## 4 Discussion
Low pH values of 4.5-5.5 favor the increase of fluorine ions in the tested medium, which corresponds to cariogenic states. These results are consistent with the research conducted by [PERSON] and [PERSON] published in [38] on resin-modified glass ionomers (Viremer, Fuji II LC, Ketac Nano), compomer (Dyract Flow) and composite (Heliomolar) and in [39]. At higher pH values of the solutions (6.0-7.5), the release of fluoride ions slower, which makes the Wave and Freedom materials a reservoir of these ions, \"sparing\" them when the saliva pH is physiological or elevated and cariogenic processes are not intensified. Earlier studies by [PERSON] et al. also showed that compomer material presents a higher fluoride release in acidic environment with pH 4.6 (pH-cycling) than in distilled/deionized water [40]. Similarly, other studies discovered that compomer material releases a significantly higher level of fluoride in acid demineralising solution than in neutral remineralising solution [41]. The results of our study are consistent with the observations of other researchers [42], indicating that the rate of fluoride release from compomer materials is dependent on the pH of the environment surrounding the material. This is undoubtedly related to the fact that the decrease in the pH of the solution increases the solubility of the material through hydrolytic degradation taking place in the matrix of the material, and more precisely at the interface between the matrix and the filler [43]. The amount of released fluoride from the material depends not only on the source and concentration of fluoride, but also on whether the fluoride ions can diffuse from the internal structure of the material [42]. The amount of released fluoride from the composite material (W) is significantly lower in the acidic environment of artificial saliva than from the compomer (F), which may be due the lower hydrophilicity of the organic monomers and the higher content of inorganic filler. Previous studies have shown that the amount of filler is negatively correlated with the process of water sorption [44]. A more extensive cross-linking of the polymer chains of the filler molecules in the composite works as a barrier through which less water diffusion takes place, thereby releasing less fluoride, especially in the environment of artificial saliva. A solution of deionized water, i.e., a solution devoid of ionic strength and other elements, proved to be the best medium for the composite material (W), where the cumulative amount of released fluoride was the highest (8.0 \(\pm\) 2.0 \(\mu\)g F/mm\({}^{2}\)). The release of fluoride ions from materials used in restorative dentistry also depends on the roughness of the material, the surface preparation, and the finishing procedure [45]. The roughness of the materials increases during the release of fluoride ions to the surrounding solutions, where the smallest change is observed in the case of composites containing inorganic silica, i.e., in the case of composite material (W). Compomer materials that combine the features of composites and glass ionomer materials are more coarse [46]. Therefore, the release of fluoride ions and hydrolytic degradation will take place much easier in these materials and the emission of fluoride ions will be higher [47]. The selection of the appropriate medium for in vitro fluoride release testing from materials is critical. Different calcium content in available experimental and commercial artificial saliva preparations may influence the obtained results, therefore it is important to be aware of this. The shape of the samples is also important--in this experiment, samples were cylindrical and had released fluoride ions from three surfaces. In real conditions in the oral cavity, these materials can release only from one surface and it is possible that amount of fluoride ions will be much lower.