## 1 Introduction
Upon contact of the material with the oral environment, dehydrated acid monomers located on the periphery of the material release protons attacking the surfaces of the FAS fillers, which in turn leads to the release of calcium, aluminum, and fluoride ions, which do not participate in the polymerization process of the material [22]. It is the ion release mechanism through water absorption after the material's polymerization--that is the main difference between composites and compomers [22]. The polymerization causes a binding process similar to the mechanism observed in composite resins; however, the subsequent watersorption leads to the ionization of acidic groups and an acid-base reaction, which releases fluoride in a similar way to glass ionomers [23]. The results of our study showed that the amounts of released fluoride from the compomer material (F) were much higher than from the composite material (W) in most of the environments in which the tested samples were placed. The matrix of the composite resin is much less hydrophilic and its incorporated fluoride ions are released only in small amounts [24], which can also be seen in the test results of this study. [PERSON] et al. observed that the fluoridated resin in the composite released the fluoride in small amounts, about 10 times less than the compomers during the first day of the study [25]. This was also confirmed by the results of other studies, in which the fluoridated resin released less fluoride ions into artificial saliva solution than a compomer [26; 27]. The amount and rate of fluoride release from the material also depends on the pH of the solution in which the material is placed [28]. In the oral cavity, the pH of the plaque can reach the critical value of 4-4.5. A pH with a value of 4-5.5 is considered as variogenic, 5.5-6 is potentially cariogenic, whereas a pH > 6 is safe [29]. The preliminary character of this study is manifested with the short-term evaluation of fluoride release and only in vitro character. The next step will be the study of long term release and preparing materials in a real tooth. The main goal of this study was to determine the fluoride concentrations at the surfaces of selected dental materials--composite and compomer (Wave and Freedom, respectively)--and to determine the level and dynamic of release of the fluoride from these materials in different storage media--denionized water, 0.9% NaCl solution, and artificial saliva (AS). Variable pH spectrum of AS (from 4.5 to 7.5), prepared accordingly to prof. [PERSON] recipe [30; 31; 32; 33], is purposed to mimic different environments of human oral cavity. Changes in the pH of human saliva, a natural physiological process, have a significant impact on the level of release of fluoride ions from the surface of dental materials. Previous experiments showed that in artificial saliva with a pH > 7, the precipitation of calcium compounds leads to the formation of turbidities. After the calcium combines with fluoride, the availability of fluoride ions is reduced. It therefore seems reasonable to evaluate the release of fluoride ions into the calcium-free artificial saliva solution. Therefore, the tests were carried out with saliva resembling solution with different pH and addition of calcium salt as well as with deionized water and saline solution. Beside the fluoride release the aim of this study is also to assess the morphological changes at the material surfaces using scanning electron microscope (SEM) and Fourier-transform infrared spectroscopy (FTIR). It is expected that the highest fluoride concentrations will be for compomer material in acidic environment. Moreover, the structure and composition of materials should not be affected after short-term release while the surface should show some changes and degradation.