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

Abstract 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.