Accepted Article This article is protected by copyright. All rights reserved. Electrophoresis manuscript No. (will be inserted by the editor) A simple electrical approach to monitor dielectrophoretic focusing of particles flowing in a microchannel Riccardo Reale · Adele De Ninno · Luca Businaro · Paolo Bisegna · Federica Caselli * Received: date / Accepted: date Abstract This paper reports an impedance-based sys- tem for the quantitative assessment of dielectrophoretic (DEP) focusing of single particles flowing in a microchan- nel. Particle lateral positions are detected in two elec- trical sensing zones placed before and after a DEP- focusing region, respectively. In each sensing zone, par- ticle lateral positions are estimated using the unbalance between the opposite pulses of a differential current sig- nal obtained with a straightforward coplanar electrode configuration. The system is used to monitor the fo- cusing of polystyrene beads of 7 or 10 µm diameter, under various conditions of DEP field intensities and flow rates which produce different degrees of focusing. This electrical approach represents a simple and valu- able alternative to optical methods for monitoring of particle focusing systems. Keywords Dielectrophoresis · Electrical impedance · Microfluidics · Position detection · Coplanar electrodes 1 Introduction A wide range of microfluidic approaches for particle ma- nipulation has emerged in last decades [1,2]. Among them, dielectrophoresis has been receiving tremendous attention [3–8]. This is primarily due to its label-free na- ture, its favorable scaling effects, the simplicity of the instrumentation and its ability to induce both negative R. Reale · A. De Ninno · P. Bisegna · F. Caselli Department of Civil Engineering and Computer Science, Uni- versity of Rome Tor Vergata, 00133 Rome, Italy E-mail: caselli@ing.uniroma2.it L. Businaro Institute for Photonics and Nanotechnologies, Italian Na- tional Research Council, 00156 Rome, Italy and positive forces [9]. As a method for particle focus- ing, dielectrophoresis does not require additional buffer inlets and precise flow control, and it enables selective position adjustment of particles in real-time [10–12]. Several applications of dielectrophoretic (DEP) par- ticle manipulation have been reported in the literature, involving e.g. sub-micrometre [13] or micrometre [14] particles, plant protoplasts [15], bacteria [16], yeast cells [17,18], parasites [19], blood cells [20], and mammalian cell lines [21]. In order to evaluate and monitor the DEP particle motion, image analysis is typically used (e.g., [22–26]), which however requires optical instrumenta- tion, thus increasing system complexity, and robust par- ticle tracking algorithms. In this work we propose an electrical approach for monitoring of DEP particle focusing in a microchan- nel. The basic principle comes from microfluidic imped- ance cytometry, a label-free technique for the electrical characterization of particles and cells (see e.g. the re- views [27–29]). In a typical impedance chip, suspended particles flow through a microchannel and integrated electrodes are used to measure the variation in channel impedance induced by the passage of a particle. The impedance change is exploited to characterize particle properties. However, identical particles flowing along trajectories with different cross-sectional positions gen- erate electric signals with different amplitudes, due to non-homogeneity of the electric field. While this phe- nomenon, known as positional dependence issue, is a drawback for accurate particle characterization [30–32], its use as a tool to determine cross-sectional positions of particle trajectories in a microchannel has recently been proposed [33]. In this work, we assess the feasi- bility of using a novel electrical metric, introduced in [34] to compensate for the positional dependence is- sue, as a mean to determine particle lateral positions This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/elps.201800423