Page 29 - 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
techniques to deposit inorganic and organic material due to its low temperature operation. However, Mg can react with immersion media and can result in limited coating thickness; organic coatings can be used to protect the metal from corrosion by temporal isolation of the media during the treatment. Organic thin film materials can be used to store drugs and deliver them in a controlled in-vivo setting. Frequently used polymers as coatings are polylactic acid (PLGA), polycaprolactone (PCL), polylactic acid (PLA), chitosan, among others; [24,87–92]. Finally, the geometry of the stent system is an important criteria to take into consideration when selecting a technique or modification of Mg surfaces. A homogenous treatment needs to be guaranteed avoiding defects than can accelerate the corrosion process or be sources of failure via fatigue, creep or fracture.
6.2.3 Modification of Magnesium surfaces by plasma and ion irradiation
Addressing the challenges of bulk-metal alloying and surface coatings another emerging area of research is the use of reactive plasmas (e.g. ionized gas) and/or energetic ions for the modification of Mg surfaces. The inter- face of biomaterials with the extreme hemodynamic environment around stents integrated with blood vessels is dependent on biomaterial surface properties that can influence cellular and biomolecular activity around the en- dothelium. Surface properties at the biointerface depend on a number of factors that include: surface chemistry, surface charge density, surface free energy (e.g. surface tension), surface topology and morphology, impurity surface composition and surface stress [45]. These properties can be modified by means of energetic particles (e.g. incident particles that carry energies with energy distributions several orders of magnitude higher than thermal energies averaging 0.025 eV). There are a variety of surface modification techniques that can be used to change the surface properties of biomaterials and these have been used in applications ranging from bone reconstruction to surface functionalization [93–95]. Most techniques used as discussed in earlier sections and many recent publications focus on thermodynamic-driven modifications of a material surface. Irradiation-driven modifications provided by ener- getic ions and plasma (e.g. ionized gas) can provide a much wider spectrum of modification alternatives that enable complete transformation of biointerfaces with much broader control of surface function [93,94]. Enhancement in biocompatibility and platelet adhesion are one of the few properties that can be influenced by plasma treatment, however some modification approaches can be unstable [96] namely due to a lack of control of some plasma-based sources. Nevertheless, modification using high energetic particles (e.g. ion and/or plasma source) of biomaterial surfaces can be advantageous compared to chemical-based or coating deposition approaches namely due to the ability to changes surface properties with extreme high fidelity in the order of a few biomolecules to the spatial scale of cell proliferation and differentiation.
New vascular therapeutic technology has recently focused on the ability to controlled engineering of tissues and in particular tissue-engineering blood vessels which could shed light on endovascular regeneration from acute or chronic injury [97]. The need for inherent multi-functional properties of a stent surface derive from the complexity in early angiogenesis steps and the management of cell recruitment to the injured site involving complex cascade of immune mediators, soluble signaling molecules, and cell-to-cell interactions [98][99]. In particular, topographical cues are known to influence both recruitment and migration of cells [100]. Furthermore, mounting evidence exists for the importance of mesoscale and nanoscale structure that can alter cell morphology, adhesion, motility, prolifer- ation, endocytosis activity, protein abundance, and gene regulation [101] Therefore, techniques that can modify and vary surface structure and morphology over different spatial scales are attractive towards establishing a strategy to design novel “smart” biomaterials.
For Mg-based stent surfaces the extent of modification depends on the ability to induce changes to the top-most surface atoms and sub-surface regions. In addition, as discussed earlier the goal for Mg-based stent materials is their ability to provide bioresorbable properties not available in traditional stent materials such as Nitinol and CoCr- based alloys. This introduces the additional functional requirement of both mechanical strength (e.g. resilience to sustain the hemodynamic forces endured by in-vivo stents) and variable corrosion resistance that allows a control- lable time-dependent degradation to non-toxic byproducts in the body. These properties can in fact be modified with plasma-based surface modification and is the focus of current research.
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