Physical and biological mechanisms of plasma interaction with living tissue

Non-thermal plasmas are already well-known for their sterilization ability; however the mechanisms of this sterilization are under debate. Short and long living active species and radicals produced by plasma, ultraviolet (UV) radiation, local thermal effects, and bombardment by charges particles are all listed as potential candidates for sterilization of various surfaces. In this work, biochemical and physical mechanisms of plasma interaction with biological materials will be discussed. Direct interaction where a surface of a microorganism is used as one of the plasma-generating electrodes is compared with indirect interaction where plasma is generated elsewhere and the plasma-treated gas is carried off to a remote location for microorganism treatment. Under these conditions, the authors show that: a) direct treatment by plasma is orders of magnitude faster than indirect treatment; and b) interaction of a microorganism with charged particles is the primary inactivation mechanism. Microorganisms selected for this study were Streptococcus, Staphylococcus, Yeast, and E. coli. Samples were treated by dielectric barrier discharge system in either continuous- wave (sinusoidal, 4-20 kHz), microsecond pulse (100 mus pulse duration, 0.1-1 kHz repetition rate), or nanosecond pulse modes (3 kV/ns rise time, 10-40 ns pulse duration, 0.1- 1 kHz repetition rate). Microorganisms were either treated directly by plasma or plasma afterglow was utilized.