DC-dielectrophoretic separation of microparticles using an oil droplet obstacle

A new dielectrophoretic particle separation method is demonstrated and examined in the following experimental study. Current electrodeless dielectrophoretic (DEP) separation techniques utilize insulating solid obstacles in a DC or low-frequency AC field, while this novel method employs an oil droplet acting as an insulating hurdle between two electrodes. When particles move in a non-uniform DC field locally formed by the droplet, they are exposed to a negative DEP force linearly dependent on their volume, which allows the particle separation by size. Since the size of the droplet can be dynamically changed, the electric field gradient, and hence DEP force, becomes easily controllable and adjustable to various separation parameters. By adjusting the droplet size, particles of three different diameter sizes, 1 microm, 5.7 microm and 15.7 microm, were successfully separated in a PDMS microfluidic chip, under applied field strength in the range from 80 V cm-1 to 240 V cm-1. A very effective separation was realized at the low field strength, since the electric field gradient was proved to be a more significant parameter for particle discrimination than the applied voltage. By utilizing low strength fields and adaptable field gradient, this method can also be applied to the separation of biological samples that are generally very sensitive to high electric potential.     

Microfluidic system for dielectrophoretic separation based on a trapezoidal electrode array

This paper presents a novel microfluidic device for dielectrophoretic separation based on a trapezoidal electrode array (TEA). In this method, particles with different dielectric properties are separated by the device composed of the TEA for the dielectrophoretic deflection of particles under negative dielectrophoresis (DEP) and poly(dimethylsiloxane)(PDMS) microfluidic channel with a sinuous and expanded region. Polystyrene microparticles are exposed to an electric field generated from the TEA in the microfluidic channel and are dielectrophoretically focused to make all of them line up to one sidewall. When these particles arrive at the region of another TEA for dielectrophoretic separation, they are separated having different positions along the perpendicular direction to the fluid flow due to their different dielectrophoretic velocities. To evaluate the separation process and performance, both the effect of the flow rate on dielectrophoretic focusing and the influence of the number of trapezoidal electrodes on dielectrophoretic separation are investigated. Now that this method utilizes the TEA as a source of negative DEP, non-specific particle adhering to the electrode surface can be prevented; conventional separation approaches depending on the positive DEP force suffer from this problem. In addition, since various particle types are continuously separated, this method can be easily applicable to the separation and analysis of various dielectric particles with high particle recovery and selectivity.     

A continuous DC-insulator dielectrophoretic sorter of microparticles

A lab-on-a-chip device is described for continuous sorting of fluorescent polystyrene microparticles utilizing direct current insulating dielectrophoresis (DC-iDEP) at lower voltages than previously reported. Particles were sorted by combining electrokinetics and dielectrophoresis in a 250 μm wide PDMS microchannel containing a rectangular insulating obstacle and four outlet channels. The DC-iDEP particle flow behaviors were investigated with 3.18, 6.20 and 10 μm fluorescent polystyrene particles which experience negative DEP forces depending on particle size, DC electric field magnitude and medium conductivity. Due to negative DEP effects, particles are deflected into different outlet streams as they pass the region of high electric field density around the obstacle. Particles suspended in dextrose added phosphate buffer saline (PBS) at conductivities ranging from 0.50 to 8.50 mS/cm at pH 7.0 were compared at 6.85 and 17.1 V/cm. Simulations of electrokinetic and dielectrophoretic forces were conducted with COMSOL Multiphysics^® to predict particle pathlines. Experimental and simulation results show the effect of medium and voltage operating conditions on particle sorting. Further, smaller particles experience smaller iDEP forces and are more susceptible to competing nonlinear electrostatic effects, whereas larger particles experience greater iDEP forces and prefer channels 1 and 2. This work demonstrates that 6.20 and 10 μm particles can be independently sorted into specific outlet streams by tuning medium conductivity even at low operating voltages. This work is an essential step forward in employing DC-iDEP for multiparticle sorting in a continuous flow, multiple outlet lab-on-a-chip device.     

Electrodeless direct current dielectrophoresis using reconfigurable field-shaping oil barriers

We demonstrate dielectrophoretic (DEP) potential wells using pairs of insulating oil menisci to shape the DC electric field. These oil menisci are arranged in a configuration similar to the quadrupolar electrodes, typically used in DEP, and are shown to produce similar field gradients. While the one-pair well produces a focusing effect on particles in flow, the two-pair well results in creating spatial traps against crossflows. Uncharged polystyrene particles were used to map the DEP force fields and the experimental observations were compared against the field profiles obtained by numerically solving Maxwell's equations. We demonstrate trapping of a single particle due to negative DEP against a pressure-driven crossflow. This can be easily extended to trap and hold cells and other objects against flow for a longer time. We also show the results of particle trapping experiments performed to observe the effect of adjusting the oil menisci and the gap between two pairs of menisci in a four-menisci configuration on the nature of the DEP well formed at the center. A design parameter, Theta, capturing the dimensions of the DEP energy well, is defined and simulations exploring the effects of different geometric features on Theta are presented.     

Continuous dielectrophoretic separation of particles in a spiral microchannel

Particle separation is a fundamental operation in the areas of biology and physical chemistry. A variety of force fields have been used to separate particles in microfluidic devices, among which electric field may be the most popular one due to its general applicability and adaptability. So far, however, electrophoresis‐based separations have been limited primarily to batchwise processes. Dielectrophoresis (DEP)‐based separations require in‐channel micro‐electrodes or micro‐insulators to produce electric field gradients. This article introduces a novel particle separation technique in DC electrokinetic flow through a planar double‐spiral microchannel. The continuous separation arises from the cross‐stream dielectrophoretic motion of particles induced by the non‐uniform electric field inherent to curved channels. Specifically, particles are focused by DEP to one sidewall of the first spiral, and then dielectrophoretically deflected toward the other sidewall of the second spiral at a particle‐dependent rate, leading to focused particle streams along different flow paths. This DEP‐based particle separation technique is demonstrated in an asymmetric double‐spiral microchannel by continuously separating a mixture of 5/10 μm particles and 3/5 μm particles.     

Dielectrophoresis for manipulation of micro/nano particles in microfluidic systems

Dielectrophoretic (DEP) force is exerted when a neutral particle is polarized in a non-uniform electric field, and depends on the dielectric properties of the particle and the suspending medium. The integration of DEP and microfluidic systems offers numerous applications for the separation, trapping, assembling, transportation, and characterization of micro/nano particles. This article reviews the applications of DEP forces in microfluidic systems. It presents the theory of dielectrophoresis, different configurations, and the applications of such systems for particle manipulation and device fabrication.     

Polarization behavior of polystyrene particles under direct current and low‐frequency (<1 kHz) electric fields in dielectrophoretic systems

The relative polarization behavior of micron and submicron polystyrene particles was investigated under direct current and very low frequency (<1 kHz) alternating current electric fields. Relative polarization of particles with respect to the suspending medium is expressed in terms of the Clausius–Mossotti factor, a parameter of crucial importance in dielectrophoretic‐based operations. Particle relative polarization was studied by employing insulator‐based dielectrophoretic (iDEP) devices. The effects of particle size, medium conductivity, and frequency (10–1000 Hz) of the applied electric potential on particle response were assessed through experiments and mathematical modeling with COMSOL Multiphysics^®. Particles of different sizes (100–1000 nm diameters) were introduced into iDEP devices fabricated from polydimethylsiloxane (PDMS) and their dielectrophoretic responses under direct and alternating current electric fields were recorded and analyzed in the form of images and videos. The results illustrated that particle polarizability and dielectrophoretic response depend greatly on particle size and the frequency of the electric field. Small particles tend to exhibit positive DEP at higher frequencies (200–1000 Hz), while large particles exhibit negative DEP at lower frequencies (20–200 Hz). These differences in relative polarization can be used for the design of iDEP‐based separations and analysis of particle mixtures.     

Continuous separation of microparticles by size with direct current-dielectrophoresis

Direct current-dielectrophoresis (DC-DEP), the induced motion of the dielectric particles in a spatially nonuniform DC electric field, is demonstrated to be a highly efficient method to separate the microparticles by size. The locally nonuniform electric field is generated by an insulating block fabricated inside a polydimethylsiloxane microchannel. The particle experiences a negative DEP (accordingly a repulsive force) at the corners of the block, where the local electric-field strength is the strongest. Thus, the particle deviates from the streamline and the degree of deviation is dependent on the DEP force (i.e., the particle size). Combined with the electrokinetic flow, mixed polystyrene particles of a few micrometers difference in diameter can be continuously separated into distinct reservoirs. For separating target particles of a specific size, it is required to simply adjust the voltage outputs of the electrodes. A numerical model based on the Lagrangian tracking method is developed to simulate the particle motion and the results showed a reasonable agreement with the experimental data.     

Dielectrophoretic investigation of plant virus particles: Cow Pea Mosaic Virus and Tobacco Mosaic Virus

This paper reports experimental results on the dielectrophoretic (DEP) behaviour on two nonenveloped plant viruses of different geometrical shapes, namely Cow Pea Mosaic Virus (CPMV) and Tobacco Mosaic Virus (TMV). The DEP properties of carboxy-modified latex beads of the same size are also reported. The DEP properties of single particles were obtained from measurement of the frequency at which the DEP force on a particle goes to zero (the crossover frequency). The DEP behaviour of particle ensembles was also measured using image processing. The dielectric properties of the particles were evaluated from the DEP data. The surface conductance was found to be 0.3 nS for CPMV, 0.38 nS for TMV, and 0.52 nS for 27 nm diameter carboxy-latex beads. Data analysis has shown that the optimal condition for separation of TMV and CPMV is a low-conductivity suspending medium - below 1 mS/m.     

Dielectrophoretic separation of micron and submicron particles: A review

This paper provides an overview on separation of micron and submicron sized biological (cells, yeast, virus, bacteria, etc.) and nonbiological particles (latex, polystyrene, CNTs, metals, etc.) by dielectrophoresis (DEP), which finds wide applications in the field of medical and environmental science. Mathematical models to predict the electric field, flow profile, and concentration profiles of the particles under the influence of DEP force have also been covered in this review. In addition, advancements made primarily in the last decade, in the area of electrode design (shape and arrangement), new materials for electrode (carbon, silicon, polymers), and geometry of the microdevice, for efficient DEP separation of particles have been highlighted.     

A continuous size-dependent particle separator using a negative dielectrophoretic virtual pillar array

We present a continuous size-dependent particle separator using a negative dielectrophoretic (DEP) virtual pillar array. Two major problems in the previous size-dependent particle separators include the particle clogging in the mechanical sieving structures and the fixed range of separable particle sizes. The present particle separator uses the virtual pillar array generated by negative DEP force instead of the mechanical pillar array, thus eliminating the clogging problems. It is also possible to adjust the size of separable particles since the size of virtual pillars is a function of a particle diameter, applied voltage, flow rate, etc. At an applied voltage of 500 kHz, 10 V(rms) (root mean sqaure voltage) sinusoidal wave and a flow rate of 0.40 microl min(-1), we separate 5.7 +/- 0.28 microm-, 8.0 +/- 0.80 microm-, 10.5 +/- 0.75 microm-, and 11.9 +/- 0.12 microm-diameter polystyrene (PS) beads with a separation purity of 95%, 92%, 50%, and 63%, respectively. The 10.5 microm- and 11.9 microm-diameter PS beads have relatively low separation purity of 50% and 63%. However, at an applied voltage of 8 V(rms), we separate 11.9 microm-diameter PS beads with a separation purity over 99%. At an applied voltage of 500 kHz, 10 V(rms) sinusoidal wave and a flow rate of 0.11 microl min(-1), we separate red blood cells (5.4 +/- 1.3 microm-diameter) and white blood cells (8.1 +/- 1.5 microm-diameter) with a separation purity over 99%. Therefore, the present particle separator achieves clog-free, size-dependent particle separation, which is capable of size tuning of separable particles.     

Dielectrophoretic manipulation of particle mixtures employing asymmetric insulating posts

A novel scheme for particle separation with insulator‐based dielectrophoresis (iDEP) was developed. This technique offers the capability for an inverted order in particle elution, where larger particles leave the system before smaller particles. Asymmetrically shaped insulating posts, coupled with direct current (DC) biased low‐frequency alternating current (AC) electric potentials, were used to successfully separate a mixture of 500 nm and 1 μm polystyrene particles (size difference of 0.5 μm in diameter). In this separation, the 1 μm particles were eluted first, demonstrating the discriminatory potential of this methodology. To extend this technique to biological samples, a mixture containing Saccharomyces cerevisiae cells (6.3 μm) and 2 μm polystyrene particles was also separated, with the cells being eluted first. The asymmetric posts featured a shorter sharp half and a longer blunt half; this produced an asymmetry in the forces exerted on the particles. The negative DC offset produced a net displacement of the smaller particles toward the upstream direction, while the post asymmetry produced a net displacement of the larger particles toward the downstream direction. This new iDEP approach provides a setup where larger particles are quickly concentrated at the outlet of the post array and can be released first when in a mixture with smaller particles. This new scheme offers an extra set of parameters (alternating current amplitude, DC offset, post asymmetry, and shape) that can be manipulated to obtain a desired separation. This asymmetric post iDEP technique has potential for separations where it is important to quickly elute and enrich larger and more fragile cells in biological samples.     

Silicon insulator-based dielectrophoresis devices for minimized heating effects

Concentration of biological specimens that are extremely dilute in a solution is of paramount importance for their detection. Microfluidic chips based on insulator-based DEP (iDEP) have been used to selectively concentrate bacteria and viruses. iDEP biochips are currently fabricated with glass or polymer substrates to allow for high electric fields within the channels. Joule heating is a well-known problem in these substrates and can lead to decreased throughput and even device failure. In this work, we present, for the first time, highly efficient trapping and separation of particles in DC iDEP devices that are fabricated on silicon using a single-etch-step three-dimensional microfabrication process with greatly improved heat dissipation properties. Fabrication in silicon allows for greater heat dissipation for identical geometries and operating conditions. The 3D fabrication allows for higher performance at lower applied potentials. Thermal measurements were performed on both the presented silicon chips and previously published PDMS devices comprised of microposts. Trapping and separation of 1 and 2 μm polystyrene particles was demonstrated. These results demonstrate the feasibility of high-performance silicon iDEP devices for the next generation of sorting and concentration microsystems.     

A review of dielectrophoretic separation and classification of non-biological particles

Dielectrophoresis (DEP) is a selective electrokinetic particle manipulation technology that is applied for almost 100 years and currently finds most applications in biomedical research using microfluidic devices operating at moderate to low throughput. This paper reviews DEP separators capable of high-throughput operation and research addressing separation and analysis of non-biological particle systems. Apart from discussing particle polarization mechanisms, this review summarizes the early applications of DEP for dielectric sorting of minerals and lists contemporary applications in solid/liquid, liquid/liquid, and solid/air separation, for example, DEP filtration or airborne fiber length classification; the review also summarizes developments in DEP fouling suppression, gives a brief overview of electrocoalescence and addresses current problems in high-throughput DEP separation. We aim to provide inspiration for DEP application schemes outside of the biomedical sector, for example, for the recovery of precious metal from scrap or for extraction of metal from low-grade ore.     

Dielectrophoresis force spectroscopy for colloidal clusters

Optical trapping-based force spectroscopy was used to measure the frequency-dependent DEP forces and DEP crossover frequencies of colloidal polymethyl methacrylate spheres and clusters. A single sphere or cluster, held by an optical tweezer, was positioned near the center of a pair of gold-film electrodes where alternating current elecroosmosis flow was negligible. Use of amplitude modulation and phase-sensitive lock-in detection for accurate measurement of the DEP force yielded new insight into dielectric relaxation mechanisms near the crossover frequencies. On one hand, the size dependence of the DEP force near the crossover frequencies indicates that the dominant polarization mechanism is a volume effect. On the other hand, the power-law dependence of the crossover frequency on the particle radius with an exponent of -2 indicates the dielectric relaxation is more likely because of ionic diffusion across the particle surface, suggesting the dominant polarization mechanism may be a surface polarization effect. Better theories are needed to explain the experiment. Nevertheless, the strong size dependence of the crossover frequencies suggests the use of DEP for size sorting of micron-sized particles.     

Sheath-assisted versus sheathless dielectrophoretic particle separation

Lab-on-chip devices are widely being used for binary and ternary cell/particle separation applications. Among the lab-on-chip methods, dielectrophoresis (DEP) is a cost-effective and label-free method, with great capabilities for size-based separation of cells and particles, which is mostly performed in sheath-assisted forms. However, the elimination of the sheath flows offers advantages such as ease of operation and higher sample throughput. In this work, we present a comparison of sheath-assisted and sheathless DEP separation of three sizes of microparticles using tilted electrodes. The sheath-assisted design was capable of separating the 5, 10, and 15 μm particles with a separation efficiency as high as 98.0% for 15 μm particles. By adding a DEP focusing region, a sheathless DEP separator was proposed, which offered higher throughputs (up to 10 times) at the cost of lowering the separation efficiency (a reduction up to 10.3% for 15 μm) compared to the sheath-assisted design. To enhance the separation efficiency, a combination of the DEP focusing accompanied by weak sheath flows from both sides was proposed. This design achieved the highest sample separation yield in the outlets (as high as 98.7% for 15 μm) with a sample throughput of more than 4.2 μL/min. This study provides insights into the choice of an appropriate platform for any application in which the yield, purity, throughput, and portability must be considered.     

Experimental study of dielectrophoresis and liquid dielectrophoresis mechanisms for particle capture in a droplet

This work presents a microfluidic system that can transport, concentrate, and capture particles in a controllable droplet. Dielectrophoresis (DEP), a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field, is used to manipulate particles. Liquid dielectrophoresis (LDEP), a phenomenon in which a liquid moves toward regions of high electric field strength under a non-uniform electric field, is used to manipulate the fluid. In this study, a mechanism of droplet creation presented in a previous work that uses DEP and LDEP is improved. A driving electrode with a DEP gap is used to prevent beads from getting stuck at the interface between air and liquid, which is actuated with an AC signal of 200 V(pp) at a frequency of 100 kHz. DEP theory is used to calculate the DEP force in the liquid, and LDEP theory is used to analyze the influence of the DEP gap. The increment of the actuation voltage due to the electrode with a DEP gap is calculated. A set of microwell electrodes is used to capture a bead using DEP force, which is actuated with an AC signal of 20 V(pp) at a frequency of 5 MHz. A simulation is carried out to investigate the dimensions of the DEP gap and microwell electrodes. Experiments are performed to demonstrate the creation of a 100-nL droplet and the capture of individual 10-μm polystyrene latex beads in the droplet.     

Continuous separation of particles using a microfluidic device equipped with flow rate control valves

We propose herein an improved microfluidic system for continuous and precise particle separation. We have previously proposed a method for particle separation called "pinched flow fractionation." Using the previously reported method, particles can be continuously separated according to differences in their diameters, simply by introducing liquid flows with and without particles into a specific microchannel structure. In this study, we incorporated PDMS membrane microvalves for flow rate control into the microfluidic device to improve the separation accuracy. By adjusting the flow rates distributed to each outlet, target particles could be precisely collected from the desired outlet. We succeeded in separating micron and submicron-size polymer particles. This method can be used widely for continuous and precise separation of various kinds of particles, and can function as an important part of microfluidic systems.     

Particle self-assembly in oil-in-ionic liquid Pickering emulsions

We have studied polydimethylsiloxane (PDMS)-in-1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]) Pickering emulsions stabilized by polystyrene microparticles with different surface chemistry. Surprisingly, in contrast to the consensus originating from oil/water Pickering emulsions in which the solid particles equilibrate at the oil-water droplet interfaces and provide effective stabilization, here the polystyrene microparticles treated with sulfate, aldehyde sulfate, or carboxylate dissociable groups mostly formed monolayer bridges among the oil droplets rather than residing at the oil-ionic liquid interfaces. The bridge formation inhibited individual droplet-droplet coalescence; however, due to low density and large volume (thus the buoyant effect), the aggregated oil droplets actually promoted oil/ionic liquid phase separation and distressed emulsion stability. Systems with binary heterogeneous polystyrene microparticles exhibited similar, even enhanced (in terms of surface chemistry dependence), bridging phenomenon in the PDMS-in-[BMIM][PF(6)] Pickering emulsions.     

Particle trapping using dielectrophoretically patterned carbon nanotubes

This study presents the dielectrophoretic (DEP) assembly of multi‐walled carbon nanotubes (MWCNTs) between curved microelectrodes for the purpose of trapping polystyrene microparticles within a microfluidic system. Under normal conditions, polystyrene particles exhibit negative DEP behaviour and are repelled from microelectrodes. Interestingly, the addition of MWCNTs to the system alters this situation in two ways: first, they coat the surface of particles and change their dielectric properties to exhibit positive DEP behaviour; second, the assembled MWCNTs are highly conductive and after the deposition serve as extensions to the microelectrodes. They establish an array of nanoelectrodes that initiates from the edge of microelectrodes and grow along the electric field lines. These nanoelectrodes can effectively trap the MWCNT‐coated particles, since they cover a large portion of the microchannel bottom surface and also create a much stronger electric field than the primary microelectrodes as confirmed by our numerical simulations. We will show that the presence of MWCNT significantly changes performance of the system, which is investigated by trapping sample polystyrene particles with plain, COOH and goat anti‐mouse IgG surfaces.