Page 26 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

CHAPTER 2
health risks if the permissible concentrations are exceeded e.g. locally during degradation [64,65].
Magnesium is a highly reactive material and the corrosion process can be induced by exposure to chloride ions in a non-oxidizing medium. The corrosion mechanism for c.p magnesium is typically localized, the surface appearance change after some time to a dark color and some cavities or pitting is produced in the surface [66]. On the other hand, due to the presence of elements and phases with different electrochemical potentials, galvanic corrosion is the prevalent mechanism for the degradation of magnesium alloys where some phases act as anodes and others as cathodes inducing the occurrence of reduction oxidation processes. This behavior can be produced also in c.p Mg in a minor extent due to the presence of impurities. Factors such as grain size, heat treatment and the environment to which it is exposed can influence the rate of degradation of the material [67,68]. In terms of corrosion resistance, corrosion resistance of Mg can be improved by the addition of elements that have electrochemical potentials close to that of Mg, about -2.37 V, decreasing the probability of galvanic corrosion processes these, elements are Y:-2.37 V; Nd:-2.43 V; and Ce:-2.48 V [7].
As hydrogen evolution is a critical by product for Mg materials used in biological systems, some alloys have been developed to decrease this rate; that is the case of Mg alloys containing Zn, Al and Mn, where H2 liberation was in the order of 0.01 mL/cm2/day according with results obtained through in vitro techniques [26].
6.2 Coatings on magnesium
The use of coatings on magnesium is another effective way to protect magnesium against corrosion or delay the deg- radation speed and at the same time in biological terms, to protect the tissue and its cells from potential cytotoxic influence of the degrading base metal. This alternative may not only decrease the degradation rate of the material but also may modify the biological response and mechanical fixation of the implant [56]. In addition, the coating obtained need to respond to physicochemical and mechanical characteristics for the application required and also to have proper biological properties such as biocompatibility and hemocompatibility [69]. Coating of magnesium or any metallic material is feasible by in situ growing on the surface (also called conversion coatings) or by deposition of another material on the primary metallic material. In Table 3 a summary of the more used techniques is presented.
6.2.1 Conversion coatings
During the exposure of magnesium in its pure form to an aqueous system (blood or culture medium), a process of alkalization occurs in the surrounding environment [70]. The increment of the local pH may affect the biological response of the cell in terms of adhesion and may induce apoptosis [71]. The production of an insulating barrier between the bare material and cells may solve these problems. The process to obtain magnesium coatings from the same material, involves chemical and electrochemical processes in which the surface of the material can change its composition, properties and/or morphology providing more protection against corrosion [69]. In conversion coat- ings a barrier is grown from the metal surface; for magnesium the result layer consists of an oxide-hydroxide layer, normally with higher oxide (MgO) than hydroxide (Mg(OH)2) and may also contain elements present in the solution used. Depending of the coatings obtained and their composition, the corrosion process of the surface-modified mag- nesium is decreased which might directly influence adhesion and function of cell. As was mentioned in a previous section, the first criteria to choose a proper coating should be both a high biocompatibility and tunable degradation [56]. Usually coatings on Mg are formed in basic aqueous electrolytes because under acidic solutions magnesium is highly reactive and easily degrades. In the formation of a new protective coating, a chain of reactions is involved: dissolutions, depositions and formation of new phases and compounds. Variables such as composition of the electro- lyte, time, and temperature, among others, can produce layers with different features such as thickness, composition and morphology. Those parameters may play a crucial role in the biological behavior as cells response
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