Abstract:
A biosensor having multiple electrical functionalities located both within and outside of the measurement zone in which a fluid sample is interrogated. Incredibly small and complex electrical patterns with high quality edges provide electrical functionalities in the biosensor and also provide the electrical wiring for the various other electrical devices provided in the inventive biosensor. In addition to a measurement zone with multiple and various electrical functionalities, biosensors of the present invention may be provided with a user interface zone, a digital device zone and/or a power generation zone. The inventive biosensors offer improved ease of use and performance, and decrease the computational burden and associated cost of the instruments that read the biosensors by adding accurate yet cost-effective functionalities to the biosensors themselves.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to applications entitled TEST STRIP WITH SLOT VENT OPENING (“Slot Vent Opening”) (attorney docket no. 7404-567), METHOD OF MAKING A BIOSENSOR (attorney docket no. 7404-480), METHOD AND REAGENT FOR PRODUCING NARROW, HOMOGENEOUS REAGENT STRIPES (“Reagent Stripes”) (attorney docket no. 7404-475), DEVICES AND METHODS RELATING TO ELECTROCHEMICAL BIOSENSORS (attorney docket no. 7404-569), SYSTEM AND METHOD FOR QUALITY ASSURANCE OF A BIOSENSOR TEST STRIP (“Quality Assurance”) (attorney docket no. 7404-456), SYSTEM AND METHOD FOR CODING INFORMATION ON A BIOSENSOR TEST STRIP (“Coding Information”) (attorney docket no. 7404-562), DISPENSER FOR FLATTENED ARTICLES (“Dispenser”) (attorney docket no. 7404-591), all of which have been filed on even date herewith and which are all incorporated herein by reference in their entireties. This application also is related to an application entitled SYSTEM AND METHOD FOR ANALYTE MEASUREMENT USING DOSE SUFFICIENCY ELECTRODES, filed Oct. 17, 2003 and given Ser. No. 10/687,958 (“Dose Sufficiency”), which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to devices, systems, and methods for measuring analytes from biological samples, such as from a sample of bodily fluid. More particularly, the present invention relates to electrically operable biosensors.  
       BACKGROUND  
       [0003]     Measuring the concentration of substances, particularly in the presence of other, confounding substances (“interferents”), is important in many fields, and especially in medical diagnosis and disease management. For example, the measurement of glucose in bodily fluids, such as blood, is crucial to the effective treatment of diabetes.  
         [0004]     Multiple methods are known for measuring the concentration of analytes such as glucose in a blood sample. Such methods typically fall into one of two categories: optical methods and electrochemical methods. Optical methods generally involve absorbance, reflectance or laser spectroscopy to observe the spectrum shift in the fluid caused by the concentration of the analytes, typically in conjunction with a reagent that produces a known color when combined with the analyte. Electrochemical methods generally rely upon the correlation between a charge-transfer or charge-movement property of the blood sample (e.g., current, interfacial potential, impedance, conductance, and the like) and the concentration of the analyte, typically in conjunction with a reagent that produces or modifies charge-carriers when combined with the analyte. See, for example, U.S. Pat. Nos. 4,919,770 to Preidel, et al., and 6,054,039 to Shieh, which are incorporated by reference herein in their entireties.  
         [0005]     An important limitation of electrochemical methods of measuring the concentration of a chemical in blood is the effect of confounding variables on the impedance of a blood sample. For example, the geometry of the blood sample must correspond closely to that upon which the impedance-to-concentration mapping function is based.  
         [0006]     The geometry of the blood sample is typically controlled by a sample-receiving chamber of the testing apparatus in which the fluid sample is received and held during its analysis. In the case of blood glucose meters, for example, the blood sample is typically placed onto a disposable test strip or biosensor that plugs into the meter. The test strip may have a sample chamber to define the geometry of the sample. Alternatively, the effects of sample geometry may be limited by assuring an effectively infinite sample size. For example, the electrodes used for measuring the analyte may be spaced closely enough so that a drop of blood on the test strip extends substantially beyond the electrodes in all directions. Regardless of the strategy used to control sample geometry, typically one or more dose sufficiency electrodes are used to assure that a sufficient amount of sample has been introduced into the sample receiving chamber to assure an accurate test result.  
         [0007]     Other examples of limitations to the accuracy of blood glucose measurements include variations in blood chemistry (other than the analyte of interest being measured). For example, variations in hematocrit (concentration of red blood cells) or in the concentration of other chemicals, constituents or formed elements in the blood, may affect the measurement. Variation in the temperature of blood samples is yet another example of a confounding variable in measuring blood chemistry. In addition, certain other chemicals can influence the transfer of charge carriers through a blood sample, including, for example, uric acid, bilirubin, and oxygen, thereby causing error in the measurement of glucose.  
         [0008]     Efforts to improve test strips have been mainly directed to making them smaller, faster, and require less sample volume. For example, it is desirable for electrochemical biosensors to be able to analyze as small a sample as possible, and it is therefore necessary to minimize the size of their parts, including the electrodes. Traditionally, screen-printing, laser scribing, and photolithography techniques have been used to form miniaturized electrodes. These methods are undesirably time-consuming, however, and screen-printing or laser scribing technologies pose limitations on the edge quality of the electrical patterns formed, such that gap widths between electrical elements normally must be 75 microns or more. Further, some of these techniques make it unworkable on a commercial scale to remove more than a small fraction, e.g., more than 5-10% of the conductive material from a substrate to form an electrical pattern.  
         [0009]     The electrode structures in available electrochemical test strips made by these techniques typically have one or perhaps two pairs of electrodes, and the measurements obtained by these electrode structures are quite sensitive to the interferents discussed above. Thus, the signal produced by the analyte desired to be analyzed must be deconvoluted from the noise produced by the interfering substances. Many approaches have been employed to attenuate/mitigate interference or to otherwise compensate or correct a measured value. Often, multiple design solutions are employed to adequately compensate for the sensitivities associated with the chosen measurement method.  
         [0010]     One approach involves removing interfering materials such as blood cells from the fluid sample before it reaches the electrodes by using perm-selective and/or size-selective membranes, filters or coatings. Multiple layers of membranes are often laminated together to achieve the ultimate goal of delivering a fluid to the electrodes which contains only low levels of interferents. Unfortunately, however, this approach suffers from incremental costs of goods, viz., coatings and membranes that must often be pre-treated prior to assembly. It also incurs additional manufacturing process steps that further increase manufacturing cost and complexity while decreasing the speed of manufacture. This approach addresses the attenuation problem by increasing the complexity and cost of the test strip, thereby reducing the burden of the meter which reads the strips.  
         [0011]     Another general approach involves the use of sophisticated excitation and signal processing methods coupled with co-optimized algorithms. While simpler, less complex test strip architectures and manufacturing processes may be realized, instrumentation costs, memory and processor requirements, associated complex coding, and calibrated manufacturing techniques are all increased by this approach. Systems employing this approach address the attenuation problem by placing a higher computational burden on the meter that reads the strips.  
         [0012]     Yet another more recent approach involves neither the strip nor instrumentation, per se, but rather exploits the measurement methodology. An example of this approach is the use of a coulometric method to attenuate the influence of hematocrit and temperature. This coulometric approach, however, requires a tight manufacturing tolerance on the volume of the sample receiving chamber in the test strips produced, since the entire sample is used during the analysis. Additionally, commercially available test strips using this technology require two separate substrates printed with electrodes, which further increases manufacturing costs. The requirement that much of the sample volume be interrogated may also limit test speed. Further, this approach requires relatively large electrodes to provide significant electrolysis of the sample in a relatively short time in order to estimate the “endpoint” of the coulometric detection.  
         [0013]     It is also well known to those skilled in the art that all of the above approaches are further supported by the initial design of reagent systems. In the detection of glucose, for example, this may involve the use of selective redox mediators and enzymes to overcome the detrimental influence of redox-active species or the presence of other sugars.  
         [0014]     It would be desirable to provide a simpler, less costly method for attenuating the influence of interferents, in a manner that does not suffer the demerits associated with the general approaches currently in wide use. It would also be desirable to provide a more functional, robust and user-friendly system for analyzing fluid samples, but without increasing the costs.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention provides a biosensor having multiple electrical functionalities located both within and outside of the measurement zone in which the fluid sample is interrogated. Incredibly small and complex electrical patterns with high quality edges provide electrical functionalities in the biosensor and also provide the electrical wiring for the various other electrical devices provided in the inventive biosensor. In addition to a measurement zone with various electrode functionalities, biosensors of the present invention may be provided with a user interface zone, a digital device zone and/or a power generation zone.  
         [0016]     The inventors of the present invention have taken an entirely different approach than the schemes discussed above for mitigating interference or otherwise correcting a value measured by a test strip. Their novel approach focuses upon (1) enhancing the quality and complexity of the electrical patterns formed on a biosensor, (2) significantly reducing the size of these electrical patterns, and at the same time (3) increasing production speeds while ( 4 ) reducing manufacturing costs. This approach decreases the computational burden and associated cost of the instruments that read the strips while at the same time adding accurate yet cost-effective functionalities to the biosensors themselves.  
         [0017]     In one form thereof, the present invention provides a biosensor for analyzing a fluid sample. The biosensor includes a biosensor body that defines a measurement zone having a sample receiving chamber in which is disposed a measurement electrode for detecting the presence or concentration of an analyte in the fluid sample. The measurement zone also includes a reagent that reacts with the fluid sample. The biosensor body further defines a user interface zone in which is disposed an electrically driven signal generator which emits a visible, audible or tactile signal upon occurrence of a triggering event.  
         [0018]     In one preferred form, the signal generator comprises a light positioned on the test strip body which illuminates (or turns off) upon the occurrence of the triggering event. In another preferred form the signal generator comprises a light disposed proximate the sample receiving chamber and which illuminates the sample receiving chamber upon the occurrence of the triggering event. In another preferred form, the signal generator is a numerical display.  
         [0019]     Any number of occurrences can constitute a “triggering event,” including but not limited to insertion of the strip into a meter, a sufficient size dose being received in the sample receiving chamber, malfunction of test, non-functional test strip, etc. Furthermore, there may be a delay between the occurrence of the triggering event and the signal generator emitting the signal.  
         [0020]     In another preferred form, the signal generator comprises an electrode set on which the OLED is coated. More preferably, the electrode set comprises a micro-electrode array with at least two electrode fingers having a gap of less than about 5 microns between them.  
         [0021]     In another preferred form, the biosensor also includes a power generation zone in which is disposed a power generator. More preferably, the biosensor additionally includes a digital information zone in which is disposed at least one digital device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The above-mentioned and other advantages of the present invention, and the manner of obtaining them, will become more apparent and the invention itself will be better understood by reference to the following description of the embodiments of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0023]      FIG. 1  is a perspective view of a biosensor or test strip in accordance with one embodiment of the present invention;  
         [0024]      FIG. 2  is an exploded perspective view of the biosensor of  FIG. 1 ;  
         [0025]      FIG. 3  is an exploded perspective view of a biosensor in accordance with a second embodiment of the present invention;  
         [0026]      FIG. 4  is an exploded perspective view of a biosensor in accordance with a third embodiment of the present invention;  
         [0027]      FIG. 5  is a plan view of a base substrate of a biosensor in accordance with a fourth embodiment of the present invention;  
         [0028]      FIG. 6  is a plan view of a base substrate of a biosensor in accordance with a fifth embodiment of the present invention; and  
         [0029]      FIG. 7  is a plan view of a base substrate of a biosensor in accordance with a sixth embodiment of the present invention. 
     
    
       [0030]     Corresponding reference characters indicate corresponding parts throughout the several views.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the specific embodiments illustrated herein and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described processes or devices, and any further applications of the principles of the invention as described herein, are contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [heading-0032]     Introduction  
         [0033]     Generally, the test strips embodied by the present invention provide for testing of an analyte in a bodily or other fluid using multiple electrode functionalities that are provided on board the test strips. In the sample receiving chamber, multiple electrode sets can be formed which perform the same or different functions. The novel electrical features of the embodiments disclosed herein extend beyond the concept of “measurement functionalities,” however. Indeed, it is helpful to view test strips embodying the present invention as having individual “zones,” each zone including electrical devices having a specific functionality. For example, in addition to a measurement zone in which fluid sample is received and analyzed, test strips disclosed herein may provide user interface, digital, and power generation zones that have been hitherto unavailable in test strip architecture.  
         [heading-0034]     General Description  
         [0035]     Zones.  
         [0036]     Turning now to  FIG. 1 , strip  200  defines a test strip body that generally has several zones, including a measurement zone  202 , a user interface zone  204 , a power generation zone  206 , a digital device zone  208  and an instrument connection zone  210 . As indicated in  FIG. 1  and as will become clear with the discussion below, the zones are not limited to specific locations on a given test strip  200 . Instead, the locations of the various zones will normally overlap to varying degrees as shown or may be discontinuous, occupying two or more different regions of the test strip body. Each zone generally has included therein electrical devices that perform a specific type or class of function.  
         [0037]     For example, the electrical devices included in the measurement zone typically have functionalities related to the measurement (or correction of measurement) of the fluid sample being interrogated. Examples of these electrical devices include macro and micro-electrode sets, dose detection electrodes, sample sufficiency electrodes, temperature correction or temperature measurement electrodes, thermistors and the like. While the measurement zone is illustrated at a dosing end  212  of the strip, it should be understood that the measurement zone may alternatively occupy other locations on the strip, e.g., a side of the strip, as is known in the art.  
         [0038]     The electrical devices in the user interface zone typically are electrically driven signal generators which emit a visible, audible or tactile signal upon occurrence of a “triggering event.” As described in more detail below, the signal generator may be a light that illuminates or turns off after a sufficiently sized sample has been received in the measurement zone, the latter event being the “triggering event.” The user interface zone is in some embodiments electrically wired to the measurement zone and/or other zones of the test strip.  
         [0039]     The power generation zone includes one or more power generators that provide power to one or more other electrical devices disposed on or in the test strip. Typically, the power generator comprises a battery, but it could also comprise a capacitor or even a solar cell, depending upon the power requirements of the electrical device the power generator is going to drive and the specific functionality of that device.  
         [0040]     Digital devices such as RFID tags, integrated circuits and the like are disposed within the digital zone and may be wired to the electrical pattern. In other embodiments, the electrical pattern that is disposed in the digital zone is itself encoded with digital information and thus comprises yet another type of digital device.  
         [0041]     Finally, the instrument connection zone includes electrical devices, typically contact pads, that electrically link to an instrument (not shown) which includes driving circuitry and metering circuitry. The driving circuitry provides a known current and/or potential through contacts  216  and monitors the current and/or voltage response over a period of time. The metering circuitry correlates the monitored current, impedance and voltage response to estimated analyte concentration or other aspect of the analyte. While the instrument connection zone is preferably disposed on a meter insertion end  214  of the strip, this need not necessarily be the case. The instrument zone could be located on a side of the strip or could be located on the end as shown, but could also include contact pads that are disposed at various locations on the top, bottom or sides of the test strip.  
         [heading-0042]     Strip Architecture and Components.  
         [0043]     With further reference to  FIGS. 1 and 2 , strip  200  is generally of a laminar structure and includes three primary layers. The base substrate layer  220  is generally a flexible polymeric material such as polyester, especially high temperature polyester materials; polyethylene naphthalate (PEN); and polyimide, or mixtures of two or more of these. A particularly preferred base substrate material is a 10 mil thick MELINEX®  329  layer available from duPont. Substrate  220  is initially coated with a conductive material such as a 50 nm layer of gold, and the complex electrical pattern  222  can be then formed therefrom by broad field laser ablation. The broad field laser ablation method is described in the METHOD OF MAKING A BIOSENSOR application incorporated above. Materials for the specific biosensor layers and the method of assembling those materials is described in the Slot Vent Opening application, also incorporated above.  
         [0044]     The electrical pattern  222  includes contacts or contact pads  216 , which, as described above, can be electrically linked to an instrument that reads strip  200 . Traces  223  run lengthwise along strip  200  and are typically used to connect electrical devices to the contact pads  216  or to connect two or more electrical devices on or in strip  200  together. For example, substrate  220  includes a measuring electrode set  228  coated by a reagent  229  and a sample sufficiency electrode set  230 , the operation of which are described in detail in the Dose Sufficiency, Slot Vent Opening, and DEVICES AND METHODS RELATING TO ELECTROCHEMICAL BIOSENSORS applications, all of which were incorporated by reference above. These electrode sets are connected to their respective contact pads by traces  230  and  232  and in turn through traces  223  as shown.  
         [0045]     User interface devices comprising L-shaped micro-electrode arrays  224  are formed on base substrate  220  and are coated with organic light emitting diodes (“OLEDs”)  226 , which illuminate upon a voltage being provided across arrays  224 . The voltage is applied or removed upon or after the occurrence of a triggering event, as described in more detail below. Similarly, micro-electrode set  234  formed on substrate  220  is coated with a second OLED  236  that illuminates or turns off upon the occurrence of the same or a different triggering event, as is also described in more detail below.  
         [0046]     A power generator  238  is provided on strip  200  and can be used to power various other electrical devices present on the strip, as explained below. Many suitable power generators are commercially available and can be employed as power generator  238 , but power generator  238  should preferably be formed as a small and especially thin material so as not to significantly increase the thickness of test strip  200 .  
         [0047]     Test strip  200  includes digital device  246 , which is shown in  FIG. 2  wired to power generator  238  by traces  248 . Digital device  246  may be an integrated circuit, an RFID tag or other digital device, as described in more detail below. Further, a portion of the electrical pattern may comprise a digital device  250 , as explained in more detail below.  
         [0048]     Laminated to base substrate  220  is a spacer layer  256 , formed, e.g., from a 4 or 5 mil thick Melinex®  329 ,  339  or  453  material available from DuPont Teijin Films. In certain embodiments, particularly those including light emitters such as OLEDs  226  and  236 , it is preferable that the spacer layer material be clear or translucent so that the OLEDs are visible when lit. The Melinex® 453 material works well for this purpose. Spacer layer  256  forms a void  258  that defines the height and perimeter of the sample receiving chamber  218  ( FIG. 1 ). The precise volume of the sample receiving chamber is defined in the Slot Vent Opening application, which was incorporated above. Spacer layer  256  also includes “cut-outs”  260  and  261  that are sized to receive digital device  246  and power generator  238 , respectively. These devices will typically be thicker than the spacer layer, such that they may protrude slightly from the top of strip  200  as shown in  FIG. 1 .  
         [0049]     A covering layer  262  overlies and is laminated to spacer layer  256 . Covering layer  262  is also preferably made from a transparent Melinex® film that is about 4-5 mils thick. Covering layer  262  overlies most of void  258  and forms the ceiling or top boundary for sample receiving camber  218 . The cover terminates short of the full length of void  258  and thereby forms a vent opening  264  as shown. Vent  264  allows air to be displaced from chamber  218  as fluid sample enters it. As can be appreciated with respect to  FIG. 1 , OLED coatings  226  and  236  are visible when lit through the covering and spacer layers.  
         [0050]     Optionally, to reduce the extent to which devices  238  and  246  protrude from strip  200 , cover layer  262  may extend further toward meter insertion end  214 , such that it is coextensive with layer  256 . The cover  262  would then be formed with a hole overlying the void  258  to form the vent. Alternatively, the cover could be formed in two pieces forming a gap therebetween, as described in the Slot Vent Opening application, incorporated by reference above. This longer spacer layer may also include cut-outs that align with cutouts  260  and  261  and reduce the extent to which devices  238  and  246  protrude from strip  200 . Typically, however, it is preferable for electrical devices in the user interface or power generation zones to be sufficiently thin such that they can be covered by covering layer  262  for protection from electromagnetic interference.  
         [heading-0051]     Electrical Pattern.  
         [0052]     The electrical patterns for use with embodiments incorporating the present invention are typically formed by broad field laser ablation, which is described in detail in the METHOD OF MAKING A BIOSENSOR application that was incorporated by reference above. This method allows several electrical functionalities to be located within and outside of measurement zone  202 —with room to spare on an already very small test strip. For example, arrow  240  in  FIG. 2  represents the approximate width of strip  200 , which is about 9 mm in the illustrated embodiment. The strip illustrated in  FIGS. 1 and 2  is preferably about 33-38 mm in length. Arrows  242  illustrate the distance from the edge of the strip to the innermost trace  223 , and this width can be configured to be about 1 mm or even as small as about 0.2 mm. Remarkably, this means that width  244 , which is the width available for components such as power generator  238  and digital device  246 , can be about 8 mm or more for a 9 mm wide strip having ten electrical traces running lengthwise along it. One of ordinary skill should readily appreciate that the electrical patterns embodied by the present invention, while complex, can nonetheless be advantageously configured into a relatively small space, such that ample room remains for other devices having relatively large footprints to be placed on the strip.  
         [heading-0053]     Measurement Zone.  
         [0054]     Generally, the measurement zone incorporating the present invention can vary widely insofar as the type and quantity of functionalities provided therein. Turning to  FIGS. 1 and 2 , the measurement zone  202  includes a sample receiving chamber  218  whose periphery is approximately indicated in  FIG. 2  by dashed line  266 . (As indicated above, the precise volume of the sample receiving chambers of various embodiments disclosed in this application can be determined with reference to the Slot Vent Opening application, incorporated by reference above.) Macro-electrode array  228  includes a working electrode and a counter electrode, each having one or more interdigitated fingers as shown. Electrode set  228  estimates the concentration of analyte based upon the reaction of the analyte with the reagent  229  coated on the electrode set. Once a sufficient sample has entered chamber  218 , a suitable potential or series of potentials across the working and counter electrodes are applied, and the impedance or other characteristic is measured and correlated to the concentration of analyte. Measuring electrodes of this type and reagent suitable for reagent layer  229  are described in the Slot Vent Opening and DEVICES AND METHODS RELATING  
         [heading-0055]     TO ELECTROCHEMICAL BIOSENSORS applications incorporated above, and need not be described in further detail herein.  
         [0056]     As mentioned, the voltage or potential is preferably not applied across electrode set  228  until the sample chamber has filled with the requisite volume of sample. In this connection, sample sufficiency electrode set  230  is provided at a downstream location in chamber  218 . When fluid has wetted electrode set  230 , its resistance or impedance (which can be intermittently monitored by applying a voltage to the contact pads  216  connected to electrode set  230 ) will drop, thereby indicating sample has reached the interior end of the chamber and sufficient sample has thus been received. A potential or series of potentials can thereafter be driven across electrode set  228  to perform the measurement. Sample sufficiency electrodes suitable for use with the present invention are disclosed in the Dose Sufficiency application that was incorporated by reference above. Additionally, once the sample sufficiency electrodes indicate that sufficient sample has been received, they can be used for other measurements, as also disclosed in the Dose Sufficiency application. It should also be understood that a single sample sufficiency electrode could be used and a voltage applied across it and one of the measurement electrodes for testing.  
         [0057]     Turning now to  FIG. 3 , a test strip  300  is shown with a sample receiving chamber having multiple, redundant functionalities. Strip  300  includes base substrate  302 , four sets of micro-electrodes  304 ,  306 ,  308  and  310 , and a set of sample sufficiency electrodes  312  formed thereon. A reagent layer whose edges are indicated by dashed lines  314  and  316  is coated onto the micro-electrode sets. Strip  300  also includes a spacing layer  318  having a void section  320 , which, in cooperation with covering layer  322  and base substrate  302 , partially defines the boundaries of the sample receiving chamber. The position of the sample receiving chamber is generally indicated by dashed line  324  on substrate  302 , although the void portion beneath the vent is not part of the sample receiving chamber. The micro-electrode sets and sample sufficiency electrodes are electrically connected to contact pads  326  through traces  328 . The architecture just described is essentially the same as that described with reference to  FIG. 1-2 , the difference being the electrical devices contained in the sample receiving chamber. Advantageously, a large central portion  330  of the base substrate  302  is not occupied by the electrical pattern and would be available to add additional user interface, power, or digital devices, as described elsewhere herein.  
         [0058]     In the embodiment shown in  FIG. 3 , identical microelectrodes are provided to make identical measurements. Sample fluid enters the sample receiving chamber  324  and is drawn in by capillary action past each of the micro-electrode arrays until it wets sample sufficiency electrode set  312 , whereupon potentials are applied across each of the microelectrode arrays  304 ,  306 ,  308  and  310 . The circuitry in the instrument (not shown) that reads the strips drives a potential across each electrode set through contacts  326  and traces  328 . Alternatively, electrodes sets  304 ,  306 ,  308  and  310  could be wired in parallel (not shown), in which case a single pair of contact pads would connect all four electrode sets to the meter. In this case, the parallel configuration of the four sets would provide an “on strip” average for the value being measured by the four electrode sets.  
         [0059]     Even though it contains five electrode sets, sample receiving chamber  324  nonetheless has a very small volume, on the order of less than about 500 nl.  
         [0060]     Turning now to  FIG. 4 , a test strip  400  is shown having a measurement zone with multiple, different functionalities. Strip  400  includes base substrate  402  with four sets of electrodes  404 ,  406 ,  408  and  410 , and a set of fault detect electrode traces  412  and  413  formed thereon. A reagent stripe  414  is coated onto electrode set  404  and micro-electrode set  406  in this embodiment. Strip  400  also includes a spacing layer  418  having a void section  420 , which, in cooperation with covering layer  422  and base substrate  402 , defines the boundaries of the sample receiving chamber. The position of the sample receiving chamber is indicated generally by dashed line  424  on substrate  402 . The electrode sets and sample sufficiency electrodes are electrically connected to contact pads  426  through traces  428 . The architecture just described is essentially the same as that described with reference to  FIG. 2 , the difference being the electrical devices contained in the measurement zone. Again, a large central portion  430  of the base substrate  402  is not occupied by the electrical pattern and would be available to add additional user interface, power, or digital devices, as described elsewhere herein.  
         [0061]     In the embodiment shown in  FIG. 4 , The first electrode pair  404  encountered by the sample includes working electrode  432 , a single-finger electrode. First electrode pair  404  also includes counter electrode pair  434 , a two-finger electrode, with one finger on either side of working electrode  432 . Each finger in first electrode pair  434  is about 250μm wide, and a gap of about 250 μm separates each counter electrode finger from the working electrode finger. The system driver connects to contacts  426  to use the first electrode pair  404  to obtain an estimated concentration of analyte in the sample.  
         [0062]     The second electrode pair  406  comprises two electrodes of five fingers each. These fingers are each about 50 μm wide with a separation of about 30 μm between them. Each electrode in the second pair connects to a conductive trace  428  to be electrically connected to a contact  426 , which contacts are used to drive and measure for a first correction factor such as hematocrit based on the analyte interaction with the second pair of electrodes.  
         [0063]     The third electrode pair  408  is also a micro-electrode configuration, with each of the two electrodes in the third pair  408  having five fingers interdigitated with the five in the other electrode. Each finger is again about 50 μm wide, with a gap of about 30 μm between them. Each electrode in the third pair  408  is connected via a conductive trace  428  to a contact  426 , which contacts are used to drive and measure for a second correction factor such as temperature based on the analyte interaction with the second pair of electrodes.  
         [0064]     The fourth set of electrodes comprises sample sufficiency electrodes  410  that signal when the sample has filled the chamber such that electrode sets  404 ,  406  and  408  can then be driven to perform their respective measurement functions.  
         [0065]     The fifth functionality in the measurement zone of strip  400  relates to fault detect traces  412  and  413  for electrode set  404 . Trace  413  connects to counter electrode  434  and is used to correct variant voltage across the pair, whereas fault detect trace  412  on working electrode  432  compensates for measured current. Additionally, traces  412  and  413  can be used to apply a potential between the primary traces and the fault detect traces to determine whether there are any defects in the primary traces. This fault detection feature is fully described in the Quality Assurance application that was incorporated by reference above.  
         [0066]     Even with five electrical devices or functionalities provided in the measurement zone, the sample receiving chamber  424  nonetheless has a very small volume, on the order of less than about 500 nl.  
         [0067]     Turning now to  FIG. 5 , a base substrate  502  for a test strip of the type described above is shown. Substrate  502  includes an electrical pattern  504  formed thereon having contact pads  506  and traces  508  leading to the electrode sets disposed in the measurement zone  510 . Measurement zone  510  includes a sample receiving chamber  512  having three branches or prongs  514 ,  516  and  518 . Branch  514  includes electrode sets  520  and  522 , branch  516  includes electrode sets  524  and  526 , and branch  518  includes electrode sets  528  and  530 . A reagent layer  532  covers electrode sets  520  and  522 , a reagent layer  534  covers electrode sets  524  and  526 , and a reagent layer  536  covers electrode set  528  and  530 . A spacing layer (not shown in  FIG. 5 ) as described above is formed with voids corresponding to and defining the branched sample receiving chamber, and a covering layer overlies the spacing layer. Vent holes are formed in the covering layer to allow air to escape each of the branches of the sample receiving chamber.  
         [0068]     One advantage of the system shown in  FIG. 5  is that it allows multiple analytes to be tested in a single test strip. For example, reagent layers  532 ,  534  and  536  can be comprised of three different reagents for testing three different analytes, e.g., a lipid panel that tests total cholesterol, HDL cholesterol and triglycerides. Reagents with appropriate enzymes and mediators for these analytes are disclosed in the Reagent Stripes application that was incorporated by reference above. Alternatively, all three reagents can be identical, in which case three of the same tests can be performed in parallel, such that each branch of the sample receiving chamber effectively receives its own fresh supply of fluid sample. By contrast, a series of electrode sets in a single-branched chamber poses the potential of contamination to the downstream electrode sets.  
         [0069]     As with the embodiments illustrated above, it should be appreciated that a large portion  538  is available in the middle of substrate  502  and could be configured to support additional electrical devices.  
         [heading-0070]     Power Generation  
         [0071]     Returning now to  FIGS. 1 and 2 , a power generator  238  is positioned centrally on strip  200 . The power generator  238  may comprise a battery such as a commercially available custom made Power Paper brand energy cell, available from Power Paper, Ltd., Kibbutz, Israel. These cells are preferably printed on a very thin substrate such as paper or thin polymer. By means of basic screen-printing techniques, different layers of conductive inks are printed to form the various components of cell  238 , which are then laminated together and in turn laminated to substrate  220 . In the embodiment illustrated in  FIG. 2 , battery  238  has a diameter of about 5.3 mm and a thickness of less than about 0.5 mm. Battery  238  is mounted to substrate  220  by ordinary adhesives or other suitable means and connects to leads  248  as show, preferably by conductive epoxy. Battery  238  produces 2.7-3.1 Volts, a current of 4-5 mA and has an “on time” of between 5-90 seconds. These parameters are sufficient for powering one of the inventive OLED circuits described below, a traditional LED, or a small piezoelectric device which produces sound, or any number of similar devices. In view of the teachings herein, which minimize the footprint of even complex electrode patterns, two or more such batteries  238  could be positioned on strip  200  and wired together to increase power production.  
         [0072]     Other power generators  238  could be substituted for the battery just described. For example, if only a short burst of energy is needed, for example to light a diode or produce a short audible sound, a super capacitor or ultra-cap modified to have a very slim profile could be used as power generator  238 . In use, for example, in one embodiment, strip  200  would be inserted into the instrument (not shown) for strip identification, strip integrity checks, temperature determination, and charging the capacitor or other power storage element. The self-powered strip is then removed from the instrument, placed at the dose site, and returned to the instrument for measurement computation and display.  
         [0073]     In view of the teachings herein, one of skill in the art would readily recognize other power generators that could be employed as power generator  238 . It is preferable, however, that the power generator be as thin as possible so as not to significantly increase the thickness of the test strip.  
         [heading-0074]     Digital Devices.  
         [0075]     Still referring to  FIGS. 1 and 2 , a digital device  246  is positioned adjacent power generator  238  and is wired thereto by traces  248 . Device  246  could be a radio frequency identification (“RFID”) tag. RFID  246  is preferably less than about 1 mm thick, more preferably less than 0.5 mm thick, and has a width of less than about 7 mm. In one embodiment, device  246  contains digital calibration data concerning the test strip and can communicate such data to an RFID reader (not shown) that is included in the instrument (not shown). Most commercially available RFID&#39;s are typically “passive,” i.e., they are powered by the radio signal emanating from the reader that reads them. Thus, if device  246  is an RFID, it need not be wired to a power generator such as power generator  238 . RFID technology is known in the art and the details thereof need not be described any further herein.  
         [0076]     As noted above, digital device  246  could be provided as an on-board integrated circuit with computing power, powered by battery  238  and connected thereto by traces  248 . Two commercially available examples include Texas Instruments MSP430C11 and MicroChip PIC 12F675 integrated low power micro-controllers for governing sample acquisition and rudimentary measurements to support dosing the strip without the strip being inserted in the meter. As yet another option, device  246  could be provided in the form of a conventional wired storage device such as a MicroChip 24AA01  1 K bit serial EEPROM, in which event it would include data such as lot code, calibration data and the like.  
         [0077]     As shown in  FIG. 2 , strip  200  also includes a digital device  250  which is comprised of a combination of contact pads  252  and conductive links  254  of electrical pattern  222 . Contact pads  252  and conductive links  254  are shown in phantom because any one (or all) of them may or may not be present in the finished test strip, depending upon the information that is to be encoded onto the test strip. Each link or contact pad can be thought of as a binary switch having a value of 0 (if not present) or 1 (if present). Any given configuration of absent/present links and contact pads may include digital information concerning lot code, expiration date, type of analyte the strip is intended to analyze and so forth. A detailed enabling description of digital device  250  is disclosed in the Coding Information application that was incorporated by reference above.  
         [0078]     Optionally, a photodiode sensor could be mounted on the test strip in the digital device zone or elsewhere to detect an environmental condition such as ambient light. The meter could then apply a voltage to the micro-electrode arrays such as micro-electrode arrays  224  so that they illuminate the measurement zone. One of skill in the art should thus appreciate that the term “digital device” for purposes of this application is somewhat broader than its common usage in the art, in that it includes devices such as a photodiode or similar devices that may be provided in the digital zone.  
         [heading-0079]     User Interface Devices.  
         [0080]     As briefly described earlier, the test strip  200  shown in  FIGS. 1 and 2  includes a user interface zone  204  that includes OLEDs coated onto micro-electrode arrays. Specifically, with reference to  FIG. 2 , OLEDs  226  are coated onto micro-electrodes  224  and OLED  236  is coated onto micro-electrode array  234 .  
         [0081]     Electrode arrays  224  are wired through traces  223  to contact pads  216 . Thus, a “triggering event” occurs when strip  200  is inserted into a meter (not shown), upon which event the circuitry of the meter recognizes that a strip has been inserted and produces a voltage across electrode sets  224 . In turn, the coatings  226  illuminate. If the strip  200  is being used in conditions of dim lighting, the OLED coating advantageously illuminates the sample receiving chamber  218  so that the user can visually confirm that the fluid sample is contacting the correct part of the strip  200  and that the sample fluid is being drawn into the strip. As noted above, the spacer and covering layers forming test strip  200  are preferably transparent or translucent such that the light emitted from the OLEDs is visible through them.  
         [0082]     OLED  236  can be configured to illuminate (or turn off) upon sufficient sample being received in the sample receiving chamber. Sample sufficiency electrodes  230  are wired through traces  223  to contact pads  216  and in turn to the meter (not shown) that reads the strips. Once the meter detects from electrodes  230  that the chamber is filled with the requisite size sample, the meter can apply a voltage across electrode set  234  through the appropriate contact pads  216  and traces  223 . OLED  236  will then illuminate, thereby providing the user a positive visual indication that the chamber has been properly filled.  
         [0083]      FIG. 6  shows a base substrate  600  of another test strip embodiment incorporated by the present invention. The test strip has a measurement zone  602 , two user interface zones  604  and  604 ′, a power generation zone  606 , and a meter connection zone  610 . This embodiment illustrates the point alluded to above, viz., that the locations of various “zones” of a particular test strip embodying the principles of the present invention may overlap, or in the case of the embodiment illustrated in  FIG. 6 , may be discontinuous or bifurcated.  
         [0084]     The sample receiving chamber  612  includes three different electrical devices or functionalities: a measurement electrode set  614 , a thermistor  616  and a sample sufficiency electrode set  618 . Electrode set  614  is connected to traces  620 , which terminate in contact pads  622  disposed at meter connection zone  610  of the strip. The sample sufficiency electrode set  618  is part of a circuit which includes a micro-electrode array  624  having an OLED  626  coated thereon and a battery  628 . Electrical devices  618 ,  624  and  628  are wired in series by traces  630 ,  632  and  634 . Traces  630  and  634  terminate in the power generation zone  606  with contact pads  636  (shown in phantom) to which the battery  628  is connected. The second or bifurcated user interface zone  604 ′ includes a traditional diode  638  wired by traces  620  to contact pads  622 .  
         [0085]     In use, the strip is dosed with a sample that is drawn into chamber  612  by capillary action. In the embodiments described above, the sample sufficiency electrodes were adapted to be driven by circuitry from a meter to which the strip is inserted. The embodiment in  FIG. 6 , however, employs a different approach. In this embodiment, sample sufficiency electrode set  618  acts as a switch in the circuit containing electrodes  618 , electrode array  624  and battery  628 . Battery  628  is a Power Paper type battery as described above that produces 2.7-3.1 Volts and a current of 4-5 mA for about 5-90 seconds. Once the aqueous fluid sample saturates sample sufficiency electrodes  618 , the circuit closes. If blood is the sample fluid, the ionic strength thereof should be sufficient to close the circuit. However, one skilled in the art would readily recognize numerous coatings that could be applied and dried onto electrode set  618  to ensure sufficient current transfer upon wetting with other fluid samples. In any event, closing the circuit is a triggering event which results in a voltage being produced across micro-electrode array  624 , which in turn causes OLED layer  626  to illuminate. In this manner, the illumination of OLED  626  provides a positive visual verification to the user that the sample chamber has been filled. Electrical device  616  is a thermistor that is used to measure the temperature of the sample receiving chamber. One thermistor suitable for device  616  is surface mount thermistor available from Vishay Intertechnology, Inc., Layern, Pa., part no. NTHS-0402N01N100KJ. Thermistor  618  is driven by electrical circuitry from a meter (not shown) through contacts  622  and traces  620 . If the temperature of the sample receiving chamber is not within a desired range for testing, the meter circuitry can apply a voltage to conventional LED  638  through contacts  622  and traces  620  to cause it to illuminate. This signals the user that the temperature of the sample is outside of a preferred range, in which event the user may then possibly repeat the test under better conditions. An LED that is suitable for mounting on substrate  600  is available from Stanley Electrical Sales of America, Inc., part no. PY1114CK. This LED is mounted to base substrate  600  preferably by a conductive epoxy. Optionally, instead of an LED, user interface zone  604 ′ may include a signal producing device that produces sound, such as a piezoelectric available from U.S. Electronics, Inc., St. Louis Mo., part number USE14240ST.  
         [0086]     Turning now to  FIG. 7 , a test strip with yet another innovative electrically driven signal generator is illustrated. Base substrate  700  of the test strip includes a measurement zone that includes a sample fluid receiving chamber  702  having disposed at least partially therein a measurement electrode set  704  and sample sufficiency electrode set  706 , whose functionality and operation are described above. Suitable spacing and covering layers (not illustrated in  FIG. 7 ) cover substrate  700  to form a test strip, as described above and in the Slot Vent Opening application incorporated by reference above. Substrate  700  includes a numerical display  712  comprised of individual segments  714  that have a shape not unlike that of the segments used for traditional LED or LCD displays. The layer or layers of the test strip (not shown) that cover display  712  are translucent or transparent such that display  712  is visible therethrough. Segments  714  include an OLED coating like that described above overlying a micro-electrode IDA, as also described above (but not shown in  FIG. 7 ). Each segment  714  has two electrodes (not shown) having two traces  708  extending therefrom and leading to respective contact pads  710 . Voltages can be applied across selective ones of the contact pads  710  to illuminate display  712  to produce any of the digits 0 to 9, a “5” being shown illuminated in  FIG. 7 .  
         [0087]     Optionally, additional digits and associated contact pads and traces can be provided with display  712  on substrate  700 . The design of the test strip with this numerical display should balance (1) the desire to keep the strips small, (2) the need to make the display large enough to be read by even those users with impaired vision, and (3) the space required from substrate  700  to accommodate the traces, contact pads, and digits. A test strip having a base substrate  700  as shown in  FIG. 7  with one digit has a length of about 33-38 mm, a width of less than about 15 mm, preferably about 9 mm, and a thickness of less than about 1 mm. The other layers that are laminated to substrate  700  can be configured and assembled in accordance with the Slot Vent Opening application, incorporated by reference above. It should be appreciated that the micro-electrode arrays and OLEDs coating them (to form segments  714  of display  712 ) do not increase the thickness of the strip.  
         [0088]     In use, the test strip having substrate  700  is inserted into a meter (not shown), a fluid sample is provided to sample receiving chamber  702 , and the meter calculates the numerical estimate of analyte concentration. Thereupon, the circuitry in the meter drives voltages across selective ones of the contact pads  710  to illuminate a number on display  712  that corresponds to the estimate of analyte concentration. If only one digit were provided in display  712  as shown in  FIG. 7 , and the analyte whose concentration is being estimated were glucose from a blood sample, the single digits could be assigned a range. For example, a “0” might correspond to a 50-100 mg/dl concentration of glucose, a “1” to 100-150 mg/dl, a 2 to 150-200 mg/dl and so on. If two digits were provided in display  712 , then the display could simply show the first two digits of the result. In such case a “10” displayed would mean 100-109 mg/dl, a “21” would mean 210-219 mg/dl, etc.  
         [0089]     Alternatively, the analyte concentration might be displayed by sequentially displaying digits. For example, “126” mg/dL might be displayed as a “1” followed by a “2”, followed by a “6”, and the sequence terminated with a unique symbol to indicate completion and avoid user confusion. In this manner, a three-digit whole number can be conveyed to the user with a single digit display.  
         [0090]     With three digits, a whole number for mg/dl concentration can be displayed all at once, as is typically done with traditional glucose meters.  
         [0091]     While  FIG. 7  embodies an electrochemical test strip, it should be understood that the innovative on-board display could be provided on test strips which employ other measurement techniques, e.g., photometric principles.  
         [0092]     Forming the test strips or biosensors as flattened articles offers several advantages, especially in terms of storing and dispensing, as described in the Dispenser application incorporated above, but it is expected that one skilled in the art can apply the teachings herein to other test devices. The inventive display as well as other features described above may be employed in other test devices that have, e.g., a cylindrical body. Examples of these other test devices include environmental, food testing and other such testing devices. Even biosensors incorporating the inventive features described herein, while generally comprising a flat and thin shape, may have portions thereof that are sized and shaped to accommodate various electrical devices, as described above.  
         [heading-0093]     OLED working examples.  
         [0094]     Polymer light-emitting devices are typically configured as a thin film (e.g., about 0.1 microns of a polymer such as polyparaphenylene vinylene) sandwiched between two different metallic electrodes. The anode is transparent and lies on a transparent substrate. The typical combination is indium tin oxide on glass. The experiments below, however, employ a light emitting polymer coated onto a micro-electrode interdigitated array (IDA) in which the electrodes are co-planar.  
       EXAMPLE I  
       [0095]     To preparing the coating, 0.012 g of tris (2,2′-bipyridyl) dichlororuthenium (II) hexahydrate (CAS Registry No. 50525-27-4) was combined with 1 ml of acetonitrile. The compound did not completely dissolve. Deionized water was then added dropwise until the ruthenium compound completely dissolved.  
         [0096]     Two functional interdigitated micro-electrode arrays (IDAs) were used. The IDAs had 750 pairs of interdigitated fingers with each finger having a width of 2 μm, a length of 6 mm, and a spacing between the next closest finger (i.e., gap width) of about 2 μm. The IDAs were custom fabricated on a silicon wafer by Premitec Inc., Raleigh, N.C. The IDAs were each coated with 2011 of the solution just described. The coated IDAs were then placed in a desiccator and allowed to dry. The reagent coatings did not dry uniformly and had a ridge around the circumference of the coating.  
         [0097]     Using a BAS 100 W electrochemical potentiostat, a 3 volt potential was applied across the micro-electrode arrays, whereupon light was emitted from the coatings. Both electrodes were tested several times with light being emitted from the coating on application of about 3 volts. A Keithley  236  “Source Measure Unit” was than setup as a better voltage source for future measurements.  
       EXAMPLE II  
       [0098]     In order to obtain a better coating than that obtained in Example I, a solution of 1% PVP 25k (BASF) was prepared in deionized water. The ruthenium compound used in Example I was then mixed with the PVP solution in a 1:1 ratio and the resulting solution was applied to several additional IDAs. The first IDA had a spacing between the interdigitated fingers of approximately 2μ/m as described above and the other had a finger spacing of approximately 21μm and 50 finger pairs. This second IDA had a finger width of 21μm, a finger length of 6 mm and was formed on a Upilex substrate also custom fabricated by Premitec. The coating composition containing the PVP produced a uniform coating on both types of IDA&#39;s.  
         [0099]     Using the Keithley SMU-236, a three (3) volt potential was applied across the IDA with the 21/m finger spacing, but this voltage was not sufficient to cause the OLED to illuminate. Three (3) volts was also applied across the IDA with the 2 μm finger spacing, which caused the OLED to illuminate with good intensity. Increasing the voltage on the 2 μm IDA increased the intensity. Voltages of about 10-20V were required to produce reasonable intensities in the IDA with the 21 μm gap width between the fingers.  
       EXAMPLE III  
       [0100]     The electrodes used in the preceding examples were left at room temperature and humidity and the experiments described above repeated at approximately 1-2 month intervals. The OLEDs still illuminated with the same voltages used in the previous examples.  
         [heading-0101]     Other OLEDs.  
         [0102]     It is anticipated that substituting other polymers in the OLED matrix used in the experiments above may improve the results, in terms of the voltage required to illuminate and the overall intensity achieved with a given voltage. One such compound is Poly(styrenesulfonate)/poly(2,3-dihydrothieno(3,4b)-1,4-dioxin), available from Aldrich. Other Poly(sodium, 4-styrenesulfonate) compounds may also perform well or better than the polymer used in the above examples. One of skill in the art would recognize that many other known light emitting compounds may work suitable as OLEDs for use in the biosensors disclosed herein.  
         [0103]     While a preferred embodiment incorporating the principles of the present invention has been disclosed hereinabove, the present invention is not limited to the disclosed embodiments. Instead, as noted above, this application is intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.