Design principles for strong and tough hydrogels

Hydrogels are crosslinked polymer networks swollen with water. Owing to their soft and water-containing nature, hydrogels are promising materials for applications in many fields, such as biomedical engineering, soft robotics and environmental studies. One of the main obstacles to the practical application of hydrogels is their low mechanical strength and toughness. Since the 2000s, many breakthroughs in the development of mechanically strong and tough hydrogels have led to enormous advances in the study of soft materials and our understanding of their failure mechanisms. Research has also been conducted on long-term mechanical stability — that is, the cyclic fatigue resistance and self-strengthening properties of hydrogels — to enable their application as load-bearing materials. This Review provides a comprehensive overview of the design principles for tough hydrogels. Strategies to obtain self-growing and reinforced hydrogels that can adapt to their surrounding mechanical environment are also presented.