MXPA06008846A - Electrochemical biosensor - Google Patents

Abstract

According to the present invention, an electrochemical sensor (10) for detecting the concentration of analyte in a fluid test sample is disclosed. The sensor (10) includes a counter electrode having a high-resistance portion (19) for use in detecting whether a predetermined amount of sample has been received by the test sensor.

Description

ELECTROCHEMICAL BIOSENSOR FIELD OF THE INVENTION The present invention relates generally to an electrochemical biosensor for use in the quantification of an analyte in a liquid sample and, more particularly, to a system for detecting an insufficient sample amount in an electrochemical biosensor.

BACKGROUND OF THE INVENTION Medical conditions such as diabetes require that a person affected by the condition regularly self-monitor the level of blood glucose concentration of that person. The purpose of monitoring the level of blood glucose concentration is to determine the level of blood glucose concentration of the person and then take a corrective action, based on whether the level is too high or too low, to bring back the level within a normal range. Failing to take corrective action can have serious medical implications for that person. A method to monitor a person's blood glucose level is with a portable test device. The portable nature of these devices makes it possible for users to conveniently test their blood glucose levels if it can be done. One type of device uses an electrochemical biosensor to collect the blood sample and to analyze the blood sample. The electrochemical biosensor includes a reagent designed to react with the blood glucose to create an oxidation current in electrodes disposed within the electrochemical biosensor - this current is indicative of the level of blood glucose concentration of the user. A predetermined amount of reagent is included within an electrochemical biosensor and is designed to react with a predetermined sample volume. If a sample volume smaller than that required by the electrochemical biosensor is collected - a condition referred to as being insufficiently filled - may result in an erroneous measurement. Because electrochemical biosensors are commonly used in a self-test environment, there is an increased likelihood that an inappropriate amount of sample can be collected. In addition, because the sample volumes are very small (typically less than about 10 μl) it is difficult for a user to visually determine if an appropriate amount of sample has been collected for the analysis. In this way, there is a need for an electrochemical biosensor that reliably detects and alerts a user in the event of an insufficient filling condition.

BRIEF DESCRIPTION OF THE INVENTION In accordance with one embodiment of the present invention, an electrochemical sensor is disclosed for detecting the concentration of analyte in a test sample of the fluid. The sensor includes a counter electrode having a high strength portion for use in detection if a predetermined amount of sample has been received by the test sensor. In accordance with another embodiment of the present invention, a method is described for evaluating whether an electrochemical test sensor is properly filled. The test sensor includes a working electrode coupled to a first branch and a counter electrode coupled to a second branch. The counter electrode includes a high strength portion and a low resistance portion. The test sensor includes a reagent disposed on the working electrode that is adapted to react with an analyte in a fluid sample to produce an electrochemical reaction that is indicative of the concentration of the analyte in the fluid sample. The method comprises applying a voltage profile through the first and second branches, measuring the current profile in the first and second branches in response to the applied voltage profile and generating an error signal due to insufficient filling when the current profile measured do not have a default profile. The brief description above of the present invention is not intended to represent each embodiment, or each aspect, of the present invention. The additional features and benefits of the present invention are apparent from the detailed description, figures and modalities set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a view with the separated parts of an electrochemical biosensor according to an embodiment of the present invention. Figure 2a is a top, oversized view of an electrode pattern of the electrochemical biosensor of Figure 1. Figure 2b is a schematic circuitry of the electrochemical biosensor of Figure 2a when the electrochemical biosensor is partially filled with the liquid sample. Figure 2c is a schematic circuitry of the electrochemical biosensor of Figure 2a when the electrochemical biosensor is appropriately filled with a liquid sample. Figure 3a is a schematic of the voltage profile applied to the test sensor of Figure 1 according to one embodiment of the present invention. Figures 3b and 3c are diagrams of the current profile of the test sensor in response to the tension profile of Figure 3a in an insufficient filling condition and an appropriate filling condition, respectively. Figure 4a is a schematic of the voltage profile applied to the test sensor of Figure 1 according to another embodiment of the present invention. Figures 4b and 4c are diagrams of the current profile of the test sensor in response to the tension profile of Figure 4a in an insufficient filling condition and an appropriate filling condition, respectively. While the invention is susceptible to various modifications and alternative forms, the specific embodiments are shown by way of example in the drawings and are described in detail in this document. However, it should be understood that it is not proposed that the invention be limited to the particular forms that are described.

DETAILED DESCRIPTION OF THE ILLUSTRATED MODALITIES Returning to the drawings and initially to Figure 1, the construction of an electrochemical sensor 10 according to an embodiment of the present invention is shown. The sensor 10 comprises an insulating base 12 on which is printed in sequence (typically by means of stencil printing techniques) a pattern of electrical conductors including first and second leads 14a, b, an electrode pattern including an electrode 16, a counter electrode, an insulating (dielectric) layer 20 including an opening 22 and a channel 25 and a reaction layer 24. The counter electrode includes a branch of low resistance counter electrode 18 (counter electrode LR) and a branch of counter electrode high resistance 19 (counter electrode HR). The reaction layer 24 includes a reagent for converting an analyte of interest (eg, glucose) into a test sample of the fluid (eg, blood) in a chemical species that is measurable electrochemically, in terms of the electrical current it produces , by means of the components of the electrode pattern. The reagent of the reaction layer 24 typically contains an enzyme such as, for example, glucose oxidase, which reacts with the analyte and with an electron receptor such as ferricyanide salt to produce an electrochemically measurable species that can be detected by the standard. of electrodes 16, 18, 19. The reaction layer 24 comprises a polymer, an enzyme and an electron receptor. The reaction layer 24 also includes additional ingredients such as a regulator and a surfactant in some embodiments of the present invention. The reaction layer 24 is disposed over the opening 22 and the channel 25 in the insulating layer 20. In this way, the portion of the reaction layer 24 exposed to the electrode pattern 16, 18, 19 is defined by an opening 22 and a channel 25 in the insulating layer 20. The working electrode 16 is electrically coupled to the first branch 14a and the counter electrode LR 18 and the counter electrode HR 19 are electrically coupled to a second branch 14b. The reaction layer 24 covers only the working electrode 16, covers the working electrode 16 and the counter electrode LR 18 or covers the working electrode 16, the counter electrode LR 18 and the counter electrode HR 19 in alternative embodiments of the present invention. When the reaction layer 24 covers only the working electrode 16, an electro active material is present on the counter electrode LR 18 to allow it to function as a counter electrode as is well known in the field.

The sensor 10 includes a cover 30 having a concave portion 32 that forms a capillary channel when it is connected to the insulating layer 20 to move the liquid sample from an inlet 34 to the interior of the test sensor 10. The downstream end of the capillary channel includes one or more openings 36 for venting the capillary channel - the fluid sample travels from the inlet 34 within the sensor 10 to the opening 36. In use, the sensor 10 collects a fluid sample (e.g., a sample of blood from a patient's finger) by contacting the capillary channel inlet 34 with the fluid sample. With reference to Figure 2a, the working electrode 16 and the counter electrode LR 18 are configured in such a way that the counter electrode LR 18 is located downstream (in terms of the flow direction of the fluid sample along the path flow rate) of the working electrode 16. This configuration offers the advantage of requiring the test fluid to completely cover the working electrode 16 in order to make contact with the counter-electrode LR 18. However, the counter electrode HR 19, which is coupled with the counter-electrode LR 18 via a resistor 40, it is placed upstream of the working electrode 16. According to one embodiment of the present invention, the resistor 40 has a resistance of about 50 kO to about 500 kO. In other embodiments, the resistance of the resistor 40 ranges from about 250 kO to about 350 kO. In yet another embodiment, the resistor 40 has a resistance of approximately 300 kO. The resistor 40 can be printed by stenciling on the insulating base 12 in a manner similar to the working electrode 16, the counter electrode LR 18, the counter electrode HR 19 and the leads 14a, b. Generally, as described below, the resistor 40 is used in the detection of an insufficient filling condition in the test sensor 10, which may result in an inaccurate measurement of the analyte of interest in the fluid sample. With reference to Figure 2b, the working electrode 16 and the counter electrode HR 19 form the illustrated circuit if the sensor 10 is insufficiently filled (ie, the counter electrode LR 18 in Figure 2a is not covered by the fluid sample). In this situation, the sensor current passes through the resistor 40. In this way, the potential V2 between the working electrode 16 and the counter electrode HR 19 is approximately the difference between the potential Va applied to the leads of the sensor 14a, b and the voltage drop Vr through the resistor 40, adopting negligible resistance along the electrode / lead pattern.

With reference to Figure 2c, the working electrode 16 and the counter electrode LR 18 form the illustrated circuit if the sensor 10 is properly filled (ie, the counter electrode LR 18 in Figure 2a is covered by the liquid sample). In this situation, the resistor 40 is electrically deviated in the circuitry. In this way, the potential V2 between the working electrode 16 and the counter electrode LR 18 is substantially the same as the potential Vx applied to the leads 14a, b of the sensor 10, adopting negligible resistance along the electrode / lead pattern . The current measured in the sensor 10 is a result of the diffusion of the electroactive species to the electrodes and the subsequent redox reactions at this point. For example, in the working electrode 16 an electron of the ferrocyanide is taken, oxidizing it to ferricyanide. In the counter electrode LR 18 (or in the counter electrode HR 19 in a situation of insufficient filling), an electron is added to the ferricyanide, reducing it to ferrocyanide. The flow of electrons in the electrical pattern that connects the two electrodes is measured and is related to the amount of ferrocyanide and therefore the amount of glucose in the sample. In a normal operation, a relatively high electric potential is applied, V2 in Figure 2c, between the electrodes (e.g., approximately 400 mV), making the oxidation and reduction reactions fast at the electrodes and depleting the region around the working electrode 16 of the mediator reduced (for example, ferrocyanide). In this way, the current is not constant but decreases with time as the reaction is limited by diffusion to the electrode surface of the reduced mediator. In general, this downward current i can be described according to equation (1): i ^ C - G - f (1) In equation (1), C is a constant, G is the concentration of the analyte (for example, glucose) in the liquid sample, t is the time elapsed since the potential of V2 is applied and k is a constant in relation to the current descent profile. If a higher electric potential is applied, an increase in the sensor current is not measured and a change for the descent with time is not measured because the sensor current is determined by diffusion to the electrode surface. If a lower electric potential is applied (eg, approximately 200 mV) between the electrodes, the oxidation and reduction reactions are slower, but sufficiently fast so that the sensor current remains dependent on diffusion. Eventually at a lower voltage (eg, less than about 200 mV), the local depletion of the reduced mediator does not occur and the sensor current causes it to vary over time. In this way, during the normal operation of the sensor 10, a change in the profile of descent of the current with time over a range of applied potentials does not occur. The operation of the test sensor 10 with insufficient filling detection will be described. If the sensor 10 is insufficiently filled (i.e., less than a quantity required for the designated reaction) the sample only covers the counter electrode HR 19 and at least a portion of the working electrode 16. In this situation of insufficient filling, the counter electrode HR 19 serves as the complete counter electrode with a high resistance due to the resistor 40. Figure 2b illustrates the circuit under this condition. The current flow through the resistor 40 causes a potential drop Vr on the resistor 40 and reduces the potential V2 available for the electrochemical reactions. If the resistance is sufficiently high, the potential V2 is reduced to a point where the reactions of the electrode surface are slow and the current measured between the leads of electrodes 14a and 14b does not descend normally with time but is essentially flat. This flat equilibrium current is a dynamic balance between the sensor current and the voltage drop Vr in the resistor. The change in the applied voltage Vx changes this equilibrium current - a lower voltage results in a lower equilibrium or a steady-state current and a higher voltage results in a higher current. The sensor current has a "step" profile if a voltage profile is applied in the form of steps. In a situation where the sensor 10 is properly filled, the sample covers the counter electrode LR 18, in addition to the counter electrode HR 19 and the working electrode 16. Figure 2c illustrates the circuitry under this condition. The branch of the circuitry between the counter electrode HR 19 and the resistor 40 to the branch 14b is electrically deviated by the direct connection between the counter electrode LR 18 and the branch 14b. The working electrode 16 and the counter electrode LR 18 form a low resistance circuit and the current of the sensor has a low profile type where the current is limited by the diffusion of electroactive species to the surface of the electrode as described above. The present invention provides an electrochemical sensor in which the electrodes are configured such that in the case of an insufficient filling condition, the result is a current response with applied time and / or voltage that is characteristic and can be distinguished from the response of a correctly filled sensor. Specifically, there are at least two ways in which a partially filled sensor 10 can be distinguished from a sensor 10 that is appropriately filled according to the alternative embodiments of the present invention. First, the sensor current of a partially filled sensor 10 does not normally fall over time, unlike the sensor current of a properly filled sensor 10. Second, the sensor current of a partially filled test sensor 10 increases with the applied voltage due to the resistor 40, while the sensor current of an appropriately filled sensor 10 (which deviates to the resistor) does not. In this way, when the amount of the test fluid entering the capillary space of the test sensor 10 is sufficient only to cover the counter electrode HR 19 and at least a portion of the working electrode 16 and when a suitable potential is applied, the current measured through the leads 14a, b is essentially constant and does not descend normally with time. Expressed in another way, a device coupled to the leads 14a, b perceives certain characteristics of the sensor current over time, which are used to determine whether an error condition has occurred due to insufficient filling. This is done by algorithmically programming the device to detect the insufficient filling condition by measuring the current at defined periods of time after the test fluid has electrically connected the counter electrode HR 19 with the working electrode 16 and / or after the test fluid has electrically connected the working electrode 16 to the counter electrode LR 18. With reference to Figures 3a, 3b and 3c, a method will be described to determine if the test sensor 10 is properly filled. At time t0, a voltage step is applied between the leads 14a, b and is kept constant up to time ti, this period is referred to as the burn-in period. Then, no voltage is applied (for example, an open circuit) during a waiting period from time ti to time t2. Finally, the voltage step is applied again during a reading period from t2 to t4. According to one embodiment of the present invention, the burn, wait and read periods are each from about 2 to about 10 seconds in duration. The applied step voltage is from about 0.3 Volts to about 0.4 Volts, according to one embodiment of the present invention. An insufficiently filled sensor 10 generates a flat current profile of the sensor during the reading period as shown, for example, in Figure 3b. A properly filled sensor 10 generates a typical downstream sensor current profile during the reading period as shown, for example, in Figure 3c. The descent factor, k, during the reading period -from time t to t4- is calculated from the two currents, Ir3 and Ir4, measured at t3 and t4, according to equation (2): (2) ln (t4) ~ ln (/ 3) In equation (2), the decay factor k, describes how fast the current i drops in equation (1), where C is a constant, G is the Glucose concentration and t is the time elapsed after the tension is initially applied. In an appropriately filled sensor 10, k is typically between about 0.30 and about 0.49, decreasing as the glucose concentration increases. The descent factor falls to zero under insufficient filling conditions. Therefore, an insufficiently filled sensor 10 is detected when verifying whether the descent factor is lower than a predetermined lower limit. With reference to Figures 4a, 4b and 4c, another method will be described to determine if the test sensor 10 is properly filled. A first voltage is applied during the burning period that occurs from time t0 to time ti and a second higher voltage is applied up to time t2. No voltage (for example, an open circuit) is applied during the waiting period from time t2 to time t3. Finally, a voltage is applied during the reading period from time t3 to time t5. According to one embodiment, the first voltage applied during the burning period from time to a is approximately 0.3 V and the second voltage applied during the burning period from time ti to time t2 is approximately 0. 6 V. During the reading period, a voltage of approximately 0.3 V is applied. The periods of burning, waiting and reading are each from approximately 2 seconds to approximately 10 seconds in length, with the first voltage of the burning period applied during about 25% to about 75% of the total burnout period, according to one embodiment of the present invention. An insufficiently filled sensor 10 generates a step current profile of the sensor Ib during the burning period as shown, for example, in Figure 4b. An appropriately filled sensor 10 generates a current profile of the sensor in the form of a descent as shown, for example, in Figure 4c. The decrease factor k during the burn period is calculated from the two currents, IM. and Ib2, measured in tx and t2, respectively, according to equation (3): ^. ^ M) - ^.) (3) In ^ -lnft) During the burning period, the descent factor is greater than about 0.2 in a properly filled sensor, but drops below about -1.0 in an insufficient filling condition. In this way, an insufficient filling condition is detected by comparing the actual descent factor with a lower limit, predetermined during the burning period. According to the alternative embodiments, the two algorithms -equations (2) and (3) - for detecting an insufficient filling condition described in connection with Figures 3a-c and 4a-c are used together to determine whether a filling condition has occurred. insufficient. The descent factor is evaluated first during the burn period as described in connection with Figures 3a-c. If an insufficient filling condition is not determined, the descent factor is then evaluated during the reading period as described in connection with Figures 4a-c. If an insufficient filling condition is not detected during the burn and read periods, an appropriate filling condition is considered to have occurred. While the invention is susceptible to various modifications and alternative forms, the specific embodiments thereof are shown by way of example in the drawings and are described in detail in this document. However, it should be understood that it is not proposed to limit the invention to the particular forms that are described, if not to the contrary, the intention is to cover all the modifications, equivalents and alternatives that are within the spirit and scope of the invention. defined by the appended claims.

Claims (35)

1. An electrochemical sensor for detecting the concentration of an analyte in a test sample of the fluid, the sensor is characterized in that it comprises: a flow path to receive the test sample of the fluid; a first shunt and a second shunt each adapted to be electrically coupled with an electric current detector; a working electrode disposed along the flow path, the working electrode is in electrical communication with the first branch; a counter electrode in electrical communication with the second branch, the counter electrode has a low resistance portion and a high resistance portion, the low resistance portion of the counter electrode is disposed along the flow path downstream of the working electrode, the high strength portion of the counter electrode is disposed along the flow path upstream of the working electrode; a resistor electrically coupled between the high resistance portion of the counter electrode and the second branch; and a reagent disposed on the working electrode, the reagent is adapted to react with the analyte to produce electrons that are transferred to the working electrode; wherein a first current profile is produced in the first and second taps in response to a voltage profile applied to the first and second taps when electrical communication occurs between only the high strength portion of the counter electrode and the working electrode, a Second current profile is produced in the first and second taps in response to substantially the same voltage profile applied to the first and second taps when electrical communication occurs between the low resistance and high resistance portions of the counter electrodes and the working electrode , the first current profile is different from the second current profile.

2. The sensor according to claim 1, characterized in that the second current profile has a descending type shape.

3. The sensor according to claim 1, characterized in that the resistor has a resistance of about 50 kO to about 500 kO.

4. The sensor according to claim 3, characterized in that the resistor has a resistance of about 250 kO to about 350 kO. The sensor according to claim 1, characterized in that the electrical communication occurs between only the high resistance portion of the counter electrode and the working electrode when a volume smaller than the predetermined volume of the fluid sample is received by the flow path . The sensor according to claim 1, characterized in that electrical communication occurs between the low resistance portion of the counter electrode and the working electrode when at least one predetermined volume of the fluid sample is received by the flow path. The sensor according to claim 1, characterized in that the first current profile and the voltage profile have similar shapes when the electrical communication occurs between only the high resistance portion of the counter electrode and the working electrode. 8. The test sensor according to claim 1, characterized in that the first and second taps are electrically coupled to the electric current detector, the detector is adapted to generate an error signal for insufficient filling when a current profile produced in the first and second derivations in response to a tension profile applied to the first and second derivations does not have a descending type shape. The test sensor according to claim 1, characterized in that the test sample of the fluid comprises blood. 10. The test sensor according to claim 1, characterized in that the analyte comprises glucose. 11. The test sensor according to claim 1, characterized in that the reagent comprises glucose oxidase. The test sensor according to claim 1, characterized in that it further comprises a reaction layer that includes the reagent, the reaction layer covers the working electrode and the low resistance portion of the counter electrode. 13. a method for evaluating whether an electrochemical test sensor is properly filled, the test sensor includes a working electrode that is electrically coupled to a first branch and a counter electrode that is electrically coupled to a second branch, the counter electrode includes a portion of low resistance and a portion of high resistance, the method is characterized in that it comprises: applying a tension profile through the first and second derivations; measure the current profile in the first and second branches in response to the applied voltage profile, and generate an error signal for insufficient filling when the measured current profile does not compare favorably with a predetermined profile. The method according to claim 13, characterized in that the predetermined profile is a profile in the form of a descent. The method according to claim 13, characterized in that it comprises forming an electrical communication between only the high resistance portion of the counter electrode and the working electrode when the test sensor is insufficiently filled. The method according to claim 13, characterized in that it comprises forming an electrical communication between the low resistance and high resistance portions of the counter electrode and the working electrode when the test sensor is properly filled. 17. The method according to claim 13, characterized in that the test sample of the fluid comprises blood. 18. The method according to claim 13, characterized in that the analyte comprises glucose. 19. An electrochemical sensor for detecting the concentration of glucose in a blood sample, the sensor is characterized in that it comprises: a flow path to receive the blood sample; a first shunt and a second shunt each adapted to be electrically coupled with an electric current detector; a working electrode disposed along the flow path, the working electrode is in electrical communication with the first branch; a low resistance counter electrode disposed along the flow path downstream of the working electrode, the low resistance counter electrode is in electrical communication with the second branch; a high resistance counter electrode disposed along the flow path upstream of the working electrode, the high strength counter electrode is in electrical communication with the second branch; a resistor electrically coupled between the high resistance counter electrode and the second branch; and a reagent disposed on the working electrode, the reagent is adapted to react with the glucose in the blood sample to produce an electrochemical reaction indicative of the concentration of the glucose in the blood sample, wherein a first current profile is produced in the first and second leads in response to a voltage profile applied to the first and second leads when an electrical communication occurs between only the high resistance counter electrode and the working electrode, a second current profile occurs in the first and second second taps in response to substantially the same voltage profile applied to the first and second taps when an electrical communication occurs between the high resistance and low resistance counter electrodes and the working electrode, the first current profile is different from the second current profile . 20. The sensor according to claim 19, characterized in that the second current profile has a descending type shape. The sensor according to claim 19, characterized in that the resistor has a resistance of about 50 kO to about 500 kO. 22. The sensor according to claim 21, characterized in that the resistor has a resistance of about 250 kO to about 350 kO. 23. The sensor according to claim 19, characterized in that the electrical communication occurs between only the high resistance counter electrode and the working electrode when a volume smaller than that predetermined of the blood sample is received by the flow path. 24. The compliance sensor with claim 19, characterized in that the electrical communication occurs between the low resistance counter electrode and the working electrode when at least one predetermined volume of the blood sample is received by the flow path. 2

5. The sensor according to claim 19, characterized in that the first current profile and the voltage profile have similar shapes when an electrical communication occurs between only the high resistance counter electrode and the working electrode. 2

6. The test sensor according to claim 19, characterized in that the first and second taps are electrically coupled with the electric current detector, the detector is adapted to generate an error signal by insufficient filling when a current profile produced in the first and second taps in response to a voltage profile applied to the first and second taps do not have a descent type shape. 2

7. The test sensor according to claim 19, characterized in that the reagent comprises glucose oxidase. 2

8. The test sensor according to claim 19, characterized in that the electrochemical reaction produces electrons that are transferred to the working electrode. 2

9. The test sensor according to claim 19, characterized in that it also comprises a reaction layer that includes the reagent, the reaction layer covers the working electrode and the low resistance counter electrode. 30. A method for evaluating whether an electrochemical sensor is properly filled, the test sensor includes a working electrode coupled to a first branch and a counter electrode coupled to a second branch, the counter electrode includes a portion of high strength and a portion of low resistance, the test sensor includes a reagent disposed on the working electrode which is adapted to react with the glucose in a blood sample to produce an electrochemical reaction which is indicative of the concentration of the glucose in the blood sample, the method is characterized because it comprises: collecting a blood sample; apply a tension profile through the first and second leads; measure the current profile in the first and second branches in response to the applied voltage profile; and generating an error signal for insufficient filling when the measured current profile is not in a predetermined manner. 31. The method according to claim 30, characterized in that the predetermined form is a descent shape. 32. The method according to claim 30, characterized in that it comprises forming an electrical communication between only the high resistance portion of the counter electrode and the working electrode when the test sensor is insufficiently filled. 33. The method according to claim 30, characterized in that it comprises forming an electrical communication between the low resistance portion of the counter electrode and the working electrode when the test sensor is properly filled. 34. The method according to claim 30, characterized in that the reagent comprises glucose oxidase. 35. The method according to claim 30, characterized in that the reagent is also disposed on the low resistance portion of the counter electrode.