A Study On Nano-structural Design And Osteoinductivity Regulation Of Silk-hydroxyapatite Bone Materials

Trauma, infection, skeletal diseases, and congenital malformations often result in large bone defects. It remains a challenge for the repair of critical-sized bone defects in clinical treatment. Autografts and allografts are considered current effective treatments, but both have potential risks. Tissue engineering is considered as a promising alternative to the conventional use of bone grafts. Hydroxyapatite(HA), especially nano-sized(nano-HA), is the main mineral constituent of natural bone and has been used as a material for bone tissue engineering due to its biocompatibility, osteoconductivity and osteoconductivity. However, some effective ways in controlling the particle uniformity and water dispersibility are still required for effective applications of HA in bone tissue regeneration.Silk fibroin(SF) has received increasing interest as a matrix material in bone regeneration due to its outstanding mechanical strength, biocompatibility and slow degradation rate. Here, SF nanoparticles with uniform size were used as templates to regulate HA biomineralization. Water dispersible core-shell HA/SF nanoparticles with uniform morphology were successfully synthesized through tuning the reaction parameters. The negative charge on the SF shell restrained the further aggregation of HA particles, endowing them good water dispersibility. The water dispersable HA/SF nanoparticles were then directly dispersed in SF solution to form uniform mixture that was transformed into silk/HA composite scaffold after freeze-drying process. Unlike silk/HA scaffolds reported previously, the present silk/HA composite scaffolds not only possessed higher HA content, but also achieved uniform distribution of HA nanoparticles at nanoscale. Improved compressive strength was also acquired, which provided better microenvironment for bone regeneration. The in vitro cell culture results suggested the critical role of the content and dispersion of HA nanoparticles in affecting stem cell behaviors. The improved distribution of HA nanoparticles at nanoscale significantly promote the growth and osteogenic differentiation of the mesenchymal stem cells(MSCs).The core-shell HA/SF nanoparticles were also used as nanocarriers to delivery osteogenic growth factor(Bone morphogenetic protein-2, BMP-2). The loading efficiency of the BMP-2 on the nanoparticles was up to 99.6%. The BMP-2 could be gradually released from the nanoparticles over a period of 21 days without burst release. The r BMSCs showed good proliferation and osteogenic differentiation on the BMP-2-loaded nanoparticles, significantly better than that on BMP-2-loaded pure silk or pure HA nanocarriers reported previously. Then, a composite scaffold system that had homogeneous distribution of HA nanoparticles and controlled BMP-2 delivery was fabricated to design improved microenvironment for bone repair. The composite scaffolds exhibited preferred ability to induce r BMSCs differentiation toward osteoblasts, which was further confirmed by in vivo animal study.In conclusion, uniform SF nanoparticles as biotemplates were used to regulate HA biomineralization for optimizing the fabrication of silk/HA composite scaffolds for bone regeneration, and designing BMP-2 nanocarriers with improved loading and releasing capacity. The influences of various factors on the differentiation of stem cells were also investigated to gain an insignt into bone regeneration, which would provide valuable information for novel scaffold fabrication for bone repair.