The Dry-Spinning Of Regenerated Silk Fibroin And Sericin By Biomimicking The Composition And Structure Of Cocoon Silks

Animal silks, especially spider dragline silks, have excellent mechanical properties and are known to be one of the toughest materials in nature. Moreover, natural cocoon silks from Bombyx mori have abound resources and the mechanical properties of the forcibly reeled silks approach those of the spider silks. Therefore, the artificial spinning of the regenerated silk fibroin (RSF) has attracted much attention in the fields of advanced materials, biology, and tissue engineering. The natual cocoon silk has an ideal core-shell structure, which is composed of two different proteins, the silk fibroin (SF) and silk sericin (SS). The mechanical properties are mainly attributed to the core of SF, and the shell of SS is generally considered to be a lubricant in the spinning process and also a protective glue layer of SF. Recently, some studies showed that SS could induce SF to form β-sheet comformation which is mainly attributed to the mechanical properties of the coccon silks. In order to obtain high performance fibers as natural cocoon silks, in this paper, composite fibers and core-shell fibers of regenerated silk fibroin (RSF) and SS were prepared from aqueous solutions by biomimicking the composition and structure of the cocoon silks. A custom built capillary spinning device and coaxial spinning devices were applied in the conventional and coaxial dry-spinning processes, respectively. Futhermore, selected as-spun fibers were post-treated to improve the mechanical properties. Rotational rheometer, Fourier transform infrared spectroscopy (FT-IR), fluorescence microscope, universal testing machine, Scanning Electron Microscope(SEM), Laser Scanning Confocal Microscope(LSCM) and Synchrotron Radiation Microfocus X-ray diffraction were employed to investigate the spinnability of the spinning dopes, the structures and the properties of the resultant fibers.To biomimic the composition of cocoon silks and the spinning process of B. mori, natural SS in the cocoon silks was partially retained to prepare RSF/SS fibers from the blend aqueous solution. It was found that the residual natural SS obviously improved the viscosity and the spinnability of the blend dope. The extrusion die swell was avoided by extending spinning line and raising take-up rate. The drastic hydrophilia of SS induced the formation of β-sheet conformation in RSF. With increasing residual natural SS (r) to16%, the mechanical properties of the as-spun blend fibers of RSF/SS s were improved gradullay. The as-spun fiber with16%natural SS exhibited a maximum breaking stress of90MPa. SEM results showed that the cracks on the smooth surface of the as-spun blend fiber disappeared after the fiber was drawn4times and then immersed for3h in the80vol%ethanol aqueous solution. However, the roughness of the fiber surface became larger after the post-treatment. Compared to the post-treated blend fiber without residual natural SS, the post-treated fibers with an r larger than0%exhibited more obvious improvement in the β-sheet content and mechanical properties. In the case of r=16%, the molecular orientataion of the post-treated fiber exceeded that of the degummed natural silk. The breaking stress, breaking strain, initial modulus and breaking energy of the fiber were up to400 MPa,40%,6.4GPa and60kJ/kg, respectively.To limit the drastic hydrophilia of natural SS on the preparation of RSF/SS blend fibers, a commercial SS with low molecular weight (LWSS) was blended with RSF aqueous solution for further dry-spinning. The addition of LWSS also improved the viscosity and the spinnability of the spinning dope. Moreover, the β-sheet content and breaking stress of the as-spun blend fibers of RSF/LWSS increased with the reducing of the RSF/LWSS mass ratio. With the decrease of the pH value of the blend dope, the fiber diameter became larger due to the elevated viscosity of the dope. However, the pH value had no obvious effects on the mechanical properties and the β-sheet content of the blend fibers. On the contrary, the diminution of the Ca2+molar concentration had limited effects on the viscosity and the spinnability of the blend dope, but improved the β-sheet content and the mechanical properties of the blend fibers. Both as-spun fibers and post-treated fibers of RSF/LWSS had smooth surfaces. The post-treatment significantly boosted the β-sheet content and the molecular orientation of the blend fibers which exhibing a breaking stress, a breaking strain, an initial modulus and a breaking energy of458MPa,26.8%,3.3GPa and49.8kJ/kg, respectively. The mainly amorphous as-spun fibers showed a low degree of crystalline orientation, but the hypocrystalline post-treated fibers performed a high degree of crystalline orientation which approached that of natural cocoon silks.To biomimic the core-shell structure of cocoon silks, coaxial RSF-LWSS fibers were prepared by coaxial dry-spinning process. LSCM results demonstrated that the fibers had similar core-shell structures to the natural cocoon silks, in which the LWSS enveloped the core RSF uniformly. The spinning device, together with the flow rate of shell dope of LWSS, had obviously effect on the spinnability of the RSF-LWSS coaxial system. The β-sheet content and the mechanical properties of the as-spun fibers increased with the mass ratio of LWSS shell to RSF core (ms/mc). The post-treated coaxial fibers with high conents of β-sheet and β-turn presented a breaking stress, a breaking strain and a breaking energy of344MPa,20%and24.6kJ/kg, respectively. The stiff shell of LWSS leads to a dramatic initial modulus of the fibers of18.6GPa, which is the maximum in this research.