Plasma-sprayed Hydroxyapatite Coatings Research Articles

Bioactivity Enhancement of Plasma-Sprayed Hydroxyapatite Coatings through Non-Contact Corona Electrical Charging.

Atmospheric plasma spray (APS) remains the only certified industrial process to produce hydroxyapatite (Hap) coatings on orthopaedic and dental implants intended for commercialization. Despite the established clinical success of Hap-coated implants, such as hip and knee arthroplasties, a concern is being raised regarding the failure and revision rates in younger patients, which are increasing rapidly worldwide. The lifetime risk of replacement for patients in the 50-60 age interval is about 35%, which is significantly higher than 5% for patients aged 70 or older. Improved implants targeted at younger patients are a necessity that experts have been alerted to. One approach is to enhance their bioactivity. For this purpose, the method with the most outstanding biological results is the electrical polarization of Hap, which remarkably accelerates implant osteointegration. There is, however, the technical challenge of charging the coatings. Although this is straightforward on bulk samples with planar faces, it is not easy on coatings, and there are several problems regarding the application of electrodes. To the best of our knowledge, this study demonstrates, for the first time, the electrical charging of APS Hap coatings using a non-contact, electrode-free method: corona charging. Bioactivity enhancement is observed, establishing the promising potential of corona charging in orthopedics and dental implantology. It is found that the coatings can store charge at the surface and bulk levels up to high surface potentials (>1000 V). The biological in vitro results show higher Ca2+ and P5+ intakes in charged coatings compared to non-charged coatings. Moreover, a higher osteoblastic cellular proliferation is promoted in the charged coatings, indicating the promising potential of corona-charged coatings when applied in orthopedics and dental implantology.

Influence of Graphene Nanoplatelets on the Performance of Axial Suspension Plasma-Sprayed Hydroxyapatite Coatings.

Axial suspension plasma spraying (ASPS) is an alternative technique to atmospheric plasma spraying (APS), which uses a suspension of much finer powders (<5-micron particle size) as the feedstock. It can produce more refined microstructures than APS for biomedical implants. This paper highlights the influence of incorporated graphene nanoplatelets (GNPs) on the behavior of ASPS hydroxyapatite (HAp) coatings. The characterization of the ASPS coatings (HAp + varying GNP contents) was carried out using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), confocal Raman microscopy (CRM), white light interferometry (WLI), and contact angle measurements. The evaluation of the mechanical properties such as the hardness, roughness, adhesion strength, and porosity was carried out, along with a fretting wear performance. Additionally, the biocompatibility of the Hap + GNP coatings was evaluated using cytotoxicity testing which revealed a decrease in the cell viability from 92.7% to 85.4%, with an increase in the GNP wt.%. The visualization of the cell's components was carried out using SEM and Laser Scanning Microscopy. Furthermore, the changes in the genetic expression of the various cellular markers were assessed to analyze the epigenetic changes in human mesenchymal stem cells. The gene expression changes suggested that GNPs upregulated the proliferation marker and downregulated the pluripotent markers by a minimum of three folds.

Effect of Substrate Preheating to 550°C on the Structure of Plasma-Sprayed Hydroxyapatite Coatings

Effect of Substrate Preheating to 550°C on the Structure of Plasma-Sprayed Hydroxyapatite Coatings

The Influence of rhBMP-7 Associated with Nanometric Hydroxyapatite Coatings Titanium Implant on the Osseointegration: A Pre-Clinical Study.

Background: Bioceramic nanometer coatings have been regarded as potential substitutes for plasma-sprayed hydroxyapatite coatings, and the association with bone morphogenetic protein (BMP) is an attempt to achieve faster osseointegration to hasten oral rehabilitation. Objective: This study aimed to investigate the effect of recombinant human bone morphogenetic protein-7 (rhBMP-7) on the osseointegration of titanium implants coated with a thin film surface of hydroxyapatite (HA). Methods: Two implants (n = 24) were placed in each white New Zealand rabbits’ femur (n = 6). Implants were placed in the right femur after standard instrumentation (A and B) and in the left femur after an over-instrumentation (C and D), preventing bone-implant contact. The distal implants were installed associated with rhBMP-7 (groups B [regular instrumentation] and D [over-instrumentation]) and, also, in the absence of without BMP (control groups A [regular instrumentation] and C [over-instrumentation]). After 4 weeks, the animals were euthanized. The bone blocks containing the implants were embedded in methyl methacrylate and sectioned parallel to the long axis of the implant, which were analyzed by image segmentation. The data were analyzed using a nonparametric statistical method. Results: We observed that Group A had a mean bone formation of 35.6% compared to Group B, which had 48.6% (p > 0.05). Moreover, this group showed 28.3% of connective tissue compared to Group A, with 39.3%. In the over-instrumented groups, rhBMP-7 (Group D) showed an enhanced and significant increase in bone formation when compared with the group without rhBMP-7 (Group C). Conclusion: We concluded that the association of rhBMP-7 to thin nanostructure HA-coated implants promoted greater new bone area than the same implants in the absence of rhBMP-7, mainly in cases of over-instrumented implant sites.

Plasma-Sprayed Hydroxyapatite Coatings and Their Biological Properties

Air plasma spraying (APS) is a common method of producing hydroxyapatite (HA) coatings for alloprosthetic implants. Modification of HA spraying potentially may diminish the risk of inflammation and local infection during bone implantation. Titanium implants were HA coated with different deposition process parameters. HA powder was deposited using APS with axial powder injection at three different distances (100, 120, and 140 mm). The surface morphology of each implant was examined and the direct contact cytotoxicity of each HA coating was evaluated according to norm ISO 10993 5:2009. The response of monocytes to HA was assessed via the activation of transcription nuclear factor. All coatings had a lamellar structure. HA sprayed at a distance of 120 mm showed the highest roughness and little phase change. The analysis of the results of the conducted research showed that plasma-spraying distance during the HA coating process had a negligible impact on biocompatibility. The results obtained for a distance of 120 mm showed a slight increase in the biological properties tested. Moreover, HA coatings sprayed at different distances were not cytotoxic and did not stimulate the NF-kB. Bare titanium was less susceptible to colonization by Staphylococcus aureus than HA-coated surfaces. HA constitutes a potentially good, low-cost, non-cytotoxic material for joint prostheses.

Fretting Wear Characteristics of Suspension Plasma-Sprayed Hydroxyapatite Coating on Titanium Substrate for Orthopedic Applications

Fretting Wear Characteristics of Suspension Plasma-Sprayed Hydroxyapatite Coating on Titanium Substrate for Orthopedic Applications

Simulated body fluids prepared with natural buffers and system for active pH regulation

The current study focuses on creating an initial highly alkaline simulated body fluid whose pH can be decreased to a physiological range by adding CO2 (protein-free simulated body fluid) or a combination of CO2 and human proteins (protein-containing simulated body fluid). The effects of dissolved human proteins, Ca2+ ions, and immersed plasma-sprayed hydroxyapatite coatings on the pH and chemical instability of prepared simulated body fluids were investigated. The physiological concentration of dissolved human proteins decreased the pH instability in prepared simulated body fluids by 60% and the physiological concentration of dissolved Ca2+ ions by 15%. The effect of immersing the hydroxyapatite coatings was negligible. In terms of chemical instability, the dissolution of Ca2+ ions caused the blurring of protein-free simulated body fluids after 0.6-1.0 h. In protein-containing simulated body fluids, this phenomenon was undetectable due to their opacity. The effects of human protein presence on the carbonated-apatite-forming ability on the surfaces of immersed hydroxyapatite coatings in the prepared simulated body fluids were also assessed. The experiments validated the bioactivity of plasma-sprayed hydroxyapatite coatings in the prepared simulated body fluids, regardless of protein presence. On the other hand, under the different experimental conditions (unregulated or regulated pH), the human protein presence had an inhibitory (unregulated pH) or indifferent/promoting (regulated pH) influence on the carbonated-apatite-forming ability. The results of the present study are discussed, as well as the strengths and shortcomings of the prepared simulated body fluids, and are compared to those of previous relevant investigations.

Zinc ion sorption onto solution-precursor plasma-sprayed hydroxyapatite coatings via raman and X-ray photoelectron spectroscopies

This paper reports the unusual ion sorption of zinc ions to the structure of solution-precursor plasma-sprayed hydroxyapatite coatings studied via X-ray photoelectron and Raman spectroscopy techniques. Raman peak deconvolution revealed no significant band shifting or peak broadening for 5 mol% and 10 mol% zinc-added hydroxyapatite coatings suggesting no distortion in the unit cell. The calculated bond length-wavenumber correlation also showed no significant changes in the bond length of the phosphate group after zinc addition. On the other hand, X-ray photoelectron spectroscopic analysis showed that a significant binding energy shift was observed for the calcium-oxygen and phosphorous-oxygen bonds when 10 mol% of zinc ion is added. It is likely that for low zinc concentration (5 mol%) in plasma-sprayed hydroxyapatite coating, zinc ions possibly inserted at the interstitial site of hydroxyapatite contrary to the presumed substitution at the calcium site. Such insertion did not disturb phosphate Raman signals and caused no shifting in the calcium-oxygen and phosphorous-oxygen Raman peaks. On the other hand, X-ray photoelectron spectroscopic analysis revealed a calcium deficient hydroxyapatite coating and significant peak shifting was observed when 10 mol% of zinc ion is added. The evolution of X-ray photoelectron spectroscopy peaks indicates that substitution of zinc ion in the calcium site is predominant for 10 mol% of zinc concentration. The substituted zinc ion forms a tetrahedral coordination with the phosphate group as revealed in X-ray photoelectron spectroscopic analysis.

Most commonly used metallic biomaterials for plasma sprayed hydroxyapatite coatings

In load bearing implants metallic biomaterials have the first preference in orthopedic surgery because of their good mechanical properties which satisfied the prerequisite of human cortical bone. However metallic implants have limitations of poor wear resistance and release of metal ions resulting in to implant failure. To overcome these limitations hydroxyapatite based coatings on metal substrate can be used. The application of hydroxyapatite coating with the second reinforcement improve the fatigue strength of composite coatings. In this paper, commonly used metallic biomaterials for application of hydroxyapatite based composite coatings are discussed. Plasma spray coating technique with variable process parameters has been considered for this review. Presently plasma spray technique is commonly used for application of HA coatings in medical industry.

Efficacy of Graphene Nanosheets on the Plasma Sprayed Hydroxyapatite Coating: Improved Strength, Toughness and In-Vitro Bioperformance With Osteoblast

Graphene nanosheet (GNS) reinforced HA coating on Ti6Al4V substrate, fabricated using plasma spray, was developed for biomedical applications. Microstructural observation corroborated that the adding GNSs homogeneously distributed in the GNS/HA coating with typical lamellar structure featured by GNSs acting as binder between the discrete HA splats. Meanwhile, GNSs were observed to get in direct contact with HA matrix and form a clean GNS-HA interface without interfacial product. Results of various indentation tests showed simultaneous improvement in both indentation yield strength (~379 MPa) and fracture toughness (0.78±0.17 MPam1/2 ), mainly attributing to synergetic toughening and strengthening mechanisms associated with the adding GNSs such as load transfer, GNS pull-out, inter-layer sliding of a GNS, crack branching, GNS bridging and crack deflection. In addition, the GNS/HA coating exhibited improved biocompatible with MG-63 cell in terms of its attachment, adhesion strength, proliferation and differentiation. Hence, the GNS/HA composite coating with improved strength, toughness and enhanced biocompatibility makes it a promising candidate for bone regeneration and implantation.

Collagen Coating of Titanium Implants Using Nonthermal Plasma

Surface modification of implants is now an essential aspect of biocompatibility. A single step process to attach biomolecules to implants would represent a major advance and plasma coating could potentially achieve this. An aqueous solution of collagen was sprayed into a non-thermal plasma and deposited onto titanium surfaces. The plasma instantly transformed the liquid aerosol into a coagulated dry coating on the implant surface. Surface analysis confirmed the presence of a thin, conformal protein layer on the metal surface. A series of titanium fixation screws were coated first with hydroxyapatite and then with a layer of collagen and were implanted into the femurs of New Zealand white rabbits and compared to various control surfaces. Measurements of the rotational torque removal force indicated that the collagen coating enhanced bone fixation and created a more reproducible well anchored, stable implant than the traditional plasma sprayed hydroxyapatite coatings. Non-thermal plasma processing offers a single step route to the biological functionalisation of implantable surfaces. The process requires no polymers, primers or linkers and offers an opportunity to control biocompatibility and to tailor the local response to the implant in vivo. This opens the door to a wide variety of novel biological surface coatings across all aspects of life sciences and medicine.

Quantitative texture analysis of a hydroxyapatite coatings plasma-sprayed on titanium substrates at different temperatures

Artificial hydroxyapatite exhibits an excellent biocompatibility with tissues of human body. However, poor mechanical properties of hydroxyapatites and low reliability in wet environments restrict their use. These limitations can be overcome by applying the hydroxyapatite as a coating onto metallic implants. X-ray diffraction analysis (restoration of orientation distribution function from pole figures and the Rietveld method) and scanning electron microscopy have been used to study thick (~330 μm) plasma-sprayed hydroxyapatite coatings. The coatings were deposited onto Ti – 2Al – 1Mn alloy substrates, one of which was held at room temperature (20°C) whereas the other substrate was preheated to 550°C. The texture of the coating deposited on substrate held at room temperature is characterized by the (001)[510] orientation, the volume fraction of which is 0.08, while the coating deposited on preheated substrate has the (001)[410] orientation, the volume fraction of which is 0.10. Results of texture analysis are qualitatively supported by the Rietveld refinement data. The problem of the formation of basal texture in plasma-sprayed hydroxyapatite coatings is discussed in terms of quantitative texture analysis in relation to the differences in the substrate temperature and spraying parameters. It was concluded that the quantitative texture analysis is of importance for deeper understanding the effect of spraying parameters on the formation of hydroxyapatite coatings.

Thermal Oxide Layer Enhances Crystallinity and Mechanical Properties for Plasma-Sprayed Hydroxyapatite Biomedical Coatings.

The stability of plasma-sprayed hydroxyapatite (HA) coatings on metallic implants in vivo remains a significant challenge for load-bearing orthopedic implants despite their excellent mechanical and osteoconductive properties. This study focuses on oxide layer formation on the surface of Ti6Al4V samples through furnace heating at 600, 700, and 800 °C for 10 min for optimization of the most effective oxide layer to increase plasma coating crystallinity and improve plasma coating bond strength with the metal surface. The 800 °C heat treatment shows an effective oxide layer which increases coating crystallinity from 64 to 75% and coating adhesive bond strength from 25.9 ± 2.3 to 30.7 ± 1.1 MPa, while simultaneously reducing the dissolution rate of HA coatings. The addition of biologically relevant dopants, MgO and SiO2, show negligible effects on crystallinity and adhesive bond strength on plasma-sprayed HA coatings and additionally show an enhancement effect on osteoblast proliferation and differentiation. Moreover, the inclusion of these additivess shows an increase in osteogenesis in a rat distal femur model after 6 and 10 weeks of implantation. Overall, this study provides a direct solution to improve the crystallinity, adhesive bond strength, and osteogenic properties of plasma-sprayed HA coatings on orthopedic implants that is more manufacturable and translational from research to an industrial scale.

Interfacial Strength of Plasma-sprayed Hydroxyapatite Coatings

Both tensile and shear adhesion strength tests were performed to evaluate interfacial strength between hydroxyapatite coatings on titanium alloy substrates subjected to anodization and heat treatment prior to deposition as well as post-deposition heat treatment. Results of the tensile adhesion strength tests were influenced by the size of the blasting media and the processing of pre- and post-heat treatments but not influenced by anodization. The finite element method (FEM) analysis of stress distribution during the shear adhesion strength test was also performed to evaluate the degree of stress singularity. The results show that the stress singularity parameters were dominant factors of stress distribution in the stress singularity fields, and they were also expected to influence the “essential” interfacial strength. In addition, the size of the blasting media had the same influence on the shear and tensile adhesion strength tests. This suggests the possibility of estimating tensile adhesion strength using the results of shear adhesion strength.

Effect of Plasma Power on Corrosion Behaviour of Plasma Sprayed Hydroxyapatite Coatings

Effect of Plasma Power on Corrosion Behaviour of Plasma Sprayed Hydroxyapatite Coatings

Influence of Yttria Stabilized Zirconia and Hydrothermal Treatment on Plasma Sprayed Hydroxyapatite Coatings

A post-treatment of hydrothermal process was conducted to evaluate its effects on the material characteristics and mechanical properties of plasma-sprayed pure HA and HA/20% YSZ coatings. Surface morphology and microstructural changes, relative element contents as well as phase transformations were analyzed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometry. Both microhardness and Young’s modulus were measured. CaCO3 was found existing before and after the whole process of post-treatment. Peaks of impurity phases such as CaO, TCP, and TTCP of as-sprayed coatings were observed to disappear while HA peaks show a tendency of getting higher and wider over time. Surface morphology of SEM analysis presents a clear deposition behavior of ultrafine HA crystallized particles and cross-sectional analysis exhibits a dense and fine structure. Mechanical properties of HA/20%YSZ coatings are found to be significantly higher than those of pure HA coatings and both of which displayed an overall increase with the heating time, indicating enhanced performances.

Influence of Substrate Surface Roughness and Post Heat Treatment on Interfacial Strength of Plasma-sprayed Hydroxyapatite Coatings

Influence of Substrate Surface Roughness and Post Heat Treatment on Interfacial Strength of Plasma-sprayed Hydroxyapatite Coatings

Influence of post heat treatment on Interfacial Strength of Plasma-sprayed Hydroxyapatite Coatings

Influence of post heat treatment on Interfacial Strength of Plasma-sprayed Hydroxyapatite Coatings

Heat and hydrothermal treatments on the microstructure evolution and mechanical properties of plasma sprayed hydroxyapatite coatings reinforced with graphene nanoplatelets

Recent demands in clinical applications drive a large amount of research to plasma sprayed hydroxyapatite (HA) composite coatings. Herein, graphene nanosheet (GNS) reinforced HA coating was fabricated using plasma spray, the effect of heat and hydrothermal treatments (hereafter referred to as thermal treatment) on microstructural evolution and mechanical properties of the composite coating were investigated. Thermally treated GNS/HA coating not only exhibited ~47.1% improvement in HA crystallinity and more denser microstructure, but also displayed increased surface roughness (3 times of that of the as-sprayed sample) due to the fact that GNSs facilitated HA nanoparticle precipitation on the coating surface. Fracture toughness of the as-sprayed HA coating increased by up to ~44.1% at 2.0 wt% GNSs owing to GNS pullout, GNS bridging and arresting of crack propagation by the embedded GNSs. As for thermally treated HA coatings, crack propagation arrested by the sintered regions was found to act as an important toughening mechanism, but thermal treatment introduced more structural defects into the GNSs and led to the reductions in their length and thickness, resulting in negligible enhancement in toughness of the GNS/HA coating.

Bioactive and Tribological Behaviour of Atmospheric Plasma Sprayed Hydroxyapatite Coatings Reinforced by Lanthanum Oxide

Lanthanum oxide (La2O3) reinforced Hydroxyapatite coating was deposited by using unique gas tunnel type plasma spray torch under optimum spraying conditions. The phase and microstructure of the as-prepared powder and coatings were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). In vitro bioactivity of the plasma sprayed lanthanum oxide reinforced hydroxyapatite coatings were investigated by using simulated body fluid solution. Results showed that there was onset of apatite formation on the surface of coatings after 15 days of immersion in SBF, while after 19 days of immersion in SBF it was indicated that a HCAp phase crystallized on their surface. Our studies demonstrate that lanthanum oxide reinforced hydroxyapatite coatings are potentially useful biomaterials with good tribological and bioactive behaviour.