Page 30 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

CHAPTER 2
6.3 Plasma and ion-surface interactions with biomaterials
The interaction of highly energetic ions from plasma sources exposed to biomaterial surfaces can be quite complex due to the inherent coupling between them and the complexity of both the plasma (e.g. ionized gas) and hemody- namic environment. The interaction is a multi-scale phenomenon that involves complex temporal and physical scales. Ion beam irradiation refers to the acceleration of charged species that implants energy onto a surface [102,103]. The charged species can be either single ions (monomeric beam), or ion clusters (cluster beam) [104–106]. In addition, the ions can either be chemically inert, which means they do not interact with the surface being bombarded, or chemi- cally reactive, in which case there is a chemical reaction that occurs upon the interaction between the irradiating species and the surface being bombarded. In some cases, either inert or chemically reactive irradiating species when combined can effectively impart energy to a surface inducing metastable phase formation. These metastable phases can introduce surface topography and surface chemistry that fine tune the physical and chemical properties of the material.
6.3.1 Methods for surface modification with plasma and ion irradiation
There is a variety of methods for the modification of biomaterial surfaces with plasmas and ion irradiation. Most conventional methods for plasma-induced surface modification have focused on plasma-induced etching or plasma- aided or enhanced coating deposition. These “top-down” methods depend largely on the interaction of plasma with a biomaterial and mostly result in the modification of one surface property of the material in question with some con- trol over spatial dimensions of the order of few hundred nm to several microns. There are three primary “top-down” methods to modify biomaterial surfaces with energetic particles extracted from a plasma or ion source: 1) ion-beam implantation, 2) ion-beam assisted deposition and 3) plasma spray deposition.
Ion-beam implantation is concerned with introducing energetic particles to a material surface and inducing changes in the bulk and near-surface structure. Given the average penetration depth of low-energy ions varies from a few nm to only about 10-20 nm, ion-beam implantation is typically done with high-energy ions usually ranging from several hundreds of keV to several MeV energies. The plasma treatment induces changes of a biomaterial’s physicochemical and biological properties given that the ions can be formed of any element in the periodic table from a high-energy ion-beam accelerator. However, upfront, it is impossible to predict that nature of these changes. For example, oxy- gen ions can be accelerated to penetrate microns into a biomaterial and induce local oxidation phases that promote biocompatibility [107]. One challenge with ion-beam implantation is its limitation to only be able to treat nearly flat surfaces since it is a line-of-sight technique. An alternative to ion-beam implantation that results in similar biomate- rial modification is achieved with plasma immersion ion implantation. In this case the ions are intrinsically coupled to a biomaterial surface by a plasma sheath as the biomaterial is “immersed” in the gaseous discharge. The ions in this configuration are not limited by the geometry of the biomaterial piece and can even modify porous substrates. The technique consists of applying a large direct current bias to the biomaterial immersed in the gaseous discharge. In this case there is no restriction with “line-of-sight” issues as in ion-implantation techniques. Yet, in plasma-immersion the biomaterial must be electrically conductive and is exposed not only to highly energetic ions from the plasma but also to radicals and electrons. This means that there is less control of surface modification and consequently biomaterial properties. Ion-beam assisted deposition (IBAD) is a well-known “top-down” technique that combines the use of energetic ions with thermal evaporation to induce changes on a biomaterial surface. The evaporation flux can be se- lected to be reactive such as oxygen or metal oxide particles that can induce chemical interactions on the surface that can influence surface properties. Plasma-spray deposition is a “top-down” technique that consists of reactants added to the gaseous state and thus induce chemical changes both in the plasma sheath and at the biomaterial surface to yield chemical and topological changes.
6.3.2 Biomaterial properties after plasma irradiation
The proper surface properties of biomaterials that are e.g. used to treat of neuro and cardiovascular traumas are
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