# Cold Atmospheric Pressure Plasma in Wound Healing and Cancer Treatment
## 4 Properties of CAP Stimulate Tissue Regeneration but May also Induce Cell Death
RONS, such as hydroxyl radicals (OH**\(\bullet\)**), hydrogen peroxides (H**\({}_{2}\)**O**\({}_{2}\)**), hyper oxides (O**\({}_{2}\)**-**\(\bullet\)**), nitric oxides (NO**\(\bullet\)**), nitrogen dioxides (NO**\({}_{2}\)\(\bullet\)**), and peroxynitrites (ONOO**\({}^{-}\)**), generated by CAP are identical with reactive species generally produced during regular metabolism. They are known to mediate critical physiological and pathological processes, such as immune response, cell adhesion, and migration, apoptosis, skeletal muscle function, and wound healing [46]. Many of the ROS-mediated responses protect cells against oxidative stress and reestablish redox homeostasis [47]. RONS and the antioxidant system are regarded as secondary messengers to control various signaling pathways [48]. Wound healing often is described as a series of three overlapping consecutive phases [49]. The initial inflammation phase is characterized by increased exudation of blood plasma, and recruitment of macrophages and granulocytes, which degrade necrotic tissue and establish an antimicrobial environment. In the subsequent proliferation phase, granulation tissue is built, and new vascular structures are assembled (neovascularization). The final remodeling phase is characterized by the maturation of collagen fibers, immigration of epithelial cells from the edge of the wound, and finally, wound closure [50]. RONS are known to be essential for the initial stage of hemostasis by mediating tissue factor (TF-mRNA), platelet recruitment, and platelet activation [51]. It has been pointed out that RONS have a widespread and pervasive effect on promoting, but also resolving inflammation [52]. In doing so, not only the presence or absence of RONS, but also the concentration seems to play an essential role. While lower concentrations of nitric oxide, for example, tend to favor growth and  
Figure 1: Cold atmospheric pressure plasma (CAP) in wound and cancer treatment. Wound healing, disinfection, and tissue regeneration are triggered by short or moderate CAP treatment (left panel). It is hypothesized that ultraviolet (UV) radiation, electromagnetic fields, and/or reactive oxygen and nitrogen species acting either alone or in concert contribute to the effects observed. Prolonged treatment for cancer therapy enhances oxidative stress from eustress to distress and hence will induce cancer cell death (right panel). EMT = epithelial-mesenchymal transition. inhibit apoptosis, higher concentrations promote cell-cycle arrest, senescence, or apoptosis [53]. Indeed, nitric oxide, which is involved in wound healing and is released primarily in the inflammatory phase, is one of the essential RONS [51; 54; 55]. Besides inflammation, its functional activity has been linked to angiogenesis, cell proliferation, matrix deposition, and remodeling [56]. During the inflammation phase of wound healing, nitric oxide is generated by neutrophils and macrophages through an enzymatic reaction catalyzed by the inducible nitric oxide synthase (NOS2) [48]. Hydrogen peroxide is also known as an active secondary messenger for a platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and tissue growth factor (TGF) [51; 57]. These examples show that temporarily elevated levels of RONS can be tolerated and have crucial physiological signaling roles. However, chronic oxidative stress, due to increased levels of RONS over more extended periods of time, may have pathological consequences, including chronic inflammation or the development of cancer. For the therapeutic use of CAP in wound healing, the role of RONS in resolving inflammation is especially intriguing [48]. Several studies have analyzed molecular changes induced by CAP treatment [58]. Transcriptome analyses of RNA from human cells treated in vitro with CAP, for example, revealed that genes associated with cellular stress response had been elevated after CAP treatment, and increased levels of anti-oxidative enzymes have been observed [59].