Measuring the Impedance of Cell Layers (part II)

Equivalent circuits and corresponding mathematical models can be applied in order to extract parameters which mirror the barrier properties of the cell layer under examination. They allow separating impedance contributions stemming from the cells from the rest of the impedance spectrum. Although cell layer are rather complex biological systems, their electronic characteristics integrated over a large collective of cells can in good approximation be modeled by basic elements. The following schematic shows an equivalent circuit which is well suited to model the setup described above.

The two main components directly attributable to the cell layer are the resistance TER and capacitance C_cl. The ohmic resistance TER describes the parallel connection of the paracellular paths, while the capacitance of both the apical and the basolateral membranes is summarized in C_cl. Further possible contributions to the cell layer's total impedance, for instance the ohmic resistance across the cell membranes can well be neglected in first order approximation. Based on these model assumptions the parallel circuit of TER and C_cl is well-suited to describe the integrated cell layer's properties.

The culture medium in the upper and lower compartment is modeled in good approximation by a simple ohmic resistance R_med. The electrodes and in particular the interface between the electrodes' metal and the culture medium exhibits a more complex impedance behavior. The so-called constant phase element (CPE) is an empiric but well-established model based on two parameters A_cpe, n_cpe. The CPE model is suited to mathematically describe the characteristic frequency-dependence of the electrode-medium interface's impedance.

The equivalent circuit and the corresponding mathematical models allow deriving an analytical expression for the total impedance of the system. The resulting function depends on five parameters (TER, C_cl, R_med, A_cpe, n_cpe). On the basis of this parametric function an algorithm can be applied to fit the experimental data. Finally, a set of best fit parameters, including the two cell layer related parameters TER and C_cl is obtained.