Fabrication of hydroxycarbonate apatite coatings with hierarchically porous structures

Hierarchically porous hydroxycarbonate apatite is known to have a high bioactivity to regenerate bone, but its application in bone graft substitutes has been restricted due to its poor mechanical properties. This drawback has been addressed by (i) depositing calcium carbonate coatings on Ti6Al4V substrates by electrophoresis; and (ii) converting the coatings to hydroxycarbonate apatite coatings with hierarchically porous structures by treatment with a phosphate buffer solution (PBS). After soaking calcium carbonate coatings in PBS for 1 day, calcium-deficient hydroxycarbonate apatite nanocrystals are deposited on the surfaces of calcium carbonate particles via a dissolution–precipitation reaction. The aggregation of the nanocrystals produces plate-like hydroxycarbonate apatite. Mesopores with a pore size of ∼3.8 nm and macropores or apertures with an aperture size of ∼1 μm are formed within and among the plates, respectively. After soaking for 9 days, the pore size of mesopores decreases and the mesopores disappear partly due to the crystal growth of hydroxycarbonate apatite. Simulated body fluid immersion tests reveal that the good in vitro bioactivity of hydroxycarbonate apatite coatings is attributed to the calcium deficiencies in apatite lattice and the hierarchically porous structures.