Subambient daytime radiative cooling textile based on nanoprocessed silk

Decreasing energy consumption is critical to sustainable development. Because temperature regulation for human comfort consumes vast amounts of energy, substantial research efforts are currently directed towards developing passive personal thermal management techniques that cool the human body without any energy consumption^1-9. Although various cooling textile designs have been proposed previously, textile-based daytime radiative cooling to a temperature below ambient has not been realized^6-13. Silk, a natural protein fabric produced by moth caterpillars, is famous for its shimmering appearance and its cooling and comforting sensation on skin^14-17. It has been recently recognized that silk, with its optical properties derived from its hierarchical microstructure, may represent a promising starting point for exploring daytime radiative cooling^18-21. However, the intrinsic absorption of protein in the ultraviolet region prevents natural silk from achieving net cooling under sunlight. Here we explore the nanoprocessing of silk through a molecular bonding design and scalable coupling reagent-assisted dip-coating method, and demonstrate that nanoprocessed silk can achieve subambient daytime radiative cooling. Under direct sunlight (peak solar irradiance >900 W m^-2) we observed a temperature of ~3.5 °C below ambient (for an ambient temperature of ~35 °C) for stand-alone nanoprocessed silks. We also observed a temperature reduction of 8 °C for a simulated skin when coated with nanoprocessed silk, compared with natural silk. This subambient daytime radiative cooling of nanoprocessed silk was achieved without compromising its wearability and comfort. This strategy of tailoring natural fabrics through scalable nanoprocessing techniques opens up new pathways to realizing thermoregulatory materials and provides an innovative way to sustainable energy.