Plasma-sprayed Hydroxyapatite Coatings Research Articles

In vivo biological responses of plasma sprayed hydroxyapatite coatings with an electric polarized treatment in alkaline solution

The plasma sprayed hydroxyapatite coatings were post-treated by an electric polarized treatment in alkaline solution (PAS). The compositions, stabilities, surface charges, bone-like apatite formation abilities of the PAS coatings were investigated. The bioactivity of the PAS coatings was characterized in vivo. The results showed that the stabilities of the PAS coatings were improved because of the increased crystallinity and the decreased impurity phase. The bone-like apatite formation abilities were also improved after the PAS treatment because of the negative charges formed on the coating surfaces. Animal experiments showed that the PAS coatings could accelerate the initial fixation of the implant. The results indicated that the PAS is a promising post-treatment method to improve the biological properties of the plasma sprayed hydroxyapatite coatings.

Chemical Inhomogeneity of Plasma Sprayed Hydroxyapatite Coatings

Hydroxyapatite(HA) coatings were fabricated using plasma spraying method.The samples were polished along the direction parallel and vertical to the coating surface,respectively.Microstructure,element distribution and phase composition of the surface and cross-section of coaings were examined by a JXA-8800R electron probe microanalyser(EPMA) with a Link ISIS300 energy spectrum analyzer and a D/max-γB X-ray diffractometer(XRD).The results show that there exists chemical inhomogeneity in terms of chemical composition and phases in the coatings,corresponding to color differences in back scattering electron(BSE) images.The micro-regions with low P and low O contents are composed of amorphous,decomposed and recrystallized Ca-P phases.The difference in moltening degrees of HA particles during plasma spraying results in the inhomogeneity.Heat treatment eliminats the chemical inhomogeneity but results in the formation or spreading of cracks within the coatings.

Adhesion strength of plasma-sprayed hydroxyapatite coatings on laser gas-nitrided pure titanium

A layer of bioceramic HA was coated on laser gas-nitrided pure titanium and grit-blasted pure titanium substrates using plasma-spraying technique, respectively. X-ray diffraction analysis showed that the microstructures of the coating were mainly composed of HA, amorphous calcium phosphate (ACP) and some minute phases of tricalcium phosphate (TCP, α-TCP and β-TCP), tetracalcium phosphate (TTCP) and calcium oxide (CaO). The experimental results showed that the 3-D TiN dendritic scaffold structure produced on the surface of pure titanium using laser gas nitriding technique in advance could anchor the HA coating and improved the interfacial adherence significantly as compared with that on the grit-blasted surfaces.

Plasma‐sprayed hydroxyapatite coating on carbon/carbon composite scaffolds for bone tissue engineering and related tests in vivo

The bioactive hydroxyapatite (HA) coatings were successfully prepared on carbon/carbon composites (C/C) by means of sand-blasting pretreatment and plasma-spraying technology. X-ray diffraction was employed to analyze the phase constitute of the coatings. Meanwhile, the bond strength between the HA coatings and C/C substrates was determined via shear test. Experimental results show that the coatings constitute HA, CaO, and other amorphous phosphates. The post heat treatment could effectively increase crystallization and purity of the coatings. Through observation and analysis by electron microprobe and scanning electron microscopy, it is concluded that the bond strength of the plasma-sprayed HA coatings on C/C is mainly determined by the interface structure and can be further improved by the post heat treatment. Meanwhile, the implantation in vivo was carried out in hybrid goats. The histological observation revealed that the osteoplaque gradually grew on the surface of the HA coatings and the pure C/C surface was covered by the fibrous tissues. No inflammation symptoms were found in the bone tissue around the implants.

Hydrothermal crystallization effect on the improvement of erosion resistance and reliability of plasma-sprayed hydroxyapatite coatings

The aim of present study is to investigate the crystallization effect of air and hydrothermal heat treatments on the erosion resistance, failure behaviors and reliability of plasma-sprayed HA coatings (HACs). Experimental results show that just 150 °C is required to see a significant increase in crystallinity and phase purity for hydrothermally-treated HACs compared with the high-temperature (600 °C) air heat treatment. The significantly improved bonding strength of hydrothermally-treated HACs can be recognized for the self-healing effect that resulted from the hydrothermal crystallization. Results of erosion test indicated that the erosion resistance decreases with increasing erosion impact angle. The maximum erosion rate occurred at 90° impact angle for all the as-sprayed and heat-treated HACs. The erosion resistance of plasma-sprayed HACs is significantly improved through the crystallization of a coating after heat treatments. Furthermore, it should be worth noting that the erosion resistance for hydrothermally-treated HACs is much better than as-sprayed and air heat-treated HACs at any impact angle. The statistical analysis of Weibull model shows that plasma-sprayed coatings are generally reliable materials with a wear-out failure model (with a Weibull modulus m > 3) of increasing failure rate (IFR). The knowledge of the minimum strength and failure behavior will be very helpful for understanding the HA coating reliability for further biological applications.

Effect of heat treatment on performance of hydroxyapatite coatings immersed in simulated body fluid

In this paper, the effect of heat treatment on the performance of hydroxyapatite (HA) coatings immersed in simulated body fluid (SBF) is researched. The electron probe microanalyser results show that different contrast appears on the planar microstructure images, and chemical inhomogeneity exists, in the plasma sprayed HA coatings. Heat treatment can eliminate the inhomogeneity, increase crystallinity of the coatings and make the element distributions tend to be homogeneous. The X-ray diffraction results show that the decomposed phase decrease, indicating that some recrystallinity chemical reactions occurred during heat treatment. The shear adhesion test shows that the bonding strength of the high crystallinity coatings increased, while the low crystallinity coatings significantly decreased (p<0⋅05) after heat treatment. However, bonding strengths of all the coatings markedly decreased (p<0⋅05) after immersion in 0⋅5SBF for 4 weeks. The scanning electron microscopy results show that the cracks, pores as well as boundaries of unmelted particles in the coatings are the origins of the rupture. There are some needle like nanocrystals distributing on the pores at the fractures of as sprayed coatings, while needle like crystals are not found at the fractures of heat treated coatings.

J0401-2-4 数種のボンドコート上にプラズマ溶射した水酸アパタイトコーティング材の疲労損傷(生体材料およびその表面改質材(2))

This paper describes the fatigue behaviors of coating composites that possesses pure titanium substrates and plasma-sprayed hydroxyapatite and titanium powder coatings. Number of cracks on the cross-section of coating in tension-tension loading is larger than that in compression-compression loading. Acoustic emission signals is also observed and these results shows the clear correlation between the total event count of AE signals and fatigue lives. The best condition of coatings, showed the least counts of AEs, is hydroxyapatite top coat and hydroxyapatite/titanium powders bond coat, each of which has 100 μm thickness.

Plasma sprayed hydroxyapatite coatings from nanostructured granules

The present study utilized nanostructured hydroxyapatite (HA) granules for plasma-sprayed coating on a titanium alloy (Ti–6Al–4V). The initial nanostructured HA powder was synthesized in aqueous solutions of gelatin. The effect of gelatin solutions of variable concentration on the particles size was investigated. The nanostructured HA particles were formed into the porous granules using a liquid immiscibility effect. Consolidation of these powders at various temperatures was carried out. The phase composition, structure, residual stresses and adhesive bond strengths of the obtained coatings from the developed feedstock were investigated by X-ray diffraction, scanning electron microscopy and mechanical tests. X-ray diffraction showed that all the HA coating is a fully crystalline phase without any decomposition. The coatings exhibited a unique phase construction and a fairly low tensile residual stress. The adhesive bond strength of a HA coating was achieved up to 37 ± 2.2 MPa. Tensile failure analysis has suggested that the failure took place at the coating/substrate interface. It is expected that the developed coatings will have much longer service life as compared to the existing ones.

Biomimetic formation of hydroxyapatite investigated by analytical techniques with high resolution

Morphology, phase and chemical compositions of atmospheric plasma-sprayed (APS) hydroxyapatite (HAp) coatings were investigated by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), proton-induced X-ray emission (PIXE) and Rutherford backscattering spectrometry (RBS). The study involved as-sprayed coatings and coatings incubated in simulated body fluid (rSBF) for up to 56 days. The results obtained using combined contributions from three complementary analytical techniques confirm that secondary Ca-deficient defect hydroxyapatite precipitated by a biomimetic process from the simulated body fluid onto the HAp coating surface after a prolonged induction time. Owing to its sensitivity proton-induced X-ray emission (PIXE) provides information on in vitro resorption of calcium phosphate ceramics and dynamic dissolution/precipitation events occurring during the incubation process.

Influence of experimental parameters on spatial phase distribution in as-sprayed and incubated hydroxyapatite coatings

In the present study, the behavior and properties of plasma-sprayed hydroxyapatite coatings [Ca(10)(PO(4))(6)(OH)(2), HAp] were investigated in relation to the spraying process. The experiments were focused on the influence of type of feedstock and spray power on the phase composition and distribution within the coatings. Depth profiles of the coatings were investigated before and after incubation in revised simulated body fluid (SBF) by X-ray diffraction and infrared spectroscopy. Besides HAp, the coatings contain oxyapatite (OAp) and carbonate apatite (CAp). Additionally, tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), CaO, and an amorphous phase were detected in the coatings. The HAp content directly depends on the used spray powder and spray power, where the influence of spray powder is much higher than the influence of the spray power. The grain size range of the spray powder strongly influences the HAp content in the coating and the formation of CaO. The in vitro behavior of the coatings in simulated body fluid mainly depends on the contents of CaO and amorphous calcium phosphate, respectively. The formation of portlandite due to the reaction of the coating with the SBF is strongly influenced by the porosity of the coatings and can be used as an indicator for the depth of interaction between fluid and coating.

In vitro dissolution of plasma-sprayed hydroxyapatite coatings with different characteristics: experimental study and modeling

The dissolution of plasma-sprayed hydroxyapatite (PHA) coatings with different characteristics, produced by various spraying conditions, in a Tris-buffered solution at pH 7.4 was experimentally studied through the measurement of the release of calcium ions. The phase composition of the coatings at surface and interface, and the porosity were evaluated. The analytical modeling revealed that the calcium dissolution process was composed of two stages. The first stage was found to be both surface and diffusion controlled. The second stage was an exactly diffusion-controlled dissolution. In the first stage, the rate of dissolution and the solubility of the coatings with minimum contents of impurity phases were mainly influenced by the contents of recrystallized HA (RHA) and amorphous calcium phosphate (ACP). It is suggested that the optimized values of the ACP and the RHA at the coating surface can tend to encourage the early fixation properties of the PHA coatings.

Nanocrystalline hydroxyapatite/titania coatings on titanium improves osteoblast adhesion

Bulk hydroxyapatite (HA) and titania have been used to improve the osseointegration of orthopedic implants. For this reason, composites of HA and titania have been receiving increased attention in orthopedics as novel coating materials. The objective of this in vitro study was to produce nanophase (i.e., materials with grain size less than 100 nm) HA/titania coatings on titanium. The adhesion of bone forming cells (osteoblasts) on the composite coatings were also assessed and compared with single-phase nanotitania and nano-HA titanium coatings. Nanocrystalline HA powders were synthesized through wet chemistry and hydrothermal treatments at 200 degrees C. Nanocrystalline titania powders obtained commercially were mixed with the nanocrystalline HA powders at various weight ratios. The mixed powders were then deposited on titanium utilizing a room-temperature coating process called IonTite. The results of the present study showed that such coatings maintained the chemistry and crystallite size of the original HA and titania powders. Moreover, osteoblasts adherent on single-phase nanotitania coatings were well-spread whereas they became more round and extended distinct filopodia on the composite and single-phase HA coatings. Interestingly, the number of osteoblasts adherent on the nanotitania/HA composite coatings at weight ratios of 2/1 and 1/2 were significantly greater compared with single-phase nanotitania coatings, currently-used plasma-sprayed HA coatings, and uncoated titanium. These findings suggest that nanotitania/HA coatings on titanium should be further studied for improved orthopedic applications.

Tribological behavior of plasma-sprayed carbon nanotube-reinforced hydroxyapatite coating in physiological solution

Wear behavior of plasma-sprayed carbon nanotube (CNT)-reinforced hydroxyapatite (HA) coating is evaluated in the simulated body fluid environment. Apart from enhancing the fracture toughness and providing biocompatibility, CNT-reinforced HA coating demonstrated superior wear resistance compared with that of hydroxyapatite coating without CNT. Initiation and propagation of microcracks during abrasive wear of plasma-sprayed hydroxyapatite coatings was suppressed by CNT reinforcement. Surface characterization and wear studies have shown that in addition to acting as underprop lubricant, CNTs provide reinforcement via stretching and splat-bridging for enhanced abrasion resistance in vitro.

Study on Silver-Containing HA Coatings Prepared by Vacuum Plasma Spraying

Vacuum plasma sprayed (VPS) hydroxyapatite (HA) coatings with silver-loaded zirconium phosphate antimicrobial have been prepared on titanium substrate. Antibacterial effects of the coatings were studied by bacteria culturing using Porphyromonas gingivalis (Pg), Fusobacterium nucleatum (Fn) and Actinobacillus actinomycetemcomitans (Aa) as microorganisms. Simulated body fluid (SBF) test was carried out to evaluate the bioactivity of the coatings as well. The results obtained showed that the coatings exhibited a marked antibacterial effect against Pg, Fn and Aa when the content of antimicrobial was equal to or more than 5%. The antibacterial capability of the coatings decreased in the following order: Pg, Fn, Aa. Bone-like apatite layer was formed on the silver-containing HA coatings after immersed in SBF, suggesting that their bioactivities were not affected obviously by the addition of silver-zirconium phosphate antimicrobial. This study indicated that silver-containing VPS HA coating is a prospective candidate as dental material.

Effect of Particle Size on Biocompatibility of Plasma-Sprayed Hydroxyapatite Coatings

Effect of Particle Size on Biocompatibility of Plasma-Sprayed Hydroxyapatite Coatings

Grit blasting of Ti–6Al–4V alloy: Optimization and its effect on adhesion strength of plasma-sprayed hydroxyapatite coatings

The effect of grit blasting parameters on the surface roughness of Ti–6Al–4V alloy as the substrate for plasma-sprayed hydroxyapatite (HA) coatings was examined using the factorial and Taguchi designs of experiments. In this study, two grit materials (Al2O3 and SiO2) each at two sizes, and two types of blasting systems (pressure and suction) were used. An equivalent surface roughness of 3.51 μm was obtained in three optimum conditions. The results of the Taguchi designed experiments were analyzed using signal to noise ratio. The tensile bonding strength of HA coatings deposited on the roughened substrates at the three different optimum conditions was measured by the standard adhesion test (ISO 13779-4). As the crystallinity of the coating at the interface, evaluated by the XRD analysis, reduced the bonding strength of the coatings was increased. These findings suggest that the substrate surface topography significantly influences the properties of the coating at the interface.

Bonding strength investigation of plasma-sprayed HA coatings on alumina substrate with porcelain intermediate layer

Hydroxyapatite (HA) has recently been used as a bone substitute in orthopedic and dental surgery applications, owing to its excellent biocompatibility. However, the poor mechanical properties of HA limit the material's application in the loading condition. In this study, HA was coated onto the surface of alumina substrate, with a view to overcome the poor mechanical properties of HA and the biocompatibility of alumina. Improvement of the bonding strength of HA coatings to alumina substrate was attempted by adding a bond coat of porcelain via plasma-spraying and by post-heat-treatment. HA–50 wt% porcelain and pure porcelain were used to manufacture two kinds of bond coat before HA coating, and then all the specimens were heat-treated at 750, 800, 850, and 900 °C for 0.5 h. The results shows that the bond strength between HA coating and Al 2O 3 substrate could be improved by employing the porcelain as the bond coat after the heat-treatment. The strengthening mechanisms of the two systems are discussed.

Effect of gun current on the microstructure and crystallinity of plasma sprayed hydroxyapatite coatings

Hydroxyapatite (HA) is a bioactive material because its chemical structure is close to the natural bone. Its bioactive properties make it attractive material in biomedical applications. Gas tunnel type plasma spraying (GTPS) technique was employed in the present study to deposit HA coatings on SUS 304 stainless steel substrate. GTPS is composed of two plasma sources: gun which produces internal low power plasma (1.3–8 kW) and vortex which produces the main plasma with high power level (10–40 kW). Controlling the spraying parameters is the key role for spraying high crystalline HA coatings on the metallic implants. In this study, the arc gun current was changed while the vortex arc current was kept constant at 450 A during the spraying process of HA coatings. The objective of this study is to investigate the influence of gun current on the microstructure, phase crystallinity and hardness properties of HA coatings. The surface morphology and microstructure of as-sprayed coatings were examined by scanning electron microscope. The phase structure of HA coatings was investigated by X-ray diffraction analysis. HA coatings sprayed at high gun current (100 A) are dense, and have high hardness. The crystallinity of HA coatings was decreased with the increasing in the gun current. On the other hand, the hardness was slightly decreased and the coatings suffer from some porosity at gun currents 0, 30 and 50 A.

In Vitro Dissolution of Amorphous Calcium Phosphate (ACP) Increased the Wear Particle Generation of Plasma-Sprayed HA Coatings

Hydroxyapatite (HA) coated total hip joint device has caused concerns of generating wear particles after long-term implantation. We designed a pin-on-disk (POD) test to examine the morphological changes and wear particle generation of plasma sprayed HA coating in vitro. HA coatings were immersed in supplemented α-calf bovine serum for 48 hours. Serum soaked HA coating exhibited significant amount of weight loss due to dissolution of amorphous calcium phosphate (ACP). POD test demonstrated the serum soaked HA coatings presented many micronsized particles on the surface while the as-received HA coatings maintained good integrity. The generation of wear particles of the serum soaked HA coatings is related to the reduction of the cohesion/adhesion of HA coatings due to the preferential dissolution of ACP.

Optimization of Dielectric Properties of Suspension Plasma Sprayed Hydroxyapatite Coatings

AbstractConventional plasma torch was adapted to spray very fine Ca5 (PO4)3OH (hydroxyapatite, HA) precursors having sizes ranging from a few hundreds of nanometers to a few micrometers. The powders were put in suspension with distilled water and antidispersive agent. A home made suspension feeder, including two peristaltic pumps, delivered the suspension into atomizer and therefrom injected into plasma jet. The suspension flow rate was electronically controlled. The resulting coatings had the thickness of a few tenths of μm. The electrical properties of the coatings including breakdown voltage and loss factor of suspension sprayed hydroxyapatite coatings sprayed onto aluminium substrates were also tested. The influence of such experimental factors as power input to plasma, pressure of atomizing gas, spray distance and suspension feed rate on the responses being the electrical properties was investigated using a 24 design of experiments (DOE). The mathematical models relating the responses with the factors were created and the significant factors were selected.