MP08: Dielectric sheath flows in microfluidic impedance cytometers

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Mikael Evander, Burak Dura, Antonio J. Ricco, Gregory T. A. Kovacs, Laurent Giovangrandi

Department of Electrical Engineering, Stanford University, USA

Impedance cytometers are being increasingly used within the biomedical community for counting and measuring the sizes and/or dielectric properties of particles and cells. One of the benefits of miniaturized impedance cytometers is the possibility to use microfluidics to achieve dielectric sheathing of the sample in a simple fashion. We show here that dielectric sheathing can be used to improve the signal amplitude and signal-to-noise ratio (SNR) for impedance detection of single particles and cells. A microfluidic impedance cytometer with sensing electrodes, dielectrophoretic focusing electrodes and fluid inlets to allow for dielectric focusing was used to perform a characterization on how low-conductivity aqueous and two-phase dielectric sheathing affect the detection sensitivity and SNR. De-ionized water or oil was used as dielectric sheath together with a conductive core of phosphate buffered saline. The relative signal (ΔI/I) from single 10 um particles was measured and analyzed for core sizes ranging from 30 to 150 um. A fivefold increase in the relative signal can be seen over the aqueous core widths. Utilizing a two-phase system, another fourfold increase is achieved. The SNR shows no significant change for the different aqueous cores. However, an improvement above 20 dB can be achieved by using the two-phase system.
Dielectric sheathing enables the use of wider channels that are less prone to clogging and easier to manufacture. The precision in cell and particle size measurements can be increased and the limit for the smallest detectable object size lowered. The technique can be taken one step further by the using two-phase dielectric sheathing. As the two-phase flow is not limited by diffusion, it is possible to achieve even higher signal amplitudes and SNR. This will be especially useful when analyzing small particles and cells that normally are difficult to detect.

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