## 1 Introduction  
The importance of fluoride in the prevention of dental caries has been the subject of clinical trials for many years. The mechanism of the action of the fluoride in this context is based on inhibiting the demineralization of hard dental tissues and supporting their remineralization [1; 2]. Fluoride exerts a significant influence on plaque bacteria; it inhibits the activity of the metabolic enzyme enolase and thereby decreases the amount of acid production. It also impairs glucose transport into their cells, by which it obstructs the formation of intracellular polysaccharide storage. All of these actions result in growth inhibition of the cariogenic micro-organisms [3]. Maintaining an appropriate level of fluoride in the oral cavity is therefore an important element in the prevention of dental caries. Dental restorative materials are available that contain fluoride ions [4] and are able to ensure their long-term release into the oral cavity [5]. In aqueous environment, fluoride ions diffuse from the resin into the inorganic part of the tooth. There, they combine with hydroxyapatite crystals and replace their hydroxyl groups, which leads to the formation of fluoroapatite [6], a structure more resistant to caries [5; 7; 8]. Compared to hydroxyapatite, the fluoroapatite crystals possess a more stable structure, lower solubility, better thermal stability, and a greater resistance to acid [3; 9]. Their movement is a complex process based on the diffusion of fluids into and out of the material as well as the diffusion of fluoride ions into the dental tissues, acting as a carrier of fluoride. The release of fluoride ions from dental materials used in restorative dentistry depends on several factors. In terms of the material itself, the number of fluoride-containing fillers, the type of resin matrix, as well as the curing mechanism of the material, including the polymerization process, all play an important role [10]. Significant in the release of fluoride from dental materials is the adsorption of fluids mainly in the matrix of the material, which allows the liquid particles to penetrate into the stable structure of the material and release fluoride ions through controlled diffusion. The properties of the resin matrix determine the diffusion rate and the volume of water absorbed [11; 12; 13]. Compomers, represented in our study by the Freedom (F) material, differ from resin composites to which the flowable Wave (W) material belongs by the type of organic monomers and inorganic fillers [14]. The composite material (W) mainly contains the monomer UDMA (urethane dimethacrylate), which is much less hydrophilic than the monomer TEGDMA (triethylene glycol dimethacrylate) [13] that is found in the compomer (F). The composite also contains HEMA (2-hydroxyethyl methacrylate), which has hydrophilic groups, absorbing water during polymerization. Previous studies have shown that the type of monomer significantly influences the process of fluoride release [15]. Furthermore, the compomer (F) includes a small number of functional monomers with carboxylic acid groups -COOH in their structure, which is why compomer materials are also known as composite resins modified with polyacids [16; 17; 18]. The material (F) does not contain any water [19], which is why the acid groups are subject to dehydration. After polymerization, they combine considerably well with the resin matrix [20], allowing an acid-base reaction with metal ions from the glass filler (strontium-fluoride glass). Compomer (F) differs from composite (W) yet another important detail, namely the inorganic component in the form of silanated reactive fillers FAS (fluoro-alumino-silicate fillers) [21], which, in addition to improving the mechanical properties, are also able to release fluoride ions. In addition, since the compomer lacks water, it is just these silanated FAS fillers that facilitate water absorption in the oral cavity by increasing the material's absorbability.