Self-stratified stretchable passive cooling interface for thermal management of on-skin electronics

Heat accumulation from Joule heating and solar irradiation severely challenges thermal safety and signal stability of long-term, outdoor on-skin electronics and human-machine interfaces. Here, we develop a self-stratified stretchable passive cooling interface (SPCI) that implements a synergistic internal-dissipation and external-blocking strategy by inducing gradient stratification via the density mismatch between Al₂O₃ microparticles and liquid metal (LM) within the elastomer. This mismatch triggers spontaneous self-stratification upon curing, yielding a multi-layer composite that features an Al₂O₃-enriched top layer for high solar reflectance (92.6%) and an LM-network-enriched bottom layer for efficient heat dissipation (thermal conductivity ≈ 1.5 W m⁻¹ K⁻¹), while retaining soft mechanics (elastic modulus ≈ 0.082 MPa) and high stretchability (> 800% elongation). The SPCI exhibits excellent cooling performance in serpentine circuits at 200 mW, reducing peak temperature by up to 8.4 °C indoors and 12 °C outdoors under solar exposure, while maintaining effective cooling under tension. When integrated into a wireless skin-interfaced photoplethysmography (PPG) platform for heart-rate monitoring, it lowers the maximum surface temperature by 8.6 °C during outdoor operation, enabling superior preservation of pulse-wave features compared with conventional elastomer encapsulation. This work establishes a scalable, mechanically compliant encapsulation interface for simultaneously mitigating internal and external thermal loads in wearable electronics and human-machine interfaces.