Investigation of calcium phosphate formation from calcium propionate and triethyl phosphate

Synthetic calcium phosphates are used in for example bone cements and implant coatings to increase biocompatibility. The common method to produce tricalcium phosphate (TCP) uses high temperatures, which creates large crystals with low specific surface areas. In order to investigate new methods to produce TCP at lower temperatures, the reaction between calcium propionate and triethyl phosphate conducted at 220 °C was studied. The method had a near 100% conversion rate, the main synthesis products were calcium phosphate and ethyl propionate. The formed calcium phosphate polymorph could be controlled depending on the water content of the precursor mixture. Anhydrous conditions created amorphous calcium phosphate. As the concentration of water increased, β-TCP was formed, followed by calcium deficient hydroxyapatite and monetite. The particle size increased with the water content, from 20 to 40 nm for amorphous calcium phosphate to tenths of micrometers for monetite. The specific surface areas varied between 209 m²/g for the amorphous product to 3.6 m²/g for the monetite product.     

Porous silicon nitride–based drug delivery carrier

Tetracalcium phosphate/monetite biocement was modified with the addition of 30 wt% highly porous silicon nitride/α-tricalcium phosphate (α-TCP) microgranules. The volume ratio of Si₃N₄ and α-TCP in microgranules was 1:1 and showed good in vitro simulated body fluid bioactivity with precipitation of hydroxyapatite particles. The intention of addition of microgranules to the biocement was to have a carrier of drug, which can be released into the body in due time. Granules prepared by the freeze granulation of starting mixture of silicon nitride and calcium phosphate and subsequent sintering at 1100°C have a suitable pore structure for the foreseen use. The pore volume was almost 1000 mm³/g with the open porosity of 77 vol%. This porosity and the biocompatible composition of silicon nitride–based granules gave a chance to fabricate a suitable composite cement for dexamethasone (DMZ) drug release into the human body. An accelerated release of dexamethasone from composite cement was observed and the full amount of DMZ was released from the composite biocement after 10 days. The presented results are a good base to adjust the total drug release time by the mixing of an appropriate amount of drug infiltrated ceramic granules with the tetracalcium phosphate/monetite cement.     

Characterization of Magnesium‐Doped Hydroxyapatite Prepared by Sol‐Gel Process

Pure and doped hydroxyapatite (HA) nanocrystalline powders (Ca_10‐xMg_x(PO₄)₆OH₂) were synthesized using sol‐gel process. For this, calcium nitrate tetrahydrate, magnesium nitrate hexahydrate, and phosphorous pentoxide were used as precursors for Ca, Mg, and P, respectively. Calculated amounts of magnesium ions (Mg⁺²) especially from 0 to 10% (molar ratio) were incorporated as dopant into the calcium sol solution. The structure and morphology of the gels obtained after mixing the phosphorous and (calcium + magnesium) sol solution were different, and their condensations in time depend on the quantities of magnesium added. The several powders resulting from the gels dried and sintered at 500°C for 1 h were characterized by thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD), and inductively coupled plasma (ICP). Additionally, their agglomeration, morphology, and particle size were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The specific surface area of each sample was measured by the Brunauer–Emmett–Teller (BET) gas adsorption technique. The results of XRD, FTIR, and ICP values ranged between 0.45 and 2.11 mg/L indicated that the magnesium added in the calcium solution was incorporated in the lattice structure of HA so prepared, while those obtained by SEM and TEM confirmed the influence of Mg on their morphology (needle and irregular shape) and crystallite size, which is about 30–60 nm. The as‐prepared powders had a specific surface area ranged between 6.37 and 27.60 m²/g.     

Synthesis and characterization of monetite and hydroxyapatite whiskers obtained by a hydrothermal method

High temperature hydrothermal syntheses, using calcium nitrate tetrahydrate, sodium dihydrogen phosphate and urea as precursors, and characterization of hydroxyapatite (HAp) whiskers are reported herein. The morphology and chemical composition of the crystals from a monetite to a hydroxyapatite phase were controlled by varying the starting concentrations of the precursors and the solution pH through the amount of urea that is decomposed during heating. X-ray diffraction (XRD) analysis, infrared spectroscopy (IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to investigate the products of the syntheses in order to find the optimum reaction conditions for obtaining the desired morphology and phase composition. Different morphologies ranging from single crystals of monetite through rods and plates of hydroxyapatite with different size distribution to whisker-like single hydroxyapatite crystal were achieved by simply varying the starting concentration of urea. Structural refinement of the hydroxyapatite whiskers confirmed a strong preferential orientation along the c-axis direction of the hexagonal crystal structure, which was significantly different from the usually observed random crystal orientation. TEM and SEM analysis of the apatite whiskers confirmed single crystal structure with the a c-axis orientation parallel to the long axis of the whiskers, with sizes up to 150 μm in length, 10 μm in width and with a thickness of about 300 nm, that grew from the same centre of nucleation, forming flaky-like particles.     

Non-stirred synthesis of Na- and Mg-doped,carbonated apatitic calcium phosphate

Hydroxyapatite-like (apatitic) calcium phosphate (Ap-CaP) powders are usually synthesized in stirred solutions containing the highly soluble salts of calcium, such as calcium nitrate, calcium acetate and calcium chloride as the Ca-source. The current study tested the ideas of simultaneously using (i) precipitated calcite (CaCO₃) powders as the Ca-source and (ii) non-stirred, static solutions for synthesizing carbonated, Na⁺- and Mg²⁺-doped apatitic calcium phosphate (Ap-CaP) powders. 0.5 M phosphate buffer (non-saline) solutions were used as the sole phosphate source. The synthesized Ap-CaP powders were found to consist of micron-size carnation-like crystalline particles. Samples were characterized by FE-SEM, XRD, FTIR, ICP-AES, dynamic light scattering (DLS), carbon percentage and BET surface area analyses.     

Composite ceramic containing a bioresorbable phase

The production of a multiphase ceramic in the system CaO-P₂O₅ from mixtures of powders of hydroxy-apatite and monetite, synthesized from calcium nitrate and ammonium hydrophosphate, is investigated. Monetite CaHPO₄ is used as the source of the resorbable phase of the composite ceramic — calcium pyrophosphate. It is established that a solid-phase reaction in which hydroxyapatite and calcium pyrophosphate interact to form tricalcium phosphate occurs during calcination. __________ Translated from Steklo i Keramika, No. 3, pp. 31–35, March, 2007.     

Development and characterization of silver and zinc doped bioceramic layer on metallic implant materials for orthopedic application

A successful electrodeposition method for preparing silver and zinc modified bioactive calcium phosphate layers onto surgical grade titanium alloy material (Ti6Al4V) was developed. The coatings were deposited on the Ti6Al4V surface by pulse current at 70 °C from an electrolyte containing adequate amounts of calcium nitrate, ammonium dihydrogen phosphate, zinc nitrate and silver nitrate. The corrosion resistivity of the bioceramic coatings was assessed in conventional Ringer׳s solution in a three electrode open cell by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results revealed the pure bioactive calcium phosphate (CaP) coated implant materials to possess the highest resistivity to corrosion, while the silver and zinc doped CaP layer showed at least one order of magnitude lower corrosion resistance. These modified CaP coatings can be further considered as antimicrobial coatings with enhanced biocompatibility. The morphology and structure of the coatings were characterized by Scanning electron microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDX) and X-ray diffraction (XRD) that confirmed the pulse current deposited CaP layer to consist of a mixture of different calcium phosphate phases such as hydroxyapatite (HAp), monetite (dicalcium phosphate, CaHPO₄) as well as other Ca-containing components, portlandite (Ca(OH)₂) and parascholzite (CaZn₂(PO₄)₂(H₂O)₂).     

Synthesis of nanoscale hydroxyapatite particles using triton X-100 as an organic modifier

Nano-sized rod like hydroxyapatite (HA) particles were synthesized using Ca(NO₃)₂·4H₂O and (NH₄)₂HPO₄ as precursors with varying contents of non-ionic surfactant viz., p-(1,1,3,3-tetramethylbutyl)phenoxypoly(oxyethylene)glycol (triton X-100) as organic modifer. The crystalline phase, chemical composition, surface area and morphology of the prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), BET surface area analysis, high resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM). The above studies indicate the presence of nanoscale hydroxyapatite (HA) phase. Triton X-100 does not appear to affect the crystallinity of the samples. Increase of BET surface area and the consequent decrease in particle size of HA were observed as a function of surfactant content. Probable mechanism for the role of the surfactant in the synthesis of nanoscale HA has also been discussed.     

Calcium phosphate bioceramics synthesized from eggshell powders through a solid state reaction

Everyday millions of tons of eggshells are produced as biowaste around the world. Most of this waste is disposed of in landfills without any pretreatment. Eggshells in landfills produce odors and promote microbial growth as they biodegrade. The present invention provides an environmentally beneficial and cost-effective method of producing calcium phosphate bioceramics (hydroxyapatite or tricalcium phosphate) from eggshell waste. In this investigation, heat treatment produced solid state reactions between eggshell powders and dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) or calcium pyrophosphate (Ca₂P₂O₇). When eggshell powders (CaO) and DCPD were heat treated at 1150 °C for 3 h, only a single hydroxyapatite (HA) phase was found; no diffraction peaks of starting materials and no β-TCP were observed. The XRD patterns of the product fabricated from raw eggshell powders (CaCO₃) and Ca₂P₂O₇ heat treated at 1100 °C for 3 h showed that almost only pure β-TCP remained with a trace amount of HA. The calcium phosphate ceramic synthesized from eggshell powders contains several important trace elements such as Na, Mg and Sr.     

High thermally stable Mg-substituted tricalcium phosphate via precipitation

Tricalcium phosphates incorporating small amounts of Mg show attractive biological performances in terms of enhanced bone apposition, bone in-growth and cell-mediated degradation. A systematic investigation on Mg-stabilized β-TCP (β-tricalcium phosphate, β-Ca₃(PO₄)₂) is presented. Microstructure, composition and thermal behaviour were investigated by means of thermogravimetry and differential thermal analysis (TG-DTA), induced coupled plasma-atomic emission spectroscopy (ICP-AES), Fourier transform infrared spectroscopy (FT-IR), N₂ adsorption isotherms, X-ray diffraction (XRD and HT-XRD), and scanning electron microscopy (SEM). Pure and Mg-substituted tricalcium phosphate precursors consisted of calcium-deficient hydroxyapatite, the specific surface area being 128 m²/g and 87 m²/g, respectively. Tricalcium phosphate nanostructured powders were obtained by thermal treatment above 800 °C. The incorporation of Mg within the calcium phosphate lattice promoted the formation of the β-TCP phase at slightly lower temperature and resulted in the stabilization of the β-polymorph at high temperature (i.e. 1600 °C).     

Synthesis of Hydroxyapatite Nanopowders via Sucrose-Templated Sol–Gel Method

The present research describes synthesis of hydroxyapatite (HAp) nanopowders using a sol–gel route with calcium nitrate and ammonium hydrogen phosphate as calcium and phosphorous precursors, respectively. Sucrose is used as template material, and alumina is added as a dopant to study its effects on particle size and surface area. Synthesized powders are characterized using X-ray diffractometry, BET surface-area analysis, and transmission electron microscopy. Results show that alumina stabilizes the HAp crystalline phase. Average particle size of mesoporous HAp samples is between 30 and 50 nm with surface area of 51–60 m²/g.     

The threonine effect on calcium phosphate preparation from a solution containing Ca/P = 1.33 molar ratio

In this study, calcium phosphate materials were prepared by a modified precipitation method using high-speed dispersing equipment. A solution with a Ca/P molar ratio of 1.33 (octacalcium phosphate stoichiometry) was transferred into the reactor vessel with different concentrations of threonine at temperature 97 °C. A white precipitant was collected after the addition of condensed ammonium solution and the samples were subsequently calcined at 900 °C. From the XRD patterns and FT-IR spectra of the uncalcined samples, three phases of octacalcium phosphate (OCP), monetite (DCP) and hydroxyapatite (HA) were obtained. Calcined samples showed two phases of β-tricalcium phosphate (β-TCP) and calcium pyrophosphate (CPP). SEM micrographs showed the different morphology of samples. The specific surface areas (ssa) were 45–53 m²/g for and 5–6 m²/g for calcined samples. From the obtained results, we found that threonine added in various amounts in the initial solution inhibits the formation of HA and consequently creates OCP and DCP.     

Preparation of needle-like hydroxyapatite by homogeneous precipitation under hydrothermal conditions

Preparation of hydroxyapatite was carried out by a homogeneous precipitation technique using hydrothermal reactions of Ca(EDTA)^2- and PO₄^3- at 130–250°C and pH 4–10. Hydroxyapatite and monetite were formed above pH 6 and below pH 4, respectively. Needle-like hydroxyapatite was formed at high temperatures above 130°C. The length of the needle-like hydroxyapatite crystal increased with increasing working solution pH and temperature.     

Performance as electrode of electrical double layer capacitor of activated carbon prepared from bamboo using guanidine phosphate and CO<Subscript>2</Subscript> activation

The electrochemical performances of an electrical double layer capacitor were investigated regarding the activated carbon prepared from bamboo by a new approach, that is, the combination of delignification, addition of guanidine phosphate, and CO₂ activation. In this study, a 1 M H₂SO₄ aqueous solution was used as the electrolyte of the capacitor. The physical properties, such as the BET specific surface area of the carbon material, depend on the preparation conditions of the activated carbon. A TEM image indicated that the addition of guanidine phosphate did not facilitate the graphitization and did not prevent activation by CO₂. The apparent reaction equation for the CO₂ activation was first-order, which is reasonable for physical activation. The electrochemical performances of the carbon material depended on the preparation conditions of the carbon material, such as the heat treatment temperature, amount of added guanidine phosphate, and CO₂ activation time. The sample prepared under the following conditions (the amount of added guanidine phosphate: 9 wt%, the heat treatment temperature: 800 °C, CO₂ activation time: 3 h) had the highest performance (153 F g^?1 at 1000 mA g^?1) because the sample had the highest BET specific surface area (2001 m² g^?1).     

Fracture surface and correlation of buckling force with aspect ratio of C60 crystalline whiskers

The structure and mechanical properties of crystalline whiskers with an average diameter of 600 nm, composed of C₆₀ molecules were studied by transmission electron microscopy combined with nanonewton force measurements used in atomic force microscopy. C₆₀ nanowhiskers with a body-centered tetragonal structure are compressed along their long axis. Young's modulus of the C₆₀ nanowhiskers was estimated to be 28 ± 5 GPa. The buckling stress was correlated to the aspect ratio of length to diameter of the C₆₀ nanowhiskers. The (100) surface was the principal fracture surface of the C₆₀ nanowhiskers.     

Characterization of Mg-substituted hydroxyapatite synthesized by wet chemical method

Mg-substituted hydroxyapatite made up of needle-like and plate-like particles containing different amounts of Mg (between 0.21 wt% and 2.11 wt%) were prepared via wet chemical precipitation method of a homogenous suspension of Mg(OH)₂/Ca(OH)₂ and an aqueous solution of H₃PO₄. According to the data of Brunauer–Emmett–Teller method and field emission scanning electron microscopy, high specific surface area Mg-substituted hydroxyapatite was obtained. Specific surface area of as-synthesized powders increased from 94.9 m² g⁻¹ to 104.3 m² g⁻¹ with increasing concentration of Mg up to 0.64 wt%. Fourier transform infrared spectroscopy, X-ray powder diffraction, differential thermal analysis, and heating microscopy, were used to evaluate thermal stability and sintering behavior of synthesis products. Increase in concentration of Mg in synthesis products (≥0.83 wt%) promoted decomposition of Mg-substituted hydroxyapatite to Mg-substituted β-tricalcium phosphate after thermal treatment.     

Structural interplay between strontium and calcium in α-CaHPO4 and β-SrHPO4

The ability of strontium ion to inhibit the abnormally high bone resorption, which occurs in pathologies characterized by loss of bone substance, has stimulated a number of research on strontium substituted/doped calcium phosphates. However, no information was available up to now on strontium substitution to calcium in the structure of α-CaHPO₄, monetite, in spite of the involvement of this phosphate in the composition of biomaterials for hard tissue substitution/repair and although it is isomorphous with α-SrHPO₄. Herein, we investigated the substitution of strontium to calcium in the structure of α-CaHPO₄, as well as the replacement of calcium for strontium in the structure of a further polymorph of SrHPO₄, namely β-SrHPO₄. To this purpose, monetite at increasing degree of strontium substitution for calcium was synthesized by means of direct synthesis in aqueous solution, as well as through thermal treatment of strontium-substituted brushite; whereas the synthesis of β-SrHPO₄ was carried out at low temperature. The results of structural refinements, spectroscopic analysis and electron microscopy investigation indicate that the method of synthesis has a great influence on the range of strontium incorporation into α-CaHPO₄, which can reach 100 at%. The morphology of the synthesized materials is also remarkably dependent on composition. The analysis of the products synthesized at low temperature shows that the upper limit of possible substitution of calcium to strontium in the structure of β-SrHPO₄ is much more limited, just up to about 20 at%. Moreover, powder X-ray analysis of β-SrHPO₄ states that it crystallizes with a monoclinic cell in the space group P2₁/c.     

Synthesis of nanostructured magnesium silicate with high surface area and mesoporous structure

Nanostructured magnesium silicate with high BET surface area and mesoporous structure was prepared by a hydrothermal method using polyethylene glycol (PEG) as surfactant and magnesium nitrate and sodium silicate aqueous solution as magnesium and silicate sources, respectively. The effects of different parameters such as hydrothermal treatment, reaction temperature, pH, ethanol/PEG ratio and etc. on the structural properties of the synthesized sample were examined. The prepared samples were characterized by X-ray diffraction (XRD), N₂ adsorption (BET) and scanning electron microscopy (SEM) techniques. The results indicated that hydrothermal treatment increased BET surface area from 290 to 394.2 m²/g and transfer amorphous phase to crystalline. Also, increasing in aging temperature, aging time, pH value and ethanol/PEG ratio did not change surface area by specific procedure, whereas increasing calcination temperature decreased surface area. Furthermore, hydrothermal treatment and increasing in pH value will change hysteresis loop. The highest BET surface area obtained in this paper is 619.8 m²/g.     

Nonenzymatic Transformation of Amorphous CaCO3 into Calcium Phosphate Mineral after Exposure to Sodium Phosphate in Vitro: Implications for in Vivo Hydroxyapatite Bone Formation

Studies indicate that mammalian bone formation is initiated at calcium carbonate bioseeds, a process that is driven enzymatically by carbonic anhydrase (CA). We show that amorphous calcium carbonate (ACC) and bicarbonate (HCO₃^?) cause induction of expression of the CA in human osteogenic SaOS‐2 cells. The mineral deposits formed on the surface of the cells are rich in C, Ca and P. FTIR analysis revealed that ACC, vaterite, and aragonite, after exposure to phosphate, undergo transformation into calcium phosphate. This exchange was not seen for calcite. The changes to ACC, vaterite, and aragonite depended on the concentration of phosphate. The rate of incorporation of phosphate into ACC, vaterite, and aragonite, is significantly accelerated in the presence of a peptide rich in aspartic acid and glutamic acid. We propose that the initial CaCO₃ bioseed formation is driven by CA, and that the subsequent conversion to calcium phosphate/calcium hydroxyapatite (exchange of carbonate by phosphate) is a non‐enzymatic exchange process.     

Deposition of calcium phosphate coatings with defined chemical properties using the electrostatic spray deposition technique

The electrostatic spray deposition (ESD) technique was used for biomedical purposes in order to deposit calcium phosphate (CaP) coatings onto titanium substrates. The relationship between various deposition parameters and the chemical properties of deposited coatings was investigated in order to be able to deposit CaP coatings with tailored chemical characteristics.The results showed that the chemical properties of the coatings were determined by both physical, apparatus-related factors and chemical, solution-related parameters. By varying the processing parameters of the technique, several crystal phases and phase mixtures were obtained, ranging from carbonate-free phases such as meta- and pyrophosphates, monetite and various tricalcium phosphates to carbonate-containing phases such as various carbonate apatites and calcite. On the basis of these results, a chemical mechanism of coating formation was proposed. Essentially, the deposition of the various crystal phases was the result of an acid–base reaction between basic CO₃²⁻ groups (originating from solvent decomposition reactions) and acidic HPO₄²⁻ groups from an intermediate monetite (CaHPO₄) phase of the CaP precipitate. The amount of carbonate incorporation (ranging from 0 to 15 wt%) determined the crystal and molecular structure of the deposited coatings.