Page 77 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

IMPROVEMENT OF CORROSION RESISTANCE OF COMMERCIAL PURE MAGNESIUM AFTER ITS MODIFICATION BY SINGLE AND TWO-STEP ANODIZATION
et al [25], topography and roughness paly and important role in the surface free energy and wettability. To probe this, they studied changes in the contact angle in samples of abraded Mg at different grades. They concluded that at high roughness there are an increment on the available surface area which the liquid can interact. This concept could be applicable to anodized samples of Mg were the porous morphology increases the contact area. Particularly this agrees with what was obtained for the NAF sample, which was the most compact and showed the highest contact angle and lower wettability in comparison with the others samples that mainly had porous surfaces.
All coatings reduced the corrosion rate and had comparable surface topologies. In general, the PEO-generated porous coatings with pore diameters ranging from 0.4 to 1.4 μm. The two-step-generated coatings appeared to comprise interconnected pores. All coatings were separated from the underlying (c.p) Mg by a dense (~0.7μm thick) barrier. The PEO process itself showed a bi-modal action, during the first seconds/minutes an initial oxide layer was formed that coincided with reaching the maximum voltage/current, while in the second phase voltage/current dropped ap- proximately ten-fold and the coating reached an equilibrium state due to passivation of the surface.
On the other side, corrosion process may also be affected by the presence of impurities in the material causing dif- ferences in the standard electrode potential and producing microgalvanic corrosion [26]. The corrosion behavior of the Mg will depend also of the immersion solution and its composition. In this study NaCl was chosen as it is a highly corrosive medium; additionally, the presence of chlorides makes it similar to SBF. Hydrogen evolution measurements were used to study the performance of the anodized Mg by the quantification of Mg consumed; for each mole of hy- drogen produced a mole of Mg was consumed; in this way, a high hydrogen evolution indicates a high Mg corrosion and viceversa [27][21]. For the case of biomedical applications, it is expected that the response can be better because body fluids can be less aggressive that the solution evaluated (0.1 M NaCl) [28]. The efficiency of the coatings was measured by the hydrogen evolution test in which samples without any modification had a much faster corrosion rate. After one month of immersion, the degradation rate for the c.p Mg was around 0.04 mm/year while coated samples with a single anodized decreased this value in around 50% and for the double-step anodized samples it was around 93%. Similar results were obtained by Xue et al [29], who found that c.p Mg anodized in silicate solution at different times decreased the degradation rate after its evaluation in NaCl solution and simulated body fluid (SBF). According with that study, the corrosion rate dropped about 500 times after the anodization of the sample during 3h. They concluded that the increment of the thickness and the formation of oxide and silicon compounds may provide a high contribution to this effect. A similar conclusion was obtained by Zaho et al [30], who used PEO to anodize c.p Mg at galvanostatic mode with a solution of silicate with borate as additive.
On the other hand biological results, in the present study showed that the coatings are protective and cells can grow in the modified surfaces which is opposite what occurs in untreated Mg where dead cells were observed. The coat- ings obtained here are also hemocompatibles and do not report strong effects in hemolysis processes and thrombo generation. There are diverse investigations around the toxicity of Mg used for implants of bone and for cardiovas- cular stents and the use of PEO is one of the alternatives to improve its biological performance [1]. For instance, and according with observed by Hoon et al [31], the biocompatibility of Mg was improved by anodization technique as results of cell adhesion, DNA measurement and functional in vitro assays evidenced for osteoblasts when compared with results for c.p Mg. Additionally, Lin et al [32], evaluated samples of a ZK60 magnesium alloy anodized with an electrolytic solution similar to those studied in this work, in which a silicate base solution with fluoride additive was used; toxicity was measured by MTT assay obtaining positive results for the cells growing on the modified surfaces. Results from hemocompatibility assays obtained in the present work, are according with those founded by Li et al. [33], who reported that even at higher concentrations of Mg2+ (10-3M/L) hemolysis is not induced.
Finally, it is possible to conclude that with the coatings obtained here, it is viable to modulate the degradation rate
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