Background

The development of silk fibroin as a biomaterial, especially biomedical tissue engineering materials, has been a hot topic in recent decades. Natural silk fiber from Bombyx mori has to undergo a series of processing steps to become a regenerated liquid silk fibroin. The liquid silk can be easily made into various forms of silk biomaterials, such as micro- or nano-particles, regenerated fibers, artificial skin, and porous matrix or 3D scaffolds, biomimetic nanofibrous scaffolds, a platform for transistors and various classes of photonic devices. The structure and properties of the final forms of silk biomaterials depend evidently on the molecular size of the regenerated liquid silk which is affected by a series of processing steps including degumming and fibroin fiber dissolution. Our investigation showed that boiling and degumming in a 0.1%-0.5% Na₂CO₃ solution used most frequently in the laboratory can not only cause a large amount of sericin degradation and hydrolysis but also leads to serious breakage of the silk fibroin peptide chain and its mechanical properties decreasing (Wang and Zhang, 2013). The low purity, high concentration, and improper storage conditions easily induce regenerated liquid silk gelling or coagulating and denaturing. The partially chain-broken polypeptides of silk fibroin can be converted into silk fibroin nanoparticles (SFNs) in acetone (Zhang et al., 2007). During this process the molecular structure of the regenerated liquid silk has changed rapidly from random-coil and a-helix form (Silk I) into anti-parallel β-sheet form (Silk II). The resulting crystalline silk nanoparticles may offer various possibilities for surface modification of industrial materials and enzyme/drug delivery system (Zhang et al., 2011).