## 4 Discussion
We chose this concentration because, in previous work, we found an H\({}_{2}\)O\({}_{2}\) content of 120 mg/L for DMEM without pyruvate directly after CAP exposure and could show that the H\({}_{2}\)O\({}_{2}\) was depleted immediately by pyruvate in DMEM with 1 mM pyruvate [80]. In consideration of other anti-oxidative components contained in DMEM, such as Fe\({}^{3+}\), which leads to the decomposition of H\({}_{2}\)O\({}_{2}\) by the Fenton reaction, we used the concentration of 150 mg/L. We could show that the effect of PAM on HaCaT cells was not solely mediated by H\({}_{2}\)O\({}_{2}\) contained in PAM. Although H\({}_{2}\)O\({}_{2}\) is an important molecule in PAM, it is not the only longer-living reactive species. As other groups have shown, after approx. 40 h, nothing of the original concentrations can be detected [37]. From our own previous work, we know that the effect of PAM on cells lasts over 21 days [13]. One possible explanation is the formation of long-lived organic hydroperoxides [84]. It has already been shown that individually added RONS in cell culture medium or buffer does not have the same effect as PAM, but that the various reactive species that are formed as a complex mixture by CAP treatment in liquids complement each other in their effect [37; 85; 86; 87]. Therefore, investigations to discover the mechanisms of action of CAP on cells in fluids are difficult and require special techniques. Simulation techniques appear to be also promising [88]. In terms of viability, we showed that PAM had a stronger inhibiting effect on HaCaT cells than the HAdase treatment. HAdase treatment led to an impairment of the outer layer of the cell and the degradation of the HA pericellular coat, but the effect of PAM extended into the interior of the cell. This is not surprising, and many effects of PAM or CAP on cells in vitro, e.g., apoptosis or DNA damage, are already known and have been described in several publications [13; 34; 35; 36; 42; 43; 44; 45; 58]. Focused on the influence of CAP or PAM on the adhesion and migration of cells, other groups have examined the expression of adhesion molecules, e.g., integrins and cadherins or CD44, on cells after CAP/PAM treatment in vitro. [PERSON] et al. found elevated integrin expression in HaCaT cells after direct or indirect Dielectric Barrier Discharge plasma treatment, whereby cadherin expression was not influenced [71; 72]. In tumors, high cell migration rates are a precondition for tumor cell invasion into the surrounding tissue. Therefore, it is interesting that reduced cell migration was shown in vitro for fibroblast cells treated by CAP using a helium plasma jet [74]. [PERSON] et al. explained this effect also by increased integrin activation and an elevated focal adhesion size. We assumed that, in cells that express HA at their surface, the RONS generated during plasma treatment first reach the cells' HA coat before they contact the adhesion molecules, e.g., integrin receptors, located in the cell membrane. Since HA is degraded by free radicals [75; 76; 77], one can also imagine that the HA coat of the cells is degraded when treated with PAM. We were able to prove this for HaCaT cells with our investigations. Furthermore, it would be interesting to find out whether other glucosaminoglycan-coupled processes in cells are also regulated by PAM treatment. As such, it is possible that the decreasing HA expression is balanced in the cells by a higher HA receptor expression. We found these considerations confirmed in investigations by [PERSON] et al. They investigated the influence of a helium jet plasma on the differentiation and adhesion of myeloma cells and found an elevated mRNA expression for the HA receptor molecule CD44 [89]. This fell in with our results such that the cells might try to compensate for a reduced HA expression by an increased expression of the corresponding receptor.