Hygroscopic Auxetic On-Skin Sensors for Easy-to-Handle Repeated Daily Use.

Despite the advance of on-skin sensors over the last decade, a sensor that solves simultaneously the critical issues for using in everyday life, such as stable performance in various environments, use over a long period of time, and repeated use by easy handling, has not yet been achieved. Here, we introduce an auxetic hygroscopic sensor that simultaneously meets all of the conditions. The auxetic structure with a negative Poisson's ratio matches with deformation of the skin in ankles; hence, a conformal contact between the sensor and the skin could be maintained during repeated movements. Sweat was absorbed in the auxetic electrode made of a hydrogel pattern coated with Ag nanowires and evaporated quickly; such hygroscopic characteristic led to excellent breathability. An electrocardiogram sensor and a haptic device were fabricated according to the proposed design for a sensor electrode. The sensors provide stable detecting performance in various environments, such as exercising, submersion in water, exposure to concentrated salt water, and continuous wearing for long time (7 days). Also, the sensors could be manually attached repeatedly without degrading the performance. This study provides new structural insights for on-skin sensors and presents future research directions.