1. Application of the liquid interface

Due to the large extent of direct and indirect damages from both gases, we had to find new ways to control or prevent them. The first approach to skin damage reduction is the addition of different interfacial liquids, which could add some beneficial properties, such as a high evaporation effect for cooling the treatment site, and antiseptic ability. Thus, liquids (ethanol, acetic acid, PBS, or saline) were applied to the mouse skin prior to the plasma treatment. Following plasma exposure, the areas of direct skin burns and the presence of indirect skin damage were evaluated [Figs. 7 and 8(a)]. The results indicate that skin damage could be ameliorated if ethanol was applied to the treatment site prior to the plasma treatment. The plasma treatment with ethanol pre-treatment resulted in minimal direct skin damage, while indirect damage was not observed. In contrast, pre-treatment with acetic acid, PBS, and saline led to extensive skin burns at the site where the plasma jet was in contact with the mouse skin. Additionally, after 48 h post-treatment, severe indirect plasma jet effects were noticed, especially in the case of saline application.A factor that could interpret demonstrated differences is that the plasma system was not connected to any ground. As a result, the mouse skin and used liquids took over the function of a ground electrode, leading to coupling. This effect was demonstrated by the I–V characterization in Figs. 3(b) and 3(c), where different waveforms are observed. Similarly, adding a liquid to the mouse skin before treatment impacted the conductivity of the treated mouse skin and, consequently, modified the “electrode” properties and, thus, the plasma–mouse interactions. The electrical conductivity of liquids is dependent on the concentration of the ionized chemical species in the solution. Among all the liquids used in this study, PBS possesses the highest electrical conductivity (10.6 S/m),5050. C. V. Chaparro, L. V. Herrera, A. M. Meléndez, and D. A. Miranda, J. Phys.: Conf. Ser. 687, 012101 (2016). https://doi.org/10.1088/1742-6596/687/1/012101 followed by 0.9% saline (1.45 S/m),5151. R. Sauerheber and B. Heinz, Chem. Sci. J. 6, 4 (2016). https://doi.org/10.4172/2150-3494.1000109 and 10% acetic acid (0.2 S/m).5252. T. H. C. Salles, C. B. Lombello, and M. A. D'Ávila, Mater. Res. 18, 509 (2015). https://doi.org/10.1590/1516-1439.310114 Ethanol and de-ionized water are not electrically conductive; however, after the plasma treatments, their conductivity was modified due to the addition of plasma-ionized species and their diffusion into the liquid. As plasma-induced skin damage cannot be explained only with electrical conductivity, thermal effects must also be considered. Since plasma was heating the skin, liquid evaporation and gas flow produced a cooling effect. With respect to surface temperature, the measured topical temperature of the mouse skin was 27–33 °C, whereas the enclosed environment for evaporation was set similarly at a medium value of 30 °C. Volume loss measurements reveal [Fig. 8(b)] that all liquids had roughly the same linear evaporation rate, except ethanol, the only liquid that evaporated completely after 240 s of plasma treatment. The proposed conclusion is that the ethanol's cooling effect and low electrical conductivity greatly reduce skin damage. Therefore, ethanol represents the best intermediate liquid stage for minimizing skin damages during plasma treatments. However, even a more efficient skin damage control was obtained in gas mixtures, where N2 gas was added to the He gas flow, as detailed below.