## 2 Materials and Methods
### Scanning Electron Microscopy Observations  
Representative adhesive treated enamel surfaces and resin/enamel interfaces were observed using scanning electron microscopy (SEM; ERA-8800 FE; Elionix, Tokyo, Japan). The enamel surfaces were treated, following the manufacturers' instructions, and rinsed with acetone and water. Additionally, the prepared specimen with #320-grit SiC paper, with and without 35% phosphoric acid pre-etching for 15 s, but not rinsed with acetone, were evaluated (baseline) to compare the morphological changes after adhesive treatment. Bonded specimens stored in 37 \({}^{\circ}\)C distilled water for 24 h were embedded in the epoxy resin (Epon 812) and longitudinally sectioned with a precision sectioning saw. The sectioned surfaces of the resin/enamel interfaces were polished to a high gloss using abrasive disks (Fuji Star Type DDC) followed by diamond pastes (DP-Paste; Struers, \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{Etching-Mode} & Pre-Etching Protocol \\ \hline \multicolumn{2}{c}{SE (self-etch)} & \multicolumn{2}{c}{Phosphoric acid pre-etching was not performed.} \\ \hline \multicolumn{2}{c}{ER (etch-and-rinse)} & \multicolumn{2}{c}{Enamel surfaces were phosphoric acid etched for 15 s. Etched surface was rinsed with water for 15 s and air-dried.} \\ \hline Adhesive & Group & Adhesive Application Protocol \\ \hline \multirow{6}{*}{CS} & \multirow{3}{*}{I} & Primer was applied to air-dried enamel surface for 20 s followed by medium air pressure for \\  & & 5 s. Bonding agent was then applied to the primed surface and a strong stream of air was \\  & & & applied over the bonding agent for 5 s. Light irradiated for 10 s. \\ \cline{2-3}  & \multirow{3}{*}{II} & Primer was applied to air-dried enamel surface for 20 s followed by medium air pressure for \\  & & & 5 s. Bonding agent was then applied to the primed surface and was gently air thinned for 5 s. \\  & & & Light irradiated for 10 s. \\ \cline{2-3}  & \multirow{3}{*}{III} & After bonding procedure of Group II without light irradiation, another layer of bonding agent was applied and was gently air thinned for 5 s. Bonding agent was light irradiated for 10 s. \\ \hline \multirow{6}{*}{GU} & \multirow{3}{*}{I} & Primer was applied to air-dried enamel surface for 10 s and then a strong stream of air was \\  & & & applied over the primer for 5 s. Bonding agent was then applied to the primed surface and \\  & & & was gently air thinned for 5 s. Light irradiated for 10 s. \\ \cline{2-3}  & \multirow{3}{*}{III} & After bonding procedure of Group II without light irradiation, another layer of bonding agent \\  & & & \\  & & & Medium air pressure was applied to the surface for 5 s. Bonding agent was applied to the \\  & & & primed surface with rubbing action for 15 s, and then a strong stream of air was applied over \\  & & & the bonding agent for 5 s. Light irradiated for 10 s. \\ \cline{2-3}  & \multirow{3}{*}{II} & Primer was applied to air-dried enamel surface with rubbing action for 20 s. \\  & & & Medium air pressure was applied to the surface for 5 s. Bonding agent was applied to the \\  & & & primed surface with rubbing action for 15 s and then gently air thinned for 5 s. \\ \cline{2-3}  & \multirow{3}{*}{I} & After bonding procedure of Group II without light irradiation another layer of bonding agent \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & Light irradiated for 10 s. \\ \hline \hline \end{tabular} Strength of air-blow for each protocol; Group I: strong air stream; Group II: gentle air stream (similar to each manufacturer’s instructions); Group III: gentle air stream was applied to each adhesive layer.  
\end{table} 
Table 2: Application protocols for pre-etching and two-step adhesive systems. Ballerup, Denmark) down to a particle size of 0.25 \(\upmu\)m, followed by ultrasonic cleaning for 20 min.