ICMCTF2013 Session D2-2: Coatings for Bio-corrosion, Tribo-corrosion, and Bio-tribology

In uncemented Ti-based implants, the tribochemical reactions occurring at the implant/bone interface were reported to be caused by a combined effect of mechanical load and a complex chemical-biological environment. These reactions are due to micromotion occurring at the implant/bone interface from daily activities of the patients, and they can lead to tissue inflammation, crack initiation and early fracture of the implant. The interplay between chemical-biological-mechanical parameters and in particular the influence of relevant proteins and osteoblastic cells, on the material degradation is of extreme clinical relevance and needs to be clarified.

The aim of this work is to evaluate the bio-tribocorrosion mechanisms of etched Ti6Al4V alloys colonized with MG63 osteoblastic-like cells. The colonized materials and control groups were characterized through cell viability/proliferation and alkaline phosphatase activity. Tribocorrosion tests were performed under a reciprocating sliding configuration at different electrochemical potentials (-0.1V and +0.5V/SCE), using a simulated body fluid and the culture medium as the electrolyte. An alumina ball was used as counterbody. Normal loads between 0.05N and 1N, were applied (153MPa and 415MPa initial contact Hertzian pressure) which allowed more detailed information on the destruction pathways of the top surface layers of the material (adsorbed proteins, passive film, etc.) to be followed. All tests were performed at a controlled temperature of 37ºC.

Several studies have been carried out using nc-TiN/a-Si₃N₄ deposited on Ti6Al4V substrates using DC and RF reactive dual magnetron sputtering in order to understand the behavior of this material when applied in human joint replacement. Characterization, mechanical and corrosive tests were conducted, including: XRD, XPS and RBS techniques to analyze the structure and composition of coatings, profilometry to analyze the topography of substrate, coating and wore Surfaces, scratch test to evaluate film adhesion and electrochemical technique as well as a reciprocating tribocorrosion test in physiological saline solution, to evaluate the corrosion susceptibility under static and reciprocating conditions respectively. All the results are analyzed and discussed.

Engineered-nanotubular structures offer exciting progress toward the design of multifunctional medical implants. To bring this to reality, the mechanical, physical, biocompatibility, and interfacial properties of such structures should be optimized. The mechanics of nanotubes is important from mechanotransduction points of view. We have observed that the fabrication of TiO₂ nanotubes with elastic modulus close to actual bone promotes osteoblast growth. In order to investigate the effect of nanotubes on behavior of osteoblast cells, series of novel experiments on measuring the mechanical properties of individual TiO₂ nanotubes followed by in-vitro cell culture tests were conducted. The nanotubes were tested in the chamber of transmission electron microscope using an in-situ atomic force microscopy stage with force resolutions better than 1 nN. Thin and thick nanotubes were tested to check if there are variations of mechanical properties as a function of size. It was shown that the nanotubular characteristics of the surface improve cell proliferation, attachment and spreading of osteoblast cells. We have developed a novel cell/surface interfacial characteristic method using focused ion beam milling (FIB) and scanning transmission electron microscopy (SEM) techniques. With this novel approach it was possible to cut through sections of cells and the underlying substrate and directly observe the biophysical interactions of osteoblasts with TiO₂ nanotubes. The results of this approach show a tight interface where TiO₂ nanotubes act as anchoring sites for osteoblasts to attach and even grow inside the hollow section of nanotubes. This tight interaction would eventually result in enhance bon-implant interlock. Based on these findings, it is possible to speculated that surface modification of Ti implants by a TiO₂ nanotube layer with elastic behavior close to the actual bone can be promising to overcome stress-shielding, a common reason for implant failures.

Every year, one in every 30 Americans has a hip prosthesis implanted, i.e. 250,000 every year implants in the US market. Health issues with regard to patient mobility, which in turn reflects an aging population, are of increasing concern,but we also see a trend toward younger and younger patients. Younger, first time recipients of hip prostheses are of concern, because of the high probability that they will be recipients of multiple prostheses replacements over their lifetimes, since some implants have approximate lifetimes of 15 years. The femoral stem, for example, should be made of 316L/316LN stainless steel, in order to optimize lifetime. Fretting corrosion, friction under small displacements, between stainless steel and bone, for instance, occurs during human gait, due to repeated loadings and un-loadings,. Some experimental investigations of fretting corrosion have been reported. As is well known, metallic alloys and especially stainless steels are covered with a passive film that inhibits corrosion and hence degradation when implanted within the body. This passive layer of few nanometers in thickness at ambient temperature, is the key of our reactive metals-based civilization according to some authors. This work is dedicated to predicting the passive layer thicknesses of stainless steel under fretting corrosion conditions, with specific emphasis on the role of proteins. The analysis is based on the Point Defect Model (PDM, micro scale) and an update of the model on the friction process (micro-macro scale) to yield the Fretting Model (FM). A genetic algorithm was used to optimize the fretting model on the experimental data, in order to derive values for important model parameters. The major results are, as expected from experimental results, albumin inhibits corrosive degradation of the steel; an incubation time is necessary for degradation of the passive film; under fretting corrosion and in the presence of a high concentration of chloride ion, passivity of the steel is also inhibited. These factors are identified as those that determine the ultimate lifetime of the prosthesis.

Commercially pure titanium and titanium alloys are predominantly used for dental implants, since these materials have excellent physical and chemical properties [1]. However, in certain cases a gray discoloration of the soft tissue can be observed using titanium implants, resulting in an undesired esthetic outcome [2, 3]. Therefore, a new coating system which combines the mechanical properties of metals with the good optical properties of ceramics would enhance the esthetics of dental implants. Above all, clinical success of dental implant systems depends strongly on their reliability. The coating should not only be esthetically pleasing and mechanically durable, but it should also withstand the corrosive environment below soft tissue.

In this study, ceramic and metallic thin films were deposited on commercially pure titanium substrates by reactive sputtering. Their color is formed by light interference phenomena. In order to characterize the corrosion properties of these systems, electrochemical impedance spectroscopy and potentiodynamic measurements were carried out using a three-electrode setup. Four coating systems (n = 3) were tested in 0.9 wt% NaCl solution and in artificial saliva (modified recipe of Fusayama et al. [4]) under different pH and temperature conditions. After the measurements, the samples were examined using a scanning electron microscope (SEM) and energy-dispersive X-ray (EDX) detector.

The roughness of the substrate not only influences the color of the investigated samples here, but also their corrosion behavior. Special focus has been given to the corrosion resistance of samples immersed in artificial saliva in order to establish a life-time model for the future application.

References

1. Albrektsson, T., et al., Acta Orthopaedica Scandinavica, 1981. (2): p. 155-170.

2. Park, S.E., et al., Clinical Oral Implants Research, 2007. (5): p. 569-574.

3. Zembic, A., et al., Clinical Oral Implants Research, 2009. (8): p. 802-808.

4. Fusayama, T., S. Nomoto, and T. Katayori, Journal of Dental Research, 1963. (5): p. 1183-1197.

An amorphous tantalum pentoxide (Ta₂O₅) coating with submicroporous topography was prepared on titanium (Ti) using a simple hydrolysis-condensation process at room temperature to enhance the initial biological responses to Ti. Surface characteristics of the test specimens were analyzed using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, glancing angle X-ray diffractometry, field emission scanning electron microscopy and t ransmission electron microscopy . The corrosion resistance was evaluated by 5-day ion release measurement in simulated blood plasma (SBP). The cytotoxicity of the materials was determined according to ISO 10993-5 specification. The biological responses, including initial fibronectin adsorption and human bone marrow mesenchymal stem cells responses, were evaluated. Results showed that the non-cytotoxic amorphous Ta₂O₅ coating with submicroporous topography was deposited on the Ti surface using a simple hydrolysis-condensation process. This Ta₂O₅ layer significantly decreased the ion release from Ti surface in SBP, and enhanced the initial fibronectin adsorption and cell proliferation. We conclude that the presence of an amorphous Ta₂O₅ coating with submicroporous topography on the Ti surface decreased the ion release and enhanced initial biological responses.

In this study, scanning electrochemical microscopy (SECM) is employed to indentify the reactivity of tribolayer coated surfaces, providing insight into the corrosion protection provided by the thin film. The substrate used in this work is a low-carbon (LC) CoCrMo alloy under two different surface conditions: (1) Polished- new implant surface and (2) tribolayer coated surfaced-retrieved implant surface.

The obtained SECM images indicate that the mechanically polished surface is heterogeneous with needle-like features (50 x 10 µm) displaying increased electrochemical activity and kinetics. The presence of the tribolayer provides some corrosion resistance although there appears to be some residual surface activity possibly due to incomplete surface coverage. The knowledge about the distribution of tribolayer is essential to determine the efficiency of the tribolayer formation on implant surface. Further studies are required to generate a clear understanding the underlying mechanisms that promote the tribolayer formation

Biocompatible metal implants for retaining normal functionality have been given special attention in the field of dentistry and orthopaedics. Ti-V alloy has shown to extend the life span of implants due to its high biocompatibility, high mechanical resistance, low density, high corrosion resistance, and atoxicity. Implants’ success rate also depends on osseointegration. Material properties such as wettability, roughness, surface energy and composition are key factors that affect osseointegration. In addition to micro and nano-roughening techniques such as sandblasting/acid etching and anodization, other techniques such as thermal oxidation, atomic layer deposition and chemical vapor deposition have been introduced to generate TiO₂ coatings for improved surface properties. TiO₂ has also been shown to improve and accelerate osseointegration through bone morphogenic protein signaling, apatite formation, and up-regulation of collagen II, osteocalcin, biglycan, collagen I, osteopontin and TGF-β1. TiO₂ layers protect the bulk Ti-V from dissociating cytotoxic aluminum and vanadium ions into the biological environment. Furthermore, TiO₂ is known to readily react with water to form hydroxyl groups on its surface, increasing surface energy and promoting cellular attachment. Recently, anodized TiO₂ nanotubes (TNTs) have shown to enhance cellular response of human mesenchymal stem cells due to increased surface area for cellular adhesion. TNTs 50 nm in diameter was formed by electrochemically anodizing Ti-V in 0.2 wt.% ammonium fluoride (NH₄F, 48% aqueous solution) in ethylene glycol with an applied voltage of 60 V DC for 3 hours. Anatase TNTs are preferred over amorphous or rutile as it resembles the crystalline structure of hydroxyapatite, which plays a key role in bone repair. Optimal parameters for forming anatase TNTs can be obtained by calcinating TNTs at 300, 450, 600 and 700 °C for two different durations, 3 and 6 hours. Annealing TiO₂ at 300 °C is known to convert amorphous into anatase TiO₂. Annealing at 450 °C is reported to promote anatase, while at 600 °C a majority of rutile with little anatase is formed. Finally, at 700 °C the rutile structure is anticipated to form. Advancing this project further will be discussed by performing surface characterizations including goniometry, scanning electron microscopy and other techniques that provide information regarding hydrophilicity, surface topography and surface composition of treated TNTs. Cellular growth is known to be more efficient on surfaces with higher hydrophilicity and higher anatase TiO₂ content rather than on surfaces which are hydrophobic, amorphous or rutile.