Development of soft dry electrodes: from materials to structure design

Bioelectric signals reflect our daily physiological activities, which can be recorded in the form of electroencephalography, electrocardiography, electromyography, etc. The traditional Ag/AgCl wet electrode is the gold standard for clinical monitoring of bioelectrical signals at present, while complicated preparation and gel evaporation limit its long-term application. Therefore, it is meaningful to research dry electrodes without conductive paste or additional adhesives. Unfortunately, the high interface impedance between electrodes and skin is a fatal defect of dry electrodes, which leads to excessive noise levels and poor signal quality. Consequently, more efforts are required to achieve conformal contact between dry electrodes and skin to reduce the contact impedance. From this perspective, we review the recent progress in capacitive electrodes, invasive microneedle electrodes, and common-contact dry electrodes. Material selection and structural design to obtain conformal contact are highlighted. Finally, we propose the future development direction of dry electrodes.