## 4 Discussion
The methods included increasing the number of applications and extending the application time. In the present study each modification of application procedure of Single Bond Universal contributed to an increase of SBS to dentin. The difference was not statistically significant in case of sole increased number of applications (2A20 and 3A20). However, in 2A40 and 3A60 groups, mean SBS results were significantly higher and almost doubled when compared to the control. It can be concluded that the chemical action of MDP monomer provided with subsequent applications in 2A20 and 3A20 groups could not fully develop in 20 s. Only after the application procedure was multiplicated within the prolonged time, did it allow Single Bond Universal to achieve higher SBS values. The considerable improvement in bond durability is attributed to an additional chemical reaction between MDP and dentin [3]. According to the adhesion-decalcification (AD) concept, the intensity and stability of molecular adhesion of calcium salts of acidic monomers to HAp is inversely proportional to their solubility [43]. The MDP possesses relatively strong decalcification ability, which enables it to form stable monomer-calcium salts, manifested as 'nano-layering' [18]. A more water-stable interface is produced, therefore contributing to bond durability. The monomer consists of a methylmethacrylate group, which allows for light-polymerization, and a phosphoric-acid group, enabling ionic interaction with calcium of HAp. Both groups are separated by a 10-fold carbon chain, which is responsible for wetting, solubility, flexibility, and the hydrophobicity-hydrophilicity balance of the monomer [44]. All of the abovementioned features of MDP may also be the reason why Single Bond Universal had the highest mean SBS compared to all other adhesives used in this study. Especially in 2A40 and 3A60 groups, where additional portions of MDP were supplied and sufficient time was ensured, the results of SBS to dentin were the highest. In the case of Prime & Bond One Select, another universal adhesive used in this study, no study group obtained significantly higher results than the control. Mean SBS values were significantly lower than those of Single Bond Universal in each respective group. This was probably due to several mechanisms operating simultaneously. One factor is the adhesive layer thickness. Previous studies showed that increased adhesive layer thickness enhanced its elasticity [45]. Resin deformation suppressed polymerization, mastication or temperature-caused stress [46]. In this study it was measured that Prime & Bond One Select obtained thinner adhesive layer than Single Bond Universal in each respective group. Prime & Bond One Select applied in separately light-cured layers was shown to present both thicker adhesive layer and improved SBS [47]. Another factor contributing to the difference in bond strength between both universal adhesives is their acidity. The pH of Single Bond Universal is higher than of Prime&Bond One Select (2.7 vs. 1.6, respectively), which makes them an ultra-mild and a moderate adhesive. As mentioned above, previous studies proved low-pH adhesives, such as Prime & Bond One Select in this study, present lower bond strength than mild ones [25]. Another factor contributing to the higher SBS of Single Bond Universal in this study is the presence of inorganic nanofiller in its composition. Prime & Bond One Select, on the other hand, is an unfilled adhesive. Studies confirmed, that adhesives created reinforced, thicker hybrid layers, due to increased viscosity, when filler was included in their composition [48]. \"Inverse\" functionalized phosphoric acid esters are components of Prime & Bond One Select. As reported by the manufacturer, they are responsible for interacting chemically with dentin [49]. However, insufficient research is available to support the actual occurrence of this phenomenon.