Spray‐Coated Fully Water‐Soluble Composites for Waterproof Breathable E‐Tattoos

Electronic tattoos (e‐tattoos), which can be conformally attached to the human skin for real‐time, continuous, and noninvasive sensing of physiological signals, hold great promise for personalized healthcare applications. However, e‐tattoos based on these water‐soluble materials are easily degraded by sweat in the course of daily exercise, leading to the inaccuracy of human signal sensing for long‐term monitoring. Herein, enhanced waterproof breathable e‐tattoos via spray deposition of a water‐soluble composite ink containing poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS), single‐walled carbon nanotubes, waterborne polyurethane, and maltitol (as a plasticizer) is developed. The sprayed e‐tattoos exhibit a distinctive 3D network structure. This unique architecture endows them with exceptional waterproof properties, excellent conformal adhesion, breathability, and biocompatibility, thereby ensuring their long‐term wearability. As epidermal electronic devices, the highly conformal e‐tattoos serve as ion‐selective interdigital electrodes and seamlessly integrated with a flexible printed circuit board. Such integration enabled wireless, continuous, and real‐time monitoring of potassium ion (K + ) concentrations in human sweat during daily activities. Therefore, the waterproof and breathable e‐tattoos with excellent comprehensive performance could provide a promising platform for next‐generation wearable epidermal electronics.