Bioimpedance and Bioelectricity Basics

Orjan G. Martinsen, Sverre Grimnes

• Publisher: Academic Press
• Year: 2014
• ISBN:
  9780124115330
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Biomedical Science
• Index: Technology & Engineering

Bioimpedance and Bioelectricity Basics, 3rd Edition paves an easier and more efficient way for people seeking basic knowledge about this discipline. This book's focus is on systems with galvanic contact with tissue, with specific detail on the geometry of the measuring system. Both authors are internationally recognized experts in the field.

The highly effective, easily followed organization of the second edition has been retained, with a new discussion of state-of-the-art advances in data analysis, modelling, endogenic sources, tissue electrical properties, electrodes, instrumentation and measurements.

This book provides the basic knowledge of electrochemistry, electronic engineering, physics, physiology, mathematics, and model thinking that is needed to understand this key area in biomedicine and biophysics.

• Covers tissue immittance from the ground up in an intuitive manner, supported with figures and examples
• New chapters on electrodes and statistical analysis
• Discusses in detail dielectric and electrochemical aspects, geometry and instrumentation as well as electrical engineering concepts of network theory, providing a cross-disciplinary resource for engineers, life scientists, and physicists

Chapter 1

Introduction

Abstract

Chapter 1 deals with the definition of the basic concepts of the book, such as bioimpedance, bioelectricity, conductor, dielectric, and so on. It gives examples of active and passive electrical properties of biomaterials, and provides an overview of what is covered in the rest of the book.

Keywords

Bioelectricity; Bioimpedance; Conductor; Dielectric

Bioimpedance, bioelectricity, and the electrical properties of tissue are much about the same things. Bioimpedance deals with some passive electrical properties of tissue: the ability to oppose (impede) electric current flow. Bioelectricity deals with the ability of tissue to generate electricity, such as done by the heart (electrocardiography). This electricity is endogenic—that is, it is generated by the tissue itself. Bioelectricity is also about how tissue can be controlled by externally applied electricity. Such electricity, together with the electricity used for measuring bioimpedance, is exogenic—that is, it refers to externally applied electricity.

Bioimpedance and bioelectrical methods use electrodes with galvanic coupling to tissue. The instrumentation uses electronic circuitry and wires coupled to the electrodes. The charge carriers flowing in the copper wires are electrons. The charge carriers in living tissue are (with some exceptions) ions. An electrode proper is the site of charge carrier conversion from ions to electrons and vice versa. It is practical to divide problems into circuit problems and field problems. Circuit problems include issues with wires, capacitors, resistors, semiconductors, batteries, and so on. The current flow is confined to the wires; a voltage difference (volt) is measured between two points in the circuitry. Field problems are related to volume conductors and quantities that are a function of position in that volume, such as the potential field Φ(xyz).

There is a duality in the electrical properties of tissue. Tissue may be regarded as a conductor or a dielectric. In frequencies of 100 kHz or less, most tissues are predominantly electrolytic conductors. Therefore, we start Chapter 2 with a look at electrolytes. Bulk electrolyte continuity is broken in two important ways: by electrode metal plates and by cell membranes. This break in continuity introduces capacitive current flow segments. At the electrodes, electric double layers are formed in the electrolyte; the cell interiors are guarded by membranes. With high-resolution techniques, it is possible to extract important capacitive (i.e., dielectric) properties even at low frequencies, such as 10 Hz. At higher frequencies, such as 50 kHz, the dielectric properties of tissue (discussed in Chapter 3) may dominate. At the highest frequencies, tissue properties become more and more equal to that of water. Pure water has a characteristic relaxation frequency of approximately 18 GHz.

In tissue and the living cell there is an inseparable alliance between electricity and chemistry. Electrolytic theory and electrochemistry therefore form an important basis for our topics; it is not possible to understand what is going on in tissue during electric current flow without knowing some electrochemistry.

Bioimpedance and bioelectricity is about biomaterials in a broad sense—materials that are living, have lived, or are potential building blocks for living tissue. The tissue of interest may be plant, fruit, egg, fish, animal, or a human body. It may also be dead biological material such as hair or nail, or excised material such as beef or a piece of stratum corneum. The basic building block is the living cell, and a prerequisite for its life is that it is surrounded by an electrolyte solution. Great caution must be imposed on the state of the biomaterial sample. A material may change completely from the living, wetted state with large contributions from interfacial counterion mechanisms, then—via a denaturation or death process—to a more or less dead and dry sample. The extreme end of the spectrums includes a sample that must be measured in a vacuum chamber. It is important to remember this when, for example, ionic versus electronic/semiconductive properties are discussed. Life is so diversified and so complex. For example, bacteria may be in dry surroundings and encapsulated in a sleeping state, and so it is difficult to give them a clear living status.

1.1. What Is Bioimpedance and Biopermittivity?

Impedance is the ratio between voltage and current. It applies to both direct current (DC) and alternating current (AC). Admittance is the inverse of impedance—that is, not impede, but admit, current flow. Immittance is the combined term for impedance and admittance, so a better and more generic term than bioimpedance is bioimmittance.

A dielectric is, traditionally, a dry insulator capable of storing electrical energy. An electrostatic field cannot penetrate a metal but may penetrate through (Greek: dia) the dielectric. The most important dielectric quantity is permittivity, or ε. Permittivity is the ability to permit storage of electric energy. Under linear conditions and for the same tissue, unity cell admittance (Y), unity cell impedance (Z=1Y),

and complex permittivity ε all contain the same information, but are presented differently. These quantities are based upon the law of Coulomb (1785) and the equations of Maxwell (1873), discussed in Chapter 9. The Maxwell equations are based on the velocity of light and the fact that light is electromagnetic radiation. There is a direct link between the electrical permittivity of a material and its optical refractive index.

Note the difference between resistance, conductance, impedance, admittance, immittance—and resistivity, conductivity, impedivity, admittivity, immittivity, permittivity. The “-ance” parameters are dependent both on the electrical properties of the sample and the measuring system geometry. The “-ivity” parameters are material constants dependent only on the electrical properties of the sample, not its geometry and dimensions (as discussed in Chapters 3 and 4).

Bioimmittance is frequency dependent. In dielectric or electrolytic models there is a choice between a step (relaxational) and sinusoidal (single-frequency) waveform excitation. As long as the step response waveform is exponential and linear conditions prevail, the information gathered is the same. At high voltage and current levels, the system is nonlinear, and models and parameters must be chosen with care. Results obtained with one variable cannot necessarily be recalculated to other forms. In some cases, one single pulse may be the best waveform because it limits heat and sample destruction.

1.1.1. The Difference between AC and DC

Impedance and admittance are basically AC parameters. It is easy to believe that AC values approach DC values when the AC frequency → 0 Hz. However, this is not necessarily true because of electrolysis. At sufficiently low frequencies, one polarity lasts long enough to generate irreversible products that change the chemical environment permanently.

1.2. What Is Bioelectricity?

Bioelectricity refers to the electrical phenomena of life processes, and is a parallel to the medical subject electrophysiology. One basic mechanism is the energy-consuming cell membrane ion pumps polarizing a cell, and the action potential generated if the cell is triggered and ion channels opened. The depolarization process generates current flow also in the extracellular volume, which again results in measurable biopotential differences in the tissue. An important part of such activity is intracellular and extracellular single cell measurement results with microelectrodes. Single neuron activity and signal transmission can be studied by recording potentials with multiple microelectrode arrays.

In addition to measure on endogenic sources, bioelectricity also comprises the use of active, stimulating, current-carrying electrodes. Electricity is used clinically for the treatment of patients (electrotherapy), and is discussed in Chapter 10. Low-energy current pulses for nerve excitation are used for pain relief, and also in implanted devices. Organ functions are activated with implanted pacemakers and external muscle stimulators. Small DC currents are used for speeding up the healing of nonunion bone fractures. High-energy methods clearly operate in the nonlinear region. We must be aware that most models treated extensively by textbooks are limited to linear cases. Many applications such as defibrillation or electroporation are performed in the nonlinear range. Defibrillation is a life-saving procedure; electroporation is used for a very short opening of cells. Surgery and ablation are performed using high-frequency currents (electrosurgery).

1.3. How Are the Quantities of Bioimpedance and Bioelectricity Measured and Controlled?

Bioelectricity experiments are performed in vivo or ex vivo with pickup electrodes and stimulation electrodes. Electrotherapeutical methods use electricity controlled by current or voltage, charge, energy, waveform, and time.

Bioimmittance is measured in vivo or in vitro. The tissue may be kept alive and perfused under ex vivo conditions. Bioimmittance can be measured with two-, three- or four-electrode systems. With four electrodes, one electrode pair is current carrying and the other pair picks up the corresponding potential difference somewhere else in the tissue. If the measured voltage is divided by the applied current, the transfer impedance is calculated. If no voltage is measured, the transfer impedance is zero. This is equivalent to the bioelectricity case in which a signal from the source, such as the heart, is transferred to the skin surface electrodes. Zero transfer impedance does not mean the tissue conducts well, only that no signal transfer occurs. With the bioimpedance two-electrode technique, the transfer factor is eliminated because current application and signal pickup occur at the same site, which means that measured impedance reflects tissue electrical properties more directly.

Single cells are measured with microelectrodes and clamp and patch techniques (see Chapters 7 and 10).

Exogenic current is usually applied with electrodes in galvanic contact with tissue. It is also possible to apply it by a magnetic field without making physical contact with the tissue. Biopotential is difficult but not impossible to measure without galvanic contact.

The technology of the instrumentation is often based on a synchronous rectifier technique because it has superior noise suppression properties, as discussed in Chapter 8. The prerequisite is a reference signal, which is always available in immittance AC measurement systems.

1.4. Models

Science is very much about the use of models, to describe and therefore predict, and to explain and therefore understand. Bioimpedance and Bioelectricity Basics emphasizes model thinking, as we see in Chapter 9. The selected model often dictates the measuring method to be used. The interpretation of the results is dependent on the angle of view and the model used. Models, however, have their shortcomings. Important models for bioimmittance are empirical and can, therefore, only describe. Because tissue behaves predominantly electrolytically, a model's treatment of DC conductivity is important. With high-energy pulses or DC, the principle of superposition often is not valid, and different contributions cannot simply be added. Many high-energy applications such as defibrillation or electropor...