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

Abstract

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

Effect of preparation conditions on the nanostructure of hydroxyapatite and brushite phases

“…It is observable that all samples are classified as B-type (where CO 3 2-substitutes phosphate position) (Ahmed et al 2014). It should be mentioned that carbonate substitution has a critical role in the crystal structure, stability and biological activity (Morales et al 2013;Minh et al 2013).…”

“…Figure 6a illustrates the morphology of the nanobrushite sample at pH 5 which appears as homogenous surface with irregular particles, a small amount of apparent porosity. This irregularity appears on the surface as roughness, which could improve the osseointegration as well as the biocompatibility leading to the positive effect on inflammatory reactions (Morales et al 2013;Müller 2010). For the sample prepared at pH 6; Fig.…”

Elucidation of differential mineralisation on native and regenerated silk matrices

“…A major finding was that the material properties such as surface roughness, specific surface area, tortuosity and secondary conformation of fibroin changed significantly with respect to the 3D matrix architecture as well as the nature of fibroin (native versus regenerated) and were critical factors in determining selective nucleation and growth of HA on fibroin (Figure 1). While the contribution of these intrinsic material properties towards modulating cellular behaviour have been highlighted [3,36], yet their direct implication on the mineralisation potential have not been addressed.…”

“…In addition, HA exhibits intimate chemical bonding between the organic-inorganic elements with an overall poor crystallinity [2]. Growing interest in the development of biomimetic synthetic HA-based composites is primarily due to; (i) developing synthetic "analogs" of HA to be able to precisely replicate the morphology and chemistry of bone tissue [3], (ii) HA coating on constructs for improving osteoconductivity and (iii) evaluating the bioactivity of novel materials by studying the nucleation and growth of "bone-like" apatite upon immersion in supersaturated fluids such as simulated body fluid (SBF) [4]. However, there are still gaps in understanding the nature of such depositions; surface features and crystallisation phase especially with respect to the morphology and chemistry of the base material.…”

“…For instance, hydrophobic crystalline fractions of silk fibroin with a 40 kDa MW are weak templates for mineralisation, whereas 2-10 kDa MW hydrophilic chains are strong templates [20]. Moreover, the specific surface area of the biomaterial plays a crucial role in determining the amount of apatite formation, as a larger hydrated layer of organic template with strong tendency for ion exchange facilitates higher precipitation of nanocrystalline HA [3]. Therefore, in the present study, we aim to address the following three specific questions with respect to in vitro mineralisation potential of silk fibroin; (i) the role of fibroin as an osteogenic substrate (in regenerated versus native form) in regulating nanocrystalline apatite synthesis, (ii) whether change in 3D morphology (lyophilised versus braid) influences HA deposition on fibroin and (iii) whether the modified 3D morphology and resultant variation in the intrinsic material properties modulate the extent of mineralisation.…”