This study examined the effects of fluoride application by low temperature atmospheric plasma on the deciduous tooth enamel. More fluoride was detected and was maintained for a longer time when the tooth enamel specimens were given the combination treatment of plasma and APF gel than the APF gel only. This means that the plasma treatment accelerates the combination of fluoride with the enamel surface.
Many types of fluoride are used by professionals in dental clinics. The APF gel, NaF solution and fluoride varnish are used mostly. In this preliminary experiment, when permanent tooth is treated with a NaF solution, varnish or APF gel, the APF gel was found to produce the most effective fluoride application; the others rarely showed an application effect. These results are supported by another study showing that the APF gel with a laser was more effective in preventing enamel erosion than a varnish with a laser . Magalhães et al.  reported that the resistance to dentin erosion was increased using a laser in combination with an APF gel than varnish. Therefore, the APF gel was used in this study. In addition, the pH of the APF gel was slightly lower than neutrality. The acid fluoride decreases the mineral loss by producing more calcium fluoride  and increases the insolubility of calcium fluoride, which suppresses demineralization and increases remineralization .
Laser irradiation alone or combined with APF decreased lesion progression in primary enamel. Many studies have focused on the application of fluoride in regard to lasers, which can be compared with plasma. Hicks et al.  reported that a treatment of deciduous teeth with APF gel and argon laser would provide a protective layer to dental caries. On the other hand, Azevedo et al.  suggested that a treatment with a Nd:YAG laser and fluoride simultaneously to the primary tooth enamel did not result in the same degree of synergy to prevent the demineralization of enamel compared to the treatment with fluoride only. Taqliaferro et al.  reported that lesion progression on deciduous tooth is reduced when processed with a carbon dioxide laser alone or combined with APF gel. However, they demonstrated that a combined treatment would not have any additional significant effect. Similarly, the effect differs according to the fluoride types, laser intensity, experimental methods, and substrates. Moreover, a high-intensity laser may cause thermal damage  and tissue ablation in the adjacent tissues . Therefore, it is inadequate as a complementary unit for applying fluoride. In addition, a laser has linearity; thus, it has some limitations when applied to the oral tissues with an irregular structure. In contrast, plasma is a form of low temperature ionized gas. Therefore, it has the advantage as a high degree of accessibility to complex oral tissues. Moreover, it causes no damage to the tissues.
EPMA is normally utilized in the qualitative and quantitative analysis of materials in metallography and geology. Recently, the utility of EPMA is being proved in the biological field. EPMA utilizes an energy dispersive spectrometer (EDS) and wavelength dispersive spectrometer (WDS) depending on the detection method of X-ray. With an EDS, it is difficult to determine the elements accurately when the peaks overlap. Furthermore, it makes it difficult to conduct accurate measurements when the content of the element in a specimen is small. In contrast, WDS can analyze even small amounts of an element.
Low frequency plasma and microwave plasma used in this experiment were effective in improving the fluoride uptake and its retentivity of enamel surface. When applying fluoride twice with subsequent the application of plasma after each fluoride application, the amount of fluoride analyzed in the enamel was similar to or greater than the amount of fluoride retained when applying fluoride 4 times in the absence of plasma. The retentivity lasted only for approximately 1 to 2 weeks when fluoride was applied 4 times in the absence of plasma. In contrast, fluoride was detected for up to 4 weeks from the group that had been given both fluoride and plasma. This means that the caries suppression effect would last longer for a deciduous tooth that is vulnerable to dental caries when the fluoride remains on tooth surface for a longer period. Fluoridation of enamel is generally considered to be a means of maintaining a fluoride ‘reservoir’, rather than being a benefit in itself. The effect of such fluoridation can be enhanced by applying the plasma and fluoride together. Despite this being an in vitro experiment, the application of fluoride using plasma would be quite effective if the aforementioned patterns last clinically.
In the process of fluoride combining to tooth enamel, calcium fluoride is formed and turned into fluorapatite. The mechanism of fluoride combining to apatite structure of tooth involves four phases: adsorption, exchange, recrystallization and crystal growth. In the adsorption phase, fluoride is easily detached from the tooth surface because hydroxyapatite of tooth does not expose the binding sites to fluoride.29 However, the high energy of electron generated from plasma can sufficiently break down the chemical bondage of hydroxyapatite. Therefore, plasma treatment could expose numerous binding sites of hydroxyapatite at Ca2 + and Ca10(PO4)6- to fluoride. So fluoride application can produce a large amount of calcium fluoride and fluorapatite when hydroxyapatite of tooth is treated with plasma. In our previous study, as for the resistance to demineralization, the calcium-to-phosphate ratio of the enamel treated with APF and nonthermal atmospheric plasma was higher than that treated with APF alone, and it increased with the frequency of treatment . So the combination treatment of nonthermal atmospheric plasma and fluoride improves retention of fluoride on the enamel and resistance to demineralization when compared with treatment with fluoride alone.
In this experiment, more fluoride was detected in the treatment using a microwave argon plasma than the low frequency helium plasma. This is because of the difference in frequency and gas. This supports the result of previous research in that argon gas would create a higher current density and more active species than helium gas . In addition, microwave plasma has a high frequency, thereby creating more energetic electrons than what a low frequency plasma would create . The hydroxyl radical (OH) is an active oxygen species, which alters the surface characteristics by being integrated with the tooth surface.33 In addition, it is believed that the affinity to fluoride is improved because the OH radicals formed in the plasma modify the structure of hydroxyapatite forming the tooth surface. These changes in the surface characteristics might influence the fluoride application effect. Plasma has an oral microbial killing effect . Therefore, it would be possible to prevent cavities in a more complex manner by making oral bacteria inactive through the application of fluoride. Overall, it will be necessary to analyze accurately the mechanism regarding to the role of plasma in the application of fluoride.