Page 131 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

GENERAL DISCUSSION AND FUTURE PERSPECTIVES
(Mg3(PO4)2·8H2O), portlandite (Ca(OH)2), calcite(CaCO3) and hydroxyapatite (Ca5(PO4) 3(OH)) with a solubility of 4.49, 27.62, 5.38, 8.44 and 58.77mg/L respectively [35]. Some of these compounds form particulate material or precipitates that can stick close to the implant or moved through the blood system, the dissolution of these ele- ments will depends of the time and general conditions of the environment about composition and pH. Additionally, organic components from the culture medium and the FBS such as vitamins, proteins (amino acids) e.g. are able to form complexes with Mg ions inhibiting the formation of insoluble salts and coating the surfaces of the implant reducing the interaction of the material with the medium retarding its degradation [34].
We consider that the static model was imprecise to study the Mg degradation. The volume/area (V/A) ratio for im- mersion testing was chosen in agreement with the ISO 10993-12(2007) in which the evaluated ranges should range from 0.17 to 0.8 mL/cm2 [37]. However, these values are not suitable to study Mg because in static conditions the buffer capacity of the medium is too low to maintain a stable pH [33]. Additionally, during the first 48h the material had the highest corrosion rate and thus increased the pH concomitantly. This caused passivation of the material and consequently a reduced solubility due to the hydroxide layer that had formed. This is, due to its hydroxide nature, thermodynamically stable at high(er) pH-values (>8.3) [38].
Discrepancies in the models used in vitro has been discussed before. For instance, Fischer et al [39], suggested that indirect evaluations of cytotoxicity of Mg sample extracts from Mg samples should be diluted at least ten fold for reliable and predictable results. Besides Mg concentration, changes in pH and osmolality strongly affect the biological evaluation of Mg as a biomaterial [40]l. In this study four cell types were evaluated with extracts from c.p Mg: two primary cultures of bone marrow-derived mesenchymal stromal cells (BMSCs) and osteoblasts and two commercial cell lines, MC3T3-E1 (pre-osteoblasts) and L929 (fibroblasts). Their results show primary cell types had an IC50 of about 35 mM, while the cells lines had an IC50 of 15 mM, below 10 mM Mg2+ was non-cytotoxic. This is corroborated by our results, albeit that in our hands the IC50 was slightly higher (~40 mM Mg2+). Also we corroborate their results that cells have a reduced proliferation at pH > 8.5.
During the static evaluation the mass transfer is irrelevant because all material is dissolved and accumulated in the medium. The periodic replenishment of part of the medium, which mimics fluid exchange, circumvents this accumulation [41][42][43].
In contrast, the influence of mass transfer, which continuously removes corrosion products, is possible under liquid flow. Under liquid flow will also affect the speed of the degradation because the chemical balance between cor- rosion and degradation is kept out of equilibrium [44]Additionally, the flow helps to avoid galvanostatic corrosion in which anodic and cathodic point are creating on the surfaces by the non-uniform presence of species. Similar results were found for Grogan et al [37] and Wang et al [45] who investigated the influence of pulsatile flow on a magnesium alloy (AZ31) compared tos static condition. As expected, flow increased the corrosion rate which means that Mg for applications in the circulatory system should be tested under flow.
We conclude that c.p Mg modified by PEO improves its corrosion resistance and with this its biological performance. Our study also provided directions for use of surface-modified Mg in cardiovascular stents. Investigations of the morphological and physicochemical properties of the surface-modified Mg showed that these surfaces may serve as a platform to deliver therapeutic cells to lesions with the benefit that the material will gradually disappear. Coat- ing controls the degradation of the material (reducing the fast reactivity and possible toxic direct effect of the Mg) without altering the balance of magnesium in the body. In general, a significant difference at the level of cytotoxic- ity and biological behavior was found between all the coatings and the bare material. However, after comparing the coatings obtained under different treatments and conditions, no significant changes were evident between them. Which makes it difficult to select one specific coating. Finally, although in this project different approaches to the real application were evaluated, in vivo studies are warranted to validate this information.
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