| Some kinds of biomedical metallic materials have been successfully developed in recent decades. Metals played an important role as biomaterials in repairing and replacing bone tissues, because they have better mechanical strength and toughness, and are more suitable for application in load bearing than ceramics or polymer. Now biomedical metallic materials include stainless steels, titanium alloys and nickel-cobalt alloys. However, these alloys have some weakness. Firstly, the release of toxic metal ions or particles caused by corrosion or wearing would decrease in biocompatibility. Secondly, mismatch between modulus of current metallic materials and bone tissues would cause new bone deformation and slowly growth.Thirdly, there are also deficiencies due to surgical extirpation of the metal sheets after healing up of fracture.Magnesium alloy is a kind of potential implant biomaterial. Magnesium alloys have better plasticity than bio-ceramics like hydroxyl apatite, while the elastic modulus and compressive yield strength of magnesium are closer to those of natural bone than is the case for other commonly used metallic implants. More important, the biocompatibility between magnesium and human body is good because that the magnesium ion is the necessary element for human. Ca2+ is the most important ion of all the positive ions in human body and one of the major compositions in bone tissue. The corrosion of the Ca-enriched phase in Mg-Ca alloys would make the local density of Ca higher. It would promote the formation of HA or another kinds of Ca3(PO4)2 ceramics which were the precursor of HA. Then it would be propitious the new hard-texture to deposit on the biomaterials. A kind of new hard-texture implant biomedical materials-MgCa alloy which can be degradation in human body had been researched in the paper. Accordingly, if used as hard texture implants, such alloy materials are expected to induce the growth of new bone texture during corrosion and degradation, and eventually be substituted entirely by new bones, achieving complete cure of the damages to hard texture.The corrosion behavior of pure Mg and Mg alloys in vitro had been studied .The corrosion process was researched by real-time measurement of the change of samples weight, pH value of SBF ,the microstructures of the immersed samples at different immersion time observed by electron microscopy, the polarization current, mechanical properties such as degree of hardness. Then the biology properties such as cytotoxicity test were studied at last. The result shows that :1. Magnesium alloy including 0~2.0%Ca had been melt. Mg-0.7Ca,Mg-2.0Ca,Mg-0.74Ca-0.35Y,Mg-1.9Ca-0.45Y and Mg-2.0Ca-1.2Y had been get by controlling the addition of the middle alloy.2. The average rate of weight loss of Mg-0.7Ca alloy was about 1.11%/d which was lower than the other alloys.The polarization currents of pure Mg, Mg-0.7Ca and Mg-2.0Ca were 2.086 mA/mm2, 2.298 mA/mm2 and 4.226 mA/mm2 which were lower 2 orders than the other alloys. The tendence of releasing H2 with the change of time for the samples match with the rate of loss weight for the samples. The cytotoxicity test shows that the leach toxicity of Mg, Mg-Ca, Mg-Zn alloys belongs to the first grade and all the samples suits for cell. Mg-0.7Ca had good combination properties such as resistant corrosion and biocompatible3. The product generated on the surface of Mg-0.7Ca during the 7days immersion was identified by X-ray diffraction and energy spectrum analysis as Ca3(PO4)2, which can promote bone's recovering. With the pH value of SBF higher, TCP changed to HA which can promote the new bone growth. The white substance also could stop the corrosion fronts .4. The result of thermodynamic calculation proves that when Ca dissolves in Mg to form solid solution or separates out as simple substance, the corrossion of the alloy will be accelerated. While Ca exists with Mg2Ca, distributing reticularly along the grain boundary, the corrosion will be restrained in the grain to protect the matrix. Also, this establishes good fundamental for the further heat treatment. If aging under high temperature with a long time, the second phase will participate sufficiently in the grain boundry to increase the erosion resistance.. |
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