Patent Publication Number: US-2010112612-A1

Title: Method for determining an analyte using an analytical test strip with a minimal fill-error viewing window

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates, in general, to analytical devices and, in particular, to analytical test strips and associated methods. 
     2. Description of the Related Art 
     The determination (e.g., detection and/or concentration measurement) of an analyte in a fluid sample is of particular interest in the medical field. For example, it can be desirable to determine glucose, cholesterol, acetaminophen and/or HbAlc concentrations in a sample of a bodily fluid such as urine, blood or interstitial fluid. Such determinations can be achieved using analytical test strips, based on, for example, photometric or electrochemical techniques, along with an associated meter. For example, the OneTouch® Ultra® whole blood testing kit, available from LifeScan, Inc., Milpitas, USA, employs an electrochemical-based analytical test strip for the determination of blood glucose concentration in a whole blood sample. 
     Typical electrochemical-based analytical test strips employ a plurality of electrodes (e.g., a working electrode and a reference electrode) and an enzymatic reagent to facilitate an electrochemical reaction with an analyte of interest and, thereby, determine the concentration of the analyte. For example, an electrochemical-based analytical test strip for the determination of glucose concentration in a blood sample can employ an enzymatic reagent that includes the enzyme glucose oxidase and the mediator ferricyanide. Further details of conventional electrochemical-based analytical test strips are included in U.S. Pat. No. 5,708,247, which is hereby incorporated in full by reference. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       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, in which: 
         FIG. 1  is a simplified exploded perspective view of an electrochemical-based analytical test strip according to an exemplary embodiment of the present invention; 
         FIG. 2  is a simplified top view of the patterned conductive layer of the electrochemical-based analytical test strip of  FIG. 1 ; 
         FIG. 3  is a simplified top view of the patterned insulating layer of the electrochemical-based analytical test strip of  FIG. 1 ; 
         FIG. 4  is a simplified see-through top view of a portion of the electrochemical-based analytical test strip of  FIG. 1  that depicts alignment of various components; and 
         FIG. 5  is a flow chart of a process for determining an analyte in a bodily fluid sample according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An analytical test strip according to the present invention includes a substrate, an enzymatic reagent layer (disposed, for example, over the substrate), and a top layer (with a first portion and an opaque second portion) disposed over the enzymatic reagent layer. 
     The analytical test strip also has a sample-receiving chamber defined therein. Moreover, the sample receiving chamber has a working portion and a non-working portion. The first portion (e.g., a transparent first portion or a translucent first portion) and the opaque second portion of the top layer are configured such that a user can view the working portion of the sample-receiving chamber through the first portion. Moreover, the user is precluded from viewing the non-working portion of the sample-receiving chamber by the opaque second portion. 
     Analytical test strips according to embodiments of the present invention can be configured, for example, as a photometric analytical test strip or as an electrochemical-based analytical test strip. An embodiment of an electrochemical-based analytical test strip according to the present invention includes an electrically-insulating substrate, a patterned conductive layer disposed over the electrically-insulating substrate, a patterned insulating layer disposed over the patterned conductive layer, an enzymatic reagent layer disposed at least over at least a portion of the patterned conductive layer, and a top layer (with a first portion and an opaque second portion) disposed over the enzymatic reagent layer. 
     The electrochemical-based analytical test strip also has a sample-receiving chamber defined therein. Moreover, the sample receiving chamber has a working portion and a non-working portion. The first portion (e.g., a transparent first portion or a translucent first portion) and the opaque second portion of the top layer are configured such that a user can view the working portion of the sample-receiving chamber through the first portion. Moreover, the user is precluded from viewing the non-working portion of the sample-receiving chamber by the opaque second portion. 
     Since a user can view only the working portion of the sample-receiving chamber, a user can readily visually verify when a bodily fluid sample has completely filled the working portion, thus providing for accurate analyte determination. It should noted that once apprised of the present disclosure, one skilled in the art will recognize that the working portion of the sample receiving portion is that portion that must be filled by a sample to enable accurate results during use of the analytical test strip, while filling of the non-working portion is not required for accurate results. 
     Since the opaque second portion blocks a user form viewing the non-working portion of the sample-receiving chamber, user visual verification is beneficially independent of whether the bodily fluid sample has or has not filled the non-working portion of the sample-receiving chamber. Therefore, a user is prevented from erroneously concluding that a sample fill-error has occurred when the working portion has been filled but the non-working portion has not been filled. This benefit leads to the first portion of the top layer also being referred to as a minimal fill-error sample viewing window. Further details, characteristics and benefits of such an analytical test strip are described with respect to the further embodiments discussed below. 
     Referring to  FIGS. 1-5 , an electrochemical-based analytical test strip  10  according to the present invention includes an electrically-insulating substrate  12 , a patterned conductor layer  14 , a patterned insulation layer  16 , an enzymatic reagent layer  18 , a patterned adhesive layer  20 , a hydrophilic layer  22 , and a top layer  24 . 
     The disposition and alignment of patterning of electrically-insulating substrate  12 , patterned conductor layer  14  (including reference electrode  14   a,  first working electrode  14   b  and second working electrode  14   c ), patterned insulation layer  16  (with electrode exposure window  17  extending therethrough) and enzymatic reagent layer  18 , and patterned adhesive layer  20  (depicted by the outermost two vertical dashed lines in  FIG. 4 ), hydrophilic layer  22  (not shown in  FIG. 4 ) and top layer  24  of electrochemical-based analytical test strip  10  are such that sample receiving-chamber  26  is formed within electrochemical-based analytical test strip  10 . 
     To ease manufacturing tolerances and provide for ready sample application and flow, the total volume of sample-receiving chamber  26  is greater than the minimal volume required for accurate use of electrochemical-based analytical test strip  10 . Therefore, the sample-receiving chamber includes both a working portion that can hold the aforementioned minimal volume and a non-working portion that is the remainder of the sample-receiving chamber. A typical, but non-limiting, volume of the working portion for the embodiment of  FIGS. 1-4  is approximately 0.95 micro-liters, while the typical, but non-limiting, volume of the total sample-receiving chamber is approximately 1.1 micro-liters. For these typical volumes the working portion constitutes approximately 86% of the sample-receiving chamber by volume. 
     In the embodiment of  FIGS. 1-4 , the extent of the working portion is essentially defined by (i) the overlap of electrode exposure window  17  with reference electrode  14   a,  first working electrode  14   b  and second working electrode  14   c  of patterned conductor layer  14 ; (ii) slightly beyond the extent of the second working electrode  14   c  (to allow for manufacturing tolerances); and (iii) the underside extent of hydrophilic layer  22 . Therefore, the working portion is essentially T-shaped in the perspective of  FIG. 4  with a “height” depicted by line A-A in  FIG. 4 . The total sample-receiving chamber in the embodiment of  FIG. 4  is essentially defined by the patterned adhesive layer and the hydrophilic layer. 
     Electrically-insulating substrate  12  can be any suitable electrically-insulating substrate known to one skilled in the art including, for example, a nylon substrate, polycarbonate substrate, a polyimide substrate, a polyvinyl chloride substrate, a polyethylene substrate, a polypropylene substrate, a glycolated polyester (PETG) substrate, or a polyester substrate. The electrically-insulating substrate can have any suitable dimensions including, for example, a width dimension of about 5 mm, a length dimension of about 27 mm and a thickness dimension of about 0.5 mm. 
     Electrically-insulating substrate  12  provide structure to the strip for ease of handling and also serves as a base for the application (e.g., printing) of subsequent layers (e.g., a carbon-based patterned conductive layer). It should be noted that patterned conductor layers employed in analytical test strips according to embodiments of the present invention can take any suitable shape and be formed of any suitable materials including, for example, metal materials and conductive carbon materials. 
     In the embodiment of  FIGS. 1-4 , patterned conductive layer  14  includes a counter electrode  14   a  (also referred to as a reference electrode), a first working electrode  14   b,  and a second working electrode  14   c  (see  FIGS. 2 and 4  in particular). Although electrochemical-based analytical test strip  10  is depicted as including three electrodes, embodiments of electrochemical-based analytical test strips, including embodiments of the present invention, can include any suitable number of electrodes. 
     Counter electrode  14   a,  first working electrode  14   b  and second working electrode  14   c  can be formed of any suitable material including, for example, gold, palladium, platinum, indium, titanium-palladium alloys and electrically conducting carbon-based materials. Details regarding the use of electrodes and enzymatic reagent layers for the determination of the concentrations of analytes in a fluid sample, are in U.S. Pat. No. 6,733,655, which is hereby fully incorporated by reference. 
     Patterned insulation layer  16  can be formed, for example, from a screen printable insulating ink. Such a screen printable insulating ink is commercially available from Ercon of Wareham, Mass. U.S.A. under the name “Insulayer.” 
     Patterned adhesive layer  20  can be formed, for example, from a screen-printable pressure sensitive adhesive commercially available from Apollo Adhesives, Tamworth, Staffordshire, UK. In the embodiment of  FIGS. 1-4 , patterned adhesive layer  20  defines outer walls of the sample-receiving chamber  26 . 
     Hydrophilic layer  22  can be, for example, a clear film with hydrophilic properties that promote wetting and filling of electrochemical-based analytical test strip  10  by a fluid sample (e.g., a whole blood sample). Such clear films are commercially available from, for example, 3M of Minneapolis, Minn. U.S.A. 
     Enzymatic reagent layer  18  can include any suitable enzymatic reagents, with the selection of enzymatic reagents being dependent on the analyte to be determined. For example, if glucose is to be determined in a blood sample, enzymatic reagent layer  18  can include oxidase or glucose dehydrogenase along with other components necessary for functional operation. Enzymatic reagent layer  18  can include, for example, glucose oxidase, tri-sodium citrate, citric acid, polyvinyl alcohol, hydroxyl ethyl cellulose, potassium ferricyanide, antifoam, cabosil, PVPVA, and water. Further details regarding enzymatic reagent layers, and electrochemical-based analytical test strips in general, are in U.S. Pat. No. 6,241,862, the contents of which are hereby fully incorporated by reference. 
     Top layer  24  includes a first portion  24   a  (e.g. a transparent or translucent first portion) and an opaque second portion  24   b.  First portion  24   a  and the opaque second portion  24   b  of the top layer are configured and aligned with the remainder of the analytical test strip such that a user can view the working portion of the sample-receiving chamber through the first portion of the top layer and is precluded from viewing the non-working portion of the sample-receiving chamber by the opaque second portion of the top layer. This configuration prevents a user from erroneously determining that a sample fill error has occurred when the working portion of the sample-receiving chamber has been filled but the non-working portion has not been filled. 
     Top layer  24  can be, for example, a clear film, with opaque second portion  24 b being created, for example, by overprinting of the clear film with an opaque ink and first portion  24   a  being simply clear film without overprinting. A suitable clear film is commercially available from Tape Specialities, Tring, Hertfordshire, UK. 
     Electrochemical-based analytical test strip  10  can be manufactured, for example, by the sequential aligned formation of patterned conductor layer  14 , patterned insulation layer  16  (with electrode exposure window  17  extending therethrough), enzymatic reagent layer  18 , patterned adhesive layer  20 , hydrophilic layer  22  and top film  24  onto electrically-insulating substrate  12 . Any suitable techniques known to one skilled in the art can be used to accomplish such sequential aligned formation, including, for example, screen printing, photolithography, photogravure, chemical vapour deposition and tape lamination techniques. 
     During use of electrochemical-based analytical test strip  10  to determine an analyte concentration in a fluid sample (e.g., blood glucose concentration in a whole blood sample), electrodes  14   a,    14   b  and  14   c  of patterned conductor layer  14  are employed to monitor an electrochemical reaction induced current of interest. The magnitude of such a current can then be correlated with the amount of analyte present in the fluid sample under investigation. During such use, a bodily fluid sample is introduced into sample-receiving chamber  26  of electrochemical-based analytical test strip  10 . 
       FIG. 5  is a flow chart of a method  500  for determining an analyte (such a glucose) in a bodily fluid sample (e.g., a whole blood sample) according to an exemplary embodiment of the present invention. At step  510 , method  500  includes introducing a bodily fluid sample into a sample-receiving chamber of an analytical test strip. 
     Method  500  also includes verifying that the bodily fluid sample has filled a working portion of the sample-receiving chamber by user visual observation of the working portion through a first portion of a top layer of the analytical test strip, while an opaque second portion of the top layer precludes user visual observation of a non-working portion of the sample-receiving chamber (see step  520  of  FIG. 5 ). Thereafter, the concentration of analyte in the bodily fluid sample is determined (for example, using an associated meter) only if during the verifying step the user has verified that the bodily fluid sample has filled the working portion, as set forth in step  530 . 
     Once apprised of the present disclosure, one skilled in the art will recognize that methods according to embodiments of the present invention, including method  500 , can be conducted using analytical test strips according to the present invention, including the electrochemical-based analytical test strip of  FIGS. 1-4 . 
     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 structures and methods within the scope of these claims and their equivalents be covered thereby.