## 4 Discussion
Optical losses due to scattering, absorption, and mode dispersion could reduce the intensity of light reaching the target bacterial biofilms, particularly in smaller-core fibers. To overcome this limitation, recent advancements have been explored, demonstrating enhanced UV-C transmission efficiency by reducing internal scattering losses [22; 23]. Future designs incorporating high-reflectivity coatings and improved collimation techniques may further improve UV-C energy delivery and enhance bacterial inactivation efficiency. In this study, a novel UV-C LED system integrated into a thin optical fiber (124 \(\upmu\)m in diameter) was developed to effectively deliver UV-C light within confined root canal structures. Since accessing the apical third of the root canal and areas approximately 3 mm near the apex requires a fiber thickness of 100 \(\upmu\)m or less, this study explored a similar approach using a 124 \(\upmu\)m fiber in a newly developed prototype device. Enhancing bactericidal efficacy requires not only stronger UV-C sources but also an optimized delivery system that minimizes energy loss while ensuring safe application. The device in this study incorporated a specialized double-lens coupling system, optimizing the transmission of 270 nm UV-C light while ensuring minimal light dispersion. Proper coupling between the LED source and the optical fiber is essential to maximize energy transfer and reduce optical losses, as misalignment can significantly decrease transmission efficiency. Studies have demonstrated that lens-based coupling systems, particularly those utilizing collimating and focusing lenses, enhance beam collimation and minimize divergence, ensuring more effective light delivery through optical fibers [22; 24]. Additionally, precise alignment of the LED with the fiber's numerical aperture is critical in reducing reflection losses and improving overall energy efficiency [22]. By optimizing these factors, UV-C light can be efficiently transmitted through the thin optical fiber, improving its bactericidal potential in confined endodontic spaces. Conventional chemo-mechanical debridement is partially effective, and UV-C LED has been proposed as an alternative or adjunct for enhancing root canal disinfection. Recent research demonstrated that 255 nm UV-C LED effectively eliminated bacterial pathogens and induced beneficial cellular responses in human embryonic palatal mesenchymal cells and gingival fibroblasts. However, its integration as an adjunct or alternative to conventional sodium hypochlorite (NaOCI) disinfection depends on the severity of endodontic infection and must be determined clinically [17]. A critical limitation in UV-C disinfection arises in surface-grown bacterial colonies or biofilms, as these structures significantly shield inner microbial cells, reducing their susceptibility to UV-induced DNA damage and diminishing the overall bactericidal efficacy [29]. Additionally, although various irrigant activation methods such as laser-activated irrigation (LAI) have shown superior biofilm removal, concerns exist regarding their impact on dentin properties. Specifically, LAI using solutions like NaOCI and EDTA can cause pronounced dental surface disintegration, more severe than observed with sonic or ultrasonic activation techniques [13]. Another promising approach is the use of UV-B source, where a combination of UV-C and UV-B has been shown to generate reactive oxygen species (ROS), further enhancing bacterial eradication [30]. This could improve antibacterial effectiveness while minimizing the required exposure time, reducing potential negative effects on the dentin structure. Additionally, recent studies suggest the potential benefits of combining UV-C irradiation with biomaterials such as TiO\({}_{2}\). When TiO\({}_{2}\) was combined with UVB irradiation, effective disinfection in complex root canals was observed, indicating potential for UV-assisted antimicrobial therapies within challenging anatomical structures [28].