Sustainable Thermochromic Hydrogels‐Based Energy‐Saving Smart Window with High Solar Modulation and Enhanced Privacy Protection

Hydrogel‐based smart windows, which actively modulate solar transmittance to control indoor temperature and brightness, have garnered significant interest due to their potential for enhancing energy efficiency. However, most existing hydrogel systems are based on synthetic polymers and chemically cross‐linked networks, which raises concerns about their environmental sustainability. In this study, we developed a novel thermochromic hydrogel entirely from natural polymers for next‐generation smart window applications, providing both energy‐saving potential and enhanced privacy. The hydrogel comprises sodium dodecyl sulfate micelles embedded within a matrix of kappa‐carrageenan and carboxymethyl chitosan, with β‐glycerophosphate employed to fine‐tune the optical characteristics. This material demonstrated outstanding anti‐freezing performance and rapid, reversible transitions between transparent and opaque states within the comfort temperature range of human body (∼34°C). At elevated daytime temperatures, the hydrogel became highly transparent, exhibiting remarkable solar modulation ( T lum = 83.54%, ∆ T sol = 78.89%). In contrast, at lower nighttime temperatures, it adopted an opaque state to enhance occupant privacy. Furthermore, the hydrogel displayed responsiveness to both external pH changes and electrical stimuli, supporting multi‐modal control. The system is simple to fabricate, cost‐effective, non‐toxic, and environmentally benign, offering great potential for sustainable energy‐saving windows and temperature‐warning devices.