Effect of Collagen Cross-Linkers Applied at Different Adhesive Steps on Dentin Microtensile Bond Strength

Aim The stability of the resin–dentin interface is compromised by collagen exposure and enzymatic degradation, making dentin biomodification with collagen cross-linkers a promising strategy. This study evaluated the effects of proanthocyanidin and theaflavin applied at different adhesive stages on immediate and six-month dentin microtensile bond strength. Methods Extracted impacted human third molars were used in this in vitro study(8 experimental groups and 2 control groups, n = 6 teeth per group). Teeth were allocated into groups according to the type of collagen cross-linker and the application step.Proanthocyanidin(PA) and theaflavin(TF)-based cross-linkers were applied at a concentration of 2% for 15 s, either as a dentin pretreatment or incorporated into subsequent adhesive steps.All groups were defined as follows: U, OptiBond Universal;E + U, etching followed by OptiBond Universal;E + PA+U, etching followed by proanthocyanidin pretreatment and OptiBond Universal;E + PAU, etching followed by proanthocyanidin-incorporated OptiBond Universal;PAFL, proanthocyanidin-incorporated OptiBond FL;PA + U, proanthocyanidin pretreatment followed by OptiBond Universal;PAU, proanthocyanidin-incorporated OptiBond Universal;E + TF+U, etching followed by theaflavin pretreatment and OptiBond Universal;E + TFU, etching followed by theaflavin-incorporated OptiBond Universal;TF + U, theaflavin pretreatment followed by OptiBond Universal; and TFU, theaflavin-incorporated OptiBond Universal. Fourier transform infrared spectroscopy(FTIR) was used to evaluate potential chemical interactions following cross-linker application, while scanning electron microscopy(SEM) was performed to assess the resin–dentin interface morphology. Microtensile bond strength(µTBS) was measured immediately and after 6 months of water storage.The significance level set at p < 0.05. Results Microtensile bond strength significantly decreased after 6 months in the U,E + U, E + PA+U,E + PAU,PAFL,PA + U, and E + TFU groups(p < 0.05).No significant aging-related reduction was observed in the PAU,E + TF+U,TF + U, and TFU groups(p > 0.05).Application of proanthocyanidin and theaflavin at specific application steps contributed to better preservation of bond strength after aging.FTIR analysis showed spectral changes in the experimental adhesive liquids after incorporation of proanthocyanidin and theaflavin.SEM revealed protocol-dependent differences in resin–dentin interface morphology. Conclusions Early-stage application of proanthocyanidin and theaflavin significantly influenced immediate and 6-month dentin bond strength and durability.