A Research On Preparation And Degradation Properties Of Porous Magnesium Alloys For Bone Repairing

For a long time,bone graft has been the main method of repairing bone defects in clinic,but there are some inevitable shortcomings such as supply limitation and body rejection to this approach.Bone tissue engineering（BTE）refers to inoculating scaffold materials with osteoblasts cultured in vitro,and then implanting the hybrid materials into the bone defects of human body untill cell spreading.The degradation process of scaffolds is accompanied by cell proliferation and differentiation,and bone generation will be achieved eventually.With the booming of BTE,there is no doubt that it will be the best choice of repairing bone defects,especially large bone defects.As one of the key points of BTE,there are more and more attention from the field of biomaterials attracted in the research on scaffold materials.In recent years,the magnesium alloy is regarded as a potential scaffold material due to its good biocompatibility,biodegradability and suitable mechanical property similar to the human bone.However,it is difficult to fabricate the porous magnesium alloy with the structure similar to cancellous bone,and the corrosion rate of magnesium scaffolds will be accelerated by larger specific surface area,which greatly hinders the application of the porous magnesium alloy as BTE scaffolds in the future.In allusion to these problems,this research used Zn and Ca as alloying elements to prepare porous magnesium alloys with three different compositions（Mg-0.3Ca,Mg-3Zn-0.3Ca and Mg-6Zn-0.3Ca）,which are prepared by negative pressure infiltration casting and similar to the structure of human cancellous bone.The effects of different compositions and thermal treatment on the microstructure and mechanical properties of the porous alloys were systematically investigated.The degradation behavior and cell compatibility of the porous magnesium alloys were also evaluated by immersion test and cell experiment in vitro.Based on the results of this research,it could provide necessary data support and theoretical guidance for the application of porous magnesium alloy in BTE.The main results of this research are as follows:（1）The porous magnesium alloy prepared by infiltration casting formed complete open-cell foam with a uniform size of pores,approximately 450μm,and it exhibited excellent mechanical property while its porosity was up to 73%.The microstructure of magnesium alloy was significantly refined by adding a small amount of Zn element.With the furtuer increase of Zn content,the volume fraction of eutectic formed byα-Mg and Ca₂Mg₆Zn₃increased rapidly,which leaded to the raise of compressive and flexural strength.After the treatment with solid solving,the grain size of Mg-6Zn-0.3Ca porous magnesium alloy increased obviously,and most of the second phase disappeared in the microstructure but there was still some second phase left along the grain boundary and within the grain.Additonally,the solid solution alloy displayed better mechanical property（2）For the as-cast porous magnesium alloy,the addition of Zn caused the formation of a large number of Mg-Zn-Ca ternary phase,and serious galvanic corrosion occured due to the difference of standard electrode potential between the second phase and matrix.The corrosion resistance of the as-cast porous magnesium alloy,including Mg-0.3Ca,Mg-3Zn-0.3Ca and Mg-6Zn-0.3Ca,decreased successively,while the corrosion resistance of solid solution Mg-6Zn-0.3Ca alloy increased to some extent.The corrosion rates of four porous magnesium alloys after degradation for 36 h were 0.90 mm/year,1.72 mm/year,3.35 mm/year,and 1.54mm/year,respectively.After fluorination treatment,corrosion resistance of the as-cast porous magnesium alloy was significantly improved.At the early stage of degradation layered uniform corrosion was dominant,but local corrosion like pitting gradually appeared with the increase of immersion time.In addition,the fluorinated porous magnesium had great mechanical integrity during the degradation process for 15 days.（3）The results of cell compatibility showed that osteoblast activity was mainly affected by the pH value and concentration of Mg²⁺,in which pH value was dominant.The untreated and fluorinated porous magnesium alloy all showed a low level of cytotoxicity over a long time.It was worth mentioning that the Mg²⁺concentration of the extract prepared from untreated Mg-6Zn-0.3Ca porous magnesium was higher than that of the untreated and fluorinated Mg-0.3Ca samples,and therefore the former showed better cell viability than the latter.What’s more,the porous magnesium alloy utilized as BTE scaffolds was conducive to the adhesion of osteoblasts on its internal and external surfaces,indicating good osteoconduction.