Thermal protection of wearable devices under outdoor conditions using radiative cooling films

Wearable electronic devices operating outdoors are vulnerable to overheating under prolonged exposure to intense solar radiation and elevated ambient temperatures. Such thermal stress compromises device performance, inducing functional failure, and poses safety risks such as skin burns. Therefore, we developed a passive radiative cooling film (RCF) providing energy-free thermal management for wearable electronics. The RCF, fabricated via electrospinning of a polyvinyl alcohol matrix embedded with silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃) nanoparticles, exhibits optimized optical properties, including a high average solar reflectance of 0.88 (0.3–2.5 μm) and thermal emissivity of 0.95 in the atmospheric window (8–13 μm). These properties enable effective radiative heat dissipation and reduce the solar heat gain. Under high-temperature outdoor conditions, RCF-integrated wearable sensors maintained an average surface temperature of 42.5 °C, which was up to 8.6 °C lower than that of sensors without the film, ensuring stable operation without thermal failure. Additionally, the RCF effectively mitigated thermal accumulation in smartphone batteries, reducing peak surface temperature from 60.0 °C to 37.1 °C under direct sunlight. These results demonstrate the potential of the RCF as a scalable and energy-efficient solution for enhancing thermal reliability and user safety of next-generation wearable outdoor electronic devices.