Flexible Water-Absorbing Silk-Fibroin Biomaterial Sponges with Unique Pore Structure for Tissue Engineering

Silk fibroin (SF) scaffolds are widely used in tissue engineering due to their biocompatibility and slow biodegradability. However, the relatively stiff mechanical properties and low permeability of these systems can limit some applications. In this study, a new type of water-stable silk sponge (ASF-PEG-S) was obtained by inducing nanoparticle (50-300 nm in diameter) formation in SF solution by autoclaving followed by freeze-drying and rinsing the dry sponges with low-molecular weight (400 Da) polyethylene glycol (PEG400) to induce SF β-sheet structure formation and thus stability in water. With further extraction, the SF nanoparticles embedded in the sponges were removed, leaving nanopores in the walls of round-shaped micro-size pores. The unique pore structure resulted in enhanced permeability and flexibility of ASF-PEG-S when compared to other types of SF sponges, especially with respect to commonly used methanol-annealed SF sponges. In addition, ASF-PEG-S absorbed water nearly 40 times more than its dry weight, while the methanol-annealed sponges absorbed half this amount. When human fibroblasts were seeded and cultured on ASF-PEG-S versus traditional SF methanol-processed sponges, improved cell encapsulation, distribution, and consistency in growth were observed, suggesting utility in tissue engineering and tissue repair applications in the future.