Page 31 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

BIODEGRADABLE MAGNESIUM-BASED SUPPORTS FOR THERAPY OF VASCULAR DISEASE A GENERAL VIEW
are important for the success of those treatments. Depending on the contacting tissue with the material, there is a growing need to improve the blood compatibility and the tissue growth and regeneration related with surface properties. Blood contact problems are key for thrombus formation, whilst conventional biomaterials surfaces, which do not have any stimuli for tissue growth, are not attractive for these applications anymore. Advanced tai- lored tools using low-energy ion irradiation which results in novel micro- and nano-morphological and chemical modifications. This may alleviate coagulation risks associated with implanted surfaces. Furthermore, synthesis by irradiation can help stimulate tissue growth and regeneration where it is necessary depending of the trauma and pathology. Ion irradiation can give tremendous advantages for surface design in order to ensure blood compat- ibility and hemostasis of inner face of stents, as well as some of those tools can also create surfaces, which will stimulate tissue (blood vessel walls) adhesion to outer face of the same stents. In the same sense, ion irradiation also can favorably modify scaffolds and grafts in order to stimulate growth tissue from tissue engineering and cell therapies point of view.
Besides situations in which the purpose is film deposition on substrates [108], ion implantation is another way in which surface properties can be modified towards a specific desired characteristic [109–111]. In this context, the chemical nature of the ions used, as well as the energy of the ion beam, the charge and energy are all charac- teristics that have significant influence on the outcome of the ion irradiation procedure. Among these, however, the dominant parameter is the energy of ion irradiation. At low ion energies, of below 100 eV, desorption and/or adsorption are phenomena that are dominant, as well as migration leading to island formation. Ion irradiation has been used for surface engineering to induce characteristics such as cleaning, smoothing, film growth or etching [112–114]. The characteristic that stands out from the current project’s point of view is the fact that irradiation of a material’s surface can induce changes to its wettability [107,115]. More importantly, for the purpose of surface properties control and bioactivity enhancement in biomaterials, the wettability can be either enhanced or reduced by manipulating the characteristics of the ion beam. The advantages that this brings for improving the biocom- patibility of medical devices, especially vascular stents are obvious: by manipulating the ion beam composition, energy, or flux, we can potentially be able to set [116] up criteria for optimum surface wettability in vascular grafts with consequences in refining our capacity to control thrombogenicity and tissue integration of these implants.
7. Problems and future challenges
The non-homogenous degradation of Mg is a challenge for its biomedical applications. This heterogeneity in most of the cases is due to the presence of impurities, secondary phases or contact with materials of other nature that may induce a localized corrosion of galvanic type. As a consequence of this, small cracks and spots of failure may start to appear that accelerate the degradation at these nano and micro niches, while simultaneously the pro- duction of hydrogen gas may locally accumulate as a degradation product. Low quantities of hydrogen gas are absorbed by the body but too high amounts cause necrosis and other damage of the tissue that surrounds the magnesium implant. The ideal situation is having a protective coating on the Mg that degrades in a controlled way [117,118]. In addition, as a passive layer of Mg is highly sensible to chloride ions (Cl-) present in physiologic fluids and culture media, this is not considered an efficient alternative for biomedical applications,: however it can be used in combination with others treatments such as coating with polymers or immobilization of drugs [119].
Corrosion phenomena can be readily investigated in vitro, e.g. through electrochemical testing potential-dynamic polarization using simulated body fluid (SBF) or NaCl as immersion solution. To date, this is the method of choice to determine the corrosion rate of a material. With the obtained data, it is possible to evaluate the efficiency of corrosion protection of the coating. However, the most critical tests are preclinical in vivo experiments in rodents of large animals such as pigs, where the material is exposed to a complete organism, with real-life variables and where elimination methods and homeostatic controls are carried out. Several studies show that results from in vitro and in vivo tests are frequently disparate for reasons that are unclear. Nevertheless, in vitro assessment of
2
29