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

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 Ca² ⁺ 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.