Systems and methods for calibrating dry electrode bioelectrical impedance sensing

Abstract

Described herein are systems and methods for calibrating dry electrode bioelectrical impedance measurements. These method and apparatuses may be used for sensing bioelectrical impedance for ambulatory and or long-term measurements. Calibration of bioelectrical impedance sensing may be performed by using measurements taken during a shorted configuration of the apparatus, in which the same current is applied to both the source and sink stimulation electrodes, to modify measurements taken in a forward and/or reverse configuration in which current is applied to either the source and/or sink.

1 . A bioelectric impedance measurement apparatus comprising:
 at least one pair of stimulation electrodes, comprising a source electrode and a sink electrode;   at least one pair of sense electrodes, comprising a first sense electrode and a second sense electrode;   a signal generator operably connected to the at least one pair of stimulation electrodes;   a controller configured to switch the apparatus between a normal mode, in which the signal generator applies a current between the source electrode and the sink electrode, and a shorted mode, in which the signal generator applies the same current to both the source electrode and the sink electrode; and   a processor configured to output a calibrated bioelectric impedance measurement based at least in part on a signal at the sense electrodes in both the normal mode and the shorted mode.   
     
     
         2 . The apparatus of  claim 1 , wherein the processor is further configured to determine the calibrated bioelectric impedance measurement based at least in part on measurements equivalent to: a voltage difference between the first and second sense electrodes in both the normal mode and the shorted mode; a ratio of voltages at the first sense electrode in the normal mode and the shorted mode; and a current across a current sense resistor in the normal mode. 
     
     
         3 . The apparatus of  claim 1 , wherein the processor is integrated with the controller. 
     
     
         4 . The apparatus of  claim 1 , wherein the controller is configured to automatically switch between the normal mode and the shorted mode. 
     
     
         5 . The apparatus of  claim 1 , controller is configured to drive a multiplexer configured to switch between the normal mode and the shorted mode. 
     
     
         6 . The apparatus of  claim 1 , wherein the controller is further configured to switch between the normal mode, the shorted mode and a reverse mode, in which the signal generator applies a current between the sink electrode and the source electrode in a direction that is reversed in relation to the normal mode. 
     
     
         7 . The apparatus of  claim 6 , wherein the processor is configured to determine a first bioelectric impedance measurement based at least in part on the signal at the sense electrodes in both the normal mode and the shorted mode and a second bioelectric impedance measurement based at least in part on the signal at the sense electrodes in both the reverse mode and the shorted mode, wherein the calibrated bioelectric impedance measurement combines the first bioelectric impedance measurement and the second bioelectric impedance measurement. 
     
     
         8 . The apparatus of  claim 7 , wherein the calibrated bioelectric impedance measurement is an average of the first and second bioelectric impedance measurements. 
     
     
         9 . The apparatus of  claim 1 , wherein the apparatus is configured as a wrist-worn device. 
     
     
         10 . The apparatus of  claim 1 , further comprising a current sense resistor in communication with the at least one pair of stimulation electrodes. 
     
     
         11 . The apparatus of  claim 1 , wherein the at least one pair of stimulation electrodes and the at least one pair of sense electrodes all comprise dry electrodes. 
     
     
         12 . A method of determining a calibrated bioelectrical impedance, the method comprising:
 applying, in a normal mode, a first current between a source electrode and a sink electrode and storing voltages from a first sense electrode and a second sense electrode;   applying, in a shorted mode, a second current simultaneously to both the source electrode and the sink electrode and storing voltages from the first sense electrode and the second sense electrode; and   outputting a calibrated bioelectric impedance measurement, wherein the bioelectric impedance measurement is based at least in part on the voltages of the sense electrodes in both the normal mode and the shorted mode.   
     
     
         13 . The method of  claim 12 , wherein estimating the calibrated bioelectric impedance measurement comprises determining the calibrated bioelectric impedance measurement based at least in part on measurements equivalent to: a voltage difference between the first and second sense electrodes in both the normal mode and the shorted mode; a ratio of voltages at the first sense electrode in the normal mode and the shorted mode; and a current across a current sense resistor in the normal mode. 
     
     
         14 . The method of  claim 13 , wherein the estimating the calibrated bioelectric impedance measurement (Z 2 ) comprises subtracting from a differential voltage of the first and second sensing electrodes in the normal mode (β N ), a differential voltage of the first and second sense electrodes in the shorted mode (β B ) multiplied by the ratio of voltages at the input of one of the sense electrodes in the normal mode and the shorted mode (γ N /γ B ) and dividing by the current across the current sense resistor in the normal mode (α N /Z 6 ), wherein:
     Z   2 =(β N −β B *(γ N /γ B )/(α N   /Z   6 )
 
 
     
     
         15 . The method of  claim 12 , further comprising automatically switching between the normal mode and the shorted mode. 
     
     
         16 . The method of  claim 12 , further comprising applying, in a reverse mode, a third current between the sink electrode and the source electrode so that current between the sink electrode and the source electrode is reversed in relation to the normal mode, and storing voltages from the first sense electrode and the second sense electrode. 
     
     
         17 . The method of  claim 16 , further comprising determining a first bioelectric impedance measurement based at least in part on the signal at the sense electrodes in both the normal mode and the shorted mode and determining a second bioelectric impedance measurement based at least in part on the signal at the sense electrodes in both the reverse mode and the shorted mode, wherein the calibrated bioelectric impedance measurement combines the first bioelectric impedance measurement and the second bioelectric impedance measurement. 
     
     
         18 . The method of  claim 17 , wherein the calibrated bioelectric impedance measurement is an average of the first and second bioelectric impedance measurements. 
     
     
         19 . The method of  claim 17 , wherein the apparatus is configured as a wrist-worn device. 
     
     
         20 . The method of  claim 12 , further comprising measuring a current through a current sense resistor. 
     
     
         21 . The method of  claim 12 , wherein the stimulation electrodes and the sense electrodes all comprise dry electrodes.