Enhancing dentin bonding with catechol-based monomers by tailoring photoinitiator systems to overcome radical inhibition.

OBJECTIVE: Although catechol-functionalized polymers show promise for improving adhesion in wet environments, these catechol groups also inhibit free-radical polymerization, slowing kinetics, reducing conversion, increasing cytotoxicity, and compromising bonding performance. This study evaluates alternative photoinitiator systems to overcome catechol-induced inhibition in both etch-and-rinse and self-etch adhesives. METHODS: Catechol methacrylamide (CMA) was incorporated into a methacrylate-based adhesive resin (0 to 5 wt%) as a partial replacement for the traditionally employed monomer 2-hydroxyethyl methacrylate (HEMA). The photopolymerization behavior of CMA-modified adhesives were investigated with varying photo-initiating systems, including type II photoinitiator systems camphorquinone (CQ)/ ethyl 4-(dimethylamino)benzoate (EDAB) and CQ/EDAB/diphenyliodonium hexafluorophosphate (DPIHP); type I systems diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO) and di-p-tolyl(2,4,6-trimethylbenzoyl) phosphine oxide (TMO). Polymerization kinetics were monitored by real-time spectroscopy, while post-cured resins and bonded interfaces were evaluated by cell viability, micro-tensile bonding strength (μTBS) and interfacial structural analyses. RESULTS: Statistically significant polymerization retardation (one-way ANOVA) was observed when CMA was incorporated at low loadings (1 wt%) in resins with the type II systems CQ/EDAB and CQ/EDAB/DPIHP, whereas type I photoinitiators (TPO, TMO) maintained fast kinetics even at 5 wt% CMA loading. The enhanced kinetics associated with TPO and TMO correlated with improved cell viability. Generally, higher CMA loadings improved bonding strength and durability in both etch-and-rinse and self-etch systems, suggesting enhanced wet adhesion (two-way ANOVA). However, these improvements were not observed in self-etch type II systems, due to the significant polymerization retardation noted above. SIGNIFICANCE: Catechol-induced inhibition is mitigated by optimizing the photoinitiator system, enabling the improvement of dentin bonding without sacrificing polymerization efficiency.