Electrochemical-sensor design

Abstract

An electrochemical test sensor adapted to assist in determining the concentration of analyte in a fluid sample is disclosed. The sensor comprises a base that assists in forming an opening for introducing the fluid sample, a working electrode being coupled to the base, and a counter electrode being coupled to the base, the counter electrode and the working electrode being adapted to be in electrical communication with a detector of electrical current, and a sub-element being coupled to the base. A major portion of the counter electrode is located downstream relative to the opening and at least a portion of the working electrode. The sub-element is located upstream relative to the working electrode such that when electrical communication occurs between only the sub-element and the working electrode there is insufficient flow of electrical current through the detector to determine the concentration of the analyte in the fluid sample.

Description

This I_r10

In Step C, the values of these two parameters are checked against their tolerance limits to determine if a short fill occurred. The tolerance limits are not constant. They change as glucose level changes. The tolerance-limit checking is described as Conditions 1 and 2 below. The criteria for a short fill are either Condition 1 or Condition 2 is true.

Condition 1 (Decay factor checking):
if |k−(a_k1+b_k1·G)|>w_k is true when G≦d_k1, or
if |k−(a_k2+b_k2·G)|>w_k is true when d_k1<G≦d_k2, or
if |k−(a_k3+b_k3·G)|>w_k is true when G>d_k2   Eq. 3
where a_k1, a_k2, a_k3, b_k1, b_k2, b_k3, w_k, d_k1, d_k2 and d_k3 are predetermined constants, G is the glucose measurement.
Condition 2 (R/B ratio checking):
if |R/B−(a_c1+b_c1·G)|>w_c is true when G≦d_c, or
if |R/B−(a_c2+b_c2·G)|>w_c is true when G>d_c   Eq. 4
where a_c1, a_c2, b_c1, b_c2, w_c, and d_c are predetermined constants, G is the glucose measurement.

The constants a_k's, b_k's, d_k's and w_k in Eq. 3 are predetermined experimentally:

• • Tests a large number of sensors at various glucose levels, G.
  • Calculates the decay factor, k, of each sensor from their I_b5 and I_b10 currents.
  • Plots all the data points in a k vs. G chart.
  • Fits a 3-piece piecewise-linear line to the data points in the k vs. G chart. These three pieces are a_k1+b_k1×G for G≦d_k1; a_k2+b_k2×G for G>d_k1 and ≦d_k2; and a_k3+b_k3×G for G>d_k2
  • Add a tolerance width of ±w_k to the three lines so that the band between the −w_k and +w_k is wide enough to enclose all the normal data points in the chart.
    The constants a_c's, b_c's, d_c and w_c in Eq. 4 are also predetermined experimentally in the same way, on a R/B vs. G chart.
    A sample calculation is as follows:
• Step A—Make three current measurements of a sensor:
  • I_b10=505.1 nA, I_r5=656.5 nA, and I_r10=561.8 nA.
• Step B—Determine the value of the decay factor k and R/B ratio:
  • The decay factor and read-to-burn ratio were calculated from the current measurements:
  • Decay factor

$k=\frac{\ln \left(I_{r5}\right)-\ln \left(I_{r10}\right)}{\ln \left(10\right)-\ln \left(5\right)}=\frac{\ln \left(656.5\right)-\ln \left(561.8\right)}{\ln \left(10\right)-\ln \left(5\right)}=0.225$

• • Read-to-Burn ratio
    R/B=I_r10/I_b10=561.8/505.1=1.11
• Step C—Check against the tolerance limits:
• The constants used in this example were:
• a_k1=0.36, b_k1=−0.0002 dL/mg, w_k=0.13, and d_k1=100 mg/dL
  The glucose reading from the sensor system is 22.9 mg/dL.
  Condition 1 was true because of the first line in Eq. 3 was true.
  if |k−(a_k1+b_k1·G)|>w_k is true when G≦d_k1
  
  |0.225−(0.36−0.0002·22.9)|=0.1304>0.13 is true when G=22.9≦d_k1=100
  No further check on Condition 2 was needed in this example, because Condition 1 was already true.
  Therefore, this sensor was determined as a short fill.

Claims

1. An electrochemical test sensor adapted to assist in determining the concentration of an analyte in a fluid test sample, the sensor comprising:

a base that assists in forming an opening for introducing the fluid test sample;

a working electrode with an exposed portion not insulated from sample by dielectric material being coupled to the base;

a counter electrode being coupled to the base, the counter electrode and the working electrode being adapted to be in electrical communication with a detector of electrical current; and

a sub-element being coupled to the base, a major portion of the counter electrode being located downstream relative to the opening and at least a the exposed portion of the working electrode, the sub-element being located upstream relative to the working electrode such that when electrical communication occurs between only the sub-element and the working electrode there is insufficient flow of electrical current through the detector to determine the concentration of the analyte in the fluid test sample a first current profile is produced in response to electrical communication between the sub-element and the working electrode, and a second current profile is produced in response to electrical communication between the sub-element, the portion of the counter electrode, and the working electrode, the first current profile having a first decay factor and the second current profile having a second decay factor, the first decay factor being distinguishable from the second decay factor.

2. The sensor of claim 1 further comprising a cover adapted to be coupled to the base to form a capillary space, the capillary space having an opening for introducing the fluid test sample therein, the capillary space forming a flow path for the fluid test sample, the working electrode and the counter electrode being situated in the flow path.

3. The sensor of claim 2, wherein the cover includes an air vent.

4. The sensor of claim 1 further comprising a reaction layer located on the surface of at least the working electrode, the reaction layer comprising an enzyme adapted to react with the analyte to produce electrons, the electrons being adapted to be transferred to the working electrode.

5. The sensor of claim 1, wherein the area of the sub-element is less than 10% of the area of the working electrode.

6. The sensor of claim 1, wherein the sub-element and the counter electrode are physically connected.

7. A method of determining whether a sufficient quantity of evaluating a fluid test sample that has been introduced to an electrochemical test sensor, the method comprising the acts of:

providing the electrochemical test sensor adapted to assist in determining the concentration of an analyte in a fluid test sample, the sensor comprising a base that assists in forming an opening for introducing the fluid test sample, a working electrode with an exposed portion not insulated from sample by dielectric material being coupled to the base, a counter electrode being coupled to the base, and a sub-element being coupled to the base, a major portion of the counter electrode being located downstream relative to the opening and at least a the exposed portion of the working electrode, the sub-element being located upstream relative to the working electrode;

introducing the fluid test sample to the test sensor;

measuring current at a plurality of time periods to obtain a plurality of current measurements; and

determining whether a sufficient quantity of the fluid test sample has been introduced to the electrochemical test sensor and, if not, notifying a user that an insufficient quantity of the fluid test sample has been introduced a decay factor from the plurality of current measurements.

8. The method of claim 7 further comprising, prior to the act of determining whether a sufficient quantity of the fluid test sample has been introduced to the electrochemical test sensor, measuring current at a plurality of time periods to obtain a plurality of current measurements.

9. The method of claim 8 7 further comprising providing a current to initiate a test sequence prior to the act of measuring current at a plurality of time periods.

10. The method of claim 8 7 further comprising, prior to after the act of measuring current at a plurality of time periods, the acts of:

converting the plurality of current measurements into a plurality of error checking parameters; and

comparing the plurality of error checking parameters decay factor with their a corresponding tolerance ranges range.

11. The method of claim 10 wherein further comprising the act of notifying a user that an insufficient quantity of the fluid test sample has been introduced occurs when one or more of the plurality of error checking parameters are the decay factor is outside of their corresponding tolerance ranges the corresponding tolerance range.

12. The method of claim 11, wherein one or more of the plurality of error checking parameters the decay factor is outside of its the corresponding tolerance range when the fluid test sample fails to contact a major the portion of the counter electrode located downstream relative to the opening and the exposed portion of the working electrode.

13. The method of claim 10, wherein the plurality of error checking parameters is derived from the plurality of current measurements.

14. The method of claim 7, wherein the act of applying the fluid test sample to the sensor electrically connects the sub-element to the working electrode.

15. The method of claim 7, wherein the act of notifying a user that an insufficient quantity of the fluid test sample has been introduced includes providing an error signal.

16. The method of claim 7, wherein the test sensor further comprises a cover adapted to be coupled to the base to form a capillary space, the capillary space having an opening for introducing the fluid test sample therein, the capillary space forming a flow path for the fluid test sample, the working electrode and the counter electrode being situated in the flow path.

17. The method of claim 16, wherein the cover includes an air vent.

18. The method of claim 7, wherein the test sensor further comprises a reaction layer located on the surface of at least the working electrode, the reaction layer comprising an enzyme adapted to react with the analyte to produce electrons, the electrons being adapted to be transferred to the working electrode, the reaction layer being removed from contact with the sub-element.

19. The method of claim 7, wherein the area of the sub-element is less than 10% of the area of the working electrode.

20. The method of claim 7, wherein the sub-element and the counter electrode are physically connected.

21. The method of claim 18, wherein the area of the sub-element is less than 10% of the area of the working electrode.

22. The electrochemical test sensor of claim 4, wherein the reaction layer is removed from contact with the sub-element.

23. An electrochemical sensor for detecting the concentration of an analyte in a fluid test sample, comprising:

a base having a counter electrode and a working electrode, the working electrode having an exposed portion for exposure to the fluid test sample, the counter electrode having an exposed-upstream portion and an exposed-downstream portion for exposure to the fluid test sample;

a reaction layer on a surface of the the working electrode including an enzyme which reacts with the analyte to produce electrons that are transferred to the working electrode; and

a cover adapted to be coupled to the base to assist in forming a capillary space, the capillary space having an opening for introducing the fluid test sample therein, the capillary space forming a flow path for the fluid test sample, the counter electrode and the working electrode being situated in the capillary space so that the exposed-downstream portion of the counter electrode is located downstream of the opening from the exposed portion of the working electrode and the exposed-upstream portion of the counter electrode is upstream of the exposed portion of the working electrode, the area of the exposed-upstream portion of the counter electrode being less than about 10% than that of the exposed portion of the working electrode.

24. The sensor of claim 23, wherein the reaction layer is not in contact with the exposed-upstream portion of the counter electrode.

25. The sensor of claim 23, wherein the exposed-downstream portion of the counter electrode and the exposed-upstream portion of the counter electrode are physically connected.

26. An electrochemical test sensor adapted to assist in determining the concentration of an analyte in a fluid test sample, the sensor comprising:

a base that provides a flow path for the fluid test sample;

a cover adapted to be coupled to the base to form a capillary space, the capillary space having an opening for introducing the fluid test sample therein;

a sub-element having an exposed portion situated in the flow path;

a working electrode having an exposed portion situated in the flow path; and

a counter electrode having an exposed portion situated in the flow path, the exposed portion of the sub-element being located upstream relative to the opening of the exposed portion of the counter electrode such that electrical communication occurs between only the sub-element and the working electrode before the working electrode is in electrical communication with the counter electrode as the fluid sample flows along the flow path,

wherein a first current profile is produced in response to the electrical communication between only the sub-element and the working electrode and a second current profile is produced in response to the electrical communication between the working electrode and the counter electrode, the first current profile being indicative of a short fill.

27. The sensor of claim 26, further comprising a reaction layer is located on the surface of at least the working electrode, the reaction layer comprising an enzyme adapted to react with the analyte to produce electrons, the electrons being adapted to be transferred to the working electrode.

28. The sensor of claim 27, wherein the reaction layer is removed from contact with the sub-element.

29. The sensor of claim 26, wherein the sub-element is the area of the sub-element is less than 10% of the area of the working electrode.

30. The sensor of claim 26, wherein the sub-element and the counter electrode are physically connected.

31. A method of evaluating a fluid test sample that has been introduced to an electrochemical test sensor, the method comprising the acts of:

providing the electrochemical test sensor adapted to assist in determining the concentration of an analyte in a fluid test sample, the sensor including a base that provides a flow path for the fluid test sample, a cover adapted to be coupled to the base to form a capillary space, the capillary space having an opening for introducing the fluid test sample therein, a sub-element having an exposed portion situated in the flow path, a working electrode having an exposed portion situated in the flow path, and a counter electrode having an exposed portion situated in the flow path, the exposed portion of the sub-element being located upstream relative to the opening of the exposed portion of the counter electrode such that electrical communication occurs between only the sub-element and the working electrode before the working electrode is in electrical communication with the counter electrode as the fluid sample flows along the flow path,

introducing the fluid test sample to the test sensor; and

determining whether a sufficient quantity of the fluid test sample has been introduced to the electrochemical test sensor and, if not, notifying a user that an insufficient quantity of the fluid test sample has been introduced.

32. The method of claim 31 further comprising:

measuring a plurality of current measurements at a at a plurality of time periods;

converting the plurality of current measurements into a plurality of error checking parameters; and

comparing the plurality of error checking parameters with corresponding tolerance ranges.

33. The sensor of claim 31, further comprising a reaction layer is located on the surface of at least the working electrode, the reaction layer comprising an enzyme adapted to react with the analyte to produce electrons, the electrons being adapted to be transferred to the working electrode, the reaction layer being removed from contact with the sub-element.

34. The sensor of claim 31, wherein the sub-element is the area of the sub-element is less than 10% of the area of the working electrode.

35. The sensor of claim 31, wherein the sub-element and the counter electrode are physically connected.