Progress on the preparation of nanocrystalline apatites and surface characterization: Overview of fundamental and applied aspects

Nanocrystalline 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.