## 4 Discussion  
A positive correlation between prolonged application time and higher resin-dentin bond strength values of simplified adhesives has been demonstrated [31; 32]. This could be attributed to both higher penetration of monomers into decalcified dentin and higher water and solvent evaporation before light curing. The conversion degree and mechanical properties of the adhesive polymer are compromised if solvent content within the adhesive solution is high [33; 34]. When water is present during the conversion from monomer to polymer, the formation of a cross-link polymer within the collagen fiber network is prevented [35; 36; 37]. Therefore, if the application procedure is prolonged, a thicker hybrid layer of higher conversion degree and better quality can be obtained [26; 31; 38]. Multiple application of SE and MM adhesives without light-uring each layer causes a minimal increase in the thickness of the adhesive layer (a physical factor), but it may have effects of chemical nature [8]. During the first application, the adhesive etches dentin, but is likely to be quickly buffered. Providing more acidic monomers during the next application may increase the ability of the adhesive to etch dentin. High-pressure or prolonged air-drying enhances solvent evaporation between successive applications of the adhesive. Therefore, the concentration of acidic monomers increases. This may contribute to a more thorough infiltration of the collagen fibers and more uniform polymerization of the adhesive [27]. Thus, without increasing the thickness, the adhesive layer of higher quality is obtained, which ensures a stronger bond between the adhesive and dentin [27]. The primary reasons for failure of composite restorations are secondary caries, marginal defects and restoration fracture [39]. Therefore, achieving a stable and durable bonding interface plays a significant role in terms of dental composite performance. There is abundant literature confirming the advantage of the etch-and-rinse over the self-etch approach when using enamel [40]. However, when dentin is regarded, the bonding strategy depends on the pH of the adhesive and its composition. Simplified adhesives are classified according to their acidity as strong (pH \(\leq\) 1), moderate (1 \(<\) pH \(<\) 2) or mild (pH = 2) [40]. [PERSON] et al. [41] added the fourth group to this classification, the so-called ultra-mild adhesives (pH \(\geq\) 2.5). Low-pH adhesives present decreased dentin bond strength values when compared to mild ones [25]. The etching pattern they produce is similar to that of 37% phosphoric acid, but the dissolved calcium phosphates are not rinsed away and become embedded in hybrid layer. Since calcium phosphates are unstable in aqueous environments and considerably weaken the interfacial integrity, the use of strong SE adhesives is not recommended [3]. Self-etch or universal, mild adhesives are proven to achieve best dentin bond strength results when phosphoric acid etching is not involved [42]. When these adhesives are applied directly on unetched dentin, the tissue is partially demineralized. A substantial amount of hydroxyapatite crystals is left around the collagen fibrils. The micromechanical interaction takes place due to in situ polymerization of the monomers that infiltrate into the collagen network. Moreover, some of the universal adhesives interact chemically with hydroxyapatite. They contain functional monomers in their composition, such as MDP. These monomers interact chemically with calcium in the residual hydroxyapatite through ionic bonding,creating a non-soluble stable nano-layer [18; 24]. It contributes to increasing mechanical strength and preventing the degradation over time of the bond interface. When bonding to dentin is regarded, SE and universal adhesives still cannot compete with the excellent performance of conventional three step etch-and-rinse adhesives. In this study the efficacy of several methods to enhance their dentin bonding performance was evaluated, as well as their impact on the adhesive layer thickness.