## 1 Introduction  
Worldwide, tumors of the oral cavity account for approximately 2% of all malignant tumors [1]. According to the German Centre for Cancer Registry Data (ZfKD) of the Robert Koch Institute, for example, 14,310 new patients with tumors of the oral cavity and throat were detected in Germany in 2018 [2], and the global incidence of lip and oral cavity cancer in 2020 accounted for 377,713 cases [1]. Radiation therapy is one important therapeutic principal for tumors of the head and neck region, and has been applied regularly [3; 4; 5; 6]. High-energy radiation from X-rays, gamma rays, neutrons, protons and other sources is used to eliminate cancer cells and to shrink tumors [7]. The ionizing radiation causes damage to the DNA of the tumor cell, both directly, through the radiation itself, and indirectly via free radicals, which are released due to the interaction of radiation with water. Having suffered irreversible damage to the DNA, the cell loses its ability to maintain cell division. If all cells of a tumor lose their proliferative potential, the tumor can be eradicated completely [8]. Despite its anticancer effects, irradiation has some injurious impacts on healthy tissue located in the radiation field [5; 6]. This results in clinical consequences such as mucositis, hyposlavation, osteoradionecrosis and radiation caries [5; 6; 9; 10; 11]. Apart from these, previous studies have shown a direct influence on the mechanical properties of the dental tissue, inter alia, the alteration of the collagenous structure [12; 13; 14]. Collagenous fibers are essential for the adhesive bond between the dentin surface and adhesive material for mounting restorative resin materials [15; 16; 17]. Restorative dental treatment is more often required in patients who have undergone radiotherapy in the head and neck area, since the risk of developing radiation caries is increased throughout life and not only during or immediately after treatment [6; 18]. Adhesively bonded composite restorations are a well-established treatment option to restore caries-affected teeth [19; 20]. Therefore, some studies recommend treatment with adhesively bonded composite fillings in these cases [21; 22]. The two approaches to adhesive systems, etch-and-rinse and the self-etch technique, establish bonding to dentin in different ways and use these collagen fibers in different manners. While the etch-and-rinse technique completely removes the smear layer through the prior use of an acid etchant and thus exposes and infiltrates the collagen fibers, the self-etch technique only demineralizes the surface of the smear layer, leaving residual hydroxyapatite crystals still attached to the collagen fibers, which are then incorporated into the bonding layer [23]. One of the recent inventions in adhesive dentistry is universal adhesives. These adhesives, also called \"multi-mode\" or \"multi-purpose\", combine the application methods of the classic etch-and-rinse and self-etch adhesives, i.e., they can be used with or without prior etching of the dentin or even with selective enamel etching [24; 25]. This flexibility of the application method is achieved by functional monomers, which are the core components of universal adhesives [26; 27; 28]. Functional monomers are acidic molecules that can fulfill various functions, including the partial dissolution of the smear layer, the demineralization of hydroxyapatite, the improvement of monomer penetration into the tooth structure and the ability to form chemical bonds with tooth substrates [27; 29; 30]. The bonding performance of various universal adhesives on human dentin In etch-and-rinse mode compared to self-etch mode based on the assessment of the microdensile bond strength (uTBS) has already been investigated in numerous studies [24; 31; 32; 33]. [PERSON] et al. [33], for example, investigated how previous etching improves the dentine penetration pattern, but without affecting the uTBS.