HAp-ZrO2-AgNP-reinforced polystyrene dental nanocomposites: Antibacterial, antiplaque and self-cleaning properties

Dental biofilms, primarily driven by Streptococcus mutans, are the main etiological agents of caries and restorative failure.

The aim of the study was to design and synthesize a novel polystyrene (PS)-based nanocomposite reinforced with hydroxyapatite (HAp), zirconia (ZrO2) and silver nanoparticles (AgNPs).

Hydroxyapatite, ZrO2 and AgNPs were synthesized via wet precipitation, sol-gel and citrate reduction methods, respectively. The nanoparticles (5 wt% total loading) were uniformly dispersed in a toluene-based PS matrix using ultrasonic-assisted solution casting. The resulting films were characterized in terms of morphology (scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM)), composition (energy-dispersive X-ray spectroscopy (EDX)), thermal stability (thermogravimetric analysis (TGA)), mechanical properties (tensile strength, Young's modulus, Vickers hardness), surface wettability (water contact angle (WCA)), antibacterial efficacy (zone of inhibition (ZOI), minimum inhibitory concentration/minimum bactericidal concentration (MIC/MBC), colony-forming unit (CFU) assay), and antibiofilm activity (crystal violet staining, confocal laser scanning microscopy (CLSM)). Statistical analysis was performed using one-way analysis of variance (ANOVA) and Tukey's honestly significant difference (HSD) test (p < 0.05).

The nanocomposite exhibited a 30% increase in tensile strength (32.5 MPa vs. 25 MPa for pure PS) and a 25% rise in elastic modulus (1.5 GPa vs. 1.2 GPa). The water contact angle increased from 85° ±2° (pure PS) to 115° ±3° (nanocomposite), confirming significantly enhanced hydrophobicity. The composite demonstrated strong antibacterial activity, with a ZOI of 15.3 mm, MIC of 6.25 mg/mL, MBC of 12.5 mg/mL, and 99.8% bacterial reduction within 24 h. Antibiofilm performance was also notable, showing a 91.2% reduction in biofilm biomass after 72 h and 74.8% decrease in biofilm thickness (from 32.5 μm to 8.2 μm). Confocal laser scanning microscopy revealed a marked shift in the live/dead cell ratio (from 4.7:1 to 0.6:1), confirming bactericidal rather than bacteriostatic action. All nanoparticles were homogeneously distributed without agglomeration, as confirmed by SEM/EDX and TEM.

The HAp-ZrO2-AgNP-reinforced PS nanocomposite integrates enhanced mechanical properties, self-cleaning surface characteristics, and potent, long-lasting antibiofilm activity against S. mutans. While further biocompatibility and clinical translation studies are warranted, this material represents a significant leap toward durable, infection-resistant dental applications.