Transparent, Highly Stretchable, Self-Healing, Adhesive, Freezing-Tolerant, and Swelling-Resistant Multifunctional Hydrogels for Underwater Motion Detection and Information Transmission

Abstract

Conductive hydrogels have emerged as fascinating materials for flexible electronics because of their integrated conductivity, mechanical flexibility, and the possibility to introduce several smart functions. However, the swelling of hydrogels in aqueous environments significantly reduces their applicability where contact with water is unavoidable. In this study, a physically cross-linked composite hydrogel is proposed, that is transparent, highly stretchable, anti-swelling, capable of autonomous self-healing, adhesive, and anti-freezing. The hydrogel is synthesized through a simple one-step photopolymerization in a novel deep eutectic solvent (DES)/water system. Dynamic physical interactions, including hydrophobic interaction, hydrogen bonding, and electrostatic interactions, confer remarkable transparency (92%), self-healing capability (up to 94%), good adhesion to a wide array of substrates (91 to 199 kPa), high toughness (1.46 MJ m⁻³), excellent elongation at break (up to 2064%), and resistance to swelling in water (equilibrium swelling ratio of 3% in water for 30 days) even in solutions at different pH (pH 1–11), and in other solvents. The incorporation of a DES contributes to exceptional anti-freezing performance. The transparent sensor achieves multifunctional sensing and human motion detection with high sensitivity and stability. Notably, the sensor demonstrates information transmission underwater through stretching and pressing, showcasing its immense potential in underwater flexible devices.