An electrical cell-substrate impedance sensing (ECIS) system was developed by our collaborator, Giaever and Keese, some 10 years ago (Nature '93) to detect the nanometer order changes in cell-to-cell and cell-to-substrate distances of cultured cells on an electrode. The ECIS system has been applied widely for the analysis of cell responses to chemical (e.g., histamine) and mechanical (e.g., shear stress) stimulation. Although the significance of resistance parameters has been well established, the relationship between electrical capacitance and cell motion has not yet been sufficiently discussed. In this study, we analyzed the relationship between the electrical impedance and cell-to-cell and/or cell-to-substrate distance using a physical model with an insulator board, punched cylindrical holes and electrode. The insulator board and holes are supposed to be the cell itself and cell-to-cell distance, respectively. We investigated the influence of the changes of the cell-to-cell distance (hole size) (A) and cell-to-substrate (insulator-electrode) distance (h) on the electrical impedance. We measured the impedance over the frequency range from 20 Hz to 1 MHz. In practical application of the ECIS, 4 kHz is commonly used to detect the impedance change. Accordingly, we also investigated the impedance changes for various values of A and h at 4 kHz. When A was reduced, serial resistance R
s was remarkably increased, while parallel capacitance C
p was decreased and series capacitance C
s changed only slightly. The vector impedance loci increased greatly when A was reduced from 2.0 mm to 1.0 mm with h of 2.0 mm. On the other hand, when h was reduced, C
p increased, and C
s significantly decreased and parallel resistance R
p, R
s decreased only slightly. Since, resistance increases mainly with a decrease in A, and the capacitance decreases mainly with a decrease in h, it is suitable to evaluate the change in A using R
p, and that in h using C
s. In other words, the polarization impedance of electrode Z
p changes significantly, especially in the low-frequency band, when A or h changes, even if the electrode area is fixed. In conclusion, the ECIS system offers a useful method to separately detect cell-to-cell and cell-to-substrate distances.
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