Functionalization of a Shape-Memory 3D-Printed Clear Aligner Resin with Silanized Chitosan Nanoparticles: Antibacterial, Optical, Polymerization, and Mechanical Evaluation

Directly 3D-printed clear aligners are increasingly adopted for orthodontic treatment, but lack intrinsic antibacterial properties. This study investigates the integration of silanized chitosan nanoparticles (CSNPs) into 3D-printed clear aligner resin to improve antibacterial activity while preserving optical, polymerization, and mechanical performance. Commercial CSNPs were surface modified with 3-(trimethoxysilyl)propyl methacrylate and characterized using FTIR, UV-Vis, TEM, EDS, XRD, DLS and zeta-potential analysis. The silanized CSNPs were incorporated into TA-28 clear aligner resin at 0.01, 0.05, and 0.1 wt%. Antibacterial activity against Streptococcus mutans (S. mutans) and Porphyromonas gingivalis (P. gingivalis) was assessed using bacterial adherence and biofilm assay. Transparency, degree of conversion, tensile and flexural properties were evaluated. Successful silanization was confirmed by characterization analyses. The incorporation of silanized CSNPs reduced bacterial adherence and biofilm formation significantly in comparison with the control group (p < 0.001). The transparency was maintained at 0.01 and 0.05 wt% but reduced significantly at 0.1 wt% (p < 0.001). While all nano-modified groups had a lower DC% than the control group, values remained above 75%. Mechanical performance showed improvements, particularly in stiffness and flexural properties, following nanoparticle incorporation. Among the tested concentrations, the 0.05 wt% CSNPs provided the most favorable overall performance, highlighting their potential for the development of next-generation bioactive clear aligners.