Use of electrochemical impedance spectroscopy (EIS) in gross failure analysis
Electrochemical Impedance Spectroscopy (EIS) is used in conjunction with continuous glucose monitors and continuous glucose monitoring (CGM) to enable in-vivo sensor calibration, gross (sensor) failure analysis, and intelligent sensor diagnostics and fault detection. An equivalent circuit model is defined, and circuit elements are used to characterize sensor behavior.
1. A method of performing diagnostics on a subcutaneous or implanted glucose sensor having at least one working electrode, comprising:
defining a vector containing values associated with one or more electrochemical impedance spectroscopy (EIS)-based parameters and values associated with one or more non-EIS-based parameters;
defining a combined threshold value for a combination of said one or more EIS-based parameters and said one or more non-EIS-based parameters;
performing, by a microprocessor, a first EIS procedure and a second EIS procedure to generate a first set of data and a second set of data, respectively, for said values associated with the one or more EIS-based parameters;
updating, by said microprocessor, the vector and said first and second sets of data; and
monitoring, by said microprocessor, the values contained in the vector to determine whether the glucose sensor has lost sensitivity based on a combined vector value of said combination of said one or more EIS-based and non-EIS-based parameters crossing said combined threshold value.
2. The method of claim 1 , wherein said one or more EIS-based parameters include double layer capacitance (Cdl).
3. The method of claim 1 , wherein said one or more non-EIS-based parameters include a period of time during which Isig is low.
4. The method of claim 1 , further comprising periodically updating the values contained in the vector for each of said one or more non-EIS-based parameters.
5. The method of claim 4 , wherein said determination of sensitivity loss is made based on comparison of one of the values contained in the vector to a corresponding threshold value.
6. The method of claim 4 , wherein the glucose sensor is determined to have lost sensitivity when a plurality of the values contained in the vector have crossed their corresponding threshold values.
7. The method of claim 6 , wherein said plurality of the values contained in the vector include values for at least one EIS-based parameter and at least one non-EIS-based parameter.
8. The method of claim 1 , wherein each of said threshold values is an absolute threshold value.
9. The method of claim 1 , wherein each of said threshold values is a relative threshold value.
10. The method of claim 1 , further comprising applying a weighting factor to each of the values contained in the vector to generate weighted vector values, and determining sensitivity loss based on said weighted vector values.
11. The method of claim 10 , wherein said weighting factor is different for each of the one or more EIS-based parameters and for each of the one or more non-EIS-based parameters.
12. The method of claim 11 , wherein, for each of said one or more EIS-based parameters and each of said one or more non-EIS-based parameters, a respective weighting factor is calculated based on each parameter's said respective threshold value.
13. The method of claim 1 , wherein said combined threshold value is an absolute threshold value.
14. The method of claim 1 , wherein said combined threshold value is a relative threshold value.
15. The method of claim 1 , wherein said one or more EIS-based parameters include membrane resistance (Rmem).
16. The method of claim 1 , wherein said one or more EIS-based parameters include membrane capacitance (Cmem).
17. The method of claim 1 , wherein said one or more non-EIS-based parameters include variability in sensor current (Isig).
18. The method of claim 1 , wherein said one or more non-EIS-based parameters include rate of change of Isig.
19. The method of claim 1 , further comprising terminating said glucose sensor by the microprocessor based on the determination that the glucose sensor has lost sensitivity.
20. A method of performing diagnostics on a subcutaneous or implanted glucose sensor having at least one working electrode, comprising:
defining a vector containing values associated with one or more electrochemical impedance spectroscopy (EIS)-based parameters and values associated with one or more non-EIS-based parameters;
defining a combined threshold value for a combination of said one or more EIS-based parameters and said one or more non-EIS-based parameters;
performing, by a microprocessor, a first EIS procedure and a second EIS procedure to generate a first set of data and a second set of data, respectively, for said values associated with the one or more EIS-based parameters;
updating, by said microprocessor, the vector and said first and second sets of data; monitoring, by said microprocessor, the values contained in the vector to determine whether the glucose sensor has lost sensitivity based on a combined vector value of said combination of said one or more EIS-based and non-EIS-based parameters crossing said combined threshold value; and
terminating said glucose sensor by the microprocessor based on the determination that the glucose sensor has lost sensitivity.