Page 88 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

CHAPTER 5
zirconia surfaces, the effect of the changes in the surface of the material by UV exposition, will be highly dependent on the time and intensity of exposure. They reported that the wettability of the material can be increased, therefore improving its biological performance. However, considering the porosity of the samples studied in the present case, the UV treatment may be not enough to ensure a proper sterilization processes, as it is very superficial and therefore it does not guarantee that within the porous structure, microbial agents (bacteria or fungi) have been completely eliminated.
On the other hand, dry-heat sterilization is generally used to avoid deterioration and/or corrosion of the material, which could be particularly important for magnesium substrates due to its high reactivity. In addition, this an effec- tive and low-cost technique. The results obtained with this technique for anodized Mg were similar to those found by other authors in other biomaterials such as titanium and zirconia [22][23]. However, as relatively high temperatures are used in this technique and although Mg does not present structural or compositional modifications at these temperatures, shielding of tensions, re-crystallization processes and release of tensions can be generated at the substrate-coating interface [24]. As a consequence of this, cracks can appear generating points of failure or defects in the coating that can affect its corrosion and mechanical resistance.
In steam autoclaving water vapor, temperature and pressure are combined to kill microorganisms, also microbial spores are neutralized [25]. Additionally, during steam autoclaving and formaldehyde treatments, in contrast to UV, vapor will penetrate all cavities of the sample, efficiently sterilizing all the surfaces. Passivation of a material is initi- ated by its oxidation and the release of corrosion products decreasing the reactivity of the material by this protective layer. For the case of Mg, during the first hours in which the material is exposed to an aqueous solution, the highest levels of corrosion rate are presented, which subsequently decrease due to the passivation of the material. The use of water vapor could produce such passivating effect on the implant material, which would be advantageous as this high corrosion stage will not affect the evaluation with cellular components due to otherwise, augmented hydrogen production and pH changes.
Sterilization with steam formaldehyde is often used when materials are thermally sensitive, as it is the case of some polymers with low melting points such as PLGA, nylon, polystyrene, poly(N-isopropyl acrylamide) hydroxylpropyl- cellulose, poly(vinylcaprolactame), polyvinyl methyl ether or heat-sensitive surgery instruments e.g. cryo-instru- ments, probes or catheters. The low temperatures (around 55 to 75oC) and mainly the chemical action (chemical crosslinking) of formaldehyde generate an atmosphere with high penetration that allows to kill bacteria and spores. This process is carried out under reduced pressure. One of the disadvantages of this method is that formaldehyde is a toxic, hazardous chemical considered carcinogenic, and poses a potential health risk for personnel that operates steam formaldehyde equipment [26][27][28]. For samples of anodized Mg, the use of an agent such as formaldehyde shows that the surface undergoes changes at a morphological level which means that in terms of surface configura- tion, the evaluated material is different from that obtained once it was anodized. Formaldehyde is an aldehyde i.e. reducing agent, thus could potentially chemically interact with the surface layer or the underlying magnesium. That would adversely alter the chemical-physical properties of the PEO-treated Mg. According with the cross-sections, none of the evaluated treatments showed evidence of degradation of the substrates. Therefore, it is considered that the coating was sufficiently protective to avoid the reaction of the Mg substrate in any of the environments employed for sterilization.
Wettability is a parameter directly related to the contact angle and the surface energy of the surfaces. According with the results of this study, the porosity of all the coatings increased the contact surface area between the drop of water and the material during the contact angle test. As the area is larger the water can spread more on the surface. In the sample of formaldehyde the surface porosity of the sample was modified, these change on the surface config- uration lead to the increase of the contact angle of the surface and consequently a decrement on the surface energy. Autoclaving increased the contact angle, which might be due to the altered surface topography i.e. the sealing of pores in the anodic film or formation of Mg(OH)2 on the coating surface, both phenomena addressed by the contact of the sample with the water during the autoclave process. Hiromoto et al [29] suggested that due to autoclaving
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