Dual mode gelation behavior of silk fibroin microgel embedded poly(ethylene glycol) hydrogels

Hydrogel formation by more than two cross-linking mechanisms is preferred for the sophisticated manipulation of hydrogel properties. Both chemical and physical crosslinks are often utilized for fabricating stimuli-responsive hydrogels or for compensating the drawbacks of the single crosslinking method. In this study, silk fibroin (SF) microgel embedded poly(ethylene glycol) (PEG) hydrogels were fabricated by dual mode cross-linking based on thiol-ene photo-click chemistry and β-sheet formation of SF. Norbornene-functionalized SF (SF-NB) was incorporated into PEG hydrogels by photo-cross-linking. The equilibrium shear modulus of SF-PEG hybrid hydrogels decreased with increasing SF-NB content. However, the incorporation of SF-NB caused stiffening of SF-PEG hybrid hydrogels gradually over 5 days and the gel modulus was maintained for 2 weeks. In contrast, the modulus of pure PEG hydrogels decreased continuously owing to hydrolytic degradation of ester bonds in the PEGNB macromers. Structural analysis revealed that such a post-gelation stiffening effect was caused by β-sheet transition in SF microgels embedded in the PEG hydrogel matrix. PEG hydrogels incorporated with 4 wt% SF microgels exhibited about 2-fold increase in shear modulus compared with the modulus on day 1 post-gelation. To evaluate the compatibility of these hydrogels as cell culture matrices, the cytotoxicity of the hydrogel was examined using in situ encapsulated A549 cells. SF-PEG hybrid hydrogels showed no apparent cytotoxicity and promoted the proliferation of encapsulated A549 cells even at a higher gel modulus compared with cells in pure PEG hydrogels. These results suggest that SF-PEG hybrid hydrogels fabricated by dual mode crosslinking serve as good candidates for three-dimensional cell culture requiring temporal control of hydrogel stiffness.