Plasma-cell interaction: Prokaryotes

Biological materials can be exposed to plasma in two different methods; the first is “direct exposure”, when the sample to be treated is in direct contact with the plasma. All plasma-generated agents, including charged particles, come in contact with the sample. The second method is “indirect or remote exposure”. In this case the sample is placed at a distance from the plasma volume or in an adjacent chamber. In this configuration, the amount of heat transmitted to the sample is minimized; the charged particles produced in the plasma do not reach the sample as they recombine on the way, as do many of the short-lived, neutral reactive species. In the following section, the potential contribution of the four main inactivation factors of plasma (heat, UV radiation, charged particles, and reactive species) are reviewed. Direct and indirect plasma exposures differentiate effects of different plasma agents by means of the vastly different half-lives of plasma species. Depending on the need for specific applications and on future understanding of plasma-cell interaction mechanisms, direct and indirect plasma treatments may be manipulated and optimized. To this end, it is conceivable to have an intermediate mode of plasma exposure in which the effects of charged particles are attenuated but nevertheless allowed. Plasma jets, discussed in Chapter 3, provide an easy route to switch between different plasma exposure regimes. In high-pressure non-equilibrium plasma discharges, reactive species are generated through various collisional pathways, such as electron impact excitation and dissociation. Reactive species play an important role in all plasma-surface interactions. Air plasmas, for example, are excellent sources of reactive oxygen-based and nitrogen-based species, such as O, O2*, O3, OH, H2O2, NO, and NO2.