Force Sensing Resistors Used as Plantar Impedance Plethysmography Electrodes

Abstract

Diabetic foot ulcers (DFUs) are an ominous consequence of diabetic foot. To develop a multidimensional lesion warning device, several variables must be considered, among which vascular parameters. To minimize the number of sensors, we investigate the use of standard flexible force sensing resistors (FSRs), FSR402 and FSR406, to detect not only plantar pressure but also bioimpedance plethysmography. Since FSRs include conductive electrodes covered by polymer film, the interface with the subject can be considered a capacitive electrode. We present a special impedance plethysmography (IPG) circuit to inject current using FSRs and measuring the resulting voltage from other FSRs contacts. IPG simulations with two modeled FSRs, two contacts each, were not successful, but using four FSRs, one capacitive contact each, shows a precision within 4% of the expected FSR resistor value. The IPG system was verified with simultaneously matching ECG-lead I, Ag–AgCl, and capacitive electrode signals alongside power spectra. For the first time, four sole pressure sensors are used also as bioimpedance electrodes to detect cardiac activity with standard components at frequencies up to 50 kHz and simulated as well as experimentally verified on one healthy subject.