Robust and Tunable Hybrid Hydrogels from Photo-Cross-Linked Soy Protein Isolate and Regenerated Silk Fibroin

Abstract

Soy protein isolate (SPI), a plant derived protein, is emerging as a potential material for biomedical applications because of its abundance in nature, ease of isolation and processing, tailorable biodegradability, low cost, and low immunogenicity. Herein we report the development and structure−property relationship of photo-cross-linked SPI and SPI/silk fibroin (SF) hybrid hydrogels for the first time. The pristine SPI hydrogels were cross-linked at two different structural conformations (i.e., closed at pH 7 and open at pH 12), and SPI/SF hybrid hydrogels were co-cross-linked at pH 7 in three different weight ratios (3:1, 1:1, and 1:3). The fabricated hydrogels were characterized using electron microscopy, X-ray diffraction, Raman and infrared spectroscopy, thermal analysis, small- and ultrasmall-angle neutron scattering, rheology, water uptake, and in vitro degradation studies. The equilibrium water swollen SPI hydrogel cross-linked at pH 7 exhibited a specific microstructure, controlled degradation in phosphate-buffered saline, and a shear storage modulus of ∼7.7 kPa, which is in the range of human lumbar nucleus pulposus and significantly higher than soy hydrogels reported by thermal treatment, pressure treatment, salt-induced cold-setting, and enzymatic cross-linking. Conversely, the SPI hydrogel cross-linked at pH 12 exhibited ordered porous microstructure, higher water uptake of ∼1946%, poor water resistance, and low mechanical properties. Increase in SF content of the SPI/SF hybrid hydrogels demonstrated improved porosity, water swelling, molecular chain mobility, elastic, and water-resistant properties. An in-depth understanding of the effect of pH and composition on the hierarchical structure and physicochemical properties of the fabricated hydrogels was established. Moreover, the pristine SPI and SPI/SF hybrid inks used for hydrogel fabrication exhibited flow properties highly suitable for 3D-printing scaffolds for tissue engineering applications. The pr