Preparation And Properties Of The Regenerated Silk Fibroin-based Films

As a natural polymer, regenerated silk fibroin (RSF) can be applied tobiomedical materials. Films can be prepared without crosslinking agentsby drying film formation, so the biocompatibility and purity of the filmscan be improved. Therefore, in this work, RSF films and hyaluronic acid(HA)/SF composite films were prepared by drying film formation method.The effect of preparation parameters including ethanol content in thesolvents, Ca2+concentration in the RSF solutions, HA content in thecomposite films and film formation temperature on the structure,morphologies and properties of RSF-based films was characterized byFTIR, XRD, SEM, TG, contact angle measurement and universal testingmachine, etc.Firstly, the effect of ethanol content in the solvents was investigated.It indicated that the hydrophobic blocks in aqueous SF solutions wasincreased, and the surface of the films became smoother with reducing theethanol content in the solvents. Correspondingly, the thermal stability ofthe films was enhanced, while water absorption decreased.Secondly, we studied the effect of Ca2+concentration in the solutionson the structure and properties of the RSF films. It indicated that theincrease of Ca2+concentration could induce the SF crystal structuraltransformation from silk I to silk II, mainly the unstable β-turn crystalschaning into thermodynamically stable β-sheet crystals, forming theregular and stable globule-like building units.Lastly, HA/SF composite films were studied. It indicated that theincrease of HA content in the composite films could induce the SF structural transformation from silk I to silk II, thus improving the thermalstability of the films. Simultaneously, due to the excellent moisturizingproperty of HA, the hydrophilicity of the films could be improved with theaddition of HA. Film formation temperature had greater effect on thecomposite films than HAcontent. At higher temperature, water in the filmsevaporated rapidly. The films tended to form more silk II crystals, so thethermal stability and mechanical strength were enhanced to different extent.However, due to the decrease of hydrated silk I structure, water absorptionand mechanical ductility were not as good as that of the films formed atlower temperature with slow drying process. Furthermore, the degradationof the films was mainly attributed to the degradation of SF and thedissolution of HA. After immersing in water, the structure of the compositefilms did not change a lot. The thermal stability of the composite filmsdeclined a little, but not significantly.The research demonstrated that we could control the secondarystructure and the properties of SF by regulating and optimizing thepreparation process of RSF-based films and the HA content. HA/SFcomposite films with stable structure could be obtained without usingcrosslinking agents. Our study will provide new route for the preparationof controllable RSF-based films with different secondary structure andhelp us gain a deeper understanding of secondary structure and propertieschanges during the regeneration of SF. Furthermore, it provides importantexperimental data for screening and using SF as advanced functionalbiomaterials, such as drug delivery matrices or tissue engineeringmaterials.