Circuit model and electric impedance characterization of cell-based sensors: (a) single-cell impedance measurements and (b) multi-cell impedance measurements. Z1: electrode-SAM-peptide impedance, Z2: counter-electrode-SAM-peptide impedance, Z3: cell membrane impedance (bottom), Z4: cell membrane impedance (top), Zcell: total single-cell impedance (Z3 + Z4), Rb: total resistance between neighboring cells, Zcells: impedance of cells, Rb//Zcells: impedance of monolayer of cells, RJ: cleft/seal resistance, Rsolution: solution resistance, and Rlimit: limiting resistance.