Abstract:
A biosensor for use in determining a concentration of a component in an aqueous liquid sample is provided including: an electrochemical cell having a first electrically resistive substrate having a thin layer of electrically conductive material, a second electrically resistive substrate having a thin layer of electrically conductive material, the substrates being disposed with the electrically conductive materials facing each other and being separated by a sheet including an aperture, the wall of which aperture defines a cell wall and a sample introduction aperture whereby the aqueous liquid sample may be introduced into the cell; and a measuring circuit.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application No. Ser. 09/709,968, filed Nov. 10, 2000, now U.S. Pat. No. 6,526,110, which is a continuation of U.S. application Ser. No. 09/314,251, filed May 18, 1999, which issued as U.S. Pat. No. 6,174,420 on Jan. 16, 2001, which is a continuation of U.S. application Ser. No. 09/068,828, which is the National Stage of PCT/AU96/00724, filed Nov. 15, 1996, now U.S. Pat. No. 6,179,974, filed on May 15, 1998, and of U.S. application Ser. No. 08/852,804, filed on May 7, 1997, which issued as U.S. Pat. No. 5,942,102 on Aug. 24, 1999, the contents of which are incorporated herein by reference in their entirely. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an electrochemical cell for determining the concentration of an analyte in a carrier. 
     BACKGROUND OF THE INVENTION 
     The invention herein describe is an improvement in or modification of the invention described in our copending U.S. application Ser. No. 08/981,385, entitled ELECTROCHEMICAL CELL, filed on Dec. 18, 1997, the contents of which are incorporated herein by reference in its entirety. 
     The invention will herein be described with particular reference to a biosensor adapted to measure the concentration of glucose in blood, but it will be understood not to be limited to that particular use and is applicable to other analytic determinations. 
     It is known to measure the concentration of a component to be analysed in an aqueous liquid sample by placing the sample into a reaction zone in an electrochemical cell comprising two electrodes having an impedance which renders them suitable for amperometric measurement. The component to be analysed is allowed to react directly or indirectly with a redox reagent whereby to form an oxidisable (or reducible) substance in an amount corresponding to the concentration of the component to be analyse. The quantity of the oxidisable (or reducible) substance present is then estimated electrochemically. Generally this method requires sufficient separation of the electrodes so that electrolysis products at one electrode cannot reach the other electrode and interfere with the processes at the other electrode during the period of measurement. 
     In our co-pending application we described a novel method for determining the concentration of the reduced (or oxidised) form of a redox species in an electrochemical cell of the kind comprising a working electrode and a counter (or counter/reference) electrode spaced from the working electrode by a predetermined distance. The method involves applying an electric potential difference between the electrodes and selecting the potential of the working electrode such that the rate of electro-oxidation of the reduced form of the species (or of electro-reduction of the oxidised form) is diffusion controlled. The spacing between the working electrode and the counter electrode is selected so that reaction products from the counter electrode arrive at the working electrode. By determining the current as a function of time after application of the potential and prior to achievement of a steady state current and then estimating the magnitude of the steady state current, the method previously described allows the diffusion coefficient and/or the concentration of the reduced (or oxidised) form of the species to be estimated. 
     Our co-pending application exemplifies this method with reference to use of a “thin layer electrochemical cell” employing a GOD/Ferrocyanide system. As herein used, the term “thin layer electrochemical cell” refers to a cell having closely spaced electrodes such that reaction product from the counter electrode arrives at the working electrode. In practice, the separation of electrodes in such a cell for measuring glucose in blood will be less than 500 microns, and preferably less than 200 microns. 
     The chemistry used in the exemplified electrochemical cell is as follows:
 
glucose+GOD→gluconic acid+GOD*  reaction 1
 
GOD*+2ferricyanide→GOD+2ferrocyanide  reaction 2
 
where GOD is the enzyme glucose oxidase, and GOD* is the ‘activated’ enzyme. Ferricyanide ([Fe(CN) 6 ] 3− ) is the ‘mediator’ which returns the GOD* to its catalytic state. GOD, an enzyme catalyst, is not consumed during the reaction so long as excess mediator is present. Ferrocyanide ([Fe(CN) 6 ] 4− ) is the product of the total reaction. Ideally there is initially no ferrocyanide, although in practice there is often a small quantity. After reaction is complete the concentration of ferrocyanide (measured electrochemically) indicates the initial concentration of glucose. The total reaction is the sum of reactions 1 and 2:
 
GOD
 
glucose+2ferricyanide→gluconic acid+2ferrocyanide  reaction 3
 
“Glucose” refers specifically to β-D-glucose.
 
     The prior art suffers from a number of disadvantages. Firstly, sample size required is greater than desirable. It would be generally preferable to be able to make measurements on samples of reduced volume since this in turn enables use of less invasive methods to obtain samples. 
     Secondly, it would be generally desirable to improve the accuracy of measurement and to eliminate or reduce variations due, for example, to cell asymmetry or other factors introduced during mass production of microcells. Likewise, it would be desirable to reduce electrode “edge” effects. 
     Thirdly, since the cells are disposable after use, it is desirable that they be capable of mass production at relatively low cost. 
     SUMMARY OF THE INVENTION 
     In a first embodiment of the present invention, a biosensor for use in determining a concentration of a component in an aqueous liquid sample is provided, the biosensor including: (a) an electrochemical cell, the electrochemical cell including a first electrically resistive substrate having a first thin layer of a first electrically conductive material on a first face, a second electrically resistive substrate having a second thin layer of a second electrically conductive material on a second face, the substrates being disposed with the first electrically conductive material facing the second electrically conductive material and being separated by a sheet including an aperture, the wall of which aperture cooperates with the electrically conductive materials to define a cell wall, and wherein the aperture defines a working electrode area in the cell, the cell further including a sample introduction aperture whereby the aqueous liquid sample may be introduced into the cell; and (b) a measuring circuit. 
     In one aspect of the first embodiment, the electrochemical cell further includes a socket region having a first contact area in electrical communication with the first thin layer of the first electrically conductive material and a second contact area in electrical communication with the second thin layer of the second electrically conductive material, whereby the electrochemical cell may be electrically connected with the measuring circuit. 
     In another aspect of the first embodiment, the measuring circuit includes a tongue plug. 
     In a further aspect of the first embodiment, at least one of the first electrically conductive material and the second electrically conductive material includes a metal. The metal may further include a sputter coated metal. 
     In still other aspects of the first embodiment, the aqueous liquid sample includes blood, and the component includes glucose. 
     In yet another aspect of the first embodiment, the measuring circuit includes an automated instrument for detecting an electrical signal from the electrochemical cell and relating the electrical signal to the concentration of the component in the aqueous liquid sample. 
     In a further aspect of the first embodiment, the electrochemical cell includes a substantially flat strip having a thickness, the strip having at least two lateral edges, and wherein the sample introduction aperture includes a notch through the entire thickness of the strip in at least one of the lateral edges thereof. 
     In a second embodiment of the present invention, a biosensor for use in determining a concentration of a component in an aqueous liquid sample is provided, the biosensor including: (a) a thin layer electrochemical cell, the cell including: (i) an electrically resistive sheet including an aperture wherein the aperture defines a working electrode area in the cell; (ii) a first electrode layer covering the aperture on a first side of the sheet; (iii) a second electrode layer covering the aperture on a second side of the sheet; and (iv) a passage for admission into the aperture of the aqueous liquid sample; and (b) a measuring circuit. 
     In one aspect of the second embodiment, the electrochemical cell further includes a socket region having a first contact area in electrical communication with the first electrode layer and a second contact area in electrical communication with the second electrode layer, whereby the electrochemical cell may be electrically connected with the measuring circuit. 
     In another aspect of the second embodiment, the measuring circuit includes a tongue plug. 
     In still other aspects of the second embodiment, the aqueous liquid sample includes blood, and the component includes glucose. 
     In a further aspect of the second embodiment, the measuring circuit includes an automated instrument for detecting an electrical signal from the electrochemical cell and relating the electrical signal to the concentration of the component in the aqueous liquid sample. 
     In yet another aspect of the second embodiment, the cell includes a substantially flat strip having a thickness, the strip having at least two lateral edges, and wherein the passage for admission into the aperture includes a notch through the entire thickness of the strip in at least one of the lateral edges thereof. 
     In a third embodiment of the present invention, an apparatus for determining a concentration of a reduced form or an oxidized form of a redox species in a liquid sample is provided, the apparatus including: (a) a hollow electrochemical cell having a working electrode and a counter or counter/reference electrode wherein the working electrode is spaced from the counter or counter/reference electrode by less than 500 μm; (b) means for applying an electric potential difference between the electrodes; and (c) means for electrochemically determining the concentration of the reduced form or the oxidized form of the redox species in the liquid sample. 
     In one aspect of the third embodiment, means for electrochemically determining the concentration of the reduced form or the oxidized form of the redox species includes: (i) means for determining a change in current with time after application of the electric potential difference and prior to achievement of a steady state current; (ii) means for estimating a magnitude of the steady state current; and (iii) means for obtaining from the change in current with time and the magnitude of the steady state current, a value indicative of the concentration of the reduced form or the oxidized form of the redox species. 
     In another aspect of the third embodiment, the cell further includes a socket region having a first contact area in electrical communication with the working electrode and a second contact area in electrical communication with the counter or counter/reference electrode, whereby the cell may be electrically connected with at least one of the means for applying an electric potential difference between the electrodes and the means for electrochemically determining the concentration of the reduced form or the oxidized form of the redox species in the liquid sample. 
     In a further aspect of the third embodiment, at least one of the means for applying an electric potential difference between the electrodes and the means for electrochemically determining the concentration of the reduced form or the oxidized form of the redox species in the liquid sample includes a tongue plug. 
     In yet another aspect of the third embodiment, at least one of the means for applying an electric potential difference between the electrodes and the means for electrochemically determining the concentration of the reduced form or the oxidized form of the redox species in the liquid sample includes an automated instrument for detecting an electrical signal from the electrochemical cell and relating the electrical signal to the concentration of the reduced form or the oxidized form of the redox species in the liquid sample. 
     In a further aspect of the third embodiment, the cell includes a substantially flat strip having a thickness, the strip having at least two lateral edges, and wherein a notch extends through a wall of the electrochemical cell and through the entire thickness of the strip in at least one of the lateral edges thereof, whereby the liquid sample may be introduced into the cell. 
     In still other aspects of the third embodiment, the liquid sample includes blood, and the redox species includes glucose. 
     In a fourth embodiment of the present invention, a method for determining a concentration of a reduced form or an oxidized form of a redox species in a liquid sample is provided, the method including: (a) providing a hollow electrochemical cell having a working electrode and a counter or counter/reference electrode wherein the working electrode is spaced from the counter or counter/reference electrode by less than 500 μm; (b) applying an electric potential difference between the electrodes; and (c) electrochemically determining the concentration of the reduced form or the oxidized form of the redox species in the liquid sample. 
     In one aspect of the fourth embodiment, step (c) includes: (i) determining a change in current with time after application of the electric potential difference and prior to achievement of a steady state current; (ii) estimating a magnitude of the steady state current; and (iii) obtaining from the change in current with time and the magnitude of the steady state current, a value indicative of the concentration of the reduced form or the oxidized form of the redox species. 
     In another aspect of the fourth embodiment, the cell further includes a socket region having a first contact area in electrical communication with the working electrode and a second contact area in electrical communication with the counter or counter/reference electrode. 
     In a further aspect of the fourth embodiment, step (b) further includes the step of: providing an automated instrument for applying an electric potential difference between the electrodes. 
     In yet another aspect of the fourth embodiment, step (c) includes the steps of: (i) providing an automated instrument for detecting an electrical signal from the electrochemical cell; and (ii) relating the electrical signal to the concentration of the reduced form or the oxidized form of the redox species in the liquid sample. 
     In a further aspect of the fourth embodiment, the cell includes a substantially flat strip having a thickness, the strip having at least two lateral edges, and wherein a notch extends through a wall of the electrochemical cell and through the entire thickness of the strip in at least one of the lateral edges thereof, whereby the liquid sample may be introduced into the cell. 
     In still other aspects of the fourth embodiment, the liquid sample includes blood and the redox species includes glucose. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be particularly described by way of example only with reference to the accompanying schematic drawings wherein: 
         FIG. 1  shows the product of manufacturing step 2 in plan. 
         FIG. 2  shows the product of  FIG. 1  in side elevation. 
         FIG. 3  shows the product of  FIG. 1  in end elevation. 
         FIG. 4  shows the product of manufacturing step 3 in plan. 
         FIG. 5  shows the product of  FIG. 4  in cross-section on line  5 — 5  of FIG.  4 . 
         FIG. 6  shows the product of manufacturing step 5 in plan. 
         FIG. 7  shows the product of  FIG. 6  in side elevation. 
         FIG. 8  shows the product of  FIG. 6  in end elevation. 
         FIG. 9  shows the product of manufacturing step 7 in plan. 
         FIG. 10  is a cross-section of  FIG. 9  on line  10 — 10 . 
         FIG. 11  shows the product of  FIG. 9  in end elevation. 
         FIG. 12  shows a cell according to the invention in plan. 
         FIG. 13  shows the call of  FIG. 12  in side elevation. 
         FIG. 14  shows the cell of  FIG. 12  in end elevation. 
         FIG. 15  shows a scrap portion of a second embodiment of the invention in enlarged section. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The construction of a thin layer electrochemical cell will now be described by way of example of the improved method of manufacture. 
     Step 1: A sheet  1  of Melinex® (a chemically inert, and electrically resistive Polyethylene Terephthalate [“TPET”]) approximately 13 cm×30 cm and 100 micron thick was laid flat on a sheet of release paper  2  and coated using a Number  2  MYAR bar to a thickness of 12 microns wet (approximately 2-5 microns dry) with a water-based heat activated adhesive  3  (ICI Novacoat system using catalyst:adhesive). The water was then evaporated by means of a hot air dryer leaving a contact adhesive surface. The sheet was then turned over on a release paper and the reverse side was similarly coated with the same adhesive  4 , dried, and a protective release paper  5  applied to the exposed adhesive surface. The edges were trimmed to obtain a sheet uniformly coated on both sides with tacky contact adhesive protected by release paper. 
     Step 2: The sheet with protective release papers was cut into strips  7 , each about 18 mm×210 mm (FIGS.  1 - 3 ). 
     Step 3: A strip  7  of adhesive-coated PET from step 2 with release paper  2 ,  5  on respective sides, was placed in a die assembly (not shown) and clamped. The die assembly was adapted to punch the strip with a locating hole  10  at each end and with for example 37 circular holes 11 each of 3.4 mm diameter at 5 mm centres equi-spaced along a line between locating holes  10 . The area of each hole  11  is approximately 9 square mm. 
     Step 4: A sheet  12  of Mylar® PET approximately 21 cm square and 135 microns thick was placed in a sputter coating chamber for palladium coating  13 . The sputter coating took place under a vacuum of between 4 and 6 millibars and in an atmosphere of argon gas. Palladium was coated on the PET to a thickness of 100-1000 angstroms. There is thus formed a sheet  14  having a palladium sputter coating  13 . 
     Step 5: The palladium coated PET sheet  14  from Step 4 was then cut into strips  14  and  15  and a die was used to punch two location holes  16  in each strip, at one end ( FIGS. 6 ,  7  and  8 ). Strips  14  and  15  differ only in dimension strips  14  being 25 mm×210 mm and strips  15  being 23 mm×210 mm. 
     Step 6: A spacer strip  7  prepared as in step 3 was then placed in a jig (not shown) having two locating pins (one corresponding to each locating hole  10  of strip  7 ) and the upper release paper  2  was removed. A strip  14  of palladium coated PET prepared as in step 5 was then laid over the adhesive layer, palladium surface downwards, using the jig pins to align the locating holes  16  with the underlying PET strip  7 . This combination was then passed through a laminator comprising a set of pinch rollers, one of Which was adapted to heat the side bearing a palladium coated PET strip  14 . The roller on the opposite side of the strip  7  was cooled. By this means, only the adhesive between the palladium of strip  14  and PET strip  7  was activated. 
     Step 7: PET strip  7  was then turned over and located in the jig with the release coating uppermost. The release coating was peeled off and second palladium coated strip  15  was placed palladium side down on the exposed adhesive surface using the locating pins to align the strips. this assembly was now passed again through the laminator of step 6, this time with the hot roll adjacent the palladium coated Mylar® added in step 7 so as to activate the intervening adhesive ( FIGS. 9 ,  10  and  11 ). 
     Step 8: The assembly from step 7 was returned to the die assembly and notches  17  punched in locations so as to extend between the circular holes  11  previously punched in the Melinex® PET and the strip edge  17 . Notches  16  extend so as to intercept the circumference of each circular cell. The strip was then guillotined to give 37 individual “sensor strips”, each strip being about 5 mm wide and each having one thin layer cavity cell ( FIGS. 12 ,  13  and  14 ). 
     There is thus produced a cell as shown in  FIGS. 12 ,  13  or  14 . The cell comprises a first electrode consisting of PET layer  12 , a palladium layer  13 , an adhesive layer  3 , a PET sheet  1 , a second adhesive layer  4 , a second electrode comprising palladium layer  13 , and a PET layer  12 . Sheet  1  defines a cylindrical cell  11  having a thickness in the cell axial direction corresponding to the thickness of the Melinex® PET sheet layer  1  together with the thickness of adhesive layers  3  and  4 . The cell has circular palladium end walls. Access to the cell is provided at the side edge of the cell where notches  16  intersect cell  11 . 
     In preferred embodiments of the invention, a sample to be analysed is introduced to the cell by capillary action. The sample is placed on contact with notch  16  and is spontaneously drawn by capillary action into the cell, displaced air from the cell venting from the opposite notch  16 . A surfactant may be included in the capillary space to assist in drawing in the sample. 
     The sensors are provided with connection means for example edge connectors whereby the sensors may be placed into a measuring circuit. In a preferred embodiment this is achieved by making spacer l shorter than palladium supporting sheets  14 ,  15  and by making one sheet  15  of shorter length than the other  14 . This forms a socket region  20  having contact areas  21 ,  22  electrically connected with the working and counter electrodes respectively. A simple tongue plug having corresponding engaging conduct surfaces can then be used for electrical connection. Connectors of other form may be devised. 
     Chemicals for use in the cell may be supported on the cell electrodes or walls, may be supported on an independent support contained within the cell or may be self-supporting. 
     In one embodiment, chemicals for use in the cell are printed onto the palladium surface of the electrode immediately after step 1 at which stage the freshly-deposited palladium is more hydrophilic. For example, a solution containing 0.2 molar potassium ferricyanide and 1% by weight of glucose oxidase dehydrogenase may be printed on to the palladium surface. Desirably, the chemicals are printed only in the areas which will form a wall of the cell and for preference the chemicals are printed on the surface by means of an ink jet printer. In this manner, the deposition of chemicals may be precisely controlled. If desired, chemicals which are desirably separated until required for use may be printed respectively on the first and second electrodes. For example, a GOD/ferrocyanide composition can be printed on one electrode and a buffer on the other. Although it is highly preferred to apply the chemicals to the electrodes prior to assembly into a cell, chemicals may also be introduced into the cell as a solution after step 6 or step 8 by pipette in the traditional manner and the solvent subsequently is removed by evaporation or drying. Chemicals need not be printed on the cell wall or the electrodes and may instead be impregnated into a gauze, membrane, non-woven fabric or the like contained within, or filling, the cavity (eg inserted in cell  11  prior to steps 6 or 7). In another embodiment the chemicals are formed into a porous mass which may be introduced into the cell as a pellet or granules. Alternatively, the chemicals may be introduced as a gel. 
     In a second embodiment of the invention a laminate  21  is first made from a strip  14  as obtained in step 5 adhesively sandwiched between two strips  7  as obtained from step 3. Laminate  20  is substituted for sheet  1  in step 5 and assembled with electrodes as in steps 6 and 7. 
     There is thus obtained a cell as shown in  FIG. 15  which differs from that of  FIGS. 9  to  11  in that the cell has an annular electrode disposed between the first and second electrode. This electrode can for example be used as a reference electrode. 
     It will be understood that in mass production of the cell, the parts may be assembled as a laminate on a continuous line. For example, a continuous sheet  1  of PET could be first punched and then adhesive could be applied continuously by printing on the remaining sheet. Electrodes (pre-printed with chemical solution and dried) could be fed directly as a laminate onto the adhesive coated side. Adhesive could then be applied to the other side of the punched core sheet and then the electrode could be fed as a laminate onto the second side. 
     The adhesive could be applied as a hot melt interleaving film. Alternatively, the core sheet could first be adhesive coated and then punched. 
     By drying chemicals on each electrode prior to the gluing step the electrode surface is protected from contamination. 
     Although the cell has been described with reference to Mylar® and Melinex® PET, other chemically inert and electrically resistive materials may be utilised and other dimensions chosen. The materials used for spacer sheet  1  and for supporting the reference and counter electrodes may be the same or may differ one from the other. Although the invention has been described with reference to palladium electrodes, other metals such as platinum, silver, gold, copper or the like may be used and silver may be reacted with a chloride to form a silver/silver chloride electrode or with other halides. The electrodes need not be of the same metal. 
     Although the use of heat activated adhesives has been described, the parts may be assembled by use of hot melt adhesives, fusible laminates and other methods. 
     The dimensions of the sensor may readily be varied according to requirements. 
     While it is greatly preferred that the electrodes cover the cell end openings, in other embodiments (not illustrated) the electrodes do not entirely cover the cell end openings. In that case it is desirable that the electrodes be in substantial overlying registration. 
     Preferred forms of the invention in which the electrodes cover the apertures of cell  11  have the advantages that the electrode area is precisely defined simply by punching hole  1 . Furthermore the electrodes so provided are parallel, overlying, of substantially the same area, and are substantially or entirely devoid of “edge” effects. 
     Although in the embodiments described each sensor has one cell cavity, sensors may be provided with two or more cavities. For example, a second cavity may be provided with a predetermined quantity of the analyte and may function as a reference cell. 
     As will be apparent to those skilled in the art from the teaching herein contained, a feature of one embodiment herein described may be combined with features of other embodiments herein described or with other embodiments described in our co-pending application. Although the sensor has been described with reference to palladium electrodes and a GOD/ferrocyanide chemistry, it will be apparent to those skilled in the art that other chemistries, and other materials of construction may be employed without departing from the principles herein taught.