Preparation of Hydroxyapatite Crystals and Their Behavior as Seeds for Crystal Growth

Samples of crystalline hydroxyapatite [Ca5OH(PO4)3 HA] were prepared by precipitation from aqueous media under a variety of experimental conditions (temperature, concentration of reagents, rates of addition of the reagents, and seeding). The resultant products showed a wide range of particle sizes, i.e., specific surface areas, from 6.39 to 50.1 m2/g. In these preparations, relatively large crystals were obtained with low rates of addition of the reagents or by seeding the precipitating medium. Small differences in supersaturation of the reaction medium can markedly affect the particle sizes and crystalline habits of the resulting products, possibly by altering the processes of nucleation and subsequent crystal growth. When these crystalline materials were used as seeds to study the crystal growth of HA, it was confirmed that the precipitation rate of calcium apatite on seed crystals is highly dependent on the surface areas available for growth, rather than on the particle sizes and amounts of the seed crystals. Small differences in the kinetic runs were observed between the various seed crystals, which can be attributed to differences in the surface properties of these crystals. Additionally, transmission electron microscopy of the seed crystals revealed that some projections form, possibly on the basal planes of the crystals, during crystal growth. Since the growth rate of these projections was greater than the mean growth rate calculated on the basis of changes in solution composition and total surface area, it appears that the kinetics of the growth process is determined, to some extent, by the geometry of the seeds.