High Entanglement in Hydrogels: From Polymer Physics to Robust Mechanics.

Entangled hydrogels offer a strategy to address the mechanical limitations of swollen networks, including conflicts between stiffness and toughness and between water content and robustness. In these systems, dense topological constraints generated by chain interpenetration act as dynamic crosslinks that complement covalent junctions, promoting stress transfer, recoverable energy dissipation, and improved fatigue and anti-swelling performance. This review first outlines the polymer-physics basis of entanglement, emphasizing tube and reptation concepts, entanglement molecular weight, and entanglement density, and the respective contributions of chemical crosslinks and entanglements to the elasticity of hydrogels. It then analyzes key factors governing the degree and stability of entanglement, including concentration, architecture and flexibility of chains, solvent conditions, and processing history. Experimental methods for probing entangled networks are summarized, covering mechanical and rheological testing together with scattering, imaging, and spectroscopic techniques that access structure and dynamics over different length and time scales. The role of entanglement in the design of robust hydrogels is discussed, highlighting how linking polymer-physics descriptors to network design has enabled robust performance in applications such as wound repair and adhesive biomedical patches, high-deformation flexible sensors, hydrogel electrolytes for energy devices, and mechanically stable environmental remediation materials. Finally, the review highlights current challenges associated with swollen-state entanglement physics, long-term durability and environmental stability, and quantitative structure-property relationships needed for predictable design and control of entangled hydrogel networks.