Methods and systems for observing sensor parameters

Abstract

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 voltage to the sensor;   (b) observing a peak instantaneous electrical current of the sensor; and   (c) observing a total current of the sensor over a period of time for a predetermined frequency;   
       so that the state of the sensor is observed. 
     
     
         2 . The method of  claim 1 , wherein observations of the peak instantaneous electrical current and/or the total current in the sensor over a period of time for a predetermined frequency are used to estimate sensor impedance magnitude. 
     
     
         3 . The method of  claim 1 , wherein observations of the peak instantaneous electrical current and/or the total current in the sensor over a period of time for a predetermined frequency are used to estimate sensor capacitance. 
     
     
         4 . The method of  claim 1 , wherein observations on the state of the sensor provide information on:
 sensor hydration;   sensor noise;   sensor offset; or   sensor drift.   
     
     
         5 . The method of  claim 3 , wherein an estimate of sensor capacitance comprises a voltage step analysis using a formula: 
       
         
           
             
               C 
               ≈ 
               
                 
                   
                     ∑ 
                     
                       n 
                       = 
                       1 
                     
                     
                       n 
                       ≤ 
                       dt 
                     
                   
                    
                   
                     
                        
                       I 
                     
                     * 
                     
                       t 
                       samp 
                     
                   
                 
                 
                    
                   V 
                 
               
             
           
         
       
       wherein:
 C comprises capacitance; 
 V comprises voltage; 
 dV comprises a controlled voltage step; 
 dt comprises a length of time for analysis; 
 t samp  comprises a length of time between samples; and 
 dI comprises a change in current. 
 
     
     
         6 . The method of  claim 1 , wherein step (a) comprises applying a voltage pulse to the sensor. 
     
     
         7 . The method of  claim 6 , wherein the method comprises:
 observing the maximum current value (counts/second) during the initial 2 seconds in response to a voltage pulse applied to the sensor, and   comparing the maximum current value to a predetermined test value.   
     
     
         8 . The method of  claim 1 , wherein step (a) comprises applying a plurality of voltages to the sensor. 
     
     
         9 . The method of  claim 1 , wherein step (c) comprises observing current in the sensor over multiple periods of time. 
     
     
         10 . The method of  claim 1 , wherein step (c) comprises observing current in the sensor over multiple frequencies. 
     
     
         11 . The method of  claim 1 , further comprising:
 performing the method on a plurality of sensors; and   comparing the information so obtained on the state of the plurality of sensors.   
     
     
         12 . The method of  claim 1 , wherein the sensor is a glucose sensor. 
     
     
         13 . The method of  claim 12 , wherein the glucose sensor comprises:
 a base layer;   at least three working electrodes disposed on the base layer;   a glucose oxidase layer disposed upon the working electrodes;   an analyte modulating layer disposed on the glucose oxidase layer, wherein the analyte modulating layer comprises a hydrogel composition; and   an adhesion promoting layer disposed between the glucose oxidase layer and the analyte modulating layer.   
     
     
         14 . A sensor system, comprising:
 an implantable sensor, the sensor including a plurality of electrodes;   a sensor electronics device, the sensor electronics device capable of being operably connected to the sensor, and the sensor electronics device including:
 a connection detection device to determine if the sensor electronics device is connected to the sensor and to transmit a connection signal; 
 a power source to supply a regulated voltage; 
 a microprocessor; and 
 a computer-readable program code having instructions, which when executed cause the microprocessor to: 
   (a) apply a voltage to the sensor;   (b) record data on a peak instantaneous electrical current of the sensor in response to the applied voltage; and   (c) record data on a total current of the sensor over a period of time for a predetermined frequency in response to the applied voltage.   
     
     
         15 . The system of  claim 14 , further comprising a monitor for displaying the recorded data from steps (b) and/or (c), wherein the data displayed on the monitor provides information on:
 sensor hydration;   sensor noise;   sensor offset; or   sensor drift.   
     
     
         16 . The system of  claim 15 , wherein recorded data from steps (b) and/or (c) are used to estimate sensor capacitance using a formula: 
       
         
           
             
               C 
               ≈ 
               
                 
                   
                     ∑ 
                     
                       n 
                       = 
                       1 
                     
                     
                       n 
                       ≤ 
                       dt 
                     
                   
                    
                   
                     
                        
                       I 
                     
                     * 
                     
                       t 
                       samp 
                     
                   
                 
                 
                    
                   V 
                 
               
             
           
         
       
     
     
         17 . The system of  claim 14 , wherein:
 step (a) comprises applying a plurality of voltages to the sensor; or   step (a) comprises: applying a voltage pulse to the sensor; and   step (c) comprises recording data on a current in the sensor over multiple periods of time; or   step (c) comprises recording data on a current in the sensor over multiple frequencies.   
     
     
         18 . The system of  claim 14 , wherein the implantable sensor is a glucose sensor comprising:
 a base layer;   at least three working electrodes disposed on the base layer;   a glucose oxidase layer disposed upon the working electrodes;   an analyte modulating layer disposed on the glucose oxidase layer; and   an adhesion promoting layer disposed between the glucose oxidase layer and the analyte modulating layer.   
     
     
         19 . The system of  claim 14 , wherein the sensor is implantable in tissue selected from the group consisting of subcutaneous, dermal, sub-dermal, intra-peritoneal, and peritoneal tissue. 
     
     
         20 . A program code storage device, comprising:
 a computer-readable medium;   a computer-readable program code, stored on the computer-readable medium, the computer-readable program code having instructions, which when executed cause a controller to:   initiate a series of voltage pulses to be applied to a sensor comprising a plurality of electrodes; and receive a signal from a detection circuit, the signal indicating:   (a) a peak instantaneous electrical current of the sensor in response to the applied voltage pulses; and   (b) a total current of the sensor over a period of time for a predetermined frequency in response to the applied voltage pulses.   
     
     
         21 . The program codes storage device of  claim 20 , including instructions, which when executed causes the controller to:
 determine the maximum current value (counts/second) during the initial 2 seconds in response to a voltage pulse applied to the sensor, and   compare the maximum current value so determined to a predetermined range of values; and   utilize a sensor signal received from the sensor to measure a physiological characteristic of a patient when the value is within the predetermined range of values.   
     
     
         22 . The program codes storage device of  claim 20 , including instructions, which when executed cause a controller to initiate a sensor observation routine; and
 transmit a first signal to a digital-to-analog converter (DAC), the DAC being coupled to an electrode of a sensor, the first signal representative of a observation sequence of voltages that the DAC is to output to the electrode of the sensor, wherein the observation sequence of voltages includes:   a first voltage applied for a first time frame;   a second voltage applied for a second time frame;   and a repeating of the application of the first voltage and the second voltage to the electrodes.   
     
     
         23 . The program code storage device of  claim 22 , including instructions, which when executed cause the controller to:
 repeat the application of the first voltage and the second voltage for a number of iterations.   
     
     
         24 . The program code storage device of  claim 23 , including instructions, which when executed cause the controller to:
 change a duration of the first amount of time and a duration of the second amount of time for at least one of the number of iterations.   
     
     
         25 . The program code storage device of  claim 22 , including instructions, which when executed cause the controller to:
 instruct the DAC to change a magnitude of the first voltage to be applied to the electrode of the sensor at least once during the repeating of the application of the first voltage; or   instruct the DAC to change a magnitude of the second voltage to be applied to the electrode of the sensor at least once during the repeating of the application of the second voltage.   
     
     
         26 . 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 (b) and the voltage measured in (f) that results from electrical resistance (R) in the sensor;   so that the state of the sensor is observed.   
     
     
         27 . The method of  claim 26 , further comprising:
 (i) changing applied sensor current to a third current comprising Ibase+deltaII, wherein deltaII comprises the difference between the Ibase and the third current;   (ii) measuring sensor voltage during the application of the third current;   (iii) observing a second voltage step between the voltage measured in (b) and/or (F and/or (ii) that results from electrical resistance (R) in the sensor; and   (iv) 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.   
     
     
         28 . The method of  claim 27 , 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).   
     
     
         29 . The method of  claim 26 , 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.   
     
     
         30 . The method of  claim 26 , further comprising:
 performing the method on a plurality of sensors made by differing manufacturing processes; and   comparing the information so obtained on the state of the plurality of sensors.