Methods and systems for observing sensor parameters
The invention disclosed herein provides methods and materials for observing the state of a sensor, for example those used by diabetic patients to monitor blood glucose levels. Typically a voltage such as a voltage pulse is applied to the sensor in order to solicit a current response from which for example, factors such as impedance values can be derived. Such values can then be used as indicators of a sensor's state, for example the state of sensor hydration, sensor noise, sensor offset, sensor drift or the like.
1. A method of observing a state of a sensor having a plurality of electrodes, the method comprising:
(a) applying a voltage to the sensor;
(b) measuring a stable-state current (Ibase) produced in response to the voltage applied to the sensor;
(c) applying the stable-state current measured in (b) to the sensor;
(d) measuring sensor voltage during the application of the stable-state current;
(e) changing applied sensor current to a second current comprising Ibase+deltaI, wherein deltaI comprises the difference between the Ibase and the second current;
(f) measuring sensor voltage during the application of the second current;
(g) observing a first voltage step between the voltage measured in (d) and the voltage measured in (f) that results from electrical resistance (R) in the sensor; so that the state of the sensor is observed;
(h) changing applied sensor current to a third current comprising Ibase+deltaII, wherein deltaII comprises the difference between the Ibase and the third current;
(i) measuring sensor voltage during the application of the third current;
(j) observing a second voltage step between the voltage measured in (d) and/or (f) and/or (i) that results from electrical resistance (R) in the sensor;
(k) calculating a voltage slope (dV/dt) using multiple voltage step measurements, wherein the voltage slope so calculated is correlated to a change in sensor capacitance (C) that results from the different currents applied to the sensor.
2. The method of claim 1 , wherein: calculating resistance (R) in the sensor using a formula R=deltaV/deltaI; and/or calculating capacitance (C) in the sensor using a formula C=deltaI/(dV/dt).
3. The method of claim 1 , wherein the sensor is an electrochemical glucose sensor implanted in vivo and the observation of the state of the sensor provides information on:
sensor hydration;
sensor noise;
sensor offset; or
sensor drift.
4. The method of claim 1 , further comprising: performing the method on a plurality of sensors made by differing manufacturing processes.
5. A method of observing a state of a sensor having a plurality of electrodes and a stable-state voltage (Vbase), the method comprising:
(a) measuring a stable-state current (Ibase) produced in response to the stable-state voltage of the sensor;
(b) forcing a current using the stable-state current (Ibase) measured in (a);
(c) changing the forced current to a second current comprising Ibase+deltaI, wherein deltaI comprises the difference between the Ibase and the second current;
(d) measuring an initially resulting voltage step (deltaV) due to resistance (R) in the sensor;
(e) changing the forced current to a third current comprising Ibase+deltaII, wherein deltaII comprises the difference between the Ibase and the third current;
(f) measuring sensor voltage during the application of the third current;
(g) observing a second voltage step between the voltage measured in (d) and/or (f) that results from electrical resistance (R) in the sensor;
(h) calculating a voltage slope (dV/dt) using the voltage step measurement in (d), wherein the voltage slope so calculated is related to a charging of a sensor capacitance (C) by the deltaI applied to the sensor;
(i) calculating resistance (R) in the sensor using a formula R=deltaV/deltaI and calculating capacitance (C) in the sensor using a formula C=deltaI/(dV/dt); so that the state of the sensor is observed.
6. The method of claim 5 , wherein the sensor is an electrochemical glucose sensor implanted in vivo and the observation of the state of the sensor provides information on:
sensor hydration;
sensor noise;
sensor offset; or
sensor drift.
7. The method of claim 5 , further comprising: performing the method on a plurality of sensors made by differing manufacturing processes.
8. The method of claim 5 , wherein step (b) comprises keeping the sensor at the same voltage while forcing the current to the sensor.
9. The method of claim 5 , wherein step (b) comprises allowing the sensor to stabilize at a slightly different voltage while forcing the current to the sensor.