Measuring the Impedance of Cell Layers (part I)

Nowadays a wide variety of well-established in vitro cell models exists. Numerous models are based on cell cultures grown on permeable membranes. The latter are available from different manufacturers as inserts for standard well plates and are routinely used as lab consumables. The inserts mainly differ in their geometric design, the membrane material, as well as the pore density and size. Albeit those technical differences all inserts comprise a porous membrane which separates an upper from a lower medium-filled compartment. These inserts are ideally suited for performing electrical measurements across cell layers cultivated on the permeable membrane.

By placing an electrode on each side of the membrane, i.e. one in the upper compartment and one in the lower, and applying a small AC voltage V_ac the electric impedance of the cell system can be measured.

Such a setup mimics the physiological location of epithelial or endothelial cell layers as interfacial tissue between two fluid compartments. Provided that the permeability of the membrane support is properly selected, the cell layer is the ion current-limiting entity. However, also the cell medium and the interface between the electrodes and the culture medium have to be taken into account. They both contribute to the measured total impedance of the system. In combination with the electric resistance and capacity of the cell layer a non-linear frequency dependence of the total impedance results.