# Dispersion of OH Radicals in Applications Related to Fear-Free Dentistry Using Cold Plasma
## 1 Introduction  
In 1879, the fourth state of matter was unveiled by Sir [PERSON] and subsequently in 1929 named \"plasma\" by [PERSON] [1; 2]. Moreover, considering the phase transition (e.g., gas to plasma), there was a suggestion that plasmas not be called the fourth state of matter [3]. Plasma research has evolved rapidly and expanded in various areas including environmental, military, agricultural, and biomedical research [4; 5]. Cold atmospheric plasmas (CAPs) enhance the generation of reactive species in their flow while the corresponding carrier gases remain at room temperature; this enhances the interaction of the reactive species with the target upon exposure of the latter to CAPs [6]. CAPs were also found useful in surface treatment, material processing, and plasma medicine [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. In the field of medicine/biomedicine, CAPs are found to have significant potential in wound healing [24; 25], cancer therapy [26; 27; 28], the inactivation of microorganisms [29], and tooth bleaching [21]. In particular, in the dentistry industry, CAPs were found effective against infected dental root canals and could remove microorganisms associated with infection of the root canal [30]. Various studies reported the effective killing of various types of viruses and bacteria by CAPs [31; 32; 33; 34; 35]. Considering the favorable characteristics of the CAPs, [PERSON] et al. [33; 36] were among the pioneers who explored the potential therapeutic use of CAPs in dentistry. In dentistry, CAP applications were related to tooth whitening/bleaching, sterilization of dental cavities, and root canal disinfection. In a recent review [37], the applications of CAPs in dentistry were reviewed. Interestingly, it was pointed out that CAP treatment of teeth was a new and painless method for the preparation of cavities for restoration, with a rather enhanced longevity, root canal disinfection, and bacterial inactivation. In particular, tooth whitening using CAPs were experimentally studied. For example, [PERSON] et al. [38] deployed a direct current air CAP to enhance tooth whitening. In a separate study, [PERSON] et al. [21] found that a helium CAP jet enhanced tooth bleaching. The combined use of hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) with CAPs was found to be three times more effective to enhance tooth bleaching, when compared with the case without CAP. In another study, 40 extracted human molar teeth were treated with carbamide peroxide and additionally exposed to CAP, a plasma arc lamp, and a diode laser, which revealed that exposure to CAPs achieved the most effective tooth bleaching [12]. In addition, [PERSON] et al. [39] applied CAP alongside H\({}_{2}\)O\({}_{2}\) gel to demonstrate enhanced tooth whitening in 10- and 20-min exposure groups. Interestingly, CAPs were shown to be able to treat irregular surfaces such as dental cavities without the need of mechanical drilling [1]. As CAPs operate at room temperature, they would not exert detrimental effects on the tissues [33]. As regards root canal disinfection, [PERSON] et al. [40] successfully inactivated _Enterococcus faecalis_ bacterium in the root canal using a CAP jet device. [PERSON] et al. [41] also examined the effectiveness of CAPs in the disinfection of the tooth root canal. From the progress in the use of CAPs in dentistry, it was hoped that CAPs could lead to fear-free and painless treatments in the near future. Enhancement in plasma treatment of teeth was attributed to reactive species generated in CAPs, such as hydroxyl (OH) radicals. In the previous work of [PERSON] et al. [42], teeth were exposed to saline solution with an air blow, 5% H\({}_{2}\)O\({}_{2}\) gel and saline solution, and a CAP microjet. The authors found that the CAP microjet provided the best tooth whitening, which was due to the OH radicals as revealed using electron spin resonance spectroscopy [42]. In particular, minor enamel surface morphological changes were observed and a rougher surface was formed upon a CAP exposure for 20 min [42].