Aqueous deposition of calcium phosphates and silicate substituted calcium phosphates on magnesium alloys

Attempts were made to deposit homogeneous films of calcium phosphates (CaPs) on two magnesium alloy systems, AZ31 and Mg-4Y, through an aqueous phosphating bath method. The deposition of silicate substituted CaPs by this aqueous method was also explored as silicate substitution is believed to increase the bioactivity of CaPs. The effect of doped and undoped coatings on the in vitro degradation and bioactivity of both alloy systems was studied. FTIR and EDX confirmed the deposition of Ca, P, and Si on both alloys and the coatings appeared to consist primarily biphasic mixtures of hydroxyapatite and β-TCP. These largely inhomogeneous coatings, as observed by SEM, were not shown to have any significant effect on maintaining the physiological pH of the culture medium in comparison to the uncoated samples, as the pH remained approximately in the 8.4-8.7 range. Interestingly, despite similar pH profiles between the coated and uncoated samples, CaP coatings affected the degradation of both alloys. These doped and undoped calcium phosphate coatings were observed to decrease the degradation of AZ31 whereas they increased the degradation of Mg-4Y. In vitro studies on cell attachment using MC3T3-E1 mouse osteoblasts showed that between the uncoated alloys, Mg-4Y appeared to be the more biocompatible of the two. Silicate substituted CaP coatings were observed to increase the cell attachment on AZ31 compared to bare and undoped CaPs coated samples, but did not have as great of an effect on increasing cell attachment on Mg-4Y.     

Enzyme-catalysed synthesis of calcium phosphates

A biomimetic method is described for the precipitation of nanosized calcium phosphates using the alkaline phosphatase (EC 3.1.3.1), which is responsible for hydrolysis of organic and inorganic phosphates in vivo. Buffered solutions containing glycerol-2-phosphate and CaCl₂ in addition to MgCl₂ and the respective enzyme were prepared for calcium phosphate precipitation. The phosphate group of glycerol-2-phosphate was cleaved through enzymatic hydrolysis. The local inorganic phosphate concentration increased resulting in the precipitation of nanosized calcium phosphates phases (Ca–P phase) composed of calcium deficient hydroxyapatite (CDHA) and hydroxyapatite (HA). At high Ca²⁺-concentration and large enzyme amounts mixed phases of HA/CDHA with an increasing quantity of HA were favoured. Under basic conditions (pH > 9) formation of HA was observed, whereas at neutral pH of 7.5 CDHA was primarily formed. The assignment of Ca–P phases was accomplished by FT-IR and Raman-spectroscopy in addition to X-ray diffractometry. The Ca–P materials exhibited BET surface areas of 173 m²/g. SEM-micrographs of the Ca–P powders showed globular-shaped agglomerates of Ca–P particles. The size of the Ca–P crystallite ranged from 9 nm to 25 nm according to transmission electron microscopy (TEM), where round-shaped, platelike and fibrelike crystallites were found. All crystallites showed diffuse ring patterns in electron diffraction confirming the nanosize of the precipitate. Using the developed technique, it was possible to synthesise 100 g of bonelike Ca–P materials in 1 day using 15 L batches with optimised parameters.     

Sintering of biphasic calcium phosphates

Biphasic calcium phosphate (BCP) discs were fabricated and then sintered using two different sintering programs to establish whether the phases present could be controlled at low and high sintering temperatures. X-ray diffraction (XRD) was used to establish the phases present after sintering and scanning electron microscopy (SEM) determined the microstructure. Sintering program 1 involved a simple heating and cooling schedule and temperatures of 1100, 1250, 1275 and 1300°C. It produced samples containing an additional alpha-tricalcium phosphate (α-TCP) phase at temperatures above 1100°C. The original ratio of hydroxyapatite/beta-tricalcium phosphate (HA/β-TCP) could not be maintained above this temperature. Sintering program 2 combined the heating and cooling schedules of the first program with a 900°C hold stage to allow α-TCP to β-TCP conversion to take place. At temperatures of 1250 and 1275°C, this program was successful in completely removing the α-TCP phase and preserving the HA:β-TCP ratio. The SEM results show that the surface morphology of the discs was not greatly affected by choice of sintering program.     

Electrolytic deposition of magnesium-substituted hydroxyapatite crystals on titanium substrate

Electrolytic deposition of magnesium-substituted hydroxyapatite (Mg-HAp) coatings on titanium was investigated and Mg-HAp crystals with up to 2 wt.% Mg²⁺ were deposited in electrolytes with various Mg²⁺ concentration. The incorporation of Mg substantially changed the morphology of the HAp crystals and decreased the crystal size and crystallinity of the HAp. The similarity with natural dentin and bone in composition and the increased specific surface of the Mg-HAp coatings on the Ti substrates were believed to benefit the bioactivity and the drug-carrying properties of the coatings.     

Doped nanocrystalline calcium carbonate phosphates

The formation of nanocrystalline calcium carbonate phosphates doped with Fe²⁺, Mg², Zn²⁺, K⁺, Si⁴⁺, and Mn²⁺ has been studied by X-ray diffraction, IR spectroscopy, differential thermal analysis, and energy dispersive X-ray fluorescence analysis. The results indicate that the synthesis involves the formation of hydroxy carbonate complexes from the three calcium carbonate polymorphs (calcite, vaterite, and aragonite) in a solution of ammonium chloride and ammonium carbonate, followed by reaction with orthophosphoric acid. This ensures the preparation of a bioactive material based on chlorophosphates, octacalcium hydrogen phosphate, and calcium chloride hydroxide phosphates containing cation vacancies. Particle-size analysis data show that the materials contain nanoparticles down to 10 nm in size. Heat treatment of the doped calcium carbonate phosphates produces calcium hydroxyapatite containing cation vacancies, which can be used as a bioactive ceramic.     

Pressure induced amorphization in calcium phosphates

Polycrystalline hydroxylapatite (HA), hydrated tricalcium phosphate (HTCP), tricalcium phosphate ( TCP), dicalcium phosphate dihydrate (DCPD) and dicalcium phosphate anhydrous (DCPA) were subjected to various pressures upto 10 GPa and the retrieved materials were examined by XRD and FTIR. These compounds showed amorphization at pressures upto 10 GPa, the pressure being lowest for HTCP and DCPA. At intermediate pressures, the amorphous phase obtained was anisotropic. Significant changes in the infrared spectra were observed in all materials except DCPA. These changes are due to the lowering of site symmetry on amorphization.     

Biomimetic bone scaffolds based on chitosan and calcium phosphates

This paper reports a novel biomimetic technique for obtaining chitosan-calcium phosphate composite scaffolds (Cs-CP) by calcium phosphate precipitation from its precursors,CaCl₂ and NaH₂PO₄ in the presence of ammonia and Cs, followed by product freeze-drying. The structural and morphological analysis showed the arrangement of CP onto Cs fibers, while Cs acting as glue to bind together the CP crystals. The presence of different forms of CP was evidenced, including hydroxyapatite, which is known to be involved in the formation of the new bone tissue in living organisms. Great mechanical properties of the scaffolds indicate their potential use in bone tissue engineering.     

Unreactiveness of calcium phosphates and hydroxypropylmethylcellulose in organic-mineral composites

Organic-mineral composites prepared by 121‡C heat treatment of aqueous suspensions of hydroxy-propylmethylcellulose (HPMC) and calcium dibasic phosphate dihydrate, nonstoichiometric hydroxyapatite, or a two-phase calcium phosphate were characterized by IR spectroscopy, scanning electron microscopy, x-ray diffraction, and chemical analysis with the aim of assessing the reactivity of the inorganic and organic phases. The results demonstrate that, under the experimental conditions used, there is no chemical interaction between the calcium phosphates studied and HPMC.     

Calcium metal to synthesize amorphous or cryptocrystalline calcium phosphates

Metallic calcium was never used before as the calcium source in synthesizing bioceramics. Amorphous calcium phosphate (ACP) powders were synthesized at room temperature, in synthetic mineralization solutions which contained Na⁺, K⁺, Mg^2 +, Cl^?, HCO₃^? and HPO₄^2 ? ions at concentrations similar to those found in human blood plasma, by using calcium (Ca) metal as the only calcium source. The experimental conditions leading to the formation of PCA (cryptocrystalline or poorly crystallized apatite) or CaCO₃ powders were also determined when using metallic Ca in aqueous synthesis in the mineralization solutions. The formation of calcium phosphate (CaP) in synthesis solutions was immediately initiated by the addition of calcium metal granules (or shots), at appropriate amounts, into the solutions while the solutions were being continuously stirred in glass bottles at room temperature (22 ± 1 °C). The synthesis reactions were reaching completion in less than 30 min with the final solution pH values ranging from 9 to 12, without a necessity for any external pH adjustment in the form of any strong base (such as NH₄OH, LiOH, NaOH or KOH) additions. ACP or PCA powders are useful for dentin and enamel re-mineralization applications or orthopedic (bone) defect-filling applications.     

Sonoelectrochemical deposition of calcium phosphates on carbon materials: effect of current density

Calcium phosphate (CaP) coatings on carbon fabric substrate were produced by sonoelectrodeposition at different current densities (5, 8, 13, 20 and 34 mA/cm²). The surface morphology and chemical composition of the coatings were characterized by SEM, Raman and FTIR spectra. The results showed that at 5 mA/cm² current density, the coating exhibits plate-like morphology, indicating an octacalcium phosphate (OCP) phase was pre-formed in the deposits and then converted into hydroxyapatite (HA). When the current density was increased to 8 mA/cm² and higher, the coatings exhibited needle-like morphology corresponding to a HA phase. Furthermore, the sonoelectrodeposited CaP coating exhibited denser and more uniform structures with smaller crystal sizes as the current density increased. Cathodic reaction mechanisms of CaP coatings on carbon in the sonoelectrochemical processes are proposed to explain the different kinds of calcium phosphate obtained.     

In vitro investigation of novel calcium phosphates using osteogenic cultures

A rat bone marrow stromal cell (RBM) culture was used to evaluate novel bioactive calcium phosphate ceramics. Three rapidly resorbable, glassy crystalline materials with the main crystalline phase Ca2KNa(PO4)2 were investigated (sample code GB 1a, GB 14, GB 9). These materials were designed to exhibit a higher degree of biodegradability than tricalcium phosphate. Additionally, a bioactive glass ceramic of low biodegradability was examined (sample code AP 40). RBM cells were cultured on the disc-shaped test substrata for 14 d. The culture medium was changed and calcium and phosphate concentrations of the medium were determined daily. Specimens were evaluated using light microscopy and morphometry of the cell-covered substrate surface, scanning electron microscopy and energy dispersive X-ray analysis. Except for GB 1a, the rat bone marrow cells attached and grew on all substrate surfaces. Of the different calcium phosphate ceramics tested, AP 40 facilitated osteoblast growth and the elaboration of the extracellular matrix to the highest degree followed by GB 9 and GB 14. The inhibition of cell growth encountered with GB 1a seemed to be related to its high phosphate ion release. © 1998 Chapman & Hall     

Phase transformations of calcium phosphates formed in wet field environments

The crystal phase and morphology of calcium phosphate salts precipitated in a wet field environment at temperatures between 30 and 70 °C and pHs between 3 and 8 were examined. Dicalcium Phosphate Dihydrate (DCPD) was the most prevalent phase precipitated. Using accelerated ageing study techniques, precipitates studied were aged, under dry conditions at 50 °C for 8 and 16 days, before being re-examined using XRD, FTIR and SEM techniques. DCPD was found to be most stable when precipitated at 40 °C and 5 pH. Considerably more phase transformation to Octacalcium Phosphate (OCP), Amorphous Calcium Phosphate (ACP) and Hydroxyapatite (HA) was seen at high temperatures and high pHs, and a greater tendency to form anhydrous salts was seen at high temperatures and low pHs. Using techniques such as XRD, FTIR and SEM the transformation of the DCPD precipitate to OCP was analysed and appeared to occur without the presence of an intermediate amorphous phase. However, transformation from OCP to HA did result in the formation of an intermediate amorphous phase.     

X-ray diffraction radial distribution function studies on bone mineral and synthetic calcium phosphates

An investigation of the molecular structure of bone mineral and synthetic calcium phosphates was carried out using radial distribution function (RDF) techniques. The X-ray data were collected using CuK and MoK radiation to insure the validity of the RDFs. Synthetic preparations of hydroxyapatite (HA) varying in their crystal size and crystallinity, and amorphous calcium phosphate (ACP), were studied, as well as bone samples from a 1-year-old chicken and 16-day embryonic chicks. Mixtures of embryonic bone and synthetic ACP were also investigated. The RDFs of bone and crystalline HA samples are similar in peak position, and show evidence of an atomic order extending to 2.5 nm and beyond. The RDF of ACP differs from that of HA, showing only short range order up to 0.9 nm, as well as small differences in peak shape. The decrease in intensity of the RDF function with increasing distance (r), observed with both HA and bone samples can be related to a decrease in crystallinity and crystal size. The RDF data indicate there is no significant amount of ACP in either very young or mature bone. The RDFs of the embryonic bone + synthetic ACP mixtures showed that a small amount of ACP can be readily detected in a sample of bone with a poorly crystalline mineral phase; from this we estimate the threshold for detection of ACP in bone to be 12% or less.     

Adsorption on apatitic calcium phosphates for drug delivery: interaction with bisphosphonate molecules

Bisphosphonates (BPs) are well established as an important class of drugs for the treatment and prevention of several bone disorders including osteoporosis. This work investigated the interaction of two bisphosphonates, risedronate and tiludronate, with several apatitic supports, a well-crystallised hydroxyapatite (HA) and nanocrystalline apatites with varying maturation times, chemical composition and surface characteristics. The purpose was to fully understand the adsorption mechanism and desorption process, by the evaluation of the effect of several physicochemical parameters (temperature, pH and concentration of calcium and phosphate ions). Whatever the nature of the BP and the structure and composition of the apatite, the adsorption of such anti-resorptive agents can be well described as an ion exchange-reaction between phosphates species on the apatitic surface and BP molecules in solution. However, the parameters of adsorption can vary depending on the physicochemical conditions of the adsorption reaction. In addition, the structure and composition of the apatitic surface also influence the adsorption properties. Finally, BPs molecules are slowly released from apatitic supports, because most of the adsorbed molecules are irreversibly bound and not spontaneously released by dilution or simple washing. Moreover, similar to their adsorption, the release of bisphosphonates is strongly affected not only by the chemical properties of the molecule, but also by the chemical and structural characteristics of the apatitic substrates. The understanding of the adsorption and release processes provides fundamental tools for the development of drug delivery systems using apatite materials.     

Morphological control of precipitated calcium carbonates and phosphates by colloidal additives

The precipitation of calcium carbonate and calcium phosphates from solutions with and without additives such as hydroxyethyl cellulose, colloidal silica and potato starch, has been studied by X-ray diffraction and scanning electron microscopy. A constant rate of 10^–6 mol s^–1 of reactant was added into the mixed solutions with and without additives. Nucleation frequency and morphology were dramatically altered by the addition of colloidal potato starch. It was found to be effective for non-specific nucleation of calcite and hydroxyapatite and specific nucleation of monetite. The nucleation frequency of calcite is increased by adding starch by about a factor of 10⁴. Starch also alters the shape of hydroxyapatite to fibre-like.     

Synthesis and characterization of nano-crystalline calcium phosphates with EDTA-assisted hydrothermal method

This study aims to prepare the biologically important hydroxyapatite (HA), dicalcium phosphate dihydrate (DCPD) and dicalcium phosphate anhydrous (DCPA) phases in simple EDTA – assisted hydrothermal method. X-ray diffraction results showed that pure DCPD, DCPA and HA nano-crystals were obtained in the Ca–EDTA/PO₄ solutions at 120 °C, 180 °C and 210 °C, respectively. Thermal gravimetric analysis of the DCPD precipitates revealed that phase transformations of DCPD to DCPA and DCPA to HA occurred at 139 °C and 195 °C, respectively, which resulted in the different Ca–P phases during hydrothermal synthesis at different temperature ranges.     

Sorption of strontium ions from solutions onto calcium and magnesium phosphates

A comparative study of the uptake of Sr ions from aqueous solutions with calcium and magnesium phosphates of various chemical compositions was made. Calcium and magnesium phosphates in the concentration range 10–5000 mg L^?1 absorb up to 280 mg g^?1 Sr, with the ability of magnesium phosphates to take up Sr ions exceeding that of calcium phosphates by an order of magnitude. The degree of decontamination of solutions from ⁹⁰Sr with the magnesium-containing samples exceeds 80%, and the distribution coefficient is (4.7–5.4) × 10³ cm³ g^?1.     

Magnetic field effects on electrochemical metal depositions

This paper discusses recent experimental and numerical results from the authors'' labs on the effects of moderate magnetic (B) fields in electrochemical reactions. The probably best understood effect of B fields during electrochemical reactions is the magnetohydrodynamic (MHD) effect. In the majority of cases it manifests itself in increased mass transport rates which are a direct consequence of Lorentz forces in the bulk of the electrolyte. This enhanced mass transport can directly affect the electrocrystallization. The partial currents for the nucleation of nickel in magnetic fields were determined using an in situ micro-gravimetric technique and are discussed on the basis of the nucleation model of Heerman and Tarallo. Another focus of the paper is the numerical simulation of MHD effects on electrochemical metal depositions. A careful analysis of the governing equations shows that many MHD problems must be treated in a 3D geometry. In most cases there is a complex interplay of natural and magnetically driven convection.     

Influence of iron and temperature on the crystallization of calcium phosphates at the physiological pH

The effect of iron and temperature on the crystallization of dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) in agarose was studied at various temperatures (27, 37 and 47 °C) at the physiological pH (7.4) by the single diffusion method. The spherulitic crystals of hydroxyapatite with a diameter of about 3 mm were observed in the presence of Fe at temperature of 47 °C. The spherulites consisted of thin, elongated needlelike crystals. Quantification of the iron incorporated in the crystals was carried out by elemental analysis. The crystals were characterized by XRD, FTIR and SEM techniques.