6.1  Introduction

Electrodes used for neural stimulation must operate under demanding conditions. Along with exhibiting biocompatibility, they have to be small enough to cause localized excitation of the target neurons and large enough to support safe delivery of charge for effective neuronal excitation. Also in most cases, the implanted electrodes are required to function for the lifetime of the implant recipient. Consequently, for any neuroprosthesis employing electrical stimulation to be successful, the implanted electrodes must function for decades without significant degradation or damage to either themselves or to the tissue. This warrants understanding the characteristics of the metal-tissue interface in an effort to optimize electrode material selection and design stimulation protocols. Seminal studies of the interface were performed as part of larger consortia developing neural prostheses for paralysis and for implantation in the visual cortex. While retinal prosthesis development has benefited from these findings, the unique structure of the retina and eye require special consideration. Hence, increasing efforts are being made to understand the safety requirements of such retinal prostheses.

Material presented here has been mainly derived from three sources: Principles of Neural Science by Kandel, Schwartz and Jessell [30], Electrochemical Methods: Fundamental and Applications by Bard and Faulkner [3] and Electrical stimulation of excitable tissue: design of efficacious and safe protocols by Merrill [39].

6.2  Electrode–Electrolyte Interface

Whenever a metal electrode is placed in an electrolyte, thermodynamic processes operate to bring the two phases in electrochemical equilibrium. This causes attraction between the charge carriers in the two phases leading to the formation of a net potential across the interface. This interface is popularly known as the electrical double layer with the principal charge carriers in the metal phase being the electrons and those in the electrolyte being the ions. The importance of this interface

lies in the fact that for any neural excitation to take place, current has to flow through tissue. Hence, the key to understanding and controlling stimulation through metal electrodes lies in understanding the different electrochemical processes that take place at the electrode–electrolyte interface.

When a metal electrode is placed in an electrolyte, a finite separation of charge occurs leading to the formation of the electrical double layer. This charge separation has several manifestations. One reason for charge redistribution at the interface is ions in the electrolyte combining with the electrode. This leads to a net transfer of electrons between the two phases causing a plane of charge at the metal electrode that is opposed by a plane of charge in the electrolyte. Other reasons for the formation of the double layer include the specific adsorption of certain chemical species and preferential orientation of polar molecules such as water. The solution side of the double layer is composed of several layers. The inner layer called the Helmholtz or Stern layer consists of solvent molecules and some other species such as specifically adsorbed ions or molecules. The locus of electrical centres of the specifically adsorbed ions defines the inner Helmholtz plane (IHP) while the locus of centres of the nearest solvated ions defines the outer Helmholtz plane (OHP). The solvated ions are said to be non-specifically adsorbed as their interaction with the charged metal is independent of the chemical properties of the ions. These ions are distributed in the three dimensional region called the diffuse layer extending from the OHP into the bulk solution. The thickness of the diffuse layer is dependent upon the total ionic concentration of the solution.

The metal electrode-solution interface has been shown to behave like a capacitor with a finite amount of charge residing in a very thin layer on the metal surface (excess or deficiency of electrons). In the solution side, the charge is made up of excess anions or cations residing close to the electrode surface. At any given potential, the double layer is characterized by its double layer capacitance Cdl (10–40 mC/cm2).

6.2.1  Basic Mechanisms of Charge-Injection Across

the Electrode–Electrolyte Interface

Before proceeding into understanding the basics of neural stimulation and electrode characterization, it is worth noting the different terminologies assigned to the electrodes employed, which vary depending upon the experimental conditions. For electrochemical characterization, a three-electrode system is employed where the electrode of interest is referred to as the working electrode, while the other two are called the counter and reference electrodes. For neural stimulation, a two-electrode system is employed where current enters the tissue through the stimulating electrode and exits the tissue through the return electrode. Neural stimulation can be further subdivided into monopolar and bipolar stimulation. Monopolar stimulation uses a small stimulating electrode and a large return electrode while in bipolar stimulation, two small electrodes are used as the source and sink in an effort to focus the current to small regions such as nerve cuff electrodes and electrodes used in cochlear implants. Measurements may contain a third