Thermal conversion of octacalcium phosphate into hydroxyapatite

Building Osteogenic Microenvironments with a Double-Network Composite Hydrogel for Bone Repair

The critical factor determining the in vivo effect of bone repair materials is the microenvironment, which greatly depends on their abilities to promote vascularization and bone formation. However, implant materials are far from ideal candidates for guiding bone regeneration due to their deficient angiogenic and osteogenic microenvironments. Herein, a double-network composite hydrogel combining vascular endothelial growth factor (VEGF)-mimetic peptide with hydroxyapatite (HA) precursor was developed to build an osteogenic microenvironment for bone repair. The hydrogel was prepared by mixing acrylated β-cyclodextrins and octacalcium phosphate (OCP), an HA precursor, with gelatin solution, followed by ultraviolet photo-crosslinking. To improve the angiogenic potential of the hydrogel, QK, a VEGF-mimicking peptide, was loaded in acrylated β-cyclodextrins. The QK-loaded hydrogel promoted tube formation of human umbilical vein endothelial cells and upregulated the expression of angiogenesis-related genes, such as Flt1 , Kdr , and VEGF , in bone marrow mesenchymal stem cells. Moreover, QK could recruit bone marrow mesenchymal stem cells. Furthermore, OCP in the composite hydrogel could be transformed into HA and release calcium ions facilitating bone regeneration. The double-network composite hydrogel integrated QK and OCP showed obvious osteoinductive activity. The results of animal experiments showed that the composite hydrogel enhanced bone regeneration in skull defects of rats, due to perfect synergistic effects of QK and OCP on vascularized bone regeneration. In summary, improving the angiogenic and osteogenic microenvironments by our double-network composite hydrogel shows promising prospects for bone repair.

Octacalcium phosphate: Innovative vehicle for the local biologically active substance delivery in bone regeneration

Disadvantages of conventional drug delivery systems (DDS), such as systemic circulation, interaction with physiochemical factors, reduced bioavailability, and insufficient drug concentration at bone defect site, have underlined the importance of developing efficacious local drug delivery systems. Octacalcium phosphate (OCP) is presumed to be the precursor of biologically formed apatite, owing to its similarity to hydroxyapatite (HAp) and readiness to convert to it. Specific crystal structure of OCP is constructed of compiled apatite layers and water layers, which make possible the incorporation of various ions in its structure, making it feasible to alter the overall effect OCP has in the system. Next to that intrinsic property, characteristics as high solubility, biodegradability and osteoconductivity have made it indispensable to tailor OCP as a carrier material. In this review, we present the main characteristics and progress done on utilizing OCP as an innovative vehicle and provide suggestions for possible research pathways and advantages for local drug delivery in bone tissue engineering. STATEMENT OF SIGNIFICANCE: Octacalcium phosphate (OCP), being a precursor to biologically formed apatite, has many assets when compared to other calcium phosphates. Owing to its highly pertinent structure, it is being used as a vehicle for biologically active substances or ions for bone regeneration. However, orchestrating drug delivery systems with OCP, in order to achieve the best possible outcome, is still a pioneering concept, and the all-encompassing data is still scarce. Although several articles have been published on this matter, to this date there is no systematic overview pointing out the benefits that OCP can bring in the field of drug delivery. Here we offer a comprehensive overview, starting from the OCP synthesis to its structure, morphology, and the biological significance OCP has.

A thermostability perspective on enhancing physicochemical and cytological characteristics of octacalcium phosphate by doping iron and strontium

Investigation of thermostability will lead the groundbreaking of unraveling the mechanism of influence of ion-doping on the properties of calcium phosphates. In this work, octacalcium phosphate (OCP), a metastable precursor of biological apatite, was used as a stability model for doping ions (Fe³⁺ and Sr²⁺) with different ionic charges and radii. After treated under hot air at different temperatures (110–200 °C), the phase, morphology, structure, physicochemical properties, protein affinity, ions release, and cytological responses of the ion-doped OCPs were investigated comparatively. The results showed that the collapse of OCP crystals gradually occurred, accompanying with the dehydration of hydrated layers and the disintegration of plate-like crystals as the temperature increased. The collapsed crystals still retained the typical properties of OCP and the potential of conversion into hydroxyapatite. Compared to the undoped OCP, Fe-OCP, and Sr-OCP had lower and higher thermostability respectively, leading to different material surface properties and ions release. The adjusted thermostability of Fe-OCP and Sr-OCP significantly enhanced the adsorption of proteins (BSA and LSZ) and the cytological behavior (adhesion, spreading, proliferation, and osteogenic differentiation) of bone marrow mesenchymal stem cells to a varying extent under the synergistic effects of corresponding surface characteristics and early active ions release. This work paves the way for understanding the modification mechanism of calcium phosphates utilizing ion doping strategy and developing bioactive OCP-based materials for tissue repair.

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OCP was used as a stability model for doping ions with different charges and radii.

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Collapse of OCP crystals occurred with structural dehydration after heat treatment.

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Fe and Sr doping altered the thermostability of OCP crystals in an opposite way.

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The thermostable difference affected the surface properties and ion release of OCP.

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Active surface and ion release of OCP synergistically mediated its biocompatibility.

Tuning the c-Axis Orientation of Calcium Phosphate Hybrid Thin Films Using Polymer Templates

The orientation of the c-axis in octacalcium phosphate (OCP) nanocrystals that were incorporated into hybrid thin films was successfully tuned using poly(vinyl alcohol) (PVA) thin-film templates of varying thicknesses. This approach was inspired by biomineralization. Thicker PVA templates enhanced the c-axis orientation of the OCP crystals perpendicular to the substrate. Using this approach with a 900 nm thick PVA template, OCP/PVA hybrid thin films (1.8 μm thick) with a c-axis orientation perpendicular to the substrate were formed. Hydroxyapatite (HAP) hybrid thin films that also exhibited a perpendicular c-axis orientation were obtained through the topotactic transformation of the OCP/PVA hybrid thin films in aqueous solution. The thickness change of the polymer templates had a significant effect on the structure of the OCP nanocrystals in the hybrid thin films. The structural control of the OCP hybrid thin films that were formed through the biomineralization-inspired approach allowed the formation of HAP hybrid thin films with controlled structures.

One-step hydrothermal synthesis of carbonated hydroxyapatite porous microspheres with a large and uniform size regulated by l-glutamic acid

Carbonated porous hydroxyapatite microspheres with an average diameter of about 64 μm and interconnected pores were hydrothermally synthesized by a one-step method.

Applications of NMR crystallography to problems in biomineralization: refinement of the crystal structure and 31P solid-state NMR spectral assignment of octacalcium phosphate

By combining X-ray crystallography, first-principles density functional theory calculations, and solid-state nuclear magnetic resonance spectroscopy, we have refined the crystal structure of octacalcium phosphate (OCP), reassigned its (31)P NMR spectrum, and identified an extended hydrogen-bonding network that we propose is critical to the structural stability of OCP. Analogous water networks may be related to the critical role of the hydration state in determining the mechanical properties of bone, as OCP has long been proposed as a precursor phase in bone mineral formation. The approach that we have taken in this paper is broadly applicable to the characterization of crystalline materials in general, but particularly to those incorporating hydrogen that cannot be fully characterized using diffraction techniques.

Effect of strontium and gelatin on the reactivity of alpha-tricalcium phosphate

The hydrolysis reaction of alpha-tricalcium phosphate (alpha-TCP) is of great interest because of its widespread use in the preparation of biomaterials for hard tissue repair. The aim of this study was to investigate how this reaction is influenced by the presence of a bioactive ion, Sr(2+), and of a biopolymer, gelatin, which were previously reported to affect the setting reaction of alpha-TCP-based cements. Hydrolysis experiments were carried out at different Sr(2+) concentrations (0, 5, 10, 20 at.%) in solutions at different gelatin concentrations (0, 0.1, 0.5, 1.0 wt.%). The results indicate that Sr(2+) delays the conversion of alpha-TCP into calcium-deficient hydroxyapatite (CDHA). The structural and morphological modifications of CDHA obtained from solutions at increasing Sr(2+) concentrations indicate that during hydrolysis strontium enters the structure of CDHA, where it partially substitutes for calcium. On the contrary, alpha-TCP hydrolysis rate increases on increasing gelatin concentration. Gelatin promotes conversion of alpha-TCP into octacalcium phosphate, and strongly interacts with the nucleating and growing crystals.

Electron microscopy of octacalcium phosphate in the dental calculus

The purpose of this study was to morphologically demonstrate the presence of octacalcium phosphate in the dental calculus by judging from the crystal lattice image and its rapid transformation into apatite crystal, as part of our serial studies on biomineral products. We also aimed to confirm whether the physical properties of octacalcium phosphate are identical with those of the central dark lines observed in crystals of ordinary calcifying hard tissues. Electron micrographs showed that crystals of various sizes form in the dental calculus. The formation of each crystal seemed to be closely associated with the organic substance, possibly originating from degenerated microorganisms at the calcification front. Many crystals had an 8.2-A lattice interval, similar to that of an apatite crystal. Furthermore, some crystals clearly revealed an 18.7-A lattice interval and were vulnerable to electron bombardment. After electron beam exposure, this lattice interval was quickly altered to about half (i.e. 8.2 A), indicating structural conversion. Consequently, a number of apatite crystals in the dental calculus are possibly created by a conversion mechanism involving an octacalcium phosphate intermediate. However, we also concluded that the calcification process in the dental calculus is not similar to that of ordinary calcifying hard tissues.

Biocompatible nanocrystalline octacalcium phosphate thin films obtained by pulsed laser deposition

We extended for the first time pulsed laser ablation to the deposition of octacalcium phosphate Ca8H2(PO4)6.5H2O (OCP) thin films. The depositions were performed with a pulsed UV laser source (lambda=248 nm, tau> or =20 ns) in a flux of hot water vapors. The targets were sintered from crystalline OCP powder and the laser ablation fluence was set at values of 1.5-2 J/cm2. During depositions the collectors, Si or Ti substrates, were maintained at a constant temperature within the range 20-200 degrees C. The resulting structures were submitted to heat treatment in hot water vapors for up to 6 h. The best results were obtained at a substrate temperature of 150 degrees C during both deposition and post-deposition treatment. High-resolution electron microscopy and XRD at grazing incidence indicated that the coatings obtained were made of nanocrystalline OCP. Cross-section TEM investigations showed that the coatings contained droplets stacked on Ti substrates as well as distributed across the entire thickness of the arborescence-like structure layers. The results of WST-1 assay, cell adherence, DNA replication, and caspase-1 activity confirmed the good biocompatibility of the coatings.

Analysis of the calcium distribution in predentine by EELS and of the early crystal formation in dentine by ESI and ESD

Predentine is a collagen-rich extracellular matrix between the odontoblasts and the dentine with a width of about 15-20 microns. Electron energy-loss spectroscopy of rat incisors shows a significantly higher calcium content in the predentine at the predentine-dentine border than in the middle region of the predentine. At the predentine-dentine border in the dentine, the calcium and the phosphate groups combine to form apatite crystallites. Electron spectroscopic diffraction with zero-loss filtering revealed that the earliest crystallites contain only Debye-Scherrer rings of apatite, which are fewer in number and more diffuse than the diffraction rings from the mature crystallites. We therefore conclude that the early crystallites still contain lattice defects, which are annealed out to some degree with crystal growth. Electron spectroscopic imaging with zero-loss filtering also showed that the earliest crystallites are chains of dots (or small islands); they build up strands composed of islands, which rapidly acquire a needle-like character and coalesce laterally to form ribbon-or plate-like crystallites. The parallel strands sometimes appear to reinforce the macroperiod of the collagen microfibrils (67 nm) by tiny holes without any crystal-substance lined up perpendicular to the parallel strands of the crystallites.