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Vol. 18 (2026)

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

https://doi.org/10.1007/s40820-025-02057-9
Xueyan Hu
Key Laboratory of Multifunctional Nanomaterials and Smart Systems, Suzhou Institute of Nano‑Tech and Nano‑Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China
Siyuan Dou
Key Laboratory of Multifunctional Nanomaterials and Smart Systems, Suzhou Institute of Nano‑Tech and Nano‑Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China
Yiming Liu
Key Laboratory of Multifunctional Nanomaterials and Smart Systems, Suzhou Institute of Nano‑Tech and Nano‑Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China
Yaru Li
Key Laboratory of Multifunctional Nanomaterials and Smart Systems, Suzhou Institute of Nano‑Tech and Nano‑Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China
Caixia Yu
Key Laboratory of Multifunctional Nanomaterials and Smart Systems, Suzhou Institute of Nano‑Tech and Nano‑Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China
Jin Wang
Key Laboratory of Multifunctional Nanomaterials and Smart Systems, Suzhou Institute of Nano‑Tech and Nano‑Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China

Corresponding Author: Jin Wang

jwang2014@sinano.ac.cn

Nano-Micro Letters, Vol. 18 (2026), Article Number: 225

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.


Highlights:
1 The hydrogel achieves an ultra-light water-based structure (0.041 g cm−3, 52.7 wt% water) by incorporating hollow foaming microspheres.
2 Sealed air pockets enable ultra-low thermal conductivity (0.034–0.039 W m−1 K−1) and over 50 °C thermal shielding under high-temperature conditions.
3 High solar reflectance (0.94) and infrared emittance (0.84) deliver up to 10.8 °C sub-ambient outdoor cooling.

Keywords

Lightweight water materials Hydrogel Passive cooling Thermal management Low density
Hu, Xueyan, Siyuan Dou, Yiming Liu, Yaru Li, Caixia Yu, and Jin Wang. 2026. “Ultra-Light Poly(N-Isopropylacrylamide) Hydrogels: Light Weight Water Materials for Passive Thermal Management via Insulation and Cooling”. Nano-Micro Letters 18 (January):225. https://doi.org/10.1007/s40820-025-02057-9.
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