Negative Impedance Capacitive Electrode for ECG Sensing Through Fabric Layer

Many applications utilize flexible capacitive electrode implementations and analog front-end optimizations for improving noncontact electrocardiogram (ECG) measurements. However, the influence of the negative impedance in the circuit front end on noncontact measurements has not been investigated. The objective of this study was to develop negative impedance capacitive electrodes for improving the degraded noncontact ECG signals induced by the high-impedance interface between the skin and electrode. A capacitive electrode with a negative impedance of -200 M Omega was designed to reduce the impedance mismatch between the body and measurement electrode. The proposed electrodes were applied on a subject's chest to continuously measure the cardiac signals through a fabric layer and further evaluate the signal-to-noise (SNR) performance for noncontact ECG measurements. The negative impedance in the circuit front end reduced the coupling effect between the skin and the measurement electrodes, thus improved the SNR in theory and in practice. Importantly, our system produced a distinguishable heartbeat signal even with a maximum cloth thickness of 1.8 mm and skin-electrode capacitance exceeding 57 pF, which is suitable for wearable applications. This study is a pilot effort toward validating the efficacy of negative impedance capacitive electrodes for noncontact physiological measurements. This novel electrode design is expected to potentially address the limitations imposed by a fabric layer and its thickness in noncontact hioelectric sensing in the future.