Abstract:
This invention relates to a non-destructive means to determine electrochemical characteristics in biosensor test strips, including first applying a cyclic oxidative and reductive electric potential onto the inspection pads connecting to the reference electrode and working electrode, on which lies a drop of enzyme reagent solution, to homogenize the electrochemical characteristics of the biosensor test strips, and then applying an inspection electric potential within a short period of time over the inspection pads connecting to the reference electrode and working electrode to measure its electrical resistance to identify any abnormal biosensor test strips if present. Afterwards, embodiments of the present invention applies a reverse electric potential, having the same time interval as the inspection electric potential, onto the inspection pads connecting to the reference electrode and working electrode to prevent degradation on mediators such as potassium ferricyanide.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 100141396, filed on Nov. 14, 2011, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an electrochemical biosensor and, more particularly, to a method for determining the electrochemical characteristics associated with a biosensor test strip. 
     2. Description of the Related Art 
     Diabetic patients routinely use over-the-counter blood glucose meters to measure their blood glucose levels. First, the patient inserts a blood glucose test strip into its associated blood glucose meter. Then, the patient places a drop of blood sample over the test strip, causing the blood sample to react with an enzyme reagent, which is placed on the reaction zone over the working electrode and the reference electrode. At this time, by applying a fixed or variable electric potential across the reaction zone, the blood glucose meter may calculate the blood glucose level based on the electrochemical characteristics generated from the measured voltage or current. 
     The accuracy of the blood glucose reading, however, depends on several factors, some of which are difficult to control. For example, the accuracy depends on the materials used in the test strips. In particular, the accuracy depends on the surface properties of the working and the reference electrodes, which tend to vary among individual test strips. Moreover, some electrochemical characteristics of the enzyme reagents are highly susceptible to manufacturing and environmental variables. These variables may negatively affect, for example, the number and sizes of the air bubbles present in the enzyme reagent and hence the homogeneous distribution of the enzyme and mediator, such as potassium ferricyanide. These variables may also negatively affect the coverage completeness of the enzyme reagent over the reaction zone. All of these may cause significant differences in the performance among test strips under the same testing conditions. Accounting for, but not limited to, the above problems, manufacturers often assign batch-specific codes to the test strips to account for the variability among each batch. This practice, however, increases the production cost and makes the glucose monitoring system less user-friendly. 
     Since physicians often refer to blood glucose readings as an aid to monitor the effectiveness of diabetes management and to give appropriate medical treatments, the accuracy of the readings is very critical. Accordingly, at the end of the manufacturing processes of the test strips, usually a number of test strips are randomly selected from a manufacturing batch to test for their conformity with the specification. This sampling method, however, cannot guarantee the quality of the entire batch of test strips, and cannot remove the defective test strips, if present, in the batch. Moreover, because the quality assurance test conducted on a selected test strip is usually destructive and non-reversible, the selected test strip loses its value after the test. This in turn increases the cost of sampling, and limits the number of test strips available for sale. Therefore, there is a need in the art to provide an accurate and non-destructive method to assure the quality of a biosensor test strip. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide an accurate and non-destructive method to assure the quality of a biosensor test strip in the manufacturing processes. 
     According to one embodiment of the present invention, the surface characteristic of a test strip is first homogenized by a homogenizing process (cyclic electric potential). Then, a quality assurance test with an inspection electric potential is conducted on the test strip during the manufacturing process after dispensing the enzyme reagent solution, but before the enzyme reagent is dried, to determine if a defective test strip is present, which may be marked and then removed. Finally, the test strip is subject to a reverse electric potential adapted to substantially restore the test strip back to its original condition before testing in the manufacturing processes. 
     According to another embodiment of the present invention, the homogenizing process comprises applying to the enzyme reagent a cyclic oxidative and reductive electric potential adapted to homogenize the electrochemical characteristic on the surface of the test strip. Afterwards, an inspection electric potential is applied to the enzyme reagent to determine the homogenized electrochemical performance. Finally, a reverse electric potential is applied to the enzyme reagent to prevent the mediator, such as potassium ferricyanide, in the enzyme reagent from degradation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The described embodiments of the present invention will be apparent through examination of the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram of voltage-current relationship, illustrating a cyclic oxidative and reductive electric potential according to an embodiment of the present invention. 
         FIG. 2  is a simplified top plan view of a test strip and associated testing methods according to embodiments of the present invention. 
         FIG. 3  is a simplified top plan view of a test strip and associated testing methods according to embodiments of the present invention. 
         FIG. 4  is a simplified top plan view of a test strip and associated testing methods according to embodiments of the present invention. 
         FIG. 5  is a simplified top plan view of a test strip and associated testing methods according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To be consistent throughout the descriptions and for clear understanding of the present invention, the following definitions are hereby provided for terms used therein: 
     The term “biosensor” refers to an analytical device, or an analytical device system, for the detection of biologically or chemically related substances or properties. For example, a blood glucose biosensor (or sometimes a blood glucose meter), may use an enzyme reagent to determine the blood glucose level. 
     The term “test strip” may refer to a device used in conjuncture with a biosensor or a component of a biosensor. A test strip may be a single-use test strip or a multi-use test strip. For example, in blood glucose testing, a single-use test strip allows the user to test the blood glucose level only once, while a multi-use test strip, having multiple reaction zones on a single test strip, allows the user to perform multiple tests on a single test strips. 
     The term “non-destructive testing” refers to conducting a test on an object without materially changing the object&#39;s properties of interest. For example, after conducting a non-destructive testing on the enzyme reagent of a test strip, the enzyme reagent may still react with a blood sample and provide an accurate blood glucose reading. 
     According to an embodiment of the present invention, before conducting a quality assurance test with an inspection electric potential on a biosensor test strip, a homogenizing process may first be applied to the test strip to homogenize certain electrochemical characteristics on the surface of the conductor tracks of the test strip. Once homogenized, a quality assurance test with an inspection electric potential may be conducted on the test strip to determine whether the test strip is defective. Finally, the test strip may be subject to a reverse electric potential to substantially restore the test strip back to its original condition before the quality assurance test with an inspection electric potential. 
     For example, to homogenize the quality of a test trip, a cyclic oxidative and reductive electric potential may initially be applied across the enzyme reagent over the reaction zone to homogenize its electrochemical characteristic. The enzyme reagent may be placed between a part of the working electrodes and a part of the reference electrodes, and may cover a part of the working electrodes and a part of the reference electrodes. The cyclic oxidative and reductive electric potential may be applied to the enzyme reagent through the working electrode and the reference electrode. 
     Once the electrochemical characteristic of the test strip is homogenized, the electrical resistance of the enzyme reagent may be measured by applying a small inspection electric potential across the working and reference electrodes. The small inspection electric potential may be applied at two inspection pads, one electrically and very closely connected to the working electrode and the other one electrically and very closely connected to the reference electrode. The inspection electric potential may be kept small to prevent it from substantially altering the electrochemical characteristic of the enzyme reagent. Preferably, the inspection electric potential is 0.35 V or less. However, the exact inspection electric potential to be applied depends at least on the materials employed by the test strip, the material of the electrodes, and the enzyme reagent. A person of ordinary skill in the art would recognize that the present invention also applies to other types of electrodes, enzyme reagents, and test strips. The scope of the present invention is not limited by the inspection electric potential to be applied. 
     As previously described, the small inspection electric potential may be applied at two inspection pads respectively, electrically connected to the working electrode and reference electrode respectively. Alternatively, an inspection pad may be located on the working electrode or the reference electrode. It should be noted that the measured resistance across the two inspection pads may depend on the inherent electrical resistance of the conductor between the inspection pads and the electrodes. The electrical resistance of the conductor tends to vary depending on the manufacturing process or on other factors. Accordingly, the inspection pads may preferably be placed near the working and reference electrodes, preferably less than 1 centimeter. In addition, to prevent the enzyme reagent from degrading during the inspection, the small voltage may preferably be applied only for a very short period of time. 
     Once the electrical resistance is measured, whether the electrical resistance is within an acceptable range may be determined. If not, the defective test strip may be identified or marked. 
     Embodiments of the present invention provide non-destructive means to determine the electrochemical characteristics in biosensor test strips. After the inspection electric potential is applied, a reverse electric potential may be applied within a short period of time at the inspection pads to substantially eliminate degradation of the enzyme reagent caused by the inspection electric potential. According to an embodiment of the present invention, the reverse electric potential may be the opposite of the inspection electric potential between the inspection pads. For example, if the inspection electric potential is +0.35 V, the reverse electric potential may be −0.35 V. In addition, the duration of the reverse electric potential may be substantially the same as the duration of the inspection electric potential. Thus, by applying a reverse electric potential for the same short period of time, one may substantially prevent the potassium ferricyanide in the enzyme reagent from degradation. According to an embodiment of the present invention, the inspection electric potential, and the reverse electric potential may be a fixed electric potential, or a variable electric potential. 
     As previously mentioned, the inspection electric potential to be applied depends on the conductive material used for the electrodes and their associated layouts. For example, if the conductive material is copper foil with gold plating, applying an inspection electric potential of 0.35 V or less for 5 seconds or less (or 0.05 V or less for 0.1 second or less), and applying an opposite reverse electric potential for the same time interval would not cause the potassium ferricyanide in the enzyme reagent to degrade, thereby maintaining its electrochemical characteristics. Similarly, if the conductive material is silver paste or carbon paste, applying an inspection electric potential of 0.35 V or less for 5 seconds or less (or 0.15 V or less for 0.1 second or less), and applying an opposite reverse electric potential for the same time interval would not cause the potassium ferricyanide in the enzyme reagent to degrade, thereby maintaining its electrochemical characteristics. 
     According to an embodiment of the present invention, the enzyme reagent may comprise glucose oxidase (GOD), glucose dehydrogenase (GDH), and/or potassium ferricyanide. In addition, the enzyme reagent may further comprise citric acid, phosphoric acid, nonionic surfactant, deionized water and/or carbon nanotube. However, the specific composition of the enzyme reagent is not material to the present invention. Instead, a person of ordinary skill in the art would recognize that the present invention is applicable to other kinds of enzyme reagent. 
       FIG. 1  is a diagram illustrating a cyclic oxidative and reductive electric potential according to an embodiment of the present invention. According to an embodiment of the present invention, the cyclic oxidative and reductive electric potential is a cyclic electric potential cyclically varying between a positive voltage and the negative voltage. The cyclic electric potential may begin with a zero voltage. The cyclic electric potential may then be increased from the zero voltage to a positive voltage (+0.45V), decreased to a negative voltage (−0.45V), and finally back to the zero voltage, thereby completing a cycle (a zero-positive-zero-negative-zero cycle). Alternatively, the cyclic electric potential may be decreased from the zero voltage to a negative voltage (−0.45V), increased to a positive voltage (0.45V), and finally back to the zero voltage, thereby completing a cycle (a zero-negative-zero-positive-zero cycle). It should be noted that the cycle may also begin with a positive voltage, such as a positive-zero-negative-zero-positive cycle, or may begin with a negative voltage, such as a negative-zero-positive-zero-negative cycle. 
     As shown in  FIG. 1 , the voltage-current relationship of the enzyme reagent tends to stabilize after the first cyclic oxidative and reductive electric potential. As illustrated, for the given voltage of −0.2 V, the corresponding current value at point  1  for the first cycle is approximately 0.4 e-5 A, and the corresponding current values at point  2  for the second cycle and point  3  for the third cycle are approximately 0.6 e-5 A. In fact, the voltage-current relationship curves tend to be the same as the number of cycle increases, typically after the first cycle. After the cyclic oxidative and reductive electric potential treatment, the electrochemical characteristics of the test strip are stabilized or homogenized, and may now provide a more accurate electrical resistance when the inspection electric potential is applied. 
       FIG. 2  is a simplified top plan view of a test strip  200  and its testing method according to embodiments of the present invention. As shown, the enzyme reagent  13  is placed between, and covers part of, the working electrode  11  and the reference electrode  12 . The working electrode  11  is electrically connected to the first inspection pad  111  and electrically connected to the first contact pad  110 . The reference electrode  12  is electrically connected to the second inspection pad  121  and electrically connected to the second contact pad  120 . 
     According to an embodiment of the present invention, the enzyme reagent  13  is first subject to a cyclic oxidative and reductive electric potential to homogenize its electrochemical characteristic. This can be accomplished by applying a cyclic electric potential to the first contact pad  110  and the second contact pad  120  by using the homogenizing device  130 , thereby causing the cyclic oxidative and reductive electric potential to pass through the enzyme reagent  13 . A person of ordinary skill in the art would recognize that there are many ways to perform this step, and the scope of the present invention is not limited to the exact method used for subjecting the enzyme reagent  13  to a homogenized condition. For example, a person of ordinary skill in the art would recognize that the cyclic electric potential may also be applied to the first inspection pad  111  and the second inspection pad  121  through the electronic device  131 . 
     Once the electrochemical characteristic of the enzyme reagent  13  is homogenized, the electrical resistance of the enzyme reagent  13  may be measured by the electronic device  131 , which applies a small inspection electric potential at the first inspection pad  111  and the second inspection pad  121 . Preferably, the inspection electric potential is 0.35 V or less. As previously mentioned, to obtain a more accurate reading of the electrical resistance, the first inspection pad  111  shall be close to the working electrode  11 , and the second inspection pad  121  shall be close to the reference electrode  12 , preferably less than 1 centimeter. Thereafter, the electronic device  131  may apply a reverse electric potential at the first inspection pad  111  and the second inspection pad  121  to substantially restore the enzyme reagent  13  to its original condition before the inspection electric potential is applied. Once the electrical resistance of the enzyme reagent  13  is measured by the electronic device  131 , the electrochemical characteristics of the test strip  200  may be determined. 
       FIG. 3  is a simplified top plan view of the test strip  300  and associated testing methods according to embodiments of the present invention. The test strip  300  is a multi-use test strip, which comprises multiple test sections, each being capable of working with a blood glucose meter to obtain the blood glucose level. As shown, the test sections  3000 ,  3001 ,  3002 ,  3003  and  3004  are located on the same test strip  300 . A test section may be removed from the test strip  300  by bending along its associated pre-break line  14 . 
     Because each of the test sections will be used to provide a blood glucose reading, a quality assurance test with an inspection electric potential may be performed on each of them. For example, regarding the test section  3001 , the electrochemical characteristic of its enzyme reagent  13  on the working electrode  11  and the reference electrode  12  may be homogenized by the homogenizing device  130 , which may apply a cyclic electric potential to the first inspection pad  111  and the second inspection pad  121 . Then, the electronic device  131  may apply a small inspection electric potential to measure the electrical resistance between the first inspection pad  111  and the second inspection pad  121 , and then may apply a reverse electric potential to restore the enzyme reagent  13 . With the electrical resistance of the enzyme reagent  13  measured, the electrochemical characteristics of the test section  3001  may be determined. 
       FIG. 4  is a simplified top plan view of the test strip  400  and associated testing methods according to embodiments of the present invention, wherein all the test sections are electrically connected and wherein the pre-break lines  141  are slightly different from the pre-break lines depicted in  FIG. 3 . As shown, the test strip  400  may comprise, but not limited to, test sections  4000 ,  4001 ,  4002 ,  4003  and  4004 . A test section may be removed from the test strip  400  by bending along its associated pre-break line  14 . Moreover, the working electrode  11  and the first contact pad  110  of each test section may be connected in series, and the reference electrode  12  and the second contact pad  120  of each test section may be connected in series. For example, the first inspection pad  111  of the test section  4002  is electrically connected to the first inspection pad  111  and the first contact pad  110  of the test section  4000 . Similarly, the second inspection pad  121  of the test section  4002  is electrically connected to the second inspection pad  121  and the second contact pad  120  of the test section  4000 . Through serial connections, the homogenizing device  130  may apply the cyclic oxidative and reductive electric potential throughout each of the test sections. In addition, an inspection electric potential may be applied on the first inspection pad  111  and the second inspection pad  121 , and the electronic device  131  may then measure the electrical resistance of the enzyme reagent at each of the test sections to assure its homogeneous quality. Finally, a reverse electric potential may be applied to each of the test sections to restore it to its original condition before inspection electric potential is applied. 
       FIG. 5  is a simplified top plan view of the test strip  500  and associated testing methods according to embodiments of the present invention. As shown, the test strip  500  may comprise, but not limited to, test sections  5001  and  5002 . A test section may be removed from the test strip  500 , in part, by bending along its associated pre-cut notches  15  and pre-formed holes  16 . Moreover, each of the test sections may share the same first contact pad  110  and the second contact pad  120 . For example, the first inspection pad  111  of the test section  5002  is electrically connected to the first inspection pad  111  of the test section  5001  and the first contact pad  110 . Similarly, the inspection pad  121  of the test section  5002  is electrically connected to the inspection pad  121  of the test section  5001  and the second contact pad  120 . Through such connections, the homogenizing device  130  may apply the cyclic oxidative and reductive electric potential throughout each of the test sections. In addition, an inspection electric potential may be applied on the first inspection pad  111  and the second inspection pad  121 , and the electronic device  131  may then measure the electrical resistance of the enzyme reagent at each of the test sections to assure its homogeneous quality. Finally, a reverse electric potential may be applied to each of the test sections on the first inspection pad  111  and the second inspection pad  121  to restore to the original condition before the inspection electric potential is applied.