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.     

Dielectrophoretically patterned carbon nanotubes to sort microparticles

This article compares the operation of a dielectrophoretic (DEP) platform before and after pattering carbon nanotubes (CNTs) between its microelectrodes. The diverse performance of the DEP system is assessed by separating 1 and 5 μm polystyrene particles. In the absence of CNTs, both particles can only be trapped by operating the system at low medium conductivities, (<10⁻³ S/m) and frequencies (<75 kHz). Alternatively, applying CNTs to the system, some CNTs coat the surface of particles and increase their overall conductivity and permittivity, whereas the rest of them are patterned between the microelectrodes and induce strong DEP forces at their free ends, which can effectively trap the coated particles. The first development extends the range of medium conductivities and frequencies at which the trapping of both particles is achievable, whereas the second development facilitates the selective deposition of particles along the surface of curved microelectrodes. Setting the medium conductivity to 2×10⁻³ S/m and the frequency to 20 MHz, most of 5 μm particles are trapped at the entry region of the first microelectrode pair, whereas most of 1 μm particles are trapped at the tips, and this distinction facilitates their separation. The trapping of 1 μm particles can be improved by decreasing the frequency to 1.5 MHz. This study demonstrates how the integration of CNTs into microfluidic systems enables them to operate beyond their capabilities.     

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.     

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.     

Dielectrophoretic deformation of breast cancer cells for lab on a chip applications

This paper presents the development and experimental analysis of a curved microelectrode platform for the DEP deformation of breast cancer cells (MDA‐MB‐231). The platform is composed of arrays of curved DEP microelectrodes which are patterned onto a glass slide and samples containing MDA‐MB‐231 cells are pipetted onto the platform's surface. Finite element method is utilised to characterise the electric field gradient and DEP field. The performance of the system is assessed with MDA‐MB‐231 cells in a low conductivity 1% DMEM suspending medium. We applied sinusoidal wave AC potential at peak to peak voltages of 2, 5, and 10 V_pp at both 10 kHz and 50 MHz. We observed cell blebbing and cell shrinkage and analyzed the percentage of shrinkage of the cells. The experiments demonstrated higher percentage of cell shrinkage when cells are exposed to higher frequency and peak to peak voltage electric field.     

Theoretical and experimental analysis of negative dielectrophoresis-induced particle trajectories

Many biomedical analysis applications require trapping and manipulating single cells and cell clusters within microfluidic devices. Dielectrophoresis (DEP) is a label-free technique that can achieve flexible cell trapping, without physical barriers, using electric field gradients created in the device by an electrode microarray. Little is known about how fluid flow forces created by the electrodes, such as thermally driven convection and electroosmosis, affect DEP-based cell capture under high conductance media conditions that simulate physiologically relevant fluids such as blood or plasma. Here, we compare theoretical trajectories of particles under the influence of negative DEP (nDEP) with observed trajectories of real particles in a high conductance buffer. We used 10-µm diameter polystyrene beads as model cells and tracked their trajectories in the DEP microfluidic chip. The theoretical nDEP trajectories were in close agreement with the observed particle behavior. This agreement indicates that the movement of the particles was highly dominated by the DEP force and that contributions from thermal- and electroosmotic-driven flows were negligible under these experimental conditions. The analysis protocol developed here offers a strategy that can be applied to future studies with different applied voltages, frequencies, conductivities, and polarization properties of the targeted particles and surrounding medium. These findings motivate further DEP device development to manipulate particle trajectories for trapping applications.     

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.     

Dielectrophoretic choking phenomenon in a converging‐diverging microchannel for Janus particles

The dielectrophoretic (DEP) choking phenomenon is revisited for Janus particles that are transported electrokinetically through a microchannel constriction by a direct‐current (DC) electric field. The negative DEP force that would block a particle with a diameter significantly smaller than that of the constriction at its inlet is seen to be relaxed by the rotation of the Janus particle in a direction that minimizes the magnitude of the DEP force. This allows the particle to pass through the constriction completely. An arbitrary Lagrangian‐Eulerian (ALE) numerical method is used to solve the nonlinearly coupled electric field, flow field, and moving particle, and the DEP force is calculated by the Maxwell stress tensor (MST) method. The results show how Janus particles with non‐uniform surface potentials overcome the DEP force and present new conditions for the DEP choking by a parametric study. Particle transportation through microchannel constrictions is ubiquitous, and particle surface properties are more likely to be non‐uniform than not in practical applications. This study provides new insights of importance for non‐uniform particles transported electrokinetically in a microdevice.     

Dielectrophoretic trapping of multilayer DNA origami nanostructures and DNA origami‐induced local destruction of silicon dioxide

DNA origami is a widely used method for fabrication of custom‐shaped nanostructures. However, to utilize such structures, one needs to controllably position them on nanoscale. Here we demonstrate how different types of 3D scaffolded multilayer origamis can be accurately anchored to lithographically fabricated nanoelectrodes on a silicon dioxide substrate by DEP. Straight brick‐like origami structures, constructed both in square (SQL) and honeycomb lattices, as well as curved “C”‐shaped and angular “L”‐shaped origamis were trapped with nanoscale precision and single‐structure accuracy. We show that the positioning and immobilization of all these structures can be realized with or without thiol‐linkers. In general, structural deformations of the origami during the DEP trapping are highly dependent on the shape and the construction of the structure. The SQL brick turned out to be the most robust structure under the high DEP forces, and accordingly, its single‐structure trapping yield was also highest. In addition, the electrical conductivity of single immobilized plain brick‐like structures was characterized. The electrical measurements revealed that the conductivity is negligible (insulating behavior). However, we observed that the trapping process of the SQL brick equipped with thiol‐linkers tended to induce an etched “nanocanyon” in the silicon dioxide substrate. The nanocanyon was formed exactly between the electrodes, that is, at the location of the DEP‐trapped origami. The results show that the demonstrated DEP‐trapping technique can be readily exploited in assembling and arranging complex multilayered origami geometries. In addition, DNA origamis could be utilized in DEP‐assisted deformation of the substrates onto which they are attached.     

Dielectrophoresis of Caenorhabditis elegans

We demonstrate for the first time the dielectrophoretic trapping and manipulation of a whole animal, the nematode Caenorhabditis elegans. We studied the effect of the electric field on the nematode as a function of field intensity and frequency. We identified a range of electric field intensities and frequencies that trap worms without apparent adverse effect on their viability. Worms tethered by dielectrophoresis (DEP) exhibit behavioral responses to blue light, indicating that at least some of the nervous system functions are unimpaired by the electrical field. DEP is useful to dynamically tether nematodes, sort nematodes according to size, and separate dead worms from live ones.     

Facile Synthesis and Enhanced Nonlinear Optical Properties of Porphyrin‐Functionalized Multi‐Walled Carbon Nanotubes

Two multi‐walled carbon nanotube (MWCNT)‐based nanohybrids, MWCNT–ZnTPP and MWCNT–TPP (TPP=5‐[4‐{2‐(4‐formylphenoxy)‐ ethyloxy}phenyl]‐10,15,20‐triphenylporphyrin, ZnTPP=5‐[4‐{(4‐formylphenyl)ethynyl}phenyl]‐10,15,20‐triphenylporphinatozinc(II)), were prepared directly from pristine MWCNTs through 1,3‐dipolar cycloaddition reactions. Covalent attachment of the porphyrins to the surfaces of the MWCNTs was confirmed by Fourier transform infrared spectroscopy, ultraviolet/visible absorption, fluorescence, Raman, and X‐ray photoelectron spectroscopy, elemental analysis, transmission electron microscopy, and thermogravimetric analysis. Attachment of the porphyrin moieties to the surface of the MWCNTs significantly improves the solubility and ease of processing of these MWCNT–porphyrin composite materials. Z‐scan studies reveal that these MWCNT–porphyrin nanohybrids exhibit enhanced nonlinear optical properties under both nanosecond and picosecond laser pulses at λ=532 nm in comparison with free MWCNTs and the free porphyrin chromophores, whereas superior optical limiting performance was displayed by MWCNT–porphyrin composite materials rather than MWCNTs/ZnTPP and MWCNTs/TPP blends, which is consistent with a remarkable accumulation effect as a result of the covalent linkage between the porphyrin and the MWCNTs.     

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.     

Poly(methyl methacrylate)/multiwalled carbon nanotube microspheres fabricated via in‐situ dispersion polymerization

Poly(methyl methacrylate)/multiwalled carbon nanotube (PMMA/MWCNT) microspheres were successfully prepared by in situ dispersion polymerization in an alcohol phase in which the acid‐treated MWCNTs were dispersed before polymerization. The PMMA and PMMA/MWCNT microspheres were monodisperse. The diameters of the microspheres decreased from about 11.6–6.0 μm as the MWCNT content was increased from 0 to 0.03 wt %. The morphology of the PMMA/MWCNT microspheres was investigated by scanning electron microscopy, atomic force microscopy, and transmission electron microscopy, and the experimental results showed that the MWCNTs were present both in the interior and on the surface of the microspheres. The synthesized PMMA/MWCNT microspheres were also characterized by electrical resistance measurements to analyze their electrical conductivity. They showed electrorheological (ER) fluid characteristics when they were dispersed in silicone oil. Their ER properties were confirmed by using optical microscopy to examine a suspension of the PMMA/MWCNT microspheres dispersed in insulating silicone oil to which an electric field of 2.5 kV/cm was applied. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 182–189, 2008     

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.     

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.     

Insulator-based dielectrophoretic focusing and trapping of particles in non-Newtonian fluids

Insulator-based dielectrophoretic (iDEP) microdevices have been limited to work with Newtonian fluids. We report an experimental study of the fluid rheological effects on iDEP focusing and trapping of polystyrene particles in polyethylene oxide, xanthan gum, and polyacrylamide solutions through a constricted microchannel. Particle focusing and trapping in the mildly viscoelastic polyethylene oxide solution are slightly weaker than in the Newtonian buffer. They are, however, significantly improved in the strongly viscoelastic and shear thinning polyacrylamide solution. These observed particle focusing behaviors exhibit a similar trend with respect to electric field, consistent with a revised theoretical analysis for iDEP focusing in non-Newtonian fluids. No apparent focusing of particles is achieved in the xanthan gum solution, though the iDEP trapping can take place under a much larger electric field than the other fluids. This is attributed to the strong shear thinning-induced influences on both the electroosmotic flow and electrokinetic/dielectrophoretic motions.     

Dielectrophoretic levitation in the presence of shear flow: implications for colloidal fouling of filtration membranes

The ability of dielectrophoretic (DEP) forces created using a microelectrode array to levitate particles in a colloidal suspension is studied experimentally and theoretically. The experimental system employs microfabricated electrode arrays on a glass substrate to apply repulsive DEP forces on polystyrene latex particles suspended in an aqueous medium. A numerical model based on the convection-diffusion-migration equation is presented to calculate the concentration distribution of colloidal particles in shear flow under the influence of a repulsive DEP force field. The results obtained from the numerical simulations are compared against trajectory analysis results and experimental data. The results indicate that by incorporating ac electric field-induced DEP forces in a shear flow, particle accumulation and deposition on the flow channel surfaces can be significantly reduced or even completely averted. The mathematical model is then used to indicate how the deposition behavior is modified in the presence of a permeable substrate, representative of tangential flow membrane filtration operations. The results indicate that the repulsive dielectrophoretic (DEP) forces imparted to the particles suspended in the feed can be employed to mitigate membrane fouling in a cross-flow filtration process.     

Ultrasonic standing wave manipulation technology integrated into a dielectrophoretic chip

Several cell-based biological applications in microfluidic systems require simultaneous high-throughput and individual handling of cells or other bioparticles. Available chip-based tools for contactless manipulation are designed for either high-precision handling of individual particles, or high-throughput handling of ensembles of particles. In order to simultaneously perform both, we have combined two manipulation technologies based on ultrasonic standing waves (USWs) and dielectrophoresis (DEP) in a microfluidic chip. The principle is based on the competition between long-range ultrasonic forces, short-range dielectrophoretic forces and viscous drag forces from the fluid flow. The ultrasound is coupled into the microchannel resonator by an external transducer with a refractive element placed on top of the chip, thereby allowing transmission light microscopy to continuously monitor the biological process. The DEP manipulation is generated by an electric field between co-planar microelectrodes placed on the bottom surface of the fluid channel. We demonstrate flexible and gentle elementary manipulation functions by the use of USWs and linear or curved DEP deflector elements that can be used in high-throughput biotechnology applications of individual cells.     

Rapid microparticle patterning by enhanced dielectrophoresis effect on a double-layer electrode substrate

We present a feasible dielectrophoresis (DEP) approach for rapid patterning of microparticles on a reusable double-layer electrode substrate in microfluidics. Simulation analysis demonstrated that the DEP force was dramatically enhanced by the induced electric field on top interdigitated electrodes. By adjusting electric field intensity through the bottom electrodes on thin glass substrate (100 μm), polystyrene particles (10 μm) were effectively patterned by top electrodes within several seconds (<5 s). The particle average velocity can reach a maximum value of about 20.0±3.0 μm/s at 1 MHz with the strongest DEP force of 1.68 pN. This approach implements integration of functional electrodes into one substrate and avoids direct electrical connection to biological objects, providing a potential lab-on-chip system for biological applications.     

DC dielectrophoresis separation of marine algae and particles in a microfluidic chip

This paper reports a microfluidic method of continuous separation of marine algae and particles by DC dielectrophoresis. The locally non-uniform electric field is generated by an insulating PDMS triangle hurdle fabricated within a PDMS microchannel. Both the particles and algae are subject to negative DEP forces at the hurdle where the gradient of local electric-field strength is the strongest. The DEP force acting on the particle or the algae depends on particles’ or algae’s volume, shape and dielectric properties. Thus the moving particles and algae will be repelled to different streamlines when passing the hurdle. In this way, combined with the electroosmotic flow, continuous separation of algae of two different sizes, and continuous separation of polystyrene particles and algae with similar volume but different shape were achieved. This first demonstration of DC DEP separation of polystyrene particles and algae with similar sizes illustrates the great influence of dielectric properties on particle separation and potentials for sample pretreatment.