Q-method for high-resolution, whole-cell patch-clamp impedance measurements using square wave stimulation

High-resolution measurements of cell impedance provide invaluable information on various cellular processes such as exocytosis, ion channel gating, or fertilization. The best recent techniques, although achieving impedance resolution at theoretical limits, have limited applicability due to their inherent constrains and high complexity. We report here a simple method of high-resolution impedance measurement, dubbed as the Q-method, based on measurement of a charge by integrating the cell current during square wave stimulation and on its decomposition into specific components related to segments of the voltage stimulus. Simple relations were derived allowing very fast and direct estimation of cell impedance parameters. The major advantages of the Q-method are its inherently low sensitivity to low-pass filtering, rejection of periodic interference signals, automatic on-the-fly adjustment of the stimulation frequency for the highest capacitance resolution, and simultaneous high-resolution low-crosstalk monitoring of membrane resistance, series resistance and parasitic capacitance in addition to membrane capacitance. Implementation of the Q-method is straightforward with any patch-clamp setup and any cell type. Theoretical grounds of the Q-method, including its resolution and the noise of individual parameters, are developed and experimentally verified.