Solvent exchange assisted fabrication of strong and swelling-resistant composite hydrogels

Hydrogels with intriguing properties have shown promising applications in biological tissues. However, their mechanical properties are still unsatisfactory at a relatively high water content, while the poor swelling resistance severely limits their applications in underwater environment and human body fluids. Here we propose a solvent exchange assisted phase separation strategy to prepare strong and tough composite hydrogels with excellent swelling resistance. During the solvent exchange process, Polyvinyl alcohol (PVA) macromolecules are aggregated and further physically crosslinked, whereas thermoplastic polyurethane (TPU) macromolecules are in-situ phase-separated in the form of rigid yet extensible microspheres with strong interfacial interaction with PVA. TPU reinforced hydrogels (PURHs) show a relatively high water content of ∼80 %, and their tensile strength, fracture strain, fracture energy and fatigue threshold can reach as high as 1.51 MPa, 638.2 %, 1896.4 J/m 2 and 515.8 J/m 2 , respectively. Benefiting from the increased crystallinity and the hydrophobicity of the PU, PURH exhibits excellent anti-swelling performance and the retention rate of its mechanical properties is kept at ∼90 % even after immersion in water for 30 days. PURHs are also biocompatible and can promote cell migration. This study provides a new method for preparation of strong and anti-swelling hydrogels that achieve trade-off between high mechanical properties and water content.