Page 76 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

CHAPTER 4
Fig.11. Hemocompatibility evaluation for Mg samples. A) Hemolysis test, B) Speed of thrombin genera- tion, C) Concentration of thrombin.
4. Discussion
In the present study, c.p Mg was anodized with the purpose to improve its corrosion resistance. Plasma electrolytic oxidation (PEO) is a simple, low-cost and reliable technique used to modify surfaces of metals like titanium, alumi- num, magnesium [13][19][20][21]. Manipulation of PEO parameters such as voltage, current density, time and com- position of electrolytic solution allow for a controlled deposit of a protective coating with tunable surface features [17]. The effect of the different morphologies and microstructure of the coatings is influenced by the additives in the electrolytes which modifies the conductivity of the solution and with this mechanism of the formation of the anodic layer [22]. Thickness is also a parameter determined by the current density or voltage, time and composition of the electrolytic solution.
The main finding of our investigations is that PEO yielded a surface coating of approximately 3-4 μm on c.p Mg, irrespective of additives and electrolyte composition. All the samples were in the same range of roughness, reason way this variable was not a differential factor when comparing the behavior between coatings. Studies carried out by Nguyen et al [23], suggested that roughness can affect directly the corrosion resistance of Mg. According with this study, smooth surfaces are more resistant than rough surfaces. This is explained due to in case of immersion of the samples in an aqueous solution, porous can accumulate medium which accelerate the degradation of the Mg at the same time that this will lead changes in the pH and high accumulation of hydrogen that consequently may cause local defects. In contrast, this effect is less aggressive in smooth surfaces. However, the results obtained here, do not confirm that, possibly because the roughness of the samples studied were in the same order of magnitude. Neither there was observed a clear correlation between wettability and corrosion resistance. In the present case, it appears that the most influential factor on the corrosion behavior is the morphology of the PEO coating, both surface and internal characteristics of the coating film.
According with the measurement of the contact angle and the calculation of the surface energy, uncoated Mg is considered as a hydrophobic material, however after the modification by PEO an improvement in the wettability of the surfaces was observed in all the specimens especially in the double-coated surfaces which presented a superhy- drophilic behavior. Those results are coherent according which what has been described by Zhang et al [24], where contact angle for samples modified by PEO were around 46o. Additionally and according to other study for Zhang
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