Underwater‐Stable Conductive Hydrogels: From Molecular Design to Next‐Generation Aquatic Sensors †

Comprehensive Summary Global priorities in ocean sustainability and biomedical innovation are accelerating the pursuit of materials that can sustain precise and adaptive sensing in complex aqueous environments. As nations invest heavily in marine technology and digital healthcare, underwater perception and communication are emerging as core capabilities for next‐generation intelligent systems. Meeting these demands requires materials that can endure dynamic ion‐rich conditions while replicating the softness, adaptability, and responsive‐ness of biological tissues. Within this context, conductive hydrogels, as a distinctive class of smart polymers, have emerged as essential building blocks for polymer composites capable of multifunctional sensing across marine and physiological environments. Their unique combination of hydrated ion transport, electronic tunability, and tissue‐like mechanics enables seamless coupling between electronic systems and biological or fluidic interfaces. However, conventional hydrogels suffer from intrinsic instability, including excessive swelling and conductive‐filler leaching, which compromise both mechanical robustness and signal fidelity. Recent advances in water‐resistant hydrogels have overcome these limitations through molecular and structural innovations. Hydrophobic modification, reinforced crosslinking, and hierarchical interpenetrating networks have yielded materials with exceptional anti‐swelling stability and long‐term conductivity under saline and high‐pressure conditions. Moreover, the stabilization of conductive interfaces via covalent anchoring, zwitterionic coordination, and hybrid ion–electron conduction ensures reliable signal transmission in dynamic underwater environments. These advances have enabled durable aquatic sensors for underwater motion tracking, physiological monitoring, and environmental perception. Beyond individual achievements, the field is evolving toward intelligent, integrated systems. The next generation of smart polymer sensors will feature multimodal perception, self‐healing, biodegradability, and AI‐assisted signal interpretation, enabling autonomous adaptation in complex aquatic environments. Looking forward, the fusion of polymer chemistry, bio‐inspired materials design, and data‐driven intelligence is expected to reshape underwater electronics into a new paradigm, where soft, sustainable, and perceptive hydrogel‐based composites serve as the material backbone of future oceanic and biomedical technologies. Key Scientists Over the past two decades, the development of underwater‐stable conductive hydrogels has been propelled by a series of landmark contributions from pioneering scientists worldwide. These milestones span from fundamental theoretical models to innovative structural designs. The concept of double‐network (DN) hydrogels, which dramatically enhanced the mechanical strength and anti‐swelling capability of hydrogels, was pioneered in 2003 by Jian Ping Gong and T. Kurokawa at Hokkaido University. [1] In 2009, Nicholas A. Peppas advanced the theoretical understanding of hydrogel swelling by developing a model based on Flory–Huggins theory . [2] In 2013, Jian Ping Gong and Tao Lin Sun introduced anti‐swelling electrolyte hydrogels through tailored cation‐anion interactions. [3] A systematic strategy for stretchable encapsulation of hydrogels with elastomers was reported in 2018 by Zhigang Suo at Harvard University. [4] This convenient and versatile strategy achieved excellent anti‐swelling capability without compromising the hydrogel's intrinsic conductivity. In the same year, Mingjie Liu developed organogel–hydrogel hybrids with outstanding anti‐swelling performance. [5] In 2019, Jian Ping Gong and Hui Guo pioneered spontaneous phase separation to form core–shell architectures for anti‐swelling. [6] In 2020, Suo's group further addressed interfacial issues, establishing the theoretical framework for hydrogel wet adhesion. [7] In 2021, Ximin He at the University of California, Los Angeles, achieved tough, anti‐swelling hydrogels by synergizing freeze‐casting and salting‐out techniques. [8] This strategy allowed for conductivity to be easily imparted by introducing polypyrrole without sacrificing the material's strength and toughness. In 2022, Shu‐Hong Yu and Huai‐Ping Cong developed electronically conductive composites with remarkable underwater stability. [9] By combining silver nanowires with a polyacrylamide matrix, they created a highly compressible, fatigue‐resistant hydrogel whose continuous conductive network provided excellent conductivity and a stable electrical response even after 1000 compression cycles in water. Most recently, in 2024, Jun Fu employed zwitterions and the Hofmeister effect to realize long‐term stability of hydrogels in seawater environments. [10] The introduction of H 2 SO 4 both enhanced anti‐swelling properties and provided conductive ions, resulting in an ionic conductivity as high as 4.35 S·m –1 and a sensing signal that showed no significant degradation after 1000 stretching cycles in seawater. In 2025, Rong Ran and Wei Cui developed a strong, anti‐swelling hydrogel using the synergistic effects of dense chain entanglement and phase separation. [11]