Electric Cell-Substrate Impedance Sensing (ECIS™) measures the change in impedance of a small electrode to AC current flow over time. The current flows between a 250 μm diameter electrode and a larger counter electrode, using standard culture medium as the electrolyte.
After inoculating cells into a well, cells anchor and spread on the base of the well and onto the active 250 μm electrode. The insulating plasma membranes of the cells on the electrode constrain the electrode’s electrical current and force the current to flow in regions beneath and between the cells. This convoluted current path causes large changes in the measured impedance. With the confluent cell layer in place, the resistance reaches a plateau. The AC current used to make the impedance measurements has no detectable effects upon the cells. This means that the measurements are non-invasive.
When using the ECIS Z Theta System, impedance can be divided into two sets of data – one due to pure resistance (R) and the other due to capacitance (C). Capacitance measured at high frequencies (> 40 kHz) can be used to ascertain what fraction of the substrate is covered by cells.