About the use of stoichiometric hydroxyapatite in compression - incidence of manufacturing process on compressibility

Literature concerning calcium phosphates in pharmacy exhibits the chemical diversity of the compounds available. Some excipient manufacturers offer hydroxyapatite as a direct compression excipient, but the chemical analysis of this compound usually shows a variability of the composition: the so-called materials can be hydroxyapatite or other calcium phosphates, uncalcined (i.e. with a low crystallinity) or calcined and well-crystallized hydroxyapatite. This study points out the incidence of the crystallinity of one compound (i.e. hydroxyapatite) on the mechanical properties. Stoichiometric hydroxyapatite is synthesized and compounds differing in their crystallinity, manufacturing process and particle size are manufactured. X-Ray diffraction analysis is used to investigate the chemical nature of the compounds. The mechanical study (study of the compression, diametral compressive strength, Heckel plots) highlights the negative effect of calcination on the mechanical properties. Porosity and specific surface area measurements show the effect of calcination on compaction. Uncalcined materials show bulk and mechanical properties in accordance with their use as direct compression excipients.