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.

For more publications and details or information check out our publications or send me an email.
Projects

Impedance analysis of the calcification of bone cells
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Master Thesis Research Project, Fabian Klemmstein, June 2020 - January 2021.

The aim of the project is to monitor the differentiation of bone cells and in particular their calcification by an analysis of changes of their impedance. Therefore, MG-63 osteosarcoma cells were seeded on an electrode array and observed for at least 21 days. Changes for the impedance of cell monolayers, with the expression of calcium initiation by a ddifferentiation medium, were especially observed for the real part of the impedance. The increase of this resistance was more pronounced for the cultivation in only a proliferation medium until the an actual differentiation could be observed after about 7 days. Subsequently, distinct difference between both media were observed that could be related to changes in the calcium mass in the cell culture. The results provided an important first insight in the process and the possibility for monitoring by impedance measurments. With the electrode array it will in the future also be possible to apply an electrical stimulus. With a more detailed analysis by equivalent circuit models, we anticipate that we will also be able to distinguish and describe adhesion characteristics. Both objectives are an important goal within the ongoing work for a better understanding of an electrical stimulation of bone cells and their growth on respective implants, as it is aim of the CRC 1270 ELAINE.

Characterization of trabecular bone based on the assessment of impedance spectra
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Doctoral Research Project, Wenzuo Wei, since August 2018.

  Wenzuo Wei  
 
Wenzuo Wei, M.Eng. (Optical Engineering), Univ. of Shanghai for Sci. and Technol., 2018.
 

The aims of the project are aligned with the objectives of the CRC 1270 ELAINE to improve our understanding of electrically active implants. Wenzuo is as a doctoral student in the project A05 - Dielectric characterization of cells, tissues and materials studying different ways for the derivation of bone tissue properties from an analysis of impedance data in a frequency range from DC to several MHz. Commonly, Cole models and equivalent circuits are applied towards such an evaluation. However, Wenzuo found that their sensitivity and also predictive potential for clinical applications is limited. Accordingly, he engaged in more advanced methods of assessment, such as the Linear Discriminant Analysis (LDA). With this statistical approach he was able to unambigiously distinguish very similar regions of trabecular bone from each other. The advantage of the approach for the classification of pathological conditions is obvious. Together with a Cole model he was further able to determine bone volume fractions, which is arguable the most important parameter to assess bone stability. By the combination with Effective Medium Approximations it is possible to determine the contributions of not just bone strucutures but also constituents, e.g. water and fat, to the overall electrical properties. Consequently, bulk but patient-specific permittivities and conductivities can be related to composition and structure of the bone. These parameters are important information for modelling efforts to describe the distribution of electric fields and currents that can be provided by electrical stimuli. Conversely, impedance measurements can be applied to monitor the success of such therpies.

Wenzuo Wei, Fukun Shi, Juergen F. Kolb, "Impedimetric Analysis of Trabecular Bone Based on Cole and Linear Discriminant Analysis," submitted for publication.

Dielectric characterization of cells, tissues and materials: CRC 1270 ELAINE - A05
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Project funding: German Science Foundation (DFG) as individual project within the Collaborative Research Centre 1270 'Elaine'", July 2017 - June 2021.

  CRC 1270 Elaine  
 
Logo of the CRC 1270 Elaine.
 

The overall goal of the CRC 1270 ELAINE is the detailed understanding of the electrical stimulation of tissues for the improvement of therapeutic options. The dielectric properties of cells and tissues determine their response to electrical stimulation. Electrical characteristics will be investigated in project A05 - Dielectric characterization of cells, tissues and materials by dielectric spectroscopy. The initial objectives are the description of trabecular bone, the evaluation of electrode interfaces, including coatings, and the assessment of bone cell growth on surfaces. The goal is to eventually develop dielectric measurements towards a non-invasive method that allows monitoring of the integration of implants with respect to patient-specific differences. Future work also aims to apply methods and procedures towards other tissue types, such as cartilage, and the detailed assessment of stimulation parameters with respect to the development of exposed cells.

Impedimetric analysis of biological cell monolayers before and after exposure to nanosecond pulsed electric fields
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Doctoral Research Project, Fukun Shi, October 2015 - July 2020.
Project funding: Chinese Scholar Council

  Fukun Shi  
 
CSC Scholar Fukun Shi, M.Sc. (Optics), South China Normal Univ., 2015.
 

The aim of the project were the investigation of changes that are induced by nanosecond pulsed electric fields (nsPEFs) on cell monolayers by impedimetric analysis. The study of cell monolayers provides an important step from the understanding of effects on individual cells towards the description of collective mechanisms of cells that are connected and interacting in a tissue. Since the treatment of tumors is the main motiviation for the application of nsPEFs, the distinction of cancer cells from 'normal' cells and respective differences of any effects was the driving objective behind the studies.Models and method for the interpreatein of impedance spectra before and after electrical stimulation, focusing on nsPEFs, were investigated to describe salient features and their development that were observed in dedicated in situ experimental studies. For the first time a non-invasive, real-time and label-free method was established to explore temporal changes and their underlying physical processes of adherent cells for characteristics of cell-cell connections and the extracellular matrix. Therefore, different procedures for a reduction and visualization of impedance data for cell monolayers were developed on the basis of complex nonlinear least squares, multivariate analysis or a deconvolution method. These approaches pave the way for a sensitive distinction and quantification of effects of electrical stimulation on different cell lines. The investigation encourages the further development into a clinical tumor diagnostic, especially for the treatment with nsPEFs but in general alos other methods of electrical stimulation.

Fukun Shi, Juergen F. Kolb, "Fukun Shi, Juergen F. Kolb, "Enhanced resolution impedimetric analysis of cell responses from the distribution of relaxation times," Biosensors and Bioelectronics 157 (2020) 112149. doi:10.1016/j.bios.2020.112149

Fukun Shi, Jie Zhuang, Juergen F. Kolb, "Discrimination of Different Cell Monolayers before and after Exposure to Nanosecond Pulsed Electric Fields based on Cole-Cole and Multivariate Analysis," J. Phys. D: Appl. Phys. 52 (2019) 495401. doi:10.1088/1361-6463/ab40d7

Fukun Shi, Anna Steuer, Jie Zhuang, Juergen F. Kolb, "Bioimpedance Analysis of Epithelial Monolayers after Exposure to Nanosecond Pulsed Electric Fields," IEEE Trans. Biomed. Eng. 66 (2019) 210-221. doi:10.1109/TBME.2018.2882299