Dielectric Spectroscopy
Electrical Properties of Cells, Tissues and Materials


Schematic representation of dielectric dispersions of biological cells. α-dispersion (mHz-kHz): due to counterion effects (perpendicular or lateral) near membrane surfaces; β-dispersion (1 kHz - 100 MHz): Maxwell Wagner effects, passive membrane capacitance, intracellular organelle membranes, protein molecule response γ-dispersion (0.1 - 100 GHz): due to dipolar mechanisms in polar media (water), salts and proteins.
Dielectric spectroscopy (also known as electrical impedance spectroscopy) is a non-invasive method to determine basic electrical parameters of materials and components of materials. Accordingly the method is frequently used in material sciences, however, can also be applied towards living tissues or cells in suspensions. Conversely, information on conductivity and permittivity, collected over a wide frequency range, yields information on the investigated tissue itself, such as for example cell membrane composition and structure. H.P. Schwan was the first to describe distinct dispersion relations in dielectric spectra of biological cells that can be exploited accordingly (H.P. Schwan, "Electrical Properties of Tissue and Cell Suspension," Advances in Biological and Medical Physics 5 (1957) 147).


Evolution of cell membrane conductivity (analyzed from dielectric spectroscopy data) after exposure to pulsed electric fields of 60 ns, 300 ns or 100 μs duration. The pulse amplitudes was adjusted for different pulse durations to deliver the same energy for each exposure.
Originally we intended to use dielectric spectroscopy as an additional method for the investigation of the effects of pulsed electric fields on cells. However, in the meantime the method became increasingly important in our research by itself. dielectric spectroscopy can be used to determine changes of the cell shape and even functional changes in the status of cells. With the appropriate model (e.g. single shell or double shell model) it is also possible to obtain information on subcellular structures, such as the nucleus.

The main part of our work aimed to understand the effects of pulsed electric fields which are able to initiate apoptosis in cancer cells. For this purpose, methods and models had to be developed for measurements on cells in electrolytes with a higher conductivity.

Measurements of samples under physiological conditions can lead to strong polarization effects at the interface between the electrode and the suspending medium due to charge accumulation. The impact from electrode polarization could be overcome by plating a layer of platinum black to the measuring electrodes To address challenges of data acquisition and analysis of dielectric spectra, different approaches are combined. Further developed models were applied to the measured dielectric parameters enabling statements about cells and their constituents, such as the cytosol or the cell membrane itself. Furthermore, information about the characteristics of the interface layer at the electrode or its coating can be derived.

We have, hence, in the meantime applied dielectric spectroscopy not only to evaluate the effects of pulsed electric fields on cells but also to investigate changes to polymers that have been subjected to submerged plasmas. More recently we have further developed methods and procedures to monitor attachment and growth of cells on metal surfaces. In related studies we describe the cohesion between cells in a monolayer.

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