Nanostructure Evolution of Biomimetic Hydrogel from Silk Fibroin and Poly(N-Vinylcaprolactam): A Small Angle Neutron Scattering Study

The nanostructure of biomimetic hydrogels fabricated using regenerated silk fibroin (RSF) protein and thermoresponsive poly(N-vinylcaprolactam) (PVCL) polymer via physical and chemical crosslinking methods have been studied using Small Angle Neutron Scattering (SANS) technique. RSF showed no gelation and/or secondary structure (Gaussian coil) change at temperatures between 12 °C and 35 °C, whereas PVCL formed physical hydrogels by molecular self assembly exhibiting swollen coil to Gaussian coil structure change above a lower critical solution temperature (LCST) of ~32 °C. RSF-PVCL blend solutions were also observed to form physical hydrogels above this LCST. However, RSF-PVCL hybrid system exhibited swollen coil to collapsed coil structure change, suggesting relatively increased gelation compared to PVCL. Contrast variation SANS experiments demonstrate that in RSF-PVCL hybrid system, RSF provokes the intrinsic structural changes in PVCL without undergoing any structural change of its own. Unlike PVCL and other biomimetic polymer hydrogels, RSF hydrogels exhibited ordered structures containing hydrophobic domains which exhibit sharp interfaces with hydrophilic domains and/or amorphous protein matrix. Further, photochemically crosslinked RSF and RSF-PVCL hybrid hydrogels showed variation in the order of secondary structures in hydrophilic domains, which can be associated to their reported difference in water uptake behavior.