Ultra-Light Poly(N-isopropylacrylamide) Hydrogels: Light Weight Water Materials for Passive Thermal Management via Insulation and Cooling.

Water, despite its abundance, high heat capacity, and environmental benignity, has long been constrained by its intrinsic density (~ 1.0 g cm-3) and fluidic nature, which limit its use as a lightweight, structurally stable material above the freezing point. Reconfiguring water into an ultra-light yet solid-like form while retaining its inherent thermal and optical advantages is therefore of great significance for next-generation cooling technologies that demand low mass, portability, and sustainability. Herein, we report an ultra-light hydrogel based on poly(N-isopropylacrylamide), in which hollow foaming microspheres are incorporated to create ultra-low-density water materials. By confining water within this composite network, the hydrogel achieves a record-low density of 0.041 g cm-3 while maintaining a high water content of 52.7 wt%. The microspheres generate sealed air pockets that serve as highly effective thermal barriers, yielding a thermal conductivity of only 0.034-0.039 W m-1 K-1 and enabling a > 50 °C temperature differential in hot-stage tests. Furthermore, the hydrogel exhibits excellent spectral properties, with high solar reflectance (0.94) and high infrared emittance (0.84), resulting in a sub-ambient cooling of up to 10.8 °C in outdoor experiments. The synergy of ultra-low density, mechanical robustness, and multifunctional thermal regulation demonstrates a viable pathway toward practical light water materials for energy-efficient, portable, and sustainable thermal management.