Formation Of Amorphous Calcium Phosphate Research Articles

Conductometric Study of the Effect of Citric Acid on Phase Evolution of Calcium Phosphate in Solutions

The influence of citric acid on phase evolution of calcium phosphate is studied by the electrical conductivity of reaction solutions. The conductance curves are divided into three distinct regions and assigned to the formation of amorphous calcium phosphate (ACP), hydroxyapatite (HA) and the solubility of HA, respectively. When more citric acid is added, the width of region I decreases because moreCa2+ions would prefer to foster the nucleation of HA from ACP. Decreasing rate of conductivity in region II becomes smaller with more addition of citric acid because the supersaturation of HA decreases in solutions. And thus, HA powders with larger grain size are synthesized with more citric acid addition.

Recovering phosphorus as struvite from the digested swine wastewater with bittern as a magnesium source

Recovering nitrogen and phosphorus through struvite (MgNH4PO4 6H2O) crystallization from swine wastewater has gained increasing interest. However, swine wastewater contains complex compositions, which may hinder the formation of struvite crystal and affect the purity of the precipitates by forming other insoluble minerals. In this work, experiments were carried out to evaluate struvite precipitation in the anaerobically digested swine wastewater, with dosing bittern as a low-cost magnesium source. Exceeded 90% phosphate removal and 23-29% ammonium reduction were obtained. FTIR, XRD and mass balance analysis were combined to analyze the species of precipitated minerals. Results showed that the precipitates were struvite, mixed with amorphous calcium phosphate (ACP) and brucite. The presence of Ca2+ diminished the percentage of struvite and gave rise to ACP formation. Controlling pH below 9.5 and bittern dosage above 1% (w/w) could inhibit ACP precipitation and harvest a highly pure struvite crystal product.

Strategy to reduce carbonate incorporation in the fabrication of hydroxyapatite nanopowders

Coexistence of carbonate ions in aqueous solution often leads to difficulty in the preparation of stoichiometric hydroxyapatite (HAp), owing to formation of carbonate-containing HAp with low crystallinity. This study is focused on examining appropriate conditions to prepare HAp with reduced incorporation of carbonate through a wet processing in aqueous solution, starting from calcium chloride (CaCl2) and trisodium phosphate (Na3PO4). Titration of Na3PO4 solution to CaCl2 solution resulted in formation of crystalline HAp with low contents of carbonate, whereas titration of CaCl2 solution to Na3PO4 solution resulted in formation of amorphous calcium phosphate that contains approximately 3 mass % of carbonate. The HAp particles prepared in a condition under coexistence of trehalose were finer and well-dispersed than those prepared in a solution free from trehalose. The addition of the Na3PO4 solution to the CaCl2 solution contributes to nucleation of HAp with minimal carbonate incorporation.

The role of prenucleation clusters in surface-induced calcium phosphate crystallization

Unravelling the processes of calcium phosphate formation is important in our understanding of both bone and tooth formation, and also of pathological mineralization, for example in cardiovascular disease. Serum is a metastable solution from which calcium phosphate precipitates in the presence of calcifiable templates such as collagen, elastin and cell debris. A pathological deficiency of inhibitors leads to the uncontrolled deposition of calcium phosphate. In bone and teeth the formation of apatite crystals is preceded by an amorphous calcium phosphate (ACP) precursor phase. ACP formation is thought to proceed through prenucleation clusters--stable clusters that are present in solution already before nucleation--as was recently demonstrated for CaCO(3) (refs 15,16). However, the role of such nanometre-sized clusters as building blocks for ACP has been debated for many years. Here we demonstrate that the surface-induced formation of apatite from simulated body fluid starts with the aggregation of prenucleation clusters leading to the nucleation of ACP before the development of oriented apatite crystals.

Time-resolved in situ studies of apatite formation in aqueous solutions

Formation of hydroxylapatite through the precipitation and evolution of calcium phosphate precursor phases under varying conditions of temperature (25–90 °C), pH (6.5–9.0), and calcium to phosphorus ratio (1.0, 1.33, 1.5, and 1.67) comparable to those found in many sediments and soils were studied. The products of low-temperature precipitation were analyzed by ex situ X-ray diffraction and SEM, as well as time-resolved in situ synchrotron X-ray diffraction. Rietveld refinement was used for quantitative evaluation of relative abundances during phase evolution. The results of ex situ investigations conducted at ambient temperature and near-neutral pH indicate formation of amorphous calcium phosphate, which over the course of experiments transforms to brushite and ultimately hydroxylapatite. The results of in situ X-ray diffraction experiments suggest a more complex pathway of phase development under the same conditions. Some of the initially formed amorphous calcium phosphate and/or crystalline brushite transformed to octacalcium phosphate. In the later stage of the reactions, octacalcium phosphate transforms quite rapidly to hydroxylapatite. This is accompanied or followed by the transformation of the remaining brushite to monetite. Hydroxylapatite and monetite coexist in the sample throughout the remainder of the experiments. In contrast to the near-neutral pH experiments, the results from ex situ and in situ diffraction investigations performed at higher pH yield similar results. The precipitate formed in the initial stages in both types of experiments was identified as amorphous calcium phosphate, which over the course of the reaction quite rapidly transformed to hydroxylapatite without any apparent intermediate phases. This is the first application of time-resolved in situ synchrotron X-ray diffraction to precipitation reactions in the Ca(OH)2-H3PO4-H2O system. The results indicate that precursors are likely to occur during the natural or induced (e.g., with application of Ca+PO4 amendments) formation of hydroxylapatite in many sedimentary environments.

Effects of glutamic acid shelled PAMAM dendrimers on the crystallization of calcium phosphate in diffusion systems

Second generation poly(amidoamine) (PAMAM) dendrimers were synthesized and peripherally modified with glutamic acid (PAMAM-MG) as a shell. The effect of the dendrimers on the crystallization of different calcium phosphate compounds was investigated in both double and one way diffusion systems. It was found that the crystals of calcium phosphate showed tape-like morphology in the presence of PAMAM-MG, and the crystals’ thickness and width decreased compared to those grown without dendritic molecules. Such a result might be due to the interaction of electric charges between dendritic molecules and octacalcium phosphate (Ca8H2(PO4)6·5H2O, OCP), which led to the adsorption of PAMAM-MG in the 100 and 010 surfaces of OCP. Moreover, PAMAM-MG showed an affinity for gelatin, and it could cause the formation of amorphous calcium phosphate (Ca9(PO4)6·nH2O, ACP) at a concentration of 5 mg/mL of PAMAM-MG. These results suggest that PAMAM-MG could be used for regulating the morphology of OCP and changing the composition of minerals in gels.

Struvite formation from the supernatants of an anaerobic digestion pilot plant

This work studied the influence of the characteristics of the supernatants on the struvite precipitation process. Eighteen experiments with the supernatants generated in an anaerobic digestion pilot plant were performed in a stirred reactor. In order to obtain the pH control during the crystallization process, a Fuzzy Logic based controller was used. High phosphorus precipitation and recovery efficiencies were obtained. The composition of the supernatants was analyzed in order to study its influence on the solids formed from those solutions. The presence of calcium reduced the percentage of phosphorus precipitated as struvite leading to the formation of amorphous calcium phosphate (ACP), which tended to be lost with the effluent of the reactor. Calcite was also formed when supernatants with high magnesium:phosphorus (Mg/P) and calcium:phosphorus (Ca/P) molar ratios were employed. Some ammonium volatilization by conversion to NH 3 occurred in all the experiments. The use of air to increase the pH to an adequate value showed to be feasible. Aeration cleaned struvite crystals from suspended solids, which makes aeration interesting for struvite separation. However, aeration slightly increased the loss of phosphorus with the effluent of the reactor and promoted ammonium volatilization.

Role of 20-kDa Amelogenin (P148) Phosphorylation in Calcium Phosphate Formation in Vitro

The potential role of amelogenin phosphorylation in enamel formation is elucidated through in vitro mineralization studies. Studies focused on the native 20-kDa porcine amelogenin proteolytic cleavage product P148 that is prominent in developing enamel. Experimental conditions supported spontaneous calcium phosphate precipitation with the initial formation of amorphous calcium phosphate (ACP). In the absence of protein, ACP was found to undergo relatively rapid transformation to randomly oriented plate-like apatitic crystals. In the presence of non-phosphorylated recombinant full-length amelogenin, rP172, a longer induction period was observed during which relatively small ACP nanoparticles were transiently stabilized. In the presence of rP172, these nanoparticles were found to align to form linear needle-like particles that subsequently transformed and organized into parallel arrays of apatitic needle-like crystals. In sharp contrast to these findings, P148, with a single phosphate group on serine 16, was found to inhibit calcium phosphate precipitation and stabilize ACP formation for more than 1 day. Additional studies using non-phosphorylated recombinant (rP147) and partially dephosphorylated forms of P148 (dephoso-P148) showed that the single phosphate group in P148 was responsible for the profound effect on mineral formation in vitro. The present study has provided, for the first time, evidence suggesting that the native proteolytic cleavage product P148 may have an important functional role in regulating mineralization during enamel formation by preventing unwanted mineral formation within the enamel matrix during the secretory stage of amelogenesis. Results obtained have also provided new insights into the functional role of the highly conserved hydrophilic C terminus found in full-length amelogenin.

A study of the formation of amorphous calcium phosphate and hydroxyapatite on melt quenched Bioglass® using surface sensitive shallow angle X-ray diffraction

Melt quenched silicate glasses containing calcium, phosphorous and alkali metals have the ability to promote bone regeneration and to fuse to living bone. These glasses, including 45S5 Bioglass((R)) [(CaO)(26.9)(Na(2)O)(24.4)(SiO(2))(46.1)(P(2)O(5))(2.6)], are routinely used as clinical implants. Consequently there have been numerous studies on the structure of these glasses using conventional diffraction techniques. These studies have provided important information on the atomic structure of Bioglass((R)) but are of course intrinsically limited in the sense that they probe the bulk material and cannot be as sensitive to thin layers of near-surface dissolution/growth. The present study therefore uses surface sensitive shallow angle X-ray diffraction to study the formation of amorphous calcium phosphate and hydroxyapatite on Bioglass((R)) samples, pre-reacted in simulated body fluid (SBF). Unreacted Bioglass((R)) is dominated by a broad amorphous feature around 2.2 A(-1) which is characteristic of sodium calcium silicate glass. After reacting Bioglass((R)) in SBF a second broad amorphous feature evolves ~1.6 A(-1) which is attributed to amorphous calcium phosphate. This feature is evident for samples after only 4 h reacting in SBF and by 8 h the amorphous feature becomes comparable in magnitude to the background signal of the bulk Bioglass((R)). Bragg peaks characteristic of hydroxyapatite form after 1-3 days of reacting in SBF.

Inhibition of the formation of oral calcium phosphate precipitates: the possible effects of certain honeybee products

We studied the effects of honeybee products on the in vitro formation of calcium phosphate precipitates. Screening tests of the in vitro formation of calcium phosphate precipitates using 20 types of honey and four types of propolis were carried out using the pH drop method. The inhibitory effect on the rate of amorphous calcium phosphate transformation to hydroxyapatite and on the induction time varied greatly among the 20 types of honey and four types of propolis. We classified them according to their effects on decreasing the rate of amorphous calcium phosphate transformation to hydroxyapatite and/or increasing the induction time. Two of the 20 honeys showed little or no inhibition, either on the rate of amorphous calcium phosphate transformation to hydroxyapatite or on the induction time. Six of the honeys reduced the rate of amorphous calcium phosphate transformation to hydroxyapatite by 12-35% and with a 2.5- to 4.4-fold increase in the induction time. The remaining 12 honeys showed even greater activity. Because four of these 12 honeys had an inhibitory effect on the rate of amorphous calcium phosphate formation, they were excluded as candidates for anticalculus agents. Furthermore, three of the four types of propolis showed an inhibitory effect that was the same as or greater than 1-hydroxyethylidene- 1,1-bisphosphonate. These results suggest that eight honeys and three types of propolis may have potential as anticalculus agents in toothpastes and mouthwashes.

Carbon Spheres Assisted Synthesis of Porous Bioactive Glass Containing Hydroxycarbonate Apatite Nanocrystals: a Material with High in Vitro Bioactivity

A hierarchically porous bioactive glass of composition 80 mol % SiO2 and 15 mol % CaO (MBGH) was synthesized using pluoronic P123 and glucose-derived amorphous carbon spheres as templates. While the carbon spheres leave behind pores of 300 nm to a few micrometers after calcination, the walls of these pores contain orderly arranged mesopores of 3−5 nm orchestrated by the pluoronic polymer. The role of carbon spheres is not only to create pores but also to aid in increased growth rate of hydroxycarbonate apatite in simulated body fluid (SBF). The negatively charged carbon spheres favor the local enrichment of calcium ions and facilitate the formation of amorphous calcium phosphate around the surface. On calcination, the walls of the macropores contain crystalline hydroxycarbonate apatite nuclei. MBGH also exhibits an accelerated growth of monoclinic-like apatite in SBF. The in vitro activity has been studied by XRD, FT-IR, FESEM, and TEM.

Precipitation of Urinary Phosphate

Struvite precipitation is an effective process for removing and recovering phosphate from separately collected fresh urine. Present study experimentally investigated the effect of reaction time, reaction pH, and initial molar Mg:P ratio on struvite precipitation. Results showed that reaction time had a negligible effect on phosphate removal, and pH influenced both phosphate removal efficiency and the composition of the precipitate. Precipitates obtained at pH 9 were crystalline struvite which was confirmed by the FTIR and X-ray diffraction analysis. Studying at a molar ratio below 1Mg:1P enhanced the formation of amorphous calcium phosphate and decreased the purity of the precipitate with respect to struvite. Struvite precipitation from fresh urine (1.57 Mg:1P) achieved at pH 9 in 1-h reaction time resulted in 96% of PO43−–P and 52.5% of NH3-N removal. High phosphate recovery obtained in this experimental study indicated that combining urine separation with struvite precipitation is a promising idea toward sustainable environment.

Complexes of Ca(II) with polymers as precursors for preparation of amorphous calcium phosphate

Ca(II) complexes with poly(ethylene glycol) (PEG), poly(N -vinyl-2-pyrrolidone) (PVP) and poly(vinyl alcohol)(PVA) were formed in aqueous solutions. The Ca(II) complexes were used as Ca precursors to react with sodium phosphates to prepare amorphous calcium phosphate (ACP). The experimental results showed that ACP precipitates could be formed in the presence of PVA or PEG while only crystalline calcium phosphate precipitates (hydroxyapatite) was formed in the presence of PVP. When Ca ions were coordinated with PEG, the viscosity of the system was not obviously influenced, which was appropriate to separate the precipitates. Hence, PEG is considered to be an optimal polymeric complexing agent to prepare ACP. The Ca(II) complexes with the polymeric complexing agents can favour ACP formation, which is attributed to the coordinated polymeric complexing agents remaining in the structure of ACP, i.e.in aggregated Ca9 (PO4)6 clusters. The existence of the coordinated complexing agents depresses the conversion process from ACP to crystalline hydroxyapatite, hence, ACP can survive in the mother solution.

Investigation of Bone Formation using Non-Crystalline Calcium Phosphate Glass in Beagle Dogs

Investigation of Bone Formation using Non-Crystalline Calcium Phosphate Glass in Beagle Dogs

Inhomogeneity of calcium phosphate coatings deposited by laser ablation at high deposition rate

Calcium phosphate coatings were deposited with a KrF excimer laser onto titanium alloy to study their homogeneity. Deposition was performed at a high deposition rate under a water vapour atmosphere of 45 Pa and at a substrate temperature of 575 °C. Samples were also submitted to annealing under the same conditions of deposition for different times just after deposition. The effects of the annealing were also investigated. The morphology of the coatings was studied by scanning electron microscopy. Their structure and phase distribution was analysed by X-ray diffractometry and infrared and micro-Raman spectroscopies. Besides the non-uniform thickness, the results reveal an inhomogeneity in the spatial distribution of calcium phosphate phases in the coatings. The phase distribution can be almost completely correlated with the deposition rate. High deposition rates (0.5 nm/pulse) occurring in the centre of deposition results in the formation of amorphous calcium phosphate, while lower deposition rates favour the presence of hydroxyapatite and alpha tricalcium phosphate. At intermediate deposition rates, beta tricalcium phosphate is found, probably because the superimposed effect of energetic particles bombardment. The annealing process promotes the crystallisation of the amorphous material. The importance of the deposition rate in the phases obtained is stated after comparing these results with a previous work where homogeneous hydroxyapatite coatings were obtained under the same conditions of laser fluence, temperature and pressure, but at lower deposition rates.

Bioactivity of three CaO-P2O5-SiO2 sol-gel glasses.

Three gel glasses containing 25 mol % of CaO and SiO(2) + P(2)O(5) contents (in mol %): 75 + 0 (S75); 72.5 + 2.5 (S72.5P2.5); and 70 + 5 (S70P5), respectively, were obtained, characterized, and studied when soaked in a simulated body fluid (SBF). The influence of composition in both textural properties (surface area and porosity) and in vitro behavior of glasses was studied. In as prepared S72.5P2.5 and S70P5 glasses, crystalline phosphate nuclei were detected through XRD and FTIR. In addition, N(2) adsorption and Hg porosimetry measurements showed that the surface area increased, whereas the pore volume and the pore diameter decreased as P(2)O(5) in glasses increased. These variations were explained on the basis of the withdrawal of calcium from the glass silica network, due to the calcium-phosphorous bonding. In vitro studies showed that the three compositions were bioactive, because an apatite layer was formed after soaking in SBF. S75 presented the highest initial reactivity but the lowest crystallization rate of the apatite-like phase. For S72.5P2.5, and S70P5 the amorphous calcium phosphate formation was slower than for S75, but the crystallization of apatite was observed after shorter periods in SBF. Furthermore, after 7 days of soaking, the layer thickness decreased as P(2)O(5) in glasses increased.

Cinétique de nucléation hétérogène et de croissance cristalline de l’hydroxyapatite lors de la précipitation spontanée des phosphates de calcium dans des solutions sucrées

The kinetics of spontaneous calcium phosphates precipitation was studied under conditions of cane juice clarification according a 2 4 factorial plan. That precipitation takes place in tow distinct steps. The first step, corresponding to formation of amorphous calcium phosphate and brushite, is very fast. The second step corresponds to the transformation of this phases into hydroxyapatite. The kinetics results of this second step showed that heterogeneous nucleation of hydroxyapatite, on the surface of the first phases formed, is essentially complete within the induction period. The crystalline growth of the nuclei of hydroxyapatite and the dissolution of the first phases formed was interpreted by an equation rate of crystal growth combined with a dissolution equation. Résumé.

Process of calcification on artificial materials

CaO, SiO2-based glasses form the bonelike apatite on their surfaces in an acellular simulated body fluid (SBF) with ion concentrations nearly equal to those of the human blood plasma. The apatite formation of the former glasses is attributed to the catalytic effect of the Si-OH groups, which are formed on their surfaces in SBF, for the apatite nucleation. The gels of SiO2, TiO2, ZrO2, Ta2O5, and Nb2O5 formed the apatite on their surfaces in SBF, but Al2O3 gel did not. This indicates that the Ti-OH, Zr-OH, Ta-OH, and Nb-OH groups besides the Si-OH groups are also effective for the apatite nucleation, but Al-OH groups are not effective. Apatite formation on self-assembled monolayer terminated with various functional groups in SBF showed that COOH and H2PO4 groups are also effective for the apatite nucleation. All these groups are negatively charged around pH 7.40. Their apatite nucleating ability is varied with their arrangements. Among the Ti-OH groups, those in anatase structure are most effective for the apatite nucleation. Transmission electron microscope attached with energy dispersive X-ray spectrometer showed that these functional groups induce the apatite nucleation not directly, but through formation of their calcium compounds and subsequent formation of amorphous calcium phosphate with low Ca/P atomic ratios.

Thermally induced crystallization of amorphous calcium phosphate in plasma-spheroidised hydroxyapatite powders

Plasma spheroidisation of hydroxyapatite powders inevitably results in the formation of amorphous calcium phosphate. The crystallisation of the amorphous phase can be thermally induced. The thermal crystallisation is studied by means of differential scanning calorimetry (DSC) and X-ray diffraction (XRD). We aim to investigate the temperature at which the crystallisation takes place and to have fundamental understandings about this phase transformation (i.e., activation energy and enthalpy).

Recent trends towards improved plasma-sprayed advanced bioceramic coatings on Ti6Al4V implants

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