Time-Dependent Effects of 222 nm UVC Photofunctionalization on Dentin Adhesion to CAD/CAM Resin Blocks

Background/Objectives: This study aimed to investigate the effect of UVC irradiation on the adhesive strength between a CAD/CAM composite resin block and dentin and to apply UVC to dental care. Methods: CAD/CAM composite resin blocks were sectioned, polished, and primed using a ceramic primer. Bovine dentin specimens were prepared and irradiated with 222 nm UVC light for no time (control, no UV), 600 s or 1200 s. After irradiation, dentin surfaces were conditioned with a tooth primer before bonding with a resin cement. Microtensile bond strength (μTBS) testing, scanning electron microscope (SEM)-based failure mode analysis, and contact angle measurements were conducted. Results: The control measured 34.32 (±3.11) MPa, the 600 s group measured 42.64 (±4.50) MPa, and the 1200 s group measured 39.75 (±7.52) MPa. There were no significant differences observed in the 1200 s group when compared with the control group. Conversely, a remarkable increase in adhesive strength was observed in the 600 s group in comparison to the control group. Failure mode analysis revealed a predominance of mixed failures in the 600 s group, which suggests enhanced interfacial bonding. Contact angle measurements demonstrated significantly enhanced surface wettability in both UVC irradiation groups relative to the control (p < 0.05). Conclusions: Photofunctionalization with 222 nm UVC improved dentin adhesion to CAD/CAM resin blocks, likely through enhanced hydrophilicity and removal of organic contaminants. Nonetheless, extended irradiation did not produce additional benefits, indicating a time-dependent response. Although 222 nm UVC irradiation demonstrates promise as a pretreatment approach in adhesive dentistry, further optimization is necessary to shorten irradiation duration and improve clinical feasibility.