A Low-Cost Bioimpedance Phase Angle Monitor for Portable Electrical Surface Stimulation Burn Prevention

Electrical skin-surface stimulation has wide clinical adoption and home use, but skin burns are a risk and existing devices do not monitor the electrode sites for skin damage or improperly connected electrodes. When a burn occurs, the low frequency phase of the skin surface impedance changes. This brief describes a novel analog processing technique that is suitable for detecting skin surface phase changes during the use of portable electrical stimulation devices. The output is a 1-bit signal indicating if the phase has changed by more than a predetermined amount, allowing the system to be incorporated into low-cost devices with limited processing capability. Computational simulations were performed using a current-sourced bipolar square-wave stimulation signal. An approximation is presented which converts a phase change in the skin-surface impedance to a voltage, which was found to have an average error of 14.1%. Due to the analog techniques used, there is a propagation delay of 232.48 ms between the phase exceeding the allowable threshold and the output being set to HIGH. These initial simulation results provide evidence for the feasibility of developing safer portable skin surface electrical stimulators that can monitor and prevent skin injury during clinical or home use.