Enhancement of mineralization on porous titanium surface by filling with nano-hydroxyapatite particles fabricated with a vacuum spray method.

Nano-hydroxyapatite (nano-HA) coating has been proved to be effective to modify the titanium surface for better bone formation. A simple and economical nano-HA coating method that filling the nano-HA particles into the porous titanium substrate by using vacuum suction was introduced in this study. Two kinds of nano-HA modified titanium surfaces, nano-HA filled type (nHA-f) and nano-HA coated type (nHA-c), were prepared by a 2-step and 3-step protocol, respectively. Nano-HA modified titanium surfaces with and without thermal treatment were compared. The physicochemical properties including morphology, elemental composition, coating thickness, adhesion strength and in vitro mineralization ability were characterized. The SEM results showed that nano-HA particles were effectively filled into the porous titanium substrate under the function of vacuum suction in the nHA-f and nHA-f-heat groups. The thickness of the nano-HA layer was about 1 μm and 7-8 μm in the nano-HA filled type and nano-HA coating type, respectively. Localized separation between the nano-HA layer and the titanium substrate was observed in the thermal-treated groups, which may increase the risks of detachment. In the in vitro mineralization test, the unheated groups (nHA-f and nHA-c) showed promoted calcium phosphate compounds deposition than the heated groups (nHA-f-heated and nHA-c-heated) because of the active biological behavior of the amorphous nano-HA; the nHA-f group with a thin nano-HA layer showed no difference to the thick coating group (nHA-c) at the early stage, which was due to the effect of the porous structure as a cage to protect the nano-HA particles from fast dissolution. In conclusion, the vacuum spray method is effective to create a thin amorphous nano-HA layer with improved adhesion strength and sustained-release ability to induce mineralization by filling the nano-HA particles into the porous titanium substrate without thermal treatment.