Page 25 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

BIODEGRADABLE MAGNESIUM-BASED SUPPORTS FOR THERAPY OF VASCULAR DISEASE A GENERAL VIEW
6.1 Magnesium Alloys
In its chemically pure form, (metallic) magnesium has limitations due to its corrosion. However, this process can be controlled by addition of alloying elements into the matrix which not only influence the corrosion resistance but also the mechanical properties. As mentioned earlier, the first use of alloyed magnesium was in the industrial sector where magnesium was mixed with other elements with the purpose of having a more resistant and light material for different applications such as in automotive and aerospace sectors, electronic devices, among others. With the pass of time, these same materials were tested for biomedical applications and as improvement of them new formula- tions were developed. For this application, one of the most crucial criteria for the choice of elements in a magnesium alloy is related with its toxicity and dissolution. For this reason, the added elements and its concentration need to be selected carefully taking care not to exceed the permissible values for the body in the toxic range and the easy distribution or elimination by excretions ways [7]. Elements such as Ca, Mn, Zn, Sn are present throughout the hu- man body and are a good option for bioabsorbable implants. In contrast, alloys that contain elements such Be, Ba, Pb, Cd and Th are toxic and require caution for biomedical use. Other elements such as Al, Bi, Li, Ag, Sr, Zr can only be used in low doses [7]. Allergic responses or hepatotoxicity have been attributed to alloys containing Al, V, Cr, Co, Ni, Cu, La, Ce, Pr [7]. Table 2 summarizes the most commonly used alloys, composition, applications and results found.
Table 2. Most used alloys elements added to Mg for biomedical applications
The solubility of alloying elements depends mainly on their atomic size with relation to Mg and also on its valence as in the case of Al, Zn, Ca, Ag, Ce, Ni, Cu and Th. The addition of alloying elements also induce grain refinement and/ or plastic deformation due to induction of a high density of dislocations and stacking faults in the microstructure [57–60]. Alloys of Mg–Al, Mg–Zn, and Mg–rare earths present a precipitation hardening due to the high solubil- ity of the secondary element in Mg [61], otherwise occurs with Mg-Ca and Mg-Si alloys which are unable to do it without being reinforced by thermic treatment which can improve properties such as corrosion resistant of the alloys [62,63]. Moreover, some of the most common alloy elements incorporated to Mg are Al, Zn, Mn and Ca and rare earths. However, the use of elements such as rare earths and Al is highly controversial because of considerable
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