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Development of Mg-Gd-(Ag, Ca) alloys as biodegradable implant materials (Dissertation)

Abstract

Mg alloys attract more and more attention as a new generation of biodegradable implant materials. However, they still have some drawbacks such as fast uncontrollable corrosion and a high amount of produced hydrogen bubbles during corrosion process. Moreover, knowledge on the effects of alloying elements and intermetallic phases in physiological environments is necessary to be further understood. Therefore, it is extremely important to develop novel Mg alloys combining proper mechanical properties, controllable homogeneous degradation properties with low degradation rate and good biocompatibility. Mg-RE (rare earth) alloys show a good combination of increased mechanical properties and bio-corrosion resistance [1-6]. Gd, Ag and Ca are used as the main alloying elements in this work. Gd has an acceptable biocompatibility and a high solid solubility in Mg. It was reported that Mg-Ag alloys showed improved ductility, a good biocompatibility and satisfactory antibacterial properties [7]. Addition of Ca contributes to the microstructure refinement of as-cast Mg alloys, and improvement of their strength and plasticity. Ca as the essential elements in human body is excellent biocompatible. In the present work, systematic studies of pure Mg, binary and ternary alloys were designed for development of Mg-Gd-Ag and Mg-Gd-Ca alloy series for biodegradable implant applications. Pure Mg, binary Mg-xGd (x=0.5, 1, 2 wt.%), Mg-xAg (x=1, 2 wt.%), Mg-xCa (x=0.4, 0.8 wt.%) and ternary Mg-2Gd-xAg (x=1, 2 wt.%), Mg-2Gd-xCa (x=0.4, 0.8 wt.%) alloys were prepared by permanent mould casting method. Their microstructure, mechanical properties, corrosion properties have been fully investigated. The phase diagrams of binary Mg-x(Gd, Ag, Ca) and ternary Mg-2Gd-x(Ag, Ca) alloys are calculated using Pandat software. The predicted phase transformations are compared with the experimental results. Solid solution (T4) heat treatment was carried out to optimize the microstructure and properties of these low-level alloys. Effects of Ag/Ca additions and solution treatment on microstructure, mechanical and corrosion behavior are studied as well. Microstructure is investigated by using optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The mechanical properties were tested in tension and compression. The corrosion behaviour is studied using weight loss method in both 0.9 wt.% NaCl solution and cell culture medium (CCM). Furthermore, the cytocompatibility was examined using primary osteoblasts by direct cell adhesion and live/dead staining test. The antibacterial properties of Ag-containing alloys were also evaluated using antibacterial assays in a dynamic bioreactor system. For binary Mg-(Gd, Ag, Ca) alloy series, the results show that these three alloying elements can contribute to grain refinement and microstructure homogenization. Their efficiency can be ordered as Ca > Ag > Gd. The additions of alloying elements (Gd, Ag and Ca) to pure Mg result in slightly improved hardness and yield strength due to grain refinement and the formations of respective intermetallic phase (IMP). Moreover, the ductility is improved by adding more Ag, but is deteriorated with increasing contents of Ca. The increased additions of Ag and Ca weaken the corrosion resistance in NaCl solution owing to an increase in cathodic kinetics caused by more formation of IMPs. However, this phenomenon is not observed in in vitro degradation in CCM, indicating degradation kinetics by completely different mechanisms. The more additions of alloying elements (Gd, Ag and Ca) and solution treatment lead to less susceptibility to pitting. In Mg-Gd alloys, Mg-2Gd alloy is considered to combine better mechanical properties (yield strength and ductility) and uniform degradation with acceptable degradation rate (DR) for further alloy development. In order to further improve the mechanical and corrosion properties of Mg-2Gd alloy, both ternary Mg-2Gd-xAg and Mg-2Gd-xCa alloy systems are developed. Both Ag and Ca contribute to the solid solution strengthening and corrosion resistance. The results show both low-level additions of Ag and Ca to Mg-2Gd alloy appear promising to improve its property profile. These additions enhance both tensile and compressive properties. The tensile yield strength (TYS), ultimate tensile strength (UTS), and elongation to fracture (El) reach to a maximum value of around 60 MPa, 140 MPa and 7%, respectively. Solution treatment is an effective way to enhance the in vitro degradation resistance significantly. The in vitro degradation rate (DR) achieves a minimum value around 0.08 mm/year in T4 condition. Both Mg-2Gd-2Ag and Mg-2Gd-0.4Ca alloys combine better mechanical properties and degradation behavior. No cytotoxicity is observed on primary osteoblasts by adding Gd, Ag and Ca. The solution-treated Mg-Gd-Ag and Mg-Gd-Ca alloys show a better cytocompatibility than pure Mg and binary alloys. Moreover, Mg-2Gd-2Ag alloy exhibits a slight antibacterial effect.
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