Sensitivity of on-skin thermometry to detecting dermal dehydration

The recent development of flexible sensors that can measure temperatures at the surface of the skin opens novel possibilities for continuous health monitoring. Here, we investigate such sensors as 3x thermometers to noninvasively detect deep dermal dehydration. Using numerical simulations, we calculate the temperature rise at the sensor at heating frequencies from 10 mHz to 10 Hz at varying levels of dehydration. The heating power in each case is limited to avoid burn injury. Our results indicate that 10-100 mHz frequencies are necessary to detect deep dermal dehydration. We show that the root-mean-square difference in temperature rise between normal and dermally dehydrated skin can be as high as 250 mK, which is detectable using lock-in techniques. Thermal contact resistance between the sensor and skin can dominate the signal when the resistance exceeds 10^-3 Km²/W. This work provides quantitative limits for sensing human dehydration using noninvasive sensors that measure the thermal conductivity of the skin structure.