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

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 presented results contribute to a facile fabrication and fundamental understanding of the structure–property relationship of SPI-based hybrid hydrogels.