## 3 Results
### SEM Observations  
Representative SEM images of treated and baseline enamel surfaces are shown in Figure 6. The baseline specimen ground with SiC papers exhibited scratch marks from the carbide polishing paper; additionally, the smear layer along with some enamel fragments was observed (Figure 6A). In the baseline specimen treated with phosphoric acid, the smear layer was completely removed, and a typical etching pattern was observed (Figure 6B). All the adhesive systems had a similar morphological appearance in the SE mode; most of the smear layer was dissolved, and a shallow etching pattern was observed. Particularly, the enamel rods were more clearly observed in OX than in the other adhesive systems (Figure 6G). Conversely, in the ER mode, although the adhesive systems showed similar morphological appearances to those of the baseline specimen treated with phosphoric acid, the spicular etching pattern appeared to be collapsed in OX (Figure 6H). Figures 7-9 show representative resin/enamel interfaces. All the adhesive systems showed excellent adaptation between the adhesive layer and the decalcified enamel substrate in both etching modes. No clear differences were observed in the thickness of the adhesive layer in the different etching modes, regardless of the adhesive system or application method used. In Group I (Figure 7), the thickness of the adhesive layers in CS and OX were similar (approximately 10 \(\upmu\)m), but GU presented with twice the thickness of the other adhesive systems. In Groups II and III (Figures 8 and 9), the thickness of the adhesive layers in CS and GU were similar and higher than that in OX. In the ER mode, the smear layer was completely dissolved, and adhesive interpenetration with enamel as resin tags was clearly observed, regardless of the adhesive system or application method used. On the contrary, enamel resin tags were not clearly seen in the SE mode, regardless of the adhesive system or application used. Part of the smear layer remained on the treated surface and created a hybrid layer (Figures 7C, 8C and 9A,C,E), which was infiltrated by the resin monomers in some instances. The ultrastructure of the adhesive layer was dependent on the adhesive system used. Unlike CS and GU, somewhat larger irregular fillers (approximately 0.1-0.5 \(\upmu\)m in size) and nano-sized fillers were observed in OX. The thickness of the adhesive layer differed with the different adhesive systems and application methods used. Figure 5: Thickness of adhesive layer in different application methods in different adhesive systems. Figure 6: Representative SEM images of the treated enamel surface. (**A**) Ground with SiC paper #320 (\(\times\)5000). (**B**) Phosphoric acid etching for 15 s (\(\times\)5000). (**C**) CS in SE mode (\(\times\)5000). (**D**) CS in ER mode (\(\times\)5000). (**E**) GU in SE mode (\(\times\)5000). (**F**) GU in ER mode (\(\times\)5000). (**G**) OX in SE mode (\(\times\)5000). (**H**) OX in ER mode (\(\times\)5000). Figure 7: Representative SEM images of resin/enamel interfaces (Group I). **(A)** CS in SE mode (\(\times\)20,000, \(\times\)1000). **(B)** CS in ER mode (\(\times\)20,000, \(\times\)1000). **(C)** GU in SE mode (\(\times\)20,000, \(\times\)1000). **(D)** GU in ER mode (\(\times\)20,000, \(\times\)1000). **(E)** OX in SE mode (\(\times\)20,000, \(\times\)1000). **(F)** OX in ER mode (\(\times\)20,000, \(\times\)1000). AL: adhesive layer, EN: enamel, RC: resin composite. The yellow arrows indicate the adhesive layer. Figure 8: Representative SEM images of resin/enamel interfaces (Group II). (**A**) CS in SE mode (\(\times\)20,000, \(\times\)1000). (**B**) CS in ER mode (\(\times\)20,000, \(\times\)1000). (**C**) GU in SE mode (\(\times\)20,000, \(\times\)1000). (**D**) GU in ER mode (\(\times\)20,000, \(\times\)1000). (**E**) OX in SE mode (\(\times\)20,000, \(\times\)1000). (**F**) OX in ER mode (\(\times\)20,000, \(\times\)1000). AL: adhesive layer, EN: enamel, RC: resin composite. The yellow arrows indicate the adhesive layer.