Toughened hydrogels inspired by aquatic caddisworm silk
Aquatic caddisworm silk is a tough adhesive fiber. Part of the toughening mechanism resides in serial, Ca2+–phosphate crosslinked nano-domains that comprise H-fibroin, the major structural protein. To mimic the toughening mechanism, a synthetic phosphate-graft-methacrylate prepolymer, as a simple H-fibroin analog, was copolymerized within a covalent elastic network of polyacrylamide. Above a critical phosphate sidechain density, hydrogels equilibrated with Ca2+ or Zn2+ ions displayed greatly increased initial stiffness, strain-rate dependent yield behavior, and required 100 times more work to fracture than hydrogels equilibrated with Mg2+ or Na+ ions. Conceptually, the enhanced toughness is attributed to energy-dissipating, viscous unfolding of clustered phosphate–metal ion crosslinks at a critical stress. The toughness of the bioinspired hydrogels exceeds the toughness of cartilage and meniscus suggesting potential application as prosthetic biomaterials. The tough hydrogels also provide a simplified model to test hypotheses about caddisworm silk architecture, phosphate metal ion interactions, and mechanochemical toughening mechanisms.