# Characteristics of 2.45 GHz Surface-Wave-Sustained Argon Discharge for Bio-Medical Applications
## 1 Introduction  
The low-temperature, non-equilibrium atmospheric pressure plasmas have attracted increasing interest as simple and less expensive plasma sources operating in open space for applications in biology, medicine, agriculture, and the food industry [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Various types of atmospheric pressure plasma sources were developed for such applications. Initially, the plasma produced at atmospheric pressure was with a temperature much higher than 40 \({}^{\circ}\)C, and mainly the thermal effects of plasma were used, as in the argon plasma coagulation device [11, 12]. Later on, the investigations were focused on developing plasma sources producing plasma at atmospheric pressure with a gas temperature low enough (below 40 \({}^{\circ}\)C, so-called cold atmospheric plasma--CAP) in order to avoid the destructive effects and damaging of heat-sensitive materials and biological systems. Two main requirements are formulated in [1] for the direct application of plasma \"on or in the human (or animal)body\" and they are the same for treatment in vivo or in vitro of any biological system, including seeds, plants, fresh fruits, etc. : (i) good stability and reproducibility of the plasma source operating at atmospheric pressure in the open space; and (ii) low temperature (<40 \({}^{\circ}\)C) at the tissue (sample) contact zone to avoid thermal destruction. Various types of plasma devices for these purposes were developed in recent years. The most widely studied and used plasma sources for bio-medical applications in various configurations and designs are based on the dielectric barrier discharge (DBD) and the atmospheric pressure plasma jets (APP) [13; 14; 15]. In [16], the plasma sources (assuming that the plasma is produced between electrodes) are categorized in three types: (i) direct plasma sources, when one of the electrodes is the sample (human body); (ii) indirect plasma sources, when the plasma is produced between two electrodes and the active plasma components are transported to the treated sample; and (iii) hybrid plasma sources--surface micro discharge (SMD) technology. A detailed review of non-thermal atmospheric pressure plasma sources for various applications and their plasma characteristics is presented in [17], and in [18] the focus is on plasma sources for medical purposes. The strategies for controlling the physical properties of the plasma for bio-medical applications and for keeping the plasma parameters bellow the damaging limits at the treatment area include the following: (i) using low electrical power; (ii) high frequency (kHz to MHz) or pulsed regime of operation; and (iii) appropriate choice of gas or gas mixture and the gas flow rate for plasma production. One of the CE certified as a medical device plasma source is the kINPen [19]. Various modifications exist under this name, and they operate in different regimes (power from 1.9 W up to 120 W; frequency from 0.8 MHz to 27.12 MHz; gases Ar, He + 2% molecular gas admixture, air with flow rates usually 3-5 slm) and one of them (the kINPen MED) is actually CE certified as a medical device. It needs about 10 years to decrease the operating temperature from about 80 \({}^{\circ}\)C [20; 21; 22] to 35-38 \({}^{\circ}\)C. In [23], the kINPen is compared with another plasma source, MicroPlaSter, which is also CE certified for medical use. The MicroPlaSter operates at 2.45 GHz and 80-110 W electrical power. Both devices use plasma, which is transported out of the ignition region by the gas flow. For MicroPlaSter, the gas flow rate is 2-5 L/min, and the distance between the device nozzle and the treated sample is fixed to 2 cm by a plastic spacer. The diameter of the treated area is 1 cm\({}^{2}\) with kINPen and 4-5 cm\({}^{2}\) with MicroPlaSter. The treatment time is also a very important parameter, and it is shown in [23] that the treatment time with kINPen can vary from 20 s to 5 min, and with MicroPlaSter, it is usually 2-5 min.