Tough Photocrosslinked Silk Fibroin/Graphene Oxide\nNanocomposite Hydrogels

Abstract

The development of\nprotein-based hydrogels for tissue engineering\napplications is often limited by their mechanical properties. Herein,\nwe present the facile fabrication of tough regenerated silk fibroin\n(RSF)/graphene oxide (GO) nanocomposite hydrogels by a photochemical\ncross-linking method. The RSF/GO composite hydrogels demonstrated\nsoft and adhesive properties during initial stages of photocrosslinking\n(<2 min), which is not observed for the pristine RSF hydrogel,\nand rendered a tough and nonadhesive hydrogel upon complete cross-linking\n(10 min). The composite hydrogels exhibited superior tensile mechanical\nproperties, increased β-sheet content, and decreased chain mobility\ncompared to that of the pristine RSF hydrogels. The composite hydrogels\ndemonstrated Young’s modulus as high as ∼8 MPa, which\nis significantly higher than native cartilage (∼1.5 MPa), and\ntensile toughness as high as ∼2.4 MJ/m³, which is\ngreater than that of electroactive polymer muscles and at par with\nRSF/GO composite membranes fabricated by layer-by-layer assembly.\nSmall-angle scattering study reveals the hierarchical structure of\nphotocrosslinked RSF hydrogels to comprise randomly distributed water-poor\n(hydrophobic) and water-rich (hydrophilic) regions at the nanoscale,\nwhereas water pores and channels exhibiting fractal-like characteristics\nat the microscale. The size of hydrophobic domain (containing β-sheets)\nwas observed to increase slightly with GO incorporation and/or alcohol\npost-treatment, whereas the size of the hydrophilic domain (intersheet\ndistance containing random coils) was observed to increase significantly,\nwhich influences/affects water uptake capacity, cross-link density,\nand mechanical properties of hydrogels. The presented results have\nimplications for both fundamental understanding of the structure–property\nrelationship of RSF-based hydrogels and their technological applications.