Self-powered rewritable visual indicator for cumulative ionic exposure via hydrovoltaic-Electrophoretic coupling

Environmental ionic exposure often accumulates over time, whereas most sensing technologies provide only instantaneous readouts and cannot directly preserve exposure history. Here, we report a self-powered rewritable visual indicator that converts ionic exposure into a persistent spatial signal through hydrovoltaic-electrophoretic coupling. Evaporation-driven ion transport within a graphene oxide hydrogel generates a hydrovoltaic output of up to ∼1.1 V, which directly drives electrophoretic migration of oppositely charged particles without external power or signal processing. As ionic concentration increases, the migration distance expands continuously to ∼4.7 mm, enabling distance-based visual encoding of exposure intensity. The resulting visual patterns retain 94.6% of their initial displacement after 60 min and remain stable over 1000 write-erase cycles, demonstrating excellent persistence and rewritability. Using airborne pollen as a representative environmental exposure scenario, we further demonstrate wearable visual indication of pollen-associated ionic exposure under practical outdoor conditions. By integrating sensing, energy generation, signal transduction, and visual readout within a single passive material platform, this work establishes a strategy for converting environmental ionic perturbations into cumulative visual information and provides new opportunities for self-powered environmental monitoring and wearable visual sensing technologies.