## 2 Materials and Methods
### Adhesive Application Protocol and Thermal Cycling  
As part of the design process for this study, we assumed that variations in clinical bonding procedures might result in the creation of different adhesive layer thicknesses. Such variation may occur in clinical situations due to misunderstanding of bonding procedures, cavity configuration, or errors in handling. We therefore designed protocols to generate adhesive layers of different thicknesses. Table 2 shows the adhesive application protocols for the adhesives. Fifteen specimens were prepared to determine the shear bond strength (SBS) to enamel in the self-etch mode (SE mode, without phosphoric acid pre-etching) and etch-and-rinse mode (ER mode, phosphoric acid pre-etching for 15 s). The bonded area was standardized as the center of the labial surface. For the ER mode, 35% phosphoric acid was applied to the enamel surface for 15 s, before applying the primer, and rinsed with water spray from a dental syringe for 15 s. For both etching modes, the primers were applied to the adherent surface following the manufacturers' instructions. The 15 specimens were divided into three groups on the basis of the application method as follows:  
Group I: a strong stream of air was applied over the bonding agent for 5 s;  
Group II: a gentle stream of air was applied over the bonding agent for 5 s (the strength of the air blow was similar to that recommended by each manufacturers' instructions); \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Code**} & **Adhesive System** & **Main Components** & **pH** & **Manufacturer** \\  & **(Lot. No.)** & **Main Components** & **(Primer)** & **(Primer)** & **(R)** \\ \hline \multirow{3}{*}{CS} & Clearfil SE Bond 2 & Primer: MDP, HEMA, water, initiators & & \\  & (Primer: 5852494) & Adhesive: MDP, HEMA, bis-GMA, & & & Kuraray Noritake \\  & (Adhesive: 5847004) & initiators, microfiller & & & Dental, Tokyo, Japan \\ \hline \multirow{3}{*}{GU} & \multirow{3}{*}{G2-Bond Universal} & Primer: 4-MET, MDP, MDTP, & & \\  & G2-Bond Universal & dimethacrylate monomer, acetone, & & & \\  & (Primer: 190711) & water, photointiator, filler & & & \\  & (Adhesive: 190711) & Adhesive: dimethacrylate monomer, & & & Japan \\  & & bis-GMA, filler, photointiator & & & \\ \hline \multirow{3}{*}{OX} & OptiBond eXTRa & Primer: GPDM, HEMA, acetone, & & & \\  & Universal & ethyl alcohol & & & \\  & (Primer: 58470004) & Adhesive: GPDM, HEMA, glycerol & & & \\  & (Adhesive: 5852494) & dimethacrylate, ethyl alcohol, sodium & & & \\ \hline \multirow{3}{*}{} & Resin Composite & Main Components & Manufacturer & \\ \hline \multirow{3}{*}{Clearfil AP-X} & bis-GMA, TEGDMA, silane barium glass filler, silane silica filler, & & & Kuraray Noritake \\  & Clearfil AP-X & silanated colloidal silica, CQ, pigments, & & Dental \\ \cline{1-1}  & (N416713) & others & & & \\ \hline \hline MDP: 10-methacryloyloxydecyl dihydrogen phosphate, HEMA: 2-hydroxyethyl methacrylate, bis-GMA: 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl] propane, 4-MET: 4-methacryloyloxyethyl trimellitate, MDTP:10-methacryloyloxydecyl dihydrogen thiophosphate, GPDM: glycerol dimethacrylate dihydrogen phosphate, TEGDMA: triethylphenylcycl dimethacrylate, CQ: dl-camphorquinone. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Materials used in this study. Group III: prepared as in Group II followed by the application of a second layer of the bonding agent and a gentle air blow for 5 s.  
The bonding agent-treated enamel surfaces were light irradiated for 10 s. The bonded specimens were created by clamping the plastic molds (height, 2.0 mm; internal diameter, 2.38 mm; Ultradent Products) in a fixture against the adherent surfaces. The resin composite was condensed into the mold and light irradiated for 20 s. All steps in the bonding procedures were conducted by a single operator over the course of the whole experiment.