Modification of a universal adhesive with CHX- or DOX-loaded TiO2 nanotubes as an anti-MMP strategy

OBJECTIVES: This study evaluated whether titanium dioxide nanotubes (TiO₂-nts), loaded with chlorhexidine (CHX) or doxycycline (DOX) and incorporated into a universal adhesive, could sustain matrix metalloproteinase (MMP) inhibition and enhance long-term bond durability without compromising polymerization or cytocompatibility. The influence of 3-aminopropyltrimethoxysilane (APTMS) surface functionalization on drug release, adhesive conversion, and mechanical performance was also investigated. METHODS: TiO₂-nts were synthesized via hydrothermal processing and either left unmodified or functionalized with APTMS. CHX or DOX was loaded into both nanotube types and incorporated into a universal adhesive (5 wt%). Nanotube morphology and functionalization were assessed using TEM, SEM/EDS, XRD, and TGA. Drug loading and release were quantified by HPLC. Degree of conversion (DC) was measured via FTIR. Microtensile bond strength (μTBS) was evaluated after 24 h and 6 months in both etch-and-rinse and self-etch modes (n = 6). MMP activity was assessed using a colorimetric assay (n = 8), and transdentinal cytotoxicity was analyzed using MTT and Live/Dead assays with human dental pulp stem cells (n = 6). Data were analyzed using one-way ANOVA and Tukey's post-hoc test (α=0.05). RESULTS: DOX exhibited higher loading efficiency and more sustained release than CHX in both nanotube types (p < 0.01). APTMS functionalization improved nanotube dispersion and preserved DC (76-82 %), while unmodified nanotubes significantly reduced DC (24-32 %, p < 0.001 vs. control). Despite lower DC, unmodified drug-loaded nanotubes maintained bond strength over 6 months with < 15 % μTBS reduction, whereas APTMS-functionalized groups exhibited greater degradation (30-40 % loss, p < 0.05). All drug-loaded adhesives inhibited MMP activity by > 60 % relative to control (p < 0.001). No significant cytotoxic effects were observed across any group (p > 0.05). SIGNIFICANCE: Incorporating TiO₂ nanotubes into adhesives highlighted a trade-off between polymerization and the duration of drug release. While drug release was short-lived, all modified adhesives reduced MMP activity and showed differences in bond stability depending on nanotube functionalization. These results provide insight into how nanotube surface chemistry influences adhesive performance and point to strategies for optimizing conversion and therapeutic longevity in future adhesive systems.