Sequential nano-amorphous calcium phosphate application modulates resin–dentin bonding performance and aging stability on demineralized dentin

Objectives This study evaluated the effect of sequentially applying a nano-amorphous calcium phosphate (NACP) solution within a two-step self-etch bonding procedure on resin–dentin bonding performance, comparing a fluoride-free and a fluoride-containing adhesive. Methods In total, 72 artificially demineralized and 60 sound bovine dentin samples were prepared. A fluoride-free adhesive (Clearfil SE Bond 2, SE) and a fluoride-containing adhesive (Clearfil SE Protect Bond, PB) were each combined with a 5 wt% NACP solution applied either pre (Np) or post primer (pN), or used without NACP as controls (p), yielding six experimental groups in total. Micro-tensile bond strength (µTBS; n = 6 teeth/group) was tested after 24 h (TC0) and after 10,000 thermocycles (TC10K). Two-way ANOVA, Tukey's post hoc test and Weibull analysis were used. Degree of conversion (DC) was measured by Raman spectroscopy, and failure modes were examined under SEM. Results Surface treatment and its interaction with time significantly affected µTBS (p < 0.001). At TC0, Np-PB and pN-PB showed higher µTBS than most groups, while Np-SE was the lowest. After TC10K, most between-group differences were no longer significant, except for pN-SE, which maintained significantly higher values than Np-SE. Weibull analysis showed declining α for Np-PB and pN-PB, increasing α for pN-SE, and decreased β for Np-PB. DC was largely unaffected except for increases in pN-SE and Np-SE after aging. Significance NACP solution influenced bonding in a sequence- and system-dependent manner. Post-primer application was associated with more stable long-term outcomes, supporting its potential as a proof-of-concept "bio-functional booster" approach for improving adhesion on demineralized dentin.