Bioinspired Programmable and Ultrastretchable Janus Helical Hydrogel Fibers for Strain-Invariant Thermoelectric Body Heat Harvesting and Sensation

Current fiber-based electronics often suffer from low stretchability and struggle to conform to complex and dynamic skin surfaces, resulting in significantly reduced performance in wearable devices. However, hydrogels with processability and adaptability permit conformity to diverse curved and uneven surfaces. Inspired by natural tendrils, we present Janus helical hydrogel fibers capable of completely maintaining the original thermoelectric performance under ultrahigh elastic strains. Janus helical fibers, composed of sodium polyacrylate (PANa) and PANa/single-walled carbon nanotube (PANa-SWCNT) hydrogels, are fabricated at scale and programmed with controllable diameters by utilizing the biological strain mismatch mechanism. The optimized fiber is ultrastretchable and has a master strain-invariant built-in temperature gradient as well as resistance, thus ensuring stable energy output even at 650% strain. The hydrogel fiber integrated with 90 pairs of p/n coils adaptively harvest heat, exhibiting a notable voltage density of 6.51 mV cm^–2, and accurately perceive environmental temperatures (−176 μV/°C) undisturbed by body movements.