Electrodes for Biopotential Recording and Tissue Stimulation

The electrical activity of a living tissue occurs at cellular level and is strictly dependent on the cell membrane. Virtually all living cells exhibit an electrical voltage difference between the cytoplasm (negative) and the extracellular surrounding (positive). Biological tissue does not have free electronic charges to move between the energy bands observed in semiconductors and metals, otherwise its electric charges are due to ions from dissociation of compounds in aqueous intra- and extracellular environments. A transducer that converts ionic current into electronic current can measure the ion flow caused by cell membrane depolarization and repolarization, which originates the action potential. Biopotential electrodes are mostly metallic and act as transducers converting ion flows into electronic current, which biomedical equipment detects and processes. This chapter explains the functioning of standard electrodes, used in the determination of half-cell potential of metals, and the importance of the electrode/electrolyte/skin interfaces in noninvasive biopotential detection. It also explains the functioning of the main types of surface and invasive electrodes used in the detection of biopotential. Stimulation electrodes are also presented.

Keywords

Biopotential; cell membrane; electrodes; half-cell potential

Contents

2.1 Introduction

Biopotential electrodes act as an interface between the biological tissue and the electronic measuring circuit, performing the transduction of ion current into electronic current. They are generally made of noble metal (silver, steel, gold) in different shapes (circular, rectangular, needle-shaped, etc.) and are coated with a salt, such as silver chloride, or polymers, such as Nafion® (fluoropolymer–copolymer based on the sulfonated tetrafluoroethylene discovered in the late 1960s by Walther Grot de Du Pont). The surface of a metal electrode is coupled to the skin through electrolyte gel. The salt and the electrolyte gel help transducing the flow of ionic charges into an electronic current.

This chapter presents the main concepts for understanding how biopotentials are registered, the functioning of the electrode/electrolyte and electrolyte/skin interfaces, the electrical characteristics of biopotential electrodes and the different types of electrodes used to register biopotential. At the end of chapter, it will be possible to understand how the transduction of a biopotential occurs, that is, how the events which begin with the exchange of ions across the cell membrane can be represented by an electrical signal captured with a metal electrode placed over the body surface or inserted into the biological tissue.

2.2 Biopotentials

The dynamic equilibrium that maintains the human body functioning involves chemical reactions to break the bonds of ingested nutrients, the construction of biopolymers such as proteins, nucleic acids, lipids, and carbohydrates and the disposal of metabolic wastes like urea and water. Although chemical reactions are always in the origin of energy distribution and synthesis of molecular constituents in living organisms, other types of energy are also needed, such as electric (biopotentials), mechanical (motion), thermal (especially in endothermic vertebrates), and even light (bioluminescence).

Electrical activity in biological tissues is dependent on the cells membrane state. Usually a difference in electrical potential is detected between the inside (cytoplasm) and the outside of the living cells. In membrane resting state, the value of this potential difference varies, according to cell type, between 5 and 100 mV, often with the interior of the cell with negative polarity relative to the outside. Figure 2.1 shows the record of the resting potential of a cell made with a microelectrode inserted through the membrane and a table with typical values of the resting potential of some biological tissues.

In biological tissue, there is no availability of free electrons and holes moving in an analogous way to what occurs in conduction and valence bands of conductive and semiconductive materials. Electric charges correspond to ions of dissociated compounds in intra- and extracellular aqueous media. Thus, the intra- and extracellular environments are conductive solutions containing charged atoms or ions, and the main are potassium (K+), sodium (Na+), and chloride (Cl−). Mechanisms of ions selective transport across the cell membrane regulate the ionic concentration in the intra- and extracellular environments. Among other factors, the cell’s transmembrane resting potential is due to the unequal concentration of ions on both sides of the membrane, active or passively partitioned by selective mechanisms of transmembrane ion transport. The effect of intra- and extracellular concentrations of potassium, sodium, and chloride ions at the ionic equilibrium potential is illustrated in Table 2.1.

The cell membrane acts as a capacitor, storing spatial distribution energy of the electrically charged ions in the intra- and extracellular environments. This potential energy is available to be quickly used and stabilizes the membrane, preventing this system to be disturbed, for example, by a subthreshold excitation.

Excitable cells, which are neurons, myocytes, and endocrine cells, can be activated when some form of energy (ionic current flow, temperature variation, pulse ultrasound, electric current, etc.) is applied to their membranes. Figure 2.2 shows a typical waveform of an action potential triggered by any stimulus, provided that it is suprathreshold. The characteristics of the active membrane change:

The propagation of the action potential across the membrane of an excitable cell occurs by the succession of depolarization and repolarization phases caused by the flow of transmembrane ionic currents, with ions flowing in and out of the cell. Figure...