Page 28 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

CHAPTER 2
to interfacial features of the implant [72]. Coatings formed by conversion have the advantage that the film will tightly adhere, because it grows from the material. One way to obtain conversion coatings is by passivation where a material protects itself from further oxidation by the growing of a MgO/Mg(OH)2 layer in a natural way (without any external source) after the contact with aqueous solution or the air. This process is commonly used in some metals like Al, Ti and Mg, however the specific process conditions and resulting material characteristics are variable. For the coating of Mg by this technique, solutions of NaOH and KOH area widely used [70,73], some studies incorporate other elements such as calcium, silicates, aluminates, borates and phosphates [74–79]. However, this kind of treatment produce generally a weak layer with only few nanometers of thickness which may not be protective enough.
Another widely used technique in the same context, is anodization, in which the material to be modified, in this case magnesium, is set up as anode in an electrolytic cell and submerged in a supporting electrolyte where through a volt- age source an electrical current is induced. In this way, an anodic layer from the material is produced in a controlled way. Parameters such as type and concentration of electrolyte, temperature, time, current density and applied volt- age determine the final morphology, thickness and adherence of the coating [34]. As during formation, the layer is growing from the material but not deposited on it, the wear resistance and hardness of anodized layers improve the substrate performance. However as the produced layer is a ceramic material it may not have the desired mechanical characteristics [56]. Usually the thickness obtained with this technique depends on the voltage applied in a direct way and can vary from 5 to 200 μm. In addition, other characteristics such the electric conductivity of the modified Mg resulting material are highly influenced by the electrolyte composition due to incorporation of some species into the anodic layer. For this reason, it is necessary to determine the ratio of the elementary cell volume of the formed ox- ide respect to that of the metal (relation of Pilling-Bedworth) to evaluate if the formed oxide is protective or not [81]. Moreover, as a modification of conventional anodization method, a technique called plasma electrolytic oxidation (PEO) or micro arc oxidation (MAO) or anodic spark deposition (ASD) has emerged. In this method, high voltages (in the order of 300V, 400V and inclusive 500V) are used to achieve the breakdown potential and the energy concen- trated at specific points which offer less resistance, results in the formation of fine spark discharges which have an important influence on the morphology of the film. If the formed coating is studied in detail, it is possible to find two structures; the barrier layer which acts like a protective film and on top of this, an irregular film that usually is a porous layer with structures that look like craters as a consequence of the multiples process of melting, solidification, crystallization, partial sintering and densification [82–85]
6.2. Deposited coatings
Another alternative for magnesium surface modification is the deposition of a coating consisting of metallic, poly- mer, ceramic or a compound material. The purpose for coating deposition is to improve the behavior of magnesium in terms of corrosion, mechanical or biological properties [69]. Some of the most used techniques here are sol-gel, plas- ma spraying among others. For the deposition of metallic materials on a Mg surface it is important to consider the effect of galvanic corrosion and changes in the interface substrate/coating, preventing the effect of stress shielding induced by the large difference in Young’s modulus between the stent scaffold material and the tissue. In the case of an endoluminal vascular stent, at the interfaces between the coating and material substrate, additional shear forces induced from the pulsating hemodynamic environment could result in detachment of the film. In addition, metal thin films must be biocompatible and biodegradable, which reduces the number of options between the metallic materials commonly used as coatings. In contrast, polymers and ceramics are a good option. Ceramics have been used primarily in applications for bone reconstruction due to their inherent bioactive properties to precipitate bio- logical apatite and thus to improve the direct link with osseous-tissue. Cathodic electro-deposition have been used with this purpose, however problems to control the purity of the coatings and the presence of unexpected phases can affect the stability for the coating [69,86]. Organic coatings can also be used with Mg but restrictions exist due to low temperatures required to deposit or immobilize them. In this situation the immersion techniques are a good option through the use of solutions, suspensions, colloids or precursors. Sol-gel is one of the most frequently used
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