Chitosan-Modified Dental Adhesives: Effects on Physical and Mechanical Properties

Objectives: The bonding of orthodontic brackets to teeth allows for efficient treatment with fixed appliances, however, current bracket bonding agents do not prevent biofilm accumulation. A companion study of our group investigated the ability of chitosan-added adhesives to control S. mutans growth. This study assessed the effect of chitosan addition (CS) on degree of conversion (DC), water sorption (WS), solubility (SL) and flexural strength (FS) of bonding agents.
Methods: Low-molecular-weight CS (Sigma-Aldrich) was added at 0.5% concentration to an experimental adhesive (BisGMA+EBP-2.5:1) either neat (i.e., EXP+CS) or solvated with 30% ethanol (i.e., EXP+CS+EtOH). Control groups consisted of respective materials without CS-addition (i.e., EXP and EXP+EtOH). For all experiments, EtOH-solvated blends were blow-dried for 5s before polymerization. The DC (%) of each adhesive blend was estimated using an ATR-FTIR approach (n=3/group). Disc-like specimens (4x1mm thick) were prepared for WS and SL measurements (mg/mm³), which were assessed using a standardized protocol based on ISO-4049 (n=5/group). For the assessment of the FS (MPa), bar-like specimens (2x2x12.5mm length) were fabricated and subjected to 3-point bending tests (EZ-LX, Shimadzu) using a standing with a 10-mm support span (n=8/group). Dependent variables (DC, WS, SL, FS) were separately analyzed by ANOVA and post-hoc Tukey tests (a=0.05).
Results: The DC ranged from 68% (EXP+CS) up to 73% (EXP+EtOH). Differences in DC% were significant only between neat/solvated pair of same adhesives, with EXP+EtOH>EXP (p=0.001) and EXP+CS+EtOH>EXP+CS (p=0.01). Likewise, WS and SL values were significantly higher for the solvated counterpart of adhesives, with EXP+EtOH>EXP (p<0.001) and EXP+CS+EtOH>EXP+CS (p<0.001). Conversely, solvated adhesives showed significantly lower FS compared to correspondent neat version, with EXP+EtOHConclusions: Addition of 0.5% CS did not affect the evaluated physical and mechanical properties of experimental dental adhesives.