Conformal sensor-based harmonic wave technique for in-vivo non-invasive monitoring skin water content

•An in-vivo non-invasive technique is developed for skin water content measurement.•Theoretical relation between thermal conductivity and water content is established.•Gender, age and BMI have little influence on water content for young population.•Water content at different depths and sites of the skin is obtained and explained. The thermal conductivity of human skin reflects the water content of skin, and thus is a strong indicator of skin health status. The current diagnosis methods for human skin can only detect the water content in a small depth beneath the skin surface, typically within the stratum corneum layer (10–20 μm), which fails to extract the water content in larger depth such as dermis, thereby limiting the timely diagnosis of some severe skin diseases (psoriasis, dry skin, skin eczema, etc.). Here we developed a conformal sensor-based harmonic wave technique which can modulate the thermal wave penetration depth and thus measure the water content in both epidermis and dermis. Variations in water content for three factors (age, site and BMI index) of different subjects were discussed. The progress made so far provides a useful approach for monitoring the water content of human skin and early warning of skin diseases.