Impedance Measurements of Cells

Aperture impedance measurements of cell volume must take into account the osmolaUty and pH of the medium. A hypotonic medium causes cells to swell a hypertonic medium causes them to shrink. Some manufacturers of aperture impedance counters deHberately provide hypertonic electrolytic media for red blood cell measurements. The shmnken red cells not only become more nearly spherical and thus less affected by orientation, but also less deformable than cells in isotonic media and thus less affected by differences in hemoglobin content. 

Impedance measurements are performed by applying an AC excitation voltage to an unknown system while measuring the current. The ratio of the excitation voltage to the current gives the complex impedance of the system. Impedance measurements of cells provide data on the intrinsic dielectric properties of the cells. 

Impedance Measurements of Cells, Figure 1 (a) Diagram showing the impedance analysis chip and a single cell flowing through the microfluidic... 

Diard J-P. 1998. Impedance measurements of polymer electrol)4e membrane fuel cells running on constant load. J Power Sources 74 244-245. 

Even though the effect of moisture on the anode kinetics is well known, interpretation of experimental results on the effect of moisture can be tricky. As Nakagawa et al. [52] pointed out, the measurement of the total cell impedance under the OCV condition is not convincing since the reduction of polarization could as well be due to the availability of H20 for the cathodic reaction. In addition, the measurement of cell performance under the constant voltage or constant current conditions may also lead to wrong conclusions about the effect of water, because the addition of H20 will... 

On the other hand, electrical impedance measurement of bacterial suspensions (.Listeria innocua) was carried out on a Si-glass chip. This method was used to perform a cell viability test. This is because the metabolic products produced from viable cells modify the ionic strength of a low-conductivity medium, significantly altering its electrical characteristics. Later work also involved the detection of the presence of small numbers of bacterial cells in a Si chip 100 L. innocua cells, 200 L. monocytogenes cells, and 40 E. coli cells [93,885], In another report, impedance spectroscopy has been used for analysis of erythrocytes in a glass-polyimide chip [886]. 

Figure 6.34. A Connections used for the EIS study of each element of a fuel cell stack running on a load. WE working electrode CE counter electrode RE reference electrode. B Two-electrode arrangement used for the measurement of Zw [37], (Reproduced by permission of ECS—The Electrochemical Society, and the authors, from Diard JP, Glandut N, Le-Gorrec B, Montella C. Impedance measurement of each cell of a 10 W PEMFC stack under load.)...

Diard JP, Glandut N, Le-Gorrec B, Montella C (2004) Impedance measurement of each cell of a 10W PEMFC stack under load. J Electrochem Soc 151(12) A2193-7... 

As to the membrane conductivity, only small losses of protonic conductivity, of the order of 5-10% after 4000 h, have been observed in well-humidified cells during PEFC life tests according to measurements of cell impedance at 5 kHz [42]. The deionized water employed in the humidification scheme [42] had very low levels of metal ions (e.g., Fe " / +, Ca + or Mg +). Such multivalent ions could exchange irreversibly with protons in the PFSA membrane, causing a drop in membrane conductivity. Deionizing the water used for PEFC humidification is therefore required, and appropriate plumbing should also be used in the humidification loop to avoid generation of ionic contaminants by corrosion processes. 

Schwan (1957) wrote an introduction to impedance measurement on cell and bacteria suspensions. A Coulter counter is a streaming blood cell counter. Two different electrolyte reservoirs equipped with impedance CC electrodes are connected via a short capillary. The reservoir with the cell suspension is on higher pressure so that the electrolyte with cells stream through the capillary. Measured impedance is dominated by the capillary because of its very small diameter. At passage, the impedance goes through a peak because of the cell membrane. The frequency of the peaks is the number of cells passed per second the impedance waveforms are characteristic for different cell types. The cell suspension must have a concentration so that the probability of two cells in the capillary at the same time is low. The electrolyte concentration must be adapted to the impedance increase caused by each cell. 

Besides ligand-induced signaling, other apphcations of this assay are the measurement of cell proliferation or cell viability (Kho et al., 2015). Chemotaxis can be measured in real time by combining this technique with the chemotaxis assay described in Section 3.2.6. Cells in the upper chamber of a CIM-plate migrate through the porous membrane into the chemoattractant-containing lower chamber and adhere to the microelectrode embedded in the bottom of the plate, leading to an increase in impedance (Iqbal et al., 2013). 

Figure 1 shows one representative time course of the impedance change of cell cultures. It is related to the value immediately after stimulation as a reference which is comparable to the unstimulated status before. The unstimulated culture which received cultivation medium without stimulant shows almost no signal change (-0.2%), whereas the stimulated cell culture has already an impedance increase of 26% after 1 h. This increase occurred in all stimulated cultures and is minimally smaller than stated in former publications with 28% to 55% measured after 1 to 4 h [3]. 

Krizaj, D., Pecar, B., 2012. Analysis of impedance measurements of a suspension of microcapsules using a variable length impedance measurement cell. J. Electr. Bioimp. 3, 42—50. 

Yeon JH, Park JK (2005) Cytotoxicity test based rai electrochemical impedance measurement of HepG2 cultured in microfabricated cell chip. Anal Biochem 341 308-315... 

In summary, impedance measurements of single cells give information on cell size, membrane capacitance, and cytoplasm conductivity. It allows the identification and differentiation of cell subpopulations at high speed. It is noninva-sive and label-free and has a wide range of biotechnological and biomedical applications. 

For the simple geometry shown in Fig. 3.3a, the cell constant is given by the distance between the electrodes divided by the electrode area, ATceii = UA. The cell constant depends on the geometry of the cell, and can be determined from an ac impedance measurement of R using a standard electrolyte solution of known conductivity. 

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