Physical, Mechanical, and Antibacterial Properties of Experimental Nanofilled Orthodontic Adhesives

Objectives: Characterize the degree of conversion (DC), biaxial flexural strength (BFS) and antibacterial properties (BIO) of experimental flowable orthodontic adhesives containing nitrogen and fluorine co-doped titanium dioxide nanoparticles (NF_TiO₂).
Methods: Experimental orthodontic adhesive resins were formulated using NF_TiO₂ (20%, 25%, 30% [v/v]). Adhesives without nanoparticles served as the control group. Unpolymerized adhesive resins (n=10/group) were individually placed onto a heated (37°C) attenuated total reflectance (ATR) monolithic diamond crystal (Golden Gate, Specac). Spectra of experimental and unaltered adhesives were obtained with a Fourier-transform infrared (FTIR) spectrometer (Nicolet IS50; 500-4,500cm^-1; resolution=4cm^-1, 10 internal scans/spectrum) before and immediately after polymerization. Disk-shaped specimens (diameter=6.0mm, thickness=0.5mm) for biaxial flexural strength (n=10/group) and for antibacterial testing (n=18/group) were fabricated and photopolymerized (40 sec/each; 385-515nm, 1,000mW/cm²; VALO). Degree of conversion values (%) were calculated from pre- and post-polymerization spectra using the two-frequency method and tangent-baseline technique. BFS was assessed using a Universal Testing Machine (Instron 68TM-5, crosshead speed=1.27 mm/min, 25°C). Antibacterial testing (BIO) was performed by growing Streptococcus mutans biofilms (UA159-ldh, 37°C, microaerophilic) on the surfaces of specimens for 24 hours and then measuring Relative Luminescence Units (RLU) with a Biotek Synergy HT multi-well plate reader. Experimental data for DC, BFS and BIO were analyzed using one-factor General Linear Models and post hoc Student-Newman-Keuls tests (SAS Software).
Results: Values of DC ranged from 74.75%±1.31% (20% NF_TiO₂) to 77.60%±1.31% (0% NF_TiO₂). BFS values ranged from 0.065±0.008 kN (25% NF_TiO₂) to 0.092±0.016 kN (0% NF_TiO₂) and BIO values ranged from 25,594.56±8,845.10 RLU (30% NF_TiO₂) to 46,868.44±14,635.23 (0% NF_TiO_2, Control) RLU.
Conclusions: Significant differences (p < 0.0001) were observed for DC, BFS and BIO. Nanofilled experimental materials were demonstrated to have superior antibacterial functionalities but were associated with lower physical and mechanical properties.