Regenerated Bombyx mori silk fibroin (SF) has excellent biological and mechanical properties, including biocompatibility, programmable biodegradability, and remarkable strength and toughness. Diverse and adaptable properties of SF are possible by varying the structural form using different processing conditions. One of the important physical forms for biomaterials is the formation of hydrogels, which has been extensively studied for a variety of polymers such as alginates, chitosan, and collagen. SF solutions at dilute concentrations ≤ 5 wt% have been previously studied with respect to hydrogel formation. The sol-gel transition depended on the concentration of the SF, temperature, and pH. In the SF hydrogel, random coil to β-sheet structural transitions were noted during the process of hydrogelation. The formation of thermodynamically stable β-sheet structure serves as physical cross-links, stabilizing SF molecules. Sufficient energy to overcome the energy barrier for β-sheet crystallization by heating, vortex shearing, and ultrasonication, facilitates the hydrogelation of SF. Due to the β-sheet formation, SF exhibits relatively slow degradation in vitro and in vivo, compared to collagens and many other biopolymers.
The radiation technique seems to be an excellent pathway for the preparation of hydrogels, because a polymer in an aqueous solution undergoes chemical crosslinking and graft copolymerization on irradiation to yield a hydrogel. Simple procedure control, no initiators and cross-linkers, no waste, and relatively low operating costs make an irradiation technique a suitable choice for the synthesis of hydrogels.
In this study, the synthesis and characterization of a SF hydrogel via chemical cross-linking reaction of SF using electron beam (E-beam) and gamma-ray (γ-ray) were investigated. Two different hydrogels were investigated: (1) physically cross-linked SF hydrogel, (2) chemically cross-linked SF hydrogel, irradiated at difference doses of γ-ray. The effect of irradiation types, dose and SF concentration on the hydrogelation of Sf was investigated. The chemically cross-linked SF hydrogel was compared with physically cross-linked on in terms of structure and gel strength. Further, swelling behavior, crystallinity and biodegradation of SF hydrogel were characterized.