In this work, silk fibroin (SF) filament with fibrils regenerated by dissolving in CaCl₂-formic acid (FA) was prepared by wet-spun method at room temperature. Different from traditional dissolved methods, SF solutions obtained by dissolving in CaCl₂-FA preserved fibrils, which have been recognized as the key to the high performance of native silk. The morphology of SF filament was analyzed, very dense filaments with smooth surface and circular, nanofibrils could be observed in longitudinal and cross-sections of filaments. Moreover, the breaking stress of samples was gradually increased with the increase of draw-down ratios. After 3 times drawing, the breaking stress and elongation at break of filament were 276.4±22.6 MPa and 40.8±3.1%, respectively. At the same time, the secondary structure of SF filament was typical β-sheet. In addition, SF filaments showed excellent degradation property, the mass lost of SF filament declined 42% after incubating in protease XIV solution. Above all, the human mesenchymal stem cells (hMSCs) adhered very well on the surface of the filaments, which demonstrated the good biocompatibility of SF filaments, was suitable for application in tissue engineering.

Keywords: silk fibroin fibers, CaCl2-formic acid, fibril, morphology, biomaterials

Silk fibroin (SF) has been explored as a versatile protein biomaterial for the formation of fibers, films and porous scaffolds for various biomedical applications, due to its biocompatibility, slow degradability and robust mechanical properties.^1-4 The biocompatibility and biodegradability of the silk fibroin protein allow silk-based biomaterials to be used in vivo.⁵ The dissolution of silk is a critical step in producing regenerated SF filaments.
The traditional dissolution solvents which degraded the SF restrict the production of regenerated SF filaments on many fields. Therefore, the way to construct the excellent mechanical properties of regenerated SF filament is still a problem for us. On the basis of our preceding work,^6-9 we report the novel preparation method of SF spinning solution which preserves the native fibrils by dissolving in CaCl₂-formic acid (CaCl₂-FA) solvent.⁶
In this paper, the SF filament was prepared by wet-spun method with water coagulation at room temperature. It was an environmentally and efficiently way to obtain filaments. Morphology, mechanical property, secondary structure, degradation property and biocompatibility of SF filaments were extensively investigated. Further more, the mechanism of SF dissolved in CaCl₂-FA solution was explained as well. Above all, in our previous studies, wet-spun silk fibroin scaffold consisted of wet-spun silk fibroin filaments with hierarchical structure for ligament tissue engineering was observed,⁷ it was a simple and efficient method of constructing anterior cruciate ligament (ACL) silk scaffolds. Buiding from these observations, the present study aimed at engineering biocompatibile SF filaments, so the human mesenchymal stem cells (hMSCs) was selected to seed on SF filaments to investigated the possibility used in vivo.