Biologically inspired flexible photonic films for efficient passive radiative cooling.

Temperature is a fundamental parameter for all forms of lives. Natural evolution has resulted in organisms which have excellent thermoregulation capabilities in extreme climates. Bioinspired materials that mimic biological solution for thermoregulation have proven promising for passive radiative cooling. However, scalable production of artificial photonic radiators with complex structures, outstanding properties, high throughput, and low cost is still challenging. Herein, we design and demonstrate biologically inspired photonic materials for passive radiative cooling, after discovery of longicorn beetles' excellent thermoregulatory function with their dual-scale fluffs. The natural fluffs exhibit a finely structured triangular cross-section with two thermoregulatory effects which effectively reflects sunlight and emits thermal radiation, thereby decreasing the beetles' body temperature. Inspired by the finding, a photonic film consisting of a micropyramid-arrayed polymer matrix with random ceramic particles is fabricated with high throughput. The film reflects ∼95% of solar irradiance and exhibits an infrared emissivity >0.96. The effective cooling power is found to be ∼90.8 W⋅m-2 and a temperature decrease of up to 5.1 °C is recorded under direct sunlight. Additionally, the film exhibits hydrophobicity, superior flexibility, and strong mechanical strength, which is promising for thermal management in various electronic devices and wearable products. Our work paves the way for designing and fabrication of high-performance thermal regulation materials.