Introduction

Measuring the electrical impedance of biologic materials over a wide frequency range (Hz–MHz) either in vitro or in vivo presents some unique difficulties. The aqueous environment, necessary for physiologic conditions, is the main source of these difficulties. First, in order to pass currents or measure voltages, electrodes must be used, so that electrochemical processes at electrodes are present. Second, conduction through an aqueous environment is ionic, not electronic, so that ionic current conduction is present. Third, the heterogeneity of biologic materials increases the complexity of measurement. And lastly, there are instrumentation problems, which are a result of inherent component limitations and interactions with electrodes. All of these problems must be considered, and methods must be employed to reduce or eliminate them. Some of the problem solutions can only be compromises, and, in order to produce meaningful measurements, limitations should be recognized. However, it should also be noted that electrical impedance of tissue has been used diagnostically, and its use in detecting tissue damage from electrical injury shows great promise.

This chapter will cover, from a bioelectrochemistry viewpoint, three primary areas in looking at the problems or difficulties in biological impedance measurements.

Electrodes

The most commonly used electrodes in biology are produced when an electrical conductor, commonly a metal, but it may be any conducting material, is immersed in a fluid solution, usually water containing ions (electrolyte). Due to free energy differences between the electrode and the solution, certain molecular structures are formed at the interface between the two phases (electrode, solution).