## 4 Discussion
It should also be mentioned that, as described in the brochure provided by the manufacturer, all etching techniques: ER, SE and SEE can be utilized with Prime & Bond One Select. The composition of this adhesive and of Xeno V (Table 1) is comparable, however, which makes Prime & Bond One Select rather a SE product with broadened indications for use. The present study showed that Prime & Bond One Select produced statistically higher SBS than Xeno V in the control group, whereas the modification of application mode resulted in comparable bonding performance of both adhesives. In the case of Xeno V, multiplying the number of applications led to an increase of bond strength, but the differences were not statistically significant (2A20, 3A20). These results are in line with other studies, which concluded that multiple applications of all-in-one adhesives did not significantly affect their bond strength to dentin [50]. The main factor responsible for the increase of SBS to dentin of Xeno V in this study was prolonged application time. Mean bond strength results in 2A40 and 3A60 groups were significantly higher than control, but not statistically different from each other. The adhesive layer was also the thickest in these groups for Xeno V, although the differences were not statistically significant. Other studies confirm that prolonging the application time of all-in-one adhesives contributes to an increase of bond strength [26; 31]. Thicker hybrid layer of higher conversion degree and better quality is obtained, which allows higher SBS results [26; 31]. When Xeno V was compared to Adper Easy One, another SE adhesive used in this study, it obtained lower SBS results in all respective study groups. The analysis of the composition and the pH of both adhesives may provide an explanation. Adper Easy One contains nanofiller which, as explained above, contributes to enhancing the thickness and quality of the adhesive layer. In this study, Adper Easy One obtained almost double the results of Xeno V in terms of adhesive layer thickness (13.31 \(\pm\) 0.43 vs. 7.40 \(\pm\) 0.82 \(\upmu\)m) in control group. In other groups the differences were not statistically significant. Also, Vitrebond(tm) Copolymer and MHP (methacrylohexyl phosphate) functional monomer are present in the composition of Adper Easy One. These chemical compounds are reported to bond chemically with hydroxyapatite by the formation of a complex with calcium ions [43], therefore enhancing SBS results. Moreover, Xeno V is a moderate adhesive in terms of acidity and Adper Easy One is a mild one. As mentioned above, mild adhesives generally show higher dentin bond strength than the ones of lower pH [25]. In the case of Adper Easy One, similarly to Xeno V, the main factor responsible for improving bond strength results was prolonged application time. SBS results in 2A20 and 3A20 groups were higher than the control, but the differences were not statistically significant. Multiplying the number of applications within a prolonged time resulted in the enhancement of bond strength (from 6.10 \(\pm\) 2.39 MPa in control group to 16.86 \(\pm\) 2.29 MPa in 3A60 group). It can be concluded that when an additional amount of MHP was delivered with subsequent applications, the etching ability increased, more complete solvent evaporation was enabled, and an adhesive layer of better quality was obtained. Previous studies confirmed, that the quality of the adhesive layer is of paramount importance for dentin bond strength [26]. In terms of adhesive layer thickness results, Xeno V presented significantly higher, almost double mean thickness values after triple application in prolonged time of 60 s when compared to control. Most of the tested adhesives, Adper Easy One, Single Bond Universal, and Prime&Bond One Select, obtained lower thickness results in 2A20 group than in control or other study groups.