In vitro mechanical integrity of hydroxyapatite coatings on Ti-6Al-4V implants under shear loading

A new test method for the mechanical behavior of coatings on metal substrates under shear loading has been developed. Finite element simulations show that this new test method provides an almost identical shear load on the coatings to that of the conventional test. Using the new method, the static and fatigue behavior of plasma-sprayed hydroxyapatite (HA) coatings on Ti-6Al-4V substrates were studied at room temperature as a function of processing conditions. The results of the static tests show that the nominal interface shear strength is in the range of 25 approximately 40 MPa. The fatigue resistance to cyclic shear loading was characterized by shear stress amplitude versus cycle-to-failure for the samples that failed within 1010(7)cycles, and by residual nominal interfacial shear strength for the samples that survived 1010(7)cycles. The experimental results indicate that a threshold level of shear stress amplitude exists for introducing fatigue damage to HA coatings. AES and XPS studies indicated that bonding between the coating and substrate does not occur over about 12 percent of the area of the interface while bonding in the contact area of the interface is mainly mechanical interlocking. A failure mechanism of interfacial microflaw coalescence is suggested.     

Wear-accelerated corrosion of Ti-6Al-4V and nitrogen-ion-implanted Ti-6Al-4V: mechanisms and influence of fixed-stress magnitude

Wear-accelerated corrosion rates at constant anodic potentials were evaluated for unimplanted and nitrogen-ion-implanted surgical Ti-6Al-4V while wearing against ultrahigh-molecular-weight polyethylene at stress levels up to 6.90 MPa (1000 psi). The ion implantation processing was found to reduce the wear corrosion rates in both saline and serum solutions at all applied stress levels. During wear testing, all of the ion-implanted surfaces remained visually unchanged from the polished condition. However, many of the unimplanted surfaces developed damage zones characterized by wear tracks and black wear debris. A surface-damage mechanism is proposed and discussed which involves disruption of the Ti-6Al-4V protective oxide film, subsequent entrapment of oxide particles in the polyethylene, then self-perpetuating damage due to the abrasive action of the embedded particles.     

The corrosion behaviour of Ti-6Al-4V, Ti-6Al-7Nb and Ti-13Nb-13Zr in protein solutions

Ti alloys are used in orthopaedic applications owing to their appropriate mechanical properties and their excellent corrosion resistance. The release of titanium and the other alloying elements into the surrounding tissue has been reported due either to passive corrosion or accelerating processes such as wear. Since the passive layer can be broken down in certain circumstances by wear it is important to study the ability of these alloys to repassivate in biological environments, in particular in the presence of proteins, and evaluate how the repassivated surface may vary from the original surface. In this study we investigated the ability of Ti-6Al-4V, Ti-6Al-7Nb and Ti-13Nb-13Zr to repassivate in phosphate buffered saline (PBS), bovine albumin solutions in PBS and 10% foetal calf serum in PBS at different pH values and at different albumin concentrations. It was found that an increase in pH had a greater effect on the corrosion behaviour of Ti-6Al-4V and Ti-6Al-7Nb than on Ti-13Nb-13Zr in PBS and that the addition of protein to the PBS reduced the influence of pH on the corrosion behaviour of all the alloys. The effect of the corrosion and repassivation was investigated by measuring changes in the surface hardness of the alloys and it was found that corrosion reduced the hardness of the surface oxides of all the alloys. In PBS the reduction was smallest for Ti-6Al-4V and largest for Ti-13Nb- 3Zr and that corrosion in protein solutions further reduced the hardness of the surface oxides. This effect was greater for Ti-6Al-4V and Ti-6Al-7Nb than for Ti-13Nb-13Zr. In conclusion, proteins in the environment appear to interact with the repassivation process at the surface of these alloys and influence the resulting surface properties.     

Self-organized nanotubular oxide layers on Ti-6Al-7Nb and Ti-6Al-4V formed by anodization in NH4F solutions

The present work reports the fabrication of self-organized porous oxide-nanotube layers on the biomedical titanium alloys Ti-6Al-7Nb and Ti-6Al-4V by a simple electrochemical treatment. These two-phase alloys were anodized in 1M (NH(4))(2)SO(4) electrolytes containing 0.5 wt % of NH(4)F. The results show that under specific anodization conditions self-organized porous oxide structures can be grown on the alloy surface. SEM images revealed that the porous layers consist of arrays of single nanotubes with a diameter of 100 nm and a spacing of 150 nm. For the V-containing alloy enhanced etching of the beta phase is observed, leading to selective dissolution and an inhomogeneous pore formation. For the Nb-containing alloy an almost ideal coverage of both phases is obtained. According to XPS measurements the tubes are a mixed oxide with an almost stoichiometric oxide composition, and can be grown to thicknesses of several hundreds of nanometers. These findings represent a simple surface treatment for Ti alloys that has high potential for biomedical applications.     

Microstructures and mechanical properties of powder injection molded Ti-6Al-4V/HA powder

Taguchi method with an L9 orthogonal array was employed to investigate the sintered properties of Ti-6Al-4V/HA tensile bars produced by powder injection molding. The effects of sintering factors at the 90% significance level: sintering temperature (1050 degrees C, 1100 degrees C and 1150 degrees C), heating rate (5 degrees C/min, 7.5 degrees C/min and 10 degrees C/min), holding time (30, 45 and 60 min) and cooling rate (5 degrees C/min, 20 degrees C/min and 40 degrees C/min) were investigated. Results showed that sintering temperature, heating rate and cooling rate have significant effects on sintered properties, whereas the influence of holding time was insignificant. It was found that a sintering temperature of 1100 degrees C, a heating rate of 7.5 degrees C/min and a cooling rate of 5 degrees C/min increased the relative density, Vicker's microhardness, flexural strength and flexural modulus. However, a further increment of sintering temperature to 1150 degrees C did not show any discernable improvement in the relative density and Vicker's microhardness, but there was a slight increase of 0.6% and 0.9% in the flexural strength and flexural modulus, respectively. Mechanically strong Ti-6Al-4V/HA parts with an open porosity of around 50% were developed.     

The machinability of cast titanium and Ti-6Al-4V

This study investigated the machinability (ease of metal removal) of commercially pure (CP) titanium and Ti-6Al-4V alloy. Both CP Ti and Ti-6Al-4V were cast into magnesia molds. Two types of specimens (with alpha-case and without alpha-case) were made for CP Ti and Ti-6Al-4V. Machinability (n = 5) was evaluated as volume loss (mm3) by cutting/grinding the 3.0 mm surface using fissure burs and silicon carbide (SiC) under two machining conditions: (1) two machining forces (100 or 300 gf) at two rotational speeds (15000 or 30000 rpm) for 1 min, and (2) constant machining force of 100 gf and rotational speed of 15000 rpm for 1, 2, 5, 10, and 30 min. As controls, conventionally cast Co-Cr and Type IV gold alloys were evaluated in the same manner as the titanium. When fissure burs were used, there was a significant difference in the machinability between CP titanium with alpha-case and without alpha-case. On the other hand, there was no appreciable difference in the amount of metal removed for each tested metal when using the SiC points.     

In vitro biocorrosion of Ti-6Al-4V implant alloy by a mouse macrophage cell line

Corrosion of implant alloys releasing metal ions has the potential to cause adverse tissue reactions and implant failure. We hypothesized that macrophage cells and their released reactive chemical species (RCS) affect the alloy's corrosion properties. A custom cell culture corrosion box was used to evaluate how cell culture medium, macrophage cells and RCS altered the Ti-6Al-4V corrosion behaviors in 72 h and how corrosion products affected the cells. There was no difference in the charge transfer in the presence (75.2 +/- 17.7 mC) and absence (62.3 +/- 18.8 mC) of cells. The alloy had the lowest charge transfer (28.2 +/- 4.1 mC) and metal ion release (Ti < 10 ppb, V < 2 ppb) with activated cells (releasing RCS) compared with the other two conditions. This was attributed to an enhancement of the surface oxides by RCS. Metal ion release was very low (Ti < 20 ppb, V < 10 ppb) with nonactivated cells and did not change cell morphology, viability, and NO and ATP release compared with controls. However, IL-1beta released from the activated cells and the proliferation of nonactivated cells were greater on the alloy than the controls. In summary, macrophage cells and RCS reduced the corrosion of Ti-6Al-4V alloys as hypothesized. These data are important in understanding host tissue-material interactions.     

Processing of HA-coated Ti-6Al-4V by a ceramic slurry approach: an in vitro study

Hydroxyapatite-coated titanium alloy composite powders (Ti-6Al-4V/HA) was produced by a ceramic slurry approach. The aim was to evaluate the stability of the coating when subjected to a physiological medium simulated body fluid (SBF). Three consolidation conditions (700 degrees C for 5 h, 700 degrees C for 8 h and 700 degrees C for 11 h) were used in the production of the Ti-6Al-4V/HA composite powders. Results showed that biodissolution followed by apatite precipitation had taken place after soaking in SBF. In addition, it was found that consolidation at 700 degrees C for 5 h resulted in a weak mechanical locking of calcium phosphate on the Ti-6Al-4V surfaces; and the formation of small crystallites, which would increase the dissolution rate.     

In vitro behavior of sintered powder injection molded Ti-6Al-4V/HA

This article reports the morphology and mechanical properties of sintered powder injection molded Ti-6Al-4V/HA parts in a simulated physiological environment. Sintered Ti-6Al-4V/HA parts were immersed in a simulated body fluid (SBF) with ion concentrations that were comparable to those of human blood plasma for a total period of 12 weeks. At intervals of 2 weeks, the immersed Ti-6Al-4V/HA parts were analyzed with the use of scanning electron microscopy (SEM), X-ray diffractometry (XRD), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Mechanical properties such as flexural strength, flexural modulus, compressive strength, and compressive modulus were also evaluated. Results showed that complete dissolution of the more soluble phases such as tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), and calcium oxide (CaO) were found after 2 weeks of immersion in SBF. ICP analysis showed that high calcium concentration release of around 200 ppm was observed in the SBF solution after 2-4 weeks of immersion, indicating that dissolution has taken place. Next, a gradual decrease in calcium concentration release in the SBF solution was observed after immersion for 4-6 weeks, with increasing amounts of calcium phosphate precipitates being observed on the Ti-6Al-4V/HA surface. Mechanical properties such as strength and modulus were found to deteriorate during 2-4 weeks of immersion, followed by gradual increment as the immersion period increased. This study also showed that parts sintered at 1150 C exhibited faster dissolution and precipitation rates than parts sintered at 1050 C in a physiological environment.     

用磁控溅射技术制备钛合金表面HA生物涂层

目的探讨磁控溅射法制备的HA生物涂层组织结构以及涂层与基体的界面结合性能. 方法利用射频磁控溅射技术在Ti-6Al-4V基体表面制备HA生物涂层,利用扫描电镜(SEM)观察HA生物涂层表面形貌和断面形貌,利用X射线衍射仪(XRD)分析涂层的相结构,利用能量分散谱仪(EDS)分析涂层的Ca/P比,采用环氧树脂E-7对接法测定HA涂层与基体的界面结合强度. 结果溅射HA生物涂层的Ca/P比为1.7, 后处理生物涂层中不存在其它钙磷杂质相,HA的晶化程度高,HA涂层与基体的界面结合强度为51.2MPa. 结论射频磁控溅射技术制备的HA生物涂层,表面形貌良好,涂层与基体的界面结合强度较高.     

用磁控溅射技术制备钛合金表面HA生物涂层

目的 探讨磁控溅射法制备的HA生物涂层组织结构以及涂层与基体的界面结合性能。方法 利用射频磁控溅射技术在Ti-6Al-4v基体表面制备HA生物涂层，利用扫描电镜(SEM)观察HA生物涂层表面形貌和断面形貌，利用X射线衍射仪(XRD)分析涂层的相结构，利用能量分散谱仪(EDS)分析涂层的Ca／P比，采用环氧树脂E-7对接法测定HA涂层与基体的界面结合强度。结果 溅射HA生物涂层的Ca／P比为1．7，后处理生物涂层中不存在其它钙磷杂质相，HA的晶化程度高，HA涂层与基体的界面结合强度为51．2MPa。结论 射频磁控溅射技术制备的HA生物涂层，表面形貌良好，涂层与基体的界面结合强度较高。     

Microstructure and mechanical properties of plasma sprayed HA/YSZ/Ti-6Al-4V composite coatings

Plasma sprayed hydroxyapatite (HA) coatings on titanium alloy substrate have been used extensively due to their excellent biocompatibility and osteoconductivity. However, the erratic bond strength between HA and Ti alloy has raised concern over the long-term reliability of the implant. In this paper, HA/yttria stabilized zirconia (YSZ)/Ti-6Al-4V composite coatings that possess superior mechanical properties to conventional plasma sprayed HA coatings were developed. Ti-6Al-4V powders coated with fine YSZ and HA particles were prepared through a unique ceramic slurry mixing method. The so-formed composite powder was employed as feedstock for plasma spraying of the HA/YSZ/Ti-6Al-4V coatings. The influence of net plasma energy, plasma spray standoff distance, and post-spray heat treatment on microstructure, phase composition and mechanical properties were investigated. Results showed that coatings prepared with the optimum plasma sprayed condition showed a well-defined splat structure. HA/YSZ/Ti-6Al-4V solid solution was formed during plasma spraying which was beneficial for the improvement of mechanical properties. There was no evidence of Ti oxidation from the successful processing of YSZ and HA coated Ti-6Al-4V composite powders. Small amount of CaO apart from HA, ZrO(2) and Ti was present in the composite coatings. The microhardness, Young's modulus, fracture toughness, and bond strength increased significantly with the addition of YSZ. Post-spray heat treatment at 600 degrees C and 700 degrees C for up to 12h was found to further improve the mechanical properties of coatings. After the post-spray heat treatment, 17.6% increment in Young's modulus (E) and 16.3% increment in Vicker's hardness were achieved. The strengthening mechanisms of HA/YSZ/Ti-6Al-4V composite coatings were related to the dispersion strengthening by homogeneous distribution of YSZ particles in the matrix, the good mechanical properties of Ti-6Al-4V and the formation of solid solution among HA, Ti alloy and YSZ components.     

Transient electric fields induced by mechanically assisted corrosion of Ti-6Al-4V

Samples of Ti-6Al-4V were immersed in physiological solution and abraded via an electrochemical scratch method to observe the development of transient electric fields a finite distance from the scratch event. Transient electric fields were detected near both potentiostatically held and freely corroding samples. Transient currents measured by a potentiostatically held PtIr microelectric probe near a potentiostatically held sample were opposite in sign to those of the mechanically induced sample currents and were found to change character with sample potential, probe potential, and distance from the scratch event. Transient probe currents measured near a freely corroding sample were of the opposite sign as the sample transient near the primary site of oxidation, but were of the same sign near the primary site of reduction. The measured transients are a direct result of the electrochemical processes ongoing during oxide fracture and repassivation and can be sensed several millimeters remote from the abrasion site. A model for the generation of these fields is presented. Possible effects that these potentials may have on cellular structures surrounding an implant are proposed.     

Wear-corrosion performance of Si-DLC coatings on Ti-6Al-4V substrate

Si-incorporated diamond-like carbon (Si-DLC) coatings ranging from 0 to 2 at % Si were deposited on Ti-alloy substrate by means of radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) technique, using a mixture of benzene (C(6)H(6)) and silane (SiH(4)) as the reaction gas. The synergy in wear and corrosion of Si-DLC coatings was investigated by tribological and electrochemical techniques. The electrolyte used in this test to simulate the corrosive environment of body fluid was a 0.89 wt % NaCl solution of pH 7.4 at 37 degrees C. This study provides quantitative data for the assessment of the effect of Si incorporation on the synergistic effect between wear and corrosion in the simulated body fluid environment. In conclusion, tribological and electrochemical measurements showed that the Si-DLC films could improve wear-corrosion resistance in the simulated body fluid environment owing to the lower friction coefficient, corrosion rate, delamination area, and water uptake.     

Corrosion behavior of cast Ti-6Al-4V alloyed with Cu

It has recently been found that alloying with copper improved the inherently poor grindability and wear resistance of titanium. This study characterized the corrosion behavior of cast Ti-6Al-4V alloyed with copper. Alloys (0.9 or 3.5 mass % Cu) were cast with the use of a magnesia-based investment in a centrifugal casting machine. Three specimen surfaces were tested: ground, sandblasted, and as cast. Commercially pure titanium and Ti-6Al-4V served as controls. Open-circuit potential measurement, linear polarization, and potentiodynamic cathodic polarization were performed in aerated (air + 10% CO(2)) modified Tani-Zucchi synthetic saliva at 37 degrees C. Potentiodynamic anodic polarization was conducted in the same medium deaerated by N(2) + 10% CO(2). Polarization resistance (R(p)), Tafel slopes, and corrosion current density (I(corr)) were determined. A passive region occurred for the alloy specimens with ground and sandblasted surfaces, as for CP Ti. However, no passivation was observed on the as-cast alloys or on CP Ti. There were significant differences among all metals tested for R(p) and I(corr) and significantly higher R(p) and lower I(corr) values for CP Ti compared to Ti-6Al-4V or the alloys with Cu. Alloying up to 3.5 mass % Cu to Ti-6Al-4V did not change the corrosion behavior. Specimens with ground or sandblasted surfaces were superior to specimens with as-cast surfaces.     

In vitro stability of plasma-sprayed hydroxyapatite coatings on Ti-6Al-4V implants under cyclic loading

The success of hydroxyapatite (HA)-coated Ti-6Al-4V implants relies on the long-term stability of HA coatings. In this study, the mechanical stability of plasma-sprayed HA coatings on Ti-6Al-4V implants under four-point cyclic bending was systematically investigated in both air and simulated body fluid (SBF) environments at room temperature. To have a clear view of the microscale damage evolution, the surface morphology change of HA coatings during cyclic loading was carefully examined by scanning electron microscopy at the same locations on the coating surfaces after four-point bending for 4, 6.5, 8.5, and 10 million cycles. Also, possible changes of other characteristics such as thickness, weight, crystallinity, and residual stress of HA coatings were measured as a function of loading cycles. Up to 10 million cycles of bending in air and SBF, we found no significant microcracking or coating spalling on the surface of coatings, and no significant changes in thickness, weight, crystallinity, or residual stress of the plasma-sprayed HA coatings. The experiment results indicate that thickness and crystallinity had no effects on the stability of the HA coatings. HA coating resistance to the cyclic four-point bending might result from the stress shielding effects of preexisting microcracks in the coatings.     

Hydroxyapatite-coated Ti-6Al-4V part 1: the effect of coating thickness on mechanical fatigue behaviour

The present investigation examines the effect of coating thickness on the fatigue behaviour of hydroxyapatite (HA)-coated Ti-6Al-4V. Uniaxial fatigue tests were conducted on grit blasted Ti-6Al-4V coupons with HA coatings deposited by atmospheric plasma-spray at thicknesses of 0, 25, 50, 75, 100, and 150 microm, as well as on grit blasted specimens that had received a stress relieving heat treatment. Coupons with 150 microm HA coatings were shown to have significantly decreased fatigue resistances, with lives similar to those of the stress relieved specimens, while coatings of thickness 25-100 microm were found to have no affect on fatigue resistance. Residual stresses generated during deposition, cracks propagating towards the substrate from within the coating, and stress relief due to heat inputs from the spraying process were all considered with respect to their potential effects on fatigue behaviour. Stress relief in the substrate due to enhanced heat transfer mechanisms was identified as the most likely source of the observed reductions in substrate fatigue life in the 150 microm coupons.     

TEM and STEM analysis on heat-treated and in vitro plasma-sprayed hydroxyapatite/Ti-6Al-4V composite coatings

A cogent understanding of the microstructure, and indeed nano-structure, of hydroxyapatite (HA) and the interface between Ti-6Al-4V and HA is crucial to its appropriateness as a biomaterials. This paper reports the analysis of plasma-sprayed HA/Ti-6Al-4V composites by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to elucidate the intricate nature of the materials following plasma spray processing and in vitro evaluation. The novel Ti-6Al-4V/HA composite coating, with approximately 48 wt% HA, had demonstrated attractive tensile adhesion strength (approximately 28 MPa) and improved Young's modulus (approximately 55 GPa). Experimental results demonstrated that amorphous calcium phosphate and fine HA grains were formed during rapid splat solidification in the as-sprayed composite coatings. Small Ti-6Al-4V grains were observed adjacent to the amorphous calcium phosphate. The coatings were further heat treated at 600 degrees C for 6 h, and significant crystallisation of the amorphous calcium phosphate phase took place. However, complete crystallisation was not achieved at this temperature, as the coatings invariably contained a small amount of amorphous calcium phosphate phase in some local regions. After immersion in simulated body fluid for 2 weeks and 10 weeks, TEM and STEM confirmed that the interfaces inside the coating maintained good microstructural integrity.     

Microvascular response of striated muscle to common arthroplasty-alloys: A comparative in vivo study with CoCrMo, Ti-6Al-4V, and Ti-6Al-7Nb

The impairment of skeletal muscle microcirculation by a biomaterial may have profound consequences. Due to excellent physical and corrosion characteristics, CoCrMo-, Ti-6Al-4V-, and Ti-6Al-7Nb-alloys are commonly used in orthopedic surgery. Yet concern has been raised with regard to the implications of inevitable corrosion product of these metals on the surrounding biologic environment, particularly in the case of CoCrMo. We, therefore, studied in vivo nutritive perfusion and leukocytic response of striated muscle to these alloys, thereby drawing conclusions on their inflammatory potential. In 28 hamsters, utilizing the dorsal skinfold chamber preparation and intravital microscopy, we could demonstrate that the implant material CoCrMo has a marked impact on local microvascular parameters. While the Ti-alloys Ti-6Al-4V and Ti-6Al-7Nb induced only a transient and moderate inflammatory response, the implantation of a CoCrMo sample led to a distinct and persistent activation of leukocytes combined with disruption of the microvascular endothelial integrity and marked leukocyte extravasation. Animals with Ti-alloys showed a clear tendency of recuperation, while in all but one CoCrMo-treated animals, a breakdown of microcirculation prior to the scheduled end of the experiment was observed. Overall, the alloy Ti-6Al-7Nb was tolerated slightly better than Ti-6Al-4V under the chosen test conditions, though this discrepancy was not statistically significant. Conclusively, the commonly used biomaterials Ti-6Al-7Nb and Ti-6Al-4V induce a considerably lower inflammatory response in the skeletal muscle microvascular system, compared to a CoCrMo-alloy. With a minimum of adverse host reaction, our results indicate that for this particular model Ti-alloys are better tolerated than CoCrMo implant materials.     

In vitro interaction between surface-treated Ti-6Al-4V titanium alloy and human peripheral blood mononuclear cells

The Ti-6Al-4V titanium alloy is widely employed as an implant material. The effects of Ti-6Al-4V samples, tested in both an untreated state and one in which the samples were subjected to a glow-discharge treatment performed with the use of air, on human peripheral blood mononuclear cells (PBMC) were studied. Apoptosis, undetectable after 24-h contact of PBMC with the two sample types, is induced after 48 h by treated samples, and, after 48 h, but in the presence of 1.5 microg/mL PHA, by both sample types. The expression of intercellular adhesion molecule-1 (ICAM-1) always increases, in comparison with control, in PBMC put in contact with the two sample types. In the same way, a remarkable increase in tumor necrosis-alpha (TNF-alpha) release in the culture medium is registered, when PBMC are put in contact with the two sample types for 24 and 48 h. Human umbilical-vein endothelial cells (HUVEC) cocultured for 48 h with PBMC, previously incubated with the two sample types for 24 h, show an increase in ICAM-1 and vascular cell adhesion molecule-1 (VCAM-1) protein expression in comparison with control (HUVEC cocultured with control PBMC), indicating that inflammatory phenomena might occur. Taken together, these results suggest that, although plasma-treated titanium alloy shows a better biocompatibility in comparison with the untreated one, attention must be paid to the careful control of the first signs of inflammation.