Bidirectional Thermochromic Hydrogel with Wide Response Temperature Regulation for Smart Windows

Thermochromic smart windows have been extensively investigated for solar regulation and building energy management to reduce building energy consumption. However, most current smart windows respond to only a single fixed temperature, failing to meet both energy-saving and privacy needs. To overcome this limitation, a series of composite hydrogels (KNSG) with bidirectional temperature-responsive properties were developed by introducing sodium dodecyl sulfate/sodium chloride (SDS/NaCl) micelles and κ-carrageenan (KCA) into a poly(N-isopropylacrylamide/glycerol (PNIPAM/Gl) matrix. These hydrogels were encapsulated between two glass panes to construct climatic-adaptable smart windows. Through synergistic optimization of SDS, NaCl, and Gl, both the upper critical solution temperature (UCST, 0.8–17.4 °C) and the lower critical solution temperature (LCST, 19.5–50.4 °C) of the material could be flexibly and broadly adjusted to match the human thermal comfort zone and various climates. The smart window achieved excellent solar modulation (ΔTSol, UCST = 76.17%, ΔTSol, LCST = 76.29%) coupled with high transparency (94.90%), effectively balancing solar modulation with natural light illumination within a comfortable temperature range. Furthermore, KNSG displayed remarkable stability and antifreezing (−28 °C), ensuring long-term usability. These dual-responsive smart windows exhibit promising practical applications in future fields for building energy efficiency, information encryption, temperature monitoring, and so forth.