Abstract:
Strips, particularly test strips and adapters for test strips, for use in meters for the electrochemical measurement of analyte in a sample material and in particular the glucose concentration of a sample of blood. The strips comprise a plurality of working connectors, for interfacing with the meter, coupled to one or more working electrodes. The strips are of particular use in adapting multi-input meters for single input use.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to strips for use with multi-input meters for the electrochemical measurement of analyte in a sample material. In particular, the invention relates to test strips and adapters for test strips for determining glucose concentration in samples of blood. 
       BACKGROUND OF THE INVENTION 
       [0002]    Devices for measuring blood glucose levels are invaluable to diabetics—especially devices that may be used by the sufferers themselves, enabling them to monitor their own glucose levels and take doses of insulin. 
         [0003]    Conventionally, at least the part of the glucose-measuring device that comes into contact with the blood sample is disposable. This is important for reasons of hygiene, ease of use, the avoidance of cross-contamination between samples, and to prevent the spread of infectious diseases. Since diabetics must frequently check their glucose levels, it is important that the cost of the disposable is minimised. 
         [0004]    Current glucose measuring devices favour an electrochemical measurement method over colorimetric methods. The general principle is that an electric current is measured between two sensor parts called the working and reference sensor parts respectively. The working sensor part includes at least one working electrode onto which is applied a layer of enzyme reagent, comprising an enzyme such as the flavo-enzyme glucose oxidase and an electron mediator compound such as ferricyanide. When a potential difference is applied across the electrodes, a current is generated by the transfer of electrons from the substance being measured (the enzyme substrate), via the enzyme and to the surface of the working electrode. The measurement of glucose using a glucose oxidase and ferricyanide test strip is based upon the specific oxidation of glucose by the glucose oxidase. During this reaction, the glucose oxidase becomes reduced. The enzyme is re-oxidized by reaction with the ferricyanide, which is itself reduced during the course of the reaction. When these reactions are conducted with a potential difference applied between the reference and working electrodes, an electrical current may be created by the electrochemical re-oxidation of the reduced mediator ion (ferrocyanide) at the working electrode surface. Thus, since the amount of ferrocyanide created during the chemical reaction described above is directly proportional to the amount of glucose in the sample positioned between the electrodes, the current generated is proportional to the glucose content of the sample. The current generated is also proportional to the area of the working electrode. Given a known area of the working sensor part, the glucose concentration can therefore be determined from the measured electric current. 
         [0005]    Because it can be very important to know the concentration of glucose in blood, particularly for people with diabetes, meters have been developed using the principles set forth above to enable a user to sample and test their blood to determine the glucose concentration at any given time. The generated current is monitored by the meter and converted into a reading of glucose concentration using an algorithm that relates current to glucose concentration via a simple mathematical formula. In general, the meters work in conjunction with a disposable strip that includes a sample chamber and at least two sensor parts disposed within the sample chamber in addition to the enzyme (e.g. glucose oxidase) and mediator (e.g. ferricyanide). A suitable disposable electrochemical test strip is that used in the OneTouch® Ultra® whole blood testing kit, which is available from LifeScan, Inc. In use, the user pricks their finger or other convenient site to induce bleeding and introduces a blood sample to the sample chamber, thus starting the chemical reaction set forth above. 
         [0006]    In electrochemical terms, the function of the meter is two fold. Firstly, it provides a polarizing voltage (approximately +0.4 V in the case of OneTouch® Ultra®) that polarizes the electrical interface and leads to current flow at the working electrode surface. Secondly, it measures the current that flows in the external circuit between the anode (working electrode) and the cathode (reference electrode). 
         [0007]    The meter described above may be considered a simple electrochemical system that operates in two-electrode mode. However, in practice, third and even fourth electrodes may be used to facilitate the measurement of glucose and/or to perform other functions in the meter. In particular, multi-input meters for use with electrochemical test strips that have two or more working electrodes are commonly used. It is also known to provide a cell having both a reference electrode and a counter electrode in which the counter electrode serves to carry the current flowing through the cell. 
         [0008]    U.S. Pat. No. 6,733,655 describes a device for measuring the concentration of a substance in a sample liquid, said device comprising a reference sensor part, a first working sensor part for generating charge carriers in proportion to the concentration of said substance in the sample liquid; and a second working sensor part also for generating charge carriers in proportion to the concentration of said substance in the sample liquid. Thus it will be seen that in accordance with the aforementioned U.S. patent that the measuring device compares the current passed by two working sensor parts as a result of their generation of charge carriers and gives an error indication if the two currents are too dissimilar—i.e. the current at one sensor part differs too greatly from what would be expected from considering the current at the other. 
         [0009]    It is not always necessary or desirable to use test strips with more than one working electrode. However, multi-input meters are often not backwards compatible with dual electrode (i.e. single reference electrode and single working electrode) test strips. A multi-input meter with an unconnected second working sensor input may interpret lack of an input as an erroneous measurement and indicate an error in the test strip. Similarly, the sensor parts of an electrochemical test strip must be matched to the meter used in order for an accurate measurement to be made, since the calculation performed by the meter to determine glucose concentration is dependent upon certain assumed information concerning the expected test strip (e.g. the working surface area of the electrodes). 
         [0010]    The restriction that a meter can only be used with particular test strips that have configurations that are matched to that meter is inconvenient to a user, who is consequently forced to use only those test strips. Thus, a user who has recently replaced his existing single working sensor meter with a multi-input meter may find that his supply of single working electrode test strips for use with his previous meter are not compatible with the multi-input meter and must be discarded and replaced with new multiple sensor test strips. Equally, a user may not always be able to obtain test strips that are designed specifically for his meter, although test strips designed for different meters may be available to him. Applicants recognize that it is desirable that the user should be able to use test strips with his meter when the test strips are not designed specifically (i.e. matched to) his meter. 
         [0011]    The provision of multiple working sensors on a test strip adds to the test strip&#39;s complexity and therefore also to the cost and difficulty of its manufacture. It is also to be expected that manufacturing defects will be more common in test strips of greater complexity. Since multiple working sensors are not required for all applications it is desirable that a user has the option of using a single working sensor test strip with any meter. However, if a multi-input meter is used, the lack of backwards compatibility with single working sensor test strips forces the user to use the more complex multiple working sensor test strips, even if they are not required for his application. 
         [0012]    Finally, the presence of multiple working sensors is problematic in cases where only a very limited quantity of sample material (e.g. blood) is available. In such cases, sufficient material may be present to fully complete the circuit in test strip having a single working electrode, but not a test strip having multiple working electrodes (all of which need to be covered by the sample material). Therefore, the lack of compatibility between multi-input meters and single working electrode test strips inhibits the use of test strips that are better suited for certain applications. 
         [0013]    Applicants have recognized that it would be desirable to permit the user of a multi-input glucose meter to use a test strip having electrodes that are not necessarily matched to the meter. 
       SUMMARY 
       [0014]    The present invention includes a strip for use with a multi-input meter for the electrochemical measurement of analyte in a sample material, a system of a strip with a meter, and a method of manufacturing such a strip. In one embodiment, the strip includes: a reference electrode; at least one working electrode; a reference connector and a plurality of working connectors for interfacing the strip to the meter; a reference link electrically coupling the reference electrode to the reference connector; and a plurality of working links electrically coupling the at least one working electrode to the plurality of working connectors, and characterised in that at least one working electrode is coupled to a plurality of the working connectors. 
         [0015]    Coupling working electrodes to multiple working connectors enables a single working electrode (or a single group of interconnected working electrodes) to provide current to more than one of the working connectors (via the plurality of working links). On connection of the strip to a multi-input meter, the total current supplied by the electrodes will be split between the working links and therefore also between the connectors. Thus, the working electrode will appear to the meter to be a plurality of electrodes, with a different one of the plurality connected to each working connector. In this way, the strip enables a multi-input meter to be used with fewer working electrodes than are normally required by the meter. 
         [0016]    Another advantage of sharing working electrodes between multiple connectors is that the total current supplied to each input of the meter will be attenuated as a function of the number of inputs interfaced to the connectors. This approach permits an otherwise inappropriately large current to be split between inputs that are configured to accept a lower current. 
         [0017]    The use of a strip according to the invention allows different configurations of working electrodes to be used with meters that are not specifically designed for those configurations. Particularly advantageously, no modification of the comparatively expensive and complex meter is required, instead all that is required is a modification of the test strip. Such modification may be performed by adapting the test strip manufacturing process in order to manufacture strips according to the present invention, or by modification of existing electrochemical test strips. For example, a strip according to certain embodiments of the present invention could be manufactured by modifying an existing multi-input test strip by adding junctions between selected working links. The modification may further include forming discontinuities in selected working links. 
         [0018]    The strip of the present invention is preferably an electrochemical test strip where, in use, the reference and working electrodes contact the sample material. Alternatively, the strip may be an adapter strip for connection between a prior art test strip and a meter. In the adapter embodiment, the reference and working electrodes mate, when in use, with the reference and working connectors of the test strip. The use of such an adapter advantageously permits existing (and unmodified) single or multiple working electrode test strips to be used with multi-input meters without modification of the test strip itself. Since the adapter does not contact the sample material, it is reusable. 
         [0019]    The at least one of the working electrodes may be coupled to all of the working connectors. 
         [0020]    The plurality of working links may have the same resistance, splitting the total current equally between the working connectors. Alternatively, the plurality of working links may have different resistances, allowing the distribution of current between the working connectors to be weighted. 
         [0021]    The one or more of the plurality of working links may have an overlay material over at least a portion of the one or more of the plurality of working links which decreases the electrical resistance of the one or more of the plurality of working links. 
         [0022]    The overlay material may include a single layer of an overlay material. Alternatively, it may be formed of several layers of the same or different materials. 
         [0023]    To control the distribution of current between the working connectors, the plurality of working links may all be made of material having the same or different resistivities and the working links may also have the same or different width, length, thickness and layout. 
         [0024]    A plurality of the working electrodes may be overlaid with an overlay material, the overlay material electrically intercoupling the overlaid working electrodes. The overlay material may entirely cover the working surfaces of the overlaid working electrodes, or it may only partially cover the working surfaces of the overlaid working electrodes. 
         [0025]    Overlaying the electrodes is advantageous since it can be used to simply convert a prior art test strip into a strip according to the present invention. In embodiments where the entire working surface of the working electrodes (i.e. the entire surface that would otherwise be exposed to the sample material) is overlaid, overlaying with a different material to that of the working electrode can be used to present a working surface to the sample that has different electrical, chemical and physical properties. What is more, the overlay material may substantially cover gaps located between adjacent overlaid working electrodes. Covering these gaps effectively enlarges the working surface of the electrodes, increasing the current that flows through the electrode. 
         [0026]    Overlaying the working electrodes thus enables the area and material of existing working electrodes&#39; effective working surfaces to be altered in addition to providing interconnection of the working electrodes (and thus also the working links). Overlaying is therefore particularly useful in modifying existing test strips for use with meters having input requirements that are not compatible with the unmodified test strips. 
         [0027]    Optionally, the overlay material may be a carbon ink. Carbon inks are suitable for screen printing, facilitating the large-scale automated modification of prior art test strips. 
         [0028]    At least one of the plurality of working links may be a split link, the split link comprising a first link portion having a first resistance and being formed of material having a first resistivity, electrically coupled to a second link portion having a second resistance and being formed of material having a second resistivity. The first and second resistivities may be different. The split link may further comprise a third link portion, wherein: the first and third link portions are separated by a gap; and the second portion at least partially overlays each of the first and third link portions such that the gap is bridged, electrically intercoupling the first and third link portions. The third link portion may be formed of material having the first resistivity. 
         [0029]    In some embodiments a plurality of the working links are split links. The plurality of split links may share the same first resistivities and may or may not share the same second resistivities. 
         [0030]    The use of a split link permits the resistance of the working links to be varied in order to apply a desired level of attenuation for each link. By selecting materials of appropriate resistivity, the resistance of each working link can be made equal, dividing the current equally between them, or can alternatively be weighted in order to weight the distribution of current between them. 
         [0031]    The formation of split links as first and third link portions, separated by a gap with a second portion bridging the gap, facilitates the strips&#39; manufacture. Large numbers of identical strip ‘blanks’ can be manufactured with only the first and third link portions in place, with the subsequent second link portion added at a later stage to bridge the first and third link portions, which can be accomplished by a suitable technique, such as, for example, by screen printing. Selecting materials of appropriate resistivities for the third link portions allows the easy customization of a strip ‘blank’ into a strip adapted for a particular meter. Since this process of customization is simply the overlaying of material to form the bridging second link portions, it is well suited for low-volume manufacturing methods. 
         [0032]    A plurality of split links may couple at least one working electrode to a plurality of working connectors via a junction, where the second link portions of the split links are located between the junction and the working connectors. Positioning the second link portions at the connector side of the junction permits a different weighting to be applied (through selection of appropriate second link portion materials) to the current available at each of the working connectors. 
         [0033]    In embodiments that use a counter electrode that is separate to the reference electrode, a counter electrode is provided and coupled to a counter connector using a counter link. 
         [0034]    In another aspect, a method of manufacturing a strip for use with a multi-input meter for the electrochemical measurement of analyte in a sample material is provided. The method includes providing a reference electrode; providing at least one working electrode; providing a reference connector and a plurality of working connectors for interfacing the strip to the measuring device; electrically coupling the reference electrode to the reference connector using a reference link; and electrically coupling the at least one working electrode to the plurality of working connectors using a plurality of working links, and characterised in that electrically coupling the at least one working electrode to the plurality of working connectors includes coupling at least one working electrode to a plurality of the working connectors. 
         [0035]    In another aspect, a system for electrochemically measuring an analyte in a sample material is provided. The system includes a strip including: a reference electrode and a working electrode, a reference connector, a first working connector, and second working connector for interfacing the strip to the measuring device; a reference link configured to electrically couple the reference electrode to the reference connector; a first working link configured to electrically couple the working electrode to the first working connector, and a second working link configured to electrically couple the working electrode to the second working connector, and a meter comprising: a first test voltage circuit capable of applying a first test voltage between the first working connector and the reference connector; a second test voltage circuit capable of applying a second test voltage between the second working connector and the reference connector; a current measurement circuit capable of measuring a first test current between the first working connector and the reference connector and a second test current between the second working connector and the reference connector. 
         [0036]    These and other embodiments, features and advantages will become apparent to those skilled in the art when taken with reference to the following more detailed description of the invention in conjunction with the accompanying drawings that are first briefly described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain features of the invention (wherein like numerals represent like elements), of which: 
           [0038]      FIG. 1  shows a prior art test strip having two working electrodes; 
           [0039]      FIG. 2  shows the prior art test strip of  FIG. 1  partially covered by a dielectric mask; 
           [0040]      FIG. 3  shows a test strip according to a preferred embodiment having two working links and connectors; 
           [0041]      FIG. 4  shows a test strip according to a preferred embodiment having three working links and connectors; 
           [0042]      FIG. 5  shows the test strip of  FIG. 3  covered by a dielectric mask; 
           [0043]      FIG. 6  shows a circuit diagram of a portion of a test strip according to a preferred embodiment. 
           [0044]      FIG. 7  shows a test strip according to a preferred embodiment wherein the working electrodes have been overlaid with an overlay material; 
           [0045]      FIG. 8  shows an adapter according to a preferred embodiment and a prior art test strip having a single working electrode; 
           [0046]      FIG. 9  shows an adapter according to a preferred embodiment and a prior art test strip having two working electrodes; 
           [0047]      FIG. 10  shows an adapter according to a preferred embodiment having split working links, and a prior art test strip having a single working electrode; and 
           [0048]      FIG. 11  shows a test strip according to a preferred embodiment having two working electrodes and split working links. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0049]    The following detailed description should be read with reference to the drawings, in which like elements in different drawings are identically numbered. The drawings, which are not necessarily to scale, depict selected exemplary embodiments and are not intended to limit the scope of the invention. The detailed description illustrates by way of example, not by way of limitation, the principles of the invention. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. 
         [0050]    As used herein, the terms “about” or “approximately” for any numerical values or ranges indicate a suitable dimensional tolerance that allows the part or collection of components to function for its intended purpose as described herein. 
         [0051]      FIG. 1  shows a prior art test strip  100 , comprising a dielectric substrate  120  upon which are provided first and second working electrodes  130 ,  135 , a reference electrode  140 , first and second working connectors  150 ,  155 , and a reference connector  160 . First and second working links  170 ,  175  connect the first and second working electrodes  130 ,  135  to the first and second working connectors  150 ,  155 , respectively, and a reference link  180  connects the reference electrode  140  to the reference connector  160 . 
         [0052]    In the context of this application, ‘dielectric’ is used to describe a substrate that has suitable electrically insulating properties. 
         [0053]      FIG. 2 . shows the prior art test strip of  FIG. 1  with a dielectric mask layer  200  applied to prevent exposure of the working and reference links  170 ,  175 ,  180  to sample material. The mask  200  defines a window  210  that exposes a working surface of the working and reference electrodes  130 ,  135 ,  140  in order that they can be contacted by sample material. 
         [0054]    An enzyme layer (not shown) is printed over the mask  200  and thus also onto the areas of the electrodes  130 ,  135 ,  140  that are exposed through the window  210  in the mask  200 , forming the reference sensor part and the two working sensor parts, respectively. A layer of adhesive is then printed onto the strip and a hydrophilic film is laminated onto the strip and held in place by the adhesive. The film defines a sample chamber over the exposed sensor parts and a thin channel to draw liquid sample material into the sample chamber by capillary action. Finally, a protective plastic cover tape is applied over the hydrophilic film, the cover tape including a transparent portion over the sample chamber. The transparent portion enables a user to tell instantly if a strip has been used and also assists in affording a visual check as to whether enough sample material has been applied. 
         [0055]    In use, the test strip  100  is inserted into a meter (not shown). The meter includes a set of contacts that electrically couple with the working and reference connectors  150 ,  155 ,  160  on insertion. The meter applies a potential difference across the reference connector  160  and each of the two working connectors  150 ,  155  and, after a predetermined period of time, the electric current flowing though each of the working connectors  150 ,  155  (and therefore also through the working electrodes  130 ,  135 ) is measured by the meter and the two measurements are compared. If the measurements differ by more than a threshold amount, an error message is displayed on the meter and the test must be repeated. However, if the measurements do not differ by more than the threshold amount, a glucose level is calculated based on the measured currents and displayed on the meter. 
         [0056]      FIG. 3  shows a test strip  300  according to a preferred embodiment. The test strip  300  includes a substrate  320  that may be made of any dimensionally stable dielectric material that is resistant to the sample material. Preferred materials for the substrate include polyester, polycarbonate, polyamide, polyethylene, polypropylene, polyvinylchloride and nylon. Other suitable materials include plastics, ceramics and glass. The test strip  300  further includes a first working electrode  330 , a reference electrode  340 , two working connectors  350 ,  355  and a reference connector  360 . The first working electrode is electrically coupled to each of the working connectors  350 ,  355  by a working link  370 ,  375  and the reference electrode  340  is electrically coupled to the working connector  360  by a reference link  380 . Suitable materials for the electrodes  330 ,  340 , connectors  350 ,  355 ,  360  and links  370 ,  375 ,  380  include carbon, gold, platinum, palladium, iridium, rhodium, conducting polymers, stainless steel and doped tin oxide. The electrodes  330 ,  340 , connectors  350 ,  355 ,  360  and links  370 ,  375 ,  380  may be, but are not necessarily, of the same material. Preferably, the electrodes  330 ,  340 , connectors  350 ,  355 ,  360  and links  370 ,  375 ,  380  are formed by screen printing carbon ink printed onto the substrate  320 . 
         [0057]    Although only a single working electrode  330  is shown in  FIG. 3 , the test strip  300  may further comprise additional working electrodes, either electrically coupled to or isolated from the first working electrode  330 . Similarly, the test strip  300  may further comprise additional working connectors and working links, either electrically coupled to or isolated from those shown in  FIG. 3 . By way of example,  FIG. 4  shows a test strip  400  according to a preferred embodiment that has three working connectors  350 ,  355 ,  456  and three working links  370 ,  375 ,  476  coupling the working connectors  350 ,  355 ,  456  to a single working electrode  330 . 
         [0058]      FIG. 5  shows the test strip  300  of  FIG. 3  with a dielectric mask layer  500  applied to prevent exposure of the working and reference links  370 ,  375 ,  380  to sample material. The mask  500  defines a window  510  that exposes a working surface of the working and reference electrodes  330 ,  340  in order that they can be contacted by sample material. The mask may be formed of any suitable dielectric material that is resistant to the sample material. Preferably, for ease of manufacture, the mask is screen printed onto the test strip. 
         [0059]    An enzyme layer (not shown) is printed over the mask  500  and thus also onto the portions of the electrodes  330 ,  340  that are exposed through the window  510  in the mask  500 , forming the reference sensor part and working sensor part, respectively. A layer of adhesive is then printed onto the strip and a hydrophilic film is laminated onto the strip and held in place by the adhesive. The film defines a sample chamber over the exposed sensor parts and a thin channel to draw liquid sample material into the sample chamber by capillary action. Finally, a protective plastic cover tape is applied over the hydrophilic film, the cover tape including a transparent portion over the sample chamber. The transparent portion enables a user to tell instantly if a strip has been used and also assists in affording a visual check as to whether enough sample material has been applied. 
         [0060]    When the test strip  300  of  FIGS. 3 and 5  is used with a multi-input meter, the current flowing between the reference and working electrodes  340 ,  330  is split between the working links  370 ,  375  connected to the working electrode  330  and thus also between the working connectors  350 ,  355 . If the working links  370 ,  375  have equal resistance and if equal voltages are applied, the current measured at each of the working connectors  350 ,  355  will be half of the current flowing between the reference and working electrodes  340 ,  330 . Since an equal current is measured at each of the electrodes, the multi-input meter will not detect an error. 
         [0061]    In one embodiment, a meter may apply a first test voltage V 1  between first working connector  350  and reference connector  360 , and a second test voltage V 2  between the second working connector  355  and the reference connector  360 , as illustrated in  FIG. 6 . As a result of first test voltage V 1  and second test voltage V 2 , the meter can measure a first test current I 1 (t) and a second test current I 2 (t) that are both proportional to an analyte concentration. The terms I 1 (t) and I 2 (t) represents the first and second test currents, respectively, as a function of time t. 
         [0062]    As show below, Equation 1 can be derived by applying Kirchoff&#39;s current law to the circuit illustrated in  FIG. 6 : 
         [0000]        I ( t )= I   1 ( t )+ I   2 ( t )  Eq. 1. 
         [0063]    In one embodiment, the first test voltage V 1  and second test voltage V 2  may be exactly the same in magnitude. However, in practice, the first test voltage V 1  and second test voltage V 2  may have a finite difference in magnitude because of the variability typically observed in electronic components. A difference voltage V diff  is a difference between the first test voltage V 1  and the second test voltage V 2 . As a result of the application of the first test voltage V 1  and second test voltage V 2 , the difference voltage V diff  is effectively applied between the first working connector  350  and the second working connector  355 . The following will describe the effects of V diff  on the current flow in the circuit of  FIG. 6  before and after a liquid sample has been applied to the sensor. 
         [0064]    For the first situation where a sample has not been applied to the sensor, I (t) is zero, hence from Eq. 1 the currents through both branches are equal in magnitude and opposite in direction I 1  (t)=−I 2  (t). The magnitude of the current I shunt  that flows between the first working connector  350  and the second working connector  355 , as a result of the difference voltage V diff , is directly proportional to the difference voltage V diff , and inversely proportional to a shunt resistance R shunt  between the first working connector  350  and the second working connector  355 , as illustrated in Equation 2. 
         [0000]    
       
         
           
             
               
                 
                   
                      
                     
                       I 
                       shunt 
                     
                      
                   
                   = 
                   
                     
                       
                          
                         
                           V 
                           diff 
                         
                          
                       
                       
                         R 
                         shunt 
                       
                     
                     = 
                     
                       
                          
                         
                           
                             V 
                             2 
                           
                           - 
                           
                             V 
                             1 
                           
                         
                          
                       
                       
                         R 
                         shunt 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0065]    The shunt resistance R shunt  may include a summation of resistance values from the first working connector  350 , first working link  370 , second working link  375 , and the second working connector  355 . A simplified representation of R shunt  is illustrated in  FIG. 6  where the first working connector  350  and the second working connector  355  are both assumed to have a negligible resistance so that R shunt =R 1 +R 2 . In the preferred embodiment, the two resistors R 1  and R 2  will have about the same value hence: 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     1 
                   
                   = 
                   
                     
                       R 
                       2 
                     
                     = 
                     
                       
                         R 
                         shunt 
                       
                       2 
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
         [0066]    For the second situation where sample has been applied, I(t) is different from zero and hence there will be a voltage drop across R common , R 1  and R 2 . Hence the effective voltage V eff  applied to the electrode is: 
         [0000]        V   eff   =V   shunt   −I ( t ) R   common   Eq. 4 
         [0067]    Since V shunt  is the voltage at the junction, as illustrated in  FIG. 6 , Equation 5 can be constructed: 
         [0000]        V   shunt   =V   1   −I   1 ( t ) R   1   =V   2   −I   2 ( t ) R   2   Eq. 5 
         [0000]    and since V 1  and V 2  are similar then each can be substituted by the nominal polarisation potential, V pol , and since I 1 (t) and I 2 (t) are very similar, each can be substituted by I(t)/2 as derived from Eq. 1. Then, Eq. 5 becomes: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     shunt 
                   
                   = 
                   
                     
                       
                         V 
                         pol 
                       
                       - 
                       
                         
                           
                             I 
                              
                             
                               ( 
                               t 
                               ) 
                             
                           
                           2 
                         
                          
                         
                           
                             R 
                             shunt 
                           
                           2 
                         
                       
                     
                     = 
                     
                       
                         V 
                         pol 
                       
                       - 
                       
                         
                           
                             I 
                              
                             
                               ( 
                               t 
                               ) 
                             
                           
                            
                           
                             R 
                             shunt 
                           
                         
                         4 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                    
                   6 
                 
               
             
           
         
       
     
         [0068]    Substituting V shunt  from Eq. 6 into the expression for V eff  (Eq. 4) results in Equation 7. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           V 
                           eff 
                         
                         = 
                           
                          
                         
                           
                             V 
                             pol 
                           
                           - 
                           
                             
                               
                                 I 
                                  
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                                
                               
                                 R 
                                 shunt 
                               
                             
                             4 
                           
                           - 
                           
                             
                               I 
                                
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                              
                             
                               R 
                               common 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             V 
                             pol 
                           
                           - 
                           
                             
                               I 
                                
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                              
                             
                               ( 
                               
                                 
                                   
                                     R 
                                     shunt 
                                   
                                   4 
                                 
                                 + 
                                 
                                   R 
                                   common 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                    
                   7 
                 
               
             
           
         
       
     
         [0069]    Hence, to ensure proper operation of the sensor, V eff  has to be sufficiently unattenuated by the terms in brackets in Eq. 7. Thus, R shunt  and R common  must be sufficiently small in magnitude so that V eff  can allow an accurate measurement of analyte. 
         [0070]    However, R shunt  must also be sufficiently large in magnitude so that I shunt  is sufficiently small (see Equation 2). If I shunt  is sufficiently large (e.g., greater than pre-determined thresholds stored in the memory of the meter), an error message may be outputted by the glucose meter incorrectly identifying the strip as defective or as already used. For example, a pre-determined threshold may be about 100 nanoamperes. Accordingly, R shunt  must also be sufficiently large in magnitude to prevent the meter from outputting an error message, but also must be sufficiently small in magnitude to allow for an accurate measurement of analyte. 
         [0071]    As there is a compromise between the requirements for R shunt , it has to be determined for suitability. The first step in the determination is: as R shunt  and R common  are dependant on the position of the junction and from Eq. 2, R common  will not contribute to increase I shunt  and from Eq. 7 R common  on has 4 times more effect than R shunt : the solution is to move the junction as close as possible to the working electrode to achieve a maximum value of R shunt  while a minimum contribution from R common . 
         [0072]    The second step in the determination process is: determine the maximum possible value of the difference |V2−V1| and configure R shunt  to be a value slightly larger than the result of dividing this voltage difference by the value of the largest current for which the system does not detect the strip as defective or already used. Thus, in an embodiment of this invention, a lower limit for R shunt  may be configured so that the resulting current I shunt  is lower than the pre-determined error thresholds of the meter. 
         [0073]    The third step in the determination process: determine a maximum possible I (t) value and configure both R shunt  and R common  so that V eff  is not sufficiently decreased to cause an inaccurate glucose measurement. Note that maximum values for I(t) may be estimated at a high glucose concentration (e.g., 600 mg/dL), a low hematocrit level (e.g., 20%), a high temperature (40 degrees Celsius), or a combination thereof. Thus, in an embodiment of this invention, an upper limit for R shunt  and R common  may be configured so that V eff  is not decreased by more than, for example, about 20% of the original value of V pol . 
         [0074]    The dimensions of the working area of the electrodes  330 ,  340  exposed through the window  510  in the mask layer  500  may be adjusted to account for the fact that the current measured at each of the working connectors  350 ,  355  is less than the total current flowing between the reference and working electrodes, as illustrated in  FIG. 5 . Increasing the working area of the electrodes  330 ,  340  will increase the measured currents and decreasing their working area will decrease the measured current. Alternatively, a correction to the measured current may be applied at the meter or may be applied to the reading displayed by the meter (e.g. manually). 
         [0075]      FIG. 7  shows the prior art test strip  100  of  FIG. 1  modified to provide a test strip  600  according to a preferred embodiment. This modification includes overlaying the working electrodes  130 ,  135  and bridging the gap  620  between them with an electrically conductive overlay material  610 . The overlay material  610  may be applied to the working electrodes  130 ,  135  and substrate  120  by any suitable method, for example by hand painting, but is preferably applied by screen printing a carbon ink onto the prior art test strip  100 . Electrically coupling the working electrodes  130 ,  135  by bridging the gap  620  between them with the overlay material  610  has the effect of electrically coupling the working links  170 ,  175  through the bridged working electrodes  130 ,  135  and the current flowing between the reference electrode  140  and working electrodes  130 ,  135  is therefore split between the working links  170 ,  175  and therefore also between the working connectors  150 ,  155 . 
         [0076]    The total current flowing through the reference electrode  140  and the working electrodes  130 ,  135  of the test strip  600  of  FIG. 7  can be adjusted by varying the effective working area of the working electrodes  130 ,  135 . The working electrodes&#39;  130 ,  135  effective working area can be increased by extending the overlay material  610  over areas of the substrate  120  that will be exposed to the sample material. In particular, bridging the gap  620  between the working electrodes  130 ,  135  with the overlay material  610  effectively increases the working electrodes&#39;  130 ,  135  working area. The overlay material  610  may be selected to have particular desired electrical, chemical and physical properties. In particular, the selection of the overlay material  610  can be used to increase or decrease the current that flows through the working electrodes  130 ,  135 . 
         [0077]      FIG. 8  shows an adapter  700  according to a preferred embodiment that, when in use, sits between a prior art test strip  710  having a single working electrode  130 , working link  170  and working connector  150 , and a multi-input meter (not shown). The adapter  700  is provided with a working electrode  730  and a reference electrode  740  that are configured to contact and form an electrical coupling with the working and reference connectors  150 ,  160  of the test strip  710 , respectively. The single working electrode  730  of the adapter  700  is electrically coupled by a pair of working links  770 ,  775  to two working connectors  750 ,  755  that are configured to interface with the working sensor inputs of the meter. The reference electrode  740  of the adapter  700  is electrically coupled by a reference link  780  to the adapter&#39;s  700  reference connector  760 , which is configured to interface with a reference connector on the meter. Preferably, the electrodes  730 ,  740  of the adapter  700  engage the connectors  150 ,  160  of the test strip  710  to releasably secure the adapter  700  to the test strip  710  during use. Once connected, the test strip  710  and adapter  700  function in the same manner as the test strip  300  of  FIG. 3 . 
         [0078]      FIG. 9  shows a variation on the adapter  700  of  FIG. 8 . The adapter  800  of  FIG. 9  is for use with the prior art test strip  100  of  FIG. 1 , which has two working electrodes  130 ,  135 , each connected to a different one of two working connectors  150 ,  155  by separate working links  170 ,  175 . The adapter  800  therefore includes two working electrodes  730 ,  835  that are configured to contact and form electrical couplings with the working connectors  150 ,  155  of the test strip  100 . Each of the working electrodes  730 ,  835  of the adapter  800  is electrically coupled to both of the working connectors  750 ,  755  of the adapter by the working links  770 ,  775  of the adapter  800 . 
         [0079]      FIG. 10  shows another adapter  900  according to a preferred embodiment. The adapter  900  is similar to the adapter  700  of  FIG. 8 , except that the working links  970 ,  975  are split links that are each divided into three working link portions  970   a - c ,  975   a - c . The split links  970 ,  975  may be divided into other numbers of portions; however, three is preferred. Although  FIG. 10  shows two split working links  970 ,  975 , other numbers of working links may be used, not all of which need be split links. 
         [0080]    The split links  970 ,  975  of  FIG. 10  each comprise a first link portion  970   a ,  975   a  and a third link portion  970   c ,  975   c . Each first portion  970   a ,  975   a  is coupled to a working connector  750 ,  755  of the adapter  900  and each of the third portions  970   c ,  975   c  is coupled to the working electrode  730  of the adapter  900  at a junction  910 . The first and third portions  970   a ,  975   a ,  970   c ,  975   c  of each link are separated by a gap, are preferably made of the same material and are preferably screen printed onto the substrate  720 . 
         [0081]    The adapter  900  of  FIG. 10 , less the second link portions  970   b ,  975   b  may be the adapter  700  of  FIG. 8  with a discontinuity formed in each of the working links  770 ,  775  to define the first and third link portions  970   a ,  975   a ,  970   c ,  975   c . These discontinuities may be formed by laser ablating, cutting, drilling or abrading the working links  770 ,  775 , or by any other suitable process. 
         [0082]    Each of the split links  970 ,  975  further includes a second link portion  970   b ,  975   b  that at least partially overlays the first and third link portions  970   a ,  975   a ,  970   c ,  975   c  and bridges the gap separating the first and third link portions. The second link portions  970   b ,  975   b  are preferably screen printed onto the adapter  900 , but may be applied by hand painting or other suitable methods. The second link portions  970   b ,  975   b  may be made of the same material as the first and/or third link portions  970   a ,  975   a ,  970   c ,  975   c . However, the second link portions  970   b ,  975   b  are preferably formed from a material having a different resistivity to that of the first and third link portions  970   a ,  975   a ,  970   c ,  975   c.    
         [0083]    The resistivity of the material used to form the second link portions  970   b ,  975   b  of  FIG. 10  may be varied across the working links  970 ,  975 . Varying the second link portion  970   b ,  975   b  material and/or the second link portions&#39;  970   b ,  975   b  dimensions and/or layout enables the resistivity of the working links  970 ,  975  to be weighted, in turn weighting the current available at each of the working connectors  750 ,  755 . 
         [0084]      FIG. 11  shows a test strip  1000  according to a preferred embodiment. The test strip  1000  includes, on a substrate  1020 , two working electrodes  1030 ,  1035  that are electrically coupled to two working connectors  1050 ,  1055  by two working links  1070 ,  1075 . The test strip  1000  further includes a reference electrode  1040  that is electrically coupled to a reference connector  1060  by a reference link  1080 . The working links  1070 ,  1075  are both split links, each split link comprising a first link portion  1070   a ,  1075   a  coupled to a working connector  1050 ,  1055  and a third link portion  1070   c ,  1075   c  coupled to a working electrode  1030 ,  1035 . Each first link portion  1070   a ,  1075   a  is spaced apart from the corresponding third link portions  1070   c ,  1075   c  by a gap and the third link portions  1070   c ,  1075   c  are intercoupled at a junction  1010 . Second link portions  1070   b ,  1075   b , at least partially overlay both the first and third link portions  1070   a ,  1075   a ,  1070   c ,  1075   c  of each of the split working links  1070 ,  1075  and bridge the gap between each working link&#39;s  1070 ,  1075  first and third portions  1070   a ,  1075   a ,  1070   c ,  1075   c . The split working links  1070 ,  1075  of the test strip  1000  of  FIG. 11  are formed in a similar manner to those of the adapter  900  of  FIG. 10  and can be similarly used to adjust the resistance of the working links  1070 ,  1075  and the division of the total working electrode  1030 ,  1035  current between the working connectors  1050 ,  1055 . 
         [0085]    While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that methods within the scope of these claims and their equivalents be covered thereby.