Aggressiveness of Self-etch Adhesives Affects Photopolymerization Efficacy and Interfacial Chemistry

Objectives: To investigate effect of static vs. dynamic application of self-etch adhesives with different aggressiveness on the polymerization efficacy and interfacial chemistry, and to understand the role of chemical interaction/reaction between self-etch adhesives and dentin in the above processes. Methods: Two self-etch adhesives Adper Easy Bond (AEB) and Adper Prompt L-Pop (APLP) were used. Each adhesive was applied onto the prepared dentin surface with (15s, dynamic application) and without (15s, static application) agitation, then light-cured for 10 s. Micro-Raman spectra and imaging were acquired from the adhesive/dentin interface to the edge of the adhesive layer at 1~micron spatial resolution. The degree of conversion (DC) of the adhesives as a function of position was calculated. SEM-EDS was used to provide the elemental distribution along the interface and adhesive layer. Results: Polymerization efficacy of the two self-etch adhesives was apparently different, depending on the application method. APLP exhibited decreasing DCs as the distance from the a/d interface became greater for both the static and dynamic applications. The DCs of APLP were to a great extent determined by the application method, with the DCs of 82.6-87.7% for the dynamic mode and 70.7-86.4% for the static mode. As for AEB, the DCs remained almost constant across the adhesive layer, and showed no significant difference between two application methods. Raman spectral analysis disclosed that the chemical interaction between the mineral apatite and adhesives was responsible for the observations. This was also verified by SEM-EDS results by tracking the distribution of elements such as P and Ca in the adhesive layer. Conclusion: The application method of self-etch adhesives on dentin might influence the polymerization efficacy depending on adhesive aggressiveness. The mineral apatite in dentin and its interaction with the acidic functional monomers play a significant role in photopolymerization.