Fabrication and in vitro characterization of magnetic hydroxycarbonate apatite coatings with hierarchically porous structures

Hydroxycarbonate apatite/Fe3O4 composite coatings (MHACs) with hierarchically porous structures were fabricated by electrophoretic deposition of CaCO3/Fe3O4 particles on Ti6Al4V substrates followed by treatment with phosphate buffer solution (PBS) at 37 °C. The effects of Fe3O4 on the conversion rate of calcium carbonate to hydroxycarbonate apatite and the porous structures and in vitro bioactivity of MHACs were investigated. After soaking CaCO3/Fe3O4 coatings in PBS, hydroxycarbonate apatite nucleates heterogeneously on the surfaces of CaCO3/Fe3O4 particles and forms a plate-like structure. Fe3O4 increases the velocity of nucleus formation of hydroxycarbonate apatite. After soaking for 1 day, the percentage of unreacted calcium carbonate for MHACs is ∼9.1%, lower than the ∼41.0% for hydroxycarbonate apatite coatings (HCACs). As the CaCO3/Fe3O4 coatings are converted to MHACs, macropores with a pore size of ∼4 μm on the coatings and mesopores with a pore size of ∼3.9 nm within the hydroxycarbonate apatite plates are formed. The mesopores remain in the MHACs after treatment with PBS for 9 days, while they disappear in the HCACs. Simulated body fluid immersion tests reveal that Fe3O4 improves the in vitro bioactivity of biocoatings. The amount of bone-like apatite precipitated on the surfaces of MHACs is greater than that on the surfaces of HCACs.