Biocompatible passive radiative cooling rapid-curing fiberglass casts

Passive daytime radiative cooling (PDRC) provides a zero-energy approach to reducing surface temperatures by reflecting solar radiation and emitting thermal energy through the mid-infrared atmospheric window. However, many high-performance PDRC materials require rigid or brittle substrates, limiting their application on flexible or curved surfaces. Here, we report a bilayer PDRC coating integrated onto a commercial fiberglass cast, a fast-curing, mechanically robust substrate commonly used for orthopedic support. The coating consists of a polyvinyl alcohol (PVA) adhesion layer and a polymethyl methacrylate (PMMA) top layer, both embedded with calcium pyrophosphate (CPP) ceramic particles derived from animal bone waste. CPP enables broadband solar reflectance and strong mid-infrared emittance while also contributing to sustainability and biocompatibility. The coating achieves over 90% solar reflectance and delivers up to 15 °C sub-ambient cooling under direct sunlight. It maintains stability under environmental stress, showing water resistance (contact angle ∼85°), UV durability, abrasion tolerance, and thermal stability exceeding 650 °C. Mechanical tests confirm enhanced flexibility without compromising structural strength. This work demonstrates a scalable, field-deployable PDRC platform suitable for wearable cooling, orthopedic comfort, and mobile thermal regulation.