Hydrothermal processing of hydroxyapatite: Past, present, and future

Apatite is a common mineral in igneous, sedimentary and metamorphic rocks, 1 which has the general chemical formula of A₁₀(BO₄)₆X₂, where A is Ca, Sr, Ba, Pb, Cd, and many rare earth elements, BO₄ is PO₄ ³⁻, VO₄ ³⁻, SiO₄ ⁴⁻, AsO₄ ³⁻, CO₃ ²⁻, X is OH⁻, Cl⁻, F⁻, CO₃ ²⁻. Water also has been reported to exist in the apatite structure in several forms. ^2-4 The presence of the elemental phosphorus which is a major component of apatite was first detected in 17th century and the phosphorus in mineral pyromorphite (lead apatite) was discovered in 1779. ^5,6 Since the first synthesis of apatite was done in the middle of the 19th century, ⁷ a large number of studies in the geochemical, crystallochemical, biological, and other fields have been conducted. Some physico-chemical properties of apatites determined by these studies are summarized in Table 1, with reference to books, ^1,5,6,8 review articles, ^2,9-11 and basic research papers. ^12-18 Most of the geochemical and crystallochemical studies were performed using natural apatites, whereas biological or recent studies were mainly done using synthesized apatites. The generally used “gem-grade” natural apatite single crystal (Durango, Mexico) shown in Figure 1 has prismatic shape, greenish color, and is considered to be formed under a hydrothermal condition. Apatite is also well known as the main inorganic constituent of hard tissues of human and animals. Because it is recognized to be one of the best biocompatible materials, there have been many reports on the biological aspects (e.g. Refs. 19-21). Various preparation methods for HAp including the hydrothermal method have been used for such studies. The hydrothermal method has priority over the other methods in the synthesis of ideal ceramic powders, due tc the effects of hydrothermal fluid on solid materials. In the present paper, hydrothermal processing, which has been recognized to be an excellent method for the preparation of ideal ceramic powders, ²²,²³ is described and its application to HAp is reviewed. Figure 1 Photograph of natural fluorapatite single crystal mined at Durango, Mexico. Table 1 Physico-chemical properties of synthetic hydroxyapatite or natural fluorapatite.