Methods of complex impedance measurements in biologic tissue.

Bioelectric impedance measurements have been used to monitor a variety of physiologic events. While important insights have been gained and useful techniques developed, there are a number of limitations to the methods usually employed. Among these are the inability to define current pathways in complex systems and the inability to distinguish between volumetric changes and materials property changes. Methods that have been used successfully in materials science can be used to address these limitations: these methods involve measurements of both real and reactive components over a wide frequency range coupled with various plotting and analytic techniques. Accurate measurement of the reactive component is inherently difficult since biologic systems are highly conductive. In addition, safety considerations have generally limited bioelectric impedance measurements in humans to frequencies above 20 kHz. For these reasons the techniques have not been widely applied in vivo; however, the techniques have been used in studies of cell suspensions and biologic tissue. This paper reviews these applications, summarizes the theory from a materials science viewpoint, discusses the instrumentation considerations for extension of the techniques to other studies, and presents more recent applications.