Ultrastiff, Tough and Healable Ionic-Hydrogen Bond Cross-linked Hydrogels and Their Uses as Building Blocks to Construct Complex Hydrogel Structures.

We report the ultrastiff and tough poly(acrylamide-co-acrylic acid)/Na-alginate/Fe3+ (P(AM-co-AA)/Na-alginate/Fe3+) hydrogel via the formation of hybrid ionic-hydrogen bond cross-linking networks. The optimal P(AM-co-AA)/Na-alginate/Fe3+ hydrogel possessed super high elastic modulus (~24.6 MPa), tensile strength (~10.4 MPa), compression strength (~44 MPa), and toughness (~4800 J/m2). The P(AM-co-AA)/Na-alginate/Fe3+ hydrogel was highly stable and maintained its superior mechanical properties in 0.5-2 M NaCl solution, aqueous solution with pH ranging from 4 to 10. The ionic cross-linking networks of the P(AM-co-AA)/Na-alginate/Fe3+ hydrogels can be locally and selectively dissociated by treating with NaOH aqueous solution with pH of 13 for 1 min and reformed by locally adding the additional Fe3+ solutions, making the hydrogels healable and cohesive. The healed hydrogels from the cutting surfaces can bear a tensile strength up to 7.1 MPa. Various complex hydrogel structures can be post constructed by using the P(AM-co-AA)/Na-alginate/Fe3+ hydrogels as building blocks via the adhesion of virgin prepared hydrogels.