Skin Impedance Estimation System for Voltage-mode Electrical Stimulator with an AC Bridge Circuit

Neuromuscular electrical stimulation is used to improve the motor function of paralyzed limbs and prevent muscle atrophy in stroke patients. The system for electrical stimulation is broadly classified into current-mode stimulators and voltage-mode stimulators. The current-mode stimulator adjusts the amplitude of the current, whereas the amplitude of the voltage is adjusted for voltage-mode stimulators. Voltagemode stimulators have the advantage that there is little risk of burns even if the electrode is partially detached. To perform arbitrary current-mode stimulation with voltage-mode stimulators, it is necessary to generate a stimulating voltage based on the skin impedance. As a primary experiment, the frequency characteristics of the electrode-skin impedance were measured using an impedance analyzer on 6 subjects, and the frequency band in which the skin impedance is equivalent to a parallel connection between resistance and capacitance was determined. A prototype bridge circuit with a skin impedance equivalent circuit implemented was designed, assembled, and tested to estimate the skin impedances of 3 subjects. The residuals were computed from the estimated skin-impedance resistance and capacitance of the bridge circuit, and the impedance-analyzer-measured resistance and capacitance. The residuals between the estimated and measured were up to 4.4 % in the resistance component, and up to 8.2 % in the capacitance component of the skin impedance measurements by the impedance analyzer.