Functional Silk Biomaterials With Vascularization Capacity For Soft Tissue Engineering

Silk is a promising biomaterial for tissue engineering because of its biocompatibility, unique mechanical properties, ease of chemical modification and biodegradability. However, the scaffolds used in different tissue regenerations have to be designed respectively based on the special requirements to achieve suitable mechanical properties, degradability and nanostructures. Engineering thick complex organs and tissues requires functional vasculatures for metabolism. So it is crucial to design scaffolds that can provide suitable environment for the vascular endothelial cell growth.Nanofiber-assisting lyophilization is a feasible way in restraining separate lamella formation in silk scaffolds. According to the self-assembly mechanism of silk protein, molecular motion, electric charge, the concentration and hydrophilic interaction can affect the fibroin solution state. In this study, the structure of silk was controlled by adjusting the self-assembly via a slowly increasing concentration process to facilitate porous structure formation. It was found that the scaffolds are soluble in water, limiting the late application.Based on silk self-assembly mechanism, an all-aqueous process was preferentially applied to avoid the use of organic solvents. Silk scaffolds were prepared by mixing silk nanofiber solution with silk solution at weight ratios of 1:15. The mixed solution was cultured at 4oC, 60 oC and 90 oC for various times, and then freeze-dried to prepare different scaffolds. With the increase of the processing temperature and time, the prepared scaffolds had improved insolubility. However, the stiffness of the scaffolds is not suitable for endothelial cell growth.Then, the silk solution was adjusted to near isoelectric point for controlling the secondary conformation transition in a lyophilization process. The insoluble silk scaffolds with lower crystal content were achieved, and had suitable stiffness for the growth of endothelial cells. In vitro and in vivo studies were conducted to verify the feasibility of the scaffolds for soft tissue repair.We cultured the rat bone marrow mesenchymal stem cells(r BMSC) on the scaffolds, using salt-leaching scaffolds and methanol-treated scaffolds as control. A series of experiments proved that these silk scaffolds have good biocompatibility, and could promote stem cell differentiation into endothelial cells. Enhanced neovascularization and tissue ingrowth in vivo without the addition of the growth factors were also achieved on the scaffolds.In conclusion, the present study provided an effective process to prepare silk scaffolds with tunable mechanical properties and secondary structures. It also offered a promising matrix with vascularization capacity for different soft tissue engineering and regenerative medicine.