Silk granular hydrogels self-reinforced with regenerated silk fibroin fibers

Granular hydrogels with high stability, strength, and toughness are laborious to develop. Post-curing is often employed to bind microgels chemically and enhance mechanical properties. Here a unique strategy was investigated to maintain microgels together with a novel self-reinforced silk granular hydrogel composed of 10 wt% 20 kDa poly(ethylene glycol) dimethacrylate microgels and regenerated silk fibroin fibers. The principle is to use the swelling of microgels to concentrate the surrounding solution and regenerate silk fibroin in situ. Self-reinforcement is subsequently one of the added functions. We showed that silk fibroin in most compositions was homogeneously distributed and had successfully regenerated in situ around microgels, holding them together in a network-like structure. FTIR analysis revealed the presence of amorphous and crystalline silk fibroin, where 50% of the secondary structures could be assigned to strong β-sheets. Swelling ratios, i.e. 10-45 vol%, increased proportionally with the microgel content, suggesting that mainly microgels governed swelling. In contrast, the elastic modulus, i.e. 58-296 kPa, increased almost linearly with silk fibroin content. Moreover, we showed that the precursor could be injected and cast into a given shape. Viscous precursors of various compositions were also placed side by side to create mechanical gradients. Finally, it was demonstrated that silk granular hydrogel could successfully be synthesized with other microgels like gelatin methacryloyl. Silk granular hydrogels represent, therefore, a novel class of self-reinforced hydrogel structures with tunable swelling and elastic properties.