Research On Fabrication And Osteoinduction Properties Of 3D Printing Polylactic Acid Bone Repair Scaffold

Bone tissue has a strong self-regeneration ability,which can fully restore the damaged part to its pre-injury composition,structure and function.Accidents,bone tumor resection,inflammation and infection can cause large bone defects(> 2 cm),which makes the bone tissue unable to heal itself.Currently,autografts(gold standard)or allografts are mainly used for the treatment of large area bone defect,but there are some problems that cannot be ignored(such as secondary surgery,cross infection,immune rejection,poor treatment effect).With the development and intersection of multiple disciplines,synthetic bone graft substitutes(bone repair scaffolds)have become ideal materials.However,the current therapeutic effect of bone repair is still unsatisfactory,because the bone repair scaffold lacks excellent osteoinduction properties.Therefore,this topic studies the bone microenvironment where the cells are located,and takes the process of bone remodeling and fracture self-healing as research guide.By combining mechanical stimulation with biochemical stimulation,the osteoinduction properties of the bone repair scaffold and the repair effect of bone defects were improved.According to the basic structural requirements of bone repair scaffold and the physiological structure of bone tissue,scaffold models with different structures were established.The fluid and mechanical properties of the scaffold model were simulated and analyzed using the two-way fluid-structure interaction method.The cell model was loaded on the inner surface of the scaffold to simulate the cell adhesion after the scaffold was implanted in vivo.The differentiation direction of cell in the scaffold was predicted by calculating the fluid shear stress(FSS)on the cell surface,so as to obtain a scaffold structure that meets the expectations of osteogenic differentiation.During the first structural design,it was found that the spherical-560 scaffold had the best osteogenic differentiation expectation,but the existing 3D printing technology could not achieve the reproduction of the structure.Therefore,the secondary redesign of the scaffold structure was carried out by combining the trabecular structure,the conclusions of the first structural design and the accuracy of 3D printing technology.The 400-900 scaffold obtained by the second structural design that meet the expectation of osteogenic differentiation and can be prepared using the existing technology.The scaffold structure obtained by simulation design was prepared using 3D printing technology.Due to its excellent biocompatibility,non-toxicity,and degradability,Polylactic acid(PLA)was used as the scaffold material.The PLA bone repair scaffolds were prepared by selective laser sintering(SLS)technology.The scaffold prepared by SLS meets the requirements and basically restores the theoretical design.Compression experiment showed that the scaffold had a certain compressive strength and can be used for the repair of cancellous bone defects.Bioactivity test indicated that the PLA scaffolds had good bioactivity.Hydroxyapatite(HA)can be rapidly formed on the surface of the PLA scaffold,which can be firmly combined with the surrounding bone tissue.When the PLA scaffold was implanted into the body,it can effectively alleviate the generation of acidic environment through the internal circulation of the human body.The scaffold needs to be implanted into the bone defect area to function,and it is mainly used for the in-situ bone repair.In order to further improve the osteogenic effect of the scaffold,bone morphogenetic protein-2(BMP-2)with excellent osteoinduction ability was loaded on the scaffold.However,the survival time of BMP-2 in vivo is too short,and the poly(lactic-co-glycolic acid)(PLGA)microspheres were used to load BMP-2 and achieve long-term sustained release.BMP-2/PLGA microspheres were prepared by double emulsion method,and the microspheres were filled on the surface of PLA scaffold by physical oscillation method.Scanning electron microscope(SEM)observation showed that the microspheres were mainly concentrated in the depressions on the surface of the scaffold,which did not affect the scaffold structure and the structure of the microspheres was not damaged.In order to evaluate and verify the in-situ osteogenic properties of BMP-2/PLGA microspheres composite scaffold,the 400-900 scaffolds(with or without loaded microspheres)were implanted in rats.At 4 weeks after implantation,the top of both the400-900 scaffold and the 400-900-microspheres scaffold were healed with the surrounding cortical bone,which had good osteogenic effects.To further evaluate the osteogenic effect inside the scaffold,Micro-CT was used to analyze the new bone at the bone defect site.The 400-900-microspheres scaffold had a higher ratio of bone volume to total volume(BV/TV)than the 400-900 scaffold,and the distribution of new bone was more uniform.At 8 weeks,the 400-900-microspheres scaffold had the highest BV/TV(0.390),and new bone almost filled the inner space of the scaffold.In vivo experiment showed that the microspheres can effectively improve the quality and quantity of new bone,and effectively shorten the time of bone regeneration.In this thesis,bone remodeling and fracture self-healing were taken as the design ideas.The osteoinduction properties of the bone repair scaffold was improved by combining mechanical stimulation with biochemical stimulation,and the bone repair scaffold with excellent osteogenic effect was prepared.The research content and results of the topic can provide useful explorations for designing bone repair scaffold with excellent osteoinduction properties by combining multiple osteogenic stimulation.