Abstract:
An easily manufactured electrochemical test strip is made with facing electrode but side by side connectors for insertion into an electrochemical test meter. Current is conducted from the electrode on one layer to a connector on the other by a conductive layer disposed adjacent the end of a spacer layer, or by displacing the layer to bring a conductive surface on it into contact with the connector.

Description:
This application claims the benefit of U.S. provisional applications Nos. 60/952,108, filed Jul. 26, 2007, 60/978,848 filed Oct. 10, 2007 and 60/979,123 filed Oct. 11, 2007, which applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This application relates to electrochemical chemical test strips of the type commonly used in testing for blood glucose levels. The test strips are disposable and are combined with a reusable meter unit for applying power to electrodes within the strip and for displaying results. 
     Electrochemical test strips are known in the art. Because test strips may be used several times a day and because each test strip can only be used once, the strips need to be made as inexpensively as possible. On the other hand, because inaccurate monitoring of blood glucose can result in significant health consequences to a user, the test strips also need to be made with fairly strict manufacturing tolerances to provide the needed level of accuracy. Further, because monitoring is commonly done by individuals of varying age and varying levels of manual dexterity, rather than by trained health care professionals, the association of the strip with the meter needs to be easy to achieve in a correct manner. 
     In the art, there are commonly two basic electrode configurations: facing electrodes in which the electrodes for performing an electrochemical measurement are disposed on different substrates in a facing arrangement across a test cell, and side-by-side electrodes, in which the electrodes are both positioned on the common substrate surface within a test cell. From these electrodes, leads are extended to allow for electrical connection between the electrodes and the meter unit. The present application relates to electrochemical test strips with electrodes in a facing configuration. 
     U.S. Pat. No. 5,437,999 discloses test strips with electrodes in a facing configuration in which contact is made with the lead from one electrode (the one on the bottom of the test cell) from the top surface of the strip, while contact is made with the lead for the other electrode (the one on the top surface of the test cell) from the bottom surface of the test strip. U.S. patent application Ser. No. 2005/0258050 discloses another test strip configuration in which contacts are made from the top and the bottom. In both of these cases, the entire surface of the substrates is metallized to provide the electrode and its associated lead formed from a single material. 
     Notwithstanding the availability of designs for strip connectors in which the connectors are targeted from the top and bottom of the strip, in some circumstances (for example to allow use of the strip with preexisting meter units having connectors adapted for one-surface connection) it may be desirable to use facing electrodes but have both of the connectors on a single surface. U.S. Pat. No. 6,071,391 discloses a facing electrode test strip in which both leads are on one of the substrates. This is accomplished by the upper electrode being conducted to its lead parts through a hole in the adhesive and/or spacer layer between the top and bottom substrates. This approach involves several extra steps in manufacturing and additional difficulties in manufacturing, since a hole needs to be made in the adhesive/spacer layer in alignment with a separately patterned underlying conductive lead and an extension from the electrode, and a conductive material then needs to be filled into the hole to make contact between the electrode and the lead on the two surfaces. US Patent Publication No. 2005/0000808 discloses a test strip construction in which the spacer layer is formed as two separate parts with a gap running from one side of the test strip to the other. A lead is formed on one substrate with an electrode connector that extends across this gap to contact an electrode on the other substrate. This electrode can be printed on the entire interior surface, and this combined with the use of a spacer with two separate pieces instead of a hole means that the positioning of the electrode connector is only important in one direction (the length of the strip). This is an improvement over the two dimensional control required in the case of a hole, but it nevertheless increases the risk of manufacturing errors which can lead to unworkable test strips and therefore to increased cost per usable strip. 
     SUMMARY OF THE INVENTION 
     The present invention provides alternative strip designs in which the electrodes in the test cell are in a facing configuration, and the connectors are disposed on a single surface wherein the test strip is easily manufactured. In accordance with the invention, a test strip comprises: 
     (a) a base substrate having disposed thereon a first electrode and connector track, said first electrode and connector track comprising a first electrode connector, and a second electrode connector, 
     (b) a dielectric layer comprising two strips of dielectric material extending longitudinally along the base substrate, said dielectric strips having an open space between them defined by inner longitudinal edges, said open space exposing a portion of the first electrode and connector track and extending less than the entire length of the base substrate, such that a first of said strips of dielectric leaves a portion the first electrode connector exposed, and the second of said strips leaves at least a portion of the second electrode connector exposed, said exposed portion of the second electrode connector being greater in length than the exposed portion of the first electrode connector, 
     (c) a spacer layer comprising two strips of insulating material disposed over the dielectric layer, said insulating strips having an open space between them exposing the inner longitudinal edges of the dielectric strips and extending less than the entire length of the base substrate such that a first of said insulating strips leaves a portion the first electrode connector exposed, and the second of said insulating strips leaves at least a portion of the second electrode connector exposed, said exposed portion of the second electrode connector being greater in length than the exposed portion of the first electrode connector, 
     (d) a conductive material disposed in contact with the exposed portion of the second electrode connector, and 
     (e) a top layer comprising a substrate having disposed thereon a second electrode and connector track, wherein a portion of the second electrode and connector track is disposed in opposition to the exposed portion of the first electrode and connector track, and a portion of the second connector track is disposed in contact with the conductive material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-E  shows the layers of a first embodiment of a test strip in accordance with the invention. 
         FIG. 2  shows a cross section through the assembled device through the working electrode. 
         FIG. 3  shows a longitudinal section through the connector end of the test strip along a line that passes through the counter connector track  12 . 
         FIG. 4  shows a further embodiment of the invention. 
         FIGS. 5-13  show side sectional views of various other embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1A-E  shows the layers of a first embodiment of a test strip in accordance with the invention.  FIG. 1  A is the base layer. The base layer comprises a substrate  10  on which are formed a working electrode and connector track  11 , and a counter connector track  12 . Working electrode and connector track  11  and counter connector track  12  are suitably formed by screen printing with conductive ink. As shown in  FIG. 1A , the working electrode and connector track  11  is in the form of a loop with an extension forming a working electrode connector  11 ′. 
       FIG. 1B  is a dielectric layer. This layer is formed from two strips  13  and  14  deposited over the base layer leaving a channel extending the length of the test strip. Strip  13  is longer than strip  14  and covers a portion of the connector part  11 ′ of the working electrode and connector track  11  that is aligned opposite the counter connector track  12 . A portion  110  of the first electrode track  11  is left exposed between the dielectric strips  12 , and  14 . 
       FIG. 1  C is a spacer layer, suitably formed from double-sided adhesive insulting film. This layer is formed from two spacer strips  15  and  16  which are the same length as strips  13  and  14  but which are narrower than these strips. Portion  110  of the first electrode track  11  remains exposed. 
       FIG. 1D  is a layer containing just a strip of conductive material  17  such as conductive glue. This layer is offset from the end of spacer strip  16  and dielectric strip  14  so that it is in contact with the counter connector strip  12 . It is therefore in the same vertical plane within the strip as layers of  FIGS. 1B and 1C  but is it shown separately as it is formed separately. 
       FIG. 1E  is a layer containing a top substrate  18  and a counter electrode and connector track  19 . The counter electrode and connector track  19  suitably is formed by screen printing with a conductive ink and has the same shape as the working electrode and connector track  11 , except that the top substrate  18  is shorter than the substrate  10  so that when the layers are assembled the ends of the counter electrode track  12  and the working electrode track  11 ′ are exposed. 
     The test strip is assembled from the layers as shown. Prior to application of the top substrate  18 , a reagent solution is placed on and dried at the exposed portion  110  of the electrode working electrode and connector track  11  to form a working electrode. The area of the working electrode is defined by the spacing between the dielectric strips of  FIG. 1B  (See  FIG. 2 ) and the dimensions of the working electrode and connector track  11  at the intersection with the gap between the dielectric layers. 
       FIG. 2  shows a cross section through the assembled device through the working electrode. In this figure, the working electrode  210 , the counter electrode  219  and the dielectric strips  13 ,  14  are shown out of scale with expanded thickness for clarity.  FIG. 2  also shows the deposition of a reagent layer  201  on top of the working electrode  210 . The spacer strips  15 / 16 , define a open channel  202  which runs the length of the device. Accordingly, no separate vent is needed to allow a liquid sample to be drawn in to the test cell by capillarity. 
       FIG. 3  shows a longitudinal section through the connector end of the test strip along a line that passes through the counter connector track  12 . As shown, the conductive material  17 , connects the counter electrode and connector  19  to the counter connector track  12 . 
     There are numerous variations which can be made in the device as depicted in  FIGS. 1-E ,  2  and  3 . For example, as shown in  FIG. 4 , a protective tape  40  can be applied over the end of the test strip to help maintain the integrity of the conductive material  17 . 
     As noted above, the channel  202  the extends along the length of the test strip, means that no separate vent needs to be formed. However, the length of this channel may tend to draw sample past the electrode area, and therefore it may be desirable to modify the channel in the region where the base substrate  10  and the top substrate  18  are exposed to limit the flow of material past the electrode. In one approach to accomplish this result, before or after assembly of the device, a hole is punched through one or both of the base layers to intersect with the channel (for example in the region marked  150  in  FIG. 1A , thus limiting the length of the channel through which the sample can flow. Another approach is to alter the surface characteristics in region  150 , for example by applying a hydrophobic coating in this region to reduce the ability of an aqueous liquid sample (such as blood) to flow past the electrode  110 . 
     The conductive material used in the test strip of the invention may be any material that does not increase the resistance in the connection between the counter electrode  19  and connector track for the counter connector  12  in a manner that cannot be compensated for in making the measurement, and that is conveniently applied in a reasonably defined location. One example of a suitable conductive material is a conductive adhesive, which can be printed into the desired pattern. Kolbe et al.,  Microelectronics Reliability , Volume 47, Issues 2-3, February-March 2007, Pages 331-334 disclose a conductive adhesive which can be printed using inkjet technology owing to the small particle size of the conductive components of the adhesive. Alternatively, the conductive adhesive may be in the form of a conductive pressure sensitive adhesive film. Films of this type may be anisotropic, i.e., they conduct only in through the thickness of the film, such as 3M™ electrically conductive adhesive transfer tape  9705  which provides a 50 μm thick film conductive in the Z-direction, or may conduct current in all direction such as 3M™ XYZ isotropic electrically conductive adhesive transfer tape  9708  or  9709 . In the case where the conductive material is an adhesive film, it may also extend back over the insulating strips  15 ,  16  if desired. 
     In the test strip of the invention, the shape of the patterned electrode and connector tracks is not critical. Various shapes for such patterned depositions are known, including L shapes. The structure show in the figures is convenient, however, since the pattern from both the working electrode and connector track  11  and the counter electrode and connector track  19  is the same. The electrodes may also be formed as coated substrates (for example gold coated polyester) with an insulating layer disposed as needed to isolate the counter electrode connector  12  from any underlying metal coating on the base layer. 
     The test strip of the invention may have a very small test cell to allow for the use of minimum volume of sample. For example, the volume of the sample may be less than 1 μl, preferably less than 500 nanoliters, and more preferably less than 200 nanoliters. 
     The test strip of the invention may use any of the various chemistries known for the detection of analytes in solution. In particular, in the case of detection of glucose, the test strip may contain glucose oxidase and a redox mediator such as ferrocyanide, an osmium compound, or a ruthenium compound. 
     The structure of the test strip of the present invention offers the advantage of requiring no special alignment of the conductive material  17  that translates the counter electrode lead to the opposing substrate for connection to a meter. T-his is achieved with a test strip comprising: 
     (a) a base substrate having disposed thereon a first electrode and connector track, and a second electrode connector, 
     (b) a dielectric layer comprising two strips of dielectric material extending longitudinally along the base substrate, said dielectric strips having an open space between them exposing a portion of the first electrode track and extending less than the entire length of the base substrate, such that a first of said strips of dielectric leaves a portion the first electrode connector exposed, and the second of said strips leaves at least a portion of the second electrode connector exposed, said exposed portion of the second electrode connector being greater in length than the exposed portion of the first electrode connector, 
     (c) a spacer layer comprising two strips of insulating material disposed over the dielectric layer, said insulating strips having an open space between them exposing the dielectric strips at the edges of the first electrode and extending less than the entire length of the base substrate, such that a first of said insulating strips leaves a portion the first electrode connector exposed, and the second of said insulating strips leaves at least a portion of the second electrode connector exposed, said exposed portion of the second electrode connector being greater in length than the exposed portion of the first electrode connector, 
     (d) a conductive material disposed in contact with the exposed portion of the second electrode connector, and 
     (e) a top layer comprising a substrate having disposed thereon a second electrode and connector track, wherein a portion of the second electrode and connector track is disposed in opposition to the exposed first electrode, and a portion of the second electrode and connector track is disposed in contact with the conductive material. 
       FIGS. 5-11  show side sectional views of various other embodiments of the invention.  FIG. 5  shows a side view with optional protective tape  40 . In  FIG. 6 , there is a spacer  65  disposed in the second part of the strip adjacent the conductive material  17  and the spacer  15  in the first part of the strip is not in the same plane as the spacer  65  in the second part of the strip. 
       FIG. 7  shows an embodiment in which conductive material  17  meets the conductive track  12  in a plane substantially mid-way across the thickness of the spacer as a consequence of the build-up of dielectric material  14  under the conductive track  12 . 
       FIG. 8  shows an embodiment in which there is no conductive material  17 . Top substrate  18  and track  19  incline down to remake contact with and continue as Track  12 . 
       FIG. 9  shows an embodiment in which the conductive material  17  is an adhesive such as conductive epoxy 
       FIG. 10  is a combination of the embodiments of  FIGS. 7 and 8 . There is no conductive material  17 , and the contact of track  19  and track  12  is in a plane substantially mid-way across the thickness of the spacer as a consequence of the build-up of dielectric material  13  under the conductive track  12 . It also shows optional cover layer  40 . 
     It may also be desirable to configure the apparatus (meter) that received the strips to depress the protective cover or the top layer of the strip when it is inserted in the meter.  FIG. 11  shows a side view of exemplary strip in the strip port connector (SPC). A structure of the SPC depresses the tape  40  (or the top surface of the strip) to secure the tape in place during measurement. 
     The use of an SPC of the type show in  FIG. 11  which compresses the strip upon insertion can render the use of the conductive material  17  and the top tape  40  superfluous. As shown in  FIG. 12 , the action of the structure on the SPC can be against the top layer itself, pressing the end portion of it downwards to make electrical contact between track  19  and track  12  when the strip is inserted in the SPC. 
       FIG. 13  shows a further embodiment of the invention. In this case, the top layer  10  is fixed in the downward position by a mechanical binding such as heat-staking (i.e. melting the plastic so the two layers bind) or Ultrasonic/vibrational welding.  FIG. 13  shows the conductive material  17  but this is optional since track  19  is in contact with track  12  at the mechanical binding site.