CA2236070C

CA2236070C - Electrochemical biosensor

Info

Publication number: CA2236070C
Application number: CA002236070A
Authority: CA
Inventors: Steven C. Charlton; Yingping Deng; Karl-Heinz Hildenbrand; Larry D. Johnson; James J. Venosky
Original assignee: Bayer Healthcare LLC
Current assignee: Ascensia Diabetes Care Holdings AG
Priority date: 1997-05-02
Filing date: 1998-04-28
Publication date: 2004-01-27
Anticipated expiration: 2018-04-28
Also published as: IL123335A; CA2236070A1; EP0877244B1; EP0877244A1; ES2223093T3; ATE269973T1; NZ329791A; DE69824664D1; NO981684D0; ZA983200B; NO325307B1; US5759364A; AU723307B2; DE69824664T2; DK0877244T3; NO981684L; JP3998807B2; AU6378398A; JPH10318970A; IL123335A0

Abstract

Disclosed is an electrochemical sensor which is made up of an insulating base plate bearing an electrode on its surface which reacts with an analyte to produce mobile electrons. The base plate is mated with a lid of a deformable material which has a concave area surrounded by a flat surface so that when mated to the base plate there is formed a capillary space into which a fluid test sample can be drawn. The side of the lid facing the base is coated with a polymeric material which serves to bond the lid to the base plate and to increase the hydrophilic nature of the capillary space.

Description

ELECTROCHEMICAL BIOSENSOR
Background of the Invention The present invention relates to an electrochemical bio-sensor that can be used for the quantitation of a specific component (analyte) in a liquid sample and, more specifically, to a method of manufacturing such a biosensor. Electrochemi-cal biosensors of the type under consideration are disclosed in U.S. patents 5,120,420 and 5,264,103. The devices dis-closed in these patents have a plastic base upon which carbon electrodes are printed which electrodes are covered with a reagent layer which comprises a hydrophilic polymer in combi-nation with an oxidoreductase specific for the analyte. There is typically a spacer element placed on the base, which ele-ment is cut out to provide a generally U shaped piece and a cover piece, so that when the base, spacer element and cover piece are laminated together, there is created a capillary space containing the electrodes and the reagent layer. In ad-dition to the oxidoreductase, there is included an electron acceptor on the reagent layer or in another layer within the capillary space. A hydrophilic polymer, e.g. carboxymethyl cellulose, is used to facilitate the drawing of the aqueous test fluid into the capillary space.
There has been developed more recently an electrochemical sensor which is comprised of two parts; a lower part (base) which carries the electrode structure with an oxidoreductase and electron acceptor evenly distributed in a hydratable poly-meric matrix on the electrodes' surface. and an upper part (lid) which is embossed to form three sides of a capillary

2 space with the base forming the fourth side upon mating of the lid and base. The base and lid are laminated together by means of a heat activated adhesive coating on the lid. The sensor is used by dipping the open end of the capillary into a small drop of test fluid, such as blood, which is drawn into the capillary tube so that it covers the enzyme and electron acceptor on the electrode's surface. Due to the hydratable nature of the polymer matrix, it disperses in the aqueous test fluid thereby allowing the oxidoreductase, which is glucose oxidase when the sensor is designed to determine the concen-tration of glucose in blood, to oxidize the analyte and the electron acceptor to shuttle the excess electrons to the work-ing electrode thereby creating a measurable current which is proportionate to the concentration of analyte in the test fluid .
The manufacture of the prior art sensors as described above involves the use of an extra part, the spacer layer, and a number of processing steps which are not required with the two part sensor (base and lid) with which the present inven-tion is involved. This type of sensor is prepared by a straight forward procedure which involves the steps of:
a) printing the electrodes onto the base material, b) coating the electrodes with the polymeric matrix containing the oxidoreductase and the electron ac-ceptor, c) coating the bifunctional adhesive layer of the pres-ent invention onto the lid,

3 d) embossing the capillary channel into the lid, and e) heat sealing the lid onto the base.
There is presented a two-fold problem in preparing a sen-sor of this type. The first relates to providing a sensor whose capillary space is rapidly filled with the test fluid and the second is to facilitate assembly of the sensor by ad-hering the lid to the base. In order to accomplish this in a manner which permits the rapid assembly of a large quantity of sensors, it was necessary to prepare a coating for application to the lid which:
i. adheres strongly to the lidstock material, ii. is pliable and extensible enough to survive emboss-ing into three sides of the lid, iii. enables rapid filling of the capillary space, iv. enables filling over a base having a relatively hy-drophobic surface (contact angle up to 90°), v. enables filling with blood having hematocrits of from 0 to 60~ when blood is the test fluid, vi. is non-tacky under ambient conditions, vii. capable of being activated and sealed by heat ap-plied through the lid from a hot plate, ca. 165°C,

4 viii. is capable of forming a good bond with the surface of the base material, ix. will not interfere when individual sensors are ex-cised from a tight array, and x. maintains the above properties for a period of time sufficient to provide a sensor with adequate shelf life.
Summary of the Invention The present invention is an electrochemical sensor for the detection of an analyte in a fluid test sample which com-prises:
a) an insulating base plate;
b) an electrode layer on the base plate in operative connection with an enzyme which reacts with the ana-lyte to produce mobile electrons; and c) a lid of deformable material which has been embossed to provide a concave area in a portion thereof while leaving a flat surface surrounding the concave por-tion in such a manner that, when mated with the base, the lid and base form a capillary space in which the enzyme is available for direct contact with the fluid test sample which is drawn into the capillary space by capillary action, wherein said sensor has a layer of a water dispersible polyure-thane over the underside of the lid to facilitate bonding of the lid to the base upon the lid and base being mated and to increase the hydrophilic nature of the capillary space to thereby increase the rate at which blood will flow into it.
Description of the Invention The construction of the sensor with which the present in-vention is concerned is illustrated by Fig. 1. The sensor 34 is comprised of insulating base 36 upon which is printed in sequence (by screen printing techniques), an electrical con-ductor pattern 38, an electrode pattern (39 and 40), an insu-lating (di-electric) pattern 42 and finally a reagent layer 44. The electrical conducting pattern 38 is optional, but its presence in the sensor is preferred to reduce the overall re-sistance of the sensor. The function of the reagent layer is to convert glucose, or another analyte, stoichiometrically into a chemical species which is electrochemically measurable, in terms of current it produces, by the components in the electrode pattern. The two parts 39 and 40 of the electrode print provide the two electrodes necessary for the electro-chemical determination.. The electrode ink, which is about 14 p (0.00055") thick, contains electrochemically active carbon.
Components of the conductor ink are typically a mixture of carbon and silver, chosen to provide a low electrical resis-tance path between the electrodes and the meter with which they are in operative connection, via contact with the conduc-tor pattern at the fish-tail end of the sensor 45. The typi-cal thickness of the entire structure is 6 a (0.00025 "). The function of the dielectric pattern is to enhance the repro-ducibility of the sensor reading by insulating the electrodes from the test sample except in a defined area 41 of the elec-trode pattern. A defined area is important in this type of electrochemical determination because the measured current is dependent both on the concentration of the analyte and the area of the electrode which is exposed to the analyte contain-ing test sample. A typical dielectric layer comprises a UV
cured acrylate modified polyurethane about 10 p (0.0004") thick. The typical thickness of the electrode structure is 6 p (0.00025"). The lid 46, which is embossed to provide a con-cave space 48 and punctured to provide air vent 50, is joined to the base 36 in a heat sealing operation. The base and lid are first aligned and then pressed together by means of a heated metal plate which is shaped such that contact is made only with the flat, non-embossed regions of the lid 52. The water dispersible polyurethane layer on the bottom surface of the lid is thereby melted and serves to fuse the lid 46 and the base 36 together upon cooling. A typical temperature for the heated plate is 165° C with the pressure being 2200 p.s.i.
Holding the lid and base together under these conditions of heat and pressure for 1~ seconds provides the desired unitary sensor with the capillary space for acceptance of the fluid test sample. The polyurethane layer bonds to the topmost ex-posed layer (dielectric 42 with dotted edges) of the base un-der the flat regions of the lid. Alternatively, the edges of the dielectric are slightly narrowed (represented by dielec-tric layer 42 with solid edges) which allows the polyurethane to bond with the material of the electrode print pattern 40.
This is a preferred configuration because the bond strength between the polyurethane adhesive and the electrode ink is greater than that between the adhesive and the dielectric ma-terial thereby providing a more leakproof capillary space.
Suitable materials for the insulating base include poly-carbonate, polyethylene and dimensionally stable vinyl and acryl polymers as well as polymer blends such as polycarbon-ate/polyethylenenthere-phthalate. The lid is typically fabri-cated from a deformable polymeric sheet material such as poly-carbonate or an embossable grade of polyethyleneterephthalate or glycol modified polyethylene terephthalate, whereas the di-electric layer can be fabricated from an acrylate modified polyurethane which is curable by ultra-violet (W) light, a polyurethane which is curable by W light or moisture or a vi-nyl polymer which is heat curable.
The water dispersible polyurethane layer on the underside of the lid serves to increase the hydrophilic nature of the capillary space and to facilitate its close adherence to the base either by bonding to the dielectric layer or to the elec-trode material.
The present invention facilitates the use of an embossed lid (46, Fig. 1) as opposed to the use of a spacer as in the prior art sensor elements in which, instead of embossing, the two sides of the capillary space are formed from a cutout in a spacer material which also carries a pressure sensitive adhe-sive to adhere the base to the spacer and the spacer to the lid. The use of the embossed lid enables one to avoid the use of an extra part, i.e. the spacer, and a number of processing steps. The steps involved in assembling the spacer containing sensor are:

i. preparing the complete electrode structures includ-ing the reagent layer; an agent to induce wicking of blood into the capillary space needs to be included in the uppermost layer;
ii. adding an additional layer containing an agent to induce wicking of blood into the capillary space;
this layer may be avoided if the agent is included in the chemistry layer;
iii. die cut a capillary channel into the spacer material which is typically a laminate of release/
liner/adhesive/spacer material/adhesive/release liner;
iv. strip the release liner from one side of the spacer material and attach the spacer to the base; and v. strip the release liner and assemble the lid to the other side of the spacer.
The present invention permits one to manufacture a sensor by:
i. printing electrodes onto the base material;
ii, coating the bifunctional layer onto the under sur-face of the lid;

iii. embossing the top and sides of the capillary space into the lid;
iv. mating the lid to the base and sealing them together by the application of heat and pressure.
The bifunctional coating of the present invention pro-vides a non-tacky adhesive as opposed to the tackiness of a pressure sensitive adhesive. Accordingly, there is no need to remove and dispose of a release liner before assembly and there is a dramatic reduction in the problems associated with the sensor assembly equipment being fouled by the adhesive ma-terial. Accordingly, the sensors of the present invention can be manufactured by mating an array of lids with a correspond-ing array of bases and then excising individual sensors from the array by a punching process. A tacky, pressure sensitive adhesive would be incompatible with this punching step due to adhesive buildup in the dies which would necessitate frequent cleaning and lost production time.
The water dispersible polyurethanes of the present inven-tion are capable of being laid down on the lid stock in a pat-tern to form a non-tacky layer under ambient conditions. They can be activated for fusing to the base at a temperature suf-ficiently low to avoid damage to the reagents in the reagent layer while forming a good bond with desirable lid materials which are long lasting thereby providing good shelf life. The coating also increases the hydrophilic nature of the interior of the capillary space due to its ionomeric property which presumably causes the surface to be significantly ionic in character. Based on these dual properties the water dispersi-ble polyurethane can be referred to as a bifunctional coating material.
The reaction of a diisocyanate with equivalent quantities of a bifunctional alcohol such as glycol gives a simple linear polyurethane. These products are unsuitable for use in the manufacture of coatings, paints and elastomers. When simple glycols are first reacted with dicarboxylic acids in a poly-condensation reaction to form long chain polyester-diols and these products which generally have an average molecular weight between 300 and 2000 are subsequently reacted with di-isocyanates the result is the formation of high molecular weight polyester urethanes. Polyurethane dispersions have been commercially important since 1972. Polyurethane ionomers are structurally suitable for the preparation of aqueous two-phase systems. Those polymers which have hydrophilic ionic sites between predominately hydrophobic chain segments are self-dispersing and, under favorable conditions, form stable dispersions in water without the influence of shear forces and in the absence of dispersants. One method of preparing cati-onic urethanes is by the reaction of a dibromide with a dia-mine. If one of these components contains a long chain poly-ether segment, an ionomer is obtained. Alternatively polyam-moniumpolyurethanes can be made by first preparing a tertiary nitrogen containing polyurethane and then quaternizing the ni-trogen atoms in a second step. Starting with polyether based NCO prepolymers, segmented quaternary polyurethanes are ob-tained. Similarly, cationic polyurethanes with tertiary sul-phonium groups can be prepared when the tert.-aminoglycol is substituted for thiodiglycol. The ionic moiety or its precur-sor can also be the diisocyanate or part of a long chain poly-etherdiol.
In order to obtain anionic polyurethanes, diols bearing a carboxylic acid or a sulphonate group are usually introduced and the acid groups subsequently neutralized, for example with tertiary amines. Sulphonate groups are usually built via a diaminoalkanesulphonate, as these compounds are soluble in water and reaction with NCO prepolymers is not adversely af-fected by water. The resulting polyurethane resins have built in ionic groups which provide mechanical and. chemical stabil-ity as well as good film forming and adhesion properties.
The most important property of polyurethane ionomers is their ability to form stable dispersions in water spontane-ously under certain conditions to provide a binary colloidal system in which a discontinuous polyurethane phase is dis-persed in a continuous aqueous phase. The diameter of the dispersed polyurethane particles can be varied between about and 5000 nm.
Solutions of polyurethane ionomers in polar solvents such as acetone, methyl ethyl ketone and tetrahydrofuran spontane-ously form dispersions when water is stirred in. The organic solvent can then be distilled off to give solvent-free sols and latices of the ionomers. Depending on the content of ionic groups and the concentration of the solution, the iono-mer dispersion is formed by precipitation of the hydrophobic segments or by phase inversions of an initially formed inverse emulsion.

In converting an organic solution into an aqueous disper-sion a 2000 molecular weight polyester based on adipic acid is reacted with excess hexamethylenediisocyanate to give an NCO
terminated prepolymer. After the addition of an equal molar amount of N-methyl-diethanolamine dissolved in acetone, the viscosity increases while polyaddition proceeds. As the vis-cosity increases, additional amounts of acetone are added to keep the mixture stirrable. The tertiary nitrogen containing segmented polyurethane is now quaternized with dimethyl sul-phate. Formation of the polyurethane ionomer results in fur-ther increase in viscosity. The ionic centers associate in a manner similar to that of soaps in paraffin oil with an appar-ent increase in molecular weight. When water is slowly added to such an ionomer solution the viscosity decreases during the addition of the first few milliliters of water. Apparently, the ionic association is reversible and any water present re-duces the ionic association to provide a clear solution in which the ionomer is molecularly dispersed. As more water is added, the viscosity increases again although the polymer con-centration decreases. This is the first phase of the forma-tion of the dispersion. Further addition of water produces a turbidity, indicating the beginning of the formation of a dis-persed phase. Further addition of water increases turbidity and finally the viscosity drops, since, due to the further de-crease of acetone concentration, the agglomerates have been rearranged to form microspheres. In this state there is a continuous water phase and a discontinous phase of polyure-thane particles which are swollen by acetone.
The final step in preparing the aqueous dispersion is the removal of acetone by distillation. The turbidity increases and the viscosity decreases due to the polymer chain recoil or shrinkage.
The physical properties of the dispersion depend on a va-riety of parameters such as chemical composition, type and amount of ionic group, molecular weight and method of prepara-tion. The diameter of the particles will vary from about 10 nm to 5 hum and the appearance of the dispersion can vary be-tween an opaque translucent sol and a milky white dispersion.
The viscosity and rheological properties can also vary within wide limits, including Bingham type viscosity and rheopexy.
Ionomers are excellent dispersants. The acetone process works well even if only a fraction of the polyurethane is ionic since this part of the material will form the outer shell of the relatively course latex particles.
The above procedure for the preparation of polyurethane dispersions is universal; all linear polyurethanes which can be synthesized in organic solvents may be modified with ionic groups. Because of its low boiling point and low toxicity, acetone is particularly suitable for preparing polyurethane dispersions.
The construction of a sensor according to the present in-vention is accomplished according to the following general ex-ample:

General Example In this example, a large number of sensor lids are fabri-cated from a rolled sheet of polycarbonate which has been un-rolled to provide a flat surface. This sheet is referred to as the lid stock since it will be the source of a multiplicity of lids.
A bifunctional coating solution, comprising an aqueous polyurethane dispersion, is spread on to one side of a poly-carbonate sheet ( 0 . 0075 " /175 a thick) using a wire wound rod or a slot die coater and air dried. The dried coating thick-ness is in the range of 0.0007 " to 0.002" (17 a to 50 p) with the wet coating thickness in the range of 0.0014" to 0.005 " (35 N to 125 p) for a typical solids content of 40% to 50%. Drying can be at ambient temperature or by forced drying under a stream of air at 70°C. The bifunctional layer has some tack for a short period after drying and when the sheet is rewound a temporary liner or interleave is introduced in contact with the coating and, the coating is in contact with the liner of the polycarbonate. After a period of a few hours, the initial tack is lost allowing the polycarbonate lid stock to be unrolled without damage to the coating. Suitable materials for the liner are polyolefins or polyethylenetere-phthalate in.a thickness of from 0.001 to 0.003" (25-75 u).
The next stage of processing involves embossing of the concave areas of the lids and the punching of various holes in the polycarbonate sheet for registration and tracking. The sheet is then slit longitudinally to give a ribbon of sensor lids in a line which is then rolled up. It is essential that the adhesive be non-tacky so that it sticks to neither the em-bossing and punching tools nor to the reverse side of the polycarbonate support while rolled in ribbon form. It is also essential that the adhesive not form gummy deposits on the punching or embossing tools which would necessitate frequent cleaning.
The base stock, typically of polycarbonate, is printed with various inks to form the electrodes and then overcoated with a dielectric layer in a predetermined pattern designed to leave a desired surface of the electrode exposed. The bifunc-tional material must adhere to the dielectric material when the lid is mated directly to the dielectric layer. In order to assemble the lidstock to the base, the continuous ribbon of lid stock is unwound and passed through a special laminator where it is registered and then combined with a strip of the base stock under the influence of heat and pressure. It is desirable for the heat sealing process to take about one sec-ond which requires an adhesive which is capable of very rap-idly forming a strong bond. After heat sealing, the continu-ous ribbon of laminate is wound onto a reel.
In order to singulate individual sensors from the lami-nate ribbon, the laminate is passed through punching equipment in which individual sensors are punched from the ribbon and then placed in a buffer preparatory to being placed into a foil blister package for storage. Here again, it is essential that the adhesive not gum or cold flow onto and form deposits in the punch mechanism. It is also essential that the adhe-sive be tack free so that the sensor not stick to the punch and reliably transfer to the buffer and from the buffer to the blister package from which it will be dispensed. Accordingly, the coating's melting point must be high enough to prevent ac-cidental melting and resultant tack from frictional heating during the melting process. Furthermore, the adhesive bond strength between the lid stock and the base stock must be strong enough to withstand delamination peel forces generated during this punch operation.
In the preferred method of using the sensors, they are packaged in a circular disk having ten individual compartments (blisters) arranged radially. The disk is made from an alumi-num foil/plastic laminate which is sealed to isolate the sen-sor from ambient humidity and from other sensors with a burst foil cover, which disk is mounted within a specially designed instrument. The sensor is kept dry by a desiccant located in-side the individual compartments. To retrieve a sensor, a knife is driven down through the burst foil into an individual elongated compartment at the end closest to the hub of the disk and then moved radially toward the perimeter of the blis-ter. , In doing so, the knife engages the rear (fish tail) of the sensor in that compartment. Radial travel of the knife pushes the tip of the sensor out through the burst foil and through parts of the instrument such that the nose of the sen-sor is completely out of the instrument and ready to receive a fluid test sample, e.g. blood. For this stage, it is essen-tial that the bond between the base and lid of the sensor withstand the sheer forces generated when the sensor bursts out through the foil. This method of providing a sensor ready for use is more fully described in U.S. Patent 5,575,403.

Finally, the sensor tip, containing the opening to the capillary space, is touched to a small drop of the fluid test sample which is typically blood produced by a finger prick.
The blood is rapidly drawn up into the capillary where the in-teraction with the enzyme is initiated and the instrument is signaled to initiate its timing sequence. It is essential that blood be drawn very rapidly into the capillary space, re-gardless of its spatial orientation in order that the timing sequence be initiated.
The bifunctional coating of the present invention is based on a water dispersible polyurethane prepared as previ-ously described. A particularly useful formulation is Disper-coll U 53 BC, from Bayer AG, which is a linear aliphatic poly-ester urethane based on hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) in aqueous dispersion with a mean particle size of 100 nm. This product, whose total weight % solids in aqueous dispersion is 40 ~ 1, has a viscos-ity at 73°F/23°C (cps/mpa) (Brookfield LVF, spindle 2, 30 rpm) of <600. The white liquid dispersion has a specific gravity of 1.2 g/cm3 and the polymer exhibits a high level of crystal-lization. The dispersion's specific gravity is 1.1, its pH is 7 and it carries an anionic particle charge. The manufactur-ers recommend that it be kept at a pH of 6-8 since acidic or highly alkaline conditions can cause a loss of properties due to hydrolytic degradation of the polymer.
Dispercoll U, the tradename for a range of aqueous, col-loidal dispersions of high molecular weight hydroxyl polyure-thane polymers, are preferred for use in the present inven-tion. Because they are prepared by the previously described ionomer process, these dispersions contain only small quanti-ties of emulsifiers while still exhibiting outstanding me-chanical and chemical stability.
Various other materials can be combined with the urethane dispersion to enhance the properties of adhesion and hydro-philicity which make it useful in the manufacture of sensors.
The addition of surfactants will increase the wettability of the surface of the dried polyurethane film to enhance the ability of test fluids such as blood to enter the capillary space. Thus, the addition of surfactants such as the sodium salt of an octylphenoxypolyethoxyethyl sulfate (Triton~ W30), a fluorocarbon such as an amine perfluoroalkyl sulfonate (Fluorad~' FC 99), a potassium salt of a fluoroalkyl carboxy-late ( FluoradTT' FC 129 ) or an anionic or a neutral f luorosur-factant such as Zonyl~ FSA or Zonyl~ FSN is useful to increase the ability of the capillary space to draw in test fluid and to avoid defects during the coating procedure by improving the wetting of the substrate. There is an optimal range of con-centration for each surfactant, i.e. enough to provide good hydrophilic nature in the capillary space of the sensor but not enough to reduce the bond strength between the lid and base. Concentrations typically range from 0.05 to 0.2$ by weight of the polyurethane dispersion.
The addition of a secondary polymer or co-polymer such as Airflex~ 400 vinylacetate-ethylene copolymer, Elvace~ 40705 or 40722 in amounts up to about 33 weight ~ of the dispersion can be used to strengthen the bond between the lid and the layer of dielectric material. Vinyl acetate copolymers, such as Flexbond~ 150 from air products can also be used in an amount ' CA 02236070 1998-04-28 of up to about 10 weight ~ of the dispersion to enhance bond strength between the lid and the dielectric layer. This is especially true when the dielectric layer contains silicone additives as defoamers.
Thickeners, such as Acrysol RM-8, which is a polyurethane associative thickener, can be added to the dispersion in order to raise its viscosity for easier coating and prevent cracking of the polyurethane layer during drying. A coloring agent, such as Erioglaucine from Sigma Chemical Company, a water soluble blue chromophore, can be added to the coating material as a visualizing agent. The following Table A sets out 8 for-mulations for the bifunctional coating which are useful in fabricating the sensor with which this invention is concerned.
TABLE A
REAGENT I FORMULATION
CONCN NUMBER
I

1x1 ' I 1 i i I I 7 $

U 54 ( _ 21.71 l I I I 1 50 I 2L3719.2414.3814.5814.58 14.5a FLEXBOND I50 ( I I 6.58I I I I i 6.63 SS 6.47 6.47 6.53 6.536.63 I

AIRFLEX A400 1 I I I t I I
55 8.75 I I

ELVACE 40705 ( I I I I i 6.63i 55 6.63 ( I

ELVACE 40722 l I 1 I I I I
55 I 6.53 I
6.53 RM-8 I I ( 1.5 1.S I I LS I 1.5 1 LS I 1.S 1.5 1.S
I ( ZONYL FSN I l0 I I - I 0.6 1 0.6 I I I 0.6 ZONI'L FSA ! I I 0.1 I ( I
I 0 S 0.45 I I

FLL10RAD FC-IZ9 I I I I I 0.6 I 0.6 I t0 ( 0.6 I 0.06 0.06I 0.06 ERIOGLAUCINE 0.06 I 0.06 0.06 I I
( S / ( 0.06 0.06 ( TOTAL I 130 (30 130 (30 30 (30 The present invention is further illustrated by the fol-lowing examples:

Example 1 - Water dispersible polyurethane U53 as the bifunc-tional Layer.
Components were combined in the order shown in Table 1 with thorough mixing.

Component FunctionConcn Amount Dry comp$a.
~ I

(%1 ( 1 ~ (%1 U 53 oohvrethanei adhesme40 291 I 99.9 ( I

erioQlancine A colorant5 I 0.58 .025 ~ f AcwsoI RM-8 I thickener1 i 8.18 .07 I

The solution was allowed to stand for a number of hours to allow entrapped bubbles to escape and was then coated onto a polycarbonate sheet {0.0075 " /190 a thick) using a #38 wire wound rod to give a wet thickness of 0.0013 inch. After dry-ing at 50°C for 3 minutes under a stream of air heated to 60°, the film of the dry composition was 0.0013 inch/35 a thick.
This film was tacky but could be rolled up with a polyethylene interleave. After a period of several hours this interleave was removed without damage to the film leaving a smooth, non-tacky surface. This material was then punched and formed in mechanical tools to generate the embossed concave shape of the lid as depicted in Fig. 1. A polycarbonate insulating base was sequentially printed with the following inks: a conductor pattern using polymer thick film composition 5085 from duPont;
a dielectric pattern using polymer thick film composition 7102T, also from duPont, a dielectric pattern using RDMSK 4954 from NorcoteTM. This dielectric normally contains a small amount of the surfactant Silwet~ 77. For experimental pur-poses, some printings were made without this surfactant. The embossed lidstock was aligned with the appropriate region of base material and heat sealed with a hot plate at a tempera-ture of 165° with the application of about 2000 p. s . i . for a time of 1 second. The lid stock was fused directly to the di-electric coating on the base.
The completed sensor was mounted vertically with the cap-illary opening facing downward and a small drop of blood (7 pL) was raised to just touch the opening. the time taken for the blood to travel vertically upwards from the front to the rear of the capillary (a distance of 4 mm) was measured by means of a video camera. The filling times are set out in Ta-ble 2.
In order to measure the peel strength of the bond between the lid and dielectric layer on the base, a 0.2 inch x 0.4 inch sealed area between the lidstock and the base was carried out by holding the base and peeling off the lidstock at an an-gle of 90°. The mean force in p.l.i. (pounds per linear inch) necessary to do this was measured and is set out in Table 2.

Peel strengthDielectricSample Filling ~ ~ ~ times ( .1.i.1 surfactanthematocrit(s) after weeks under the follmving storaee conditions.

I,airial 4 ~ 40 50 8 none ~ 40 ~ 0.46 0.25 ~ 0.38 1.23 I .075% 40 I 0.36 0.35 I 0.48 1.15 L-77 ~

In this experiment the base had no reaction layer. Nev-ertheless, there was exhibited strong bond strength and a very rapid fill time over the storage conditions of 4° to 40°. In-' CA 02236070 1998-04-28 clusion of the surfactant Silwet~ L-77 in the dielectric layer had a negligible effect.
Example 2 - Water dispersible polyurethane combined with the secondary polymer Flexbond~ 845.
The film was prepared and tested as in the previous exam-ple with the results being set out in Tables 3 and 4. The test results show good bond strength and rapid blood filling even at high hematocrit when stored at refrigerator tempera-tures. The vinyl-acrylate copolymer is optional in conduction with the bifunctional coating material of the present inven-tion. It can be used advantageously in situations such as those in which the dielectric layer does not form a long last-ing bond with the polyurethane.

Component i FunctionConcn Dry compsn.
I
Amount (%) LT 53 Dolvuretnaneachcswe 40 93.7 ; ~ 278 I

Flexbond 845 adhesae 55 6.2 ! i I
I3.4 I

erioalauciae coiotattt~ .025 I I I
0.6 ~

Acrvsol RM-8 thiciceaer1 .06 ! I I
7.5 Peel strengthDielect Sample F~Iing times (s) after ~ surfactant( 13 works wader .Gi.) hematocritthe following storage conditions.

( Inirial I 4 ( 40 I 50 7.7' I .075% L-77 1.02 O.GS ! 3.6 I 3.6 I 60 I 1.12 t 0.97 I 2.5 ! 4.3 'Dielectric contained no suriaaaat.

Example 3 - Water dispersible polyurethane U53 with the sur-factant Triton~ W30.
This film was prepared as in the previous examples with the components being added in the order shown. Fill times with the superscript 1 correspond to sensors without the rea-gent layer which were tested after 10 weeks storage. Those sensors with the superscript 2 had printed layers of glucose oxidase/potassium ferricyanide in polyethylene oxide) beneath the dielectric layer and partially exposed through the opening in this layer. The results of this experiment are set out in Tables 5 and 6.

Component ~ FuncnonI ConcnAmount Dry compm.
I

i (%) ( ) ~

U ~3 voiwret'tsaneadhesn~e40 99.8 . 291 I

enoQlaucme I colorant5 i .025 0.58 I

Tritons W 30 suriacranti 10 0.13 i i 1.5 I

Acrvsol~ RM-8 thickenert 1 .07 I !
8.18 I

Peel strengthDielectric~ Sampie F7ling times (s) s weeks ~ after 10' orl3 ( .i.i.) surfactant~ under the following hematocriitstorage conditions.

I I I ~ I 4 _x__40 ~ 50 10.6 1 none I 40 I 0.42' I 0.28' f 0.69'0.68' I .075% L-77I 40 I 0.32' I 0.19' I 0.44'0.75' I

I .05% 1 40 ~ 0.58= ! 0.37' I 0.77'0.4 L-77 I t' I .05% ~.,-77 ( 60 I I 0.38' I 0.84' I 0.54=

These data demonstrate high bond strength and rapid fill times under all storage conditions tested even with extremely high hematocrit blood. The presence of surfactant in the di-electric causes a slight improvement in fill times. This ex-ample demonstrates good fill times for both normal and high hematocrit blood even after the sensor was stressed. Four comparison sensors with a reagent layer were assembled using lidstock which had been punched and formed as described but which did not have the bifunctional coating. These sensors were held together by double faced adhesive tape. The fill time for these freshly prepared sensors was more than 11 sec-onds indicating that the bifunctional coating of the present invention is critical for rapid filling of the capillary space.
Example 4 - Water dispersible polyurethane U53 combined with secondary co-polymer Airflex~ 400 vinyl-ethylene emulsion and FluoradTM FC99.
The film was prepared and tested as in the first example except that the base had a reaction layer over the printed electrodes. The results are set out in Tables 7 and 8.

Component FunctionConcn AmountDry compsa.

t U 53 nolwretaaneadhesoe ! 40 248.2 83.1 ! . I

Airflex~~ adhesive! 50 39.6 16.6 400 f I

eriostlaucme coioraatI 5 0.59 I .025 f I

Fiuorad''"t surfactantI 10 2.88 I 0.24 FC 99 I f Act~sol~ Rl4i-8thickenert 1 8.71 I .0?3 ' !

Peel strengthI DielectricSample F'Oling times ' ~ (s) after I3 weeks under t .Li.l ~ surfactanthematocritt6e following ~ storage eonditiotu.

I I I Dial ( 4 ~ 40 I 50

5.7' ( .0S% 40 I 0.66 I 0.5~ 0.27 I 0.76 I .05% 60 I 0.91 I 0.51 L-77 I 0.36 ~
I 0.79 ~ Peel measured strcngtit to aielectnc without surfactant.

From the data presented in Tables 7 and 8 it can be de-termined that a combination of a different surfactant and a different secondary polymer gives excellent fill times with blood of normal and high hematocrit without requiring refrig-crated storage.
Example 5 - Water dispersible polyurethane U54 combined with Elvase~ 40722 vinylacetate-ethylene emulsion, Flexbond~ 150 polyvinylacetate based general purpose pressure sensitive emulsion, and surfactant FluoradTM FC129.
The data generated using this formulation are set out in Tables 9 and 10.

Component ~ FuactionConcn Amo~mt Dry compm.
~ ~ ~ (%1 (%) 1 U 54 ooivuzztaaaeadhesive50 Z 15.7 75.5 I I i I

Flacoond~ 150 I acdesnreI 55 20.8 8.0 I I

Elvacc.8~ 40722I adaesivef 5~ 4I.7 16:0 ? I

erioQlauciae I coioraatI 5 0.61 0.022 I I

FluoradTx FC I surmcsantI 10 4.53 I 0.32 Ace~SOl~ RM-8 I thickenerI 1 I 16.6 I .12 Peel strength Samplc Fi'Iling Dielectric hematocrittunes ~ ~ (s) tD.Li.I after ~ surfaasnt 8 ~ weeks ttadcr the foliowing storaee conditions.

I I Itlitial4 ( 40 ~ 500 I

9.4 I none I 40 I 0.62 0.?7 I
0.55 I
0.78 I

( .0S% 40 I 0.41 0.64 0.27 !
0.46 I

This example demonstrates excellent performance with a different water soluble polyurethane.

Claims

CLAIMS:

1. An electrochemical sensor for the detection of an analyte in a fluid test sample which comprises:
a) an insulating base plate;
b) an electrode layer on said base plate in operative connection with an enzyme which reacts with the analyte to produce mobile electrons;
and c) a lid of deformable material which has been embossed to provide a concave area in a portion thereof while leaving a flat surface surrounding the concave portion in such a configuration that, when mated with the base, the lid and base form a capillary space in which the enzyme is available for direct contact with fluid test sample which is drawn into the capillary space by capillary action, wherein said sensor has a polymeric layer over the underside of the lid to facilitate bonding of the lid to the base upon their being mated and to increase the hydro-philicity of the capillary space.

2. The sensor of Claim 1 wherein the polymeric layer comprises a surfactant stabilized polyurethane.

3. The sensor of Claim 1 wherein the polymer layer comprises a water dispersible polyurethane ionomer.

4. The sensor of Claim 3 wherein the polyurethane ionomer is anionic.

5. The sensor of Claim 4 wherein the polyurethane ionomer is a linear aliphatic polyester urethane based on hexamethylenediisocyanate and isophorone diisocyanate.

6. The sensor of Claim 1 wherein the polymer is a vinylacetate-ethylene co-polymer.

7. The sensor of Claim 1 wherein there is an electrical conducting layer on the surface of the insulating base plate and the electrode layer is on the surface of the conducting layer and in electrically conductive contact therewith.

8. The sensor of Claim 1 wherein the lid is mated to the base with direct contact between the lid and the base being at the surface of the electrode layer and the under-side of the lid.

9. The sensor of Claim 1 wherein there is a layer of dielectric material patterned over the electrode layer so that only a portion of the electrode, as predetermined by the pattern of the dielectric layer, is available for direct contact with the fluid test sample.

10. The sensor of Claim 9 wherein the lid is configured so that its edges mate with the dielectric layer.

11. The sensor of Claim 9 wherein the dielectric layer is configured so that it leaves a portion of the electrode layer exposed for direct contact with the flat surface of the lid.

12. The sensor of Claim 1 wherein the enzyme is an oxidoreductase.

13. A method of making an electrochemical sensor for the detection of an analyte in a fluid test sample which comprises:
a) providing an insulating base plate which has on its surface an electrode layer in operative connection with an enzyme which reacts with the analyte to produce mobile electrons; and b) mating the base plate with a lid of a deformable material a part of which encompasses a concave area with a flat surface surrounding the concave portion in such a configuration that, when mated with the base, the lid and base form a capillary space in which the enzyme is available for direct contact with the fluid test sample in the capillary space and the lid has a polymeric layer over the underside thereof to facilitate bonding of the lid to the base and increase the hydrophilicity of the capillary space; and c) heating the base plate and lid while applying pressure therebetween to fuse the base plate to the lid.

14. The method of Claim 13 wherein there is a layer of dielectric material patterned over the electrode layer so that only a portion of the electrode, as predetermined by the pattern of the dielectric layer, is available for direct contact with the fluid test sample.

15. The method of Claim 14 wherein the dielectric layer is configured so that the flat surface of the lid mates solely with the dielectric layer.

16. The method of Claim 14 wherein the dielectric layer is configured so that it leaves at least a portion of the electrode layer exposed for direct contact with the flat surface of the lid.

17. The method of Claim 13 wherein the polymeric layer on the underside of the lid comprises a surfactant stabilized polyurethane.

18. The method of Claim 13 wherein the polymer layer com-prises a water dispersible polyurethane ionomer.

19. The method of Claim 18 wherein the polyurethane ionomer is anionic.

20. The method of Claim 19 wherein the anionic polyurethane ionomer a.s a linear aliphatic polyester urethane based on hexamethylene diisocyanate and isophorone diisocyanate.