Research On Biodegradable Magnesium-Stronsium Alloy For Bone Substitutes And Biological Properties

In this dissertation, a new biodegradable magnesium-stronsium alloy for bone substitutes was designed. Sr is a known osteopromotive element which can activate osteoblastic cell replication and inhibit expression by osteoblasts. The component, microstructure, mechanical procperties of the material were analysised thoroughly. The as-cast ingots were hot extruded and MAO treated to tailor the degradation of the material. The morphology, preparation, phase composition and corrosion resistance of the MAO coating was characterized. The in vitro and vivo degradation evaluation was investigated by immersion test, weight loss, cytotoxicity test and animal implantation. The main conclusions were drawn as follows:The microstructure of as-cast Mg-Sr exhibited the typical metallographic microstructural feature of a-Mg grains with intermetallic phases precipitating along the grain boundaries. The result of XRD indicates the constitution of a-Mg matrix and second phase present as Mg17Sr2 in the alloys. After extrusion, the microstructure was more homogeneous and the intermetallic phases were broken and detected within the grains.The surface morphologies of the MAO coatings prepared on the as-cast Mg-Sr alloys exist many micropores on the surface of the coating. It can be seen from the X-ray diffraction (XRD) patterns that only small peaks corresponding to MgO and MgF2 can be detected. It is indicated that the Sr, Ca and P elements in the two coatings should be in the form of amorphous state. The XPS results indicated that the Ca, Sr and P were in the form of Ca2+, Sr2+ and PO3-4 to incorporate into the MAO coating. The drop time of Sr-CaP coating was higher than CaP coating, indicating a better corrosion resistance.The corrosion rate of different material was as-cast Mg-Sr alloy> as-cast Mg-Sr alloy with coating> as-extruded Mg-Sr alloy. Corrosion assessment by pH variation, weight loss measurement, Mg and Sr concentration shows the same result. Laminar corrosion products with some cluster were randomly scattered on the surface of the Mg-Sr substrate. The cells viability of different Mg-Sr samples showed cytotoxicity of Grade 0-1, suggesting acceptable bio-safety for the cellular applications, the as-cast alloy with coating showed the mild degradation without gas bubble and was incorporated by new bone. In conclusion, the as-cast Mg-Sr alloy with coating is potential to be used for bone substitute alternative.