A Flexible Wireless Passive Platform for Decoupled Electrolyte and Temperature Sensing Toward Heat‑Stress Assessment

Strenuous exercise and heat exposure can significantly elevate core body temperature and induce profuse sweating, leading to dehydration, electrolyte imbalance, and an increased risk of heat stress. Simultaneous yet decoupled monitoring of thermal and sweating-related signals is therefore essential for reliable physiological assessment under intense motion. Here, we report a flexible, wireless, and fully passive dual-mode sensing platform that enables independent detection of sweat accumulation and skin temperature on a single LC resonant architecture. The system integrates a sweat-responsive electrochemical module and a thermosensitive conductive composite, both inductively coupled to a flexible serpentine coil for battery-free near-field interrogation. We establish an orthogonal "frequency-magnitude" decoding strategy: sweat-induced dielectric changes dominantly drive resonance frequency shifts, while temperature-dependent resistive losses primarily modulate the reflection coefficient magnitude (S₁₁​). To further enhance robustness against signal coupling and motion artifacts, a machine-learning-assisted spectral decoder is introduced, enabling high-accuracy recognition of joint physiological states. Owing to its lightweight, maintenance-free operation, and robust performance under vigorous motion, this platform provides a promising strategy for heat-stress assessment and real-time physiological monitoring in demanding scenarios such as military training and competitive sports.