Magnetic hydroxyapatite nanotubes on micro-arc oxidized titanium for drug loading

A novel coating which comprised magnetic hydroxyapatite (HA) nanotubes as an outer layer and porous TiO₂ as the inner layer was fabricated on Ti by a hybrid process of micro arc oxidation (MAO), hydrothermal treatment (HT) and electrochemical treatment (ET). The microstructure of HA nanorods after HT was investigated, and their morphology evolution from nanorods to nanotubes by prolonging ET was observed. The formation mechanism of HA nanotubes was explored, and magnetism property of the composite coating was evaluated. Monocrystallized HA nanorods doped with 1.19 At% Fe were initially formed on microporous TiO₂ by MAO and HT, and they had a mean diameter of 300 nm. The mismatching between Ca and Fe ions induced some lattice distortions on the basal-faceted surfaces of HA. During ET process, Na⁺ and H⁺ ions should concentrate on the negatively charged basal planes of HA inducing further lattice distortion and local dissolution. By prolonging the ET process, dissolution of basal planes increased, resulting in the formation of HA nanotubes with an average outer diameter of about 300 nm, and wall thickness of about 30 nm. The composite coating showed magnetism behavior with a value of 0.3 emu g^-1 at applied field of 4 T. A strategy to fabricate a magnetic HA nanotube coating on Ti was given, and the unique nanostructure should have a potential application in drug loading.