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.     

Performance characterization of an insulator-based dielectrophoretic microdevice

Dielectrophoresis (DEP), the motion of particles in nonuniform electric fields, is a nondestructive electrokinetic (EK) transport mechanism can be used to concentrate and separate bioparticles. Traditionally, DEP has been performed employing microelectrodes, an approach that is expensive due to the cost of microelectrode fabrication. An alternative is insulator-based DEP (iDEP), an inexpensive method where nonuniform electric fields are created with arrays of insulating structures. This study presents the effects of operating conditions on the dielectrophoretic behavior of polystyrene microparticles under iDEP. Experiments were performed employing microchannels containing insulating structures that worked as insulators. The parameters varied were pH (8-9) and conductivity (25-100 microS/cm) of the bulk medium, and the magnitude of the applied field (200-850 V/cm). Optimal operating conditions in terms of pH and conductivity were obtained, and the microdevice performance was characterized in terms of concentration factor and minimum electric field required (minimum energy consumption). This is the first report on improving iDEP processes when EOF is present. DEP and EOF have been studied extensively, however, this study integrates the effect of suspending medium characteristics on both EK phenomena. These findings will allow improving the performance of iDEP microdevices achieving the highest concentration fold with the lowest energy consumption.     

Characterization of electrokinetic mobility of microparticles in order to improve dielectrophoretic concentration

Insulator-based dielectrophoresis (iDEP), an efficient technique with great potential for miniaturization, has been successfully applied for the manipulation of a wide variety of bioparticles. When iDEP is applied employing direct current (DC) electric fields, other electrokinetic transport mechanisms are present: electrophoresis and electroosmotic flow. In order to concentrate particles, iDEP has to overcome electrokinetics. This study presents the characterization of electrokinetic flow under the operating conditions employed with iDEP; in order to identify the optimal conditions for particle concentration employing DC-iDEP, microparticle image velocimetry (μPIV) was employed to measure the velocity of 1-μm-diameter inert polystyrene particles suspended inside a microchannel made from glass. Experiments were carried out by varying the properties of the suspending medium (conductivity from 25 to 100 μS/cm and pH from 6 to 9) and the strength of the applied electric field (50–300 V/cm); the velocities values obtained ranged from 100 to 700 μm/s. These showed that higher conductivity and lower pH values for the suspending medium produced the lowest electrokinetic flow, improving iDEP concentration of particles, which decreases voltage requirements. These ideal conditions for iDEP trapping (pH = 6 and σ _m = 100 μS/cm) were tested experimentally and with the aid of mathematical modeling. The μPIV measurements allowed obtaining values for the electrokinetic mobilities of the particles and the zeta potential of the glass surface; these values were used with a mathematical model built with COMSOL Multiphysics software in order to predict the dielectrophoretic and electrokinetic forces exerted on the particles; the modeling results confirmed the μPIV findings. Experiments with iDEP were carried out employing the same microparticles and a glass microchannel that contained an array of cylindrical insulating structures. By applying DC electric fields across the insulating structures array, it was seen that the dielectrophoretic trapping was improved when the electrokinetic force was the lowest; as predicted by μPIV measurements and the mathematical model. The results of this study provide guidelines for the selection of optimal operating conditions for improving insulator-based dielectrophoretic separations and have the potential to be extended to bioparticle applications. Figure Comparison of experimental measurements and mathematical modeling of electrokinetic and dielectrophoretic effects on microparticles

Recent advances in direct current electrokinetic manipulation of particles for microfluidic applications

Microfluidic devices have been extensively used to achieve precise transport and placement of a variety of particles for numerous applications. A range of force fields have thus far been demonstrated to control the motion of particles in microchannels. Among them, electric field‐driven particle manipulation may be the most popular and versatile technique because of its general applicability and adaptability as well as the ease of operation and integration into lab‐on‐a‐chip systems. This article is aimed to review the recent advances in direct current (DC) (and as well DC‐biased alternating current) electrokinetic manipulation of particles for microfluidic applications. The electric voltages are applied through electrodes that are positioned into the distant channel‐end reservoirs for a concurrent transport of the suspending fluid and manipulation of the suspended particles. The focus of this review is upon the cross‐stream nonlinear electrokinetic motions of particles in the linear electroosmotic flow of fluids, which enable the diverse control of particle transport in microchannels via the wall‐induced electrical lift and/or the insulating structure‐induced dielectrophoretic force.     

Simultaneous concentration and separation of microorganisms: insulator-based dielectrophoretic approach

Microanalytical methods offer attractive characteristics for rapid microbial detection and concentration. There is a growing interest in the development of microscale separation techniques. Dielectrophoresis (DEP), a nondestructive electrokinetic transport mechanism, is a technique with great potential for microbe manipulation, since it can achieve concentration and separation in a single step. DEP is the movement of particles due to polarization effects in nonuniform electric fields. The majority of the work on dielectrophoretic manipulation of microbes has employed alternating current fields in arrays of microelectrodes, an approach with some disadvantages. An alternative is to employ insulator-based DEP (iDEP), a dielectrophoretic mode where nonuniform fields are produced by employing arrays of insulating structures. This study presents the concentration and fractionation of a mixture of bacteria and yeast cells employing direct current-iDEP in a microchannel containing an array of cylindrical insulating structures. Negative dielectrophoretic trapping of both types of microorganisms was demonstrated, where yeast cells exhibited a stronger response, opening the possibility for dielectrophoretic differentiation. Simultaneous concentration and fractionation of a mixture of both types of cells was carried out analogous to a chromatographic separation, where a dielectropherogram was obtained in less than 2 min by applying an electric field gradient and achieving concentration factors in the order of 50 and 37 times the inlet concentration for Escherichia coli and Saccharomyces cerevisiae cells, respectively. Encouraging results were also obtained employing a sample of water taken from a pond. The findings demonstrated the great potential of iDEP as a rapid and effective technique for intact microorganism concentration and separation.     

Fabrication and evaluation of a ratchet type dielectrophoretic device for particle analysis

Dielectrophoresis is an electrokinetic phenomenon that utilizes an asymmetric electric field to separate analytes based on differences in their polarizabilities relative to that of the suspending medium. One dielectrophoretic device architecture that offers interesting possibilities for particle transport without the use of external flow is the ratchet geometry. This paper describes the fabrication and evaluation of a novel dielectrophoretic ratchet device using a series of fine particles as test probes. The asymmetrical electric field required to selectively transport target analytes was produced using electroformed electrodes which offer the possibility of reducing convective heating and which can be used to construct a device in which all particles located within the fluidic channel are exposed to the applied field. Initial tests of this device were conducted using magnetite and polystyrene fine particles to demonstrate selective particle collection and a separation based on differences in the electrical properties of the analytes employed.     

Controlled microparticle manipulation employing low frequency alternating electric fields in an array of insulators

Low frequency alternating current insulator-based dielectrophoresis is a novel technique that allows for highly controlled manipulation of particles. By varying the shape of an AC voltage applied across a microchannel containing an array of insulating cylindrical structures it was possible to concentrate and immobilize microparticles in bands; and then, move the bands of particles to a different location. Mathematical modeling was performed to analyze the distribution of the electric field and electric field gradient as function of the shape of the AC applied potential, employing frequencies in the 0.2-1.25 Hz range. Three different signals were tested: sinusoidal, half sinusoidal and sawtooth. Experimental results demonstrated that this novel dielectrophoretic mode allows highly controlled particle manipulation.     

T cell activation on a single-cell level in dielectrophoresis-based microfluidic devices

The gentle and careful in vitro processing of live cells is essential in order to make them available to future therapeutic applications. We present a protocol for the activation of single-T cells based on the contact formation with individual anti-CD3/anti-CD28 presenting microbeads in a lab-on-chip environment. The chips consist of microfluidic channels and microelectrodes for performing dielectrophoretic manipulation employing a.c. electric fields. The dielectrophoretic guiding elements allow the assembly of cell-bead pairs while avoiding ill-defined physical contacts with their environment. After overnight cultivation of the manipulated cells, 77% of the bead-associated T cells expressed the activation marker molecule CD69. Physiological stress on the cells was shown to be mainly due to the single-cell cultivation and not to the manipulation in the chips. The same approach could be useful for the in vitro regulation of stem cell differentiation.     

The electrokinetic properties of latex particles: comparison of electrophoresis and dielectrophoresis

A comprehensive study of the AC and DC electrokinetic properties of submicrometre latex particles as a function of particle size and suspending medium conductivity and viscosity is presented. Electrophoretic mobility and dielectrophoretic cross-over results were measured for particle diameters ranging from 44 to 2000 nm. The zeta potentials of the particles were calculated from the electrophoretic mobility data for different suspending medium conductivities, using various models, with and without the inclusion of surface conduction. The dielectrophoretic data was analysed to derive values for the Stern layer conductance and zeta potentials.     

Parasitic trap cancellation using multiple frequency dielectrophoresis, demonstrated by loading cells into cages

This paper presents a method of using multiple frequencies to counteract electric field distortions that interfere with the dielectrophoretic (DEP) manipulation of particles or cells. To demonstrate the technique, simulations were performed for a scenario in which cells were to be loaded into a cage whose walls created parasitic trapping sites that prevented cells from entering it. By employing negative DEP on one electrode in conjunction with positive DEP on another, these traps could be almost completely cancelled. The model predictions were validated experimentally: multiple frequency DEP was used to load many cells into a cage in a matter of seconds in fluid flows of up to 300 microm s(-1), which could not be done with single frequency DEP. Actively cancelling field distortions permits the presence of features that would otherwise be prohibited near regions of dielectrophoretic manipulation, significantly expanding the environments in which dielectrophoresis can be used.     

Quantification of pH gradients and implications in insulator-based dielectrophoresis of biomolecules

Direct current (DC) insulator-based dielectrophoretic (iDEP) microdevices have the potential to replace traditional alternating current dielectrophoretic devices for many cellular and biomolecular separation applications. The use of large DC fields suggest that electrode reactions and ion transport mechanisms can become important and impact ion distributions in the nanoliters of fluid in iDEP microchannels. This work tracked natural pH gradient formation in a 100?μm wide, 1?cm-long microchannel under applicable iDEP protein manipulation conditions. Using fluorescence microscopy with the pH-sensitive dye FITC Isomer I and the pH-insensitive dye TRITC as a reference, pH was observed to drop drastically in the microchannels within 1?min in a 3000?V/cm electric field; pH drops were observed in the range of 6-10 min within a 100?V/cm electric field and varied based on the buffer conductivity. To address concerns of dye transport impacting intensity data, electrokinetic mobilities of FITC were carefully examined and found to be (i) toward the anode and (ii) 1 to 2 orders of magnitude smaller than H? transport which is responsible for pH drops from the anode toward the cathode. COMSOL simulations of ion transport showed qualitative agreement with experimental results. The results indicate that pH changes are severe enough and rapid enough to influence the net charge of a protein or cause aggregation during iDEP experiments. The results also elucidate reasonable time periods over which the phosphate buffering capacity can counter increases in H? and OH? for unperturbed iDEP manipulations.     

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.     

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.     

A 3-D dielectrophoretic filter chip

The paper presents a 3-D filter chip employing both mechanical and dielectrophoretic (DEP) filtration, and its corresponding microfabrication techniques. The device structure is similar to a classical capacitor: two planar electrodes, made from a stainless steel mesh, and bonded on both sides of a glass frame filled with round silica beads. The solution with the suspension of particles flows through both the mesh-electrodes and silica beads filter. The top stainless steel mesh (with openings of 60 mum and wires of 30 mum-thickness) provides the first stage of filtration based on mechanical trapping. A second level of filtration is based on DEP by using the nonuniformities of the electric field generated in the capacitor due to the nonuniformities of the dielectric medium. The filter can work also with DC and AC electric fields. The device was tested with yeast cells (Saccharomyces cerevisae) and achieved a maximal trapping efficiency of 75% at an applied AC voltage of 200 V and a flow rate of 0.1 mL/min, from an initial concentration of cells of 5 x 10(5) cells/mL. When the applied frequency was varieted in the range between 20 and 200 kHz, a minimal value of capture efficiency (3%) was notticed at 50 kHz, when yeast cells exhibit negative DEP and the cells are repelled in the space between the beads.     

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.     

Insulator-based dielectrophoresis of microorganisms: theoretical and experimental results

Dielectrophoresis (DEP) is the motion of particles due to polarization effects in nonuniform electric fields. DEP has great potential for handling cells and is a non-destructive phenomenon. It has been utilized for different cell analysis, from viability assessments to concentration enrichment and separation. Insulator-based DEP (iDEP) provides an attractive alternative to conventional electrode-based systems; in iDEP, insulating structures are used to generate nonuniform electric fields, resulting in simpler and more robust devices. Despite the rapid development of iDEP microdevices for applications with cells, the fundamentals behind the dielectrophoretic behavior of cells has not been fully elucidated. Understanding the theory behind iDEP is necessary to continue the progress in this field. This work presents the manipulation and separation of bacterial and yeast cells with iDEP. A computational model in COMSOL Multiphysics was employed to predict the effect of direct current-iDEP on cells suspended in a microchannel containing an array of insulating structures. The model allowed predicting particle behavior, pathlines and the regions where dielectrophoretic immobilization should occur. Experimental work was performed at the same operating conditions employed with the model and results were compared, obtaining good agreement. This is the first report on the mathematical modeling of the dielectrophoretic response of yeast and bacterial cells in a DC-iDEP microdevice.     

Measuring internal electric fields in organic light-emitting diodes using electroabsorption spectroscopy

A widely applicable electroabsorption technique to measure internal electric fields in organic light-emitting diodes is presented. The technique exploits the change in the a.c. electroabsorption response in the presence of a d.c. electric field. The electroabsorption signal is modulated at the fundamental frequency of the a.c. test signal, in addition to the usual modulation at the second harmonic frequency, when a d.c. bias is present. In metal/organic film/metal devices employing different metal contacts there is a built-in electric field in the organic film caused by the difference in work function between the two contacts. The electroabsorption response at the fundamental frequency of the applied a.c. bias is measured as a function of an external d.c. bias. The electroabsorption signal is nulled when the applied d.c. bias cancels the built-in electric field established by the different metals. We apply this technique to measure changes in metal–polymer Schottky barrier heights as a function of the contact metal. In metal/multiple organic films/metal structures the electroabsorption signals from the constituent organic films are identified spectroscopically and measured at both the fundamental and second harmonic frequency of the a.c. test signal. The amplitudes of the electroabsorption responses are then used to determine the a.c. and d.c. electric fields present in the organic layers. We apply this technique to determine the d.c. electric field distribution within a multi-layer organic light-emitting diode. These results highlight the general applicability of electroabsorption methods to probe internal electric fields in organic light-emitting diodes. © 1997 John Wiley & Sons, Ltd.     

Insulator-based dielectrophoresis for the selective concentration and separation of live bacteria in water

Insulator-based dielectrophoresis (iDEP) was utilized to separate and concentrate selectively mixtures of two species of live bacteria simultaneously. Four species of bacteria were studied: the Gram-negative Escherichia coli and the Gram-positive Bacillus subtilis, B. cereus, and B. megaterium. Under an applied direct current (DC) electric field all the bacterial species exhibited negative dielectrophoretic behavior. The dielectrophoretic separations were carried out in a glass microchannel containing an array of insulating posts. The insulating posts in the microchannel produced nonuniformities in the electric field applied along the channel. Mixtures of two species of bacteria were introduced into the microchannel and the electric field was applied. The bacterial species exhibited different dielectrophoretic mobilities under the influence of the nonuniform field. From these experiments a trapping order was established with E. coli trapping at the weakest applied electric field, while the Bacillus species were trapped at different characteristic threshold fields. At stronger applied electric fields, the two different species of bacteria in the microchannel were dielectrophoretically trapped into two spatially distinct bands. The results showed that iDEP has the potential to selectively concentrate and separate different species of bacteria.     

Brownian Motion and Large Electric Polarizabilities Facilitate Dielectrophoretic Capture of Sub-200 nm Gold Nanoparticles in Water

Dielectrophoresis can move small particles using the force resulting from their polarization in a divergent electric field. In liquids, it has most often been applied to micrometric objects such as biological cells or latex microspheres. For smaller particles, the dielectrophoretic force becomes very small and the phenomenon is furthermore perturbed by Brownian motion. Whereas dielectrophoresis has been used for assembly of superstructures of nanoparticles and for the detection of proteins and nucleic acids, the mechanisms underlying DEP of such small objects require further study. This work presents measurements of the alternating-current (AC) dielectrophoretic response of gold nanoparticles of less than 200 nm diameter in water. An original dark-field digital video-microscopic method was developed and used in combination with a microfluidic device containing transparent thin-film electrodes. It was found that the dielectrophoretic force is only effective in a small zone very close to the tip of the electrodes, and that Brownian motion actually facilitates transport of particles towards this zone. Moreover, the fact that particles as small as 80 nm are still efficiently captured in our device is not only due to Brownian transport but also to an effective polarizability that is larger than what would be expected on basis of current theory for a sphere in a dielectric medium.     

Effect of adsorbed polymers on electrophoresis of dispersed particles in strong electric fields

In this study, the effect of adsorbed non-ionic polyethylenoxides (PEO) and polyvinil-pyrrolidones (PVP) of different molecular mass on the electrophoresis of polystyrene, graphite and aluminum oxide particles both in weak (5–20 V/cm) and strong (50–400 V/cm) electric fields has been studied. The measurements in strong fields have been performed in short electric pulses using image analyzing equipment in which we have determined the deviation of particles from their normal sedimentation trajectory. We have shown that the adsorption of PEO and PVP strongly decreases the electrophoretic velocity (V_ef) of particles in weak electric fields, and this decrease is bigger the adsorbed amount is higher. This is explained by the shift of the shear plane toward solution due to polymer adsorption. At the same time the polymer adsorption only slightly changes the V_ef of particles in strong fields. It means that the non-linear (“cubic”) electrophoresis is independent of the position of the shear plane, i.e. the zeta-potential value. It proves our idea that the electrophoretic velocity in strong electric fields is determined mainly by the surface conductivity of particles, i.e. it is not a function of the electrokinetic potential but rather the Dukhin number that characterizes the polarization of the electric double layer.