Formation Of Octacalcium Phosphate Research Articles

Adherent octacalciumphosphate coating on titanium alloy using modulated electrochemical deposition method

Our aims in this study were (1) to develop an electrochemical method of depositing adherent octacalciumphosphate (OCP) and other calcium phosphate coatings on titanium alloy (Ti6Al4V) substrates of different shapes and surface preparations, (2) to determine the properties of the coating (composition, morphology, thickness, dissolution), and (3) to observe transformation of OCP to carbonatehydroxyapatite (CHA) in simulated body fluid (SBF). Titanium (Ti)-alloy plates, tensile bars with four types of surfaces (grit-blasted with apatitic abrasive, chemically textured, arc-deposited, and Co-Cr-beaded) and dissolution cylinders were electrochemically coated with the use of modulated pulse time electric fields programmed with a custom-made dual microprocessor. Modulated electrochemical deposition (MECD) was carried out with pH and temperature conditions favorable for OCP formation. Coatings were characterized using X-ray diffraction, FT-IR, scanning electron microscopy, tensile strength tests, and solubility tests. XRD and FT-IR analyses showed that pure, uniform OCP coatings were produced on Ti6Al4V surfaces with coating-to-substrate tensile strengths greater than 7,000 psi. Coatings on Ti arc-deposited surfaces, chemically textured surfaces, and Co-Cr-beaded surfaces all gave tensile strengths ranging from 5,000 to 7,000 psi, with no coating shadows in the crevices. Dissolution of OCP coating in 100 mL of 0.1 M Tris buffer solution was determined from the amount of calcium (Ca) released onto the buffer, which was 7.7 +/- 1.0 ppm Ca at pH 7.3 after 4 h, and 22 -/+ 1.4 ppm Ca at pH 3 after 2 h. We found that OCP crystal size can be controlled by the current density and relative pulse time modulation. Our study demonstrated the following: (1) Highly adherent calcium phosphate (e.g., OCP) coating of uniform compositions (e.g., OCP) on Ti-alloy substrates can be obtained at low temperatures with the use of MECD by optimizing pulse time modulation of the electric field, reaction pH, temperature, and electrolyte composition; and (2) OCP readily transforms to CHA when exposed to SBF.

Mineralisation of two phosphate ceramics in HBSS: role of albumin

The role of albumin in the mineralisation process of commercial hydroxyapatite (HAp) and synthesised biphasic (HAp-tricalcium phosphate) ceramics in a bufferless simulated inorganic plasma (HBSS) was investigated by conventional in vitro tests and static and dynamic wettability measurements. Albumin was either pre-adsorbed or solubilised in HBSS. It was found that calcium complexation by albumin plays a key role in early mineralisation kinetics, so that mineralisation is favoured when albumin is pre-adsorbed and hindered when it is dissolved in HBSS. In the biphasic ceramic this picture is complicated by the fact that albumin, in solution, seems to promote the dissolution of tricalcium phosphate, and simultaneously compete for calcium with the ceramic. It also appears that albumin has a stabilising effect of octacalcium phosphate present in deposits on commercial HAp. The same effect may be present in the case of the biphasic ceramic, at earlier mineralisation times, when octacalcium phosphate appears as a precursor of HAp. Octacalcium phosphate formation on commercial apatite is accompanied by carbonate substitution in phosphate positions.

Formation of octacalcium phosphate in vitro.

Formation of octacalcium phosphate in vitro.

Effects of Nickel on Calcium Phosphate Formation

We have investigated the effect of nickel on calcium phosphate formation from aqueous solutions. The calcium phosphates prepared under different reaction conditions (pH, temperature, and nickel concentration) were characterized by X-ray diffraction, FTIR spectroscopy, and chemical analysis. The apatite compounds were also studied thermogravimetrically. From the combined results of the techniques employed we have determined that nickel favors the formation of brushite and amorphous calcium phosphate. We have found, as well, that the presence of nickel in the solution inhibits calcium hydroxyapatite (CaHAP) and octacalcium phosphate formation. However in the synthesis performed at basic pH and 95°C the apatitic phase (HAP) could be obtained. The present results suggest that the presence of nickel may modify the precipitation of oral calcium phosphate.

Effects of Ca addition on the formation of octacalcium phosphate and apatite in solution at pH 7.4 and at 37°C

Effects of Ca addition on formation of octacalcium phosphate (OCP) and apatite were studied in solution at pH 7.4 and at 37°C. Ca solution was added at 0.02–0.1 ml/min into reacting calcium phosphate solution with initial solution conditions of Ca: 0.55–0.91 mM; PO 4: 6.3–13.9 mM; Ca/PO 4=0.043–0.14. The ratio of OCP to apatite in the product decreased as the speed of Ca addition decreased and this tendency became prominent as the Ca/PO 4 increased. The Ca addition at an appropriate speed was crucial to maintain the degree of supersaturation to OCP (DS(OCP)) during reaction at a certain level, thereby regulated the formation and transition of OCP. The lifetime of OCP increased and the transition from OCP to apatite became slow, with an increase in the duration of Ca addition. The morphology and lattice dimension of apatites obtained by slow transition were different from those of apatite obtained by fast transition. It was inferred that in vivo crystallization of apatite and property of the resultant apatite crystal are affected, in part, by the activity of the influx of Ca 2+ ions into the calcifying region.

Surface Induced Calcium Phosphate Nucleation and Growth

AbstractCalcium phosphate nucleation on colloidal oxide surfaces, such as TiO2, SiO2 and Al2MO3, has been studied as a model system to understand the role of surface chemistry on crystal nucleation kinetics and phase formation. The nucleation induction times have been measured using Constant Composition (CC) technique and calcium phosphate phases formation have been determined mainly by X-ray diffraction. The results indicated TiO2 not only significantly reduces the nucleation induction time and interfacial energy but also stimulates octacalcium phosphate formation over dicalcium phosphate dihydrate. SiO2 and Al2O3 have little effect on both nucleation kinetics and phase formation.

Effects of CO 2-3 ion on the formation of octacalcium phosphate at pH 7.4 and 37°C

The effects of carbonate (CO 2- 3) ion on the formation of octacalcium phosphate (OCP) were studied at pH 7.4 and 37°C. The reaction was carried out in solutions with various amounts of KHCO 3 (0–20mM) and with an initial solution Ca/P ratio of 0.075–0.25. CO 2- 3 ion changes the OCP/apatite ratio of the product, depending upon the Ca/P ratio of the solution, that is, with the increase in CO 3 concentration, the amount of OCP increases and then decreases, while the amount of apatite decreases and then increases: when the Ca/P of the solution is high, a smaller amount of CO 2- 3 ion disturbs the crystal growth of OCP. The morphology of the products is plate-like under various amounts of CO 3, but the thickness decreases and the crystal size becomes small with increasing CO 3 concentration. These results indicate that CO 2- 3 ion significantly affects the product and morphology of final crystals. Possible roles of CO 2- 3 ion in the biological crystallization are discussed.

The low temperature formation of octacalcium phosphate

The low temperature formation of octacalcium phosphate (Ca 8(HPO 4) 2(PO 4) 4·5H 2O) was investigated. Octacalcium phosphate (OCP) was formed by the hydrolysis of α-tricalcium phosphate (α-TCP), and by reaction between monocalcium phosphate monohydrate (MCPM) and tetracalcium phosphate (TetCP). Relationships between phase formation, microstructural evolution, and variations in solution chemistry were examined. Hydrolysis of α-TCP to form OCP occurs more rapidly at elevated temperatures. At the highest temperature studied, 70°C, initial precipitation of OCP occurs in about one hour, but its inevitable hydrolysis to the more stable HAp phase takes place over several days. At room temperature, nearly three days are required to initiate OCP formation, yet remains a final product phase for well over a period of months. When the initial solution pH is less than 7, pure phase OCP is the final product, while HAp forms when initial pH values are higher than this. Furthermore, OCP precipitates faster at the highest initial pH values where its formation is observed. OCP formation by reaction between MCPM and TetCP is also dependent on temperature and time of reaction. For temperatures between 40 and 55°C and at reasonable times (less than four days) the product is phase pure OCP. After that time, inevitable hydrolysis of the OCP product to HAp occurs. Between 30 and 40°C, DCPD (CaHPO 4·2H 2O) is simultaneously present with OCP, and below 30°C, HAp and DCPD coexist within this aforementioned time span. Conversely, between 55 and 60°C, DCP (CaHPO 4) and OCP are the product phases (in the allowed time of reaction, before the eventual degradation of OCP), and above 60°C HAp and DCP are the final products.

Immobilized DPP and other proteins modify OCP formation.

Osteonectin, gamma-carboxyglutamic acid-containing (Gla) protein, and dentin phosphoprotein were covalently attached to sepharose beads and inoculated in solutions at two different degrees of supersaturation with respect to both octacalcium phosphate (OCP) and hydroxyapatite. In both solutions, the inhibitory activity towards de novo formation of calcium phosphate that these proteins display when freely dissolved in solution was completely eliminated when they were immobilized on the sepharose at concentrations of up to 5 micrograms/mg wet beads. In the solution that was more highly supersaturated with respect to OCP, the immobilized dentin phosphoprotein, moreover, was found to induce de novo formation of OCP in proportion to the concentration of the protein immobilized. For example, at 10 micrograms/ml of the immobilized dentin phosphoprotein, the induction period was reduced more than 50%. However, in the solution considerably less supersaturated with respect to OCP, none of the immobilized proteins were capable of inducing OCP or apatite deposition. These findings suggest that the immobilized dentin phosphoprotein could work as a nucleating substrate for the OCP phase in solutions where calcium and phosphate concentrations are sufficiently higher than equilibrium saturation levels for the OCP phase.

The phase transformation of calcium phosphate dihydrate into octacalcium phosphate in aqueous suspensions

The transformation of dicalcium phosphate dihydrate into octacalcium phosphate (OCP) under carefully controlled conditions in calcium phosphate solutions of pH 6.4–7.0 has been investigated using a pH-stat technique. During the initial dissolution of DCPD, with roughening of the crystal surface, the nucleation of OCP takes place and transformation continues with an appreciable consumption of alkali, to maintain the pH, during the growth of these OCP nuclei. In solutions undersaturated with respect to DCPD, the transformation reactions are facilitated due to the increased DCPD surface roughness produced by the more extensive initial dissolution. An increase in the amount of DCPD seed crystals also increases the rate of OCP formation. The transformation is markedly inhibited by the presence of magnesium ions which have been shown in previous constant composition experiments to have a greater influence in reducing the rate of OCP crystal growth as compared with that of DCPD.

Formation of octacalcium phosphate and subsequent transformation to hydroxyapatite at low supersaturation: a model for cartilage calcification.

By means of X-ray powder diffraction (XRD), octacalcium phosphate (OCP, Ca8H2(PO4)6 X 5H2O, Ca/P = 1.33) has been shown to be a metastable precursor of hydroxyapatite (HA, Ca10(PO4)6(OH)2, Ca/P = 1.67) during de novo HA formation in aqueous solutions containing [CaCl2] = 1.0-2.0 mM and [Na2HPO4] less than or equal to 3 mM, kept at 37 degrees C, 300 mosM and stirred gently at 150 rpm. At the precipitation boundary with initial pH = 7.4, OCP is stable for at least 24 hours when [Ca] = 1.0 mM, and is less stable when [Ca] = 2.0 mM, transforming into a mixture of OCP and HA within 24 hours. Almost complete transformation to HA within 24 hours takes place with high [Ca] and high [Pi]. Statistical analysis of the pH 7.4 precipitation boundary data supports the XRD findings: although [Ca][Pi] values vary significantly (P less than 0.001) with [Ca] (2.70 +/- 0.05 mM2 for [Ca] = 1.0 mM and 2.00 +/- 0.10 mM2 for [Ca] = 2.0 mM), [Ca]1.33[Pi] values do not vary with [Ca] suggesting that the initial precipitation process is 6(1.33 Ca + Pi)----OCP. With initial pH = 7.6, a different precipitation boundary with lower [Ca][Pi] values has been determined. These findings strongly support the fact that rat epiphyseal cartilage fluid which has [CaUF][PiUF] = 2.6 mM2 (UF = ultrafiltrate) and pH 7.6 [2] should be able to support de novo calcification.

The mineralization of collagen in vitro

The kinetics of crystallization of calcium phosphates on bovine tendon collagen (type I) has been studied using the constant composition method. During the experiments, the supersaturation was maintained constant by the potentiostatically controlled addition of solutions containing calcium and phosphate ions at the molar ratio of the phase being precipitated on the collagen substrate. In solutions supersaturated with respect to hydroxyapatite, this phase grows directly on the collagen surfaces with a typical plate-like apatitic morphology. The kinetics of the reaction reflects a polynucleation mechanism with a rate of growth dependent upo the fourth power of the relative supersaturation with respect to hydroxyapatite. At higher supersaturations, the constant composition results indicate the formation of octacalcium phosphate at the collagen surface but only following an initial induction period. This phase subsequently hydrolyzes to hydroxyapatite. Calculated surface energies of the nucleating phases are close to those anticipated for inorganic calcium salts. Crystal nuclei sizes, estimated using classical nucleation theory, verified the possibility of nucleation of both octacalcium phosphate and hydroxyapatite in the 64 nm collagen spacings.

A thermodynamic analysis of the secondary transition in the spontaneous precipitation of calcium phosphate.

A thermodynamic analysis has been made of the secondary transition stage in the spontaneous precipitation of calcium phosphate following the amorphous-crystalline transformation. The first formed crystalline material has a solubility similar to that of octacalcium phosphate (OCP) and the computed thermodynamic solubility product remains invariant in the pH range 7.00--8.60. The duration of the secondary stage is sensitive to pH and the transition appears to occur by hydrolysis of the first formed OCP-like phase to a more basic apatitic phase with a tricalcium phosphate (TCP) stoichiometry. The crystalline material at the end of this transition has an invariant solubility product, in the pH range 7.00 to 8.60, when the TCP-like molecular formula is assumed. Changes in the solution chemistry which accompany the solid-to-solid transitions are consistent with the above conclusions. The results of this study are also consistent with those of a previous study which suggest that the stability of the amorphous calcium phosphate phase is dependent upon the instability of the solution phase with respect to OCP formation.