## 4 Discussion  
This study aimed to determine the influence of the thickness of the adhesive layer formed by different application modes on the immediate enamel bond performance and bond durability of a new-generation two-step adhesive system and to compare it with those of conventional two-step SE adhesive systems. A major concern regarding the experimental protocol was the standardization and reproduction of an adhesive layer of similar thickness with each bonded specimen. It is practically difficult to reproduce the same thickness with a limited error range of a few micrometers, and the adhesive thickness cannot be monitored in each bonded specimen during the SBS test. The different application methods used in this study produced adhesive layers of different thicknesses within a certain range, which were clinically accepted bonding procedures. Although the CV was dependent on the method of application, significant differences in film thickness were observed among the three groups. GU showed thicker films than the other adhesive systems; additionally, a significant difference in thickness was observed between GU and the other adhesives in Group I. This can be explained by the components of the primer and bonding agent in GU. The primer contains nanofillers and functional monomers that resemble universal adhesives. On the other hand, the bonding agent in GU does not contain any functional monomers, solvent, or HEMA. These factors might influence the viscosity of the primer and bonding agent and the polymerization performance of the resin monomer. In particular, the nanofillers in the primer in GU remained on the enamel surface and increased the thickness of the adhesive layer even when a strong stream of air was applied in Group I [16]. Therefore, the adhesive layer of GU in Group I might have been thicker than those in the other adhesive systems. The bond performances of all the tested adhesive systems were dependent on the application method used. Therefore, the first null hypothesis, that the enamel bond durability of the new two-step adhesive system under thermal stress would not differ with the different application methods used, was rejected. All the adhesives in Group II showed superior immediate and durable bond performances to those in Groups I and III, regardless of the etching mode. Therefore, the optimal adhesive thickness for the effectiveness of the enamel bond in the tested two-step adhesive systems might be similar to that seen in Group II (approximately 40 to 60 \(\upmu\)m). In the 24 h group, all the adhesives showed significantly lower SBS values in Group I when compared to those in Group II in both etching modes. A thicker adhesive layer might act as an elastic buffer zone during the polymerization process of the resin composite [18; 19]. Additionally, the stress distribution at the interface might be better, owing to the increase in the size of the plastic zone and the improved elasticity [20; 21]. During bond strength testing, load stress might generate cracks and create plastic deformation zones near the ends of these cracks. Crack propagation and fracture are thought to be related to the size of the plastic deformation zone [20; 21]. Therefore, the thicker adhesive layer in Group II allows for a plastic deformation zone that is larger than that in Group I; thus, the stress distribution at the interface might be more dispersed. Furthermore, the thickness of the adhesive in the oxygen inhibition layer in Group I is much higher than those in the other groups; thus, the thinner oxygen layer might prove to be weak under load stress [22]. There are only a few studies on the effect of the thickness of the adhesive layer on the enamel bond performance, in contrast to similar work on the dentin. Moreover, the efficacy of the double-layer technique and an extra layer of hydrophobic resin on the enamel bond performances of two-step adhesive systems is still controversial. [PERSON] et al.