Harnessing sacrificial bond kinetics for hydrogel self-strengthening

Biological tissues can self-renew and upgrade by recombining building blocks like amino acids from their environment. This reflects an open, dynamic growth process that allows adaptation to mechanical changes. Artificial materials, however, are usually based on closed, static systems. They cannot interact with the environment or grow structurally, making them progressive damage until failure in cyclic loading due to the absence of a structural reconstruction mechanism.

To meet practical requirements, it is crucial to endow artificial materials with self-strengthening capabilities akin to biological structures to withstand cyclic fatigue behaviors encountered in daily use. In recent years, researchers have created various partially or fully self-healing materials. The self-stiffening materials during mechanical use — by incorporating composites or liquid crystal elastomers — have also been developed. Additionally, using mechanochemical approaches to reshape polymers have been explored.

 Mechanochemistry uses physical forces (e.g., grinding, extrusion, and shearing) instead of traditional energy sources (heat, light, and electricity) to break and recombine polymer bonds, achieving structural reconstruction. This interdisciplinary field connects polymer chemistry, materials science, and mechanics, offering a foundation for designing force-responsive soft materials. However, reshaping bulk solid materials using these molecular mechanisms to enhance their mechanical properties remains a significant challenge.

As a typical example, dual-network (DN) hydrogels, promising flexible materials, achieve enhanced toughness by incorporating an energy dissipation mechanism via sacrificial bonds. In the initial design, the two networks are constructed by short-chain and long-chain molecules crosslinked by covalent bonds, respectively. When stretched, the short-chain network breaks and dissipates energy but suffers permanent damage. After the first loading cycle, the gel cannot fully recover, and accumulated damage eventually leads to failure. To enable recovery, sacrificial covalent bonds are replaced with non-covalent (typically physical) bonds. Under stress, the covalent-bonded network maintains structural integrity, while the physical network breaks bonds to dissipate energy. Under unloading, the physical network reforms, providing both enhanced toughness and self-healing capabilities.