Simultaneous optical and impedance analysis of single cells: A comparison of two microfluidic sensors with sheath flow focusing

Systems with disposable microfluidic cartridges are demanded for in vitro point-of-care diagnostics. Here, we developed two microfluidic sensors for flow cytometry cell differentiation based on the combined analysis of electrical impedance and optical properties of microparticles and blood cells. Both microdevices incorporate 1D sheath flow focusing to confine particles in a defined plane. For one sensor, a novel approach based on electroplating of gold was used in combination with lithography in SU-8. Electrodes for differential measurements are integrated into the flow channel perpendicularly to the plane of the substrate. For the manufacturing of the second sensor, we applied double-etched glass structures with integrated sputtered Pt electrode pairs. Stability of hydrodynamic focusing was benchmarked by measuring the coefficients of variations of calibration beads with specified size and fluorescence intensities. Sensitivity of electrical impedance measured in the MHz frequency range allowed detection of beads with a diameter of 1 mu m when using side scatter as trigger signal. Measured particles were moving at a velocity of up to 6 m/s. Differentiation of leukocytes was achieved by simultaneous detection of impedance signals, correlated with intrinsic electrical properties of cells, and fluorescence measurements using standard protocols for immunophenotyping of white blood cells. The resulting separation of cell clusters is similar to that of microdevices with 2D hydrodynamic focusing and routine flow cytometers.