A general strategy for strengthening and toughening physical hydrogels via anti-Hofmeister sequence solid salting-out

In recent years, many strategies have been developed to enhance the mechanical properties of hydrogels. However, due to the complex process, the addition of non-green crosslinking agents and the use of high concentration salt solutions will lead to increased costs, resource waste and environmental pollution. In addition, the synergistic improvement of strength and toughness of hydrogels is still challenging. Herein, a general green strategy of solid salting-out to improve the strength and toughness of hydrogels is reported. This strategy can induce the directional phase separation of water molecules in the hydrogel, promoting the close layer-by-layer self-assembly of the PVA molecular chain. Compared with liquid salting-out, solid salting-out has a more significant improvement in the strength of hydrogels, and solid salt blocks can be recycled and reused. A PVA hydrogel prepared by the solid salting-out method showed both high strength and toughness that were 1962- and 1900-fold of those before solid salting-out, respectively. The strength and toughness of the hydrogel can be further improved to 109.06 MPa and 125.73 MJ m−3 by inducing the collaborative self-assembly of cellulose nanofibers and PVA molecular chains. As a general strategy, solid salting-out could enhance various hydrogels composed of materials such as silk, gelatin, and cotton staple cellulose. Thus, this study not only provides a simple method for the strengthening and toughening of hydrogels but also has potential applications in the fields of biological implants because of the favorable biocompatibility of the prepared hydrogels.