Water: The soul of hydrogels

Hydrogels are soft and wet materials with a three-dimensional porous network structure, capable of swelling to retain a large volume of water and maintaining semi-solid integrity. In general, their diverse structures are mainly determined by the type of polymer matrix, the method and degree of crosslinking, and the three-dimensional structure. However, all different hydrogels share water as their core theme. The water content and organization, both at the surface and within the hydrogels, are crucial factors influencing their many physical properties. Over the past years, their formulations and applications have made transformative advances. But the construction of novel hydrogel systems requires understanding how water molecules or solutes interact with the hydrogel. Herein, this review reexamines hydrogels from the perspective of water and summarizes the states, distribution, and behavior of water within hydrogels, as well as the hydrogel properties imparted by water. We also enumerate the techniques for detecting water in hydrogels and discuss the latest progress in the regulation and design of water-hydrogel systems and their unique role in key applications. Thus, the role of water within hydrogels extends far beyond merely acting as a solvent; it is one of the key factors bridging the structure–function relationship in hydrogels.