Crystalline HA Research Articles

Electric discharge assisted pulsed laser deposition of hydroxylapatite

Hydroxylapatite (Ca10(PO4)6(OH)2, HA) is a calcium phosphate used as coating for dental and orthopaedic titanium implants, because its composition and structure are similar to the mineral part of bone. Pulsed laser deposition has been successfully applied, but requires temperatures higher than 450 °C, which may affect the coating–substrate interface. A HA target was ablated with an ArF laser in a water vapour pressure of 45 Pa. The material was deposited on a titanium substrate, placed at 4.7 cm from the target. In this region an autonomous electric discharge was maintained with a DC power supply (0–1000 V, 800 W), giving an ionisation current between 0 and 80 mA. The presence of the HA phase in the coatings was assessed by X-ray diffraction. Their Ca/P ratio was measured by energy dispersion of X-rays, while the OH− and CO32− contents were evaluated by Fourier transform infrared spectroscopy. At higher ionisation currents, crystalline HA coatings can be obtained at lower temperatures (as low as 300 °C). This effect is due to both the higher incorporation of OH− in the coatings (higher H2O dissociation by the ionisation current) and the higher mobility and ionisation of the particles on the substrate (provided by the electron bombardment of the coating during its growth). It is only necessary to use a high ionisation current during the first growth steps in order to provide the seeds for crystallisation.

Effect of Hydrothermal Treatment on Microstructural Feature and Bonding Strength of Plasma-Sprayed Hydroxyapatite on Ti-6Al-4V

The effects of hydrothermal treating temperature on bonding strength pertaining to the microstructural evolution of plasma-sprayed hydroxyapatite coatings (HACs) on Ti-6Al-4V substrate were investigated. On the basis of the observed microstructure, the as-sprayed amorphous and impurity phases, such as α-Ca3(PO4)2, Ca4P2O9 and CaO transform into crystalline HA after performing the hydrothermal treatment. Additionally, ultrafine crystallized particles can be recognized that the nucleation and grain growth occur by hydrothermal treatment at elevated temperatures of 175°C and 200°C. Furthermore, the hydrothermal treatment between 100°C and 150°C shows a significant decrease in microcracks, corresponding to an increase in bonding strength. This study reveals that even a small variation of hydrothermal heating temperature under 200°C can cause significant changes of the microstructural morphologies. Combining the experimental results of spraying defects, microstructural evolution and bonding strength, the knowledge of the hydrothermal-treated HACs will be helpful for the understanding and prediction in the biological stability and mechanical stability of HACs in the long-term clinical use.

Mechanical properties of pulsed laser-deposited hydroxyapatite thin film implanted at high energy with N + and Ar + ions. Part I: nanoindentation with spherical tipped indenter

We report here a comparison between the effects of ion beam implantation treatment using nitrogen and argon ions, on the mechanical characteristics of HA films grown by pulsed laser deposition, using a KrF ∗ excimer laser. Crystalline and stoichiometric HA films were grown on Ti–5Al–2.5Fe alloy substrate, previously coated with a TiN buffer layer. After deposition, the film were implanted with ions of N + and Ar + of high energy (1–1.5 MeV range) and dose set at 10 16 at cm −2. From the load–displacement curves determined by nanoindentation tests using a spherical tipped nanoindenter ( R=5 μm), we put into evidence an enhancement of the mechanical characteristics (hardness and elastic modulus) of the HA films after implantation, especially for those implanted with N + ions. Moreover, using various applied normal loads (ranging from 1 to 100 mN) in different implanted areas, a good reproducibility of nitrogen implantation effect are observed.

Micro- and nano-testing of calcium phosphate coatings produced by pulsed laser deposition

Micro- and Nano-testing methods have been explored to study the thin calcium phosphate coatings with high adhesive strength. The pulsed laser deposition (PLD) technique was utilised to produce calcium phosphate coatings on metal substrates, because this type of coatings exhibit much higher adhesive strength with substrates than conventional plasma-sprayed coatings. Due to the limitations of the conventional techniques to evaluate the mechanical properties of these thin coatings (1 μm thick), micro-scratch testing has been applied to evaluate the coating-to-substrate adhesion, and nano-indentation to determine the coating hardness and elastic modulus. The test results showed that the PLD produced amorphous and crystalline HA coatings are more ductile than titanium substrates, and the PLD coatings are not delaminated from the substrates by scratch. Also, the results showed that the crystalline HA coating is superior in internal cohesion to the amorphous one, even though the lower elastic modulus of amorphous coating could be more mechanically compatible with natural bone.

Plasma sprayed graded titanium hydroxyapatite coatings

A series of graded titanium-hydroxyapatite (HA) coatings have been produced by single gun plasma spray technology, and their performance was compared with that of pure HA coating. The results show that deformed HA and titanium particles exhibit an alternate distribution in respective networks formed within the coatings. In comparison with a pure HA coating, less cracks appeared on the surface of the graded coatings and the shear bond strength was increased by 50-80%. The graded coating with two interlayers and a thin outer HA layer contained crystalline HA, an amorphous phase, α-tricalcium phosphate (TCP), β-TCP, tetracalcium phosphate (TTCP), TiO2 and TiN; while no Ti-HA compounds were formed. As compared with pure HA coating, the graded coating contained more decomposed phases TCP and TTCP, resulting from titanium catalysing HA decomposition.

Hydroxyapatite nanostructure material derived using cationic surfactant as a template

This article reports the successful synthesis of mesoporous hydroxyapatite, Ca10(PO4)6(OH)2 (denoted HA) using cationic surfactant as the template. The wide-angle (2θ > 10°) diffraction data revealed characteristic peaks of HA, where a hexagonal lattice structure can be deduced. The lattice structure is found in space group P63/m, the parameters of which are in excellent agreement with reference data; i.e.a = b = 0.9418 nm, c = 0.6884 nm. For small angle diffraction (2θ < 10°), the characteristic peaks occur at 2θ values of 3.30, 5.75, 7.25 and 8.30°, indicating the presence of atomic planes with a periodical spacing of 2.677 nm. Nitrogen adsorption indicated a pore size distribution of approximately 3 nm, and a corresponding pore volume of 0.0113 cm3 g−1, hence the volume ratio of the mesopores was found to be ∼0.036. SEM micrographs reveal a rod-like structure of HA, possessing a thickness of about 50–100 nm and length ranging from 500 to 1000 nm, while TEM micrographs revealed that nano-channels are formed within the rod-like structure. These nano-channels align in a lengthwise direction within the rods, consistent with the cavities being generated by the removal of “organic” CTAB templating structure during calcination. The channels have dimensions of around 3.5 nm and the spaces between the nano-channels are filled with an ordered crystalline HA structure. Comparing the pore size and the spacing between neighboring channels, the pore volume ratio of each rod was calculated to be ∼0.029, which is in agreement with the result from gas adsorption. A probable mechanism is that CTAB–PO43− mixtures form rod-like micelles, which contain many PO43− groups on the surface, and in the presence of Ca2+, Ca9(PO4)6 clusters are preferentially condensed on the rod-shaped micellar surface due to the conformation compatibility between the identical hexagonal shapes of the micelles and Ca9(PO4)6 clusters. The micelles act as nucleating points for the growth of HA crystals. During the thermal incubation stage, CTAB–HA complexes are produced and they coalesce to form a stable three-dimensional rod-like structure. The morphology of the final product shows about 10 layers of HA crystal grows on one micelle during the reaction in water.

664 液相析出法によりチタン基板上に形成した HA 薄膜の特性

In the present study, liquid phase deposition method was employed to produce a dense crystalline HA film on titanium substrate in metastable calcium phosphate solution. A 100x10x0.02(mm)^3 titanium foil was employed as a substrate. Prior to HA coating, the substrate was immersed in 5M NaOH solution at 60℃ for 24h. The temperature of the substrate was kept constant at 50,60,and 70℃ by electrically heated with DC power source in metastable calcium phosphate solution. The X-ray diffractogram indicated that the film deposited on the titanium substrate was composed of HA. The amount of HA deposited during 3h-treatment increased with an increase in the substrate temperature from 50 to 70℃. These results suggests that the dense HA film can be formed by a simple chemical and thermal treatments. This technique is useful especially to produce uniform HA coatings onto complex-shaped and porous dental implants.

Piezoelectric properties of collagen-nanocrystalline hydroxyapatite composites

In this paper we did a study of the physicochemical, dielectric and piezoelectric properties of anionic collagen and collagen-hydroxyapatite (HA) composites, considering the development of new biomaterials which have potential applications in support for cellular growth and in systems for bone regeneration. The piezoelectric strain tensor element d14, the elastic constant s55, and the dielectric permittivity ɛ11 were measured for the anionic collagen and collagen-HA films. For the collagen-HA composite film (Col-HACOM) the main peaks associated to the crystalline HA is present. For the nanocrystalline composite, nanometric HA powder (103 nm particle size) (HAN), obtained by mechanical milling were used. For the composite film (Col-HAN) the HA and CaH(PO4)2H2O phases were detected. One can see that the HA powder (HAN) present the main peaks associated to crystalline HA. The IR spectroscopy measurements on HA-COM and HAN powders, Col-HACOM and Col-HAN composite films and collagen film (Col) presents the main resonances associated to the modes of (PO4)3−, (CO3)2−. The IR spectra of Collagen Film (Col) shows the bands associated to amide I (C=O), amide II (N–H) and amide III (C–N) vibrational modes. The scanning electron photomicrography of the Col-HACOM and Col-HAN films, respectively, shows deposits of HA on the surface of collagen. It also shows that HACOM crystals has a dense feature, whereas the HAN crystals has soft porous surface. Energy-dispersive spectroscopy (EDS) analysis showed that the main elements of the hybrid sponge were carbon, oxygen, calcium, and phosphorus. The EDS of HACOM crystal, present in the Col-HACOM composite showed a molar ratio Ca/P = 1.71, whereas the Col-HAN composite the molar ratio of calcium and phosphorus (Ca/P = 2.14) and the amount of carbon were greater. The piezoelectric strain tensor element d14 obtained for the anionic collagen was around 0.102 pC/N. The collagen composite with nanocrystalline HA crystals (Col-HAN) present a better result (d14 = 0.040 pC/N) compared to the composite with the commercial ceramic (d14 = 0.012 pC/N). This is an indication that the nanometric particles of HA present little disturbance on the organization of the collagen fibers in the composite. In this situation the nanometric HA are the best candidates in future applications of these composites.

Design, Synthesis, and Characterization of Hydroxyapatite Particulates

AbstractNew hydrothermal technology for the preparation of hydroxyapatite designer particulates has been developed. Phase diagrams were constructed for the pH dependent equilibrium between monetite and HA from 50 to 200°C. Thermodynamic calculations were completed using temperature dependent functions for the relevant solution phase and solid-liquid equilibria and solute species activity coefficients. Model accuracy was evaluated through experiment at 50, 100, and 200°C and pHs between 2.2 and 8.9. The thermodynamic calculations and experimental results are in good agreement. Stoichiometric, crystalline HA has also been prepared by heterogeneous reaction of Ca(OH)2 powder and aqueous (NH4)2HPO4 at room temperature using mechanochemical-hydrothermal methods. This method appears to have very good reproducibility in terms of crystallinity and chemical composition.

Immersion behavior of RF magnetron-assisted sputtered hydroxyapatite/titanium coatings in simulated body fluid.

The focus of the present study was on the dissolution/degradation behavior of a series of magnetron-sputtered, single-layered HA/Ti coatings on Ti-6Al-4V substrate immersed in SBF. Changes in coating morphology, crystal structure, and adhesion strength with immersion time are characterized. XRD, FTIR, and LVSEM results consistently indicate that highly crystalline monolithic HA coating is very dissolvable in SBF. The monolithic HA coating is largely delaminated in 3 weeks and entirely peeled off the substrate in 7 weeks. The dissolution is even greater for 95HA/5Ti coating, which severely disintegrated in only 1 week. The amorphous-like coatings sputtered from targets comprising 10 vol % or more Ti, however, appeared almost intact, and their adhesion strengths, which were all higher than 60 MPa, did not change much (within 10%) even after 14 weeks of immersion. The coatings from targets comprising roughly 10-50 vol % Ti combine advantages of high and nondeclining adhesion strength, high resistance to SBF attack, and possibly much higher bioactivity (with large amounts of Ca, P, etc., dissolved in the coatings) than that of Ti.

Osseo-mechanical induction of extra-cortical plates with reference to their surface properties and geometric designs

The purpose of this investigation was to determine which geometric and surface properties encouraged optimal ingrowth and bonding of bone to an extra-cortical plate. Forty-eight titanium extra-cortical plates were attached onto the left and right femora of adult rabbits. The plates were of six different designs and the osseoconductive effects of four surfaces were examined. A roughened titanium surface, a plasma sprayed HA coating of low crystallinity (57%) and a solution precipitated calcium phosphate coating were compared with a plasma sprayed crystalline hydroxyapatite coating (crystallinity 85%). Thin sections were prepared by grinding and polishing. Bone formation and the interface around the plates were investigated histologically and computer and morphometric analyses were used to quantify new bone formation, bone apposition onto the plate, bone porosity and the condition of the HA coating. The study found that a hydroxyapatite coating (with the exception of the solution precipitated coating) had significantly greater interfacial contact with bone when compared to a roughened titanium surface, and that significantly more bone attached to a crystalline HA coating compared with the HA coating of lower crystallinity although significantly more bone formed in the vicinity of the lower crystalline HA coating. Differences in the bony reaction induced by the various geometric designs were evident and the optimal plate design requires either holes or slots along its length as this encouraged bone ingrowth into the plate.

In vitro stability of a highly crystalline hydroxylapatite coating in a saturated citric acid solution.

A novel pressurized hydrothermal post-plasma-spray process has been developed to convert the crystalline non-HA and amorphous components of plasma-sprayed hydroxylapatite coatings back into crystalline HA. The process, known commercially as MP-1, was used to produce coatings comprising approximately 96% crystalline HA. The in vitro solubility of the coating in saturated citric acid solution has been measured to simulate the effect of implant detoxification procedures, which use citric acid as a cleaning medium. The MP-1 coating solubility in saturated citric acid solution (pH = 1) was compared to that of coatings with crystalline HA contents ranging from 37.5-82%. All coatings showed an initial sharp rise in coating dissolution, which correlated with crystalline HA content, followed by a steady state dissolution rate. After 60 s at 25 degrees C, the MP-1 coating showed a 65% decrease in solubility compared to a highly amorphous coating (AM-2). All coatings showed very similar steady state dissolution rates, except for AM-2, which was significantly higher. SEM analysis showed that the AM-2 coating surface was degraded substantially more than the other coatings, resulting in partial coating exfoliation. A mechanism of coating dissolution is proposed, in which the initial rapid leaching of soluble phases from the coating leaves behind a porous layer of highly crystalline HA at the coating surface. The stability of this porous crystalline layer leads to steady state, diffusion-limited dissolution of the remainder of the coating. The observed two-regime dissolution profile can be accurately represented by a 2-parameter model, which predicts the initial sharp rise in coating dissolution followed by a slower, steady state loss in coating mass. Model parameters were determined from experimental solubility data, and were shown to correlate with the percentage of crystalline HA in the coatings. The present data suggest that the treated coating is significantly more resistant to degradation from aggressive detoxification procedures such as citric acid burnishing.

Two-dimensional infrared correlation spectroscopy of synthetic and biological apatites

Two-dimensional (2D) infrared (IR) correlation spectroscopy was used to monitor the ν1, ν3 phosphate contour (900–1200 cm-1) of maturing poorly crystalline hydroxyapatite in synthetic (synthesized at constant and variable pH) and biological (calcified turkey leg tendon) systems. The 2D IR plots of the mineral prepared at variable pH exhibit peaks at 961, 999, 1018, 1036, 1095, 1126, and 1150 cm-1. The peaks at 961, 999, and 1095 cm-1 represent vibrations of PO3-4 in an apatitic/stoichiometric environment of poorly crystalline HA, while those at 1018, 1036, and 1126 cm-1 arise from PO3-4 in a nonstoichiometric/acid phosphate environment of poorly crystalline HA. The 2D IR analysis suggests that the intensities of peaks associated with PO3-4 in a nonstoichiometric/acid phosphate environment decrease as the reaction progresses. The 2D IR plots of the mineral formed at constant pH showed only bands characteristic of PO3-4 in a stoichiometric/acid phosphate environment. Analysis of the 2D IR plots of the mineral from calcified turkey leg tendon reveals peaks at 1019, 1039, 1075, 1126, and 1147 cm-1. The peaks at 1019, 1039, and 1126 cm-1 are characteristic of PO3-4 in a nonstoichiometric/acid phosphate environment of poorly crystalline HA, while the band at 1075 cm-1 is characteristic of PO3-4 in an apatitic/stoichiometric environment of poorly crystalline HA. Thus, the in vitro experiment in which the mineral is formed at variable pH is a better model of the mineral phase in calcified turkey leg tendon. In addition, the asynchronous plots from both the synthetic and biological minerals revealed those peaks which were noncorrelated. Also, this method of data analysis provided enhanced resolution of the highly overlapped ν1, ν3 phosphate contour commonly seen in Fourier transform-IR spectra of calcified tissue. © 1996 John Wiley & Sons, Inc.

Pulsed laser deposition and processing of biocompatible hydroxylapatite thin films

Calcium phosphate-based ceramics were deposited on Ti-6Al-4V and Si substrates by pulsed laser deposition. The calcium phosphate phase (amorphous, α- or β-tricalcium phosphate, tetracalcium phosphate, or hydroxylapatite), Ca/P ratio and surface morphology of the films were related to deposition conditions. Deposition of crystalline films required both elevated substrate temperature and a reactive gas environment. At substrate temperatures < 400°C, films were amorphous. Above 400°C, the gaseous environment during deposition determined the crystalline phase of the films. In Ar or in vacuum at 500–600°C, films were amorphous. In O 2, deposition of α-tricalcium phosphate was favored between 400 and 700°C and β-tricalcium phosphate was favored above 700°C. Films deposited in low pressures of O 2 (<0.05 Torr) were P-deficient. The correct stoichiometry was approached as the pressure of O 2 in the chamber during deposition increased. In water vapor-enriched inert gas environments, deposition of hydroxylapatite was observed at temperatures between 400 and 700°C and tetracalcium phosphate was observed at temperatures above 700°C. Adhesion of crystalline HA films deposited at 600°C in Ar/H 2O was excellent, but films that were amorphous (deposited either at room temperature or in a non-reactive gas environment) and tricalcium phosphate films did not adhere well to the substrates.

Pulsed laser deposition of hydroxylapatite thin films on Ti‐6Al‐4V

AbstractPulsed laser deposition was used to deposit thin films of calcium hydroxylapatite (Ca10(PO4)6(OH)2), or HA, on polished substrates of Ti‐6Al‐4V. Thin films of pure, crystalline HA, uncontaminated by other calcium phosphate phases, were deposited over a range of temperatures between 400 and 800°C. The HA films were polycrystalline with a preferred (001) crystallographic orientation, as determined by transmission electron microscopy and x‐ray diffraction. Adhesion of the HA films to the Ti‐6Al‐4V substrates was excellent when films were deposited at temperatures ≤600°C; in a scratch test, mean pressures of ca. 1010 Nm−2 produced conformational cracking in a film deposited at 600°C, but no decohesion from the substrates.