**Research on Mg-Zn-Ca Alloy as Degradable Biomaterial**

B.P. Zhang1,2, Y. Wang2 and L. Geng2 *1National Engineering Laboratory for Carbon Fiber Technology, Institute of Coal Chemistry, Chinese Academy of Sciences 2School of Materials Science and Engineering, Harbin Institute of Technology China* 

### **1. Introduction**

182 Biomaterials – Physics and Chemistry

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Magnesium and magnesium alloys are light metals, which characterized a low density, high specific strength and strong specific stiffness. The fracture toughness of magnesium is greater than that of ceramic biomaterials such as hydroxyapatite. The Young's elastic modulus and compressive yield strength of magnesium are closer to those of cortical bone. Especially, Mg2+ is present in large amount in the human body and involved in many metabolic reactions and biological mechanisms. The human body usually contains approximately 35g per 70kg body weight and the human body's daily demand for Mg is about 350 mg/day. Due to the excellent biomechanical properties and biocompatibility, magnesium alloys used to be introduced as implants into orthopedic and trauma surgery in recently years [1~3].Various magnesium alloys have been investigated as biodegradable materials and some of them have been shown good biocompatibility. For example, AZ31, AZ91, WE43, LAE442, Mg-Ca and Mg-Zn have been investigated for bone implant application [4~8]. It has been shown that magnesium enhances osteogenesis response and increases newly formed bone. However, some magnesium alloys containing aluminum or heavy metal elements which have latent toxic effects on the human body. Thus, several problems such as inadequate strength, rapid corrosion and toxic ions must be solved before this unique metal is widely used in biomedical fields.

It is well known that pure magnesium has poor mechanical properties and the mechanical properties of magnesium can be effectively improved by the appropriate selection of alloying elements [1]. But, based on the aforementioned considerations, the range of alloying elements used in the degradable magnesium alloys is rather limited, Zn, Mn, Ca and perhaps a very small amount of low toxicity RE can be tolerated in the human body and can also be retard the biodegradation. Therefore, Mg-Ca binary alloys attract attention of researchers because Ca is an important element of human bones. The mechanical properties and biocompatibility of Mg-Ca binary alloy can be adjusted by controlling the Ca content and processing treatment. However, an inadequate mechanical properties as well as lower corrosion resistances of Mg-Ca binary alloys are the biggest drawback of these alloys [7][8]. Fortunately, in latest recent years, Mg-Zn system is paid more attention because Zn is one of abundant nutritional elements in human body [9] [10]. Additionally, it is a great potential

Research on Mg-Zn-Ca Alloy as Degradable Biomaterial 185

X-ray diffraction (XRD, Philips-X'Pert) using Cu Ka radiation was employed for the identification of the constituent phases in the as-cast Mg-Zn-Ca alloy and their corrosion products after immersion. Microstructure observations of the alloys were conducted on Olympus optical microscope. The specimens for optical microscopy were etched with a solution of 8 vol. % acetic acid for 30 s, thoroughly flushed with water and alcohol, and then

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

Electro-chemical measurements and immersion tests were performed in a Hank's simulated body fluid to evaluate the in-vitro degradation properties. The chemical composite of Hank's simulated body was listed in table 2.The pH value of Hank's solution was adjusted with HCl and NaOH to 7.2~7.4, to avoid precipitation or formation of sediments in the solution. Its temperature was controlled around 37 ± 0.5 0C, which is equal to the human

A typical three-electrode system consisting of graphite rod as counter electrode, saturated calomel electrode (SCE) as a reference electrode and specimen (1cm2 exposed areas) as a working electrode was used. Potentiodynamic polarization experiments were carried out at a scan rate of 0.5 mV/s. The electrochemical measurements of specimens with thickness of 4 mm and a gauge diameter of 15mm were machined from the ingot and ground with 2000

The immersion tests were carried out in Hank's solution according to ASTM-G31-72 [21]. Samples were removed after 30 days of immersion, rinsed with distilled water, and were cleaned with chromic acid to remove the corrosion products. The degradation rates (in units of mm year-) were obtained according to ASTM-G31-72. An average of five measurements was taken for each group. The pH value of the solution was also recorded in the immersion

L-929 cells were adopted to evaluate the cytotoxicity of Mg-Zn-Ca alloys. The cells were cultured in Dulbecco's modied Eagle's medium (DMEM), 10% fetal bovine serum (FBS), 100 Uml-1penicillin and 100 mg ml-1 streptomycin at 37 oC in a humidied atmosphere of 5% CO2. The cytotoxicity tests were carried out by indirect contact. Extracts were prepared using DMEM serum free medium as the extraction medium with the surface area of extraction medium ratio 1.25 ml/cm2 in a humidified atmosphere with 5% CO2 at 37 oC for

body normal temperature. Pure Mg (>99.99%) was also tested as a contrast.

grit SiC paper, and they were rinsed with distilled water and dried by hot air.

**2.2 Composition and microstructure characterization** 

dried by hot air.

**2.3 Mechanical properties** 

**2.4 In vitro degradation tests** 

examined by SEM (SEM, Hitachi-3000N).

**2.4.1 Electrochemical measurements** 

tests at absolute group for 144 hours.

**2.5 Cytotoxicity assessments** 

**2.4.2 Immersion tests** 

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 case of the MTT tests with cells.

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 alloys for use as bone implant materials.
