One approach to studying the interfacial electrochemical phenomena is the use of electrochemical impedance spectroscopy (EIS). Here, the electrical characteristics of an interface or electrochemical geometry can be interrogated using both frequency-based and time-based impedance methods. The Gilbert Lab has published work using both approaches to study novel hard coatings for medical devices and to investigate the effects of voltage, geometry, solution, and material combination on impedance behavior. More recently, the lab has focused on development of time-based methods that utilize microimpedance methods to explore small volume and small area impedance characteristics of metallic biomaterials surface to, for example, explore the local impedance changes due to anodization, the study of changes in impedance in regions that have experienced corrosion or mechanically assisted corrosion damage.

During corrosion of metallic biomaterials, the solution chemistry plays an important role that has been understudied. Inflammatory species including hydrogen peroxide and other super-oxide anions may play a significant role in the corrosion behavior of these alloys. The Gilbert lab has discovered that the combination of voltage history (the variation of voltage over time) and solution chemistry can result in new and previously unknown corrosion processes. One such process resulted in a patent whereby the beta phase of Ti-6Al-4V can be selectively dissolved from the surface with a combination of cathodic activation of the oxide followed by anodic polarization when performed in a solution containing hydrogen peroxide. (Patent: “Method of preparing Biomedical Surface”, Patent #8,012,338, September 6, 2011.)

Other studies are ongoing to investigate the role of inflammation, infection and solution chemistry effects on the electrochemical behavior of metallic biomaterials.

In this work the structure, chemistry and electrical and electrochemical properties of the nanometer thick oxide thin films on alloy surfaces are studied. The Gilbert lab has focused on electrochemical atomic force microscopy methods, impedance methods and analytical modeling of the semiconducting electrochemical behavior of these surfaces to study phenomena include voltage-driven oxidation (anodization). Mott-Schottky analysis of the voltage dependence of the semiconductor properties.

Our work has used ex-situ and in-situ AFM methods to study protein adsorption onto biomaterial surfaces and to explore how electrochemical state (by way of voltage control) of the surface can alter the rate of protein adsorption (e.g., fibrinogen on stainless steel or titanium) or the conformation of the adsorbed proteins.

Our work in this area grew out of the question: what effect does an electrochemically active metallic biomaterial surface have on viable cells cultured on these surfaces? The Gilbert Lab has developed electrochemical cell culture systems that can simultaneously apply and measure electrochemical phenomena while also culturing living cells on the surface. We have found, for example, that Cp-Ti surfaces can keep cells alive when the electrochemical voltage of the surface is kept more positive than – 300 mV vs Ag/AgCl, while cells cultured on Cp-Ti surfaces held at voltages below -400 mV will undergo cell death by way of what appears to be apoptosis within 1 to 16 hours. Thus, electrochemically active surfaces engaged in reduction reactions can result in rapid changes in cell behavior leading to apoptosis when the surface is cathodically biased. This approach is leading to both implant-based and particle approaches to therapeutic treatment of infection and/or cancer therapy.

Recent work has shown that other cells, including bacteria, and other materials (Co-Cr-Mo alloy) can induce similar effects. That is, there appears to be a general killing effect of net cathodic electrochemical conditions, with the associated reduction reactions present, to induce cell death.

The Gilbert Lab is studying methods to develop controlled electrochemical conditions using bimetallic particles engaged in high rate redox reactions to deliver a strong inhibitory and killing effect on bacteria and cancer cells. We have found that cathodic voltages and relatively low current densities can result in killing of bacterial in biofilms on the surface of metallic implant biomaterials. With the appropriate design and materials, one can develop such electrochemical conditions at implant surfaces and may be able to treat nascent infection at the early time points after implantation. Additionally, it may be possible to treat metallic biomaterial-based infections be application of electrochemical stimuli in-situ.

Recent design modifications of modular implants and the introduction of large-head metal-on-metal devices has dramatically increased the incidence of modular taper corrosion clinically and has also given rise to significant detectable levels of metal ions in patients with severely corroding tapers. Other clinical manifestations of the process include osteolysis and pseudotumor formation all of which is leading to increased rates of failure and revision.

Indentation testing consists of driving a rigid indenter into the surface and measuring the load-deflection behavior. From this one can determine modulus, hardness and energy dissipation.

The Gilbert lab, in its efforts to study device-based failure and corrosion mechanisms, has developed a number of instrumented performance tests to study, for example, fretting corrosion of modular hip replacements, spinal instrumentation and other devices engaged in corrosion and mechanical effects. These tests have been able to develop methods to study new materials and designs and can demonstrate the observed clinical mechanisms of corrosion present.

In spinal devices, there has been a progressive increase in use of multi-segmental spine rod with screw fixation systems for various spinal conditions like scoliosis, spinal stenosis syndrome and post-traumatic spine instability. These constructs typically consist of multiple screws and rods with connectors, all of which lead to multiple points of metal-metal contact and high cyclic-load transmission. These constructs can have crevice-like geometries that result in restricted local environment. When these factors are combined (local fluids, fretting and restricted geometries) they may lead to significant increase in corrosion rates. Gilbert Lab has developed a highly controlled fretting corrosion performance test method for spinal devices that can control, monitor and assess the electrochemical processes present at fretting interfaces in terms of current and voltage response.