Research On Mg-Zn-Ca Alloys As Degradable Biomaterial

In this paper, Zn and Ca elements, which have no toxicity, are chosen as alloying elements to develop Mg-Zn and Mg-Zn-Ca alloys for biomedical application by using casting and extrusion technology. The effects of Zn and Ca on microstructure, mechanical properties and in-vitro corrosion properties of the mentioned alloys have been systematic studied by XRD, OM, SEM, TEM, tensile testing and electrochemical measurement. The in vitro and in vivo biocompatibility of the Mg-Zn-Ca alloys was also evaluated. At the same time, the surface modification of Mg-Zn-Ca alloys by using micro-arc oxidation and sealing technology were studied.Thermodynamic calculations showed that Zn reduces stacking fault energy of the Mg-Zn alloys. Microstructure and tensile test showed that Zn not only has a minor role in grain refinement in magnesium alloy, but also has a strong solid solution strengthening effect and improves the tensile strength of Mg-Zn alloys. Tensile strength was significantly increased with 4.0wt.% Zn content. The addition of Ca element further refines the microstructure of the as-cast Mg-Zn alloys by forming Ca2Mg6Zn3 and Ca2Mg5Zn13.The yield strength, tensile strength and elongation of the as-cast Mg-Zn-Ca alloys increased at the beginning, but then decreased with the increase of Ca content, and the highest performance was obtained at 0.2wt.% Ca. The maximum of the yield strength, ultimate tensile strength and elongation obtained in this study were 58.1MPa, 225MPa, and 17.5%, respectively. After hot extrusion, the grain size of Mg-Zn-Ca alloys was refined remarkably and their mechanical properties were significantly enhanced.The corrosion test showed that an increasing content of Zn leads to a high corrosion potential. Thus, the corrosion resistance of the as-cast Mg-Zn alloys improved remarkably with an increasing of Zn content. But, when the Zn content exceeded 4wt.%, the corrosion resistance decreased due to precipitated the coarse MgZn phase. Ca element has a great influence on the corrosion resistance of Mg-Zn-Ca alloy, the corrosion resistance of the as-cast Mg-Zn-Ca increased first and then decreased with increasing Ca content. The alloy has the best corrosion resistance when Ca content was 0.2 wt. %. Hot extrusion greatly increased the corrosion resistance of Mg-Zn-Ca alloys. The study on the corrosion mechanism showed that the corrosion modes of Mg-Zn-Ca alloy were pitting corrosion and filiform corrosion.The electrical parameters of the micro-arc oxidation greatly influenced the corrosion resistance of the Mg-Zn-Ca alloy. The optimal oxidation parameters were voltage 400V, frequency 500Hz, 8% duty cycle, micro-arc oxidation time of 10 minutes. The reasonably sealing process can effectively increase the corrosion resistance of Mg-Zn-Ca alloy in simulated body fluid.In vitro experiments showed that Mg-Zn and Mg-Zn-Ca alloys with a certain concentration range of alloying elements have good cell compatibility.In vivo test showed that the surface modified Mg-Zn-Ca alloy have a good biocompatibility. There are no inflammations appeared after 1 month implantation. Microstructure showed that there were new born formed on the surface of the Mg-Zn-Ca implant as well as the bone tissue. With the degradation of magnesium implant, the implant and the newborn bone became well conjunct. After 2-3 months implantation, stained tissue revealed that there were a large number of new bones in the bone tissue. With the degradation of Mg-Zn-Ca alloy, there was new degradation layer, which contains Ca and P was found at the interface between the bone tissue and Mg-Zn-Ca implants.