Paintable In Situ Gelling Bioelectronic Interfaces for Epidermal Electrophysiological Monitoring

Noninvasive epidermal electrophysiological monitoring is frequently constrained by unstable electrode-skin contact interfaces over soft, textured, and dynamically deforming skin. The predefined geometries of conventional electrodes limit their capacity to conform to local microtopography. Here, we introduce an in situ gelation strategy performed directly on the epidermis to construct a conformal ionic bioelectronic interface. A fluidic precursor comprising gelatin and tannic acid is initially stabilized against premature gelation via competitive solvent-solute interactions. Upon application, solvent evaporation and gelatin reassembly induce rapid network formation. This process directly yields a highly conformal and gently detachable interface with remarkably low contact impedance. The resulting hydrated dynamic network provides mechanical compliance, robust interfacial adhesion, stretch-tolerant ionic conduction, and cooling-induced interfacial weakening that enables gentle removal. The on-skin-gelled interface reduces skin-electrode contact impedance to 30 kΩ at 10 Hz, compared with 191 kΩ for commercial conductive pastes, and increases the signal-to-noise ratio from 24.9 to 27.8 dB. In proof-of-concept demonstrations, the interface supports within-subject electromyography-based gesture recognition with a classification accuracy of 93.4% and electroencephalography recordings of eye-state-dependent α-rhythm modulation as well as short-term sleep-related waveforms. These findings underscore the potential of on-skin in situ gelation as a versatile approach to fabricating paintable ionic bioelectronic interfaces for advanced electrophysiological monitoring.