Application of Low-Temperature Plasma Processes for Biomaterials

Abstract

Physical plasma is defined as a gas in which part of the particles that make up the matter are present in ionized form. This is achieved by heating a gas leading to dissociation of the molecular bonds and subsequently ionization of the free atoms. Thereby, plasma consists of positively and negatively charged ions and negatively charged electrons as well as radicals, neutral and excited atoms and molecules (Raizer, 1997; Conrads and Schmidt, 2000). On the one hand, plasma is a natural phenomenon as more than 90 % of the universe is in the plasma state, for example in fire, in the polar aurora borealis and perhaps most importantly in the nuclear fusion reactions of the sun. On the other hand, plasma can be created artificially and has found applications in technology like plasma screens or light sources. The use of high temperature plasma for energy production is still the focus of ongoing research. For the modification of biomaterial surfaces, low temperature plasma which is sometimes also called cold plasma is used. It is characterized by a low degree of ionization at low or atmospheric pressure (Roth, 1995; Roth 2001; Hippler et al., 2008). To create low temperature plasmas, a compound is first transformed into a gas and then ionized by applying energy in the form of heat, direct or alternating electric current, radiation or laser light. Commonly used plasma gas sources are oxygen, nitrogen, hydrogen or argon. Two typical research plasma reactors for different applications are shown in Fig. 1. Depending on the nature and amount of energy, low temperature plasmas are characterized by a non-equilibrium between electron temperature and gas temperature. Thus the main parameters which define the characteristics of a plasma and thereby its applicability are its temperatures, types and densities of radicals and its level of ionization. In material science, possible applications of low-temperature plasmas include the modification of surface properties like electrochemical charge or amount of oxidation as well as attachment or modification of surface-bound chemical groups. Consequently, properties like hardness, resistance to chemical corrosion or physical abrasion, wettability, the water absorption capacity as well as the affinity toward