## 4 Discussion  
The success of root canal therapy depends on a sufficient reduction in bacteria and their by-products from the root canal system [1]. However, achieving optimal disinfection through traditional methods, namely the combination of mechanical instrumentation and irrigation with a disinfecting solution, is difficult due to the complexity of the root canal system [25]. Previous studies have shown that laser irradiation through optical fibers can successfully disinfect root canal walls; however, these techniques are limited by the potential harm to surrounding tissues [14; 15; 16; 26]. In this study, the effectiveness of UV-C LED light, known for its sterilizing effect, delivered through thin optical fibers and potentially applicable for root canal treatment, against _E. faecalis_ was evaluated, specifically utilizing the UV-C band (200-280 nm) among the various ultraviolet wavelengths, including UV-C (200-280 nm), UV-B (280-320 nm), and UV-A (320-400 nm) [27]. The experimental results demonstrated that UV-C LED irradiation exerted significant bactericidal effects on _E. faecalis_ in a time-dependent manner. As shown in the results, bacterial viability remained unchanged at 0 and 10 s but began to decline from 30 s onwards, with the most pronounced reductions observed at 120 and 180 s. The optical density (OD600) measurements revealed a progressive decline in bacterial counts over increasing exposure times, reinforcing the time-dependent nature of UV-C bactericidal activity. These findings are consistent with previous studies confirming the efficacy of UV-C in eradicating _E. faecalis_ biofilms [28]. Although UV-C LED irradiation exhibited effective antimicrobial properties, power efficiency remains a critical factor for practical clinical application. Previous research indicates that optical fiber-mediated LED transmission can result in power losses, significantly impacting energy delivery to target sites [23; 24]. In this study, a statistically significant bactericidal effect was observed at 120 s, indicating that the reduced energy delivery through the thin optical fiber is compensated by prolonged exposure time. The prototype device utilized in this study operated at 270 nm, with a power of 100 mW, lower than conventional laser-based disinfection methods. These conventional lasers, particularly Er/YAG and Nd/YAG systems, have been widely used in endodontics due to their deep penetration and effective biofilm disruption [15; 16]. However, these systems operate at significantly higher power levels (200-3000 mW), which increases the risk of excessive heat generation, potentially leading to dental microcracks or thermal damage to adjacent tissues [15; 16]. In the present study, UV-C LED at 270 nm effectively sterilized the superficial layers of dentin by utilizing lenses, precise alignment, and coupling techniques to reduce light loss. UV-C irradiation provides an alternative antimicrobial approach by disrupting bacterial DNA replication through pyrimidine dimer formation, with peak germicidal efficiency occurring between 255 and 280 nm [17; 28]. However, due to its short wavelength, its penetration depth is limited, reducing its direct impact on deeper bacterial colonies [17; 28]. Unlike longer-wavelength laser systems, which can penetrate deeper but pose risks of thermal damage and structural alterations in dental walls, UV-CLED operates with minimal heat generation, potentially lowering the risk of collateral damage to surrounding tissues. The integration of UV-C LED with fiber optic technology has been explored to optimize antimicrobial disinfection, particularly in confined anatomical spaces such as root canals [17; 18; 28]. Compared to free-space UV irradiation, fiber optic-guided UV-C delivery provides higher precision and better energy concentration within targeted regions, reducing unintended collateral damage to adjacent tissues. However, one major limitation of fiber optic-based UV-C delivery is energy attenuation as light propagates through the fiber.