Curvature-induced Dielectrophoresis

Xiangchun Xuan
Department of Mechanical Engineering, Clemson University, Clemson, SC 29634, USA

Dielectrophoresis is a powerful tool that has been widely used to manipulate (e.g., focus, trap, concentrate, and sort) particles and cells in microfluidic devices [1,2]. Traditional electrode-based dielectrophoresis (eDEP) arises from the non-uniform high-frequency AC electric field between pairs of electrodes that are fabricated within a microchannel [3,4]. This method requires complicated microchannel fabrication, and is prone to device fouling due to the electrode degradation [5]. Such problems are not encountered in insulator-based dielectrophoresis (iDEP) devices, where both AC and DC electric fields can be applied through the electrodes that are positioned virtually outside a microchannel [6,7]. However, in-channel insulating obstacles such as hurdles, posts, and ridges are required to create the electric field gradients. The locally amplified electric field around these micro-obstacles may cause adverse impacts on both the sample and the device due to potential Joule heating and particle clogging issues. These drawbacks are overcome in a recently developed new method from our group that exploits the curvature of insulating walls to electrically control particle and cell motions in curved microchannels via dielectrophoresis [8-10]. This focus is aimed to introduce the fundamentals of curvature-induced dielectrophoresis (c-iDEP) and review the recent developments of this method in particle and cell manipulations.

Fig. 1: Illustration of the electrokinetic and dielectrophoretic motions a particle experiences in a microchannel turn. Also shown are the electric field lines and contour (the darker the higher).

Fig. 1 shows the electric field lines (short arrows indicate the directions) and contour (the darker the higher) in a constant-width microchannel turn. Due to the variation in path length for electric current, electric field attains the maximum and minimum values near the inner and outer corners, respectively. Therefore, particles are subjected to a dielectrophoretic motion, UDEP, when moving electrokinetically through a channel turn. In DC and low-frequency AC electric fields, UDEP usually points toward the outer corner where the electric field is lower (i.e., negative dielectrophoresis). This cross-stream dielectrophoretic motion competes with the streamwise electrokinetic particle motion, UEK, leading to a particle deflection that varies with the intrinsic particle properties including size, charge and electric conductivity [8-10].

Due to c-iDEP, particles migrate away from the inner corner of a 90° turn, which agrees quantitatively with a full numerical model [11]. Such a cross-stream motion yields a size-dependent focusing of particles to the center of a serpentine microchannel, which has been applied to filter yeast cells from E. coli bacteria [12]. If the applied electric field is sufficiently large, big particles can even bounce between the two sidewalls of a serpentine microchannel, enabling a size-based continuous separation [13]. When particles are re-suspended in a less conductive medium, they experience positive dielectrophoresis and tend to move toward the sidewalls of a serpentine microchannel [14]. As a spiral microchannel keeps turning, particles are pushed toward the outer sidewall with every loop by c-iDEP [15]. This phenomenon has been exploited to implement a continuous size-based particle separation in a double-spiral microchannel, where a mixture of particles are focused to the outer wall of the first spiral and then deflected to size-dependent flow paths in the second spiral [9]. It has also been demonstrated in a similar double-spiral microchannel to realize a binary separation of particles by surface charge as well as a ternary separation of particles by charge and size simultaneously [16].

Compared to the traditional eDEP and iDEP techniques, c-iDEP eliminates the need for in-channel fabrication of microelectrodes or micro-insulators and hence reduces the probability of device fouling. Moreover, the use of a curved microchannel can significantly reduce the footprint of a particle/cell focuser or separator, which should benefit its integration into lab-on-a-chip devices with applications to various areas.

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