Recyclable near-body temperature eutectic system with double positive temperature coefficient effect for personalised thermal regulation

Wearable temperature sensors are essential for medical and personal thermal management applications but often face challenges in achieving accuracy, flexibility and multifunctionality. To address these limitations, we developed a biodegradable polymer-based quaternary composite that leverages a binary eutectic fatty acid system and graphene nanoplatelets (GNPs) to deliver self-regulating heating and temperature sensing capabilities. The incorporation of polycaprolactone (PCL), lauric acid (LA) and myristic acid (MA) facilitates precise thermal control by enabling a tuneable phase transition range of 30 to 60 °C, while GNPs enhance electrical conductivity and thermal response. Notably, the material exhibits a distinct double positive temperature coefficient (PTC) effect, maintaining PTC behaviour up to 80 °C without transitioning to a negative temperature coefficient (NTC) effect. This double PTC behaviour enables precise thermal regulation, with self-regulating heating at ∼36 °C under low-power operation (∼100-250 mW), demonstrating stable power consumption and effective heat absorption through its phase change properties. The composite also supports operation under practical voltages, 5 V (standard power bank), making it well-suited for wearable systems. Additionally, the material demonstrates excellent recyclability through a simple dissolution and recasting process, retaining its stable thermal response even after recycling. These attributes make the composite highly suitable for electronic skins, smart thermal regulation and overcurrent protection fuses. The integration of PCL and fatty acids (FAs) enhances recyclability, promoting sustainable and long-term applications in personal thermal management systems.