Effect of self-etching primer containing N-acryloyl aspartic acid on enamel adhesion

Objectives: A previous study demonstrated that N-acryloyl aspartic acid (N-AAsp) aqueous solution is useful as a self-etching primer for dentin adhesion. The purpose of this study was to evaluate whether N-AAsp solution can be used for enamel etching, when compared with phosphoric acid. Methods: N-AAsp was synthesized by the reaction of DL-aspartic acid and acryloyl chloride. The experimental self-etching primers were prepared by dissolving N-AAsp in water at various concentrations up to 20 wt%. The tensile bond strength of resin to bovine enamel was measured after the enamel was treated with the N-AAsp primers. The 40 wt% H₃PO₄ was used for comparison. The treated enamel surfaces and the resin-enamel interfaces were also observed with a scanning electron microscope. Results: The tensile bond strength to enamel increased linearly with the increasing N-AAsp concentration and reached the same value as that to enamel etched with 40 wt% H₃PO₄ when N-AAsp concentration was 20 wt%, even though the etching ability of 20 wt% N-AAsp was inferior to 40 wt% H₃PO₄. Regardless of the concentration of N-AAsp, the bond strength to enamel was the same as that to dentin reported in our previous study. Significance: The 20 wt% N-AAsp aqueous solution showed enamel bonding equivalent to 40 wt% H₃PO₄ in addition to the advantage of eliminating the rinse step with water. Based on the results obtained in this study along with those obtained in a previous study, it was concluded that 20 wt% N-AAsp aqueous solution is a promising self-etching primer to simultaneously treat enamel and dentin.