## 4 Discussion
A recent study also showed that an increased incidence angle using a helium-driven discharge at short treatment distance improved the water contact angle and showed a rather homogenous and wide spread of ROS compared to a vertical treatment [34]. Another factor which was determined to characterize the effective area is the (V)UV irradiation. The UV irradiation, in accordance with ROS, decreased (from 0\({}^{\circ}\) to 50\({}^{\circ}\)) by 21% and 9%, respectively, with increasing tilting. Thus, both factors may influence the antimicrobial efficacy. The VUV irradiation remained nearly constant. UV radiation leads to the formation of DNA lesions [36] which in consequence results in the induction of DNA repair mechanisms in bacteria but obviously also inactivates microorganisms [37]. Nevertheless, UV photons seem to have only minor influence on bacterial inactivation [38; 39]. Furthermore, [PERSON] et al. [40; 41] described for a He/O\({}_{2}\) plasmaa change in plasma effluent composition due to UV or, in particular, VUV photons, resulting in a synergistic effect with ROS and thereby a higher inactivation efficiency. This demonstrates once again the complexity of plasma and the thereby mediated processes. In the herein introduced study, a first attempt was made to describe the manifold influencing parameters which are necessary to develop a contactless handheld device for plasma treatment. The microorganism _S. aureus_ DSM 799/ATCC 6538 is a relevant wound pathogen and was chosen for this reason as a test organism. Nevertheless, in wounds it is not only one microorganism that is treated but rather it is a mixture of different species in different amounts. Thus, it has to be mentioned that other microorganisms may be easier or harder to inactivate [2]. This may influence the effective area and the treatment time. The different parameters to detect the effective area of the kINPen(r) MED-\(\beta\) were used for the design of a top unit for the device. The combined concept (Figure 13a) comprised a distance measurement via IR laser signal propagation time sensors with a focus on the working point in the effluent and an replaceable ring aperture. This--disposable or autoclavable, as requested--aperture can illuminate the effective area of the kINPen(r) MED-\(\beta\) for the pre-defined treatment angle of, e.g., 0\({}^{\circ}\) or 45\({}^{\circ}\). Each aperture will be manufactured and provided according to the requirements of the medical personnel either for perpendicular or tilted treatment. Thus, the operator can adjust the selected aperture based on the wound accessibility and space for handling the device at the bedside. The provided measurements (inhibition zone, ROS, VUV, UV) led to a better understanding of the cocktail component contributions and distributions which allowed for the deduction of the dimensions (length and width) for the illumination of the effective area. However, the technical solution requires a defined spatial dimension at each given angle. Taking into account the complexity of the plasma cocktail components, a minimal effective area was defined to determine the illumination dimensions for the aperture at each given angle (Figure 13b). The resulting dimensions for the investigated treatment angles were calculated with an elliptical fit. A clear impact of the treatment area was shown over the investigated range of treatment angles. This motivates an adjusted aperture for application conditions. While the treatment angle of 0\({}^{\circ}\) and 25\({}^{\circ}\) changed the effective area from 107 mm\({}^{2}\) to 132 mm\({}^{2}\), the shape remained rather constant. For 50\({}^{\circ}\) however, the shape changed significantly while the area increased only slightly (140 mm\({}^{2}\), +31% in total). A simplified laboratory prototype to test handling and user compliance was realized by adjusting four LEDs in a circular aperture to visualize the effective treatment area for a vertical treatment position (Figure 14a).