Method and electronics unit for detecting in-vivo properties of a biosensor
A method for detecting in-vivo properties of a biosensor. In the inventive method, a sensitivity-to-admittance relation is provided and a raw current in the biosensor is measured. An in-vivo current response is also measured at first and second operating points. A time constant τ is determined by the electrical capacitance C of the working electrode and the electrical resistance R M of the membrane by τ=R M ·C. The first and second operating points are selected below and above τ, respectively. An analyte value in a sample of a body fluid is determined by using the raw current and compensating sensitivity drift in the biosensor, which in turn is compensated by using the measured value for the raw current and a corrected value for the sensitivity. The failsafe operation of the biosensor is monitored by using the in-vivo current response measured at the first and second operating points.
1. A method for detecting in-vivo properties of a biosensor adapted for determining a value of an analyte in a body fluid sample, the biosensor having a working electrode covered by a membrane and an enzyme for providing a reaction with the analyte, the method comprising:
a) providing a sensitivity-to-admittance relation of the biosensor;
b) measuring a raw current in the biosensor;
c) measuring an in-vivo current response indicative of the in-vivo admittance of the biosensor, wherein the in-vivo current response is measured at first and second operating points and a time constant τ is determined by the electrical capacitance C of the working electrode and the electrical resistance R M of the membrane by τ=R M ·C, wherein the first operating point is selected below τ and the second operating point is selected above τ;
d) determining an analyte value in a sample of a body fluid by using the raw current and compensating an in-vivo sensitivity drift in the biosensor, wherein the in-vivo sensitivity drift is compensated by using the measured value for the raw current and a corrected value for the sensitivity, whereby the sensitivity is determined by using the sensitivity-to-admittance relation from step a); and
e) monitoring a failsafe operation of the biosensor by using the in-vivo current response measured at the first and second operating points.
2. The method according to claim 1 , wherein the first operating point is selected for providing a first characteristic value related to the electrical resistance of the membrane and wherein the second operating point is selected for providing a second characteristic value related to the electrical capacitance of the working electrode.
3. The method according to claim 2 , wherein the in-vivo current response of the biosensor is determined by applying a potential step to an electrical potential difference at the biosensor, wherein the potential step comprises applying an additional electrical potential between the working electrode and a reference electrode of the biosensor over a time interval.
4. The method according to claim 1 , wherein the second operating point is selected above 3τ.
5. The method according to claim 4 , wherein the second operating point is selected above 5τ.
6. The method according to claim 1 , wherein the failsafe operation of the biosensor is monitored by using at least one of (i) the sensitivity determined from the sensitivity-to-admittance relation of the biosensor, (ii) the electrical capacitance C of the working electrode, and (iii) the electrical resistance R M of the membrane.
7. The method according to claim 6 , wherein a structural modification of the biosensor is determined by monitoring alterations of at least two of (i) the sensitivity determined from the sensitivity-to-admittance relation, (ii) the electrical capacitance C of the working electrode, and (iii) the electrical resistance R M of the membrane.
8. The method according to claim 1 , wherein the sensitivity-to-admittance relation is obtained during a calibration of the biosensor, wherein the calibration of the biosensor is selected from at least one of a multiple calibration, an initial calibration, and a factory calibration.
9. The method according to claim 1 , wherein the biosensor is a fully or partially implantable biosensor for continuously monitoring the analyte.
10. The method according to claim 1 , wherein the analyte comprises glucose and the analyte value is determined by using glucose oxidase or glucose dehydrogenase as the enzyme.
11. An electronics unit for detecting in-vivo properties of a biosensor by performing a method according to claim 1 .
12. The electronics unit according to claim 11 , wherein the electronics unit is further adapted for:
applying an electrical potential between the working electrode and a reference electrode of the biosensor; and
measuring the raw current generated thereby, wherein the electronics unit comprises a direct current measuring unit configured for measuring the raw current.
13. A system for operating a biosensor for electrochemically detecting an analyte value in a sample of a body fluid, the system comprising a biosensor operable by performing a method according to claim 1 .