Moisture sensor based on heat transfer possessing insusceptibility to coating materials on skin

This paper presents a thermal-based skin moisture sensor that is insusceptible to thin coating materials on skin. To realize this sensor, numerical simulations are performed to evaluate the influence of surface-coating materials, such as Vaseline moisturizer, on the heat transfer from a heat source in the sensor to the skin. The simulation results show that the influence of the thin Vaseline layer on the temperature change of the heat source is negligible, but the existence of air gaps between the sensor and the skin drastically interrupts the heat transfer. Considering the simulation results, the thermal-based skin moisture sensor is designed and fabricated with a microelectromechanical systems (MEMS) pressure sensor to avoid heat transfer interruption due to air gaps. The measurement results of water content in a water-absorbing polymer, covered with a 10-μm-thick polyethylene film, show that the moisture sensor can assess the water content of materials that are covered with a thin layer. Finally, we demonstrate the hydration measurement of skin with Vaseline coating. The measurement results confirm the potential of the moisture sensor to measure the water content of skin covered with moisturizing substances.