**4. Classification of magnesium alloys considered as potential biomaterials due to their structure (amorphous or crystalline) and alloying elements (rare earth elements, noble metals etc.)**

In the context of resorbable orthopedic implants, research was initially carried out on technical magnesium alloys, for example AZ31, AZ91, WE43, LAE442. Unfortunately, magnesium alloys containing aluminum (AZ31) and heavy metals have been excluded as biomaterials because these additives have a toxic effect on the human body. The research was limited to alloys containing biocompatible elements and/or small amounts of rare earth elements, that are tolerated by the human body in appropriate concentrations [15].

#### *Amorphous and Crystalline Magnesium Alloys for Biomedical Applications DOI: http://dx.doi.org/10.5772/intechopen.94914*

As regards magnesium alloys for resorbable implants with a crystalline structure, the following groups of alloys has been examined: Mg-Ca, Mg-Zn, Mg-Zn-Ca, Mg-Mn, Mg-Si, Mg-Zr, Mg-Zn-Zr, Mg-Zn-Y, Mg-Zn-Zr-Y, Mg-Zn-Mn.

Rare earth elements (REE) are added in order to improve mechanical properties and creep resistance at elevated temperatures [35, 36]. Gadolin (Gd) and yttrium (Y) increase the strength properties during precipitation hardening. Neodymium (Nd) improves tensile strength at ambient and elevated temperatures. Yttrium and strontium (Sr) reduce the texture, and thus anisotropy, in rolled and extruded semi-finished products [35].

Magnesium alloys with the addition of rare earth elements, such as Mg-Y, Mg-Gd [37] and Mg-Nd, have been designed for use as biomaterials. ZW21 and WZ21 alloys (with the addition of Y and Zn) show promising mechanical and corrosion properties. For example, they are ductile (up to 20% elongation) and their tensile strength ≈ 270 MPa. Alloys such as AE21 and WE43 are used for stents [38, 39].

Noble metals as an additive to magnesium and calcium alloys have been studied mainly by the authors of this chapter [12, 40]. There is no information in the literature on the influence of Au and Pt addition on the degradation rate and mechanical properties of magnesium alloys. There are several sources for adding silver to magnesium alloys [41, 42].

The mechanical and corrosion properties of the alloy can be regulated by the structural and chemical composition of the alloy. Compared to their crystalline counterparts, magnesium-based metallic glasses may be more resistant to corrosion, due to their single-phase structure, which may result in a more uniform alloy corrosion. An example confirming the higher corrosion resistance of the amorphous material in physical fluid compared to the crystalline material with the same chemical composition is shown in **Figure 3**.

#### **Figure 3.**

*Results of structural tests of Mg36.6Cu36.2Ca27.2 alloy with amorphous structure (a) and crystalline structure (b) and surface images after 1.5 h of immersion in physiological fluid with amorphous structure (c) and crystalline structure (d) [43].*

Magnesium alloys with amorphous structure, such as bulk metallic glasses (e.g. rods, plates) in the following phase systems: Mg-Cu-Y (-Ag, -Pd, -Gd), Mg-Ni-Y (-Nd), Mg-Cu-Gd (-Zn, -Y), Mg-Zn-Ca were obtained. In addition, studies are also carried out on Mg-based metallic glass without rare earth elements. The Laws [44] obtained bulk metallic glasses based on Mg-Cu-Ca, Mg-Ag-Ca, Mg-Cu-Ag-Ca alloy systems. However, for applications in implantology, magnesium alloys should have a biocompatible chemical composition. Therefore, the group of alloys based on the Mg-Zn-Ca phase system is most frequently considered as a new biomaterial for resorbable orthopedic implants [45]. In 2005, Gu et al. were the first to obtain bulk metallic glass in the Mg-Ca-Zn system, which was characterized by good strength properties and high glass transition capacity [46].

In the process of designing new degradable biomaterials, elements with potential toxicological problems should be omitted whenever possible and, if they are absolutely necessary, they should be reduced to the minimum. Calcium and zinc are essential elements in the human body; therefore, these elements should be the first choice for alloying additives in biomedical magnesium alloys. The concentration of calcium should not exceed 2 wt.%, and zinc – 6 wt.%, due to the corrosive properties of these magnesium alloys [47].

The most commonly used chemical elements for magnesium alloys are: Zn, Zr, Ca, Sr., Yb, Al, Li, Mn and rare earth elements (REEs) (Ce, Er, La, Gd, Nd, Y). The following are the additions, the influence of which on the properties of magnesium alloys is described in detail:


Noble metals, such as gold and silver, were used as alloying additives in pure magnesium to increase its ductility. However, the alloys had low tensile strength [51]. Another source mentions that the addition of silver, as a substitute for calcium, improves the corrosion properties, strength and has an anti-bacterial effect [49].

**Figure 4** shows variation in the open-circuit potential with time and polarization curves for pure Mg and Mg65Zn20.1Ca1.7Yb13Sr0.2 alloy in Ringer solution at 37°C.

In the OCP plot (**Figure 4a**), various levels of recorded curves are visible, which results from differences in chemical compositions. The steady-state for *Amorphous and Crystalline Magnesium Alloys for Biomedical Applications DOI: http://dx.doi.org/10.5772/intechopen.94914*

**Figure 4.**

*Variation of the open-circuit potential with time (a) and polarization curves (b) for pure Mg and Mg65Zn20.1 Ca1.7Yb13Sr0.2 alloy in Ringer solution at 37°C.*

pure magnesium is in the range of approx. -1.65 V, while for Mg65Zn20.1Ca1.7Yb13 Sr0.2 alloy – slightly above −1.4 V. This shift towards positive values indicates favorable behavior of samples with alloy additions. Potentiodynamic measurements (**Figure 4b**) also show the differences between the studied alloys. The values of corrosion current density were slightly higher for magnesium alloy, as compared to pure Mg. However, significant differences in Ecorr by approx. 0.25 V are observed, which indicates that it is recommended to use alloying elements to improve corrosion resistance. E. Mostaed et al. [52] showed similar results regarding the differences in electrochemical tests between pure magnesium and the ZK60 alloy.

Designing of magnesium alloys as biomedical materials is a great challenge, due to rapid degradation of Mg in the environment of bodily fluids and insufficient implant-bone connection in orthopedic applications [50]. These disadvantages can be limited due to an appropriate selection of alloying additions. The purpose of optimal chemical composition of a new class of Mg-based biodegradable materials is to obtain optimal strength, ductility, resistance to fatigue and corrosion by modifying the structure and phase distribution [50, 53]. Currently, efforts are being made to select alloying additions that would promote osseointegration, understood as the fusion of the implant with the newly formed bone tissue and biodegradation without adverse effects on the functioning of body organs [50].
