Dry electrodes based on carbon nanotubes-graphene hybrid nanostructures for long-term ECG monitoring

Abstract

Dry electrodes for long-term ECG monitoring based on composites of hybrid nanostructures from single-walled carbon nanotubes (SWCNT) and reduced graphene oxide (RGO) flakes in a polydimethylsiloxane (PDMS) matrix under laser irradiation have been developed. A method for forming liquid dispersion for uniform homogenization of carbon nanomaterials within the composite volume has been developed. It was established that exposure of the composite to laser irradiation with a wavelength in the near-infrared spectral range and intensity of 15 kW/cm² leads to a 13-fold reduction in resistance to 8 ± 2 kΩ due to the formation of SWCNT/RGO hybrid nanostructures. It was obtained that the formation of conductive networks in the PDMS matrix resulted in increased defect density due to the formation of bonds between carbon nanomaterials. The study of the dependence of the applied force on the elongation of the formed composites with different compositions showed that the sample based on the PDMS/SWCNT/RGO composite possessed the highest strength. The elastic modulus value of the fabricated PDMS/SWCNT/RGO composite electrodes (180.0 ± 5.3 kPa) correlates well with the elastic modulus of skin. The study of the influence of mechanical pressure on the impedance of the formed electrodes showed that an increase in pressure (0–100 mmHg) leads to a decrease in impedance across the entire frequency range of 10–1000 Hz. The PDMS/SWCNT/RGO composite electrodes demonstrated high stability of impedance-frequency dependence in the same frequency range during prolonged contact (7 days) with a sweat-simulating suspension compared to conventional Ag/AgCl electrodes. In addition, the high stability of the PDMS/SWCNT/RGO composite electrodes was confirmed by a 14-day recording of impedance changes during their on-skin placement. During ECG measurements, it was found that the signal quality recorded by PDMS/SWCNT/RGO composite electrodes was comparable to that from Ag/AgCl electrodes, while the amplitudes of main peaks were sharper. The biocompatibility of PDMS/SWCNT/RGO composite electrodes was demonstrated in studies of connective tissue cell viability on their surface. It was established that the cell count on PDMS/SWCNT/RGO composite corresponded to the cell count on the control sample. Thus, the developed PDMS/SWCNT/RGO composite electrodes can be used for wearable devices, including continuous ECG monitoring for at least 7 days. The absence of a gel layer in dry PDMS/SWCNT/RGO composite electrodes represents their advantage over commercial analogues in long-term ECG monitoring.