Plasma-sprayed Hydroxyapatite Coatings Research Articles

Mechanical performance of bioceramic coatings obtained by high-velocity oxy-fuel spray for biomedical purposes

Plasma-sprayed hydroxyapatite coatings were used for many years to improve the osseointegration of joint implants. However, it was observed that their mechanical properties did not meet the requirements. Consequently, as in previous studies, an HAp–TiO2 mixture was considered. But, contrarily to these studies, where TiO2 is the majority phase (80–90wt.% into the mixture), in this work the feasibility of coatings containing only 20wt.% of TIO2 is proposed. Composite coatings were obtained via high-velocity oxy-fuel (HVOF) spray at four different sets of conditions. Bond strength tests, fracture toughness calculations and scratch tests together with preliminary cell viability tests were performed and compared with those of pure hydroxyapatite coatings and a powder mixture of 60wt.%HAp–40wt.%TiO2 (considered in previous studies and which showed mechanical and biological limitations, respectively). Results showed that one of the proposed 80–20 cases displayed a best combination of mechanical and biological properties than the proposed alternatives.

Plasma-Sprayed Apatite Coatings: Review of Physical-Chemical Characteristics and Their Biological Consequences

Hydroxyapatite (HA) is widely used as a coating on orthopedic medical implants to improve anchoring to bone tissue and facilitate bone repair. The use of plasma-sprayed HA coatings on metal implants has raised as much controversy as interest over the last decade. Although faster and stronger fixation and enhanced bone growth have been reported, the long-term performance of HA-coated implants has been questionned This ambivalent behaviour is related to the thermal decomposition of HA during the plasma spraying of HA powder, which has several observable consequences for the physical-chemical and biological properties of the coatings. This review presents the current knowledge on the characteristics of plasma-sprayed HA coatings and their biological consequences. Various schemes of chemical reactions and physical-chemical processes that can occur during HA-particle plasma spraying, HA-coating formation, or after implantation of HA coatings are presented. Standard and emerging methods developed for the characterization of HA coatings are reviewed, with a focus on Raman spectroscopy, which allows high-quality imaging. Finally, several promising trends that have been explored in recent years to improve the mechanical resistance of coatings and their adhesion on the substrate, to improve the coating microstructure and crystallinity, or to confer a therapeutic activity (e.g., antibacterial activity) on plasma-sprayed HA coatings are reviewed.

プラズマ溶射によるチタンへのアパタイト被覆とその評価

プラズマ溶射によるチタンへのアパタイト被覆とその評価

Improvement of Osteoblast Adhesion through Polarization of Plasma-Sprayed Hydroxyapatite Coatings on Metal

Hydroxyapatite coatings (HACs) have long been applied to orthopedic and dental implants made of titanium and its alloys because of their high biocompatibility and osteoconductivity. We have recently demonstrated that the charged surfaces on HAC induced by polarization enhance mineral deposition in simulated body fluid and osteoconductive capabilities in vivo. The present study evaluates the effects of the electrical polarization of HAC on surface characteristics and osteoblast adhesion. It was found that electrical polarization has no effect on surface roughness and crystallinity. Morphological observations and quantitative analyses of adhered osteoblasts on HACs revealed that the cell areas positively stained for actin, which indicates the degree of cell spreading, were distinctly larger on negatively and positively charged HAC than that on uncharged HAC.

Effect of Power and Stand-Off Distance on Plasma Sprayed Hydroxyapatite Coatings

Plasma sprayed hydroxyapatite coatings were deposited onto mild steel substrates under several power and stand-off distance conditions. The microstructure of the coatings was observed and imaged using scanning electron microscopy (SEM). The crystal structure and crystallinity of the coatings were analyzed using X-ray diffraction (XRD). The Ca/P ratio of the coating was measured using energy dispersive X-ray spectroscopy (EDX). The deposition efficiency of the coatings was measured by weighing the substrate before and after coating. The porosity, surface roughness, and microhardness of the deposited coatings were characterized by using ImageJ, surface profilometer, and Vickers microhardness tester, respectively. The results indicated that microhardness and deposition efficiency increased with increasing power and stand-off distance. Porosity, surface roughness, and crystallinity decreased as the Ca/P ratio steadily decreased.

A Taguchi Design Study for Optimisation of Plasma Sprayed Hydroxyapatite Coatings

Plasma sprayed hydroxyapatite coatings were deposited onto mild steel substrates. A Taguchi L9 design of experiment protocol was used to optimise the coating process parameters. The effect of three factors: (i) power and secondary gas flow rate (X1), (ii) powder feed rate and carrier gas flow rate (X2), and (iii) stand-off distance (X3) on the coating responses was studied. The responses of the plasma sprayed hydroxyapatite coatings were evaluated in terms of porosity, deposition efficiency, microhardness, crystallinity, and surface roughness. A regression analysis established relationships between process parameters and responses. Higher power, lower powder feed rate and the middle stand-off distance of 11 cm lead to optimum attributes of low porosity, high deposition efficiency, high microhardness, high crystallinity, and high surface roughness.

Suspension thermal spraying of hydroxyapatite: Microstructure and in vitro behaviour

In cementless fixation of metallic prostheses, bony ingrowth onto the implant surface is often promoted by osteoconductive plasma-sprayed hydroxyapatite coatings. The present work explores the use of the innovative High Velocity Suspension Flame Spraying (HVSFS) process to coat Ti substrates with thin homogeneous hydroxyapatite coatings.The HVSFS hydroxyapatite coatings studied were dense, 27–37μm thick, with some transverse microcracks. Lamellae were sintered together and nearly unidentifiable, unlike conventional plasma-sprayed hydroxyapatite. Crystallinities of 10%–70% were obtained, depending on the deposition parameters and the use of a TiO2 bond coat. The average hardness of layers with low (<24%) and high (70%) crystallinity was ≈3.5GPa and ≈4.5GPa respectively. The distributions of hardness values, all characterised by Weibull modulus in the 5–7 range, were narrower than that of conventional plasma-sprayed hydroxyapatite, with a Weibull modulus of ≈3.3.During soaking in simulated body fluid, glassy coatings were progressively resorbed and replaced by a new, precipitated hydroxyapatite layer, whereas coatings with 70% crystallinity were stable up to 14days of immersion. The interpretation of the precipitation behaviour was also assisted by surface charge assessments, performed through Z-potential measurements.During in vitro tests, HA coatings showed no cytotoxicity towards the SAOS-2 osteoblast cell line, and surface cell proliferation was comparable with proliferation on reference polystyrene culture plates.

Drug loading and release of Tobramycin from hydroxyapatite coated fixation pins

This paper evaluates the loading and release properties of Tobramycin incorporated by adsorptive loading from a solution into plasma sprayed and biomimetically coated Hydroxyapatite (HA) fixation pins. The aim of this study is to contribute towards designing a functional implant surface offering local release of the antibiotic agent to prevent post-surgical infections. Cathodic arc deposition is used to coat stainless steel fixation pins with a bioactive, anatase phase dominated, TiO₂ coating onto which a HA layer is grown biomimetically. The loading and release properties are evaluated by studying the subsequent release of Tobramycin using high performance liquid chromatography and correlated to the differences in HA coating microstructure and the physical conditions under loading. The results from these studies show that a dual loading strategy consisting of a solution temperature of 90 °C and a pressure of 6 bar during a loading time of 5 min release a sufficient amount of Tobramycin to guarantee the inhibition of Staphylococcus aureus up to 2 days for plasma sprayed HA coatings and for 8 days for biomimetic coatings. The present study emphasizes the advantages of the nanoporous structure of biomimetically deposited HA over the more dense structure of plasma sprayed HA coatings in terms of antibiotic incorporation and subsequent sustained release and provides a valuable outline for the design of implant surfaces aiming for a fast-loading and controlled, local drug administration.

Improvement in mechanical properties of plasma sprayed hydroxyapatite coatings by Al2O3 reinforcement

Thermal sprayed hydroxyapatite coatings suffer from poor mechanical properties like tensile strength, wear resistance, hardness, toughness and fatigue. The mechanical properties of hydroxyapatite coatings can be enhanced via incorporation of secondary bioinert reinforcement material. In this study an attempt has been made to improve the mechanical properties of plasma sprayed hydroxyapatite by reinforcing it with 10, 20 and 30% Al2O3. The plasma sprayed coatings have been characterized using FE-SEM/EDAX, XRD, AFM and FTIR spectroscopy. Corrosion studies have been done in simulated body fluid and abrasive wear studies have been performed on flat specimens on a disk wear tester. Microhardness, tensile strength and wear resistance are found to be increased with increasing Al2O3 content. All types of coatings show superior resistance against corrosion in simulated body fluid.

552 プラズマ溶射ハイドロキシアパタイトコーティング材のスクラッチ試験による密着性評価(材料力学II,学術講演)

552 プラズマ溶射ハイドロキシアパタイトコーティング材のスクラッチ試験による密着性評価(材料力学II,学術講演)

0306 水酸アパタイト溶射皮膜の疲労はく離進展挙動に及ぼす環境中摩耗の影響(OS08-1 バイオエンジニアリング(1)医療と福祉を支えるバイオエンジニアリング,オーガナイズドセッション 8:「バイオエンジニアリング(1)医療と福祉を支えるバイオエンジニアリング」)

0306 水酸アパタイト溶射皮膜の疲労はく離進展挙動に及ぼす環境中摩耗の影響(OS08-1 バイオエンジニアリング(1)医療と福祉を支えるバイオエンジニアリング,オーガナイズドセッション 8:「バイオエンジニアリング(1)医療と福祉を支えるバイオエンジニアリング」)

Modeling of phenomena occurring in plasma jet during suspension spraying of hydroxyapatite coatings

The phenomena occurring after injection of water–ethanol suspension of fine hydroxyapatite powder were simulated numerically. The simulation started with the calculation of the map of velocity and temperature of working plasma gases. The map was calculated by taking into account the evaporation of the liquids included in the suspension. The content of the chemical species included in plasma jet was also calculated. The suspension was injected through a mechanical injector into the anode-nozzle of SG-100 torch. The plasma was generated with the use of working gases composed of 45slpm of Ar and 5slpm of H2 and with the electric power input of 30kW. The initial droplets of suspension were supposed to be spherical with a diameter equal to that of the injector i.e. 500μm. The trajectory of suspension's droplets was calculated until the evaporation of liquids. Then, the simulation of movement and heating of solid hydroxyapatite (HA) started. The HA powder was in-house synthesized and had a bimodal size distribution with two maxima around 3 and 10μm. The equations describing the momentum and heat transfers from hot gas to the solids took into account the small size of solid particles. In particular, the effects of thermophoretic force and of the modification of drag coefficient by non-continuum phenomena were taken into account in the calculation of the trajectories of small particles. Similarly, the non-continuum effect was considered in calculation of heat transfer. The obtained trajectories were tentatively correlated with the microstructure of suspension plasma sprayed HA coatings.

Recrystallization of Amorphous Phases via Post-Coating Heat-Treatment of Plasma-Sprayed Hydroxyapatite Coatings

Plasma-sprayed hydroxyapatite coating suffer from amorphous phases which dissolve in body fluid environment. An attempt has been made to reduce these amorphous phases by post-coating heat treatment of plasma-sprayed specimens. The specimens after post-coating heat treatment were analyzed using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The results showed that amorphous calcium phosphates converted into crystalline hydroxyapatite at 500°C, and calcium oxide was present even after heat treatment at 900°C. A very fine crystalline hydroxyapatite phase of around 100 nm in size was formed at 900°C.

Effect of Al2O3 Reinforcement and Al2O3–13 wt% TiO2 Bond Coat on Plasma Sprayed Hydroxyapatite Coating

In present work an attempt has been made to enhance mechanical properties of plasma sprayed hydroxyapatite coating by addition of 10 wt% aluminum oxide. A bond coat of Al2O3-13TiO2 has been applied to improve strength of hydroxyapatite composite coating. Mechanical properties of coatings with addition of alumina and with incorporation of bond coat have been investigated in accordance with the ASTM C 633-79. Results indicate that the tensile bond strength of hydroxyapatite coating increased by 13 per cent by alumina reinforcement and 25 per cent by both reinforcement and application of bond layer respectively. The area of adhesive failure was found to decrease for hydroxyapatite-alumina composite coating and hydroxyapatite composite coating with bond coat as compared to pure hydroxyapatite coating. The microharness of specimens was found to increase with reinforcement, however the highest hardness was recorded for bond coat. The microhardness was found to decrease with distance from substrate coating interface. Defence Science Journal, 2012, 62(1), pp.193-198 , DOI:http://dx.doi.org/10.14429/dsj.62.1039

Nanocrystalline structure of the surface layer of plasma-sprayed hydroxyapatite coatings obtained upon preliminary induction heat treatment of metal base

Biocompatible nanocrystalline hydroxyapatite (HA) coatings for intrabone titanium implants have been obtained by plasma spraying. The HA coatings have an average grain size within 10–30 nm and are characterized by improved characteristics of morphological heterogeneity that is acquired due to the induction heat treatment (IHT) of substrates prior to plasma spray deposition. Based on the data of scanning electron microscopy with computer-aided processing of images, it is established that the average grain size depends on the IHT temperature.

Antibacterial and biological characteristics of silver containing and strontium doped plasma sprayed hydroxyapatite coatings

Infection in primary total joint prostheses is estimated to occur in up to 3% of all surgery. As a measure to improve the antimicrobial properties of implant materials silver (Ag) was incorporated into plasma sprayed hydroxyapatite (HA) coatings. To offset potential cytotoxic effects of Ag in the coatings strontium (Sr) was also added as a binary dopant. HA powder was doped with 2.0wt.% Ag2O, 1.0wt.% SrO and was then heat treated at 800°C. Titanium substrates were coated using a 30kW plasma spray system equipped with a supersonic nozzle. X-ray diffraction confirmed the phase purity and high crystallinity of the coatings. Samples were evaluated for mechanical stability by adhesive bond strength testing. The results show that the addition of dopants did not affect the overall bond strength of the coatings. The antibacterial efficacies of the coatings were tested against Pseudomonas aeruginosa. Samples that contained the Ag2O dopant were found to be highly effective against bacterial colonization. In vitro cell–material interactions using human fetal osteoblast cells were characterized by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay for cell viability, field emission scanning electron microscopy for cell morphology and confocal imaging for the important differentiation marker alkaline phosphatase (ALP). Our results showed evidence of cytotoxic effects of the Ag–HA coatings, characterized by poor cellular morphology and cell death and nearly complete loss of functional ALP activity. The addition of SrO to the Ag–HA coatings was able to effectively offset these negative effects and improve performance compared with pure HA-coated samples.

Fatigue Behavior and Apatite Precipitation of Plasma-Sprayed HAp Coating on Commercially Pure Titanium Substrate in Simulated Body Fluid (SBF)

Plasma sprayed Hydroxyapatite (HAp) coating on commercially pure titanium (cp-Ti) is widely used as implant materials. In this study, fatigue behavior of as-sprayed HAp top coat with HAp/Ti bond coat specimen under ambient environment (A-HTi) as well as under simulated body fluid, SBF, environment (I-HTi) at 36.5°C was investigated by four point bending fatigue test at a stress amplitude of 170 MPa under various frequencies. In order to investigate apatite precipitation during fatigue loading, the test specimen was immersed in SBF at 36.5°C during the fatigue test. For comparison, the test specimen was immersed in SBF at 36.5°C for a day to a week without fatigue loading and then the fatigue test of the immersed specimen was carried out under ambient environment (I-A-HTi). The fatigue loading would not influence the apatite precipitation in the HAp coating layer of the specimen. The fatigue lives of the I-HTi and I-A-HTi specimens were shorter compared to that of A-HTi specimen. The shorter fatigue lives of the I-HTi and I-A-HTi specimens would result from the attack of SBF on titanium substrate. However, the apatite precipitation in the coating layer up to one week immersion did not significantly influence the delamination between HAp top coat and cp-Ti substrate under the bending fatigue.

Mechanical, In vitro Antimicrobial, and Biological Properties of Plasma-Sprayed Silver-Doped Hydroxyapatite Coating

Implant-related infection is one of the key concerns in total joint hip arthroplasties. To reduce bacterial adhesion, we used silver (Ag)/silver oxide (Ag(2)O) doping in plasma sprayed hydroxyapatite (HA) coating on titanium substrate. HA powder was doped with 2.0, 4.0, and 6.0 wt % Ag, heat-treated at 800 °C and used for plasma spray coating using a 30 kW plasma spray system, equipped with supersonic nozzle. Application of supersonic plasma nozzle significantly reduced phase decomposition and amorphous phase formation in the HA coatings as evident by X-ray diffraction (XRD) study and Fourier transformed infrared spectroscopic (FTIR) analysis. Adhesive bond strength of more than 15 MPa ensured the mechanical integrity of the coatings. Resistance against bacterial adhesion of the coatings was determined by challenging them against Pseudomonas aeruginosa (PAO1). Live/dead staining of the adherent bacteria on the coating surfaces indicated a significant reduction in bacterial adhesion due to the presence of Ag. In vitro cell-material interactions and alkaline phosphatase (ALP) protein expressions were evaluated by culturing human fetal osteoblast cells (hFOB). Our results suggest that the plasma-sprayed HA coatings doped with an optimum amount of Ag can have excellent antimicrobial property without altering mechanical property of the Ag-doped HA coatings.

Effect of Biomimetic Nanocrystalline Apatite Surface on Bone Ingrowth

The Success of Hydroxyapatite-Coated Acetabular Components Has Not Been Consistent. Plasma-Sprayed Hydroxyapatite Coatings Work Well on Nonporous Substrates but Do Not Coat the Inner Surfaces of Open-Porous Substrates. Solution Deposition Can Generate Consistent Bioceramic Coats on Porous Surfaces that More Closely Mimic the Trabecular Pattern and Biochemistry at the Bone Interface. we Compared Bone Response to the Following Implants: Porous-Coated Ti6al4v Cylinders with 1 of 3 Treatments: Plasma Sprayed with Hydroxyapatite (HA), Coated with a Solution-Deposited Biomimetic Apatite Coating (BA), and Untreated (Control). Bilateral Femurs in 36 Rabbits Were Implanted with One of the above Implants. Bone Ingrowth for HA and BA Surfaces Was Significantly Higher than that for Control Surfaces. No Fragmentation or Debris Production Was Evident in the Apatite Coat of the BA Group. A Biomimetic Coat of Solution-Deposited Apatite May Be Resistant to Coating Delamination and Particle Generation.

Performance of atmospheric plasma sprayed HA coatings under dry and wet fatigue conditions

Hydroxyapatite (HA) is widely used as a bio-medical coating on press-fit (cementless) orthopaedic implants to enhance the biological response of the device. Presently atmospheric plasma spray (APS) is the most widely applied technology for the deposition of such bio-medical coatings. There have been concerns however, regarding the high deposition temperatures to which the HA precursor powder is exposed during APS processing, as this can result in changes to the HA powder’s crystallinity. Furthermore, concerns have been raised regarding the affect these crystalline alterations may have on the solubility of the deposited HA coating, and thus, the affect they may have on the integrity of the coating at the metal interface. The aim of this study is to evaluate the fatigue performance of APS HA coatings, carried out under both dry and wet fatigue conditions. The wet fatigue conditions were facilitated using a Simulated Body Fluid (SBF) solution, which enabled the in vitro simulation of the HA coating response in a typical in vivo environment. SEM and XRD examination of both the reference (as received) and dry samples demonstrated the coating properties were almost identical after 10 million cycles, while the evaluation of the wet samples suggested complete failure of the HA coating had occurred during testing. In conclusion, HA coating delamination during the wet fatigue testing was attributed to significant weakening of the coating, due to material loss from dissolution as a result of exposure to the SBF solution. Significantly the dissolution was found to occur both at the interface and within the coating.