Transparent Ionic Skin: Minimal [EMIM]Cl Enhances Nanocellulose Hydrogel Conductivity for Superior Wearable Sensing

ABSTRACT Growing focus on health and quality of life is driving increasing demand for skin‐like wearable sensors in human motion monitoring and healthcare. Unlike traditional e‐skin, ionic skin utilizes a polymer network scaffold with mobile ions, effectively overcoming the issue of poor dispersion of conductive fillers in polymer matrices. As an ionic liquid with facile synthesis and low cost, 1‐ethyl‐3‐methylimidazolium chloride ([EMIM]Cl) forms strong interactions with both polymers and water molecules. Cellulose is a natural polymeric material with advantages such as low cost, environmental friendliness, and renewability. 2,2,6,6‐Tetramethylpiperidinyl‐1‐oxyl (TEMPO)‐oxidized cellulose nanofibrils (TOCNs) exhibit excellent biocompatibility. In this work, the TOCN‐[EMIM]Cl ionic hydrogel was formed by mixing a TOCN dispersion with [EMIM]Cl ionic liquid, followed by Ca 2 + cross‐linking. By adjusting the [EMIM]Cl content from 0 to 3 wt.%, the conductivity of the TOCN‐[EMIM]Cl hydrogel increased from 9.43 × 10 −5 to 4.13 × 10 −4 S cm −1 . The obtained ionic skin exhibits high transparency, with a sensitivity of 2.11 kPa −1 , rapid response/recovery times (< 50 ms), and excellent cyclic stability (> 5000 cycles). Stable and distinguishable signal outputs have been achieved for human joint movements (wrist, elbow, and knee), demonstrating significant potential in flexible wearable sensors and health monitoring applications.