Application of electrochemical impedance spectroscopy in sensor systems, devices, and related methods
A diagnostic Electrochemical Impedance Spectroscopy (EIS) procedure is applied to measure values of impedance-related parameters for one or more sensing electrodes. The parameters may include real impedance, imaginary impedance, impedance magnitude, and/or phase angle. The measured values of the impedance-related parameters are then used in performing sensor diagnostics, calculating a highly-reliable fused sensor glucose value based on signals from a plurality of redundant sensing electrodes, calibrating sensors, detecting interferents within close proximity of one or more sensing electrodes, and testing surface area characteristics of electroplated electrodes. Advantageously, impedance-related parameters can be defined that are substantially glucose-independent over specific ranges of frequencies. An Application Specific Integrated Circuit (ASIC) enables implementation of the EIS-based diagnostics, fusion algorithms, and other processes based on measurement of EIS-based parameters.
1. A method of performing sensor diagnostics on a subcutaneous or implanted sensor of a system, the subcutaneous or implanted sensor including at least one working electrode and the system including a microprocessor that is operatively coupled to the subcutaneous or implanted sensor and performs the sensor diagnostics, the method comprising:
periodically performing an electrochemical impedance spectroscopy (EIS) procedure to generate multiple sets of impedance-related data for the at least one working electrode, the impedance-related data comprising impedance magnitudes and phase angles;
calculating a slope of the impedance magnitudes across a frequency band of the multiple sets of impedance related data;
accessing a phase angle of the phase angles of the multiple sets of impedance related data;
detecting a pullout of the subcutaneous or implanted sensor based on at least one of: the slope of the impedance magnitudes or the phase angle; and
notifying an alert based on detecting the pullout of the subcutaneous or implanted sensor.
2. The method of claim 1 , further comprising:
performing a plurality of tests on the multiple sets of impedance based on a pullout of the subcutaneous or implanted sensor not being detected; and
calculating respective values of a plurality of parameters based on the multiple sets of impedance-related data, the plurality of parameters including impedance, current, phase angle, and Nyquist slope, wherein the plurality of tests are performed based on the respective values.
3. The method of claim 2 , wherein the plurality of tests include a test, and wherein the test fails based on at last one of: a percent increase in real impedance at 1 kHz in a predetermined period is greater than 30%, or a change in the phase angle at 0.1 Hz in the predetermined period is greater than 10°.
4. The method of claim 2 , wherein the plurality of tests include a test, and wherein the test fails based on: a percentage increase in impedance magnitude at 1 kHz in a predetermined period is greater than 30% and a percentage decrease in current at 1 kHz in the predetermined period is greater than 30%.
5. The method of claim 2 , wherein the plurality of tests include a test, and wherein the test fails when a percentage increase in current at 1 KHz is less than or equal to 30% and a percentage decrease in impedance magnitude at 1 kHz in a predetermined period is less than or equal to than 30%.
6. The method of claim 2 , wherein the plurality of tests include a test, and wherein the test fails when the Nyquist slope at a low frequency band is globally decreasing.
7. The method of claim 6 , wherein the low frequency band is from 0.1 Hz to 1 Hz.
8. The method of claim 2 , wherein the plurality of tests include a test, and wherein the test fails when the phase angle at 0.1 Hz is decreasing over time.
9. The method of claim 2 , wherein the plurality of tests include a test, and wherein the test fails when at least two of the following are met:
current is less than 10 nA;
imaginary impedance at 1 kHz is less than −1500 Ohm; and
the phase angle at 1 KHz is less than −15°.
10. The method of claim 2 , wherein the plurality of tests include a test, and wherein the test fails when a percent increase in a ratio of the impedance to the current at 0.1 Hz in a predetermined period is greater than 30%.
11. A system for performing sensor diagnostics, the system comprising:
a subcutaneous or implanted sensor including at least one working electrode; and
a microprocessor operatively coupled to the subcutaneous or implanted sensor and configured to:
periodically perform an electrochemical impedance spectroscopy (EIS) procedure to generate multiple sets of impedance-related data for the at least one working electrode, the impedance-related data comprising impedance magnitudes and phase angles;
calculate a slope of the impedance magnitudes across a frequency band of the multiple sets of impedance related data;
accessing a phase angle of the phase angles of the multiple sets of impedance related data;
detect a pullout of the subcutaneous or implanted sensor based on at least one of: the slope of the impedance magnitudes or the phase angle; and
notify an alert based on detecting the pullout of the subcutaneous or implanted sensor.
12. The system of claim 11 , wherein the microprocessor is further configured to:
perform a plurality of tests on the multiple sets of impedance based on a pullout of the subcutaneous or implanted sensor not being detected; and
calculate respective values of a plurality of parameters based on the multiple sets of impedance-related data, the plurality of parameters including impedance, current, phase angle, and Nyquist slope, wherein the microprocessor performs the plurality of tests based on the respective values.
13. The system of claim 12 , wherein the microprocessor performs a test, and wherein the test fails based on at least one of: a percent increase in real impedance at 1 kHz in a predetermined period is greater than 30%, or a change in the phase angle at 0.1 Hz in the predetermined period is greater than 10°.
14. The system of claim 12 , wherein the microprocessor performs a test, and wherein the test fails based on: a percentage increase in impedance magnitude at 1 kHz in a predetermined period is greater than 30% and a percentage decrease in current at 1 kHz in the predetermined period is greater than 30%.
15. The system of claim 12 , wherein the microprocessor performs a test, and wherein the test fails when a percentage increase in current at 1 kHz is less than or equal to 30% and a percentage decrease in impedance magnitude at 1 KHz in a predetermined period is less than or equal to than 30%.
16. The system of claim 12 , wherein the microprocessor performs a test, and wherein the test fails when the Nyquist slope at a low frequency band is globally decreasing.
17. The system of claim 16 , wherein the low frequency band is from 0.1 Hz to 1 Hz.
18. The system of claim 12 , wherein the microprocessor performs a test, and wherein the test fails when the phase angle at 0.1 Hz is decreasing over time.
19. The system of claim 12 , wherein the microprocessor performs a test, and
wherein the test fails when at least two of the following are met:
current is less than 10 nA;
imaginary impedance at 1 kHz is less than −1500 Ohm; and
the phase angle at 1 KHz is less than −15°.
20. The system of claim 12 , wherein the microprocessor performs a test, and wherein the test fails when a percent increase in a ratio of the impedance to the current at 0.1 Hz in a predetermined period is greater than 30%.