Ionically Toughened Cellulose Hydrogel for Self‐Regulating Smart Windows and Wearable Sensors

ABSTRACT Cellulose‐based hydrogels exhibit excellent optical transparency, biocompatibility, and flexibility, offering promising applications in biomedicine, energy‐efficient buildings, and smart electronics. However, their practical use is limited by inherent drawbacks such as poor mechanical strength and fixed thermoresponsive range. In this study, we introduced poly(acrylic acid) (PAA) and CaCl 2 into the hydroxypropyl cellulose (HPC) hydrogel network to form a multiple dynamic crosslinking network, resulting in a synergistically toughened HPC‐PAA‐Ca 2+ thermochromic hydrogel with a tunable phase transition temperature range of 10°C–42°C. The photothermal regulation property enables its use in energy‐efficient smart windows, demonstrating a cooling effect of 13.1°C in a model house. The mechanical performance of the hydrogel was enhanced by carboxyl‐Ca 2+ coordination bonds, achieving a mechanical stress of 280 kPa, elongation of 460%, toughness of 0.56 MJ/m 3 , and excellent compression recovery (rebound rate > 95%). The incorporation of Ca 2+ also endows HPC‐PAA‐Ca 2+ hydrogel with high electrical conductivity of 1.48 S/m, enabling its use as a sensitive strain sensor capable of accurately monitoring human joint movements (elbow, wrist, and fingers). This work advances the development of multifunctional cellulose‐based hydrogels and expands their potential applications in smart materials.