RESUMEN
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 ß-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 ß-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.
RESUMEN
Structural characteristics and thermal and solution properties of the regenerated silk fibroin (SF) prepared from formic acid (FU) were compared with those of SF from water (AU). According to the turbidity and shear viscosity measurement, SF formic acid solution was stable and transparent, no molecular aggregations occurred. The sample FU exhibited the beta-sheet structure, while AU random coil conformation using Fourier transform infrared (FTIR), X-ray diffraction (XRD), and differential scanning calorimetry. The effects of methanol treatment on samples were also examined. According to the measurement of crystallinity (XRD) and crystallinity index (FTIR), the concept of long/short-range ordered structure formation was proposed. Long-range ordered crystallites are predominantly formed for methanol treated SF film while SF film cast from formic acid favors the formation of short-range ordered structure. The relaxation temperatures of SF films measured by dynamic thermomechanical analysis supported the above mechanism due to the sensitivity of relaxation temperature on the short-range order.
