Apparatus for electrodermal activity measurement with current compensation

Abstract

An apparatus for measuring electrodermal activity can include a first electrode in contact with a first portion and a second electrode in contact with a second portion of a stratum corneum, and in electronic communication with the second electrode through the stratum corneum. A processing module is electrically coupled to the first electrode and the second electrode and is operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V− (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current (d) measure a resulting current producing an amplified output voltage (e) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage and the compensation current to desaturate the output voltage.

1 . An apparatus for measuring electrodermal activity, comprising:
 a first electrode in contact with a first portion of a stratum corneum of skin;   a second electrode in contact with a second portion of the stratum corneum and in electronic communication with the first electrode through the stratum corneum; and   a processing module electrically coupled to the first electrode and the second electrode and operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V−, (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current, (d) measure a resulting current and produce an amplified output voltage, (e) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage, and the compensation current to desaturate the output voltage.   
     
     
         2 . The apparatus of  claim 1 , wherein the processing module is further configured to adjust the compensation current if the conductance of the stratum corneum is too low. 
     
     
         3 . The apparatus of  claim 1 , wherein the processing module is further configured to measure a compensated value of conductance from which a tonic level conductance is removed. 
     
     
         4 . The apparatus of  claim 1 , wherein the processing module is configured to reverse the polarity of at least one of the first electrode and the second electrode after a predetermined period of time to substantially reduce electrolysis. 
     
     
         5 . The apparatus of  claim 1 , wherein the processing module is configured to allow a tuning of the compensation current that is subtracted from the current between the first electrode and the second electrode prior to amplification. 
     
     
         6 . The apparatus of  claim 1 , wherein the stratum corneum is a stratum corneum of any portion of person's skin. 
     
     
         7 . The apparatus of  claim 6 , wherein the apparatus is configured to measure a tonic level conductance of the stratum corneum in the range of about 0.05 μS to about 50 μS. 
     
     
         8 . The apparatus of  claim 1 , wherein the apparatus is configured to measure the conductance of the stratum corneum with a resolution of 0.0001 uS. 
     
     
         9 . A wearable device for measuring electrodermal activity, comprising:
 a housing configured to be removably associated with a user;   a first electrode, at least a portion of the first electrode disposed outside the housing and configured to contact a first portion of a stratum corneum of skin when the housing is associated with the user;   a second electrode, at least a portion of the first electrode disposed outside the housing and configured to contact a second portion of the stratum corneum of the skin when the housing is associated with the user;   a processing module disposed in the housing and electrically coupled to the first electrode and the second electrode, the processing module operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V− (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current, (d) measure a resulting current and produce an amplified output voltage, (c) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage, and the compensation current to desaturate the output voltage;   a communications module disposed in the housing, the communications module coupled to the processing module and configured to at least one of a display an electrodermal activity of the user, and communicate electrodermal activity data from the processing module to an external device; and   a power source disposed in the housing and configured to provide electrical power to the processing module and the communications module.   
     
     
         10 . The wearable device of  claim 9 , wherein the processing module is further configured to decrease the compensation current if the conductance of the stratum corneum is too low. 
     
     
         11 . The wearable device of  claim 10 , wherein the wearable device is a wrist band. 
     
     
         12 . The wearable device of  claim 11 , wherein the processing module is configured to measure a tonic level conductance of the stratum corneum of the wrist of the user in the range of about 0.05 μS to about 50 μS. 
     
     
         13 . The wearable device of  claim 19 , wherein the wearable device is at least one of a head band, an arm band, a foot band, an ankle band, and a ring. 
     
     
         14 . The wearable device of  claim 9 , wherein the processing module is further configured to reverse the polarity of at least one of the first electrode and the second electrode after a predetermined period of time to substantially reduce electrolysis. 
     
     
         15 . The wearable device of  claim 9 , further comprising a heart beat sensor. 
     
     
         16 . The wearable device of  claim 15 , further comprising at least one of an accelerometer, a temperature sensor, a blood oxygen sensor, and a glucose sensor. 
     
     
         17 . The wearable device of  claim 16 , wherein the communications module is configured to communicate at least one of heart beat data, an accelerometer data, a temperature data, a blood oxygen data, and a glucose data to the external device. 
     
     
         18 . The wearable device of  claim 17 , where the communication module is configured to communicate with the external device via at least one of a USB, USB 2.0, IEEE 1394, BLUETOOTH®, low powered BLUETOOTH®, and Wi-Fi. 
     
     
         19 . A method for measuring electrodermal activity, comprising:
 disposing a first electrode and a second electrode on a stratum corneum of a user;   biasing the first electrode at a first voltage and a second electrode at a second voltage;   measuring a current flowing between the first electrode and the second electrode;   subtracting a compensation current from the current flowing between the first electrode and the second electrode to get an input current;   transforming the input current into an amplified output voltage;   measuring an output voltage;   measuring a conductance of the stratum corneum;   determining if the output voltage is saturated; and   increasing the compensation current to change the output voltage such that the output voltage is not saturated.   
     
     
         20 . The method of  claim 19  further comprising:
 determining if the conductance of the stratum corneum is too low; and 
 decreasing the compensation current if the conductance is too low. 
 
     
     
         21 . The method of  claim 20 , wherein the measured conductance is a tonic level conductance having a value in the range of about 0.05 μS to about 50 μS. 
     
     
         22 . The method of  claim 20 , wherein the measured conductance is a phasic level conductance having a resolution up to 0.0001 μS.