Mapping alternating current electroosmotic flow at the dielectrophoresis crossover frequency of a colloidal probe

AC electroosmotic (ACEO) flow above the gap between coplanar electrodes is mapped by the measurement of Stokes forces on an optically trapped polystyrene colloidal particle. E²‐dependent forces on the probe particle are selected by amplitude modulation (AM) of the ACEO electric field (E) and lock‐in detection at twice the AM frequency. E²‐dependent DEP of the probe is eliminated by driving the ACEO at the probe's DEP crossover frequency. The location‐independent DEP crossover frequency is determined, in a separate experiment, as the limiting frequency of zero horizontal force as the probe is moved toward the midpoint between the electrodes. The ACEO velocity field, uncoupled from probe DEP effects, was mapped in the region 1–9 μm above a 28 μm gap between the electrodes. By use of variously sized probes, each at its DEP crossover frequency, the frequency dependence of the ACEO flow was determined at a point 3 μm above the electrode gap and 4 μm from an electrode tip. At this location the ACEO flow was maximal at ～117 kHz for a low salt solution. This optical trapping method, by eliminating DEP forces on the probe, provides unambiguous mapping of the ACEO velocity field.     

Three-dimensional focusing of particles using negative dielectrophoretic force in a microfluidic chip with insulating microstructures and dual planar microelectrodes

The focusing of biological and synthetic particles in microfluidic devices is a prerequisite for the construction of microstructured materials, as well as for medical applications. In the present study, a microdevice that can effectively focus particles in three dimensions using a combination of insulator-based and metal-electrode dielectrophoresis (DEP) has been designed and fabricated. The DEP force is employed to confine the particles using a negative DEP response. Four insulating microstructures, which form an X-pattern in the microchannel, were employed to distort the electric field between the insulators in a conducting solution, thereby generating regions with a high electric-field gradient. Two strips of microelectrodes on the top and bottom surfaces were placed in the middle of the microchannel and connected to an electric pole. Two sets of dual-planar electrodes connected to the opposite pole were placed at the sides of the microchannel at the top and bottom surfaces. The results of a transient simulation of tracks of polystyrene particles, which was performed using the commercial software package CFD-ACE? (ESI Group, France), demonstrate that the three-dimensional focusing of particles was achieved when the applied voltage was larger than 35?V at a frequency of 1 MHz. Furthermore, the focusing performance increased with the increased strength of the applied electric field and decreased inlet flow rate. Experiments on particle focusing, employing polystyrene particles 10 μm in diameter, were conducted to demonstrate the feasibility of the proposed design; the results agree with the trend predicted by numerical simulations.     

Development of a new contactless dielectrophoresis system for active particle manipulation using movable liquid electrodes

This study presents a new DEP manipulation technique using a movable liquid electrode, which allows manipulation of particles by actively controlling the locations of electrodes and applying on–off electric input signals. This DEP system consists of mercury as a movable liquid electrode, indium tin oxide (ITO)‐coated glass, SU‐8‐based microchannels for electrode passages, and a PDMS medium chamber. A simple squeezing method was introduced to build a thin PDMS layer at the bottom of the medium chamber to create a contactless DEP system. To determine the operating conditions, the DEP force and the friction force were analytically compared for a single cell. In addition, an appropriate frequency range for effective DEP manipulation was chosen based on an estimation of the Clausius–Mossotti factor and the effective complex permittivity of the yeast cell using the concentric shell model. With this system, we demonstrated the active manipulation of yeast cells, and measured the collection efficiency and the dielectrophoretic velocity of cells for different AC electric field strengths and applied frequencies. The experimental results showed that the maximum collection efficiency reached was approximately 90%, and the dielectrophoretic velocity increased with increasing frequency and attained the maximum value of 10.85 ± 0.95 μm/s at 100 kHz, above which it decreased.     

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 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.     

Dielectrophoretic cell trapping for improved surface plasmon resonance imaging sensing

The performance of conventional surface plasmon resonance (SPR) biosensors can be limited by the diffusion of the target analyte to the sensor surface. This work presents an SPR biosensor that incorporates an active mass‐transport mechanism based on dielectrophoresis and electroosmotic flow to enhance analyte transport to the sensor surface and reduce the time required for detection. Both these phenomena rely on the generation of AC electric fields that can be tailored by shaping the electrodes that also serve as the SPR sensing areas. Numerical simulations of electric field distribution and microparticle trajectories were performed to choose an optimal electrode design. The proposed design improves on previous work combining SPR with DEP by using face‐to‐face electrodes, rather than a planar interdigitated design. Two different top‐bottom electrode designs were experimentally tested to concentrate firstly latex beads and secondly biological cells onto the SPR sensing area. SPR measurements were then performed by varying the target concentrations. The electrohydrodynamic flow enabled efficient concentration of small objects (3 μm beads, yeasts) onto the SPR sensing area, which resulted in an order of magnitude increased SPR response. Negative dielectrophoresis was also used to concentrate HEK293 cells onto the metal electrodes surrounded by insulating areas, where the SPR response was improved by one order of magnitude.     

Dielectrophoretic tweezers as a platform for molecular force spectroscopy in a highly parallel format

We demonstrated the application of a simple electrode geometry for dielectrophoresis (DEP) on colloidal probes as a form of molecular force spectroscopy in a highly parallel format. The electric field between parallel plates is perturbed with dielectric microstructures, generating uniform DEP forces on colloidal probes in the range of several hundred piconewtons across a macroscopic sample area. We determined the approximate crossover frequency between negative and positive DEP using electrodes without dielectric microstructures-a simplification over standard experimental methods involving quadrupoles or optical trapping. 2D and 3D simulations of the electric field distributions validated the experimental behavior of several of our DEP tweezers geometries and provided insight into potential improvements. We applied the DEP tweezers to the stretching of a short DNA oligomer and detected its extension using total-internal reflection fluorescence microscopy. The combination of a simple cell fabrication, a uniform distribution of high axial forces, and a facile optical detection of our DEP tweezers makes this form of molecular force spectroscopy ideal for highly parallel detection of stretching or unbinding kinetics of biomolecules.     

AC dielectrophoretic deformable particle-particle interactions and their relative motions

This paper develops a numerical simulation model to research the deformable particle-particle interactions caused by dielectrophoresis (DEP) under AC electric fields. The DEP force is calculated by using Maxwell stress tensor method, and the hydrodynamic force is obtained by calculating the hydrodynamic stress tensor. Simulation results show that the DEP interactive motion will facilitate the particles forming particle chains that are parallel to the electric field, and the particles with low shear modulus present a lower x-component velocity. Also, the electric field intensity and particles radius have some effects on the DEP motions, and for different particles, smaller particles with larger electric field intensity easily reach a larger velocity. The numerical research may provide universal guidance for biological cells manipulation and assembly.     

Dielectrophoretic assembly of grain-boundary-free 2D colloidal single crystals

Dielectrophoresis (DEP) force-assisted assembly of a colloidal single photonic-crystal monolayer in a microfluidic chamber was demonstrated. Negative DEP force with a high-frequency AC electric field induced the compression of colloidal microspheres to form a colloidal crystal domain at the center of a hexapolar-shaped electrode. While typical assembly by monotonic DEP force forms multicrystalline domains containing crystal defects, repetitions of the DEP/relaxation cycle significantly facilitated crystal growth of 10μm monodispersed polystyrene microspheres, allowing a grain-boundary-free single-crystal monolayer domain of ca. 200μm in size. Microsphere size as well as size distribution affected the formation of the single-crystal domain. A simple method was used to immobilize the single-crystal domain on the glass substrate without losing its crystallinity.     

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.     

Front Cover: Enhancement of dielectrophoresis-based particle collection from high conducting fluids due to partial electrode insulation

Dielectrophoresis (DEP) is a successful method to recover nanoparticles from different types of fluid. The DEP force acting on these particles is created by an electrode microarray that produces a nonuniform electric field. To apply DEP to a highly conducting biological fluid, a protective hydrogel coating over the metal electrodes is required to create a barrier between the electrode and the fluid. This protects the electrodes, reduces the electrolysis of water, and allows the electric field to penetrate into the fluid sample. We observed that the protective hydrogel layer can separate from the electrode and form a closed domed structure and that collection of 100 nm polystyrene beads increased when this occurred. To better understand this collection increase, we used COMSOL Multiphysics software to model the electric field in the presence of the dome filled with different materials ranging from low-conducting gas to high conducting phosphate-buffered saline fluids. The results suggest that as the electrical conductivity of the material inside the dome is reduced, the whole dome acts as an insulator which increases electric field intensity at the electrode edge. This increased intensity widens the high-intensity electric field factor zone resulting in increased collection. This informs how dome formation results in increased particle collection and provides insight into how the electric field can be intensified to the increase collection of particles. These results have important applications for increasing the recovery of biologically-derived nanoparticles from undiluted physiological fluids that have high conductance, including the collection of cancer-derived extracellular vesicles from plasma for liquid biopsy applications.     

Floating electrode dielectrophoresis

In practice, dielectrophoresis (DEP) devices are based on micropatterned electrodes. When subjected to applied voltages, the electrodes generate nonuniform electric fields that are necessary for the DEP manipulation of particles. In this study, electrically floating electrodes are used in DEP devices. It is demonstrated that effective DEP forces can be achieved by using floating electrodes. Additionally, DEP forces generated by floating electrodes are different from DEP forces generated by excited electrodes. The floating electrodes' capabilities are explained theoretically by calculating the electric field gradients and demonstrated experimentally by using test-devices. The test-devices show that floating electrodes can be used to collect erythrocytes (red blood cells). DEP devices which contain many floating electrodes ought to have fewer connections to external signal sources. Therefore, the use of floating electrodes may considerably facilitate the fabrication and operation of DEP devices. It can also reduce device dimensions. However, the key point is that DEP devices can integrate excited electrodes fabricated by microtechnology processes and floating electrodes fabricated by nanotechnology processes. Such integration is expected to promote the use of DEP devices in the manipulation of nanoparticles.     

Gene delivery by electroporation after dielectrophoretic positioning of cells in a non-uniform electric field

We report the use of dielectrophoresis (DEP) to position U-937 monocytes within a non-uniform electric field, prior to electroporation (EP) for gene delivery. DEP positioning and EP pulsing were both accomplished using a common set of inert planar electrodes, micro-fabricated on a glass substrate. A single-shell model of the cell's dielectric properties and finite-element modeling of the electric field distribution permitted us to predict the major features of cell positioning. The extent to which electric pulses increased the permeability of the cell membranes to fluorescent molecules and to pEGFPLuc DNA plasmids were found to depend on prior positioning. For a given set of pulse parameters, EP was either irreversible (resulting in cytolysis), reversible (leading to gene delivery), or not detectable, depending on where cells were positioned. Our results clearly demonstrate that position-dependent EP of cells in a non-uniform electric field can be controlled by DEP.     

Dielectrophoretic manipulation and separation of microparticles using curved microelectrodes

This paper presents the development and experimental analysis of a dielectrophoresis (DEP) system, which is used for the manipulation and separation of microparticles in liquid flow. The system is composed of arrays of microelectrodes integrated to a microchannel. Novel curved microelectrodes are symmetrically placed with respect to the centre of the microchannel with a minimum gap of 40 μm. Computational fluid dynamics method is utilised to characterise the DEP field and predict the dynamics of particles. The performance of the system is assessed with microspheres of 1, 5 and 12 μm diameters. When a high‐frequency potential is applied to microelectrodes a spatially varying electric field is induced in the microchannel, which creates the DEP force. Negative‐DEP behaviour is observed with particles being repelled from the microelectrodes. The particles of different dimensions experience different DEP forces and thus settle to separate equilibrium zones across the microchannel. Experiments demonstrate the capability of the system as a field flow fraction tool for sorting microparticles according to their dimensions and dielectric properties.     

Assembly of long carbon nanotube bridges across transparent electrodes using novel thickness-controlled dielectrophoresis

The assembly of carbon nanotubes (CNTs) across planner electrodes using dielectrophoresis (DEP) is one of the standard methods used to fabricate CNT-based devices such as sensors. The medium drag velocity caused by electrokinetic phenomena such as electrothermal and electroosmotic might drive CNTs away from the deposition area. This problem becomes critical at large-scale electrode structures due to the high attenuation of the DEP force. Herein, we simulated and experimentally validated a novel DEP setup that uses a top glass cover to minimize the medium drag velocity. The simulation results showed that the drag velocity can be reduced by 2–3 orders of magnitude compared with the basic DEP setup. The simulation also showed that the optimum channel height to result in a significant drag velocity reduction was between 100 μm and 240 μm. We experimentally report, for the first time, the assembly and alignment of CNT bridges across indium tin oxide (ITO) electrodes with spacing up to 125 μm. We also derived an equation to optimize the CNT's concentration in suspensions based on the electrode gap width and channel height. The deposition of long CNTs across ITO electrodes has potential use in transparent electronics and microfluidic systems.     

Multistep manipulations of poly(methyl‐methacrylate) submicron particles using dielectrophoresis

A microfluidic chip for multistep manipulations of PMMA submicron particles (PMMA‐SMPs) based on dielectrophoresis (DEP) has been developed that includes four main functions of focusing, guiding, trapping, and releasing the SMPs. The structure of the DEP chip consists of a top electrode made of indium tin oxide, a flow chamber formed by optically clear adhesive tape and bottom electrodes with different patterns for different purposes. The bottom electrodes can be divided into three parts: a fish‐bone‐type electrode array that provides the positive DEP force for focusing the suspended nanoparticles (NPs) near the inlet in the flow chamber; the second is for switching and guiding the focused NPs along the electrode surface to the target area, like a flow passing along a virtual channel; and a trapping electrode in the downstream for trapping and releasing the guided NPs. According to the simulation and experimental results, NPs can be aligned along the electrode of the focusing electrode and guided toward the target electrode by means of a positive DEP force between the top and bottom electrodes, with the effects of Brownian motion and Stokes force. In order to demonstrate the sequence of DEP manipulations, a PMMA‐NP suspension is introduced to the DEP chip; the size of the PMMA‐SMPs is about 300 nm. Furthermore, a LabVIEW program developed for sequence control of the AC signals for the multistep manipulations. Consequently, the DEP chip provides an excellent platform technology for the multistep manipulation of SMPs.     

Embedded passivated-electrode insulator-based dielectrophoresis (EπDEP)

Here, we introduce a new technique called embedded passivated-electrode insulator-based dielectrophoresis (EπDEP) for preconcentration, separation, or enrichment of bioparticles, including living cells. This new method combines traditional electrode-based DEP and insulator-based DEP with the objective of enhancing the electric field strength and capture efficiency within the microfluidic channel while alleviating direct contact between the electrode and the fluid. The EπDEP chip contains embedded electrodes within the microfluidic channel covered by a thin passivation layer of only 4 μm. The channel was designed with two nonaligned vertical columns of insulated microposts (200 μm diameter, 50 μm spacing) located between the electrodes (600 μm wide, 600 μm horizontal spacing) to generate nonuniform electric field lines to concentrate cells while maintaining steady flow in the channel. The performance of the chip was demonstrated using Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacterial pathogens in aqueous media. Trapping efficiencies of 100 % were obtained for both pathogens at an applied AC voltage of 50 V peak-to-peak and flow rates as high as 10 μl/min.     

Short-focus nematic liquid crystal microlens array with a dielectric layer

ABSTRACT

A short-focus microlens array using dielectric layer and inhomogeneous electric field over a homogeneous nematic liquid crystal (LC) layer is proposed. The top substrate has a planar indium tin oxide (ITO) electrode which is coated on the inner surface. The bottom substrate has strip ITO electrodes which are embedded in the dielectric layers. The inhomogeneous electric field generates a required gradient refractive index profile within the LC layer which, in turn, causes the focusing effect. Due to the thinner LC layer (15 μm), the spherical aberration should be negligible. Moreover, the fabrication process of the proposed microlens array can be easily carried out because of the layer-by-layer configuration. The simulation results show that the focal length of the LC microlens can be continuously tuned from infinity to 0.988 mm with the change of applied voltage.     

Dielectrophoretic fluidic cell fractionation system

This paper presents the development and experimental verification of a DEP fluidic system capable of fractionation of intact biological cells in suspension into purer subpopulations. This was accomplished by employing a specially shaped nonuniform electric field, synthesized by microfabricated planar microelectrode arrays, housed on an insulating glass substrate. To improve the efficiency of cell sorting, the microelectrodes are individually biased by a variable frequency alternating current (ac) voltage source, which allows us to exploit both positive and negative dielectrophoresis (DEP) to affect cell separation. Furthermore, through suitable establishment of a cell stream supported by sheath flow, such fractionation is achieved in a continuous fashion. The proposed DEP fluidic fractionation may be configured to operate in three (3) different modes. In this work, however, a detailed account is only presented for one mode of operation. The simulation of the electric field and force profiles, together with the experimental results obtained on model cells (plant protoplasts), confirm our theoretical predictions and furthermore demonstrate improvements in both separation efficiency and throughput over a wide range of frequencies (10 Hz to 5 kHz).     

High-throughput dielectrophoretic manipulation of bioparticles within fluids through biocompatible three-dimensional microelectrode array

Dielectrophoresis (DEP) has been deemed as a potential and ideal solution for bioparticle manipulation. A 3-D carbon micro-electro-mechanical system (MEMS) fabricated from the latest developed carbon-MEMS approach has advantages of offering low-cost, biocompatible and high-throughput DEP manipulation for bioparticles. In this paper, a typical process for fabrication of various 3-D microelectrode configurations was demonstrated; accurate numerical analysis was presented on electric field gradient distribution and DEP force based on various microelectrode array configurations. The effects of electrode edge angle, electrode edge-to-edge spacing and electrode height on the electric field distributions were investigated, and optimal design considerations and rules were concluded through analysis of results. The outcomes demonstrate that the sharp edge electrode is more effective in DEP manipulation and both electrode edge-to-edge spacing and electrode height are critical design parameters for seeking optimal DEP manipulation. The gradient magnitude increases exponentially as the electrode spacing is reduced and the electric field extends significantly as the electrode height increases, both of which contribute to a higher throughput for DEP manipulation. These findings are consistent with experimental observations in the literature and will provide critical guidelines for optimal design of DEP devices with 3-D carbon-MEMS.