Purpose: This study investigated whether double-application influences the bond strength of single-step self-etch adhesives and the mechanical properties of the resin-dentin area. Materials and Methods: Three single-step selfetch adhesives (EXL-683, experimental, 3M ESPE; Clearfil Tri-S Bond, Kuraray Medical; G BOND, GC) were applied on dentin surfaces by using the single-application or double-application method. The manufacturers' instructions stated use of the single-application method. Resin composite (Clearfil AP-X, shade A3, Kuraray Medical) was then placed and light cured for 40 seconds. The microshear bond test was carried out and the bond strength data were analyzed using one-way ANOVA with post hoc. The hardness (H) of the bonding layer, resin-dentin interface and dentin beneath the interface at depths of 10 μm and 100 μm were measured with a nanoindentation device (ENT-1100, Elionix). The H data were analyzed using two-way ANOVA, one-way ANOVA and the independent t-test at a significance level of α=0.05. Results: The mean bond strength with singleapplications of EXL-683, Clearfil Tri-S Bond and G BOND were 39.1, 36.9 and 30.0 MPa, respectively, while with the double-application, they were 46.9, 40.2 and 32.2 MPa. Double-application for EXL-683 to dentin showed significantly higher bond strength than single-application α<0.05. However, there was no significant difference between the single-and double-application methods for the other two adhesives a>0.05. The H of the bonding layer was significantly influenced by both the adhesive material and application method α<0.001; the H of the bonding layer with a double-application was significantly improved compared to a single-application. The H of the resin-dentin interface was significantly affected by the application method α<0.001, but not by the adhesive material (α>0.05). Neither the H value of dentin at 10 μm nor a depth of 100 μm was affected by the adhesive material or application method (α<0.05). Conclusions: The double application of EXL-683 did improve the bond strength to dentin. The hardness of the bonding layer and resin-dentin interface was significantly affected by the double application for each material used.
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