## 1 Introduction  
The The kINPen(r) MED (neoplas tools GmbH, Greifswald, Germany) is a cold atmospheric-pressure plasma (CAP) jet which is CE-certified as a medical device, class IIa [1, 2]. The generated CAP is an excited and conductive gas state with low temperature (<40 \({}^{\circ}\)C) which emits electromagnetic radiation, like ultraviolet, visible and infrared radiation, and other electromagnetic fields and generates charged particles and reactive species [3; 4; 5]. The most important species--in particular for the antimicrobial efficacy of CAP--are reactive oxygen (ROS) and nitrogen species (RNS) [6; 7]. These species include atomic oxygen (O), ozone (O\({}_{3}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), nitrite (NO\({}_{2}\)'), nitrate (NO\({}_{3}\)'), nitric oxide (NO\({}^{\bullet}\)), hydroxyl radicals (HO\({}^{\bullet}\)), superoxide anion radicals (O\({}_{2}\)\({}^{\bullet}\)') and singlet oxygen (\({}^{1}\)O\({}_{2}\)) [5; 8; 9]. The kINPen(r) is able to inactivate clinically relevant microorganisms which were isolated from patients with acute or chronic wounds, such as _Staphylococcus aureus_, _Klebsiella pneumonia_ or _Proteus mirabilis_[10; 11]. However, the kINPen(r) has not only an antimicrobial efficacy, it also stimulates the cell proliferation and migration and thereby tissue regeneration in the course of wound healing. The antiseptic efficacy and wound healing support on human or animal skin was confirmed for the kINPen(r) in clinical studies and animal trials ([12; 13; 14; 15; 16; 17; 18; 19], for reviews please refer to [20; 21]).  
The intensity of the treatment with the kINPen(r) MED is controlled by the treatment time. Thus, 30-60 s/cm\({}^{2}\) is the recommended treatment condition and the procedure is repeated every few days until the desired effect is achieved. The plasma treatment is currently realized by a sterile disposable spacer which mechanically guarantees a pre-defined distance of at least 7 mm between the skin surface and nozzle. This spacer is made of biocompatible plastic and can easily be placed on the tip of the pen-shaped handset of the kINPen(r) MED (Figure 1). However, the distance defined by the spacer and a perpendicular treatment with the kINPen(r) MED are recommendations [2; 22] the attending doctor or qualified personnel cannot comply with absolutely due to lack of space or patient/wound positioning, as well as sensitive wound tolerance levels. Furthermore, the effective area of the kINPen(r) MED cannot be seen with the naked eye.  
Thus, a concept was developed for a contact-free spacer to visualize the effective treatment area and to detect the treatment distance. In order to visualize the effective area, the relevant wound pathogen _Staphylococcus aureus_ DSM 799/ATCC 6538 (DSM--German Collection of Microorganisms and Cell Cultures, ATCC--American Type Culture Collection) was used to determine the zone of antibacterial efficacy. In addition, the area of ROS formation and the impact of (vacuum) ultraviolet ((V)UV) irradiation were determined. The effective area was analyzed depending on the incidence angle, which was the prerequisite for the design of the novel concept. Thus, the introduced construction enables a contactless plasma treatment with an unobstructed, illuminated view over the effective area and a free energy exchange with the surrounding air.