# Microstructural, Electrochemical and In Vitro Analysis of Mg-0.5 Ca-xGd Biodegradable Alloys
###### Abstract
Gadolinium (Gd) presents a solubility of 23.49 wt.% at the eutectic temperature [20] in Mg and plays a key role in strengthening the solid solution when alloyed to Mg. [PERSON] et al. identified that Gd is one of the rare earth elements with the best biocompatibility of all REEs (rare earth elements) [21]. [PERSON] et al. [22] noted that the Mg-Gd binary system has mechanical properties very close to those of the biological bone compared with the conventional biocompatible Co-Cr and Ti alloys. [PERSON] et al. [23] investigated the effects of Gd on a solid solution and reported that it was an effective solid solution enhancer in Mg compared with Al and Zn. Studies conducted by [PERSON] et al. [24] suggested that the molten Mg-20 Gd alloy contained mostly the supersaturated \(\alpha\)-Mg solid solution, while the as-cast Mg-20 Gd alloy consisted of \(\alpha\)-Mg + Mg5 Gd. [PERSON] et al. [25] showed that increasing Gd will lead to the age-hardening response and tensile strength of Mg-\(\times\)Gd-Y-Zr (x = 3 wt.%, 6 wt.%, 10 wt.%, 12 wt.%) alloys and developed a Mg-10 Gd3Y-0.5 Zr alloy with an ultimate tensile strength of 390 MPa and a yield strength of 245 MPa. [PERSON] et al. [23] published that the properties of Mg alloys had been improved by the addition of REEs due to the formation of metastable REEs containing phases along the grain boundaries. Alloying with Gd more than 10 wt.% increases the mechanical strength due to the noble behavior of Mg5 Gd precipitated in grain boundaries. Research findings have also shown that Gd improves the rate of degradation of Mg alloys by stabilizing the resulting corrosion products [26], but at concentrations greater than 5 wt.%, the rate of degradation and biocompatibility could decrease. At the same time, [PERSON] et al. [27] highlighted the corrosion resistance of the Mg-10 Gd alloy implanted in rats and observed that alloys with a high percentage of Gd (10 wt.%) implanted for up to 36 weeks led to increased concentrations of Gd in various rat organs, such as the spleen, liver, lung and kidney. Among several other alloy elements, calcium (Ca) improves the biocompatibility and mechanical properties of magnesium-based alloys [28; 29]. Additionally, the most major issue for the application of MgCa as implantable elements is still the high corrosion and electro-corrosion rate in body fluids [30; 31; 32]. Percentages higher than 1 wt.% Ca have also been shown to reduce certain characteristics such as castability, corrosion behavior and mechanical properties due to the higher volume fraction of the Mg\({}_{2}\)Ca phases [33; 34], which also has an effect on the biocompatibility of the alloy [35]. [PERSON] et al. presented in their studies that the report through Mg\({}^{2+}\) and Ca\({}^{2+}\) ions has an important part to play in the maintenance of the cellular activities of the embryonic mesenchymal cells that give rise to striated muscle fibers [36]. Upon first contact of multi-nucleated fibers with high Mg and Mg-Ca corrosion compounds, the effect was to reduce cell viability, while at the same time the myotubes became capable of adaptation. A significant percentage of Mg\({}^{2+}\) ions can substantially increase cell proliferation and, at the same time, the presence of Ca\({}^{2+}\) ions is extremely important for the formation of fibers [37]. The properties of Mg-Ca alloys are affected by mechanical or thermo-mechanical processing [38; 39; 40]. Therefore, alloying the Mg-Ca alloys (<1 wt.% Ca) with a third element, such as Gd, can be the right path to follow in order to improve the mechanical properties and biocompatibility of the alloy being studied in this paper. In this research paper, the authors studied a novel Mg-0.5 wt.% Ca experimental material which was alloyed with a controlled content of 0.5, 1.0, 1.5, 2.0 and 3.0 wt.% Gd in order to improve the biodegradability rate needed for a symmetrical balance between alloy degradation and bone healing, followed by in vitro 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) tests to determine its biocompatibility.