Degradation characteristics of hydroxyapatite coatings on orthopaedic TiAlV in simulated physiological media investigated by electrochemical impedance spectroscopy
Introduction
Metallic materials have found wide application in restorative surgery as basic biomaterials for manufacturing implant prostheses for skeletal replacements and fixtures. In this case, metallic materials which combine good mechanical characteristics, high corrosion resistance and good compatibility with biological materials are chosen. Amongst the various materials currently employed, the alloy Ti–6Al–4V has found extensive biomedical applications due to its good mechanical properties and ability for osseointegration [1], combined with an excellent corrosion behavior due to passivity. Passivity is due to the very stable and tenaciously adherent oxide films spontaneously formed over the surface [2], [3], [4], [5], [6], which reform very rapidly if removed or mechanically damaged. These films are fairly unreactive, though transient microscopic breakdown of the passive state induced by the presence of chloride ions in the environment has been recently observed in vitro [7], [8]. When the prostheses are placed in the human body, the passive films undergo further transformations, namely thickening of the passivating film and stoichiometric changes, as well as metal dissolution [9], [10], [11], [12]. Both passivation and metal dissolution are processes of an electrochemical nature.
Enhanced biocompatibility of titanium-base materials is achieved by coating them with ceramic biomaterials, and through this they have become the most widely used material combination for dental and orthopaedic implants [13], [14]. The material response is governed by ion leaching and by corrosion with the release of particles. These processes are not only dependent on solubility (especially in the case of surface reactive biomaterials, such as glasses, glass ceramics or calcium phosphate ceramics), but also on intercellular turnover, cellular activity, bacteria, pH, fretting due to biomechanical situation, electrochemical processes at the interface and other factors. They usually imply a change in the chemical composition and physical properties of the interface. In addition to it, the processes undergone at the buried interface between the metal and the ceramic or polymeric material must be known and characterized. Amongst the materials employed, hydroxyapatite (HA) coatings are specially attractive, as a bone-like material is introduced in the interface between the metal and the living tissue, which results in improved osseointegration [15], [16], [17], [18]. The stability of these biomaterials still originate from their electrochemical passivity, and their ability to avoid passivity breakdown in the highly aggressive physiological environment. But at this stage, there is scarce information on the electrochemical behavior of ceramic coated biomaterials in the scientific literature.
This investigation was aimed to investigate the effect of plasma-sprayed HA-coatings on Ti–6Al–4V substrates regarding the corrosion behavior of the metallic biomaterials during experiments in physiological media. Electrochemical impedance spectroscopy (EIS) was the major investigative technique, since it has the potential to discover new information on the processes occurring, whilst not interfering significantly with the mechanisms operating. Based on the properties of the coating and the corrosion reaction at the coating/substrate interfaces, much more information can be collected than by routine DC electrochemical methods [19], [20]. The spectra of bare and coated substrates were recorded, and the data were analyzed to evaluate the equivalent circuit parameters in each case with the goal of determining the degradation characteristics of ceramic-coated biomaterials.
Section snippets
Materials
Ti–6Al–4V alloy matching the ASTM F136-84 was used as the substrate. Metal substrate was annealed at 750°C for 2 h, and air cooled. Then the substrate was alumina grit blasted to a surface roughness RA (mean roughness, DIN 4762)=1.66 μm and RZ (roughness depth, DIN 4768)=5.61 μm, and passivated in nitric acid. Some specimens were then coated with HA. HA-coatings with a thickness of 50±10 μm on Ti–6Al–4V were produced by plasma spraying. Samples were beam shaped (180×30×6 mm3), and coatings performed
Polarization experiments
In order to compare the susceptibility to corrosion of the HA-coated specimen in relation to the bare alloy material, the anodic polarization potentiodynamic curves of the two materials in Ringer's physiological solution were first recorded. Prior to the beginning of the polarization procedures, the samples were kept in the solution for 55 min in order to establish the free corrosion potential (Ecor). Subsequently, the potentiodynamic polarization curves were obtained with a scan rate of 1 mV/s
Conclusions
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Electrochemical impedance spectroscopy is a very useful technique for studying the corrosion behavior of surgical implant alloys, even when they are coated with a ceramic material such as hydroxyapatite.
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Over the frequency range applied the equivalent circuit employed for the description of the coated samples provides the best fitting of the experimental data. Two-layer models satisfactorily describe the electrochemical behavior of the system by considering a porous film on the metallic
Acknowledgements
This work was initiated within the framework of a Collaborative Research Programme (Acción Integrada No. HP1995-0092 and HP1996-0109) between Spain and Portugal. Support for work conducted at the University of La Laguna by the Ministerio de Ciencia y Tecnologı́a (Spain) under contract BQU2000-0867 is gratefully acknowledged.
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