The underlying cause of electrical stimulation-induced tissue trauma is debated. Our focus has been to study effects of generating electrochemical by-products at the electrode-electrolyte interface, using the pulse-clamp technique coupled with voltammetry to analyze charge transfer. The platinum-H(2)SO(4) system has been a standard for analyzing electrochemistry on platinum-stimulating electrodes, even though the chemical differences between H(2)SO(4) and the living body are obvious. Experiments were designed to determine whether phosphate-buffered saline (PBS) could serve as a more accurate emulation of living tissue. It had been rumored that platinum's performance in PBS deviates from that in H(2)SO(4) at larger potentials. Voltammetry in PBS was performed in two potential ranges. In a conventional potential range (-0.6 V to +0.9 V versus Ag/AgCl), characteristic peaks appear very similar to published voltammograms of platinum in H(2)SO(4). However, in an extended range (-1.0 V to +1.7 V versus Ag/AgCl), platinum exhibited additional electrochemical activity: one oxidation peak and two reduction peaks. Therefore, voltammetry was performed in NaCl and a sodium phosphate mixture (i.e. PBS components) to separate their activity. The altered electrochemical performance of platinum in PBS suggests that certain reactions on platinum at potentials outside the water window will not reflect what happens in vivo.

Objective Neural prostheses employing platinum electrodes are often constrained by a charge/charge-density parameter known as the Shannon limit. In examining the relationship between charge injection and observed tissue damage, the electrochemistry at the electrode-tissue interface should be considered. The charge-storage capacity (CSC) is often used as a predictor of how much charge an electrode can inject during stimulation, but calculating charge from a steady-state i-E curve (cyclic voltammogram) over the water window misrepresents how electrodes operate during stimulation. We aim to gain insight into why CSC predictions from classic i-E curves overestimate the amount of charge that can be injected during neural stimulation pulsing. Approach In this study, we use a standard electrochemical technique to investigate how platinum electrochemistry depends on the potentials accessed by the electrode and on the electrolyte composition. Main Results The experiments indicate: 1) platinum electrodes must be subjected to a “cleaning” procedure in order to expose the maximum number of surface platinum sites for hydrogen adsorption; 2) the “cleaned” platinum surface will likely revert to an obstructed condition under typical neural stimulation conditions; 3) irreversible oxygen reduction may occur under neural stimulation conditions, so the consequences of this reaction should be considered; and 4) the presence of the chloride ion (Cl−) or proteins (bovine serum albumin) inhibits oxide formation and alters H adsorption. Significance These observations help explain why traditional CSC calculations overestimate the charge that can be injected during neural stimulation. The results underscore how careful electrochemical examination of the electrode-electrolyte interface can result in more accurate expectations of electrode performance during applied stimulation.

The aim of this tutorial is to encourage members of the neuroprosthesis community to incorporate electron transfer processes into their thinking and provide them with the tools to do so when they design and work with neurostimulating devices. The focus of this article is on platinum because it is the most used electrode metal for devices in commercial use. The i(V e) profile or cyclic voltammogram contains information about electron transfer processes that can occur when the electrode-electrolyte interface, V e, is at a specific potential, and assumed to be near steady-state conditions. For the engineer/designer this means that if the potential is not in the range of a specific electron transfer process, that process cannot occur. An i(V e) profile, recorded at sweep rates greater than 0.1 mVs(-1), approximates steady-state conditions. Rapid transient potential excursions, like that seen with neural stimulation pulses, may be too fast for the reaction to occur, however, this means that if the potential is in the range of a specific electron transfer process it may occur and should be considered. The approach described here can be used to describe the thermodynamic electron transfer processes on other candidate electrode metals, e.g. stainless steel, iridium, carbon-based, etc.

The application of a train of cathodic-first/charge-balanced/biphasic pulses applied to a platinum electrode resulted in a positive creep of the anodic phase potential that increases with increasing charge injection but reaches a steady-state value before 1000 pulses have been delivered. The increase follows from the fact that charge going into irreversible reactions occurring during the anodic phase must equal the charge going into irreversible reactions during the cathodic phase for charge-balanced pulses. In an oxygenated electrolyte the drift of the measured positive potential moved into the platinum oxidation region of the i(V e) profile when the charge injection level exceeds k = 1.75. Platinum dissolution may occur in this region and k = 1.75 defines a boundary between damaging and non-damaging levels on the Shannon Plot. In a very low oxygen environment, the positive potential remained below the platinum oxidation region for the highest charge injection values studied, k = 2.3. The results support the hypothesis that platinum dissolution is the defining factor for the Shannon limit, k = 1.75. Numerous instrumentation issues were encountered in the course of making measurements. The solutions to these issues are provided.

A high surface area, Pt-Ir coating was electrodeposited onto the contacts of Advanced Bionics HiFocus 1j electrode arrays. The coating was imaged using optical and scanning electron microscopy (SEM) and the impedance was directly measured using clinically relevant voltage transients of biphasic pulses as short as 17 μs. The coating, which SEM showed to completely cover the underlying substrate, decreased the polarization impedance of 17 μs and 32 μs pulses by 91% and 93%, respectively.