Originally published May 1998
Electrochemical phenomena play an important role in the performance of metallic biomaterials. When a metal is introduced to the body, a wide variety of processes and interactions with the biological environment can take place. Such processes include metal ion release; oxide formation, either as a passive oxide film or as a particulate oxide; and corresponding reduction reactions that typically involve oxygen reduction but may also include biochemical species such as proteins. These phenomena vary microscopically over the alloy surface and depend on the local environment, local alloy composition and structure, and local mechanical factors.
Metallic biomaterial productslike these femoral caps made from a CoCrMo alloymust function under conditions of considerable stress and motion. Photo: Carpenter Technology Corp.
Metallic biomaterials are normally considered to be highly corrosion resistant, owing to the presence of an extremely thin passive oxide film that spontaneously forms on their surfaces. These films serve as a barrier to corrosion processes in alloy systems that would otherwise experience very high corrosion rates. That is, in the absence of passive films, the driving force for corrosion for typical implant alloys (e.g., titanium-based, cobalt chromium (CoCr)based, and stainless-steel alloys) is very high, and corrosion rates would also be high. Figure 1 reproduces an atomic-force-microscope (AFM) image of a polished and etched surface of a titanium-6% aluminum-4% vanadium (Ti-6Al-4V) alloy showing the oxide film that spontaneously forms on its surface. Note its domed-shape appearance and the difference between the alpha-phase oxide and the beta-phase oxide (raised portion in the center of image). The properties of these passive oxide films depend to a large extent on their structure and chemistry, which are themselves dependent on the substrate's prior thermal, mechanical, and electrochemical history.
Figure 1. Atomic-force-microscope image of oxide film on Ti-6Al-4V. Note the dome-shaped morphology and the difference between the alpha and beta oxide films. Beta grains are shown at center top, with alpha grains surrounding the beta. The image is 4 µm on each side, with a z range of 23 nm.
The in vivo environment is complicated by several factors that are not common to other applications. For instance, most metallic biomaterials are designed to function under conditions of considerable stress and motion, typically cyclic stress (i.e., fatigue) and wear- or fretting-type motion. When stresses and motion are sufficiently large, localized abrasion of the surface oxide film can occur, changing the local electrochemical environment and dramatically increasing the rate of corrosion. In studies previously published by the author, it was found that fretting of oxides will increase corrosion currents by two to three orders of magnitude and decrease the open circuit potential of the alloy by up to 500 mV.13 Such changes can have a profound impact on the nature of the oxide film and on the chemistry of the local solution adjacent to the alloy.
Furthermore, the role and effect of viable cells, proteins, enzymes, and catalysts present in the body are poorly understood. Most traditional electrochemical techniques used to evaluate metallic biomaterial performancefor example, polarization tests or electrochemical impedance testsdo not allow for testing of this complex, microscopically varying interaction between material and environment, nor do they investigate the effect of mechanical factors on corrosion. Standard polarization tests do not provide any detailed microscopic information on the distribution of corrosion over a surface or other information regarding the nature of the oxide film. Nor is it easy to gather detailed information regarding the electrical properties of the oxide films that form on these surfaces using available impedance spectroscopy methods. These latter techniques are typically performed at a fixed potential and do not obtain information through the range of potentials that can be experienced by the alloy in vivo.
This article will present three techniques that have been developed and used by the author to provide more detailed and quantitative information concerning the electrochemical behavior of metallic biomaterials. The three techniques are scanning electrochemical microscopy (SECM), high-speed electrochemical scratch testing, and step-polarization impedance spectroscopy. Each provides unique information relating to local electrochemical and mechanical phenomena, and to the measurement of electrical properties of surface oxides.
Scanning Electrochemical Microscopy
SECM is a technique that has evolved from other scanning probe technologies.46 The basic instrumentation features a three-dimensional translation stage and an electrochemical cell that allows for two independent working electrodes, the sample, and the probe (see Figure 2). Movement actuators attached to the probe can precisely place it (with accuracy to 1 µm or less) within a 2 x 2-mm area and within approximately 1 to 10 µm from the surface of the sample.
Figure 2. Schematic of the scanning electrochemical microscope. The probe is moved by a series of dc motors and piezoelectric translation actuators with positioning. The probe and sample are independently biased to cause a variety of different electrochemical reactions to take place.
Both the sample and the probe can be polarized to whatever potentials are desired in order to react with the solution environment. For instance, if the sample is anodically polarized to corrode (i.e., release ions into solution) and the probe is cathodically polarized to reduce any corrosion product (i.e., lower its valence) coming from the sample, then, as the probe is scanned over the surface of the sample, any variation in corrosion rate should be detected by the probe. If this information is plotted with respect to x-y position, then a three-dimensional imagerepresenting probe current versus positionwill result. Thus, in this scenario, the distribution of corrosion on a surface can be imaged while it is actually taking place.
Figure 3. SECM image of ion release from CoCr through a pinhole in the TiN/A1N superlattice coating. The peaks in the figures represent regions of high current and high ionic release. (a) SECM image with sample at +1000 mV and probe at 700 mV. (b) SECM image of identical region with sample at +500 mV and probe at 700 mV. Note clear sign of pinhole in (a) (dome in center) but not in (b).
An example of such an image is depicted in Figure 3a, which shows an SECM image of a cobalt-chromium alloy (ASTM F 15377) that has had a 5-µm intermetallic coating applied (TiN/AlN superlattice coating). The sample has been polarized to above its breakdown potential, 1000 mV (versus AgCl), and the probe has been polarized to 700 mV to reduce any ionic species present in solution. The sample was subjected to these high potentials for more than 16 hours, and several regions of the surface have developed pinholes in the coating. The image shows one such pinhole, approximately 200 µm in diameter, as it is corroding. In Figure 3b, the same region is imaged; however, the sample potential has been reduced to 500 mV (AgCl), which causes a corresponding drop in probe currents since less ionic dissolution is taking place. An SEM image of this same surface is shown in Figure 4. It can be seen that significant amounts of corrosion have taken place through the pinhole.
Figure 4. Scanning-electron-microscopy image of superlattice-coated CoCr after SECM imaging, showing the pinholes that have formed due to transpassive polarization of the substrate.
A wide variety of imaging methods can be used in SECM (see Figure 5). These include monitoring of ion release, monitoring of reduction-oxidation of solution-dissolved species, and competitive reduction of a species in solution. Furthermore, because of the ability to independently control both the sample and probe potentials, one can obtain complete information concerning which combinations of sample and probe potentials will result in probe current changes. These so-called "current-potential response surfaces" are useful not only in providing qualitative information for imaging purposes, but also in quantifying (with the appropriate calibrations) the type and amount of species being detected so that data regarding spatial distribution of concentrations (i.e., diffusion fields) can be obtained.8,9
Figure 5. Schematic of potential imaging modes for the SECM. From left to right: ion-release mode, solution reduction-oxidation mode (positive feedback mode), competitive reduction mode.
An example of one such current-potential response surface is shown in Figure 6. This is an image of the probe current (10-µm PtIr tip) resulting from sweep voltammetry (at 50 mV/sec) while the probe is positioned approximately 10 to 20 µm above a polarized cobalt-chromium-molybdenum (CoCrMo) sample immersed in pH-7 saline. During sweeping of the probe voltage, the sample is held at a fixed potential; the sample potential is then increased, and the probe-voltage sweep is repeated. From these curves, it can be seen that imaging can take place when the sample is either negatively biased below about 300 mV or polarized above 500 to 600 mV due to changes in the probe current response with potential.
Figure 6. Current-potential response surface for CoCrMo in physiological saline solution. Arrow indicates the potentials for sample and probe where imaging was performed in Figure 3a.
SECM has many potential applications to biomaterials research. These include the study of corrosion heterogeneity in different microstructures, the monitoring of electroactive species distribution over a surface, and even the potential to monitor biologically based electrochemical reactions near implant surfaces or near viable cell membranes. For example, it has recently been reported that using high-sweep-rate cyclic voltammetry to monitor species such as epinephrine or other biological species released from cells in culture can be performed with the microelectodes used in SECM.10 With the scanning capabilities of SECM, one may be able to spatially and temporally resolve electrochemical events at or near viable cells while they are situated on a material surface. The future of this technique is quite promising, and may provide new insights into corrosion behavior or other electrochemical processes relevant to biomaterials research.
Electrochemical Scratch Tests
As stated earlier, most alloys used in medicine and dentistry rely on a surface oxide film (a so-called passive film) to provide protection against corrosion. In the absence of mechanical factors, these oxide films are in fact highly corrosion resistant. However, when stress, fretting, or wear are present, the possibility of mechanically assisted corrosion significantly increases, and the rate of corrosion can increase as well.11,12
The author has investigated mechanically assisted corrosion processes with a number of techniques. The one that shows the most promise as a tool to evaluate the combined role of mechanical and electrochemical factors is the electrochemical scratch test.13,14 In this test, a passivating alloy is first immersed in solution and polarized to a fixed potential (see Figure 7). Next, a diamond stylus attached to a calibrated load cell is brought into contact with the sample surface at a known contact stress. High-speed piezoelectric actuators are then used to apply a scratch of fixed length (typically 20 to 70 µm, and ranging in width from 1 to 10 µm, depending on applied stress) in approximately 1 to 2 milliseconds. The actuators and computer control and acquisition systems are a part of the scanning electrochemical microscope described earlier.
Figure 7. Schematic of scratch test. A controlled load is applied to a diamond stylus while it is moved a fixed length over the surface in 1 to 2 milliseconds. Then, the film reforms anodically and ions are released from the scratch region. = oxide thickness; = oxide density; z = cation valence; F = 96,500 C/mole; A = scratch area; Mw = molecular weight of oxide; = time constant for repassivation; = overpotential for ionic dissolution; ba = Tafel slope.
Scratching of the surface results in removal of the surface oxide as well as localized deformation of the surface alloy. Immediately after scratching, the alloy undergoes three processes: ionic dissolution through the scratched region, reestablishment of the electrical double layer, and repassivation of the oxide film layer (i.e., the film reforms). These are highly transient events, lasting only about 3 to 10 milliseconds. The current transient and other relevant parameters are monitored with computer data-acquisition techniques; a typical example for a Ti-6Al-4V alloy in phosphate-buffered saline is shown in Figure 8. The characteristics of the current transient are used to describe the behavior of the surface during a scratch test, with peak current and time constant the primary features used.
Figure 8. Current transient response of a Ti-6Al-4V surface to a controlled scratch. The area scratched is approximately 300 µm2, which yields a current density of 10 A/cm2.
To date, this technique has been employed to assess the effect of sample potential, applied scratch load, solution chemistry, and addition of proteins on the electrochemical response to scratches for CoCrMo and titanium alloys.13,14 From the effect of potential, one can determine the potential below which an oxide film does not form when the surface is scratched: between 600 and 800 mV versus silversilver chloride (AgCl) for titanium, and between 400 and 500 mV (AgCl) for CoCr. Above these potentials, an oxide film will form, whose thickness can be directly approximated using a Faraday's lawtype calculation. That is, the charge associated with the current transient is equated with the volume of film that reforms. If the area scratched (Ao) is known, then the thickness () can be determined as shown below: