Output Circuits for Cutaneous Muscle Stimulators

Abstract

The purpose of this project was to analyze and evaluate the output circuit stages of four non-invasive muscle stimulators. The stimulators were labeled Constant Voltage Transformer Isolated, Constant Current Transformer Amplified, Microstim Plus, and the Constant Current stimulator. The impedance of the human skin under constant current pulses was studied in 10 human subjects, in which the maximum impedances were computed. Accuracy of circuit simulations of the four devices was determined by comparing the output waveforms of the simulation to those of the hardware through models of passive loads. The evaluation of the four circuits was based on output range, power efficiency, quiescent power, output regulation, cost, weight, volume, and comfort level of the stimulation. The results showed that skin impedance for humans ranged from 5965 ohms to 1270 ohms. An increase in current pulse intensity caused the skin impedance to decrease in value with a trend that follows a power law. The simulation for the Constant Voltage Transformer Isolated was the most accurate due to the simplicity of the circuit. The Microstim Plus stimulator had the lowest quiescent power, the smallest size, weight, and cost, and provided the most comfortable stimulation. The Constant Current stimulator regulated the best for current pulses of 25 mA or less over a range of resistive loads. Electrical isolation safety and isolation design improvements for the Constant Current Transformer Amplified stimulator are presented.