# Wearable Plasma Pads for Biomedical Applications
## 1 Introduction  
Non-thermal atmospheric-pressure plasmas (NAPPs) have been studied for potential applications in dermatology and wound healing [1; 2; 3; 4; 5; 6]. Plasma components such as ultraviolet (UV) rays, electric current (EC), reactive oxygen species (ROS), and reactive nitrogen species (RNS) are involved in skin treatment. In the most recent review papers [1; 7; 8; 9; 10], plasma sources for cell and tissue research have been introduced. At least three different NAPP sources have been developed for biomedical applications [11; 12; 13; 14]. Direct plasma sources--primarily dielectric barrier discharge (DBD) technology--use the human body as a counter electrode [6; 15]. Indirect plasma sources generate plasma between two electrodes within a delivery device and the active components created by the plasma are transported to the target area [16; 17; 18; 19; 20; 21]. These indirect delivery devices can vary from thin plasma needles to plasma jets and large torches, with different arrangements of electrodes. A recent approach uses the so-called hybrid plasma source, which combines the benefits of both of the above techniques and enables large and flexible plasma designs. Thus far, the plasma sources used for medical purposes have been based on both direct and indirect approaches. Future trends seem to be shifting toward hybrid plasmas, as these enable large and flexible designs. In the early stage of floating electrode DBD (FE-DBD) plasma generation, a high voltage (10-30 kV) with a low frequency (200 Hz-1 kHz) is applied to an electrode protected by a 1-mm-thick quartz plate as a dielectric barrier [2; 19]. Technical (operating voltage) parameters for DBD sources developed at the Leibniz Institute for Plasma Science and Technology (INP Greifswald) are 10 kV at 20 kHz for surface DBD in ambient air [7]. High-voltage (10-30 kV) non-thermal plasma discharge generation in ambient air to treat living animals and humans requires high-level safety precautions. Safety and guaranteed non-injurious regimens are essential in plasma medicine applications. Our laboratory developed two types of FE-DBD devices [18]. One uses a 1-mm-thick quartz plate and the other uses a copper wire coated with 1-mm-thick foam rubber for insulation against high voltage. The applied voltage is reduced to the range of 2-3 kV at a higher frequency of 40-60 kHz using a direct current to alternative current (DC-AC) inverter. Further development of the FE-DBD involves a reduced operating voltage. The other parameter involved in voltage reduction is dielectric barrier thickness. A thin film of polyimide can reduce thickness by replacing the solid dielectric plate, which is fragile. In this study of hybrid-DBD plasma, the operating voltage was reduced to 1 kV at a frequency of 50 kHz using a flexible 125-\(\upmu\)m-thick polyimide film. The lower voltage enables development of a miniaturized wearable power supply with a DC-AC inverter. For the last two decades, the focus of biomedical plasma research has been on plasma jets and DBD devices. Plasma jets have been developed in pencil structures, while DBD devices comprising a finite surface area have been produced in plane structures [3; 16; 17; 18; 19]. Both devices are utilized for direct skin treatments by physicians and other healthcare professionals; however, these devices require prolonged treatment. For example, in the case of atopic eczema, itching decreases after a 4 h plasma treatment, or after applications of several minutes per day for 30 days [3]. Long-term plasma treatment is required for wound healing and treatment of skin diseases. It is also important to note that these devices can only cover a limited area in a single treatment. However, because they are not limited by treatment time and the size of the treatment area, DBDs integrated in an adhesive device for application on human extremities have been proposed [20; 21; 22].