Enzyme-Directed Biomineralization Coating on TiO₂ Nanotubes and its Positive Effect on Osteogenesis

Hydroxyapatite (HA)-coated TiO₂ nanotubes (TNTs) have been reported to enhance osteogenesis. However, the nanoscale topography of TNTs usually vanishes due to the uncontrollable mineralization on the surface. In this study, TNTs with different diameters­(small, 25 nm; medium, 55 nm; and large, 85 nm) were fabricated by anodization in 3 different voltages. Enzyme-directed biomineralization was adopted to deposit calcium phosphate on the above TNTs. The surface structures and properties of the coatings were characterized by scanning electron microscopy, dispersive X-ray spectrometry, X-ray diffraction, and Fourier-transform infrared spectroscopy. The osteogenesis effect of the hybrid TNT/HA and the original TNTs were evaluated. The results showed that hydroxyapatite deposited homogeneously along the TiO₂ nanotubes while preserving the intrinsic nanotopography. Mechanically, alkaline phosphatase­(ALP) played a critical role in the mineralization and large nanotube size is more favorable for the mineralizing process because of more ALP absorption. Besides, the hybrid nanosurface TNT/HA coating was found to improve the adhesion, proliferation, and osteogenic differentiation of MC3T3-E1 cells compared to pure TNTs. Our study suggests that the hybrid TNT/HA coating constructed by enzyme-directed biomineralization on TiO₂ nanotubes is a promising modification strategy for titanium implants.