Sleeping Heart Monitoring Using Hydrogel-Textile Capacitive ECG Electrodes

In the application on sleeping heart monitoring by using capacitive electrocardiogram (cECG), a raw cECG signal with high quality is usually difficult to be obtained due to the low coupling capacitance composed of human skin, pajamas, bed sheet and sensing electrodes. This is mainly caused by a low relative dielectric constant of bed sheet and pajamas. In order to overcome this challenge, this study proposed a cECG system containing hydrogel-textile electrode for sleeping heart monitoring. The hydrogel layer was applied in an array pattern onto conductive textile to become a sensitive electrode in order to increase the coupling capacitance and lower the equivalent impedance as well, which was helpful to improve the quality of raw cECG signals. The reference electrode was linked to the circuit of the signal acquisition circuit to decrease the common-mode noises. The system incorporates eight membrane pressure sensors to capture the body’s sleep postures and detect the cECG signals from different sleep postures. The experimental results on tests with three different human sleep postures showed that the signal-noise ratio of the proposed system was close to the wet-electrode based ECG system. The sleeping heart monitoring experiments were conducted involving eight subjects with no constraint conditions through a period of two hours, the R-wave of the cECG signal was analyzed using RR interval extraction algorithms and the heart rate (HR) was calculated. The results showed that all subjects’ HRs during sleep state falls into a normal range, indicating that no signs of tachycardia and bradycardia can be observed.