Design Strategies and Perspectives on the Toughening of Hydrogels via Fully Physical Cross-Linking

Abstract

Conventional hydrogels are inherently brittle and mechanically weak, limiting their application in load-bearing or dynamic environments. Although extensive development has been made in hydrogel toughening, the most dominant techniques rely upon chemical cross-linking, which restrains their adaptability and functionality because of the permanence of covalent bonds. While dynamic covalent bonds have been introduced to enhance reversibility in covalently cross-linked systems, they often require harsher conditions, display delayed responsiveness, and involve more complex chemistry. Given these challenges, physical cross-linking methods─such as metal–ligand coordination cross-links, crystalline region formation, electrostatic interactions, hydrophobic association, polymer chain entanglement, host–guest interaction, and hydrogen bonding─have been considered promising strategies to enhance both toughness and dynamic features. These characteristics provide high versatility and practicality, enabling advanced applications in areas such as soft robotics and tissue engineering. This review presents a comprehensive analysis of strategies and perspectives for toughening hydrogels via fully physical cross-linking and highlights emerging applications that exploit the unique advantages of reversible physical networks.