Study On Stereolithography Process And Mechanical/Biological Performance Of Hydroxyapatite Porous Bone Scaffolds

Bone tissue engineering provides a new method for the treatment of bone defects,in which the scaffold structure plays an important role in the successful construction of the engineered bone tissue.This field involves multi-disciplinary knowledge such as material science,manufacturing science and biomedicine.Although not been applied to clinical practice,it has a broad development prospect.Considering the mechanical and biological properties,the key to promoting the development of this technology is to design and prepare porous structures.In this paper,a structural design method of bone scaffolds was proposed,and a non-uniform porous bone scaffold was constructed.A stereolithography model of hydroxyapatite paste scanned by a single line laser was established,and the influence of process parameters on the cured depth and width was revealed.The influence of process parameters on forming quality and the influence of porous structure on mechanical properties were studied.A multi-field model of bone scaffold was established to study the distribution of pore wall strain,fluid shear stress and cell phase concentration.The biocompatibility of the hydroxyapatite bone scaffold was evaluated,and the influence of porous structure on cell proliferation was revealed to build a prediction model of cell proliferation performance.A structural design method of bone scaffolds was presented to construct a non-uniform porous bone scaffolds;the changes of bone density,the distribution of porous structure and the influencing factors of unit discontinuity phenomenon was analyzed.The results indicated that,with the increase of the iteration number,the number of units in the dead zone of strain energy density increased rapidly at first and then slowly.Bone density converged slowly to maximum or minimum value,or to the dead zone of strain energy density.The channels on the longitudinal section distributed along longitudinal orientation with better continuity,while the channels on the transverse section distributed as an irregular network with worse continuity.With the increase of the space coefficient or the minimum-restricted pore diameter,the number of discontinuous units decreased.According to the distribution law of laser in time and space,a stereolithography model of hydroxyapatite paste scanned along a single line was established.The influence of laser power and scanning speed on cured depth and width was studied.The results indicated that,with the increase of laser power or the decrease of scaning speed,the cured depth increased slowly,which was consistent with the theoretical analysis.The larger the transmission depth was,the smaller the width-depth ratio was,and the thinner the parabola of the cross-section profile was;on the contrary,the larger the width-depth ratio was,the flatter the parabola of the cross-section profile was.With a line fitting method,the penetration depth of hydroxyapatite paste was 35.1 μm,and the critical exposure was 11.4mJ/cm2.The effects of processing parameters on forming quality and the effects of porous structure on mechanical properties of bone scaffolds were studied.The results indicated that,with the increase of laser power or the decrease of scanning speed,the width of the pore on the side of bone scaffolds decreases continuously.The smaller the scanning space was,the larger the overlapping area of two cured lines was,and the higher the over-cured area was,which resulted in the increase of the roughness of the upper surface.The result of orthogonal experiment showed that,the optimal layer thickness,laser power,scanning speed and scanning space were 100μm,60mw,7m/s and 50μm,respectly.The compressive strength of hydroxyapatite bone scaffold was 20MPa-37MPa,which was equivalent to that of cancellous bone,far less than that of dense bone.The effective elastic modulus decreased with the increase of porosity,and could be adjusted between 1.8GPa-7.1GPa to match the original bone.A multi-field model was established to study the distribution of pore wall strain,fluid shear stress and cell phase concentration in bone scaffolds.The results indicated that,the distribution of pore wall strain in the non-uniform porous bone scaffold,constructed in this dissertation,was more uniform than that of the conventional uniform circular porous structure.The influence of the inlet velocity on pore wall strain was weakened from the central area to the peripheral area.The fluid shear stress decreased from peripheral area to central area,and a high fluid shear stress was easy to appear at the top corner.The cell phase concentration was higher on the ground of the transverse channel in scaffolds,and was almost zero at the bottom side outlets.With the increase of the inlet velocity,the cell phase tended to be uniform at first,then easy to accumulate.The biocompatibility of hydroxyapatite bone scaffolds was evaluated.The effects of cell phase concentration,pore wall strain and fluid shear stress on cell proliferation in scaffolds were studied.A prediction model of cell proliferation performance in bone scaffolds was established.The results indicated that,the cytotoxicity of hydroxyapatite scaffolds was 0 level,which represented the scaffolds had good biocompatibility.When the inlet velocity was more than 3mm/s,even in the high cell phase concentration area,it was difficult to get a higher cell coverage ratio.The increase of pore wall strain could improve the cell coverage ratio.The difference of cell coverage ratio,caused by pore wall strain,gradually increased with the increase of incubation time.After the pore wall strain exceeded 4600με,the cell coverage ratio did not increase further,but decreased slightly.Under the external load in this study,1×10-3Pa-5×10-3Pa was a suitable range of fluid shear stress for cell proliferation.The error of the test value and predicted value of cell coverage ratio in bone scaffold was 1.5%-14.5%.The predicted value was more accurate on the first day,with a error of 1.5%-2.9%.