Enhancing Osteogenic Response in Selective Laser Melting-Fabricated Titanium through Hierarchical Micro/Nano Surface Engineering via Acid and Iodine Treatments

Selective laser melting (SLM) enables the fabrication of patient-specific titanium (Ti) implants with complex geometries. However, the as-built SLM-Ti surfaces retain partially melted particles that may hinder favorable cellular responses. Because surface topography plays a critical role in regulating osteogenic cell behavior, controlled surface modification is needed to optimize these responses. In this study, SLM-Ti samples were treated with a mixed-acid solution for 1, 3, or 6 h (designated A1, A3, and A6, respectively) to regulate their macro- and microtopography. Surface characteristics were quantified using conventional roughness parameters (Ra and Rz) and fractal dimension (Df), which provided a quantitative measure of hierarchical surface complexity and guided surface optimization. The responses of MC3T3-E1 preosteoblasts to untreated and acid-treated samples were assessed by measuring cell proliferation, alkaline phosphatase (ALP) activity, and extracellular matrix mineralization. Among the acid-treated groups, A1 (Ra = 9.3 μm; Df = 1.187) showed the most favorable combination of cell proliferation and ALP activity, indicating enhanced early osteogenic differentiation. Accordingly, A1 was selected as the optimal macro-/microstructured surface for subsequent hierarchical modification. The A1 surface was then subjected to sequential NaOH-CaCl2-heat-ICl3 treatment to produce the A1-Io surface, which contained an iodine-containing calcium-deficient calcium titanate nanostructured layer superimposed on the microstructured surface. This hierarchical macro-/micro-/nanoarchitecture increased Df to 1.359, reflecting greater multiscale surface complexity. Compared with A1 alone, A1-Io enhanced late-stage extracellular matrix mineralization and further improved cell proliferation and ALP activity. These findings demonstrate that controlled acid etching followed by iodine treatment provides a quantitatively guided strategy for enhancing the osteogenic performance of SLM-fabricated titanium implants.