Electrochemistry in Action-Engineering the Neuronal Response to Electrical Microstimulation

Brain neuromodulation has revolutionized the medical treatment of neurological diseases and injuries; however, existing therapies are limited in their clinical scope of application. Most existing therapies are delivered through implanted macroelectrodes that reside either on top of or directly inside the brain. Estimates of the effective electric field spread from these devices generally span from thousands to millions of individual neurons. Unfortunately, some neurological diseases and injuries require stimulation fields of higher precision. Next-generation microneuromodulation devices (˜102 – 103 μm2 surface area) have been developed with hundreds of closely spaced channels. These devices may be able to provide electrical microstimulation in the form of biphasic, charge-balanced small amplitude square waves that provide salient, behaviorally relevant information to human subjects. However, there is a lack of knowledge incorporated into their safety and clinical use. Neuromodulation is a field of science, medicine, and bioengineering that encompasses implantable and non-implantable technologies, electrical or chemical, that act upon neural interfaces to improve life for humanity. Our research groups collaboratively investigate neuromodulation performed via electrical microstimulation. Our primary development target is brain neuromodulation. In this article we highlight the application of electrochemistry to the field of neuromodulation.

Impedance Analysis of Electrochemical Systems

Interpretation of impedance spectroscopy data requires both a description of the chemistry and physics that govern the system and an assessment of the error structure of the measurement. The approach presented here includes use of graphical methods to guide model development, use of a measurement model analysis to assess the presence of stochastic and bias errors, and a systematic development of interpretation models in terms of the proposed reaction mechanism and physical description. Application to corrosion, batteries, and biological systems is discussed, and emerging trends in interpretation and implementation of impedance spectroscopy are presented.

Impedance Response of a Thin Film on an Electrode: Deciphering the Influence of the Double Layer Capacitance

The different contributions of the interfacial capacitance are identified in the case of passive materials or thin protective coatings deposited on the electrode surface. The method is based on a graphical analysis of the EIS results to determine both the passive-film capacitance in the high-frequency domain and the double-layer capacitance in the low-frequency domain. The proposed analysis is shown to be independent of the physicochemical origins of the frequency dispersion of the interfacial capacitances which results, from an analysis point of view of the experimental results, in the use of a constant-phase element However, for a correct evaluation of the thin-film properties such as its thickness, the high-frequency data must be corrected for the double-layer contribution. In particular, it is shown that if the double-layer capacitance gives a frequency-dispersed response, it is necessary to correct the high-frequency part for the double-layer constant-phase elements. This is first demonstrated on synthetic data and then used for the determination of the thickness of thin oxide film formed on Al in neutral pH solution.

Characterization of damaged skin by impedance spectroscopy: mechanical damage

PURPOSE

Electrochemical impedance spectroscopy is a convenient method that has been used to characterize skin barrier function, which affects drug delivery into and through the skin. The objective of this study was to relate changes in skin barrier function arising from mechanical damage to changes in the impedance spectra. These observations are compared in a companion paper to changes in chemically damaged skin.

METHODS

Electrical impedance and the permeation of a non-polar compound were measured before and after human cadaver skin was damaged by needle puncture.

RESULTS

The impedance responses of all skin samples were consistent with an equivalent circuit model with a resistor and constant phase element (CPE) in parallel. Pinhole-damaged skin exhibited a lower resistance pathway acting in parallel with the skin resistance without changing the CPE behavior. The characteristic frequency of the impedance scans determined after needle puncture increased by an amount that could be predicted. The flux of 4-cyanophenol increased by a small but significant amount that did not correlate with the hole resistance calculated under the assumption that the resistance of the surrounding skin did not change.

CONCLUSIONS

Skin impedance measurements are sensitive to irreversible damage caused by exposure to puncture with a needle.

Corrosion of tungsten microelectrodes used in neural recording applications

In neuroprosthetic applications, long-term electrode viability is necessary for robust recording of the activity of neural populations used for generating communication and control signals. The corrosion of tungsten microwire electrodes used for intracortical recording applications was analyzed in a controlled bench-top study and compared to the corrosion of tungsten microwires used in an in vivo study. Two electrolytes were investigated for the bench-top electrochemical analysis: 0.9% phosphate buffered saline (PBS) and 0.9% PBS containing 30 mM of hydrogen peroxide. The oxidation and reduction reactions responsible for corrosion were found by measurement of the open circuit potential and analysis of Pourbaix diagrams. Dissolution of tungsten to form the tungstic ion was found to be the corrosion mechanism. The corrosion rate was estimated from the polarization resistance, which was extrapolated from the electrochemical impedance spectroscopy data. The results show that tungsten microwires in an electrolyte of PBS have a corrosion rate of 300-700 μm/yr. The corrosion rate for tungsten microwires in an electrolyte containing PBS and 30 mM H₂O₂ is accelerated to 10,000-20,000 μm/yr. The corrosion rate was found to be controlled by the concentration of the reacting species in the cathodic reaction (e.g. O₂ and H₂O₂). The in vivo corrosion rate, averaged over the duration of implantation, was estimated to be 100 μm/yr. The reduced in vivo corrosion rate as compared to the bench-top rate is attributed to decreased rate of oxygen diffusion caused by the presence of a biological film and a reduced concentration of available oxygen in the brain.

JES Classics: ECS Science at Its Best: Perspectives on Newman’s Work on Resistance for Flow of Current to a Disk

In the late 1960s, John Newman began publishing a series of papers on the electrical characteristics of a disk electrode embedded in a semi-infinite insulating plane. His work revealed the influence that nonuniform current distributions have on experimental measurements obtained with the rotating disk electrode, commonly used in the electrochemical community.