A micro-to-macroscale and multi-method investigation of human sweating dynamics

Sweat secretion and evaporation from the skin dictate the human ability to thermoregulate and thermal comfort in hot environments and impact skin interactions with cosmetics, textiles and wearable electronics/sensors. However, sweating has mostly been investigated using macroscopic physiological methods, leaving micro-to-macroscale sweating dynamics unexplored. We explore these processes by using a coupled micro-imaging and transport measurement approach used in engineering studies of phase change processes. Specifically, we used a comprehensive set of 'macroscale' physiological measurements (ventilated capsule sweat rate (SR), galvanic skin conductance and dielectric epidermis hydration) complemented by three microscale imaging techniques (visible light, midwave infrared and optical coherence tomography imaging). Inspired by industrial jet cooling devices, we also explore an 'air jet' (versus cylindrical) capsule for measuring SR. To enable near-simultaneous application of these methods, we studied forehead sweating dynamics of six supine subjects undergoing passive heating, cooling and secondary heating. The relative dynamics of the physiological measurements agree with prior observations and can be explained using imaged microscale sweating dynamics. This comprehensive study provides new insights into the biophysical dynamics of sweating onset and following cyclic porewise, transition and filmwise sweating modes and highlights the roles of stratum corneum hydration, salt deposits and microscale hair.