Page 129 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

GENERAL DISCUSSION AND FUTURE PERSPECTIVES
applications with the prospect of clinical applications to treat CVD.
In the first part of this thesis, new chemical formulations to produce surface-coatings on commercial pure magne- sium (c.p Mg) by PEO were studied. One of the novelties of this project was the use of two organic additives in the modification process of the Mg to improve the corrosion resistance of the material. In chapter 3, the effect to add hexamethylenetetramine (HMT) and mannitol (MAN) to the electrolytic solution of the anodizing was studied. It is important to clarify that although it is well known that the use of organic compounds during the anodization process can result in coatings that are more resistant to corrosion, HMT and MAN were not used before for similar applications or in c.p Mg [13]. In addition, the chemical composition of the obtained coatings was not affected by the addition of extra elements different from those used in the base solution (i.e silicate) and magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2) which are normally generated as the result of oxidation of the material. These coatings were compared with others obtained from a studied formula where sodium fluoride (NAF) was used as an additive to the electrolytic solution, where the fluoride (F), which is an element that can be highly toxic at certain concentrations, was incorporated into the obtained anodic film [14][15]. . Additionally, an optimization of the process was carried out after the modification of the parameters such as current density (potentiostaic mode) and voltage (galvanostatic mode).during the anodizing. This experiments allowed us to conclude that the role of NAF, HMT and MAN as additives directly influenced the formation mechanisms of the coatings affecting the thickness, surface mor- phology and structural organization [16].
Once coatings with different characteristics were obtained, one per group was selected taking into account the uni- formity and morphology of the anodic films and their possible contribution to decrease the corrosion rate of the material. More detailed studies were performed with those samples and were described in chapter 4. The chosen samples were for NAF coating obtained by treatment at 104.16 mA.cm-2 for 600s, for HMT samples treated at 320 V for 600s and for MAN treatment at 380V for 600s. In addition to the samples mentioned above, two-step anodized samples were prepared. This combinations were considered in the study due to previous investigations in materials such as titanium (Ti), aluminum (Al) and zirconium (Zr) has been shown an effective increase in the ordering of the morphology after the anodization of samples with a pretreatment with a determined pattern serving as guide for self-organization[17][18][19]. The two-step anodized treatment consisted of a first anodizing with NAF solution to generate a compact layer which should be more protective and resistant to corrosion[16]. Consequently, samples were anodized for second time with either HMT or MAN in order to create a porous surface which is more attractive for biological interactions like cell adhesion [20][21].. Our results show that during the second step of anodization a competition between the old film dissolution and new film formation occurred. This influenced the morphology of the coatings and the corrosion resistance was higher compared to samples that were anodized only once. This cor- roborates findings by other authors [22][23][24]. For all the coatings, in particular for two-step anodized samples, the highest release of hydrogen production was observed during the first 48h of exposure to an aqueous solution. This was expected because when samples were in contact with the NaCl solution, the initialized degradation process first increased and then attenuated by the re-passivation of the samples, confirming literature [25]. As stated, this effect was less pronounced in samples with a single anodization step. The behavior of the samples treated with MAN solution differs from the others with a more gradual degradation profile. That means that Mg2+ was released at a constant rate during the first month. Additionally, coated samples showed an improvement in the wettability of the material converting hydrophobic surfaces of Mg to hydrophilic features. This is crucial because the first reaction that occurs on the surface of the implant after an implantation, is the deposition of proteins. This deposition promotes attachment of cells and possibly also their proliferation and differentiation andor maturation [26]. Our results also showed that surface-coated materials were hemocompatible i.e. did not increase coagulation of blood, nor caused hemolysis in contrast to c.p Mg which was coherent with studies reported previously in literature [27][28].
8
127