Wearable nanocomposite hydrogel temperature sensor based on thermally-switchable and mechanical-deformation-insensitive structural colors

Smart and flexible nanocomposite materials with structural colors have great potentials in the field of wearable sensors. However, the mechanochromic property of conventional photonic materials severely interferes with the optical signal outputs that are tunable by non-strain stimuli, which greatly impedes their applications in wearable photonic sensors. Herein, a wearable nanocomposite hydrogel temperature sensor based on thermally-switchable structural colors that are independent of mechanical deformations, is fabricated by the facile in-situ gelation of N-isopropyl acrylamide (NIPAM), nanoclay discs, carbon black, and monodisperse silica or polymeric NPs in their aqueous dispersions. The resulting non-assembled arrangements of the monodisperse NPs account for uniform structural color independent of mechanical deformations and observation angles. Simultaneously, the temperature-induced reconfiguration of the physically cross-linked poly-NIPAM hydrogel matrix, leads to super-large volume changes and in turn wide-range structural color shifts in almost the entire visible spectrum. The smart hydrogel matrix with homogenous polymer chain length by dynamic physical cross-linkings among exfoliated nanoclay discs, makes the photonic hydrogel robust, stretchable, and self-healable. The combination of mechanical-deformation-insensitive and thermally-switchable structural colors in the nanocomposite hydrogel enables skin-based wearable temperature sensing with naked-eye detection. This work opens a new platform for diverse non-invasive wearable photonic sensors beyond the current strain sensing.