Synergistic color-changing and conductive photonic cellulose nanocrystal patches for sweat sensing with biodegradability and biocompatibility

) and progressive color changes from blue, green, yellow, to ultimately red upon sweat wetting. Interestingly, this system simultaneously demonstrates selective and sensitive electrical sensing functions, as well as satisfactory biocompatibility, biodegradability, and breathability. Importantly, a proof-of-concept demonstration of a skin-adhesive patch is presented, where the optical and electrical dual-signal sweat sensing allows for intuitive visual and multimode electric localization of sweat accumulation during physical exercises. This innovative interactive strategy for monitoring human metabolites could offer a fresh perspective into the design of wearable health-sensing devices, while greatly expanding the applications of CNC-based photonic materials in medicine-related fields.