Anisotropic Tough and Non‐Swelling Hydrogels Based on Hofmeister‐Effect for Underwater Monitoring

Abstract Hydrogels are widely employed in wearable electronics, soft robotics, and human‐machine interfaces owing to their skin‐like softness, strain sensitivity, and biocompatibility. However, synthetic hydrogels often fall short of matching the mechanical robustness and fatigue resistance of natural load‐bearing anisotropic tissues with high water content. Additionally, most hydrogels are prone to swelling, which limits their application in humid environments. Herein, an anisotropic composite hydrogel with outstanding mechanical properties and non‐swelling properties is developed by using polyvinylalcohol (PVA) and carboxylated carbon nanotubes (CNT) through ice templates and salting‐out strategies. The composite hydrogel (PVA/CNT‐S) has anisotropic orientation structure with high water content (65%), exceptional tensile strength (3 MPa) and toughness (4 MJ·m −3 ), and electrical conductivity (0.11 S·m −1 ) along the alignment axis. With a swelling ratio of 0.8%, the hydrogel maintains structural integrity after one month in aqueous environments and endures 1400 underwater fatigue cycles without fracture. The PVA/CNT‐S can be used as a strain sensor with fast response (500 ms) and stable sensing performance (during 10 000 tensile cycles), enabling real‐time monitoring of robotic fish swimming and human motion under water. Moreover, it supports underwater Morse code communication and water level monitoring, offering a versatile platform for submersible sensing, distress signaling, and early alarming applications under aquatic environments.