Relation of lattice ion solution composition to octacalcium phosphate dissolution kinetics

The kinetics of dissolution of octacalcium phosphate (OCP) has been investigated under conditions of constant relative undersaturation with respect to OCP, σ_OCP=−0.57, at pH 4.500, and ionic strength (I), 0.15 mol l⁻¹. The molar calcium/phosphate ratio (R) in the solutions was varied from 0.1 to 10. The dissolution rate decreased by 160% as R increased from 0.1 to 10. The ζ-potential of the OCP surfaces was measured in solutions equilibrated with respect to OCP at pH values ranging from 5.0 to 11. Under stoichiometric conditions (R=1.33), OCP was positively charged at pH values from 5.0 to 10. As the solution calcium concentration was increased, ζ became more positive over the entire pH range studied. At R=0.1, two isoelectric points were apparent at pH values of about 6.3 and pH 9.5. This behavior may be related to the solubility product (K_sp) of OCP. The relationship between surface characteristics and dissolution rate are discussed in terms of kink density and the kinetic ionic ratio model developed previously (J. Zhang, G.H. Nancollas, J. Colloid Interf. Sci., 200 (1998) 131.     

Formation of octacalcium phosphate by heterogeneous nucleation on a titania surface

Biocompatibility of the surfaces of titanium dental implants can be improved by covering them with calcium phosphate crystals, which makes the surface bioreactive. Possibly the most effective bioreactive foreign material that improves osteointegration and adsorption/binding of extracellular proteins and structural proteins is crystalline octacalcium phosphate {2×[Ca₄H(PO₄)₃·2.5H₂O] or Ca₈(HPO₄)₂(PO₄)₄·5H₂O, OCP}. In this work the building up of OCP crystals on the surface of TiO₂ anatase is examined in the process of heterogeneous nucleation from constant-composition solutions of CaCl₂ and KH₂PO₄ at constant pH (pH 6.8) and ionic strength (I=0.05 M), in dense titania suspensions. Constant relative supersaturation with regard to the calcium phosphate formation was maintained by the controlled addition of the reagent solutions, according to the desired speed of crystallization. The surface saturation value of calcium ion adsorption was measured by detecting the pH decrease during CaCl₂ addition in a separate experiment. The OCP crystallization was also conducted on the surface of an evaporated titanium layer, and on titanium metal disks. The surface of the disks was modified by the laser ablation method in order to increase the oxide layer thickness. Calcium phosphate crystals formed on the surface of the modified titanium disks, but not in an appreciable amount on the surface of the evaporated titanium layer.     

The role of polycarboxylic acids in calcium phosphate mineralization

The role of two polyelectrolytes, poly-L-glutamate and poly-L-aspartate, in the growth of calcium phosphate crystal phases, has been investigated at constant supersaturation. Both molecules are strong inhibitors of HAP growth when present in the solution phase but also act as hydroxyapatite and (octacalcium phosphate)-like crystal nucleators when adsorbed on germanium surfaces. The structure of the polymers in solution is presented and various adsorption models are analyzed. A "train-loop" structure of these long, flexible chain polymers on the crystal surface is consistent with all the adsorption (experimental and theoretical), inhibition, and electrophoretic mobility results.     

The nucleation of calcium phosphate from solution

Dicalcium phosphate was precipitated from homogeneous solution in the pH range 5–6 by using the hydrolysis of cyanate to control the precipitation process. Turbidimetric determination of the onset of precipitation enabled the critical supersaturation to be determined. In this region the average critical supersaturation ratio was found to be 2.3.     

Formation of calcium phosphates in gelatin with a novel diffusion system

The present paper demonstrated a novel and simple diffusion system to precipitate calcium phosphates in gelatin gel. In this system, a gelatin cup was specially used as the membrane separating reservoirs of calcium and phosphate ions. Relative to the conventional diffusion system, the novel one in our experiment decreased the time required for the deposition from 5-7 days to 20 h and increased the amount of the precipitated mineral phases significantly. The influence of pH values and concentrations of calcium and phosphate solutions buffered with Tris-HCl and NaOH, respectively, was investigated. The results showed that precipitation of the mineral phase at low pH values (7 for calcium and 11 for phosphate) and concentrations (200 mM for calcium and 15 mM for phosphate) resulted in the formation of plate-like octacalcium phosphate (OCP) crystals. With increasing the pH values of calcium and phosphate solutions to 8 and 12, respectively, spherical amorphous calcium phosphate (ACP) particles were obtained uniquely. Furthermore, flower-like hydroxyapatite (HAP) aggregates composed of many nano-sized needles were formed from the solutions with high pH values (8 for calcium and 12 for phosphate) and concentrations (500 mM for calcium and 37.5 mM for phosphate). The novel diffusion system is proposed to play an important role in both studying the process of biological mineralization and synthesizing calcium phosphates in different forms.     

The seeded growth of calcium phosphates. Surface characterization and the effect of seed material

The kinetics of growth of calcium phosphate on seed materials hydroxyapatite, “amorphous” calcium phosphate, enamel, and dentine has been studied at 25°C and at constant physiological pH in stable supersaturated solutions of calcium phosphate. The grown material was characterized chemically, by x-ray powder diffraction and specific surface area measurements and by scanning electron microscopy. In addition, dissolution kinetic studies have been made of the material grown at different times in the crystal growth experiments and of “amorphous” calcium phosphate. The crystallization experiments were made over a range of initial supersaturation to obtain information about the nature of the calcium phosphate phases formed. The specific surface area of the grown phases show marked changes during the growth experiments. The results of the dissolution experiments indicate that the solid material formed in the early stages of crystallization has a stoichiometry corresponding to OCP.     

医用CVIC/C复合材料表面仿生沉积生物活性钙磷涂层

为了在医用化学气相渗透工艺(CVI)C/C复合材料表面制备生物活性钙磷涂层,用阴极声电化学工艺处理CVIC/C复合材料,再将试样浸泡于过饱和钙磷溶液中,使其诱导钙磷晶体生长.XPS研究表明,经声电工艺处理后,CVIC/C复合材料表面发生了改性;SEM,XRD和FTIR研究表明,在过饱和钙磷溶液中,未经声电化学处理的CVIC/C复合材料不具有诱导钙磷晶体生长的功能,而改性的CVIC/C表面能够诱导钙磷晶体生长,形成片状五水磷酸八钙(OCP)涂层.同时讨论了OCP沉积的形成机理.     

Formation of composite scaffolds based on chitosan and calcium phosphate

Composite materials based on chitosan and calcium phosphates with the structure and properties close to those of natural human tissues were developed. The possibility of formation of biological apatite, i.e., octacalcium phosphate, was demonstrated. The increase in the strength characteristics was caused by crystallization of the octacalcium phosphate phase on the surface of the material pore space. The materials resemble the extracellular matrix of the body in their structure and composition and can find wide use in the regenerative medicine as a scaffold for osteogenic factors.     

Biomimetic gelatin-octacalcium phosphate core-shell microspheres

Calcium phosphate/polymeric microparticles synthesized through a biomimetic approach are regarded with increasing interest for their various potential applications, including tissue engineering and regenerative medicine. Herein we report the synthesis and characterization of gelatin/octacalcium phosphate core/shell microspheres. Deposition of the calcium phosphate shell on the polymeric microspheres was obtained through bio-inspired mineralization on the surface of functionalized gelatin microparticles. Gelatin microspheres stabilized by alginate dialdehyde were prepared using an inverse microemulsion. Functionalization was achieved by enriching the microspheres composition with calcium ions or, alternatively, with alendronate, a bisphosphonate widely employed for the treatment of bone diseases. Functionalization and synthesis of the inorganic phase in the microemulsion environment were key factors for the achievement of a complete coating of the microspheres with calcium phosphate. The inorganic shell is constituted of small crystals of octacalcium phosphate, which control gelatin and alendronate release.     

In situ formation of porous mineral–polymer scaffold for tissue engineering

A principle of three-dimensional printing of composite materials with structure and properties close to natural human tissues was implemented. It was found that the inorganic component, dicalcium phosphate dihydrate, crystallizes in the bulk of alginate matrix. The subsequent chemical treatment of experimental samples in buffered solutions promoted the formation of octacalcium phosphate on the surface of pore space of the printed material.     

Hydroxyapatite of platelet morphology synthesized by ultrasonic precipitation from solution

Hydroxyapatite (HA) synthesis by precipitation with urea from aqueous solutions of calcium nitrate and ammonium hydrogenphosphate is studied. Ultrasonication during the synthesis decreases the size of platelet HA crystals from several micrometers to 200–300 nm. At low calcium concentrations in solution, the crystallizing phase is carbonate-hydroxyapatite, whereas at high calcium concentrations, octacalcium phosphate (OCP) precedes hydroxyapatite crystallization.     

Thermochemistry of the mixed calcium phosphate Ca8P2O7(PO4)4

The calcium mixed phosphate Ca₈P₂O₇(PO₄)₄ has been synthesized by thermal decomposition of octacalcium phosphate previously prepared by precipitation in ammoniacal phosphate solution. The enthalpy of formation at 298.15 K referenced to β-tricalcium phosphate and calcium pyrophosphate is determined. β-Tricalcium phosphate was prepared by two methods: precipitation in ammoniacal aqueous medium and high temperature solid-state reaction. Calcium pyrophosphate was prepared by high temperature solid-state reaction. All the compounds are characterized by chemical analysis, X-rays diffraction and IR spectroscopy. The enthalpy of formation +10.83 ± 0.63 kJ mol⁻¹ is obtained by solution calorimetry at 298.15 K in nitric acid.     

In situ phosphorus K-edge X-ray absorption spectroscopy studies of calcium–phosphate formation and transformation on the surface of bacterial cellulose nanofibers

In this work, for the first time, in situ formation and transformation process of embryo calcium phosphate (Ca–P) minerals on three-dimensional bacterial cellulose nanofibers was investigated. Combined with XRD, X-ray absorption near-edge structure results revealed that the embryo precursor was amorphous calcium phosphate which was subsequently converted to β-tricalcium phosphate, octacalcium phosphate, and finally to the more thermodynamically stable form of hydroxyapatite. The methodology reported herein may be extended to the studies of Ca–P and other minerals on various substrates.     

A Dendritic Amphiphile for Efficient Control of Biomimetic Calcium Phosphate Mineralization

The phase behavior of a dendritic amphiphile containing a Newkome‐type dendron as the hydrophilic moiety and a cholesterol unit as the hydrophobic segment is investigated at the air–liquid interface. The amphiphile forms stable monomolecular films at the air–liquid interface on different subphases. Furthermore, the mineralization of calcium phosphate beneath the monolayer at different calcium and phosphate concentrations versus mineralization time shows that at low calcium and phosphate concentrations needles form, whereas flakes and spheres dominate at higher concentrations. Energy‐dispersive X‐ray spectroscopy, X‐ray photoelectron spectroscopy, and electron diffraction confirm the formation of calcium phosphate. High‐resolution transmission electron microscopy and electron diffraction confirm the predominant formation of octacalcium phosphate and hydroxyapatite. The data also indicate that the final products form via a complex multistep reaction, including an association step, where nano‐needles aggregate into larger flake‐like objects.

     

Preparation of quaternized dimethylaminoethylmethacrylate grafted nonwoven fabric for the removal of phosphate

Dimethylaminoethylmethacrylate (DMAEMA) grafted polyethylene/polypropylene (PE/PP) nonwoven fabric was prepared by radiation-induced graft polymerization. Grafting conditions were optimized and about 150% DMAEMA grafted samples were used for further experiments. DMAEMA graft chains were later quaternized with dimethyl sulphate for the removal of phosphate ions. Adsorption experiments were conducted with quaternized DMAEMA grafted fabric for phosphate removal at low (0.5–25 ppm) and high phosphate concentrations (50–1000 ppm). Adsorbed phosphate amounts at pH 7 were found to be 63 mg phosphate/g polymer and 512 mg phosphate/g polymer for low (25 ppm) and high phosphate concentrations (1000 ppm) respectively showing the efficiency of the adsorbent material in removing phosphate. The pH effect on phosphate adsorption showed that the quaternized DMAEMA grafted nonwoven fabric can adsorb phosphate over a wide pH range (5.00–9.00) indicating that adsorbent material can effectively remove different forms of phosphate ions, namely H₂PO₄^?, HPO₄^2? and PO₄^3? in aqueous solution at this pH range where the species exist. Competitive adsorption experiments were also carried out with two concentration levels at pH 7 to investigate the effect of competing ions. Phosphate adsorption on quaternized DMAEMA grafted nonwoven fabric was found to be higher than the other competing ions at two concentration levels. At high concentration level, the adsorption order was phosphate>nitrite>bromide>sulphate>nitrate whereas at low concentration level, the order was phosphate?sulphate>bromide>nitrite>nitrate.     

Influence of polyaspartic acid and phosphophoryn on octacalcium phosphate growth kinetics

Polyaspartic acid (PAA) and phosphophoryn (PPn) have been suggested to adsorb specifically on the (100) and (010) faces, respectively, of octacalcium phosphate (Ca₄H(PO₄)₃ · 2.5H₂O, OCP). In this study, the extent of adsorption and influence of these molecules on OCP crystal growth has been investigated. For kinetic studies, protein effects on crystal growth were examined in solutions sustained at a constant level of supersaturation at pH 6.00 and ionic strength of 0.08 mol l⁻¹. The maximum adsorbed mol surface concentration for PPn was 100-fold less than that for PAA. Inhibitory effects interpreted in terms of mol surface coverage showed PPn to retard OCP growth more effectively than PAA. However, when considering percentage of crystal face covered by protein, PAA and PPn showed similar maximum adsorption concentrations onto the (100) and (010) crystal faces, respectively. PAA inhibited OCP growth by 20% when only 1% of the (100) face (1% total crystal area) was covered. PPn had to reach over 200% (010) face coverage (or 28% total crystal area) before a similar level of crystal growth inhibition was obtained. This difference in inhibitory effect may be the result of a more effective β-strand conformation of the shorter PAA molecule or may indicate that growth at the (100) face is rate controlling and, therefore, less than 1% coverage of this face is needed before a significant decrease in rate is observed.     

An understanding of renal stone development in a mixed oxalate-phosphate system

The in vivo formation of calcium oxalate concretions having calcium phosphate nidi is simulated in an in vitro (37 degrees C, pH 6.0) dual constant composition (DCC) system undersaturated (sigma DCPD = -0.330) with respect to brushite (DCPD, CaHPO 4 . 2H 2O) and slightly supersaturated (sigma COM = 0.328) with respect to calcium oxalate monohydrate (COM, CaC2O4 . H2O). The brushite dissolution provides calcium ions that raise the COM supersaturation, which is heterogeneously nucleated either on or near the surface of the dissolving calcium phosphate crystals. The COM crystallites may then aggregate, simulating kidney stone formation. Interestingly, two intermediate phases, anhydrous dicalcium phosphate (monetite, CaHPO4) and calcium oxalate trihydrate (COT), are also detected by X-ray diffraction during this brushite-COM transformation. In support of clinical observations, the results of these studies demonstrate the participation of calcium phosphate phases in COM crystallization providing a possible physical chemical mechanism for kidney stone formation.     

Crystal morphology, composition, and dissolution behavior of carbonated apatites prepared at controlled pH and temperature

A range of carbonated apatites was prepared by aqueous precipitation at 37, 60, and 85°C and at controlled pH values varying from 6.00–9.50 in 0.25 increments. The products were analyzed for Ca, P, Sr, Mg, Na, F, and carbonate. Their initial dissolution rates were measured in a pH 4.5, 0.01 mol · liter⁻¹ acetate buffer. Information about crystal morphologies and crystal defects was obtained by x-ray diffraction and high-resolution transmission electron microscopy (TEM). The molar ratios of the products, together with their Sr and Mg contents, increased with increasing pH. Initial dissolution rates of the products, when adjusted for carbonate content, were in the order 37 > 60 > 85°C whereas apparent particle sizes determined by TEM and x-ray diffraction were ordered 37 < 60 < 85°C. Carbonated apatites precipitated at pHs of 7.0 or less were observed to have planar defects parallel to (100) that were identified as unit-cell-thick intergrowths of octacalcium phosphate. Carbonated apatites precipitated at higher pHs and noncarbonated apatites did not have these defects. A crystal growth mechanism is proposed to account for the presence of the (100) defects.     

Photodecarboxylation of ketoprofen in aqueous solution. A time-resolved laser-induced optoacoustic study

The photodecarboxylation reaction of 2-(3-benzoylphenyl)propionate (ketoprofen anion, KP-) was studied in water and in 0.1 M phosphate buffer solutions in the pH range 5.7-11.0 by laser-induced optoacoustic spectroscopy (LIOAS, T range 9.5-31.6 degrees C). Upon exciting KP- with 355 nm laser pulses under anaerobic conditions, two components in the LIOAS signals with well-separated lifetimes were found (tau 1 < 20 ns; 250 < tau 2 < 500 ns) in the whole pH range, whereas a long-lived third component (4 < tau 3 < 10 microseconds) was only detected at pH < or = 6.1. The heat and structural volume changes accompanying the first step did not depend on pH or on the presence of buffer. The carbanion resulting from prompt decarboxylation within the nanosecond pulse (< 10 ns) drastically reduces its molar volume ([-18.9 +/- 2.0] cm3/mol) with respect to KP- and its enthalpy content is (256 +/- 10) kJ/mol. At acid pH (ca 6), a species is formed with a lifetime in the hundreds of ns. The enthalpy and structural volume change for this species with respect to KP- are (181 +/- 15) kJ/mol and (+0.6 +/- 2.0) cm3/mol, respectively. This species is most likely a neutral biradical formed by protonation of the decarboxylated carbanion, and decays to the final product 3-ethylbenzophenone in several microsecond. At basic pH (ca 11), direct formation of 3-ethylbenzophenone occurs in hundreds of ns involving a reaction with the solvent. The global decarboxylation reaction is endothermic ([45 +/- 15] kJ/mol) and shows an expansion of (+14.5 +/- 0.5) cm3/mol with respect to KP-. At low pH, the presence of buffer strongly affects the magnitude of the structural volume changes associated with intermolecular proton-transfer processes of the long-lived species due to reactions of the buffer anion with the decarboxylated ketoprofen anion.     

Homogeneous nucleation of droplets from supersaturated vapor in a closed system

Kinetic equations describing homogeneous nucleation kinetics within standard model are solved numerically under the condition of a constant number of molecules in the considered system. It has consequences to decrease the supersaturation of the supersaturated vapor during the process of the formation of small droplets of a new phase. The decrease of supersaturation occurs in a short time and reaches some value which remains unchanged for a relatively long time (quasistationary regime), especially at lower initial supersaturations. This time interval decreases with increasing value of the initial supersaturation. In the quasistationary regime the nucleation rate reaches its stationary value. At higher initial supersaturation, the rate of formation of nuclei goes to some maximum value corresponding to the stationary nucleation rate and then decreases with time due to the decrease of supersaturation.