Effect of ammonium carbonate on formation of calcium-deficient hydroxyapatite through double-step hydrothermal processing

Double-step hydrothermal processing is a process where powder compacts of calcium phosphates are exposed to vapor of solvent solution, followed by being immersed in the solution. In the present study, we investigated the effects of ammonium carbonate on formation of calcium-deficient hydroxyapatite (CDHA) through double-step hydrothermal processing. The synthesized CDHA has high crystallinity when the solution has relatively low concentration of the ammonium carbonate ranging from 0.01 to 0.25 mol dm⁻³. Carbonate content in the prepared samples were distinctly increased with increasing the concentration of ammonium carbonate to indicate formation of carbonate-containing calcium-deficient hydroxyapatite (CHAp) with low crystallinity. Morphology of the CHAp formed on the compacts varied progressively from rods and rosette-like shape to irregular shape with increase in the initial concentration of the ammonium carbonate in the solution. Application of ammonium carbonate in the double-step hydrothermal processing allows fabrication of irregular-shaped CDHA containing carbonate ions in both phosphate and hydroxide site, with low crystallinity, when the initial concentration of ammonium carbonate was 0.5 mol dm⁻³ and more.     

Carbonate-containing hydroxyapatite derived from calcium tripolyphosphate gel with urea

Carbonate containing hydroxyapatite (CO₃HAp) is one of the candidate materials as a bioresorbable bone substitute. In the present work, CO₃HAp was efficiently prepared by a hydrothermal treatment of calcium tripolyphosphate gel with urea at 140 ^∘C for 24 h. Chemical potential plots of the CO₃HAp for estimation of its dissolution behavior suggested that the CO₃HAp is more soluble than hydroxyapatite (HAp) and is as soluble as octacalcium phosphate (OCP) and/or β -tricalcium phosphate (TCP). This material is expected to be applied to bioresorbable materials such as bone fillers.     

Ribbon-like and rod-like hydroxyapatite crystals deposited on titanium surface with electrochemical method

Homogeneous coatings were attained by electrochemical method in electrolytes containing Ca²⁺ and PO₄³⁻ ions with Ca/P ratio being 1.67. SEM observation showed that the hydroxyapatite (HAp,Ca₁₀(PO₄)₆(OH)₂) crystals prepared with higher concentration electrolyte (4 × 10^− 2 M Ca²⁺) are ribbon-like with thickness of nanometer size, a morphology seldom reported previously. In an electrolyte of lower concentration (6 × 10^− 4 M Ca²⁺), the HAp crystals formed are rod-like with a hexagonal cross section and diameter of about 70–80 nm. XRD patterns and IR spectra confirmed that the coatings consist of HAp crystals. TEM micrographs and SAD indicated that the longitude direction for both ribbon-like and rod-like crystal is [002], and the flat surface of the ribbon is (110). HRTEM showed that the ribbon-like crystal is a mixture of HAp and octacalcium phosphate (OCP, Ca₈H₂(PO₄)₆.5H₂O).     

Biomimetic mineralization of calcium phosphate crystals in polyacrylamide hydrogel: Effect of concentrations of calcium and phosphate ions on crystalline phases and morphology

Calcium phosphate crystals were synthesized in polyacrylamide (PAAm) hydrogel, and the effects of the concentrations of calcium and phosphate ions on the crystalline phases and morphology were investigated. PAAm hydrogels containing diammonium hydrogen phosphate ((NH₄)₂HPO₄) were transformed into calcified materials by diffusion of calcium ions from calcium nitrate (Ca(NO₃)₂) aqueous solution into the gels. Several kinds of calcium phosphate crystals were precipitated at various Ca(NO₃)₂ concentrations (0.5–4.0 mol·dm^? 3), or (NH₄)₂HPO₄ contents (3.6–21.6 mmol) in the gels. The crystalline phases were mainly determined by the (NH₄)₂HPO₄ content in the gels. When the (NH₄)₂HPO₄ content was ≥ 10.8 mmol, hydroxyapatite (HAp) formed near the interfaces between Ca(NO₃)₂ solution and the gels, whereas octacalcium phosphate (OCP) formed in gels with ≤ 10.8 mmol (NH₄)₂HPO₄. HAp crystals were granular in form and about 200 nm in diameter, and OCP crystals were spherulitic with diameter 10–70 μm.     

Synthesis of organic derived hydroxyapatite scaffold from pig bone waste for tissue engineering applications

Micro porous hydroxyapatite (HAp) had drawn great attention in the field of tissue engineering due to its numerous applications such as tissue regeneration, dental, drug delivery, and adsorption and desorption of substances etc. The chemical synthesis of HAp is often faced with the high cost of starting materials and often lacks the presence of beneficial ions which can promote biological reactions. This paper examined a novel application of pig bone waste for the synthesis of HAp via heat treatment between 600 and 1000 °C. Thus synthesized powder was characterized by X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FT-IR), Thermogravimetric analysis (TGA), Scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDX) and Transmission electron microscopy (TEM). XRD results revealed the main characteristic peaks of single phase HAp powder, while the presence of various functional groups such as PO₄^3?, CO₃^2? and OH^? corresponding to HAp were observed by FT-IR analysis. The elemental composition of as-synthesized HAp powder as observed by EDX showed the presence of Ca and P in addition to some beneficial metals such as Na, K, Mg and Si which plays vital roles in biological applications. SEM and TEM observation confirmed the microscopic sctructure of the as-synthesized HAp to be rod-like morphology with 38–52 nm in length. Porous HAp scaffold up to 65% porosity could be prepared using ammonium bicarbonate as pore forming agent. Therefore, bio-waste such as pig bones can be utilized in the synthesis of porous hydroxyapatite scaffold which can serve as an alternative for the conventional chemical method.     

Biomimetic synthesis of hierarchical crystalline hydroxyapatite fibers in large-scale

Crystalline hierarchical hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂, HAp)] fibers were successfully synthesized via a biomimetic route by using cotton cloth as a natural bio-template. The effects of pH value, aging time, ultrasonic cleaning time, and calcination temperature on the purity and morphology of the resulting hydroxyapatite (HAp) were monitored by scanning election microscope (SEM), X-ray diffraction (XRD), and infrared spectrophotometer (IR) to obtain an optimized reaction condition, namely, pH 9, ultrasonic cleaning for 1 min, aging for 24 h, and calcination at 600 °C for 4 h. We found that the natural cellulose could not only control the morphology of HAp but also lower its phase transformation temperature. The impact of this method lies in its low cost and successful production of large-scale patterning of three-dimensional hierarchical HAp fibers.     

Effects of pH and temperature on microstructure and morphology of hydroxyapatite/collagen composites synthesized in vitro

Hydroxyapatite/collagen composites (HAp/Col) were synthesized, in vitro, using the self-organization mechanism, by the co-precipitation of collagen, extracted from pork skin, and aqueous H₃PO₄ and Ca(OH)₂. The effects of pH and temperature, on the microstructure and morphology of HAp/Col composites, were extensively studied.It was shown that the yield of the composite is closely related to the pH value, during preparation. At high pH, the white precipitate of the composites was formed in a large quantity. In contrast, the yield was quite low, at low pH, owing to the dissolution of the composite precipitates. The electrophoretogram of the as-prepared composite shows that the α1(I) chain was dominant and that there was no protein residue in the supernatant of the reaction solution. Accordingly, it is reasonable to assume that the collagen molecules had completely reacted with hydroxyapatite.A closer look at the structural evolution of HAp/Col composites revealed that, at pH = 5, there was no sign of interfacial interaction between hydroxyapatite and collagen, during the first 36 h. In contrast, under alkaline conditions, the amount of α(I) chains was indeterminate, at the start of precipitation, while the concentration of β-chains decreased gradually. The chemical structure of HAp/Col, as determined by FTIR spectra, revealed that the bending vibration of phosphate contours can be attributed to the dissolution of HAp, at low pH. The crystalline of HAp was readily discerned, for all samples, as revealed by the XRD patterns. The morphology of the HAp/Col, prepared under neutral to alkaline conditions (7 ≤ pH ≤ 9), exhibited a compact, coral reef-like structure. In summary, HAp/Col composites can be synthesized, in vitro, under conditions native to animal physiology. The desirable conditions for synthesis are a higher pH (8 to 9) and a temperature of 40 °C.     

Nanocrystalline hydroxyapatite bioceramic using microwave radiation: Synthesis and characterization

In this work, we synthesized bioactive hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) ceramic powder in the lower-end of nano-regime using microwave radiation, which offers several advantages. The powder was synthesized using calcium nitrate tetrahydrate and sodium phosphate dibasic anhydrous as the starting materials. EDTA served as the complex reagent. The pH of the final suspension was adjusted to 9 by adding ammonium hydroxide. Applied microwave power of 600 W, pH of the suspension, mole ratio of Ca/P in the staring chemicals, and the chelating effect of EDTA served as the factors in the synthesis of nanocrystalline HAp powder. The synthesized powder was studied using various characterizing techniques viz., XRD, SEM, HR-TEM, EDS, TG/DTA and FT-IR to determine powder morphology, particle-size, crystallinity, phases, elemental composition and thermal behavior. Results confirmed highly crystalline nano-powder (5–30 nm) with elemental composition of Ca and P in HAp phase and possessed mixed (elliptical and rod-shape) morphology. Using the Scherrer formula, the average crystallite size was found to be 12 nm. The FT-IR confirmed that the powder is of typical apatite structure. Thermal analysis showed a remarkably lower initial dehydroxylation temperature, compared to micron sized HAp, as reported in literature.     

Bone cell–material interactions on metal-ion doped polarized hydroxyapatite

The objective of this work is to study the influence of Mg²⁺ and Sr²⁺ dopants on in vitro bone cell–material interactions of electrically polarized hydroxyapatite [HAp, Ca₁₀(PO₄)₆(OH)₂] ceramics with an aim to achieve additional advantage of matching bone chemistry along with the original benefits of electrical polarization treatment relevant to biomedical applications. To achieve our research objective, commercial phase pure HAp has been doped with MgO, and SrO in single, and binary compositions. All samples have been sintered at 1200 °C for 2 h and subsequently polarized using an external d.c. field (2.0 kV/cm) at 400 °C for 1 h. Combined addition of 1 wt.% MgO/1 wt.% SrO in HAp has been most beneficial in enhancing the polarizability in which stored charge was 4.19 μC/cm² compared to pure HAp of 2.23 μC/cm². Bone cell–material interaction has been studied by culturing with human fetal osteoblast cells (hFOB) for a maximum of 7 days. Scanning electron microscope (SEM) images of cell morphology reveal that favorable surface properties and dopant chemistry lead to good cellular adherence and spreading on negatively charged surfaces of both Sr²⁺ and Mg²⁺ doped HAp samples over undoped HAp. MTT assay results at 7 days show the highest viable cell densities on the negatively charged surfaces of binary doped HAp samples, while positive charged doped HAp surfaces exhibit limited cellular growth in comparison to neutral surfaces.     

Carbonate and fluoride incorporation in synthetic apatites: Comparative effect on physico-chemical properties and in vitro bioactivity in fetal bovine serum

CO₃- and/or F-substituted apatites have been considered as potential bone substitution material for dental and orthopedic applications. The objective of this study was to compare physico-chemical properties and in vitro bioactivity in fetal bovine serum (FBS) of apatites containing CO₃ and/or F. The results showed that CO₃ and F in apatites have opposite effects on crystallinity and solubility. Calcium deficient hydroxyapatite (HA) and F-substituted apatite (FA) partially transformed to beta-tricalcium phosphate (β-TCP) at temperature 950 °C. After immersion in FBS for 10 days, calcium deficient HA, FA, and CO₃-substituted apatite (CHA) and CO₃- and F-substituted apatite (CFA) pellet surfaces all showed formation of apatite.     

Microwave accelerated synthesis of nanosized calcium deficient hydroxyapatite

Rapid synthesis of calcium deficient hydroxyapatite (CDHA, Ca_10–x(HPO₄)_x(PO₄)_6–x(OH)_2–x) with Ca/P ratio 1.5 was done by precipitation using calcium nitrate tetra-hydrate and phosphoric acid and subsequently subjecting to microwave irradiation in a domestic microwave oven for 15 min. Transmission electron microscopy analysis shows needle like morphology of CDHA having length 16–39 nm and width 7–16 nm. The synthesized CDHA has the characteristic HPO^2–₄ vibration band at 875 cm^–1 in Fourier transform infrared (FT-IR) spectra. The X-ray powder diffraction (XRD) analysis shows a pattern corresponding to stoichiometric hydroxyapatite (HA) with broad peaks suggesting that CDHA particles were nanosized. Fourier transform Raman spectroscopy (FT-Raman) do not indicate any fluorescence band that is characteristic of non-stoichiometric HA. The thermal decomposition of CDHA to beta tricalcium phosphate (-TCP) was also studied for the additional confirmation. The nanosized CDHA was found to be stable up to 600°C.     

Biomimetic synthesis of enamel-like hydroxyapatite on self-assembled monolayers

Hydroxyapatite (HAp) crystals mimicking tooth enamel in chemical composition and morphology were formed on sulfonic-terminated self-assembled monolayer (SAM) in 1.5SBF with F⁻ at 50 °C for 7 days. F⁻ ions showed a marked effect on the composition and morphology of deposited HAp crystals. In the absence of F⁻ ions, HAp containing CO₃²⁻ were formed on SAM, and worm-like crystals of 200–300 nm in length aggregated to form a spherical morphology. When F⁻ was added, HAp crystals containing both CO₃²⁻ and F⁻ were formed on SAM. Needle-shaped crystals of high aspect ratio and 1–2 μm in length grew elongated along the c-axial direction. In addition, these needle-shaped crystals grew in bundles, mimicking HAp crystals in tooth enamel. After the process of ripening, the needles in bundle grew to large size of up to 10 μm in length, and still kept no crystal–crystal fusion like enamel HAp crystals. The formation of enamel-like HAp can be attributed to the substitute of F⁻ for OH⁻ by disturbing the normal progress of HAp formation on SAM. The results suggest potential applications in preparing a novel dental material by a simple method.     

Microwave-assisted hydrothermal synthesis of zinc oxide particles starting from chloride precursor

Zinc oxide (ZnO) was synthesized using a microwave assisted hydrothermal (MAH) process based on chloride/urea/water solution and under 800 W irradiation for 5 min. In the bath, Zn²⁺ ions reacted with the complex carbonate and hydroxide ions to form zinc carbonate hydroxide hydrate (Zn₄CO₃(OH)₆·H₂O), and the conversion from Zn₄CO₃(OH)₆·H₂O to ZnO was synchronously achieved by a MAH process. The as-prepared ZnO has a sponge-like morphology. However, the initial sponge-like morphology of ZnO could change to a net-like structure after thermal treatment, and compact nano-scale ZnO particles were finally obtained when the period of thermal treatment increased to 30 min. Pure ZnO nanoparticles was obtained from calcination of loose sponge-like ZnO particles at 500 °C. The analysis of optical properties of these ZnO nanoparticles showed that the intensity of 393 nm emission increased with the calcination temperature because the defects were reduced and the crystallinity was improved.     

Adsorption of gum Arabic on bioceramic nanoparticles

Surface modification agents can be used to tailor the surface chemistry and biological activity of bioceramic nanoparticles in very intriguing ways. However, the specific modes of interactions between macromolecules and nanoparticles can be difficult to characterize. The aim of this study was to investigate the adsorption of gum Arabic on hydroxyapatite (HAp) and magnetic nanoparticles (MNP) using the bicinchoninic acid (BCA) test. Gum Arabic (GA) is a natural gum that has been widely used as an emulsifying agent and shows promise for dispersing nanoparticles in aqueous solutions. The adsorption of GA onto HAp nanoparticles followed a Langmuir isotherm with an adsorption plateau occurring at 0.2 g GA/g HAp. The adsorption of GA onto MNP attained a maximum value of 0.6 g GA/g MNP, after which it decreased to approximately 0.2 g GA/g MNP. The maximum adsorption density of GA on both MNP and HAp is equivalent when normalized to the specific surface area (4 × 10^− 3 g GA/m²). Adsorbed GA molecules were displaced from the surface of HAp and MNP in the presence of phosphate ions.     

The impact of Cu(II) ions doping in nanostructured hydroxyapatite powder: A finite element modelling study for physico-mechanical and biological property evaluation

The synthesis of doped nanostructured materials with multifunctional properties and improved biocompatibility have immense potential for biomedical applications. In this present study, a facile wet chemical precipitation method was employed to synthesize hydroxyapatite (HAp) and different concentrations copper doped HAp, and Cu_x-HAp (x = 1, 2, and 4 mol%) nano materials. Sophisticated analytical and spectroscopic techniques were employed to confirm its physico-chemical properties, and morphological features. The synthesized HAp, Cu_x-HAp were further studied as a drug nanocarrier using doxorubicin hydrochloride (DOX) as a model drug, which results a maximum drug release of ～34.3% (at pH 4.5) for 1 mol% of Cu-HAp. The nanostructured materials were further used to fabricate scaffolds by employing gel-casting method. The finite element modeling theoretical approach was adopted, to correlate the force distribution over the developed scaffold during mechanical characterization. The in vitro study confirmed the nontoxic behavior of the HAp and Cu_x-HAp scaffolds using MG-63 cell line. The developed scaffold effectively facilitates and simulates the new cell attachment, growth, and proliferation on its surface with adequate (～7.87 MPa) compressive strength properties. The enhanced biocompatibility with improved mechanical stability of Cu_x-HAp nanomaterials could address some of the critical challenges in biomedical applications.     

Fabrication of titania/hydroxyapatite composite granules for photo-catalyst

The titania/hydroxyapatite composite granular photo-catalyst with novel microstructure was fabricated by the process based on the liquid immiscibility effect and followed by precalcination and hydrothermal treatment from commercially available powders of α-Tri-calcium phosphate and TiO₂. XRD, SEM, BET, optical microscopy and UV-vis spectrophotometer were applied to characterize the prepared photo-catalyst. Microstructure analysis indicated that the granule was weaved by rod-shaped hydroxyapatite crystals whose surface was covered by nano-sized TiO₂. In the composite granules, the active surface of anatase was retained effectively. With the hybridization of TiO₂ and HAp, a 16-nm blue-shift of absorption edge could be observed and the crystallinity of anatase could be enhanced by precalcination. The granules with the rod-shaped hydroxyapatite crystals performing as scaffold work as three-dimensional high porous, size-controllable small reactor. The phase and microstructure transformation of the granule before and after hydrothermal treatment was investigated and its decomposition ability was evaluated by using Methylene blue as a target pollutant compound.     

Hydrothermal synthesis of composites of well-crystallized hydroxyapatite and poly(vinyl alcohol) hydrogel

Composites of hydroxyapatite (HAp) and poly(vinyl alcohol) (PVA) hydrogel were fabricated by the hydrothermal treatment of calcium phosphate powder. Alpha-tricalcium phosphate (α-TCP) or beta-tricalcium phosphate (β-TCP) powder was dispersed in PVA hydrogel and exposed to water vapor at 120 °C, 140 °C or 160 °C for 6 h. Low crystallinity HAp was formed in specimens prepared from α-TCP and PVA hydrogel prior to hydrothermal treatment, which was caused by hydrolysis of α-TCP. This allowed specimen shape to be retained after hydrothermal treatment. β-TCP showed less reactivity in forming HAp in the PVA hydrogel, which led to the formation of large rod-shaped crystals approximately 15 μm in length. Specimens from β-TCP and PVA were too soft to retain their shape after hydrothermal treatment. HAp with controlled morphology was prepared using different types of tricalcium phosphate precursor. The application of α-TCP allowed the in situ fabrication of HAp/PVA composites.     

Bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid

The purpose of this study was to investigate bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid (SBF). Calcium phosphates were electrodeposited on commercially pure titanium substrates in the modified SBF at 60 °C for 1 h maintaining the cathodic potentials of − 1.5 V, − 2 V, and − 2.5 V (vs. SCE). Subsequently, the calcium phosphate coatings were transformed into apatites during immersion in the SBF at 36.5 °C for 5 days. The apatites consisted of needle-shaped crystallites distributed irregularly with different grain sizes. As the coatings were electrodeposited at higher cathodic potential, the crystallite of the apatites got denser and the grain sizes of the apatites became bigger during subsequent immersion in the SBF. However, as the coatings were electrodeposited at higher cathodic potential, the coatings were transformed into apatites with lower crystallinity and the Ca/P atomic ratio of the apatites got higher than 1.67, that of stoichiometric hydroxyapatite, after subsequent immersion in the SBF. In addition, CO₃²⁻ ions contained in the modified SBF were incorporated in the calcium phosphate coating during electrodeposition and had an influence on transforming the calcium phosphate into bonelike apatite during subsequent immersion in the SBF showing that CO₃²⁻ incorporated in the apatites disturbed crystallization of the apatites. These results revealed that the coating electrodeposited at − 2.0 V (vs. SCE) in the modified SBF containing CO₃²⁻ ions was the most bioactive showing transformation into carbonate apatite similar to bone apatite.     

Preparation of anatase nanocrystals deposited on hydroxyapatite by hydrothermal treatment

TiO₂-deposited hydroxyapatite (HAp) crystals have been successfully synthesized by hydrothermal treatment of nearly neutral suspension of HAp powders in 10 vol.% of TAS-FINE™ (Titanium amine complex) solutions at 120 or 180 °C. Resultant products were characterized by XRD, SEM–EDX, Vis-Raman and TEM. SEM and TEM observation showed that small rodlike TiO₂, identified as anatase crystals of 100–150 nm in length, were deposited on HAp plates/crystals. The anatase crystals seem to be formed by heterogeneous nucleation on the surfaces of HAp crystals. A higher temperature and a longer reaction period promoted the cystallization of anatase TiO₂.     

Preparation and characterization of selenite substituted hydroxyapatite

Selenite-substituted hydroxyapatite (Se-HA) with different Se/P ratios was synthesized by a co-precipitation method, using sodium selenite (Na₂SeO₃) as a Se source. Selenium has been incorporated into the hydroxyapatite lattice by partially replacing phosphate (PO₄^3 ?) groups with selenite (SeO₃^2 ?) groups. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM) techniques reveal that substitutions of phosphate groups by selenite groups cause lower carbonate groups occupying at phosphate sites and change the lattice parameters of hydroxyapatite. The powders obtained are nano-crystalline hydroxyapatite when the Se/P ratios are not more than 0.1. The particle shape of Se-HA has not been altered compared with selenite-free hydroxyapatite but Se-incorporation reduces the crystallite size. The crystallinity was reduced as the Se/P ratios increased until amorphous phase (Se/P = 0.3) appeared in the Se-HA powder obtained, and then another crystal phase presented as calcium selenite hydrate (Se/P = 10). In addition, the sintering tests show that the Se-HA powders with the Se/P ratio of 0.1 have thermal stability at 900 °C for 2 h; hence they have great potential in the fabrication of bone repair scaffolds.