polymer may be present therein, particularly at the interface with the hydrophobic matrix 46. Similarly, small amounts of hydrophilic polymer may be present in the hydrophobic matrix 46, particularly at the interface with hydrophilic segments 44.

(26) In the embodiment shown in FIG. 4, inventive membrane 42 is shown in combination with a sensor 50, which is positioned adjacent to the membrane. It is noted that additional membranes or layers may be situated between membrane 42 and sensor 50, as will be discussed in further detail below. Diffusion of the sample along paths 52 through membrane 42 into association with a working electrode 54 of sensor 50 causes development of a signal that is proportional to the amount of analyte in the sample. Determination of the analyte may be made by calculations based upon similar measurements made on standard solutions containing known concentrations of the analyte. For example, one or more electrodes may be used to detect the amount of analyte in the sample and convert that information into a signal; the signal may then be transmitted to electronic circuitry required to process biological information obtained from the host. U.S. Pat. Nos. 4,757,022, 5,497,772 and 4,787,398 describe suitable electronic circuitry that may be utilized with implantable devices of the present invention.

(27) The present invention solves a need in the art by providing a reliable membrane for controlling glucose diffusion therethrough. As shown in FIG. 4, glucose can traverse along hydrophilic segments 44 from the side 48 of the membrane in contact with a body fluid to the side 56 proximal to sensor 50. The hydrophilic microdomains 44 are likely distributed substantially evenly throughout the membrane. Furthermore, these microdomains are likely substantially uniform in size throughout the membrane. The size and order to dimensions of these microdomains is considerably less than the that of the working electrode 54 of sensor 50. As such, the electrode is in association with a useful amount of both the hydrophobic 46 and hydrophilic 44 regions of the membrane to allow effective control over the amount of glucose diffusing to the electrode. Moreover, as shown in FIG. 4, the number of paths available for glucose to permeate the membrane and diffuse from side 48 to the sensing side 56 would be greater for the inventive membrane than for prior art membranes. Consequently, more accurate and reproducible glucose readings are attainable across the entire inventive membrane.

(28) FIG. 5 shows a preferred embodiment of the present invention wherein membrane 42 is used in combination with a proximal membrane layer 58 that comprises an enzyme that is reactive with the analyte. In this instance, diffusion of the sample from side 48 through the membrane 42 into contact with the immobilized enzyme in layer 58 leads to an enzymatic reaction in which the reaction products may be measured. For example, in one embodiment the analyte is glucose. In a further embodiment, the enzyme immobilized in layer 58 is glucose oxidase.

(29) As described above, glucose oxidase catalyzes the conversion of oxygen and glucose to hydrogen peroxide and gluconic acid. Because for each glucose molecule metabolized, there is proportional change in the co-reactant O.sub.2 and the product H.sub.2O.sub.2, one can monitor the change in either the co-reactant or the product to determine glucose concentration. With further reference to FIG. 5, diffusion of the resulting hydrogen peroxide through layer 58 to the sensor 50, (e.g. electrochemically reactive surfaces), causes the development of an electrical current that can be detected. This enables determination of the glucose by calculations based upon similar measurements made on standard solutions containing known concentrations of glucose.

(30) In addition to glucose oxidase, the present invention contemplates the use of a layer impregnated with other oxidases, e.g. galactose oxidase or uricase. For an enzyme-based electrochemical glucose sensor to perform well, the sensor's response must neither be limited by enzyme activity nor cofactor concentration. Because enzymes, including glucose oxidase, are subject to deactivation as a function of ambient conditions, this behavior needs to be accounted for in constructing sensors for long-term use.

(31) When the membrane of the present invention is combined with an enzyme layer 58 as shown in FIG. 5, it is the enzyme layer that is located more proximally to the sensor 50 (e.g. electrochemically reactive surfaces). It is noted that enzyme-containing layer 58 must be of sufficient permeability to 1) freely pass glucose to active enzyme and 2) to permit the rapid passage of hydrogen peroxide to the sensor (electrode surface). A failure to permit the rapid passage of glucose to the active enzyme or hydrogen peroxide from the active enzyme to the electrode surface can cause a time delay in the measured signal and thereby lead to inaccurate results.

(32) Preferably, the enzyme layer is comprised of aqueous polyurethane-based latex into which the enzyme is immobilized.

(33) It is noted that while the inventive membrane 42 may itself contain immobilized enzymes for promoting a reaction between glucose and oxygen, it is preferred that the enzyme be located in a separate layer, such as layer 58 shown in FIG. 5. As described above, it is known that enzyme actively reacting with glucose is more susceptible to irreversible inactivation. Therefore, a disadvantage of providing enzyme in a layer that is semi-permeable to glucose, is that the calibration factors of the sensor may change over time as the working enzyme degrades. In contrast, when enzyme is dispersed throughout a membrane freely permeable to glucose (i.e. layer 58 in FIG. 5), such a membrane is likely to yield calibration factors that are more stable over the life of a sensor.

(34) In one preferred embodiment of the invention, the first polymer of the membrane includes homopolymer A and the second polymer includes copolymer AB.

(35) In another embodiment, the first polymer includes copolymer AB and the second polymer includes copolymer AB. Preferably, the amount of B in copolymer AB of the first polymer is different than the amount of B in copolymer AB of the second polymer. In particular, the membrane may be formed from a blend of two AB copolymers, where one of the copolymers contains more of a hydrophilic B polymer component than the blended targeted amount and the other copolymer contains less of a hydrophilic B polymer component than the blended targeted amount.

(36) In yet another embodiment of the invention, the first polymer includes homopolymer A and the second polymer includes homopolymer B.

(37) As described above, the invention also provides a polymeric membrane for regulating the transport of analytes that includes at least one block copolymer AB, wherein B forms a network of microdomains which are not photomicroscopically observable when hydrated at 400× magnification or less. In one embodiment, the ratio of A to B in copolymer AB is 70:30 to 90:10.

(38) For each of the inventive embodiments herein described, homopolymer A is preferably a hydrophobic A polymer. Moreover, copolymer AB is preferably a hydrophobic-hydrophilic copolymer component that includes the reaction products of a hydrophobic A polymer and a hydrophilic B polymer. Suitable materials for preparing membranes the present invention are described below.

(39) For purposes of the present invention, copolymer AB may be a random or ordered block copolymer. Specifically, the random or ordered block copolymer may be selected from the following: ABA block copolymer, BAB block copolymer, AB random alternating block copolymer, AB regularly alternating block copolymer and combinations thereof.

(40) In a preferred embodiment, the sensor, membrane, and methods of the present invention may be used to determine the level of glucose or other analytes in a host. The level of glucose is a particularly important measurement for individuals having diabetes in that effective treatment depends on the accuracy of this measurement.

(41) In particular, the invention provides a method of measuring glucose in a biological fluid that includes the steps of: (a) providing (i) a host, and (ii) an implantable device for measuring an analyte in a hydrophilic body fluid, where the device includes a polymeric membrane having a matrix including a first polymer and a second polymer dispersed throughout the matrix, wherein the second polymer forms a network of microdomains which are not photomicroscopically observable when hydrated at 400× magnification or less; and a proximal layer of enzyme reactive with the analyte; and (b) implanting the device in the host. In one embodiment, the device is implanted subcutaneously.

(42) The invention also provides a method of measuring glucose in a biological fluid that includes the following steps: (a) providing (i) a host, and (ii) an implantable device for measuring an analyte in a hydrophilic body fluid, that includes a polymeric membrane including a matrix including a first polymer and a second polymer dispersed throughout the matrix, wherein the second polymer forms a network of microdomains which are not photomicroscopically observable when hydrated at 400× magnification or less; and a proximal layer of enzyme reactive with the analyte, the device being capable of accurate continuous glucose sensing; and (b) implanting the device in the host. Desirably, the implant is placed subcutaneously in the host.

(43) Glucose sensors that use, for example, glucose oxidase to effect a reaction of glucose and oxygen are known in the art, and are within the skill of one in the art to fabricate (see, for example, U.S. Pat. Nos. 5,165,407, 4,890,620, 5,390,671, 5,391,250, 6,001,067 as well as copending, commonly owned U.S. patent application Ser. No. 09/916,858. It is noted that the present invention does not depend on a particular configuration of the sensor, but is rather dependent on the use of the inventive membrane to cover or encapsulate the sensor elements.

(44) For the electrochemical glucose sensor to provide useful results, the glucose concentration, as opposed to oxygen concentration, must be the limiting factor. In order to make the system sensitive to glucose concentration, oxygen must be present within the membrane in excess of the glucose. In addition, the oxygen must be in sufficient excess so that it is also available for electrochemical reactions occurring at the amperometric electrode surfaces. In a preferred embodiment, the inventive membrane is designed so that oxygen can pass readily into and through the membrane and so that a reduced amount of glucose diffuses into and through the membrane into contact with an immobilized glucose oxidase enzyme. The inventive membrane allows the ratio of oxygen to glucose to be changed from a concentration ratio in the body fluid of about approximately 50 and 100 parts of glucose to 1 of oxygen to a new ratio in which there is a stoichiometric excess of oxygen in the enzyme layer. Through the use of the inventive membrane, an implantable glucose sensor system is not limited by the concentration of oxygen present in subcutaneous tissues and can therefore operate under the premise that the glucose oxidase reaction behaves as a 1-substrate (glucose) dependent process.

(45) The present invention provides a semi-permeable membrane that controls the flux of oxygen and glucose to an underlying enzyme layer, rendering the necessary supply of oxygen in non-rate-limiting excess. As a result, the upper limit of linearity of glucose measurement is extended to a much higher value than that which could be achieved without the membrane of the present invention. In particular, in one embodiment the membrane of the present invention is a polymer membrane with oxygen-to-glucose permeability ratios of approximately 200:1; as a result, 1-dimensional reactant diffusion is adequate to provide excess oxygen at all reasonable glucose and oxygen concentrations found in a subcutaneous matrix [Rhodes, et al., Anal. Chem., 66: 1520-1529 (1994)].

(46) A hydrophilic or “water loving” solute such as glucose is readily partitioned into a hydrophilic material, but is generally excluded from a hydrophobic material. However, oxygen can be soluble in both hydrophilic and hydrophobic materials. These factors affect entry and transport of components in the inventive membrane. The hydrophobic portions of the inventive membrane hinder the rate of entry of glucose into the membrane, and therefore to the proximal enzyme layer while providing access of oxygen through both the hydrophilic and hydrophobic portions to the underlying enzyme.

(47) In one preferred embodiment, the membrane of the invention is formed from a blend of polymers including (i) a hydrophobic A polymer component; and (ii) a hydrophobic-hydrophilic copolymer component blended with component (i) that forms hydrophilic B domains that control the diffusion of an analyte therethrough, wherein the copolymer component includes a random or ordered block copolymer. Suitable block copolymers are described above. One is able to modify the glucose permeability and the glucose diffusion characteristics of the membrane by simply varying the polymer composition.

(48) In one preferred embodiment, the hydrophobic A polymer is a polyurethane. In a most preferred embodiment, the polyurethane is polyetherurethaneurea. A polyurethane is a polymer produced by the condensation reaction of a diisocyanate and a difunctional hydroxyl-containing material. A polyurethaneurea is a polymer produced by the condensation reaction of a diisocyanate and a difunctional amine-containing material. Preferred diisocyanates include aliphatic diisocyanates containing from 4 to 8 methylene units. Diisocyanates containing cycloaliphatic moieties, may also be useful in the preparation of the polymer and copolymer components of the membrane of the present invention. The invention is not limited to the use of polyurethanes as the hydrophobic polymer A component. The material that forms the basis of the hydrophobic matrix of the inventive membrane may be any of those known in the art as appropriate for use as membranes in sensor devices and having sufficient permeability to allow relevant compounds to pass through it, for example, to allow an oxygen molecule to pass through the inventive membrane from the sample under examination in order to reach the active enzyme or electrochemical electrodes. Examples of materials which may be used to make a non-polyurethane type membrane include vinyl polymers, polyethers, polyesters, polyamides, inorganic polymers such as polysiloxanes and polycarbosiloxanes, natural polymers such as cellulosic and protein based materials and mixtures or combinations thereof.

(49) As described above, the hydrophobic-hydrophilic copolymer component includes the reaction products of a hydrophobic A polymer component and a hydrophilic B polymer component. The hydrophilic B polymer component is desirably polyethylene oxide. For example, one useful hydrophobic-hydrophilic copolymer component is a polyurethane polymer that includes about 20% hydrophilic polyethylene oxide. The polyethylene oxide portion of the copolymer is thermodynamically driven to separate from the hydrophobic portions of the copolymer and the hydrophobic A polymer component. The 20% polyethylene oxide based soft segment portion of the copolymer used to form the final blend controls the water pick-up and subsequent glucose permeability of the membrane of the present invention.

(50) The polyethylene oxide may have an average molecular weight of from 200 to 3000 with a preferred molecular weight range of 600 to 1500 and preferably constitutes about 20% by weight of the copolymer component used to form the membrane of the present invention.

(51) It is desired that the membrane of the present invention have a thickness of about 5 to about 100 microns. In preferred embodiments, the membrane of the present invention is constructed of a polyetherurethaneurea/polyetherurethaneurea-block-polyethylene glycol blend and has a thickness of not more than about 100 microns, more preferably not less than about 10 microns, and not more than about 80 microns, and most preferably, not less than about 20 microns, and not more than about 60 microns.

(52) The membrane of the present invention can be made by casting from solutions, optionally with inclusion of additives to modify the properties and the resulting cast film or to facilitate the casting process.

(53) The present invention provides a method for preparing the implantable membrane of the invention. The method includes the steps of: (a) forming a composition including a dispersion of a second polymer within a matrix of a first polymer, the dispersion forming a network of microdomains which are not photomicroscopically observable when hydrated at 400× magnification or less; (b) maintaining the composition at a temperature sufficient to maintain the first polymer and the second polymer substantially soluble; (c) applying the composition at the temperature to a substrate to form a film thereon; and (d) permitting the resultant film to dry to form the membrane. In one embodiment, the forming step includes forming a mixture or a blend. As described above, in preferred embodiments, the first polymer is a polyurethane and the second polymer is polyethylene oxide. In general, the second polymer may be a random or ordered block copolymer selected from the following: ABA block copolymer, BAB block copolymer, AB random alternating block copolymer, AB regularly alternating block copolymer and combinations thereof.

(54) In one embodiment, the composition comprised of a dispersion of the second polymer within the matrix of a first polymer is heated to a temperature of about 70° C. to maintain the first and second polymers substantially soluble. For example, the combination of a hydrophobic polymer A component and a hydrophobic-hydrophilic copolymer AB component is desirably exposed to a temperature of about 70° C. to maintain the polymer and copolymers substantially soluble. In particular, the blend is heated well above room temperature in order to keep the hydrophilic and hydrophobic components soluble with each other and the solvent.

(55) The invention contemplates permitting the coated film formed on the substrate to dry at a temperature from about 120° C. to about 150° C. The elevated temperature further serves to drive the solvent from the coating as quickly as possible. This inhibits the hydrophilic and hydrophobic portions of the membrane from segregating and forming large undesired structures.

(56) The membrane and sensor combinations of the present invention provide a significant advantage over the prior art in that they provide accurate sensor operation at temperatures from about 30° C. to about 45° C. for a period of time exceeding about 30 days to exceeding about a year.

EXAMPLES

Example 1

(57) A Method for Preparing a Membrane of the Present Invention

(58) The inventive membrane may be cast from a coating solution. The coating solution is prepared by placing approximately 281 gm of dimethylacetamide (DMAC) into a 3 L stainless steel bowl to which a solution of polyetherurethaneurea (344 gm of Chronothane H (Cardiotech International, Inc., Woburn, Mass.), 29,750 cp @ 25% solids in DMAC) is added. To this mixture is added another polyetherurethaneurea (approximately 312 gm, Chronothane 1020 (Cardiotech International, Inc., Woburn, Mass.), 6275 cp @ 25% solids in DMAC). The bowl is then fitted to a planetary mixer with a paddle-type blade and the contents are stirred for 30 minutes at room temperature. Coatings solutions prepared in this manner are then coated at between room temperature to about 70° C. onto a PET release liner (Douglas Hansen Co., Inc., Minneapolis, Minn.) using a knife-over-roll set at a 0.012 inch gap. The film is continuously dried at 120° C. to about 150° C. The final film thickness is approximately 0.0015 inches.

(59) Observations of Membrane Using Photomicroscopy at 400× Magnification or Less

(60) A ¼″ by ¼″ piece of membrane is first immersed in deionized water for a minimum of 2 hours at room temperature. After this time, the sample is placed onto a microscope slide along with one drop of water. A glass cover slide is then placed over the membrane and gentle pressure is applied in order to remove excess liquid from underneath the cover glass. In this way, the membrane does not dry during its evaluation. The hydrated membrane sample is first observed at 40×-magnification using a light microscope (Nikon Eclipse E400). If air bubbles are present on the top or bottom of the film, the cover glass is gently pressed again with a tissue in order to remove them. Magnification is then increased to 200×; and the hydrated membrane is continuously observed while changing the focus from the top to bottom of the film. This is followed by an increase in magnification to 400×, with the membrane again being continuously observed while changing the focus from the top to bottom of the film.

(61) Results

(62) Based on the results of an optical micrograph of a sample membrane prepared by using a room temperature coating solution and drying of the coated film at 120° C., the micrograph being captured as described above, it was noticed that both circular and elliptical domains were present throughout the hydrated section of membrane. At the same magnification, the domains were not observable in dry membrane. Giving that in an electrochemical sensor, the electrodes included therein are typically of the same size and same order of dimensions as the observed circular and elliptical domains, such domains are not desired. These domains present a problem in that they result in a locally high concentration of either hydrophilic or hydrophobic material in association with the electrodes.

Example 2

(63) Optimizing the Coating Solution Conditions

(64) This example demonstrates that preheating the coating solution to a temperature of 70° C. prior to coating eliminates the presence of both the circular and elliptical domains that were present throughout the hydrated cross-section of a membrane prepared using a room temperature coating solution and drying of the coated film at 120° C. Example 2 further demonstrates that, provided the coating solution is preheated to about 70° C., either a standard) (120° or elevated (150° C.) drying temperature were sufficient to drive the DMAC solvent from the coated film quickly to further inhibit the hydrophilic and hydrophobic portions of the polyurethane membrane from segregating into large domains.

(65) In particular, the invention was evaluated by performing a coating experiment where standard coating conditions (room temperature coating solution and 120° C. drying temperature of the coated film) were compared to conditions where the coating solution temperature was elevated and/or the drying temperature of the coated film was elevated. Four experimental conditions were run as follows:

(66) SS-room temperature solution and standard (120° C.) oven temperature.

(67) SE-room temperature solution and elevated (150° C.) oven temperature.

(68) ES-preheated (70° C.) solution and standard (120° C.) oven temperature.

(69) EE-preheated (70° C.) solution and elevated (150° C.) oven temperature.

(70) Results

(71) Samples of each of the four membranes listed above were then hydrated for 2 hours, and then observed under the microscope. Performance specifications were achieved when the micrograph of the membrane prepared under a given condition showed an absence of circular and/or elliptical domains that result in an undesirable, discontinuous hydrophilic and hydrophobic membrane structure. Table 1 below summarizes these results where (+) indicates a membrane meeting desired performance specifications and (−) is indicative of a membrane showing the undesirable circular and/or elliptical domains. In summary, for both the ES and EE conditions, where the coating solution was preheated to 70° C. prior to coating on a substrate, no hydrated domains were observed at a 200× magnification. Furthermore, regardless of the drying temperature used for the coated film, when the coating solution was not preheated (conditions SS and SE), the hydrated structures were observed. Therefore, it is likely that preheating the coating solution effectively inhibits the hydrophilic and hydrophobic segments of the polyurethane from segregating into large domains.

(72) TABLE-US-00001 TABLE 1 Coating Condition Result SS − SE − ES (Inventive) + EE (Inventive) +

Example 3

(73) Evaluation of the Inventive Membranes for their Permeability to Glucose and H.sub.2O.sub.2

(74) Membranes prepared under the EE condition described in Example 2 were evaluated for their ability to allow glucose and hydrogen peroxide to get through the membrane to a sensor. In particular, a series of polyurethane blends of the present invention were generated wherein the percentage of Chronothane H in a coating blend was varied. Furthermore, one of these blends (57.5% Chronothane H in coating blend) was prepared under both the EE condition and the SS condition as described in Example 2. FIG. 6 shows that the sensor output generated with a series of polyurethane blends of the present invention was dependent upon the percentage of the Chronothane H. In particular, the sensor output increased as the percentage of Chronothane H in the coating blend increased. With further reference to FIG. 6, when the percentage of Chronothane H in the coating blend was 57.5%, the sensor output was three times greater for the membrane prepared under the optimized EE coating condition as compared to the non-optimized SS coating condition.

(75) Furthermore, FIG. 7 demonstrates that, regardless of the percent Chronothane H in the coating blend, an inventive membrane prepared under the EE condition shows a fairly constant percent standard deviation of sensor output. Moreover, a membrane prepared with 57.5% Chronothane H in the coating blend under the SS condition showed a percent standard deviation of sensor output approximately twice that of an EE membrane prepared with the same percentage of Chronothane H in the blend. It is noted that given that the sensor output is a true measure of the amount of glucose getting through the membrane to the sensor, the results indicate that the permeability of glucose and H.sub.2O.sub.2 is relatively constant throughout a given inventive membrane prepared under optimized coating conditions (i.e., EE conditions). This is important from a manufacturing standpoint.

(76) Having described the particular, preferred embodiments of the invention herein, it should be appreciated that modifications may be made therethrough without departing from the contemplated scope of the invention. The true scope of the invention is set forth in the claims appended hereto.