Stimuli-responsive Protein-based Hydrogels by Utlizing beta-Sheet Conformation of Silk Fibroin as Cross-links

Abstract

Stimuli responsive polymers can provide a variety of applications for biomedical fields such as drug delivery, biotechnology, and chromatography. The interest in these polymers has exponentially increased due to their promising potential. Stimuli-responsive polymers have been utilized in various forms: hydrogels, micelles, modified interfaces, and conjugated solutions. Among them, hydrogels have gained strong attention as biomaterials due to their biocompatibility and biodegradability in the swollen state. The introduction of stimuli-responsive characteristic into hydrogels should provide more versatile applications such as targeted drug delivery, micro or nano scale actuating valves, artificial organs responding to stimuli, and protein or DNA purification. In many applications, better biological materials are needed, particularly the incorporation of two or more functionalities into one material.

One strategy is to develop interpenetrating polymer networks (INPs) in hydrogels. Novel protein-based complex hydrogels were prepared by blending gelatin (Gel) with Bombyx mori silk fibroin (SF) and introducing beta-sheet conformation of SF in their complex networks. The influence of solvent-induced SF crystallization on the properties and structures of these binary protein complexes was determined as functions of blend composition and preparation history. Rheological tests confirmed that the fine beta-sheet crystalline structure successfully governed the Gel/SF complex networks, increasing their viscoelastic properties and sustaining their physical form as hydrogels even at body temperature. The helix-coil transition of gelatin in the Gel/SF complex hydrogels was determined by DSC and rheological tests to be reversible between ambient and body temperatures, so these hydrogels exhibit reversible IPNs/semi-IPNs transitions. This reversible temperature-responsive conformational change of gelatin molecules in Gel/SF complex hydrogels could promote an abrupt swelling increase and a temperature-triggered protein release from the networks at body temperature, which could be utilized for a targeted drug delivery.

These hydrogels show a temperature-responsive gelatin release profile: at 20 °C they exhibited no gelatin release and maintained their hydrogel dimensions, but at 37 °C they showed time-dependent gelatin release and their hydrogel dimensions decreased.

Protein-synthetic polymer hybrid interpenetrating networks (IPNs) of poly(N-isopropylacrylamide) (PNIPAAm) with Bombyx mori (B. mori) silk fibroin (SF) are described. In these IPNs, SF has the beta-sheet crystalline structure, and shows improved storage and loss moduli. The IPN hydrogels show volume phase transition behavior at the same temperature and NaCl concentration as pure PNIPAAm hydrogels. The PNIPAAm/SF IPNs retain the swelling kinetics of PNIPAAm and show increased deswelling kinetics, with a mechanism whereby the internal water molecules are rapidly released through the induced beta-sheet networks. The IPNs with SF beta-sheet structure successfully decrease the formation of a skin layer observed in conventional PNIPAAm hydrogels. Therefore, the proposed IPN hydrogels can provide three benefits; improved mechanical property, biocompatibility, and deswelling rates.