As global warming intensifies, it’s critical to develop passive personal thermal management strategies for human sustainable energy-saving development and thus keep people cool outdoor on a scorching summer day. Recently, researchers have designed various types of radiative cooling textiles with mid-infrared (MIR) transparency to enhance human body heat dissipation to the ambient environment. In an outdoor situation, the sky (3K) represents a far better heat sink as compared to the ambient environment, thus textiles with high MIR emissivity have the potential to achieve sub-ambient temperature.
Although a variety of radiative cooling materials with significant MIR emissivity have been reported, they all exhibit weak water vapor permeability, inadequate wear-ability and comfort, limiting the applications on textile. Sub-ambient radiative cooling textiles with good wear-ability and comfort have not been realized.
Silk, a natural animal fibre, has been one of the most favourable fabrics for centuries, with its luster appearance and cooling sensation. Due to the self-assembled hierarchical structure of silk and its main component of fibroin protein, natural silk exhibits good reflectivity in the range of the solar spectrum and high emissivity in the MIR, suggesting a potential radiative cooling performance. However, its protein component intrinsically induces the high absorption in the ultraviolet (UV) region (Fig. 1a, b), thus lowing the overall reflectivity to sunlight and prohibiting it from achieving a sub-ambient temperature under the sunlight in the daytime (Fig. 1c).
Fig. 1 a, Schematic of net heating performance of natural silk due to high absorption in the UV wavelength. b, Schematic of UV absorption by protein molecular of silk. c, The reflectivity spectrum of natural silk between 0.3-2.5 um wavelength range, the corresponding reflectivity in the sunlight wavelength (0.3-2.5 um) is about 86%. d, Schematic of net cooling performance of nano-processed silk for enhancing the UV reflectance. e, Schematic of enhanced anti-UV property by connecting nanoparticles with silk through a coupling reagent. f, The reflectivity spectrum of nano-processed silk between 0.3-2.5 um wavelength range, the corresponding reflectivity in the sunlight wavelength (0.3-2.5 um) is about 95%.
We propose a scalable coupling reagent assisted dip-coating method to process natural silk. Due to the introduction of inorganic oxide nanoparticles with high refractive index and appropriate size, the reflectivity in the sunlight range of the obtained nano-processed silk (NP-silk) can be significantly enhanced, reaching as high as 95% (Fig. 1d-f). In the meantime, the incorporation of nanoparticles into silk does not affect the high emissivity of silk in the MIR range as the particle sizes are much smaller than the thermal wavelength. Consequently, this designed NP-silk shows potential for excellent sub-ambient radiative cooling performance.
During outdoor tests under direct sunlight (peak solar irradiance > 900 W/m2), a temperature of ~ 3.5 °C below ambient (ambient temperature ~ 35 °C) for our nano-processed silk was achieved. We also observe an 8 °C temperature reduction of a simulated skin when covered with nano-processed silk, as compared to that with natural silk. Combining the large-scale screen-printing technique, the prepared one-face NP-silk also presents a stable washability and comfortable wear-ability. The results presented in this work provide a sustainable energy-saving method for personal thermal management, and may inspire further developments of materials and devices of passive cooling to reduce energy consumption.
More details can be found in our article published in Nature Nanotechnology (https://www.nature.com/articles/s41565-021-00987-0).
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