Hydrogels May Not Always Absorb Water: Strategies to Achieve Antiswelling and Negative Swelling

The intrinsic high water affinity of hydrogels makes them easy to overswell in water, which can cause reduced mechanical performance and structural deformation. A promising solution to those issues is enabling antiswelling or negative swelling. Recently, extensive efforts have been spent on relevant studies. The results show it is possible to eliminate or even reverse hydrogel swelling by modulating critical forces during the swelling process. The design and working principle of a series of state-of-the-art strategies are discussed, whose success reveals that the comprehensive performance and applicability of hydrogel functional materials can be greatly enhanced via controlled water absorbance. The achievement of antiswelling and negative swelling provides additional dimensions to tune the performance of hydrogels. As of today, several feasible strategies have been developed including restricting osmotic swelling, creating dense crosslinks, and setting up phase separation. Without having to swell significantly in water, hydrogels could be more stable and robust, which is critical to practical applications.