An ORMOSIL-containing orthodontic acrylic resin with concomitant improvements in antimicrobial and fracture toughness properties

Abstract

Global increase in patients seeking orthodontic treatment creates a demand for the use of acrylic resins in removable appliances and retainers. Orthodontic removable appliance wearers have a higher risk of oral infections that are caused by the formation of bacterial and fungal biofilms on the appliance surface. Here, we present the synthetic route for an antibacterial and antifungal organically-modified silicate (ORMOSIL) that has multiple methacryloloxy functionalities attached to a siloxane backbone (quaternary ammonium methacryloxy silicate, or QAMS). By dissolving the water-insoluble, rubbery ORMOSIL in methyl methacrylate, QAMS may be copolymerized with polymethyl methacrylate, and covalently incorporated in the pressure-processed acrylic resin. The latter demonstrated a predominantly contact-killing effect on Streptococcus mutans ATCC 36558 and Actinomyces naselundii ATCC 12104 biofilms, while inhibiting adhesion of Candida albicans ATCC 90028 on the acrylic surface. Apart from its favorable antimicrobial activities, QAMS-containing acrylic resins exhibited decreased water wettability and improved toughness, without adversely affecting the flexural strength and modulus, water sorption and solubility, when compared with QAMS-free acrylic resin. The covalently bound, antimicrobial orthodontic acrylic resin with improved toughness represents advancement over other experimental antimicrobial acrylic resin formulations, in its potential to simultaneously prevent oral infections during appliance wear, and improve the fracture resistance of those appliances.

Conflict of interest statement

Competing Interests: The authors have the following competing interests: David Lang of Lang Dental Manufacturing Co. Inc., supplied the Ortho-Jet acrylic for this study. John D. Blizzard is employed by Quadsil Inc. There are no patents, products in development or marketed products to declare. This does not alter the authors' adherence to all the PLoS ONE policies on sharing data and materials, as detailed online in the guide for authors.

Figures

Figure 1. Percentage distribution (mean and standard deviation) of live bacteria within the biovolumes of interest (i.e. the volume of biofilm from the base to the 24th µm of a z-stack), and within the biomass at the basal layer of biofilms that were grown on orthodontic acrylic disks prepared from the control (0 wt% QAMS) and the four experimental groups (0.4–6 wt% QAMS) (N = 6).

A. Streptococcus mutans biofilms. B. Actinomyces naeslundii biofilms. For each parameter (upper case letters for biovolume and numerals for bottom of biofilm), groups labeled with the same category of descriptors (i.e. upper case letters, lower case letters, numeric) are not statistically significant (P>0.05).

Figure 2. Representative BacLight-stained 48-h S. mutans biofilms grown on the surfaces of orthodontic acrylic resins with different QAMS concentrations.

A. 0 wt%. B. 0.4 wt%. C. 2 wt%. D. 4 wt%. E. 6 wt%. Green: live bacteria; Red: dead bacteria. The first column represents merged CLSM images of the biofilms. The second column represents 3-D plots of each biofilm within a biovolume extending from the 1st (basal) to the 13th layer of a Z-stack (i.e. 24 µm thick). The 3rd column represents the percentage distribution of live and dead bacteria within the biomass at each level of the biofilm (Z-step  = 2 µm). For C–E, the blue dotted line denotes the levels with 50% dead biomass.

Figure 3. Representative BacLight-stained, 48-h A. naeslundii biofilms grown on the surfaces of orthodontic acrylic resins with different QAMS concentrations.

A. 0 wt%. B. 0.4 wt%. C. 2 wt%. D. 4 wt%. E. 6 wt%. Green: live bacteria; Red: dead bacteria. Column designations are the same as Figure 2. For the scatter plots in the right column of C–E, the blue dotted line denotes the levels with 50% dead biomass.

Figure 4. Representative merged images illustrating the adhesion and growth of C. albicans on orthodontic acrylic surfaces containing A. 0 wt% QAMS (control).

B. 0.4 wt% QAMS. C. 2 wt% QAMS. D. 4 wt% QAMS. E. 6 wt% QAMS. F. Two-dimensional analysis of the growth of C. albicans on the surfaces of orthodontic acrylic resins with different QAMS concentrations (N = 6). C. albicans did not form biofilms on acrylic surfaces containing QAMS. Percentage distributions of the fungal biomass with the field of examination are shown as black columns, and the percentage distributions of live fungus within the biomass are depicted as green columns. For each parameter, groups identified with the same category of descriptors (i.e. upper case letters for biomass and lower case letters for live fungi) are not statistically significant (P<0.05). Values represent means and standard deviations.

Figure 5. Colorimetric assays for examining the amount of A. Remnant quaternary ammonium charges present on the surface of the acrylic (fluorescein staining) after a 7-day water-aging period, and B. Cumulative amount (in mM) quaternary ammonium moieties present in the leachate after the 7-day water-aging period. Columns indicated by different symbols are significantly different from those without symbols (P

Figure 6. Effects of QAMS concentration on the flexural properties of the orthodontic acrylic resins (N = 10).

A. Ultimate flexural strength. B. Flexural modulus. For each parameter, the ISO requirement is included for comparison (dotted line). For the ultimate flexural strength, groups with the same upper case letter designators are not statistically significant (P>0.05). For flexural modulus, groups with the same lower case letter designators are not statistically significant (P>0.05). Values represent means and standard deviations.

Figure 7. Effects of QAMS concentration on the toughness properties of the orthodontic acrylic resins (N = 10).

A. Maximum stress intensity factor (Kmax). B. Work of Fracture (Wf). For each parameter, the ISO requirement is included for comparison (dotted line). Linear regression models provide an excellent fit for the relation between Kmax and QAMS concentration, and the relation between total fracture work and QAMS concentration. For Kmax, groups with the same upper case letter designators are not statistically significant (P>0.05). For total fracture work, groups with the same lower case letter designators are not statistically significant (P>0.05). Values represent means and standard deviations.

Figure 8. Effects of QAMS concentration on A. Water sorption; B. Water solubility; C. Sorption diffusion coefficients of QAMS-free and QAMS-containing orthodontic acrylic resins (N = 10).

Values represent means and standard deviations. ISO requirements for water sorption and water solubility are included for comparison (dotted lines). For each parameter, groups designated with different categories of descriptors (upper case letters for water sorption, lower case letters for water solubility, numerals for diffusion coefficients), are not statistically significant (P>0.05). D. A water sorption isotherm representative of all the five groups.