A Portable Device Performing Continuous Impedance Spectroscopy for Skin Conductivity

Electrodermal activity is widely used in psychophysiological studies to quantify emotional responsiveness and stress. Existing devices typically assess skin conductivity using either constant current/voltage or a fixed-frequency excitation. However, skin behaves like a complex impedance system which is frequency-dependent, but single-frequency methods only produce a single conductivity value. We propose a method to assess skin impedance at multiple frequencies simultaneously and continuously to observe temporal changes in the impedance at each frequency for the improved analysis of electrodermal activity. Furthermore, we present an implementation of this method in a wearable device to highlight its performances. We achieved the measurement of 16 frequencies simultaneously within the range of 10–1000 Hz at a rate of eight samples per second. The device was evaluated using static impedances made from discrete electronic components and demonstrated an average error of 1.9%. An in vivo experiment was conducted on seven volunteers resting for 30 min. Measurements were taken consecutively with our device and the Sciospec ISX-3 at the same body location, without removing the electrodes, when electrodermal activity was minimal. The average difference between the two instruments on living tissues was 3.8%. Performing continuously impedance spectroscopy could enable a more robust and a more universal way to quantify electrodermal activity. Additionally having a complete spectrum of impedance may help improve the specificity of the analysis of electrodermal activity by better differentiating between sweat gland activity and changes in the electrode/skin interface using equivalent circuit modeling.