Impedance spectroscopy (IS) represents a powerful label-free method for cell analysis. The technique allows quantitative measurement of the inherent electrical and dielectric properties of cells; such as membrane capacitance, membrane resistance, cytoplasmic conductivity and permittivity. These properties directly reflect the cellâs biological structure and metabolic state. Widespread use of IS has been limited by the technical complexity and time consuming nature of the measurement process. Measurements are typically performed on suspensions of cells, giving a population averaged value for the properties of the cell. This represents a significant drawback making the identification of cell sub-populations and cell sorting impossible. AC electrokinetic techniques such as electrorotation have allowed researchers to measure cell dielectric properties, at the single cell level, however it takes many minutes to measure each cell.
Recently, we have developed a high throughput micro impedance cytometer technology, capable of rapidly measuring the dielectric properties of individual cells in a flow though format similar to that of a traditional flow cytometer (>1000 cells per second). The general principle of the single cell impedance cytometry system is shown in the figure. The impedance of single cells is measured using micro-electrodes fabricated within a micro-channel (typical dimensions are 20Î¼m by 40Î¼m). The system can also measure the optical properties of the cell simultaneously with impedance, allowing independent identification of cell phenotype or physiological state through the use of fluorescent probes (e.g. DNA intercalating dyes, other cell markers).
We have demonstrated that the micro impedance cytometer system can differentiate between the three major human leukocyte sub-populations (monocytes, neutropils, T-lymphocytes), without the need for labelling, based on known differences in their cell membrane dielectric properties.