Although water is abundant in resources, has high heat capacity and is environmentally friendly, it has been limited in its application as a lightweight and structurally stable material due to its inherent density (~ 1.0 g cm⁻³) and fluid properties, which prevent it from exceeding the freezing point. Therefore, reconfiguring water into an ultra-light and solid-like form while retaining its inherent thermal and optical advantages is of great significance for next-generation cooling technologies, which require low mass, portability and sustainability. This paper reports a lightweight super-hydrated hydrogel based on poly(N-isopropylacrylamide), in which hollow foam microspheres are incorporated to form an ultra-low-density water material. By confining water within the composite network, the hydrogel achieves a record-low density of 0.041 g⁻³ while maintaining a high water content of 52.7 wt%. The microspheres form sealed airbags, serving as efficient thermal barriers with a thermal conductivity of only 0.034–0.039 W m⁻¹ K⁻¹, and the temperature difference in thermal grade tests is >50°C. Additionally, the hydrogel exhibits excellent spectral properties, with a high solar reflectivity (0.94) and an infrared emissivity (0.84), achieving sub-environmental cooling of up to 10.8°C in outdoor experiments. The synergistic effect of ultra-low density, mechanical robustness and multifunctional thermal regulation demonstrates a feasible path to realizing practical lightweight water materials, enabling energy-saving, portable and sustainable thermal management. This research was published in Nano-Micro Letters under the title "Ultra-Light Poly(N-isopropylacrylamide) Hydrogels: Lightweight Water Materials for Passive Thermal Management via Insulation and Cooling".

Reference Information: DOI: 10.1007/s40820-025-02057-9