Process‑oriented surface modification strategies to optimize interface-only bonding performance of 3D‑printed dental polymers

Current bonding strategies for fixed lingual retainers fabricated with 3D-printed resin (3DR) commonly rely on resin encapsulation or oversized retention pads to compensate for adhesive limitations. While these approaches may enhance bond strength, they compromise the key advantages of additive manufacturing, including streamlined computer-aided design, improved hygiene, and a low-profile structure. To address these limitations, this study proposes an interface-only bonding protocol between additively manufactured resin components that eliminates bulk coverage and pads, placing full reliance on the engineered adhesive interface within the same 3D-printed polymer system. The effectiveness of process-oriented, targeted surface modification strategies, non-thermal handheld plasma (NTHP) and resin matrix-penetrating primer (RMPP), was compared with conventional airborne-particle abrasion (APA) and universal adhesive (UA). Surface characteristics were analyzed using contact profilometry for roughness, water contact angle measurements for wettability, and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) for surface chemical composition of the additively manufactured polymer. Interfacial stability was assessed through shear bond strength (SBS) testing and failure mode analysis before and after 10,000 thermal cycles. The results demonstrate that APA significantly increases surface roughness, while NTHP improves wettability without inducing detectable topographical changes. ATR-FTIR analysis confirmed the presence of distinict urethane dimethacrylate and 10- methacryloyloxydecyl dihydrogen phosphate functional groups in the RMPP and UA groups, respectively, indicating different interfacial chemistries at the 3D-printed polymer–polymer bonding interface. Although pre-aging SBS values were comparable across all groups, the APA–RMPP combination achieved the highest bond stability after aging. In contrast, UA-treated groups exhibited the lowest post-aging values, likely due to increased hydrolytic degradation. These finding suggest that the synergistic effect of micromechanical retention from APA and chemical penetration from RMPP enhances the durability of interface-only bonding between 3D-printed dental polymers of the same material, offering a process-driven, minimalist surface engineering strategy for reliable integration of additively manufactured orthodontic components into modern digital workflows.