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 aqueous NaOH 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 of up to 7.1 MPa. Various complex hydrogel structures can be constructed by using the P­(AM-co-AA)/Na-alginate/Fe³⁺ hydrogels as building blocks via the adhesion of as-prepared hydrogels.