Skin stimulation and recording: Moving towards metal-free electrodes

When one thinks about electrodes, especially ones meant for humans, one typically thinks of some kind of metal. Whether on the skin or in the brain, metal electrodes are characteristically expensive, stiff, non-efficient in electron-ion transduction, and prone to toxic metal ion by-products during stimulation. In order to circumvent these disadvantages, electrically-conductive laser-induced graphene (LIG) and mixed electron-ion conducting polymer (poly(3, 4‐ethylenedioxythiophene) polystyrene sulfonate – PEDOT:PSS) was leveraged to create a metal-free electrode combination that allows for an economical, soft, and organic electrode for applications on human skin. Compared to clinical-standard silver – silver chloride (Ag/AgCl) skin electrodes, the metal-free hydrogel electrodes show notable improvement in electrochemical stability and prolonged stable potentials during long-term DC stimulation (0.5–24 h). Recording and stimulation performance on human participants rivals that of Ag/AgCl, thus fortifying the notion that they are an appropriate progression to their noble metal counterparts.