Abstract: BACKGROUND: Titanium and its alloy are widely used in the field of oral implant and bone defect repair with its good mechanical properties and biocompatibility. However, insufficient osseobinding efficacy due to biological inertia of material surfaces remains a key clinical problem restricting their long-term stability. Therefore, it is particularly important to improve the osteogenic activity of titanium through surface modification.
OBJECTIVE: To explore the in vitro osteogenic properties of titanium surface titanium dioxide nanotubes and polydopamine composite coating.
METHODS: Titanium dioxide nanotube arrays were constructed on the surface of medical pure titanium by anodic oxidation, which was recorded as nanotube group. The nanotube samples were immersed in hydrochloric acid dopamine solution to construct polydopamine functional layer, which was recorded as nanotube-polydopamine group. The surface morphology, surface roughness, and water contact angle of the pure titanium group, nanotube group, and nanotube-polydopamine group were detected. Passage 3 rat bone marrow mesenchymal stem cells were inoculated on the surface of the three groups of samples. CCK-8 assay, live/dead fluorescence staining, and scanning electron microscopy were performed. After osteogenic induction culture, alkaline phosphatase staining and Alizarin Red S staining were performed. 
RESULTS AND CONCLUSION: (1) Scanning electron microscopy showed that the surface of the pure titanium group was relatively flat. The surface of the nanotube group formed a highly ordered titanium dioxide nanotube array structure, and the surface of the nanotubes of the nanotube-polydopamine group was deposited with a polydopamine coating. There was no significant difference in the surface roughness of the three groups of samples (P > 0.05). The water contact angle of the nanotube group was smaller than that of the pure titanium group and the nanotube-polydopamine group (P < 0.05), and the water contact angle of the nanotube-polydopamine group was smaller than that of the pure titanium group (P < 0.05). (2) CCK-8 assay and live/dead fluorescence staining results showed that samples from the pure titanium group, nanotube group, and nanotube-polydopamine group all had good biocompatibility and no obvious cytotoxicity. Scanning electron microscopy showed that compared with the pure titanium group and the nanotube group, the nanotube-polydopamine group promoted the extension and cell-to-cell adhesion of rat bone marrow mesenchymal stem cells. (3) The results of alkaline phosphatase staining showed that the alkaline phosphatase activity of the nanotube-polydopamine group was higher than that of the pure titanium group and the nanotube group (P < 0.05). The results of Alizarin Red S staining showed that the extracellular matrix mineralization level of the nanotube-polydopamine group was stronger than that of the pure titanium group and the nanotube group (P < 0.05). The results show that titanium modified with titanium dioxide nanotube-polydopamine composite coating on the surface of pure titanium can effectively promote the osteogenic differentiation of bone marrow mesenchymal stem cells. 

Key words: titanium, surface modification, polydopamine, titanium dioxide, nanotube, bone marrow mesenchymal stem cell, osteogenic differentiation, biocompatibility