## 3 Applications of Cold Atmospheric Pressure Plasma
### Plasma Inactivation Mechanism  
As claimed above, CAPP produces different reactive species (RONS, positive and negative ions, atoms, molecules, etc.) playing a crucial role in the inactivation of microbial targets. [PERSON] et al. reviewed the mechanism of bacteria inactivation via CAPP. The bacteria are eroded by cell bombardment of charged particles, breaking the appropriate chemical bonds and opening the cell membrane to the penetration of reactive agents into the inner volume of bacteria. The etching process (facilitated, in particular, by RONS species) subsequently causes formation of molecular fragments leading to morphological changes of the cell (e.g., formation of deep channels in the bacteria), RONS cause damage by oxidation of cytoplasmic membrane, protein and DNA and thus complete bacterial destruction [57]. To increase the use of CAPP technology in industry, it is important to provide theoretical studies about the pathogen inactivation process. To describe the inactivation mechanism at the atomic level is difficult based on only experimental research [123]. A better understanding of plasma dynamics and chemistry, and the transport of reactive species in the plasma can be achieved using numerical simulations. Most computational studies evaluate the plasma physics and gas phase chemistry (e.g., spatial-temporal profile of the electric field, density of reactive species, radiation intensity). Various kinetic models (classical reactive molecular dynamics, nonreactive molecular dynamics, density functional theory) are used to describe the biomolecular systems at the molecular or atomic scale [124]. These models save time, money and other resources by analyzing a large amount of data at the same time and provide valuable correlations. The study of [PERSON] et al., based on reactive force field molecular dynamics simulation, found that ROS (O, OH, HO\({}_{2}\) and H\({}_{2}\)O\({}_{2}\)) reacted with the S. cerevisiae glucan structure by hydrogen abstraction reaction to cleave the chemical bonds (C-O and C-C), which resulted in cell wall destruction. Their results showed that O (highest activity) and OH induced the largest number of hydrogen abstraction reactions. The activity of HO\({}_{2}\) and H\({}_{2}\)O\({}_{2}\) were lower, however, the number of main chain and branched chain fractures were bigger. Consequently, the destructive effect of H\({}_{2}\)O and H\({}_{2}\)O\({}_{2}\) was more efficient [123].  
The bacterial cell membranes, barriers protecting proteins and nucleic acids can block ROS from entering the interior of cells. However, ROS with high permeability are able to enter the cell interior and cause cell death. Therefore, the permeability of ROS into the bacterial membrane plays a crucial role in the bacterial inactivation mechanism [125] as was shown by [PERSON] et al. Their molecular dynamics simulation described the behavior of various ROS at the membrane-water interface. They showed that the cell membrane acts as a weak barrier to O\({}_{2}\) (hydrophobic ROS), which remains within the lipid bilayer, and plays the most important role in oxidation reactions. On the other hand, the cell membrane serves as a strong barrier to protect internal substances from the hydrophilic ROS (OH, HO\({}_{2}\), H\({}_{2}\)O\({}_{2}\)). The permeability of the cell membrane to HO\({}_{2}\) was markedly enhanced after the CAPP treatment, which enhanced the bacteria-killing properties of the plasma. They also concluded that the arrangement of phospholipid molecules (the main constituent of the cell membrane having hydrophilic head and hydrophilic tail) in the cell depends on the ROS generated by the plasma. High amount of ROS resulted in decreased cell membrane thickness and disordered phospholipid arrangements [125].