Toughening hydrogels with small molecules: tiny matter, big impact

Hydrogels are three-dimensional, crosslinked networks of hydrophilic polymers with mesh sizes ranging from nanometers to microns. Different from solvent-free networks such as elastomers, hydrogels can retain significant amounts of water within their structure, enabling the diffusion of small molecules (e.g., ions, solvents, monomers) throughout the network. These small molecules often play a pivotal role in altering the hydrogel structure by driving processes such as ionic crosslinking, phase separation, and crystallization. Extensive studies have demonstrated that small molecules can influence hydrogel networks either through direct interactions or by inducing changes in the surrounding environment, both of which can lead to substantial enhancement in mechanical performance; yet this area has not been comprehensively reviewed. To address this gap, this paper summarizes how small molecule-induced structural transformations contribute to hydrogel toughening and outlines design strategies that employ these effects. The review also highlights cutting-edge applications and discusses challenges and opportunities in developing tough hydrogels with the aid of small molecules. We hope this review provides helpful insights into the impact of small molecules on macromolecular networks and fosters the development of next-generation soft materials.