Unblended polydimethylsiloxane dielectrics for non-contact electrocardiograph bioelectrodes

Abstract

Dielectric materials play crucial roles in the non-contact recording of electrocardiograms (ECGs), allowing cardiac physiological signals to be monitored and abnormalities detected early. Previous dielectrics are rigid, unstable, require expensive fabrication, and induce severe noise. PDMS is a polymer with remarkable biomedical properties suitable for non-contact bioelectrodes. However, the poor dielectric capability of PDMS has led to costly attempts to improve it. This research introduces an affordable technique to explore and characterize unblended PDMS films as dielectrics for non-contact ECG bioelectrodes by varying the weight mix ratio of Sylgard 184 TM silicone elastomer and its crosslinker. Capacitance and relative permittivity were measured and estimated using the parallel-plate technique in the frequency range of the LCR meter. Skin impedance values were also measured for different skin conditions using the Hioki impedance analyzer test frequency of 4 Hz - 1 MHz. Non-contact ECG measurements are affected by morphology of the dielectric, bioelectrode conductor, contact area, and skin conditions. To investigate the impact of these factors on skin impedance, the skin bioelectrode interface was modeled using equivalent circuits for wet, direct contact, and non-contact modes and analyzed using least-squares non-linear curve fit. Finally, the proposed approach was verified by recording ECG using different bioelectrodes and dielectrics. Due to susceptibility to motion artifacts and electrical interferences, digital filters were introduced to improve the quality of ECG recorded. The results demonstrate the effectiveness of the proposed method for improving the dielectric performance of unblended PDMS films, with a steady increase in capacitance (50.53 pF to 102.86 pF) and relative permittivity (0.19 to 0.69) observed with an increase in the proportion of the crosslinker. Good agreement was found between the measured skin impedance and equivalent circuit models, but differences exist in estimated circuit parameters. The unblended PDMS films with the highest mix ratio (10:2) successfully recorded visible P-QRS-T peaks in ECG. Skin conditions, bioelectrode conductivity, dielectric thickness and porosity, and the OpenBCI board filtering parameters affect the ECG recordings. This study confirms the potential of unblended PDMS films for non-contact bioelectrodes to detect heart abnormalities early.