Low temperature consolidation of nanocrystalline apatites. Toward a new generation of calcium phosphate ceramics

Biomimetic nanocrystalline apatites analogous to bone mineral can be prepared by different ways. These non-stoichiometric compounds possess a high reactivity related to the presence of a metastable hydrated layer on the surface of the nanocrystals. The processing of such unstable phases by conventional techniques at high temperature strongly alters their physico-chemical and biological properties. Therefore, several low temperature routes have been investigated taking advantage of the structural characteristics of these compounds. Self-setting, injectable cements leading to nanocrystalline apatites have been developed. Solid mesoporous ceramic-like materials can also be obtained at low temperature by drying aqueous suspensions of nanocrystalline apatites. Among the most promising routes, however, are pressure sintering and spark plasma sintering at temperatures lower than 300°C. These techniques produce ceramics retaining most of the characteristics of the nanocrystals. The consolidation mechanism is thought to implicate the high mobility of ions within the hydrated layer. This work should be helpful for the preparation of a new generation of resorbable highly-reactive bioceramics