Coral-Inspired Asymmetrically Porous Radiative Cooling Biofilm with Thermoplastic Polyurethane-Enhanced Mechanical Tolerance as Building Energy-Saving Envelope

Passive radiative cooling (PRC) is an emerging energy-free cooling strategy that reflects solar radiation and emits heat into outer space to achieve a cooling temperature below ambient. Despite notable progress, large-scale commercialization of PRC films for practical applications still remains a challenge, specifically in terms of their ability to envelope complex surfaces and the mechanical tolerance for prolonged utilization. Herein, we fabricated an asymmetrically coral-like porous cellulose acetate/thermoplastic polyurethane (CA/TPU) film with controllable size and thickness inspired by natural coral. The asymmetrical structure with coral-shaped pores enhances the scattering of sunlight across the apertures, and the introduction of hydroxyapatite nanorods (HAP NRs) strengthens the infrared emission. Additionally, the cross-linking between the polymer molecular chains of CA and elastomer TPU enables the PRC biofilm outstanding mechanical tolerance and flexibility which can be stretched to 236.2%. As a result, the PRC biofilm exhibits high solar reflectivity of 97.26% and infrared emissivity of 97.22%, contributing to a subambient temperature drop of 9.2 °C under solar radiation of 1550 W/m2. The PRC biofilm is expected to be utilized as an energy-saving envelope structure for buildings, vehicles, and textiles in practical applications.