Abstract: BACKGROUND: Micro-arc oxidation technology has been used to improve the osseointegration of titanium implants. A variety of porous coatings containing different active elements are constructed on the surface of titanium. In recent years, studies have found that salidroside has good bone-promoting activity. Polydopamine, as an excellent binder on the surface of the material and a carrier for secondary adsorption, has the characteristics of loading active molecules.  
OBJECTIVE: To further improve the bone-promoting ability of the micro-arc oxidation coating on the titanium surface, a composite coating with excellent osseointegration was constructed on the porous surface.
METHODS:  The micro-arc oxidation coating was made on the surface of pure titanium, which was marked as MAO group, and the polydopamine coating was further prepared on the surface of micro-arc oxidation coating, which was marked as PDA group. Salidroside (1, 2, 4 g/L) was loaded on the surface of polydopamine coating, which was divided into Sal-1 group, Sal-2 group and Sal-4 group. The micro-morphology, surface element content and distribution of each coating were analyzed by scanning electron microscope and X-ray energy dispersive spectrometer. The proliferation activity of cells on the surface of each group was evaluated by CCK-8 assay and fluorescence staining, and the abilities of bone differentiation and mineralization were observed by alkaline phosphatase staining and alizarin red staining. The potential bone-promoting targets of salidroside were predicted by computer molecular docking technique.  
RESULTS AND CONCLUSION: (1) Scanning electron microscope showed that the surface of the five groups of specimens had porous micron morphology and the average pore diameter was similar, and X-ray energy dispersive spectrometer analysis showed that salidroside could be loaded on the surface of micro-arc oxidation coating by polydopamine carrier. (2) CCK-8 experiment showed that the survival rate of MC3T3-E1 cells in Sal-2 group and Sal-4 group was higher than that in MAO group on the 3rd and 5th days of culture (P < 0.05). Fluorescence staining showed that there were significant differences in MC3T3-E1 morphology among the five groups, and compared with MAO group, Sal-2 group and Sal-4 group had better cell morphology, larger cell spreading area and better connection with neighboring cells. (3) The activity of alkaline phosphatase in Sal-2 group and Sal-4 group was higher than that in MAO group on the 4th and 7th days (P < 0.05). Alizarin red staining showed that calcium nodules appeared in all groups on the 21st day, especially in Sal-2 group and Sal-4 group. (4) Computer molecular docking results show that extracellular regulated protein kinases 2, c-Jun N-terminal kinase 3, estrogen receptor β, and progesterone receptor may be the more active targets for salidroside to promote bone formation. (5) The results showed that salidroside was loaded on the surface of titanium micro-arc oxidation coating by polydopamine, and the bone-promoting activity of micro-arc oxidation coating was improved by proper amount of salidroside.

Key words: material, pure titanium, surface modification, micro-arc oxidation, polydopamine, salidroside, osteogenic activity, molecular docking