Implant-associated infections remain among the most critical complications in orthopedic and dental implantology, frequently leading to implant failure and necessitating revision surgery. These infections are particularly problematic during the early post-implantation phase, when the host immune response and bone regeneration processes are not yet fully established. Bacterial adhesion and biofilm formation during this vulnerable period highlight the importance of evaluating the intrinsic antibacterial properties of implant surfaces. Additive Manufacturing （AM） has emerged as a transformative technology in implant fabrication, enabling the production of highly complex, patient-specific three-dimensional structures with precisely controlled micro- and nano-topographies. However, the unique surface architectures fabricated with AM, which are potentially important in modulating host responses and infection dynamics, are not yet matched by in vitro evaluation systems capable of capturing such biologically relevant interactions. In particular, conventional assays lack the capacity to simultaneously assess bacterial behavior and host cellular responses. In this study, we developed a novel in vitro antibacterial evaluation platform incorporating a co-culture model of bacteria and primary osteoblasts, designed to better simulate the early biological environment encountered by implants. We first established a quantitative bacterial colony counting method based on image-based area correction and optimized both the bacterial seeding density and co-culture conditions to ensure reliable measurement of bacterial proliferation and osteoblasts viability. The system was then applied to AM-fabricated titanium substrates with or without unidirectional microgrooves. Under mono-culture conditions, bacterial viability did not differ significantly between groove and flat substrates. In contrast, in the co-culture setting, only the groove substrates exhibited a marked antibacterial effect, characterized by reduced bacterial survival alongside sustained osteoblasts morphology. This response was not detectable using conventional monoculture- based assays. These findings indicate that the antibacterial performance of 3D-structured AM implants is highly contextdependent, becoming apparent only under biologically relevant, cell-interactive conditions. The proposed co-culture model provides a robust and physiologically meaningful platform for evaluating implant surface antibacterial properties, with significant implications for the preclinical assessment of next-generation AM biomaterials.

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 Recently, non-equiatomic Ti-Nb-Mo-Ta-W alloy has been proposed as a refractory high-entropy alloy that suppresses elemental segregation designed using a thermodynamic calculations and empirical parameters. In this study, we purposed to investigate the effect of rapid solidification by laser irradiation for metal 3D printing on the microstructure formation of this alloy. Thermal diffusion simulation result showed that both temperature gradient （G） and solidification rate （R） during solidification after laser irradiation were extremely high compared to those during solidification by conventional methods. In the melt-pool, a dendritic or cellular microstructures with the size of submicron order were formed along the normal direction of the melt-pool edge, which was finer in size than the microstructure formed by arc melting. These findings suggest that laser irradiation for metal 3D printing has a benefit in the refinement of microstructure.

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 It is essential to develop biomedical implant materials with low Young’s modulus for alleviating stress shielding arising from the gap of Young’s modulus between metal materials and bone tissue. Metastable β-type titanium alloys offer promising characteristics, including good biocompatibility, specific strength, corrosion resistance, and low Young’s modulus. Among these alloys, Ti-Nb alloy system exhibits low Young’s modulus when e/a is in the range of 4.26 ～ 4.30, that was reported in previous studies using single crystal. The purpose of this study was to clarify whether Young’s modulus decreases in Ti-42Nb （wt.%） alloy, where e/a = 4.27, developed even by laser-powder bed fusion （L-PBF） by comparing the Young’s modulus of Ti-42Nb alloy and Ti-15Mo-5Zr-3Al （wt.%） alloy. This study revealed that Ti-42Nb alloy has lower Young’s modulus than Ti-15Mo-5Zr-3Al alloy when fabricated by L-PBF, demonstrating the validity of Ti-42Nb alloy as a material for biomedical implant with low Young’s modulus.

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