Calcium phosphate crystallization under terrestrial and microgravity conditions

Calcium phosphate crystalline powders grown under terrestrial and space (EURECA 1992-1993 flight) conditions in the Solution Growth Facility are analyzed and compared by optical and electron microscopy (scanning and transmission), electron and X-ray microdiffraction and microanalyses. On earth, only small, micrometer size scale, spherolites of hydroxyapatite (HAP) grow. In space, the HAP spherolites reach hundreds of micrometer. Also, octacalcium phosphate (OCP) spherolites up to 3 mm have been obtained. Computer modelling of diffusion in a real chamber has been performed. It suggests high spatial supersaturation gradients at zero gravity which may provide much higher local supersaturations on earth, where convection takes place. The analyses suggest that the dramatic difference between the terrestrial and space samples should come from much lower supersaturation in space.     

The mechanism of biological mineralization

The control of supersaturation and the nucleation and growth of crystals in calcium phosphate systems are important in relation to the physiological deposition of bone and tooth. Other calcium salts such as the carbonate and oxalate hydrates are significant components of pathological mineral deposits. The use of a highly reproducible seeded growth technique has enabled kinetic studies to be made of the crystal growth of these minerals. Under conditions of relatively high supersaturation, secondary nucleation may be induced upon the surface of the seed crystals. In the case of the calcium phosphates, temperature, supersaturation, surface concentration, pH, ionic strength and presence of foreign ions are very important in determining the nature of the phase which grows upon the added seed crystals. Kinetic considerations are of overriding importance in determining the course of the reactions. It is not possible to predict the phase which forms purely on the basis of thermodynamic solubility data. Thus, in solutions appreciably supersaturated with respect to both dicalcium phosphate dihydrate (DCPD) and hydroxyapatite (HAP), the addition of low concentrations of HAP seed results in the exclusive formation of DCPD whereas this phase is absent when the seed concentration is increased. The kinetic results for calcium oxalate and phosphates are discussed in terms of the important problems relating to tooth mineralization and the origin and growth of renal calculi.     

Growth and characterization of nano‐crystalline hydroxyapatite at physiological conditions

Pure, stable stoichimetric nano crystalline hydroxyapatite material was crystallized by double diffusion technique at physiological conditions, temperature at 37°C and pH at 7.4. The sample was sintered at 400°C, 750°C and 1200°C with equal interval of time. They were characterized by X‐ray diffraction studies, Fourier Transformation Infra‐Red analysis, Thermogravimetric analysis, Scanning Electron Microscopic studies and Atomic Force Microscopic studies. The X‐ray analysis confirmed that the grown crystals are to be the pure form of hydroxyapatite. Infra‐red studies confirmed CO free hydroxyapatite. Thermogravimetric studies showed the thermal stability of the hydroxyapatite crystals even at 1200°C. The presence of pores in the sintered sample was traced by scanning electron microscopy. Atomic force microscopy revealed the presence of nano crystalline HAP of size 0.958 nanometer in the samples grown using this technique. At higher temperature the deagglomeration of bulk phases and agglomeration of nano phases leads to the nano crystalline HAP were observed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Long-term Crystal Growth under Microgravity during the EURECA-1 Mission (I) THM Growth of AlxGa1−xSb

Long-term crystal growth experiments were successfully performed under microgravity conditions during the first flight of the unmanned mission EURECA-1 in the automatic mirror furnace (AMF). Two crystals of Al_xGa_1−xSb with [001] and [111] orientation respectively were grown from gallium solution by the travelling heater method. The grown length of the single crystals is 4.0 mm and 4.2 mm respectively. The space-grown samples show a high microscopic homogeneity which indicates the absence of time dependent convection. From pulse markers a constant growth rate of 0.6 ± 0.1 μm/min is measured which is lower than 0.8 ± 0.1 μm/min obtained in earth grown reference samples. Details about the experiment performance and the growth results are given.     

Effect of Buoyancy-driven Convection on Surface Morphology and Macromorphology of Potassium Bichromate Crystals

Surface topography and macromorphology of potassium bichromate crystals grown from unstirred aqueous solution depend on diffusion and convection. Micromorphology of as-grown surfaces is a source of quantitative data on the local values of the degree of supersaturation σ at various parts of crystals. Micromorphology and values of growth velocities of faces, differently located in space with respect to the gravitational force show that upper parts of the crystals grow at about 5–25% lower values of σ than the lower ones. The cause of convection changes of size and number of faces is the characteristic distribution of supersaturation around the crystal induced by movement of the solution.     

Long-term Crystal Growth under Microgravity during the EURECA-1 Mission (II) THM Growth of Sulphur-doped InP

Long-term crystal growth experiments were successfully performed under microgravity conditions during the first flight of the unmanned EURECA-1 mission in the automatic mirror furnace (AMF). Two crystals of sulphur-doped InP with [001] and [111] orientation respectively were grown from indium solution by the travelling heater method (THM). The absence of time dependent buoyancy-driven convection is documented by the lack of type I striations in the space-grown crystals. The sulphur concentration is measured by spatially resolved photoluminescence. As expected, the macrosegregation can be described by a pure diffusion-controlled model which is in good agreement with the findings from the first German spacelab mission D1. Compared to the earth-grown reference samples, both of the space-grown InP crystals show strong disturbances such as inclusions and type II striations. The morphological instabilities are similar to growth disturbances already known from the space-grown MD-ELI-01 from the D1 mission.     

Influence of solution conditions on deposition of calcium phosphate on titanium by NaOH-treatment

The present paper demonstrated a biomimetic method to coat calcium phosphate (Ca-P) on the surface of titanium induced by NaOH-treatment from a simple supersaturated hydroxyapatite solution (SHS). The influence of pH value and calcium ions concentration on the precipitation process was investigated. It is necessary for the solution to be supersaturated than the critical concentration of octacalcium phosphate (OCP) to get Ca-P coatings on titanium surface. In the precipitating process, it seems that amorphous calcium phosphate (ACP) precipitated first, then OCP, and finally hydroxyapatite (HA). The system was in continuous evolution and the phase transitions occurred in sequence.     

Recent research on pseudobiological hydroxyapatite crystal growth and phase transition mechanisms

We have studied the growth and phase transition mechanisms of hydroxyapatite (HAP) and its interaction with a growth factor protein in a simulated physiological environment. Using atomic force microscopy (AFM) and real-time phase shift interferometry, we performed in situ observations of growth in simulated human body fluid solutions seeded with millimeter-sized HAP single crystals produced by hydrothermal synthesis, and we measured the normal growth rate. The step kinetic coefficient (derived from the velocity of growth steps) and the edge free energy (calculated from the variation in the normal growth rate with the degree of supersaturation) both deviated greatly from the standard values for typical inorganic salt crystals and were found to be close to those of protein crystals. This suggests that the growth units of HAP crystals are clusters rather than simple ions and that growth proceeds through the accumulation of these clusters. Observations using dynamic light scattering confirmed the presence of clusters with a diameter of about 1 nm in simulated body fluids. Ab initio analysis of the cluster energy stability indicated that calcium phosphate clusters based on Ca₃(PO₄)₂ units achieve an energy minimum for clusters of the form [Ca₃(PO₄)₂]₃. These clusters have S₆ symmetry, and, when they are used to build a HAP crystal structure, their structure is likely to become slightly modified, resulting in the formation of C₃ structures.Since these clusters would also be the building blocks of amorphous calcium phosphate (ACP), they provide vital clues to the phase transition from ACP to HAP. Using time-resolved static light scattering, we tracked the ACP–HAP phase transition process and found that the degree of coarseness inside a cluster aggregate changes abruptly within a specific time interval and HAP is formed and deposited in the final stages. This suggests that an ACP aggregate changes into HAP as its internal structure becomes regularized. Using this phase transition process, we produced a complex of HAP and the FGF-2 growth factor protein. This complex releases the FGF-2 into a physiological saline solution over a period of at least a week, meaning that it can be used as a pharmacologically active third-generation biomaterial.     

Chemical bond analysis of the crystal growth of KDP and ADP

The bond valence model is employed to calculate the bond strength of constituent chemical bonds formed between growth units in both potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) crystals, with the aim to predict and control the crystal shape. After calculating and comparing the relative growth rates of all selected planes, which are related to the crystallographic structure and chemical bond strength, the natural morphology of both KDP and ADP can be conveniently deduced. To check the calculated results, the crystallites of KDP and ADP are grown under different growth conditions; their morphologies accord well with our theoretical calculations. Meanwhile, the influence of ethanol anti-solvent on crystal morphology is microscopically interpreted, which alters the crystal morphology by changing the supersaturation of the growing solution and influencing the bonding process. When ethanol is added into the growing solution, the crystal shape becomes slender.     

Hydroxyapatite synthesis from biogenic calcite single crystals into phosphate solutions at ambient conditions

Hydroxyapatite is one of the most important bone substitute biomaterials. Here, it has been successfully overgrown on biogenic seed crystals at ambient conditions. Single crystals of calcite from Atrina rigida, Paracentrotus lividus and Heterocentrotus mammillatus have been soaked in phosphate solution with different concentrations and pHs for 2 months. X-ray powder diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy have been used to characterize soaking precipitates. The results show that the conversion of calcite to hydroxyapatite occurs to an extent which depends on composition and morphology of seed crystals, and starting concentration and pH of phosphate solutions. In the same experimental conditions, synthetic calcite single crystals did not convert to hydroxyapatite. The morphological observations suggest for hydroxyapatite formation, a mechanism that involves a superficial dissolution of calcite and a subsequently overgrowth of hydroxyapatite. Moreover, the final architectural assembly of the hydroxyapatite crystals resembles the shape of the starting biogenic seed crystals.     

HRTEM simulation in determination of thickness and grain misorientation for hydroxyapatite crystals

High-resolution transmission electron microscopy (HRTEM) and HRTEM simulation by the Bloch wave method (JEMS) are used to determine the structure and thickness of micro-and nanocrystals of biominerals and hydroxyapatite grown from aqueous solutions. It is established that thin (from one to several lattice parameters) crystals, including hydroxyapatite in mineralized biological tissues, are usually formed in low-temperature (up to 40°C) solutions. Relatively thick (up to several tens of lattice parameters) crystals grow only in high-temperature (~95°C) aqueous solutions. HRTEM simulation showed that crystals with a thickness exceeding one lattice parameter consist of nanograins misoriented with respect to one another along various directions within an angle of 0.7°.     

Inhibitory effect of polyelectrolytes on crystallization kinetics of hydroxyapatite

Modelling of the biologic materials, well-organized multifunctional structures and systems found in nature has attracted the interest of scientists working in many scientific disciplines. A new and rapidly growing field of biomimetics has stimulated an increased focus on biological materials as the researchers attempt to mimic the features, characteristics and growth of these naturally-occurring materials. This review discusses the principal features of biomineralization in relation to the controlled crystallization of inorganic materials and biomimetic routes to the formation of nanometer hydroxyapatite particles. This approach can be compared with biologic mineralization and has the potential for providing much greater control of particle size and distribution than would conventional methods. The constant-composition method has been used to study the influence of polyelectrolytes on the kinetics of crystal growth of hydroxyapatite (HAP), the thermodynamically most stable calcium phosphate phase, on HAP seed crystals at pH 7.4 and 37 °C. The results indicate that polyelectrolyte concentration and the larger number of negatively charged functional groups markedly affect the growth rate. The fit of the Langmuir adsorption model to the experimental data supports a mechanism of inhibition through molecular adsorption of polymers on the surface of growing crystals. This system may allow insights into biomineralization processes.     

A novel thermolysis method of colloidal protein precursors to prepare hydroxyapatite nanocrystals

A novel thermolysis method of colloidal protein precursors is introduced to prepare hydroxyapatite (HAP) nanocrystals. The colloidal protein precursors are sonochemically synthesized from saturated Ca(OH)₂ aqueous solution, Ca(H₂PO₄)₂ aqueous solution and bovine serum albumin (BSA) molecules. The colloidal protein precursors are amorphous and composed of 15‐90 nm near spherical calcium phosphate nanoparticles and BSA molecules. The particle size analysis shows the volume particle size distribution is from 9.0 nm to 222.6 nm and the volume‐averaged particle size is 45.8 nm. During the calcination procedure BSA molecules are burningly removed and the HAP nanocrystals can be obtained at 500 °C. The effects of BSA concentration on the properties of samples are discussed. Results show that BSA combustion can promote the transformation of crystalline HAP from amorphous material. Moreover, the increase of BSA concentration reduces the crystalline sizes of HAP crystals and the crystallinity of product. With BSA concentration of 5 g/L, the obtained HAP nanocrystals are mainly 25∼100 nm similar spherical nanoparticles besides some 40∼70 nm×75∼150 nm short rod‐like crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In situ time-resolved X-ray diffraction study of octacalcium phosphate transformations under physiological conditions

The present study is focused on the in vitro investigation of octacalcium phosphate (OCP) transformations when mixed with physiological solution (aqueous isotonic 0.9% NaCl) by the energy dispersive X-ray diffraction technique. This technique allowed in situ monitoring of the processes taking place in this system in real time. It was demonstrated that when OCP is mixed with physiological solution, it undergoes two simultaneous processes: (1) amorphous-into-crystalline OCP transformation (37%), and OCP average grain size growth from 22 up to 35 nm; (2) partial phase transformation of OCP (33%) into hydroxyapatite (HA), and HA average grain growth up to 5 nm. Both processes take place simultaneously with the same characteristic time (2.90±0.25) h.     

The combined effects of supersaturation and Ba2+ on the batch cooling crystallization of potassium dihydrogen phosphate

The combined effects of supersaturation and Ba²⁺ on potassium dihydrogen phosphate (KDP) were investigated in batch cooling suspension crystallization. Growth size, morphology, and impurity Ba²⁺ adsorbed in the KDP crystals were measured with changing Ba²⁺ concentration and supersaturation. Significant changes in shapes and volume of the grown crystals have been observed. The results further confirmed that the size and shape of crystals were greatly determined by supersaturation. Ba²⁺ ions significantly modified the growth habit of KDP crystals. The concentration of Ba²⁺ ions adsorbed in the crystals increases with the increasing Ba²⁺ ions in the solutions and supersaturation. The foggy phenomena caused by the addition of Ba to the KDP solution were also described. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influences of supersaturation on LPE growth of GaN single crystals using the Na flux method

The dependency of LPE growth rate and dislocation density on supersaturation in the growth of GaN single crystals in the Na flux was investigated. When the growth rate was low during the growth of GaN at a small value of supersaturation, the dislocation density was much lower compared with that of a substrate grown by the Metal Organic Chemical Vapor Deposition method (MOCVD). In contrast, when the growth rate of GaN was high at a large value of supersaturation, the crystal was hopper including a large number of dislocations. The relationship between the growth conditions and the crystal color in GaN single crystals grown in Na flux was also investigated. When at 800 °C the nitrogen concentration in Na–Ga melt was low, the grown crystals were always tinted black. When the nitrogen concentration at 850 °C was high, transparent crystals could be grown.     

Study of synthetic hydroxyapatite by the method of high-resolution transmission electron microscopy: Morphology and growth direction

Hydroxyapatite crystals grown from aqueous solutions have been studied by the methods of high-resolution electron microscopy and transmission electron diffraction. Processing of the experimental electron micrographs with the use of the Digital Micrograph program and the study of the corresponding Fourier trans-forms showed that the submicron microcrystals grow mainly along the [0001] direction. The (0001) and the $(01\bar 10)$ planes are perpendicular and parallel to the long edge of the crystals, respectively. The good accord between the experimental electron-microscopy images and the electron microscopy images calculated by the EMS program was attained only for crystals with the thicknesses ranging from one to five lattice periods. This allows us to state that hydroxyapatite grows from aqueous solutions in the form of very thin (with the thickness of the order of several lattice parameters) platelike crystals.     

Effect of supersaturation on deuteration distribution in K(H1‐xDx)2PO4 crystals

The homogeneity of deuterium distribution in potassium dideuterium phosphate single crystals was studied by Raman and infrared spectroscopy analyses. Results indicated that the pyramidal section exhibited more homogeneous deuterium distribution than the prismatic section. Supersaturation slightly affected the deuterium homogeneity in the pyramidal section of crystals grown rapidly from 80% deuterated solution. Deuterium homogeneity in the prismatic section decreased with increasing supersaturation level of the growth solution.     

A novel method to synthesize large-sized hydroxyapatite rods

A simple and controllable method was developed to synthesize large-sized hydroxyapatite (HAP) rods. Firstly, HAP crystals were reprecipitated from the supersaturated HAP solution in the presence of gelatin. After calcination at 600 °C for 4 h, large-sized HAP rods with good purity and crystallinity were obtained. Furthermore, due to the protection of gelatin at a high concentration, the HAP rods had a perfect monodispersity. The average length of rod-like HAP crystals could be controlled in the range of 0.65–5 μm by simply changing the weight ratios of HAP to gelatin from 0.2:8 to 2:8.     

Earth optimization of space experiment on growth of germanium by floating-zone technique with the use of rotating magnetic fields

The results are considered of the earth experiments on growth of high-purity and Ga-doped germanium single crystals 15 mm in diameter and 60 mm in length, which were performed in a Zona-4 “space furnace” under the technological regimes close to those existing in space orbits. It is shown that the use of a magnetohydrodynamic (MHD) factor [weak (0.15–0.2 mT) rotational (400 Hz) magnetic fields] during crystallization of semiconductors by the floating-zone technique is a very promising method for control of dopant distributions and electrophysical properties in a growing crystal. It is shown that in such magnetic fields, the effective coefficient of Ga distribution in Ge decreases by 10%. The shift of the donor-acceptor balance of the residual dopants in a compensated semiconductor during growth with the MHD-stirring of the melt was first established in growth of undoped germanium single crystals. It was also established that magnetic fields produce different effects on the resistivity microinhomogeneity in undoped and doped crystals. The mechanisms of the MHD effect on the properties of the grown crystals are discussed as well as the perspectives of performing analogous experiments aboard spacecrafts. It is predicted that, under the microgravitation conditions, the effects revealed in terrestrial experiments would be more pronounced.