Morphology and structural characteristics of hydroxyapatite whiskers: Effect of the initial Ca concentration, Ca/P ratio and pH

Hydrothermal homogeneous precipitation combines the best characteristics of the hydrothermal and homogeneous precipitation methods, and allows long and uniform hydroxyapatite (HA) whiskers, with a high aspect ratio and high crystallinity, to be obtained. Their morphology and structural characteristics depend on the initial Ca/P ratio (iCa/P) and pH (ipH), as well as the initial calcium concentration (i[Ca]). Variation in these values had no effect on constitution, which was crystallographically indistinguishable from HA. Ca/P ratio steadily improved with increases in both ipH and iCa/P, but was independent of i[Ca]. Uniform whiskers were obtained at high iCa/P and low ipH, or at high ipH and low iCa/P. Whiskers with a mean length of 96–140 μm and an aspect ratio of 96–136 were obtained at ipH = 2–3 and iCa/P = 1.67–2. At a low ipH and low iCa/P, irregular plate-like particles and branch-like whiskers were formed, while a high ipH favoured the formation of lath-like HA at high iCa/P. Preferred growth along the c-axis was more intense at higher iCa/P and ipH as well as at low i[Ca]. However, under these conditions, the crystal growth habit was also changed, showing preferred growth along both the c- and a-axes. The increase in whisker width over the general value obtained was abrupt at low i[Ca] and high iCa/P.