Lightweight and hierarchically porous hydrogels for wearable passive cooling under extreme heat stress

Increasing extreme heat stress puts humans at risk of heat stroke and dehydration in outdoor environments. However, current personal thermal management (PTM) approaches in hot summers suffer from low cooling efficiency. Here, we designed a lightweight, hierarchically porous hydrogel (HPHG) with low density, robust breaking strength, high evaporation enthalpy, high solar reflectance, and infrared emissivity, and controlled evaporation speed. The HPHG shows strong and prolonged passive cooling: sub-ambient temperature cooling up to 22.5°C under direct sunlight and a prolonged cooling time of >15 h. The HPHG can also be fabricated into a lightweight cooling vest (<350 g), achieving an average temperature drop of 11°C compared to air temperature. The concept of HPHG proposed in this study opens an avenue for hydrogel as a lightweight and wearable material in PTM and solves the bottleneck problem of passive cooling under extreme heat stress in an outdoor environment.