**2.3 Mechanical properties**

184 Biomaterials – Physics and Chemistry

alloying element to improve the mechanical properties and corrosion resistance of Mg alloys [11][12]. And the addition of other alloying element can further improve the mechanical properties of Mg-Zn alloys [13] [14]. Zn/Mn-containing magnesium alloys, e.g. Mg2Zn0.2Mn [15] and Mg-1.2 Mn-1.0 Zn [16] ternary alloys are studied, the results indicate that Zn/Mn-containing magnesium alloys have satisfactory mechanical properties and can be potential biodegradable alloys. But, the degradation rates of Zn/Mn-containing magnesium alloys are so fast. After 9 weeks implantation, about 10~17% Mg-Mn-Zn magnesium implant has degraded. After 18 weeks implantation, about 54% Mg-Mn-Zn alloy has degraded [16]. The results studied by H.X. Wang at al [17] indicate that the Mg-Zn-Ca alloys coated with Ca-decient hydroxyapatite have an excellent corrosion resistance in Kokubo's simulated body uid (SBF), but the chemical composition of Mg-Zn-Ca alloys was not reported. L.Mao et al [18] studied the effects of Zn on microstructure and mechanical properties of biomedical Mg-Ca-Zn alloys. The results show that the microstructure is refined and the mechanical properties can be improved evidently with Zn content increasing. The mechanical properties of bending and compression can meet the requirements for hard tissue metal implants. However, the effect of Ca on microstructure and mechanical properties of biomedical Mg-Ca-Zn alloys, the corrosion resistance and cytotoxicity were not studied. Xuenan Gu et al[19] reported that the Mg66Zn30Ca4 bulk metallic glasses sample presents a more uniform corrosion morphology than as-rolled pure Mg and Mg70Zn25Ca5 samples. Both indirect cytotoxicity and direct cell culture experiments were carried out using L929 and MG63 cell lines. The results show higher cell viabilities for Mg-Zn-Ca extracts than that for as-rolled pure Mg. In addition, L929 and MG63 cells were found to adhere and proliferate on the surface of Mg66Zn30Ca4 sample. Unfortunately, the cytotoxicity was tests by MTT, according Janine Fischer et.al[20] research, in the case of Mg materials, the use of MTT test kits leads to false positive or false negative results, because Mg is a very reactive element. It is conceivable that Mg in the highly alkaline environment may be able to open the ring form of the tetrazolium salt and bind to it, which could lead to a change in colors similar to the formation of formazan in the

It is reasonable to speculate that the Mg-Zn-Ca alloys with a proper Zn and Ca content can exhibit a superior combination of mechanical properties, corrosion resistance and biocompatibility. In this paper, Zn and Ca, which have no toxicity, are chosen as alloying elements to successfully improve the mechanical properties of magnesium. The effects of Zn and Ca content on mechanical properties, in-vitro corrodible property and cytotoxicity of Mg-Zn-Ca alloys have been systematic investigated to assess the feasibility of Mg-Zn-Ca

Mg-Zn-Ca alloys were prepared from high purity Mg (99.99 %), Purity Zn (99.8 %), and an Mg-26.9 wt. % Ca master alloy. Melting and alloying operations were carried out in a steel crucible under the protection of a mixed gas atmosphere of SF6 (0.3 vol. %) and CO2 (Bal.). Purity Zn and master alloy were added into the pure Mg melts at 720 oC. The melts were kept for 10min at 720oC to ensure that all the required alloying elements were dissolved in the melt alloy, and then the melts were cooled down to 700 oC and poured into a steel mold

case of the MTT tests with cells.

**2. Materials and methods** 

**2.1 Materials** 

alloys for use as bone implant materials.

which had been pre-heated to 200 oC.

Tensile tests were carried out using an Instron-5569 universal testing machine at a constant crosshead speed of 1.0mm/min at room temperature. The tensile specimens with diameter of 6mm and gauge length of 30mm were cut by electric-discharge machining from the ingot. The Young's modulus was get from the tensile test. The fracture morphologies were examined by SEM (SEM, Hitachi-3000N).
