Dielectrophoretic Separation Research Articles

Microfluidic separation of live and dead yeast cells using reservoir-based dielectrophoresis.

Separating live and dead cells is critical to the diagnosis of early stage diseases and to the efficacy test of drug screening, etc. This work demonstrates a novel microfluidic approach to dielectrophoretic separation of yeast cells by viability. It exploits the cell dielectrophoresis that is induced by the inherent electric field gradient at the reservoir-microchannel junction to selectively trap dead yeast cells and continuously separate them from live ones right inside the reservoir. This approach is therefore termed reservoir-based dielectrophoresis (rDEP). It has unique advantages as compared to existing dielectrophoretic approaches such as the occupation of zero channel space and the elimination of any mechanical or electrical parts inside microchannels. Such an rDEP cell sorter can be readily integrated with other components into lab-on-a-chip devices for applications to biomedical diagnostics and therapeutics.

Low-voltage lab-on-chip for micro and nanoparticles manipulation and detection: experimental results

In this paper, a low voltage fully integrated Laboratory-on-Chip (LoC) for dielectrophoretic manipulation and capacitive sensing of nano and micro particles is presented. The proposed system is intended to design an implantable LoC. The lowest static power consumption of the implemented Integrated circuit is 650 μA with a voltage supply of −1.10 and +1.8 V. Three different sizes of carboxyl-modified polystyrene particles (diameters of 0.22, 0.97 and 2.04 μm) where tested experimentally with three different electrode architectures to achieve dielectrophoretic mixing and separation. U-shaped, L-shaped and octagonal electrodes are used to perform the separation and mixing operations. The biosensing part is designed with a charge based capacitive sensor with an integrated sigma-delta modulator at its output stage. It was tested experimentally with algae and ethanol. The chip size is 1.2 by 1.2 mm and it is connected to a 15 × 30 cm microfluidic design. An efficient particle manipulation was achieved by applying a voltage of 1.7 V peak to peak in the microchannel with 90 and 180° dephased signals.

SEPARATING PLASMA AND BLOOD CELLS BY DIELECTROPHORESIS IN MICROFLUIDIC CHIPS

In this paper, a dielectrophoretic (DEP) micro separator is studied for plasma-blood separation. DEP forces created by non-uniform electric fields are used as deflected forces to deplete blood cells from side walls at a given inlet flow rate ( Qin ). Then one can extract plasma through a microchannel on side wall at certain extraction flow rate ( Qp ). In this experiment, saline isotonic solution is chosen as dilute solution for whole blood. The minimum dilute ratio (whole blood: saline dilute) is found to be 1:3 for DEP to substantially deplete blood cells from side walls. Exraction of plasma from whole blood sample by DEP force is also investigated. Experimental results show blood cells do not enter side channel by DEP force at inlet flow rate Qin =0.5 μ1/ min when plasma extract flow rates is Qp ≤ 0.3 μ1/ min . By calculating pure plasma extraction volume fraction, the efficiency in current experiment can reach as high as 20% if dilute ratio 1:3 of whole blood sample is considered.

Dielectrophoretic Field-Flow Microchamber for Separation of Biological Cells Based on Their Electrical Properties

We describe the development, fabrication and testing of a microfluidic chamber for dielectrophoretic field-flow separation of biological cells based on their electrical properties. The chamber was constructed from a single Pyrex wafer with interdigitated Au electrodes, a spacer, and a top cover glass, making the events in the chamber observable under most optical microscopes. The dimensions were optimized based on numerical computations of the electric field, its gradient and the fluid-flow velocity profile. The electrodes were fabricated using photolithography. A double-sided self-adhesive tape of 100 μm thickness was used as a spacer, with an opening of 80 mm length and 20 mm width cut in its middle to form a channel of 100 μm height, and with water-resistant acrylic glue of the tape holding the glass plates together and providing a tight seal. The glue loses its adhesive properties above 70 °C, allowing for easy disassembly of the chamber in hot water and its thorough cleaning. A 1:1 mixture of normal and 50 °C -heat-treated CHO cells was used to test the chamber. A 93% efficiency of separation was obtained, confirming the usefulness of the chamber in separating cells with sufficient differences in electrical properties of their membranes.

Micro‐ and Nanofluidics – Applications in Biotechnology

AbstractCover illustration: Micro‐ and Nanofluidics – Applications in Biotechnology. This issue of BTJ edited by Hikmet Geckil and Utkan Demirci covers recent advances on BioMEMs (microelectromechanical systems used in biology) and lab‐on‐a‐chip devices for cell and fluid manipulation, e.g. picoliter sequencing and PCR, as well as cytometry and imaging technologies. The cover image provided by Gurkan Yilmaz shows fluorescent particles moving inside a spiral channel in an dielectrophoretic particle separation experiment displayed by a fluorescent microscope. DOI: http://dx.doi.org/10.1002/biot.201000204

Negative dielectrophoresis‐based particle separation by size in a serpentine microchannel

Dielectrophoresis has been widely used to focus, trap, concentrate, and sort particles in microfluidic devices. This work demonstrates a continuous separation of particles by size in a serpentine microchannel using negative dielectrophoresis. Depending on the magnitude of the turn-induced dielectrophoretic force, particles travelling electrokinetically through a serpentine channel either migrate toward the centerline or bounce between the two sidewalls. These distinctive focusing and bouncing phenomena are utilized to implement a dielectrophoretic separation of 1 and 3 μm polystyrene particles under a DC-biased AC electric field of 880 V/cm on average. The particle separation process in the entire microchannel is simulated by a numerical model.

Continuous dielectrophoretic bacterial separation and concentration from physiological media of high conductivity

Biological sample processing involves purifying target analytes from various sample matrices and concentrating them to a small volume from a large volume of crude sample. This complex process is the major obstacle for developing a microfluidic diagnostic platform. In this study, we present a microfluidic device that can continuously separate and concentrate pathogenic bacterial cells from complex sample matrices such as cerebrospinal fluid and whole blood. Having overcome critical limitations of dielectrophoretic (DEP) operation in physiological media of high conductivity, we utilized target specific DEP techniques to incorporate cell separation, medium exchange, and target concentration into an integrated platform. The proposed microfluidic device can uptake mL volumes of crude biological sample and selectively concentrate target cells into a submicrolitre volume, providing ~10(4) fold of concentration. We designed the device based on the electrokinetic theory and electric field simulation, and tested the device performance with different sample types. The separation efficiency of the device was as high as 97.0% for a bead mixture in TAE buffer and 94.3% and 87.2% for E. coli in human cerebrospinal fluid and blood, respectively. A capture efficiency of 100% was achieved in the concentration chamber. With a relatively simple configuration, the proposed device provides a robust method of continuous sample processing, which can be readily integrated into a fully automated microfluidic diagnostic platform for pathogen detection and quantification.

Continuous separation of breast cancer cells from blood samples using multi-orifice flow fractionation (MOFF) and dielectrophoresis (DEP)

Circulating tumor cells (CTCs) are highly correlated with the invasive behavior of cancer, so their isolations and quantifications are important for biomedical applications such as cancer prognosis and measuring the responses to drug treatments. In this paper, we present the development of a microfluidic device for the separation of CTCs from blood cells based on the physical properties of cells. For use as a CTC model, we successfully separated human breast cancer cells (MCF-7) from a spiked blood cell sample by combining multi-orifice flow fractionation (MOFF) and dielectrophoretic (DEP) cell separation technique. Hydrodynamic separation takes advantage of the massive and high-throughput filtration of blood cells as it can accommodate a very high flow rate. DEP separation plays a role in precise post-processing to enhance the efficiency of the separation. The serial combination of these two different sorting techniques enabled high-speed continuous flow-through separation without labeling. We observed up to a 162-fold increase in MCF-7 cells at a 126 µL min(-1) flow rate. Red and white blood cells were efficiently removed with separation efficiencies of 99.24% and 94.23% respectively. Therefore, we suggest that our system could be used for separation and detection of CTCs from blood cells for biomedical applications.

Selective Parallel Integration of Individual Metallic Single-Walled Carbon Nanotubes from Heterogeneous Solutions

The dielectrophoretic separation of individual metallic single-walled carbon nanotubes (SWNTs) from heterogeneous solutions and their simultaneous deposition between electrodes is achieved and confirmed by direct electric transport measurements. Out-of-solution guided parallel assembly of individual SWNTs was investigated for electric field frequencies between 1 and 200 MHz. At 200 MHz, 19 of the 22 deposited SWNTs (86%) displayed metallic behavior, whereas at lower frequencies the expected random growth distribution of 1/3 metallic SWNTs prevailed. A threshold separation frequency of 188 MHz is extracted from a surface-conductivity model, and a conductivity weighting factor is introduced to elucidate the separation frequency dependence. Low-frequency experiments and numerical simulations show that long-range nanotube transport is governed by hydrodynamic effects whereas local trapping is dominated by dielectrophoretic forces. The electrokinetic framework of dielectrophoresis in low-concentration solutions is thus provided and allows a deeper understanding of the underlying mechanisms in dielectrophoretic deposition processes for long and large-diameter SWNT-based low-resistance device integration.

Continuous dielectrophoretic separation of particles in a spiral microchannel

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

Dielectrophoretic separation of colorectal cancer cells.

Separation of colorectal cancer cells from other biological materials is important for stool-based diagnosis of colorectal cancer. In this paper, we use conventional dielectrophoresis in a microfluidic chip to manipulate and isolate HCT116 colorectal cancer cells. It is noticed that at a particular alternating current frequency band, the HCT116 cells are clearly deflected to a side channel from the main channel after the electric activation of an electrode pair. This motion caused by negative dielectrophoresis can be used to simply and rapidly separate cancer cells from other cells. In this manuscript, we report the chip design, flow conditions, dielectrophoretic spectrum of the cancer cells, and the enrichment factor of the colorectal cancer cells from other cells.

A microfluidic system with non‐embedded tilting inter‐castellated electrode for dielectrophoretic separation

A dielectrophoretic (DEP) separation device with non‐embedded intercastellated electrode design is proposed. The dielectrophoretic force is enhanced by a tilting electrode where an electric field gradient along the flow direction is generated. The maximum strength of the electric field is reached at the outlet where the separation occurs. Yeast cells with different dielectric properties (viable vs. non‐viable) were used to study the separation efficiency under different operation conditions. The effect of medium conductivity on the DEP spectrum was modeled by a smear‐out sphere model for yeast cells. The results show that low medium conductivity has high DEP force and is advantageous for separation. The number of collected yeast cells was calculated based on spectrophotometer readings. The optimal collection frequencies for viable and non‐viable cells are 10 MHz and 5 MHz, respectively. The different DEP characteristics for viable and non‐viable cell is the primary reason for such a discrepancy. Good collection efficiency in yeast cells shows that a non‐embedded tilting intercastellated electrode is an efficient way to separate different dielectric property materials.

Glutaraldehyde enhanced dielectrophoretic yeast cell separation

We introduce a method for improved dielectrophoretic (DEP) discrimination and separation of viable and nonviable yeast cells. Due to the higher cell wall permeability of nonviable yeast cells compared with their viable counterpart, the cross-linking agent glutaraldehyde (GLT) is shown to selectively cross-link nonviable cells to a much greater extent than viable yeast. The DEP crossover frequency (cof) of both viable and nonviable yeast cells was measured over a large range of buffer conductivities (22 muScm-400 muScm) in order to study this effect. The results indicate that due to selective nonviable cell cross-linking, GLT modifies the DEP cof of nonviable cells, while viable cell cof remains relatively unaffected. To investigate this in more detail, a dual-shelled oblate spheroid model was evoked and fitted to the cof data to study cell electrical properties. GLT treatment is shown to minimize ion leakage out of the nonviable yeast cells by minimizing changes in cytoplasm conductivity over a large range of ionic concentrations. This effect is only observable in nonviable cells where GLT treatment serves to stabilize the cell cytoplasm conductivity over a large range of buffer conductivity and allow for much greater differences between viable and nonviable cell cofs. As such, by taking advantage of differences in cell wall permeability GLT magnifies the effect DEP has on the field induced separation of viable and nonviable yeasts.

Controlling the Carbon Nanotube-to-Medium Conductivity Ratio for Dielectrophoretic Separation

The surface conductivity of colloidal nanotubes, induced by ionic surfactants, is known to affect alternating current dielectrophoresis, which has been actively investigated with regard to separating single-walled carbon nanotubes according to electronic type. The nanotube-to-suspending medium conductivity ratio is a primary factor for determining the dielectrophoretic behavior of semiconducting nanotubes. In this study, our theoretical and experimental analysis revealed that the suspending medium conductivity also plays an important role in controlling the conductivity ratio. This work elucidates the effects of several surfactant systems on the conductivity ratio and therefore the degree of separation between metallic and semiconducting nanotubes. The equimolar mixture of anionic and cationic surfactants was more effective than a nonionic polymer in reducing the conductivity ratio because the conductivity of colloidal nanotubes was decreased and that of the suspending medium was increased. Besides, the surfactant mixture provided a better dispersion of nanotubes. The dielectrophoretic separation was carried out using microelectrodes with a gap size of 4 mum at an electric field frequency of 10 MHz. The complete separation of nanotubes at the reduced conductivity ratio was confirmed by Raman spectroscopy and electrical transport measurements.

Cover Picture: Electrophoresis 9'09

AbstractIssue no. 9 is an Emphasis Issue with 7 articles on various aspects of “Microfluidics and Miniaturization” while the remaining articles are grouped into sections on “Detection Sensitivity Enhancement and Stacking”, “Binding Studies” and “Other Methodologies”.In addition, issue no. 9 has two Fast Track articles. The first on proteome alteration of early‐stage differentiation of mouse embryonic stem cells into hepatocyte‐like cells, and the second on dielectrophoretic separation of small particles in a sawtooth channel.

Insulator‐based dielectrophoretic separation of small particles in a sawtooth channel

Insulator-based dielectrophoretic separation of small particles in a sawtooth channel is studied in the limit of dilute concentration. Pathlines for the movements of infinitesimal particles are constructed and the geometric changes of these pathlines are used to establish the criterion for blocking and trapping particles with different physical properties. The sharp corners of the sawtooth channel create much stronger dielectrophoretic force than channels with smooth corners for blocking particle movements. Particle blocking and trapping depend on particle properties and the geometry of the device. It is shown that once the channel geometric aspect ratios are specified, the blocking criterion depends on only a single dimensionless parameter C defined in terms of the particle mobility ratio (dielectrophoretic versus electrokinetic), the applied voltage and the spacing between the teeth. Selective blocking and trapping of particles can be realized by varying the geometry of the channel progressively. High-resolution separation can be achieved by tuning the differential in the parameter C to a desired level.

Cover Picture: J. Biophoton. 4/2009

AbstractAC Dielectrophoretic separation of 10 μm non‐fluorescent microspheres from 40 nm red fluorescent nanoparticles on a microelectrode array. The leftmost 3 × 3 microelectrodes are activated. The rightmost column of microelectrodes are used as control. (Top Left) Red Fluorescence image before experiment showing a red haze. (Top Right) Red Fluorescence image of the 40 nm nanoparticles concentrating on the electrodes at the conclusion of the experiment. (Bottom Left) Bright Field image of 10 μm microspheres distributed in a random pattern before the experiment. (Bottom Right) Bright Field image of 10 μm microspheres concentrating in between the microelectrodes at the conclusion of the experiment (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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 (microPIV) was employed to measure the velocity of 1-microm-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 microS/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 microm/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 sigma(m) = 100 microS/cm) were tested experimentally and with the aid of mathematical modeling. The microPIV 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 microPIV 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 microPIV 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.

Electrohydrodynamic‐mediated dielectrophoretic separation and transport based on asymmetric electrode pairs

This paper presents a system for continuous separation and transport of micron and submicron particles in fluidic environment based on dielectrophoretic fractionation in concert with AC electrothermal (AC ET)-induced fluidic pumping action. In this system, high frequency AC signals are used to energize asymmetric electrode pairs. AC ET-driven fluidic pumping is utilized as an alternative to the commonly used external pressure-driven fluid flow. Distinct collection sites for negative-dielectrophoretic and positive-dielectrophoretic particle populations are identified. The coupling effects of dielectrophoretic force and viscous drag from AC ET fluid flow on particle motions are investigated theoretically and numerically. We demonstrate that these two forces can be efficiently coupled to achieve continuous separation and transport of particle mixture in a fluidic medium when the dielectric properties of the particles and the fluidic environment are different. The combination of dielectrophoretic separation and AC ET pumping function provides a promising approach to further miniaturize and integrate these mechanisms into lab-on-chip devices.

Continuous separation of submicron particles using Angled electrodes

Dielectrophoretic separation of particles is achieved by the generation of electric forces on the particles by non-uniform electric fields. This paper presents a technique based on negative dielectrophoresis in a novel design of electrode array for the non-contact separation of polarisable particles. Angled electrodes are used to generate a lateral force in a microfluidic channel separating a mixed stream of particles into distinct streams of constituent components and achieving a high degree of spatial separation.