International audienceNanocrystalline apatites play an important role in biomineralisation and they are used as bioactive biominerals for orthopaedic applications. One of the most interesting characteristics of the nanocrystals, evidenced by spectroscopic methods, is the existence of a structured surface hydrated layer, well developed in freshly formed precipitates, which becomes progressively transformed into the more stable apatitic lattice upon ageing in aqueous media. The hydrated layer is very fragile and irreversibly altered upon drying. Several routes leading to different apatite compositions are found in biological systems. The loosely bound ions of the hydrated layer can be easily and reversibly substituted by other ions in fast aqueous ion exchange reactions. These ions can either be included in the growing stable apatite lattice during the ageing process or remain in the hydrated layer. The adsorption properties of nanocrystals appear to be strongly dependent on the composition of the hydrated layer and on ageing. The surface reactivity of the apatite nanocrystals can play a part in different biomaterials and could explain the setting reactions of biomimetic calcium phosphate cements and the possibility of obtaining adherent nanocrystalline coatings on different substrates

Calcium phosphate synthesized by a wet-mechanochemical reaction between Ca(OH) 2 and H 3 PO 4 is identified by X-ray diffractometry as an apparently single phase of hydroxyapatite (HAp). We examined more detail on the local structure of the product by high-resolution 1 H and 31 P solid-state double nuclear magnetic resonance (NMR) spectroscopy. Adsorbed and structural waters, as well as two kinds of OH groups in HAp, were observed in the inversion recovery 1 H magic angle spinning (MAS) spectra. HAp and other calcium phosphate were observed in 1 Hf 31 P cross polarization (CP)/MAS spectra. The 31 P differential cross polarization (DCP)/ MAS and 1 H inversion recovery f 31 P CP/MAS measurements revealed different chemical interactions between 1 H and 31 P and, hence, could distinguish between ordered and disordered HAp. This micro-inhomogenity results from interiors and near-surface of nanoparticles as well as Ca 2+ -deficiency.

Nanocrystalline apatitic calcium phosphates play a crucial role in calcified tissues and biomaterials. One of the most interesting characteristics of biomimetic apatite nanocrystals is the existence of a surface hydrated layer essentially related to their formation process in solution. This hydrated layer shows specific spectroscopic characteristics. It seems to exist in its nascent state only in wet samples and is altered on drying. This surface-hydrated layer progressively disappears as the stable apatite domains develop. The surface ions can be rapidly and reversibly exchanged in solution, mainly with selected bivalent species. The exchange reactions clearly reveal the existence of two domains: the relatively inert apatite core and the very reactive surface-hydrated domains. The structure of the hydrated layer has been shown to be reversibly affected by the constituting ions. Such a surface layer in bone apatite nanocrystals could participate actively in homeostasis and probably other regulation processes. The specificity of biomimetic apatite nanocrystals also opens interesting possibilities in materials science. The mobility of the mineral ions on the crystal surface, for example, allows strong bonding and interactions either with other crystals or with different substrates. Inter-crystalline interactions have been described as a "crystal fusion" process in vivo and they could be involved in the setting reaction of biomimetic calcium phosphate cements. Ceramic-like materials using the surface interaction capabilities of the nanocrystals can be produced at very low temperature (below 200 C). The surface-hydrated layer could also be involved in interactions with macromolecules and polymeric materials or in the coating of implants. The ion exchange and adsorption capabilities of the nanocrystals could probably be used for drug release, offering a range of possible behaviours.