Biomimetic nanocrystalline apatites (BNA) Ca10-x-Z(PO4)6-(HPO4)x(OH)2-x-2Z, (H2O)n were synthesized by precipitation in aqueous medium, then consolidated by Spark Plasma Sintering (or SPS). They are constituted of nanocrystals involving an apatitic core and a surface phospho-calcic hydrated layer containing "non-apatitic" phosphate, hydrogenphosphate and calcium ions, highly "labile" (easily exchangeable), responsible for their high reactivity. The chemical composition, structure and morphology of the nanocrystals of BNA evolve upon maturation in solution, and they tend toward greater thermodynamic stability. Although the amount of non-apatitic chemical species decreases upon maturation, their presence is still significant after a long maturation. Low temperature (150°C) SPS sintering of maturated BNA allowed us to obtain highlycohesive, porous ceramics. The sintering phenomenon observed in such conditions suggests a "crystal fusion" consolidation process, involving the high surface reactivity of the nanocrystals by way of their hydrated layer. The mechanical properties (elastic modulus between 12 and 35 GPa, flexure strength close to 10 MPa) of the ceramics obtained are close to those of bone mineral. Moreover, the nanometer-scale dimensions of the crystals, beneficial to bioresorption after implantation in osseous site, as well as the presence of labile nonapatitic ionic species, favorable to bioactivity, are preserved after SPS. These properties confer to BNA ceramics a particularly promising potential in view of applications in the field of bone tissue engineering.
