International audienceNanocrystalline calcium phosphate apatites constitute the main inorganic part of hard tissues, and a growing focus is devoted to prepare synthetic analogs, so-called "biomimetic", able to precisely mimic the morphological and physico-chemical features of biological apatite compounds. Both from fundamental and applied viewpoints, an accurate characterization of nanocrystalline apatites, including their peculiar surface features, and a deep knowledge of crystallization aspects are prerequisites to attempt understanding mineralization phenomena in vivo as well as for designing innovative bioactive materials that may then find applications in bone tissue engineering, either as self-supported scaffolds and fillers or in the form of coatings, but also in other domains such as drug delivery or else medical imaging. Also,interfacial phenomena are of prime importance for getting a better insight of biomineralization and for following the behavior of biomaterials in or close to their final conditions of use. In this view,both adsorption and ion exchange represent essential processes involving the surface of apatite nanocrystals, possibly doped with foreign elements or functionalized with organic molecules of interest. In this review paper, we will address these various points in details based on a large literature survey. We will also underline the fundamental physico-chemical and behavioral differences that exist between nanocrystalline apatites (whether of biological origin or their synthetic biomimetic analogs) and stoichiometric hydroxyapatite

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ture of the predominant species in the gel precursor. This phenomenon allows us to direct the crystallization reaction to the desired aluminophosphate phase simply by controlling the gel precursor pH. ExperimentalThe reaction gels of composition n-DPA:P 2 O 5 :Al 2 O 3 :40H 2 O, using pseudoboehmite alumina (Condea, Pural SB), phosphoric acid 85 % (Aldrich), and n-DPA (n-dipropylamine 99 %, Aldrich) were prepared in a similar way to that described by Davis et al. [16]. A 1 L batch reactor was filled with 500 mL of the reaction gel and stirred at 350 rpm. A heating blanket was used to achieve and maintain reflux (99 C). The heating rate was 5 /min. During heating, the gel pH increased spontaneously from 3.5 to 6. The control of the pH to within ±0.1 pH units was achieved using a pH-stat unit, by adding 3 M HNO 3 . At the end of each experiment, the reactor was quenched with cold water and the products recovered by slurrying the reactor contents with water, decanting the supernatant liquid, filtering, washing with deionized water, and drying overnight at room temperature. The products were characterized by quantitative X-ray diffraction (XRD) using a Rigaku Rotaflex RU-200B instrument with Cu Ka radiation (1.5418 ) and AlPO 4 hydrates of approximately 95 % purity as standards.

Bio-inspired apatite nanoparticles precipitated in the presence of citrate ions at increasing maturation times are characterized in terms of structure, size, morphology, and composition through advanced X-ray total scattering techniques. The origin of the platy crystal morphology, breaking the hexagonal symmetry, and the role of citrate ions is explored. By cross-coupling the size and shape information of crystal domains with those obtained by atomic force microscopy on multidomain nanoparticles, a plausible mechanism underlying the amorphous-to-crystal transformation is reconstructed. In the present study, citrate plays the distinct roles of inducing the platy morphology of the amorphous precursor and controlling the thickness of the Ca-defi cient apatite nanocrystals. These fi ndings can open new scenarios also in bone mineralization, where citrate might have a broader role to play than has been thought to date.

Demineralization of dental hard tissue is a widespread problem and the main responsible for dental caries and dentin hypersensitivity. The most promising strategies to induce the precipitation of new mineral phase are the application of materials releasing gradually Ca2+ and PO43− ions or mimicking the mineral phase of the host tissue. However, the design of formulations covering both processes is so far a challenge in preventive dentistry. In this work, we have synthesized innovative biomimetic amorphous calcium phosphate (ACP), which has been, for the first time, doped with fluoride ions (FACP) to obtain materials with enhanced anti-caries and remineralizing properties. Significantly, the doping with fluoride (F) did not vary the physico-chemical features of ACP but resulted in a faster conversion to the crystalline apatite phase in water, as observed by in-situ time-dependent Raman experiments. The efficacy of the as synthesized ACP and FACP samples to occlude dentinal tubules and induce enamel remineralization has been tested in vitro in human molar teeth. The samples showed good ability to partially occlude the tubules of acid-etched dentin and to restore demineralized enamel into its native structure. Results demonstrate that ACP and FACP are promising biomimetic materials in preventive dentistry to hinder demineralization of dental hard tissues.