A smart electrolyte-replenishing semi-dry electrode based on a temperature-responsive hydrogel for sustainable electrophysiological signal acquisition

Abstract

Semi-dry electrodes combine the advantages of conventional wet and dry electrodes for wearable devices for electrophysiological signal monitoring but face severe difficulties in providing sustainable signal acquisition with low impedance. Herein, we developed a new semi-dry electrode that can achieve smart electrolyte-replenishment for long-term stability and highly comfortable electrophysiological signal acquisition. By combining an Ag nanoparticle-loaded sponge (ANLS) with a poly(N-isopropylacrylamide) (PNIPAM)/polyvinyl alcohol (PVA) hydrogel (PPH), a heterogenous interpenetrating network (H-IPN) conductive sensing layer (ANLS-PPH) was obtained, which was then integrated with an outer protective polydimethylsiloxane (PDMS) layer and Ag/AgCl connection central layer to fabricate a three-layer semi-dry electrode. Most importantly, PPH enabled smart electrolyte-replenishment triggered by body temperature. This controlled, external force-free release of electrolytes ensured optimal user comfort while significantly lowering scalp–electrode impedance to 8–18 kΩ. Furthermore, owing to the outer PDMS layer, this semi-dry electrode could retain water without it evaporating into the air, thereby maintaining low contact impedance for at least 6 hours. Finally, the H-IPN structure endowed this semi-dry electrode with highly enhanced mechanical performance (258.5 kPa tensile strength) and provided stability to the electrode structure during signal acquisition. Thus, this semi-dry electrode could stably acquire electroencephalogram (EEG) and electrocardiogram (ECG) electrophysiological signals for a long time, exhibiting a high degree of consistency with conventional wet electrodes (97.9% of spectral correlation). This study presents a smart electrolyte-replenishing semi-dry electrode for sustainable electrophysiological signal acquisition, inspiring new strategies for wearable electrophysiological devices.