Mechanically Robust and Underwater‐Stable Hydrogels via Synergistic Molecular Locking and Topological Entanglement

ABSTRACT Highly entangled hydrogels typically suffer from swelling‐induced dilution of chain entanglements in aqueous environments, leading to compromised energy dissipation and mechanical integrity. Herein, we report an underwater‐stable and highly entangled poly(acrylamide–acrylic acid) hydrogel coordinated with Fe 3+ (EH‐P(AM‐AC)@Fe 3+ ), which integrates exceptional toughness with sustained energy dissipation. Microstructural analysis reveals that dynamic Fe 3+ –carboxylate coordination acts as molecular locking that constrain polymer chain mobility, effectively densifying the network and preserving the topological entanglement structure. This synergistic interplay maintains the integrity of the slippage‐based dissipation mechanism under aqueous conditions. The hydrogel exhibits outstanding mechanical performance both in air and underwater, including a compressive strength exceeding 500 MPa, a tensile strength above 15.6 MPa, and an ultrahigh load‐to‐weight ratio (>12 000; 0.8 g supporting 10 kg). Stress relaxation and creep analyses demonstrate that molecular locking and entanglement coupling suppress chain slippage and disentanglement, increasing the relaxation activation energy. The hydrogel shows remarkable damage tolerance, with cutting and puncture resistances of 250 and 75 MPa, respectively. The hydrogel also exhibits excellent underwater sensing stability, maintaining stable and consistent signal output under turbulent conditions. These findings provide a general strategy for designing robust, underwater‐stable hydrogels with promising applications in flexible protection and bioinspired robotics.