Flexible planar capacitive devices for hydration and sweat sensing

Skin is one of the most complex structures in the body, with many physiological functions. Skin acts as the barrier or an interface between the external environment and internal organs. Hydration within the skin is varied, known as the skin's water-loading. Perspiration occurs when watery fluid is secreted through the eccrine and apocrine glands. Flexible epidermal sensors are fabricated, which can be used to measure skin hydration and perspiration (sweat) as these sensors need to be skin-conformable. Polyimide and polydimethylsiloxane are used as they are flexible and skin compliant, and the sensing layer is formed on them. The sensitivity of hydration sensors was in the range of 0.002-0.0046/%, while for sweat sensors, it was in the range of 0.092-0.116 mu l(-1). Stability tests indicated that external factors such as environment or physical deformation and skin curvature do not affect the performance of the as-prepared sensors. The sensitivity and stability results of the planar capacitor are highly suitable for flexible hydration and sweat-sensing applications. The proposed sensors offer an outstandingly good option for incorporation into wearable systems for physical personal health monitoring. In the future, we plan to integrate these sensors on a single substrate to create a multimodal device.