Dielectrophoresis

A review is presented of the present status of the theory, the developed technology and the current applications of dielectrophoresis ͑DEP͒. Over the past 10 years around 2000 publications have addressed these three aspects, and current trends suggest that the theory and technology have matured sufficiently for most effort to now be directed towards applying DEP to unmet needs in such areas as biosensors, cell therapeutics, drug discovery, medical diagnostics, microfluidics, nanoassembly, and particle filtration. The dipole approximation to describe the DEP force acting on a particle subjected to a nonuniform electric field has evolved to include multipole contributions, the perturbing effects arising from interactions with other cells and boundary surfaces, and the influence of electrical double-layer polarizations that must be considered for nanoparticles. Theoretical modelling of the electric field gradients generated by different electrode designs has also reached an advanced state. Advances in the technology include the development of sophisticated electrode designs, along with the introduction of new materials ͑e.g., silicone polymers, dry film resist͒ and methods for fabricating the electrodes and microfluidics of DEP devices ͑photo and electron beam lithography, laser ablation, thin film techniques, CMOS technology͒. Around three-quarters of the 300 or so scientific publications now being published each year on DEP are directed towards practical applications, and this is matched with an increasing number of patent applications. A summary of the US patents granted since January 2005 is given, along with an outline of the small number of perceived industrial applications ͑e.g., mineral separation, micropolishing, manipulation and dispensing of fluid droplets, manipulation and assembly of micro components͒. The technology has also advanced sufficiently for DEP to be used as a tool to manipulate nanoparticles ͑e.g., carbon nanotubes, nano wires, gold and metal oxide nanoparticles͒ for the fabrication of devices and sensors. Most efforts are now being directed towards biomedical applications, such as the spatial manipulation and selective separation/enrichment of target cells or bacteria, high-throughput molecular screening, biosensors, immunoassays, and the artificial engineering of three-dimensional cell constructs. DEP is able to manipulate and sort cells without the need for biochemical labels or other bioengineered tags, and without contact to any surfaces. This opens up potentially important applications of DEP as a tool to address an unmet need in stem cell research and therapy.

The movement and behaviour of particles suspended in aqueous solutions subjected to non-uniform ac electric fields is examined. The ac electric fields induce movement of polarizable particles, a phenomenon known as dielectrophoresis. The high strength electric fields that are often used in separation systems can give rise to fluid motion, which in turn results in a viscous drag on the particle. The electric field generates heat, leading to volume forces in the liquid. Gradients in conductivity and permittivity give rise to electrothermal forces and gradients in mass density to buoyancy. In addition, non-uniform ac electric fields produce forces on the induced charges in the diffuse double layer on the electrodes. This causes a steady fluid motion termed ac electro-osmosis. The effects of Brownian motion are also discussed in this context. The orders of magnitude of the various forces experienced by a particle in a model microelectrode system are estimated. The results are discussed in relation to experiments and the relative influence of each type of force is described.

probe is kept at a fixed location while the interference is measured as a function of the position of the optical delay line. In this way, it is even possible to measure the transmission function of a certain stretch of photonic structure by carrying out this procedure on either end of the stretch and comparing the results.

Dielectrophoretic trapping of molecules is typically carried out using metal electrodes to provide high field gradients. In this paper we demonstrate dielectrophoretic trapping using insulating constrictions at far lower frequencies than are feasible with metallic trapping structures because of water electrolysis. We demonstrate that electrodeless dielectrophoresis (EDEP) can be used for concentration and patterning of both single-strand and double-strand DNA. A possible mechanism for DNA polarization in ionic solution is discussed based on the frequency, viscosity, and field dependence of the observed trapping force.

BackgroundThe microfabricated impedance spectroscopy flow cytometer used in this study permits rapid dielectric characterization of a cell population with a simple microfluidic channel. Impedance measurements over a wide frequency range provide information on cell size, membrane capacitance, and cytoplasm conductivity as a function of frequency. The amplitude, opacity, and phase information can be used for discrimination between different cell populations without the use of cell markers.The microfabricated impedance spectroscopy flow cytometer used in this study permits rapid dielectric characterization of a cell population with a simple microfluidic channel. Impedance measurements over a wide frequency range provide information on cell size, membrane capacitance, and cytoplasm conductivity as a function of frequency. The amplitude, opacity, and phase information can be used for discrimination between different cell populations without the use of cell markers.MethodsPolystyrene beads, red blood cells (RBCs), ghosts, and RBCs fixed in glutaraldehyde were passed through a microfabricated flow cytometer and measured individually by using two simultaneously applied discrete frequencies. The cells were characterized at 1,000 per minute in the frequency range of 350 kHz to 20 MHz.Polystyrene beads, red blood cells (RBCs), ghosts, and RBCs fixed in glutaraldehyde were passed through a microfabricated flow cytometer and measured individually by using two simultaneously applied discrete frequencies. The cells were characterized at 1,000 per minute in the frequency range of 350 kHz to 20 MHz.ResultsCell size was easily measured with submicron accuracy. Polystyrene beads and RBCs were differentiated using opacity. RBCs and ghosts were differentiated using phase information, whereas RBCs and fixed RBCs were differentiated using opacity. RBCs fixed using increasing concentrations of glutaraldehyde showed increasing opacity. This increased opacity was linked to decreased cytoplasm conductivity and decreased membrane capacitance, both resulting from protein cross-linking.Cell size was easily measured with submicron accuracy. Polystyrene beads and RBCs were differentiated using opacity. RBCs and ghosts were differentiated using phase information, whereas RBCs and fixed RBCs were differentiated using opacity. RBCs fixed using increasing concentrations of glutaraldehyde showed increasing opacity. This increased opacity was linked to decreased cytoplasm conductivity and decreased membrane capacitance, both resulting from protein cross-linking.ConclusionsThis work presents label-free differentiation of cells in an on-chip flow cytometer based on impedance spectroscopy, which will be a powerful tool for cell characterization. © 2005 Wiley-Liss, Inc.This work presents label-free differentiation of cells in an on-chip flow cytometer based on impedance spectroscopy, which will be a powerful tool for cell characterization. © 2005 Wiley-Liss, Inc.

We describe an extremely simple method by which one can obtain thin, strongly adherent films of single-walled nanotube (SWNT) bundles on flexible, transparent plastic substrates such as poly(ethylene terephthalate) (PET: "overhead transparency"). These films show low sheet resistance, >80% optical transparency and robust flexibility, e.g., they can be bent and folded to a crease while retaining full electrical connection across the crease. We believe this could open new opportunities in the design and development of next-generation transparent optoelectronic devices.

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.

This paper reviews the current understanding of electrocoalescence of water droplets in oil, highlighting particularly the mechanisms proposed for droplet–droplet and droplet–interface coalescence under the influence of an applied electric field, as well as various factors influencing the ...

Miniaturization to the micrometer and nanometer scale opens up the possibility to probe biology on a length scale where fundamental biological processes take place, such as the epigenetic and genetic control of single cells. To study single cells the necessary devices need to be integrated on a single chip; and, to access the relevant length scales, the devices need to be designed with feature sizes of a few nanometers up to several micrometers. We will give a few examples from the literature and from our own research in the field of miniaturized chip-based devices for DNA analysis, including dielectrophoresis for purification of DNA, artificial gel structures for rapid DNA separation, and nanofluidic channels for direct visualization of single DNA molecules. The length scales of the fundamental building blocks of biology overlap with the length scales realizable using micro-and nanofabrication technology from the microelectronics industry. The persistence length (L p ) of DNA is given for standard physiological conditions. The gene size corresponds to the typical size of 1 kbp excluding any non-coding regions (introns). The chromosome size range corresponds to metaphase condensed chromosomes. The radius of gyration is indicated for DNA molecules ranging in size from 100 kbp to 100 Mbp C.

We present the principle of cell characterization and separation by dielectrophoretic field-flow fractionation and show preliminary experimental results. The operational device takes the form of a thin chamber in which the bottom wall supports an array of microelectrodes. By applying appropriate AC voltage signals to these electrodes, dielectrophoretic forces are generated to levitate cells suspended in the chamber and to affect their equilibrium heights. A laminar flow profile is established in the chamber so that fluid flows faster with increasing distance from the chamber walls. A cell carried in the flow stream will attain an equilibrium height, and a corresponding velocity, based on the balance of dielectrophoretic, gravitational, and hydrodynamic lift forces it experiences. We describe a theoretical model for this system and show that the cell velocity is a function of the mean fluid velocity, the voltage and frequency of the signals applied to the electrodes, and, most significantly, the cell dielectric properties. The validity of the model is demonstrated with human leukemia (HL-60) cells subjected to a parallel electrode array, and application of the device to separating HL-60 cells from peripheral blood mononuclear cells is shown.

The phenomena of dielectrophoresis and electrorotation, collectively referred to as AC electrokinetics, have been used for many years to study, manipulate and separate particles on the nanotechnology, that is the precise manipulation of particles on the nanometre scale. In this paper we present the principles of AC electrokinetics for particle manipulation, review the current state of AC Electrokinetic techniques for the manipulation of particles on the nanometre scale, and consider how these principles may be applied to nanotechnology.

Dielectrophoresis, the movement of particles in non-uniform AC electric fields, was used to rapidly separate viable and non-viable yeast cells with good efficiency. Known mixtures of viable and heat-treated cells of Saccharomyces cerevisiae were separated and selectively isolated using positive and negative dielectrophoretic forces generated by microelectrodes in a small chamber. Good correlations with the initial known relative compositions were obtained by direct microscopic counting of cells at the electrodes after initial dielectrophoretic separation (r = 0.995), from methylene blue staining (r = 0.992) and by optical absorption measurements (r = 0.980) of the effluent after selectively flushing out the viable and non-viable cells from the chamber. Through measurement of cell viability by staining with methylene blue and plate counts, for an initial suspension of approx. 1.4 x 10 7 cells per ml containing 60% non-viable cells, the dielectrophoretically separated non-viable fraction contained 3% viable cells and the viable fraction 8% dead cells. The separation efficiency is increased by dilution of the initial suspension or by repeat operation(s). Cell viability was not affected by the separation procedure.

Colloidal particles suspended in water respond to direct (DC) or alternating current (AC) fields in a variety of ways, including directional motion along or across the field direction, field-gradient dependent response and induced particle-particle interaction. We review here some of these effects and their applications in new techniques for particle manipulation and assembly, making of novel biomaterials and designing of new self-propelling microdevices. The coupling of the counterionic layer mobility, fluid flows and the resulting particle motion are the basis not only of the classic electrophoretic effects, but also of the recent developments in AC electrohydrodynamics and induced charge electrophoresis of asymmetric particles. We also discuss how dielectrophoresis (particle interaction with external AC field gradients), could be used to manipulate and assemble objects on any size scale. We discuss the interactions leading to the assembly of such structures, ways to simulate the dynamics of the process and the effect of particle size and conductivity on the type of structure obtained. Finally, we demonstrate how an additional level of complexity can be engineered to turn miniature semiconductor diodes into prototypes of self-propelling micromachines and micropumps. The diodes suspended in water propel themselves electro-osmotically when a uniform alternating electric field is applied across the container. Semiconductor diodes embedded in channel walls could serve as distributed microfluidic pumps and mixers powered by a global external field.

DC-Dielectrophoresis (DC-DEP), the induced motion of the dielectric particles in a spatially non-uniform DC electric field, is applied to separate biological cells by size. The locally non-uniform electric field is generated by an insulating hurdle fabricated within a PDMS microchannel. The cells experience a negative DEP (accordingly a repulsive) force at the corners of the hurdle where the gradient of local electric-field strength is the strongest. The DC-DEP force acting on the cells is proportional to the cells’ size. Thus the moving cells deviate from the streamlines and the degree of deviation is dependent on the cell size. In this paper, we demonstrated by using this method that, combined with the electroosmotic flow, mixed biological cells of a few to tens of micrometers difference in diameter can be continuously separated into different collecting wells. For separating target cells of a specific size, all that is required is to adjust the voltage outputs of the electrodes.

This paper provides an overview of the electrokinetic phenomena associated with particles and cells in microchannel systems. The most important phenomena covered include electrophoresis, dielectrophoresis, and induced-charge electrokinetics. The latest development of these electrokinetic techniques for particle or cell manipulations in microfluidic systems is reviewed, in terms of the basic theories, mathematical models, numerical and experimental methods, and the key results/findings from the published literatures in the most recent decades. Some of the limitations associated with the negative field effects are discussed and the perspectives for the future investigations are summarized.

In alternating current electrokinetics, electric fields are used to generate forces on particles. Techniques have been applied for the manipulation of particles and the measurement of their dielectric properties. The fields are typically generated by microelectrode structures fabricated on planar surfaces. One particular design, using interdigitated bar electrodes, is used both in dielectrophoretic field flow fractionation and travelling wave dielectrophoresis. This paper presents a Fourier series analysis of the dielectrophoretic force on a particle generated by this type of electrode array, for both dielectrophoresis and travelling wave dielectrophoresis. Simple expressions are derived for the force at a distance of the order of the electrode spacing from the electrodes. A full analytical expression is given for the dielectrophoretic force in two dimensions. Comparisons are made with previously published experimental observations.

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.

Using gold electrodes lithographically fabricated onto microscope cover slips, DNA and proteins are interrogated both optically (through fluorescence) and electronically (through conductance measurements). Dielectrophoresis is used to position the DNA and proteins at well-defined positions on a chip. Quadrupole electrode geometries are investigated with gaps ranging from 3 to 100 m; field strengths are typically 10 6 V/m. Twenty nanometer latex beads are also manipulated. The electrical resistance of the electronically manipulated DNA and proteins is measured to be larger than 40 M under the experimental conditions used. The technique of simultaneously measuring resistance while using dielectrophoresis to trap nanoscale objects should find broad applicability.

Dielectrophoresis (DEP), the motion of a particle caused by an applied electric field gradient, can concentrate microorganisms non-destructively. In insulator-based dielectrophoresis (iDEP) insulating microstructures produce non-uniform electric fields to drive DEP in microsystems. This article describes the performance of an iDEP device in removing and concentrating bacterial cells, spores and viruses while operated with a DC applied electric field and pressure gradient. Such a device can selectively trap particles when dielectrophoresis overcomes electrokinesis or advection. The dielectrophoretic trapping behavior of labeled microorganisms in a glass-etched iDEP device was observed over a wide range of DC applied electric fields. When fields higher than a particle-specific threshold are applied, particles are reversibly trapped in the device. Experiments with Bacillus subtilis spores and the Tobacco Mosaic Virus (TMV) exhibited higher trapping thresholds than those of bacterial cells. The iDEP device was characterized in terms of concentration factor and removal efficiency. Under the experimental conditions used in this study with an initial dilution of 1 Â105 cells/ml, concentration factors of the order of 3000Â and removal efficiencies approaching 100% were observed with Escherichia coli cells. These results are the first characterization of an iDEP device for the concentration and removal of microbes in water. D

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.

the lateral motion induced on particles by non-uniform electric fields, is a sensitive function of the electrical conductivity of the particle suspending medium. This dependence is exploited in a new technique for separating bioparticles from suspended mixtures. The bioparticles are first immobilised by positive dielectrophoresis at electrodes in a separation chamber, and the conductivity of the liquid flowing through the chamber is then gradually and continuously increased so as to produce a conductivity gradient with time. The bioparticles are released from the electrodes according to their own dielectric properties and as a function of flow rate and medium conductivity. This is demonstrated for pure suspensions and mixtures of the bacteria Bacillus subtilis, Escherichia coli and Micrococcus luteus.

We report ultra-high density assembly of aligned single walled carbon nanotubes (SWNTs) two dimensional arrays via ac dielectrophoresis using high quality surfactant free and stable SWNT solutions. After optimization of frequency and trapping time, we can reproducibly control the linear density of the SWNT between prefabricated electrodes from 0.5 SWNT/µm to more than 30 SWNT /µm by tuning the concentration of the nanotubes in the solution. Our maximum density of 30 SWNT/µm is the highest for aligned arrays via any solution processing technique reported so far. Further increase of SWNT concentration results dense array with multiple layers. We discuss how the orientation and density of the nanotubes vary with concentrations and channel lengths. Electrical measurement data show that the densely packed aligned arrays have low sheet resistances. Selective removal of metallic SWNTs via controlled electrical breakdown produced field effect transistors (FET) with high current onoff ratio. Ultra-high density alignment reported here will have important implications in fabricating high quality devices for digital and analog electronics.

Dielectrophoresis (DEP) is a nondestructive electrokinetic mechanism with great potential for the manipulation of bioparticles. DEP is the movement of particles induced by polarization effects in nonuniform electric fields. Since the 1960s, this technique has been successfully used for the manipulation of microbioparticles, such as microorganisms. Moreover, due to the advances in microfabrication techniques, that allowed progressively smaller microstructures to be constructed, DEP can now be used for the manipulation of nanobioparticles. The first research studies on the DEP of nanobioparticles started in the 1990s. Since then, many research groups have carried out outstanding work with DEP of nanobioparticles such as macromolecules, virus, and spores. However, the need of a critical report that integrates these findings is evident. The aim of the present review is to depict the state-of-the-art on the use of DEP for the separation of nanobioparticles and the potential trends of novel applications of this technique. This review compiles and analyzes the significant findings obtained by many researchers. This publication is intended to provide the reader with state-of-the-art information on many research studies focused on DEP to handle nanobioparticles.

Droplet-based programmable processors promise to offer solutions to a wide range of applications in which chemical and biological analysis and/or small-scale synthesis are required, suggesting they will become the microfluidic equivalents of microprocessors by offering off-the-shelf solutions for almost any fluid based analysis or small scale synthesis problem. A general purpose droplet processor should be able to manipulate droplets of different compositions (including those that are electrically conductive or insulating and those of polar or non-polar nature), to control reagent titrations accurately, and to remain free of contamination and carry over on its reaction surfaces. In this article we discuss the application of dielectrophoresis to droplet based processors and demonstrate that it can provide the means for accurately titrating, moving and mixing polar or non-polar droplets whether they are electrically conductive or not. DEP does not require contact with control surfaces and several strategies for minimizing surface contact are presented. As an example of a DEP actuated general purpose droplet processor, we show an embodiment based on a scaleable CMOS architecture that uses DEP manipulation on a 32 3 32 electrode array having built-in control and switching circuitry. Lastly, we demonstrate the concept of a general-purpose programming environment that facilitates droplet software development for any type of droplet processor. † Lab on a Chip special issue: The Science and Application of Droplets in Microfluidic Devices.

The dielectrophoresis (DEP) phenomenon is used to separate platelets directly from diluted whole blood in microfluidic channels. By exploiting the fact that platelets are the smallest cell type in blood, we utilize the DEP-activated cell sorter (DACS) device to perform sizebased fractionation of blood samples and continuously enrich the platelets in a label-free manner. Cytometry analysis revealed that a single pass through the two-stage DACS device yields a high purity of platelets (,95%) at a throughput of ,2.2610 4 cells/second/microchannel with minimal platelet activation. This work demonstrates gentle and label-free dielectrophoretic separation of delicate cells from complex samples and such a separation approach may open a path toward continuous screening of blood products by integrated microfluidic devices.

Under suitable conditions, a DNA molecule in solution will develop a strong electric dipole moment. This induced dipole allows the molecule to be manipulated with field gradients, in a phenomenon known as dielectrophoresis (DEP). Pure dielectrophoretic motion of DNA requires alternate current (AC) electric fields to suppress the electrophoretic effect of the molecules net charge. In this paper, we present two methods for measuring the efficiency of DEP for trapping DNA molecules as well as a set of quantitative measurements of the effects of strand length, buffer composition, and frequency of the applied electric field. A simple configuration of electrodes in combination with a microfluidic flow chamber is shown to increase the concentration of DNA in solution by at least 60-fold. These results should prove useful in designing practical microfluidic devices employing this phenomenon either for separation or concentration of DNA.

Dielectrophoresis and electrorotation are commonly used to measure dielectric properties and membrane electrical parameters of biological cells. We have derived quantitative relationships for several critical points, defined in Fig. A 1, which characterize the dielectrophoretic spectrum and the electrorotational spectrum of a cell, based on the single-shell model (Pauly,

This article reviews progress in the application of electrophoretic techniques for the separation of nanoparticles. Numerous types of nanoparticles have recently been synthesised and integrated into different products and procedures. Consequently, analytical methods for the efficient characterisation of nanoparticles are now required. Several studies have revealed that gel electrophoresis can readily be used for separating nanoparticles according to their size or shape. However, many other studies focused on separation of nanoparticles by CE. In some cases nanoparticles could be separated by CZE, simply using pure buffer as the BGE. In other studies, buffer additives (most often SDS) were used, enabling fast separations of metallic nanoparticles by size. Other CE methods also allowed for separation of nanoparticle conjugates with biomolecules. Dielectrophoresis is yet another electrophoretic technique useful in separation and characterisation of nanoparticles; particularly nanotubes...

The unique electronic and structural properties of metallic and semiconducting single-walled carbon nanotubes (SWNTs) have led to the vision of carbon-nanotube-based transistors or interconnects for future nanoscale electronics. Such potential applications are, however, challenged by the inability to selectively produce metallic or semiconducting SWNTs instead of a mixture. Over the last two years, progress has been made by exploring various concepts for the separation of metallic from semiconducting SWNTs. [2] So far, chemical routes, which could easily be scaled up, do not provide full spatial separation, but rather, selective enrichment. On the other hand, dielectrophoresis (DEP), a physical process, has been shown to separate metallic from semiconducting tubes with high selectivity but low yield. Here, we show that DEP is developing towards a bulk-separation method, allowing, for the first time, the production of thin films of only metallic SWNTs and the measurement of their corresponding optical spectra.

The study of the dielectric properties of micrometer-or nanometer-scale particles is of particular interest in present-day applications of biomedical engineering. Electrokinetics utilises electrically energised microelectrode structures within microfluidic chambers to noninvasively probe the physiological structure of live cancer cells.

In this study, we demonstrated a micro-fluidic system with multiple functions, including concentration of bacteria using dielectrophoresis (DEP) and selective capture using antibody recognition, resulting in a high capture efficiency of bacterial cells. The device consisted of an array of oxide covered interdigitated electrodes on a flat silicon substrate and a y16 mm high and y260 mm wide micro-channel within a PDMS cover. For selective capture of Listeria monocytogenes from the samples, the channel surface was functionalized with a biotinylated BSAstreptavidin-biotinylated monoclonal antibody sandwich structure. Positive DEP (at 20 V pp and 1 MHz) was used to concentrate bacterial cells from the fluid flow. DEP could collect y90% of the cells in a continuous flow at a flow rate of 0.2 ml min 21 into the micro-channel with concentration factors between 10 2 -10 3 , in sample volumes of 5-20 ml. A high flow rate of 0.6 ml min 21 reduced the DEP capture efficiency to y65%. Positive DEP attracts cells to the edges of the electrodes where the field gradient is the highest. Cells concentrated by DEP were captured by the antibodies immobilized on the channel surface with efficiencies of 18 to 27% with bacterial cell numbers ranging from 10 1 to 10 3 cells. It was found that DEP operation in our experiments did not cause any irreversible damage to bacterial cells in terms of cell viability. In addition, increased antigen expression (antigens to C11E9 monoclonal antibody) on cell membranes was observed following the exposure to DEP.

In this study we report on an experimental method based on dielectrophoretic analysis to identify changes in four Escherichia coli isogenic strains that differed exclusively in one mutant allele. The dielectrophoretic properties of wild-type cells were compared to those of hns, hha, and hha hns mutant derivatives. The hns and hha genes code respectively for the global regulators Hha and H-NS. The Hha and H-NS proteins modulate gene expression in Escherichia coli and other Gram negative bacteria. Mutations in either hha or hns genes result in a pleiotropic phenotype. A two-shell prolate ellipsoidal model has been used to fit the experimental data, obtained from dielectrophoresis measurements, and to study the differences in the dielectric properties of the bacterial strains. The experimental results show that the mutant genotype can be predicted from the dielectrophoretic analysis of the corresponding cultures, opening the way to the development of microdevices for specific identification. Therefore, this study shows that dielectrophoresis can be a valuable tool to study bacterial populations which, although apparently homogeneous, may present phenotypic variability.

The dielectric properties of normal erythrocytes were compared to those of cells infected with the malarial parasite Plasmodium falciparum. Normal cells provided stable electrorotation spectra which, when analyzed by a single-shelled oblate spheroid dielectric model, gave a specific capacitance value of 12 " 1.2 mFrm 2 for the plasma membrane, a cytoplasmic permittivity of 57 " 5.4 and a cytoplasmic conductivity of 0.52 " 0.05 Srm. By contrast, parasitized cells exhibited electrorotation spectra with a time-dependency that suggested significant net ion outflux via the plasma membrane and it was not possible to derive reliable cell parameter values in this case. To overcome this problem, cell membrane dielectric properties were instead determined from dielectrophoretic crossover frequency measurements made as a function of the cell suspending medium conductivity. The crossover frequency for normal cells depended linearly on the suspension conductivity above 20 mSrm and analysis according to the single-shelled oblate spheroid dielectric model yielded values of 11.8 mFrm 2 and 271 Srm 2 , respectively, for the specific capacitance and conductance of the plasma membrane. Unexpectedly, the crossover frequency characteristics of parasitized cells at high suspending medium conductivities were non-linear. This effect was analyzed in terms of possible dependencies of the cell membrane capacitance, conductance or shape on the suspension medium conductivity, and we concluded that variations in the membrane conductance were most likely responsible for the observed non-linearity. According to this model, parasitized cells had a specific membrane capacitance of 9 " 2 mFrm 2 and a specific membrane conductance of 1130 Srm 2 that increased with increasing cell suspending medium conductivity. Such conductivity changes in parasitized cells are discussed in terms of previously observed parasite-associated membrane pores. Finally, we conclude that the large differences between the dielectrophoretic crossover characteristics of normal and parasitized cells should allow straightforward sorting of these cell types by dielectrophoretic methods.

We elucidate on the low mobility and charge traps of the chemically reduced graphene oxide (RGO) sheets by measuring and analyzing temperature dependent current-voltage characteristics. The RGO sheets were assembled between source and drain electrodes via dielectrophoresis. At low bias voltage the conduction is Ohmic while at high bias voltage and low temperatures the conduction becomes space charge limited with an exponential distribution of traps. We estimate an average trap density of 1.75x10 16 cm -3 . Quantitative information about charge traps will help develop optimization strategies of passivating defects in order to fabricate high quality solution processed graphene devices.

We introduce the integration of a novel dielectrophoresis (DEP)-assisted filter with a compact disk (CD)-based centrifugal platform. Carbon-electrode dielectrophoresis (carbon-DEP) refers to the use of carbon electrodes to induce DEP. In this work, 3D carbon electrodes are fabricated using the C-MEMS technique and are used to implement a DEP-enabled active filter to trap particles of interest. Compared to traditional planar metal electrodes, 3D carbon electrodes allow for superior filtering efficiency. The system includes mounting modular 3D carbon-DEP chips on an electrically interfaced rotating disk. This allows simple centrifugal pumping to replace the large footprint syringe pump approaches commonly used in DEP systems. The advantages of the CD setup include not only a reduced footprint, but also complexity and cost reduction by eliminating expensive precision pumps and fluidic interconnects. To demonstrate the viability of this system we quantified the filter efficiency in the DEP trapping of yeast cells from a mix of latex and yeast cells. Results demonstrate selective filtering at flow rates up to 35 ml min À1 . The impact of electrode height, DEP chip misalignment and particle sedimentation on filter efficiency and the advantages this system represents are analyzed. The ultimate goal is to obtain an automated platform for bioparticle sorting with application in different fields such as point-of-care diagnostics and cell-based therapies. † Electronic supplementary information (ESI) available: -DEP spectra of viable yeast cells, non-viable yeast cells and latex particles when contained in a medium with conductivity equal to 31.2 mS cm À1 in the frequency range from 30 kHz to 1 MHz. -Model of the electric field generated around 40 and 70 mm high electrodes inside a 100 mm high channel filled with a medium with conductivity equal to 100 mS cm À1 . Electrodes are polarized at 10 V pp . See

We demonstrate high yield fabrication of field effect transistors (FET) using chemically reduced graphene oxide (RGO) sheets suspended in water assembled via dielectrophoresis. The two terminal resistances of the devices were improved by an order of magnitude upon mild annealing at 200 0 C in Ar/H 2 environment for 1 hour. With the application of a backgate voltage, all of the devices showed FET behavior with maximum hole and electron mobilities of 4.0 and 1.5 cm 2 /Vs respectively. This study shows promise for scaled up fabrication of graphene based nanoelectronic devices.