Research On Finely Controllable Fabrication Of Hierarchically Porous Hydroxyapatite Bone Scaffolds

Recently,due to the explosive development of additive manufacturing(also known as 3D printing)has become one of the most focused research topics,especially in mechanical engineering(like printing of metals)and medical science(e.g.cell printing).Bone tissue engineering is commonly regarded as the most potential route for the repair of bone defects,in which scaffold is the key.Therefore,more and more researchers are trying various ways to make scaffolds with good biocompatibility,and the vast majority of them are definitely through 3D printing or its combination with other technologies.However,with the development of bone tissue engineering,the requirement of bio-imitability also becomes increasingly demanded,which is difficult for traditional fabricating methods and even 3D printing technology to satisfy such needs.So it is far from cease to explore new methods and techniques in order for more biomimetic scaffolds for bone tissue engineering,such as in terms of materials and structure.In order to fabricate porous scaffolds with better biological performance,this thesis intensively researched on the optimization of fabrication process of scaffolds from the aspects of structure and composition,based on extrusion deposition technology which uses commercially available hydroxyapatite(HA)as matrix material.In addition,a novel method to produce hierarchically porous scaffolds with optimized microstructures,i.e.multi-scaled pores(nm-?m-mm)was developed by combination of gas-foaming.On the other hand,a new 3D bio-printing system was set up,which is typically suitable for multi-materials,to make highly biomimetic bone scaffolds.Both of them are expected to establish solid foundation for promoting the biological performance of scaffolds in bone tissue engineering.First of all,the fabrication of porous scaffolds with micron graphite as porogen to form additional in-rod porosity is introduced in details.To ensure that graphite was totally burnt out and to maximize the amount of in-rod porosity,the sintering profile of hydroxyapatitegraphite composed scaffolds(HA-G)was optimized.Furthermore,X-ray diffraction(XRD)and energy dispersive spectroscopy(EDX)were employed to confirm the complete oxidization of graphite respectively.And the hierarchical structures of HA-G scaffolds were analyzed by scanning electron microscopy(SEM).The bulk porosity of HA-G scaffolds can be finely controlled by adjusting the content of graphite and statistically analysing the drying and sintering shrinkage of scaffolds,which makes this novel technique stable and controllable.Secondly,to study the effect of particle size of pore former on the performance of HAG scaffolds,nano-sized graphite was introduced as another kind of porogen to achieve hierarchical porosity.The physicochemical properties of these scaffolds were carefully investigated.In view of the importance of mechanical strength in bone tissue engineering,the optimal content of graphite was determined on the basis of strength and in-rod porosity to clarify our objectives and to reduce the workload and cost of experiment at the same time.Specifically,micro-morphologies,the distribution of pore size and porosity,crystallinity after sintering,biodegradability and hydrophilicity etc.were characterized.The results show that HA-G scaffold is advantageous to normal scaffolds in physicochemical properties due to hierarchically porous structures.Then,considering the possible cytotoxicity caused by residual graphite in HA-G scaffolds,cell counting kit-8(CCK-8)was used to evaluate the cytotoxicity of graphite composed scaffolds.Besides,the adhesion and spreading status of pre-osteoblasts on scaffolds were observed by SEM,as well as the viability of the cells imaged by fluorescence.In addition,the ability that promotes the differentiation of myoblasts into myotubes was also researched through biological evaluation(7 days).From the point of view of the physicochemical properties and bioactivity,micron graphite is the best option as porogen.Finally,since 3D printing of multi-materials in bone scaffolds is increasingly desired,a self-developed multi-nozzle 3D printing system was successfully set up after intensive investigation of precision,controllability,stability,synchronization and coordination.In this paper,the control of precision and other aspects of this printing system was detailedly analysed.It is worth mentioning that we also built a database according to our experience and testing results to greatly facilitate the adjustment of parameters that suit new materials.This system is highly expected to provide practical and technical reference for the achievement of successful production of highly biomimetic scaffolds.