# Microstructural, Electrochemical and In Vitro Analysis of Mg-0.5 Ca-xGd Biodegradable Alloys
###### Abstract  
The subject of Mg-based biodegradable materials, used for medical applications, has been extensively studied throughout the years. It is a known fact that alloying Mg with biocompatible and non-toxic elements improves the biodegradability of the alloys that are being used in the field of surgical applications. The aim of this research is to investigate the aspects concerning the microstructure, electrochemical response (corrosion resistance) and in vitro cytocompatibility of a new experimental Mg-based biodegradable alloy--Mg-0.5%Ca with controlled addition of Gd as follows: 0.5, 1.0, 1.5, 2.0 and 3.0 wt.%--in order to establish improved biocompatibility with the human hard and soft tissues at a stable biodegradable rate. For this purpose, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), light microscopy (LM) and X-ray diffraction (XRD) were used for determining the microstructure and chemical composition of the studied alloy and the linear polarization resistance (LPR) method was used to calculate the corrosion rate for the biodegradability rate assessment. The cellular response was evaluated using the 3-(4,5-dimethyltizol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) test followed by fluorescence microscopy observation. The research led to the discovery of a dendritic \(\alpha\)-Mg solid solution, as well as a lamellar Mg\({}_{2}\)Ca and a Mg\({}_{5}\)Gd intermetallic compound. The in vivo tests revealed 73-80% viability of the cells registered at 3 days and between 77 and 100% for 5 days, a fact that leads us to believe that the experimental studied alloys do not have a cytotoxic character and are suitable for medical applications. Mg-Ca-Gd alloys; SEM; EDS; microstructure; electrochemical evaluation; in vitro test +
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In addition to orthopedic applications in the form of pins, screws, rods and plates, Mg alloys have been investigated in cardiovascular applications as biodegradable Mg alloy stents [6; 7; 8; 9; 10; 11] due to their biodegradability and mechanical properties. In addition, the ideal cardiovascular stent biomaterial should promote endothelialization with minimal neo-intimal hyperplasia. In this sense, any kind of injury caused by the implantation of the biomaterial into the living tissue will initiate an inflammatory reaction to the material, with fibroblasts being a key contributor to new tissue formation post-injury (i.e., depositing new collagen and facilitating healing or fibrous encapsulation), even though it may not necessarily be in direct contact with the implanted cardiovascular devices. Unfortunately, biodegradable magnesium-based alloys also have a number of disadvantages, such as low corrosion resistance and high hydrogen release during bone healing, which can cause subcutaneous inflammatory reactions [12; 13]. Furthermore, the presence of various impurities that accompany the manufacturing process of these materials, such as Fe, Ni, Cu, etc., at very low concentrations leads to a decrease in biocompatibility and a lack of corrosion properties [14; 15]. In order to improve these aspects, scientific studies have continued to combine Mg with biocompatible elements that do not produce toxic effects (Ca, Sr, Mn, etc. ), but also rare earth (RE) elements [16]. The conceptual alloy design for these alloys with elements of the rare earth category was based on the necessity and improvement of the properties of Mg-Ca, Mg-Sr and Mg-Mn systems, etc., by increasing mechanical resistance, decreasing density and improving the rate of degradation in environments such as simulated biological fluids. [PERSON] et al. [17] and [PERSON] et al. [18] established that rare earth elements are divided into two classes: light RE elements (Y, Ce, La, etc.) and heavy RE elements (Gd, Sm, Sc, etc.) and research conducted by [PERSON] et al. identified that some of the most effective alloys used in the medical field are the Mg-Gd and Mg-Y binary alloys [19].