Orientation control for a-axis or c-axis oriented crystal films of hydroxyapatite, and a possible growth mechanism derived from X-ray diffraction

Hydroxyapatite (HA), the main component of living hard tissues, owes its specificity to electrical anisotropy resulting from its hexagonal crystal structure. The strong demand for technology to control the crystal orientation prompted us to develop techniques to fabricate uniaxial a- or c-axis crystalline HA films. C-axis-oriented HA was grown epitaxially on a ZnO (001) film, on an Al2O3(001) single crystal with a hexagonal lattice by pulsed laser deposition, and on the amorphous native oxide layer of a Si wafer, confirming the importance of hexagonal lattice formation as a first layer for c-axis orientation. In contrast, a-axis-oriented HA film was grown on Au and Pt films of which low reactivity enable HA constituent elements to migrate unrestrictedly before undergoing nucleation. The ability to separately produce a- and c-axis HA contributes to device development and to fundamental biochemical research on bones and teeth in the living body.