Heart Monitor Using Flexible Capacitive ECG Electrodes

Noninvasive sensors capable of measuring weak biopotential signals, such as electrocardiogram (ECG) and EEG, and communicating results wirelessly to a host computer are developing rapidly. Some of them utilize capacitively coupled electrodes in an attempt to place the sensor at a distance from the skin. This article demonstrated the fabrication and development of a capacitively coupled ECG electrode prototype using custom high specific capacitance electrodes and custom high-performance electronics. Two ultrathin capacitive electrodes were fabricated on a flexible polyimide substrate (2 x 2 in) protected by a guard ring to reduce noise. The detection and amplification circuitry consisted of operational amplifiers (OpAmps) that filtered and conditioned the ECG signal. R-peaks in the ECG were readily detected and quantified using both simulated signals from ECG databases and real signals from human subjects. Heart rates and heart rate variability calculated from our monitor measurements were comparable with commercial rigid wearable sensors, including a smartwatch and an ECG monitor that uses standard clinical ionic electrodes. The prototype monitor was tested on human subjects during rest and moderate exercise and showed appropriate responses. The challenge of high-gain low-noise amplification was met by the development of highly thinned OpAmps whose operation was shown to be equivalent to commercially available rigidly packaged OpAmps, demonstrating that high-performance Si-electronics can be used to produce high-fidelity signals from weak biopotentials.