Paracrystalline Disorder from Phosphate Ion Orientation and Substitution in Synthetic Bone Mineral

Enhancing the zircon yield through the addition of calcium phosphates into ZrO2-SiO2 binary systems: synthesis and structural, morphological, mechanical and in vitro analysis

The crystallization of ZrSiO4 is generally accomplished by the addition of mineralizers into ZrO2-SiO2 binary oxides. The current investigation aimed to investigate the effect of adding calcium phosphates into ZrO2-SiO2 binary oxides on the yield of ZrSiO4. The concentration of calcium phosphate additions were varied to obtain ZrSiO4 that fetches improved mechanical and biological properties for application in hard tissue replacements. The findings highlight the significant role of Ca2+ and P5+ in triggering the ZrSiO4 formation via their accommodation at the Zr4+ and Si4+ sites. Especially, calcium phosphate additions trigger the t- → m-ZrO2 transition beyond 1000 °C, which consequently reacts with SiO2 to promote ZrSiO4 formation. Calcium phosphates are accommodated at the lattice sites of ZrSiO4 with a maximum limit of 20 mol%, beyond which the crystallization of β-Ca3(PO4)2 is noticed. The optimum amount of 20 mol% of calcium phosphates displayed a better strength than that of all the investigated specimens. More than 80% of cell viability in MG-63 cells was invariably determined in all the calcium phosphate-added ZrSiO4 systems.

A novel route to enhance the dissolution of apatite: Structural incorporation of hydrogen phosphate

Potential use of hydroxyapatite nanoparticles (HANPs) [Ca₁₀(PO₄)₆(OH)₂] as slow P-release fertilizer (SRF) has recently attracted wider attention. However, commercially available HANP (with Ca/P ratio = 1.667) is the least soluble calcium phosphate and thus limits its full potential as an SRF in agronomic applications. In this research, we sought to enhance the dissolution rate of HANPs by enriching hydrogen phosphate (HPO₄^2-) species in the phosphate (PO₄^3-) structural sites. Seven different types of pure crystalline HANPs were synthesized at a range of Ca/P ratio from 1.46 (at pH 6.0) to 2.10 (at pH 12.0). Complementary results from FTIR and solid-state ³¹P MAS NMR spectroscopies showed that HPO₄^2- species is most abundant in HANPs crystallized at pH 6.0 and gradually depleted at higher pH products. The rate of depletion of HPO₄^2- species is proportional to the increase in carbonate incorporation into the HANP lattice, which preferentially forms B-type carbonated HANPs. The enhanced dissolution rate of HANPs due to hydrogen phosphate incorporation was tested using a flow-through macro-dialysis system that limits the partial transition of HANPs to other solid phases, which otherwise interfere with dissolution. The results show that the dissolution rate of HANPs increased with decreasing pH of synthesis and was highest in HANPs at pH 6.0. The dissolution rate differed by ten times between HANPs synthesized at pH 7.0 and 10.0. Overall, the atom-efficient synthetic route developed and the ability to tune the dissolution rate of HANPs are significant steps forward in improving the P-release efficiency of a potent SRF and is expected to contribute to efforts toward enhancing agricultural sustainability.

Investigating pair distribution function use in analysis of nanocrystalline hydroxyapatite and carbonate-substituted hydroxyapatite

Hydroxyapatite (HA) is a complex material, which is often nanocrystalline when found within a biological setting. This work has directly compared the structural characteristics derived from data collected using a conventional laboratory-based X-ray diffractometer with those collected from a dedicated pair distribution function (PDF) beamline at Diamond Light Source. In particular, the application of PDF analysis methods to carbonated HA is evaluated. 20 synthetic samples were measured using both X-ray diffraction (XRD) and PDFs. Both Rietveld refinement (of laboratory XRD data) and real-space refinement (of PDF data) were used to analyse all samples. The results of Rietveld and real-space refinements were compared to evaluate their application to crystalline and nanocrystalline hydroxyapatite. Significant relationships were observed between real-space refinement parameters and increasing carbonate substitution. Understanding the local order of synthetic hydroxyapatite can benefit several fields, including both biomedical and clinical settings.

Internalization of Fluoride in Hydroxyapatite Nanoparticles

Hydroxyapatite (HAP) is a cost-effective material to remove excess levels of fluoride from water. Historically, HAP has been considered a fluoride adsorbent in the environmental engineering community. This paper substantiates an uptake paradigm that has recently gained disparate support: assimilation of fluoride to bulk apatite lattice sites in addition to surface lattice sites. Pellets of HAP nanoparticles (NPs) were packed into a fixed-bed media filter to treat solutions containing 30 mg-F/L (1.58 mM) at pH 8, yielding an uptake of 15.97 ± 0.03 mg-F/g-HAP after 864 h. Solid-state ¹⁹F and ¹³C magic-angle spinning nuclear magnetic resonance spectroscopy demonstrated that all removed fluoride is apatitic. A transmission electron microscopy analysis of particle size distribution, morphology, and crystal habit resulted in the development of a model to quantify adsorption and total fluoride capacity. Low- and high-estimate median adsorption capacities were 2.40 and 6.90 mg-F/g-HAP, respectively. Discrepancies between experimental uptake and adsorption capacity indicate the range of F^- that internalizes to satisfy conservation of mass. The model was developed to demonstrate that F^- internalization in HAP NPs occurs under environmentally relevant conditions and as a tool to understand the extent of F^- internalization in HAP NPs of any kind.

Hydrothermal synthesis of perfectly shaped micro- and nanosized carbonated apatite

Herein we present a Ca[EDTA]-based synthesis and comparative study of perfectly shaped plate-like, rod–like, and prism-like carbonated apatites.

New insights into the application of pair distribution function studies to biogenic and synthetic hydroxyapatites

Biogenic and synthetic hydroxyapatites are confounding materials whose properties remain uncertain, even after years of study. Pair distribution function (PDF) analysis was applied to hydroxyapatites in the 1970's and 1980's, but this area of research has not taken full advantage of the relatively recent advances in synchrotron facilities. Here, synchrotron X-ray PDF analysis is compared to techniques commonly used to characterise hydroxyapatite (such as wide angle X-ray scattering, Fourier-transform infrared spectroscopy and thermogravimetric analysis) for a range of biogenic and synthetic hydroxyapatites with a wide range of carbonate substitution. Contributions to the pair distribution function from collagen, carbonate and finite crystallite size were examined through principal component analysis and comparison of PDFs. Noticeable contributions from collagen were observed in biogenic PDFs when compared to synthetic PDFs (namely r < 15 Å), consistent with simulated PDFs of collagen structures. Additionally, changes in local structure were observed for PDFs of synthetic hydroxyapatites with differing carbonate content, notably in features near 4 Å, 8 Å and 19 Å. Regression models were generated to predict carbonate substitution from peak position within the PDFs.

HRTEM study of individual bone apatite nanocrystals reveals symmetry reduction with respect to P63/m apatite

In this study we present the first crystal structure model for bone apatite based on the analysis of individual nanocrystals by high resolution transmission electron microscopy (HRTEM). Crystallographic image processing of the obtained HRTEM images from different projections indicates symmetry reduction with respect to P6₃/m stoichiometric apatites and the presence of threefold symmetry along the c axis. Based on HRTEM observations and the measured Ca/P = 2 ratio we propose a structural model with phosphate-to-carbonate substitution and O vacancies localized along c axis, which explains the observed loss of 6₃ screw axis parallel, and the shift of mirror plane perpendicular to the c axis. Also, the presence of non-equivalent (010) surfaces has been proven. These results on the atomic structure of bone apatite nanocrystals contribute to the understanding of their biochemically controlled nucleation processes.

Altered Bone Mechanics, Architecture and Composition in the Skeleton of TIMP-3-Deficient Mice

Tissue inhibitor of metalloproteinases-3 (TIMP-3) maintains a healthy extracellular matrix by regulating matrix metalloproteinases (MMP), disintegrin-metalloproteinases (ADAM), and disintegrin-metalloproteinases with ThromboSpondin-like motifs (ADAMTS) activity. Currently, there is a need for a comprehensive understanding of the effects of TIMP-3 on the bone quality and integrity. In this study, we examined the mechanical, morphological, and compositional properties of TIMP-3 knock out (Timp-3 ^-/-) mouse bone. We hypothesize that the lack of TIMP-3 plays an important role in maintaining the overall bone integrity. Mechanical properties of humeri, lumbar vertebrae, and femurs from Timp-3 ^-/- mice were determined using 3-point bending, compression, and notched 3-point bending, respectively. Morphological properties of the humeral cortical and trabecular bone and the caudal vertebrae cortical bone were evaluated using micro-computed tomography, while the composition of the femoral cortical and trabecular bone was examined using Fourier transform infrared spectroscopic imaging. Our results revealed that the integrity of the Timp-3 ^-/- bone is compromised due to changes in its composition, structure, and mechanics. Reductions in the yield and ultimate load and stress capacity, and loss in bone fracture toughness were attributed to reduced density and thickness, and increased porosity of cortical bone. Thin trabeculae were dense, highly connected, and closely packed in Timp-3 ^-/- bone. Furthermore, altered cortical and trabecular bone mineralization and increased compositional heterogeneity were found in Timp-3 ^-/- bone, all being indicative of high bone remodeling. In conclusion, this study suggests that the lack of TIMP-3 is detrimental to bone development and maintenance.