Kinetics Of Hydroxyapatite Formation Research Articles

Effect of natural phosphate content on the growth kinetics of hydroxyapatite crystals grown from kaolin clay

In the present study, the kinetics of hydroxyapatite formation in different kaolin-natural phosphate mixtures was investigated using X-ray diffraction (XRD) and differential thermal analysis (DTA) measurements. The XRD patterns revealed the presence of both kaolinite and hydroxyapatite phases in all of the mixtures and full disappearing of kaolinite phase in the heat-treated samples. According to the DTA curve results, both the peak temperature formation of hydroxyapatite and the relative fraction-transformed increased as the heating rate increased and the amount of phosphate in the mixture decreased. The activation energy of hydroxyapatite phenomenon was estimated using Kissinger and Ligero analytical methods. The obtained values of the activation energy were in the range of ∼196.5 and 210.6 kJ/mol. The growth morphology parameters n and m were both found to be near to half unity indicating that the surface nucleation with plate-like growth was dominant in hydroxyapatite formation controlled by diffusion.

Bioactive glass (type 45S5) nanoparticles: in vitro reactivity on nanoscale and biocompatibility

Bioactive glasses represent important biomaterials being investigated for the repair and reconstruction of diseased bone tissues, as they exhibit outstanding bonding properties to human bone. In this study, bioactive glass (type 45S5) nanoparticles (nBG) with a mean particle size in the range of 20–60 nm, synthesised by flame spray synthesis, are investigated in relation to in vitro bioreactivity in simulated body fluid (SBF) and response to osteoblast cells. The structure and kinetics of hydroxyapatite formation in SBF were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) revealing a very rapid transformation (after 1 day) of nBG to nanocrystalline bone-like carbonated HAp. Additionally, calcite is formed after 1 day of SBF immersion because of the high surface reactivity of the nBG particles. In the initial state, nBG particles were found to exhibit chain-like porous agglomerates of amorphous nature which are transformed on immersion in SBF into compact agglomerates covered by hydroxyapatite with a reduced size of the primary nanoparticles. In vitro studies revealed high cytocompatibility of nBG with human osteoblast cells, indicated through high lactatedehydrogenase (LDH) and mitochondrial activity as well as alkaline phosphatase activity. Hence, this study contributes to the understanding of the structure and bioactivity of bioactive glass (type 45S5) nanoparticles, providing insights to the phenomena occurring at the nanoscale after immersion in SBF. The results are relevant in relation to the understanding of the nanoparticles’ bioreactivity required for applications in bone tissue engineering.

ChemInform Abstract: Kinetics of Hydroxyapatite Formation at Low Temperature.

The influence of temperature on the kinetics of formation of calcium-deficient hydroxyapatite by an acid-base reaction has been determined. Calcium-deficient hydroxyapatite is compositionally similar to bone apatite. Reactants used in this study were tetracalcium phosphate, a calcium phosphate more basic than hydroxyapatite, and dicalcium phosphate, which is more acidic than hydroxyapatite. The kinetics of hydroxyapatite formation are initially controlled by the surface areas of the reactants; however, eventually the rate of hydroxyapatite formation becomes diffusionally controlled. The origin of diffusion control appears to be associated with the epitaxial formation of hydroxyapatite on the surface of the dicalcium phosphate reactant. The microstructure of hydroxyapatite formed at 5°, 15°, and 25°C were generally similar and are characterized by the formation of a porous acicular product which covers the surfaces of the reactants. The microstructure of the hydroxyapatite formed at 38°C was distinct from this and exhibited a pseudomorphic relationship with the dicalcium phosphate reactant. Seeding with hydroxyapatite accelerated the initial reaction but did not appear to have major long-term effects on the fractional degree of reaction or on microstructural development. Reaction was also accelerated in a neutral salt solution of high ionic strength.

Hydroxyapatite?Polyphosphazane Composites Prepared at Low Temperatures

Owing to their similarities to bone apatite, calcium phosphate bioceramics, such as hydroxyapatite (HAp), are used as biomaterials for hard tissue replacements. Composites of bioceramics and biomedical polymers can mimic bone structure and properties. The characteristics of composites comprising HAp and a biomedical polymer and prepared at low temperatures are described. The kinetics of HAp formation in the presence of a polyphosphazene polymer that carries carboxylic acid moieties (acid-PCPP) were established at temperatures from 25° to 50°C. Evolution in the compositions of the solids present, solution chemistry, and microstructure development were established as functions of reaction time and temperature. The polymer participated in HAp formation affecting its rates of nucleation and growth through the formation of calcium cross links. The presence of polymer also enhanced ductility.

Formation of hydroxyapatite–polyphosphazene polymer composites at physiologic temperature

Aspects of the formation of bone analog composites at 37 degrees C are described. The composites are composed of hydroxyapatite (HAp) and the calcium salt of a biocompatible polymer and are capable of forming under in vivo conditions. Composite formation involves the formation of monolithic HAp from particulate calcium phosphate precursors while Ca ions liberated to the aqueous medium in which this reaction is occurring form crosslinks with the acidic polymer. The reactants are poly[bis(carboxylatophenoxy)phosphazene] (acid-PCPP), tetracalcium phosphate [Ca4(PO4)2O, TetCP], and anhydrous dicalcium phosphate (CaHPO4, DCPA). The effects of the proportion of polymer (5, 10, or 15 wt %) on the kinetics of HAp formation were studied. Compositional evolution of the solid calcium phosphates present was followed by X-ray diffraction and infrared spectroscopy analyses. HAp formation through a dissolution-precipitation process provided a mildly alkaline medium suitable for deprotonation of the acid-PCPP and for the formation of the calcium crosslinks, as monitored by infrared spectroscopy. Concurrence of crosslinking of the polymer and HAp formation was established, indicating true composite formation can be realized at physiologic temperature.

Growth of nano-scale hydroxyapatite using chemically treated titanium oxide nanotubes

A vertically aligned nanotube array of titanium oxide was fabricated on the surface of titanium substrate by anodization. The nanotubes were then treated with NaOH solution to make them bioactive, and to induce growth of hydroxyapatite (bone-like calcium phosphate) in a simulated body fluid. It is shown that the presence of TiO 2 nanotubes induces the growth of a “nano-inspired nanostructure”, i.e., extremely fine-scale (∼8 nm feature) nanofibers of bioactive sodium titanate structure on the top edge of the ∼15 nm thick nanotube wall. During the subsequent in-vitro immersion in a simulated body fluid, the nano-scale sodium titanate, in turn, induced the nucleation and growth nano-dimensioned hydroxyapatite (HAp) phase. The kinetics of HAp formation is significantly accelerated by the presence of the nanostructures. Such TiO 2 nanotube arrays and associated nanostructures can be useful as a well-adhered bioactive surface layer on Ti implant metals for orthopaedic and dental implants, as well as for photocatalysts and other sensor applications.

Phase transformations in plasma sprayed hydroxyapatite coatings

In the present study, the kinetics of hydroxyapatite formation in different kaolin-natural phosphate mixtures was investigated using X-ray diffraction (XRD) and differential thermal analysis (DTA) measurements. The XRD patterns revealed the presence of both kaolinite and hydroxyapatite phases in all of the mixtures and full disappearing of kaolinite phase in the heat-treated samples. According to the DTA curve results, both the peak temperature formation of hydroxyapatite and the relative fraction-transformed increased as the heating rate increased and the amount of phosphate in the mixture decreased. The activation energy of hydroxyapatite phenomenon was estimated using Kissinger and Ligero analytical methods. The obtained values of the activation energy were in the range of ∼196.5 and 210.6 kJ/mol. The growth morphology parameters n and m were both found to be near to half unity indicating that the surface nucleation with plate-like growth was dominant in hydroxyapatite formation controlled by diffusion.

Effects of magnesium on the formation of calcium-deficient hydroxyapatite from CaHPO4.2H2O and Ca4(PO4)2O.

Calcium-deficient hydroxyapatite (HA) with a Ca/P molar ratio of 1.50 was synthesized in various concentrations (0.01-75 mM) of MgCl2 at 37.4 degrees C by reaction between particulate CaHPO4.2H2O and Ca4(PO4)2O. The effects of magnesium on the kinetics of HA formation were determined using isothermal calorimetry. All reactions completely consumed the precursor phases as indicated by X-ray diffraction analysis and a constant enthalpy of reaction (240 kJ/mol). Magnesium concentrations below 1 mM had no effect on the kinetics of HA formation. Magnesium concentrations between 1 and 2.5 mM affected the reaction path but did not affect the time required for complete reaction. Higher concentrations extended the times of complete reaction due to magnesium adsorption on the precursor phase(s) and HA nuclei, and stabilization of a noncrystalline calcium phosphate (NCP). HA formation in the presence of magnesium resulted in separation of the following two events: initial formation of HA nuclei and NCP, and consumption of CaHPO4.2H2O. This was indicated by the appearance of an additional calorimetric peak. Variations in calcium, magnesium, and phosphate concentrations and pH with time were determined. Increasing the magnesium concentration resulted in elevated calcium concentrations. After an initial decrease in magnesium owing to its adsorption onto HA nuclei and precursor(s), a period of slow reaction at constant magnesium concentration was observed. Both the magnesium concentration in solution and the proportions of precursors present decreased prior to any evidence of a crystalline product phase. This is attributed to the formation of NCP capable of incorporating magnesium. This noncrystalline phase persisted for more than 1 year for reactions in magnesium concentrations about 2.5 mM. Its conversion to HA resulted in the release of magnesium to the solution.

Low temperature synthesis of a self-assembling composite: hydroxyapatite-poly[bis(sodium carboxylatophenoxy)phosphazene

The present study was undertaken to investigate the low temperature formation of a hydroxyapatite-polyphosphazene polymer composite likely to be biocompatible. The temperature range studied (25 to 60°C) was selected to bracket physiological temperatures. The composite precursors consisted of CaHPO4·2H2O, Ca4(PO4)2O, and poly[bis(sodium carboxylatophenoxy)phosphazene]. The results indicate that a synergistic relationship exists in the formation of a polyphosphazene network and hydroxyapatite (HAp) matrix phase during composite synthesis. Calcium from the HAp precursors participates in the formation of a Ca crosslinked polymeric network which influences the rate of HAp formation and its morphology. The mechanistic paths taken during composite formation were followed by determining variations in the concentration of species in solution (at physiological temperature), rates of heat evolution, and microstructural development. These analyses indicate that the polymer controls the kinetics of hydroxyapatite formation and the composite microstructure. Low reaction temperatures and a high proportion of polymer facilitate the formation of a highly interconnected composite. The presence of the polyphosphazene allows a metastable calcium phosphate solution to persist for extended periods prior to the formation of hydroxyapatite. The degree of supersaturation and the length of the induction period increase with an increase in polyphosphazene content. The temperature dependence of these induction periods obeyed an Arrhenius relationship.

Formation of hydroxyapatite in serum

The kinetics of the formation of calcium-deficient and carbonated hydroxyapatite at 38°C were investigated by isothermal calorimetry. Hydroxyapatite (HAp) was formed by reaction of the particulate calcium phosphates CaHPO4 and Ca4 (PO4)2O. Compared with its rate of formation in DI water, the formation of calcium-deficient HAp is significantly inhibited in serum. When serum is diluted with DI water, the extent of inhibition varies with the extent of dilution. When collagen or HAp seeds are present the extent of inhibition in serum is reduced. The kinetics of HAp formation were also examined in various concentrations of albumin to establish the extent to which inhibition is associated with the presence of this plasma protein. While HAp formation is inhibited in albumin, the extent of inhibition is not as great as in serum. The formation of carbonated HAp is also inhibited in serum and albumin. However, the extent of inhibition is significantly reduced. The variations in sodium and carbonate in solution during HAp formation indicate that these species are incorporated at different rates, with carbonate incorporation being more rapid. Elevated sodium concentrations in solution result in solution pH values near 12. The reduction in the inhibition of HAp formation may be associated with the reaction to carbonated HAp occurring at elevated pH or with the influence of pH on protein adsorption.

Kinetics of Hydroxyapatite Formation at Low Temperature

The influence of temperature on the kinetics of formation of calcium‐deficient hydroxyapatite by an acid‐base reaction has been determined. Calcium‐deficient hydroxyapatite is compositionally similar to bone apatite. Reactants used in this study were tetracalcium phosphate, a calcium phosphate more basic than hydroxyapatite, and dicalcium phosphate, which is more acidic than hydroxyapatite. The kinetics of hydroxyapatite formation are initially controlled by the surface areas of the reactants; however, eventually the rate of hydroxyapatite formation becomes diffusionally controlled. The origin of diffusion control appears to be associated with the epitaxial formation of hydroxyapatite on the surface of the dicalcium phosphate reactant. The microstructure of hydroxyapatite formed at 5°, 15°, and 25°C were generally similar and are characterized by the formation of a porous acicular product which covers the surfaces of the reactants. The microstructure of the hydroxyapatite formed at 38°C was distinct from this and exhibited a pseudomorphic relationship with the dicalcium phosphate reactant. Seeding with hydroxyapatite accelerated the initial reaction but did not appear to have major long‐term effects on the fractional degree of reaction or on microstructural development. Reaction was also accelerated in a neutral salt solution of high ionic strength.