Mimicking the Mechanical Properties of Cartilage Using Ionic- and Hydrogen-Bond Cross-Linked Hydrogels with a High Equilibrium Water Content above 70%

The fabrication of synthetic hydrogels, which simultaneously possess excellent mechanical properties and a high equilibrium water content (EWC) and can be potentially used to mimic the cartilage, one of the load-bearing tissues with a modulus of 1 MPa, a fracture strength of tens of MPa, an elongation at break of 100%, and an EWC of 75%, remains a difficult and challenging task. In this work, multiphysical cross-linked P(AAw-co-AMx)/XGy-GGz/Fe3+/B hydrogels were designed and prepared using acrylic acid (AA) and acrylamide (AM) as polymerizable monomers; xanthan gum (XG) and guar gum (GG) as macromolecular sources of hydrogel bonds; and ferric (Fe3+) and tetraborate (B4O72–) ions as sources of ionic cross-linking. Under optimized conditions, the synergistic effects of multiple ionic hydrogen bond cross-linking networks endow the obtained P(AAw-co-AMx)/XGy-GGz/Fe3+/B hydrogels with MPa levels of elastic moduli (1.4–3.3 MPa) and fracture strengths (1.5–2.6 MPa) and higher elongations at break of about 138–171% simultaneously with high EWCs above 70% (72–76%), as well as excellent self-recovery ability. Cytotoxicity tests confirmed that the P(AAw-co-AMx)/XGy-GGz/Fe3+/B hydrogels exhibited excellent cytocompatibility and had no effect on the cell morphology, cell cycle regulation, and apoptosis level. More importantly, after culturing in Eagle's minimum essential medium for 72 h, the P(AA0.4-co-AM2.0)/XG1.0-GG1.0/Fe3+/B and P(AA0.6-co-AM2.4)/XG1.0-GG1.0/Fe3+/B hydrogels could retain excellent elastic moduli and fracture strengths as well as EWCs close to those of the cartilage.