Some compositional and structural features of mature bone mineral particles remain unclear. They have been described as calcium-deficient and hydroxyl-deficient carbonated hydroxyapatite particles in ...which a fraction of the PO
lattice sites are occupied by HPO
ions. The time has come to revise this description since it has now been proven that the surface of mature bone mineral particles is not in the form of hydroxyapatite but rather in the form of hydrated amorphous calcium phosphate. Using a combination of dedicated solid-state nuclear magnetic resonance techniques, the hydrogen-bearing species present in bone mineral and especially the HPO
ions were closely scrutinized. We show that these HPO
ions are concentrated at the surface of bone mineral particles in the so-called amorphous surface layer whose thickness was estimated here to be about 0.8 nm for a 4-nm thick particle. We also show that their molar proportion is much higher than previously estimated since they stand for about half of the overall amount of inorganic phosphate ions that compose bone mineral. As such, the mineral-mineral and mineral-biomolecule interfaces in bone tissue must be driven by metastable hydrated amorphous environments rich in HPO
ions rather than by stable crystalline environments of hydroxyapatite structure.
Nanocrystalline apatites are major constituents of hard tissues, and attempts are made worldwide to prepare synthetic analogs. However the impact of synthesis/postsynthesis parameters is often ...disregarded. Based on an updated knowledge on such compounds, we inspected the effects of synthesis parameters (maturation time, temperature, pH, nature of counter-ions) and post-treatments (re-immersion in aqueous media, thermal treatment) on physicochemical characteristics. Great modifications were noticed during the 3 first days of maturation, where a progressive evolution of the apatite phase (localized in the core of the nanocrystals) toward stoichiometry was observed at the expense of the non-apatitic surface layer which progressively disappears. Similar trends were also evidenced for maturation run under increasing temperatures, studied here in the range 20–100 °C. pH impacted more specifically the chemical composition. The nature of the counter-ion in the starting phosphate salt influenced composition and nonstoichiometry, depending on its (in)ability to be incorporated in the lattice. Freeze-drying allowed to preserve a high surface reactivity, although further evolutions were noticed after re-immersion. Effects of a thermal treatment of dried samples were unveiled, suggesting a denaturation of the hydrated layer on the nanocrystals. This work underlines the necessity of a good control of synthesis/postsynthesis parameters for the production of biomimetic apatites.
A biomaterial that is both bioactive and capable of controlled drug release is highly attractive for bone regeneration. In previous works, we demonstrated the possibility of combining activated ...carbon fiber cloth (ACC) and biomimetic apatite (such as calcium-deficient hydroxyapatite (CDA)) to develop an efficient material for bone regeneration. The aim to use the adsorption properties of an activated carbon/biomimetic apatite composite to synthetize a biomaterial to be used as a controlled drug release system after implantation. The adsorption and desorption of tetracycline and aspirin were first investigated in the ACC and CDA components and then on ACC/CDA composite. The results showed that drug adsorption and release are dependent on the adsorbent material and the drug polarity/hydrophilicity, leading to two distinct modes of drug adsorption and release. Consequently, a double adsorption approach was successfully performed, leading to a multifunctional and innovative ACC-aspirin/CDA-tetracycline implantable biomaterial. In a second step, in vitro tests emphasized a better affinity of the drug (tetracycline or aspirin)-loaded ACC/CDA materials towards human primary osteoblast viability and proliferation. Then, in vivo experiments on a large cortical bone defect in rats was carried out to test biocompatibility and bone regeneration ability. Data clearly highlighted a significant acceleration of bone reconstruction in the presence of the ACC/CDA patch. The ability of the aspirin-loaded ACC/CDA material to release the drug in situ for improving bone healing was also underlined, as a proof of concept. This work highlights the possibility of bone patches with controlled (multi)drug release features being used for bone tissue repair.
Calcium phosphate apatites are inorganic compounds encountered in many different mineralized tissues. Bone mineral, for example, is constituted of nanocrystalline nonstoichiometric apatite, and the ...production of “analogs” through a variety of methods is frequently reported. In another context, the ability of solid surfaces to favor the nucleation and growth of “bone-like” apatite upon immersion in supersaturated fluids such as SFB is commonly used as one evaluation index of the “bioactivity” of such surfaces. Yet, the compounds or deposits obtained are not always thoroughly characterized, and their apatitic nature is sometimes not firmly assessed by appropriate physicochemical analyses. Of particular importance are the “actual” conditions in which the precipitation takes place. The precipitation of a white solid does not automatically indicate the formation of a “bone-like carbonate apatite layer” as is sometimes too hastily concluded: “all that glitters is not gold.” The identification of an apatite phase should be carefully demonstrated by appropriate characterization, preferably using complementary techniques. This review considers the fundamentals of calcium phosphate apatite characterization discussing several techniques: electron microscopy/EDX, XRD, FTIR/Raman spectroscopies, chemical analyses, and solid state NMR. It also underlines frequent problems that should be kept in mind when making “bone-like apatites.”
Bone is a natural nanocomposite. Its mineral component is nanocrystalline calcium phosphate apatite, whose synthetic biomimetic analogs can be prepared by wet chemistry. The initially formed ...crystals, whether biological or synthetic, exhibit very peculiar physicochemical features. In particular, they are nanocrystalline, nonstoichiometric, and hydrated. The surface of the nanocrystals is covered by a non-apatitic hydrated layer containing mobile ions, which may explain their exceptional surface reactivity. For their precipitation in vivo or in vitro, for their evolution in solution, for the 3D organization of the nanocrystals, and for their consolidation to obtain bulk ceramic materials, water appears to be a central component that has not received much attention. In this mini-review, we explore these key roles of water on the basis of physicochemical and thermodynamic data obtained by complementary tools including FTIR, XRD, ion titrations, oxide melt solution calorimetry, and cryo-FEG-SEM. We also report new data obtained by DSC, aiming to explore the types of water molecules associated with the nanocrystals. These data support the existence of two main types of water molecules associated with the nanocrystals, with different characteristics and probably different roles and functions. These findings improve our understanding of the behavior of bioinspired apatite-based systems for biomedicine and also of biomineralization processes taking place in vivo, at present and in the geologic past. This paper is thus intended to give an overview of the specificities of apatite nanocrystals and their close relationship with water.
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.
Experimental thermodynamic properties of phosphate apatites are reviewed and commented. Predictive methods are developed and tested here for apatite compounds, allowing reliable estimations of ΔHf°, ...ΔGf° and S°, not only for end-members but also for solid solutions, non-stoichiometric and hydrated samples. A periodic table of recommended thermodynamic properties for 33 apatite systems is generated, and relative stabilities are pointed out.
•Experimental thermodynamic properties of phosphate apatites are reviewed.•Predictive thermodynamic methods are investigated for estimating missing data.•An additive model is developed to estimate ΔHf°, ΔGf° and S° for phosphate apatites.•End-members, solid solutions, nonstoichiometric and hydrated samples are considered.•A periodic table of recommended thermodynamic properties for 33 systems is proposed.
Apatite minerals represent a major class of ionic compounds of interest to many disciplines including medical sciences, geology, anthropology, cosmology, environmental and nuclear sciences. Yet, these compounds have not received great attention from a thermodynamic viewpoint, and some diverging data – often drawn from molecular modeling assays – were reported. In this contribution, an extensive literature overview of available experimental-based data on M10(PO4)6X2 apatites with M=Ca, Ba, Sr, Mg, Cd, Pb, Cu, Zn and X=OH, F, Cl or Br has first been made, on the basis of standard formation energetics (ΔHf° and ΔGf°) as well as entropy S° and molar heat capacity Cp,m°. The case of oxyapatite was also included. From this overview, it was then possible to identify general tendencies, evidencing in particular the primary role of electronegativity and secondarily of ionic size. Using the experimental data as reference, several predictive thermodynamic methods were then evaluated, including the volume-based-thermodynamics (VBT) method and a more advanced additive contributional model. In particular, the latter allowed obtaining estimates of thermodynamic data of phosphate apatites within a maximum of 1% of relative error, generally within 0.5%. Fitted hi, gi and si contributive parameters are given for bivalent cations and monovalent anions, so as to derive, by simple summation, coherent estimates of ΔHf°, ΔGf° and S° for any apatite composition, at T=298K. The model was shown to also lead to consistent estimates in cases of solid-solutions or even non-stoichiometric or hydrated phosphates apatites. Ultimately, a periodic table of recommended thermodynamic properties of 33 phosphate apatites end-members (at T=298K and 1bar) was established, with the view to serve as an easily readable reference database.
The incorporation of foreign ions, such as Mg2+, exhibiting a biological activity for bone regeneration is presently considered as a promising route for increasing the bioactivity of bone-engineering ...scaffolds. In this work, the morphology, structure, and surface hydration of biomimetic nanocrystalline apatites were investigated before and after surface exchange with such Mg2+ ions, by combining chemical alterations (ion exchange, H2O−D2O exchanges) and physical examinations (Fourier transform infrared spectroscopy (FTIR) and high-resolution transmission electron microscopy (HRTEM)). HRTEM data suggested that the Mg2+/Ca2+ exchange process did not affect the morphology and surface topology of the apatite nanocrystals significantly, while a new phase, likely a hydrated calcium and/or magnesium phosphate, was formed in small amount for high Mg concentrations. Near-infrared (NIR) and medium-infrared (MIR) spectroscopies indicated that the samples enriched with Mg2+ were found to retain more water at their surface than the Mg-free sample, both at the level of H2O coordinated to cations and adsorbed in the form of multilayers. Additionally, the H-bonding network in defective subsurface layers was also noticeably modified, indicating that the Mg2+/Ca2+ exchange involved was not limited to the surface. This work is intended to widen the present knowledge on Mg-enriched calcium phosphate-based bioactive materials intended for bone repair applications.
The present study focuses on tailoring the relative content of Fe3+ and Fe2+ ions incorporation into hydroxyapatite (HA) lattice, employing a hydrothermal approach in a closed vessel to minimize Fe2+ ...oxidation and secondary phase formation. Citrate molecules are used to regulate nanoparticle formation/stability, creating a mild reducing environment, while the impact of a stronger reducing agent, hydroxylamine, is explored. Fe3+ insertion was found to be less favoured than Fe2+, possibly due to charge imbalance. Iron doping significantly alters stoichiometry and crystallinity of HA, with Fe3+ enhancing OH− depletion. Morphological analysis reveals differences among samples, as induced by the different Fe ions incorporation: particularly Fe2+ ion incorporation is found to maintain rod-like structures, which changes upon Fe3+ presence. Overall, this study provides insights into controlled doping of HA with iron ions, vital for developing stable, redox-responsive nanomaterials applicable in cancer therapy and other applications where surface activity plays a relevant role.
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