Stretchable Skin Hydration Sensor Based on Hygroscopic and Ion Conductive Polymer Composites

Transepidermal water loss (TEWL) is water emission from the skin induced by the diffusion of water molecules. Because TEWL is sensitive to damage on the outermost skin layer, it has been utilized as an indicator of skin barrier function. Here, we demonstrate a stretchable hydration sensor to monitor TEWL based on a porous thermoplastic polyurethane (TPU) film coated with ion conductive polyvinyl alcohol (PVA)/lithium chloride (LiCl) composite layers. The stretchable hydration sensor with the large surface area and the hygroscopic PVA/ LiCl layer coated on the porous structure exhibits a remarkable relative current change (Delta I/I0 (%) similar to 107) under a wide humidity range (10-95 % relative humidity (RH)). Moreover, the highly deformable characteristics of the device up to 50 % strain makes it an appropriate tool for detecting water emission from a curved skin surface. For a proof-of-concept demonstration of monitoring skin barrier function, the hydration sensor can precisely perceive the change in TEWL in response to various external treatments (heat, damage, oil treatment, and applying cosmetics). Finally, the stretchable hydration sensor array enables non-contact finger motion perception through water loss detection, making it suitable for use in touchless human-machine interfaces.