Double-Network Physical Cross-Linking Strategy To Promote Bulk Mechanical and Surface Adhesive Properties of Hydrogels

Development of mechanically strong and adhesive hydrogels with self-recovery and self-healing properties is important for many applications but has proven to be very challenging. Here, we reported a double-network design strategy to synthesize a fully physically cross-linked double-network (DN) hydrogel, consisting of the first gelatin network and the second poly(N-hydroxyethyl acrylamide) network where both networks were mainly cross-linked by hydrogen bonds. The resultant gelatin/pHEAA hydrogels exhibited high mechanical property (tensile stress of 1.93 MPa, tensile strain of 8.22, tearing energy of 4584 J/m2), fast self-recovery at room temperature (toughness/stiffness recovery of 70.2%/68.0% after 10 min resting), and good self-healing property (self-healed tensile stress/strain of 0.62 MPa/3.2 at 60 °C for 6 h). More importantly, gelatin/pHEAA hydrogels also exhibited strong surface adhesion on different hydrophilic solid surfaces, as indicated by high adhesion energy (i.e., interfacial toughness) of 645 J/m2 on glass, 867 J/m2 on Al, 702 J/m2 on Ti, and 579 J/m2 on ceramics. Surface adhesion can be largely retained after multiple, repeatable adhere on/peel off actions. Reversible and strong mechanical properties in bulk and on solid surfaces are likely attributed to reversible hydrogen bondings and physical coordinate bonds between the networks and between networks and surfaces. This work demonstrates our design principle that multiple physical bonds in both networks offer excellent mechanical recoverability, self-healing, and self-adhesive properties, while DN structure provides strong and tough mechanical properties via efficient energy dissipation by sacrificed bonds, which offers a new possibility to develop next-generation hydrogels with desirable properties used for soft robotics, wearable electronics, and human–machine interfaces.