Toughening Double-Network Hydrogels by Polyelectrolytes.

The Hoffmeister effect of inorganic salts is verified as a promising way to toughen hydrogels, however, the high concentration of inorganic salts may be accompanied by poor biocompatibility. In this work, it is found that polyelectrolytes can obviously elevate the mechanical performances of hydrogels through the Hoffmeister effect. The introduction of anionic poly(sodium acrylate) into poly(vinyl alcohol) (PVA) hydrogel induces the aggregation and crystallization of the PVA to boost the mechanical properties of the resulting double-network hydrogel: elevation of 73, 64, 28, 135, and 19 times in the tensile strength, compressive strength, Young's modulus, toughness, and fracture energy compared with poly(acrylic acid), respectively. It is noteworthy that the mechanical performances of the hydrogels can be flexibly tuned by the variation of polyelectrolyte concentration, ionization degree, relative hydrophobicity of the ionic component, and polyelectrolyte type in a wide range. This strategy is verified to work for other Hoffmeister-effect-sensitive polymers and polyelectrolytes. Also, the introduction of urea bonds into the polyelectrolyte can further improve the mechanical properties and antiswelling capability of hydrogels. As a biomedical patch, the advanced hydrogel can efficiently inhibit hernia formation and promote the regeneration of soft tissues in an abdominal wall defect model.