Cold Plasma Techniques for Pharmaceutical and Biomedical Engineering

Abstract

Plasmas can be defined as the state of ionized gas consisting of positively and negatively charged ions, free electrons and activated neutral species (excited and radical), and are generally classified into two types, thermal (or equilibrium) plasma and cold (or nonequilibrium) plasma, based on the difference in characteristics. The thermal plasma is the state of fully ionized gas characterized by a high gas temperature and an approximate equality between the gas and electron temperature (Tg ≈ Te) and can be generated under atmospheric pressure. The energetic of this plasma is very high enough to break any chemical bond, so that this type of plasma can be excluded from most of organic chemistry, let alone from the field of pahramceutical science. In contrast, the cold plasma is most characterized by a low gas temperature and a high electron temperature (Tg << Te), and easily generated by electric discharges under reduced pressure. The field of plasma chemistry deals with occurrence of chemical reactions in the cold plasma including atmosphere pressure glow discharge plasma. One of the characteristics of surface treatment by cold plasma irradiation is the fact that it is surface limited (ca. 500-1000 A) so that only the surface properties can be changed without affecting the bulk properties. In recent years, biomedical applications of cold plasma are rapidly growing due to the fact that the use of cold plasmas is very useful to treat heat-sensitive objects such as polymeric materials and biological samples. The demonstrations of plasma technology in the biomedical field have created a new field at the intersection of plasma science and technology with biology and medicine, called “Plasma Medicine”. (Fridman et al., 2008) When the cold plasma is irradiated onto polymeric materials, the plasma of inert gas emits intense UV and/or VUV ray to cause an effective energy transfer to solid surface and gives rise to a large amount of stable free radicals on the polymer surface. In view of the fact that surface reactions of plasma treatment are initiated by such plasma-induced radicals, study of the resulting radicals is of utmost importance for understanding of the nature of plasma treatment. Thus, we have undertaken plasma-irradiation of a wide variety of polymers, synthetic and natural, and the surface radicals formed were studied in detail by electron spin resonance (ESR) coupled with the aid of systematic computer simulations. On the basis