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.     

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.     

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.     

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.     

Dielectrophoretic force microscopy of aqueous interfaces

A novel scanning probe microscopy technique has allowed dielectrophoretic force imaging with nanoscale spatial resolution. Dielectrophoresis (DEP) traditionally describes the mobility of polarizable particles in inhomogeneous alternating current (ac) electric fields. Integrating DEP with atomic force microscopy allows for noncontact imaging with the image contrast related to the local electric polarizability. By tuning the ac frequency, dielectric spectroscopy can be performed at solid/liquid interfaces with high spatial resolution. In studies of cells, the frequency-dependent dielectrophoretic force is sensitive to biologically relevant electrical properties, including local membrane capacitance and ion mobility. Consequently, dielectrophoretic force microscopy is well suited for in vitro noncontact scanning probe microscopy of biological systems.     

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.     

The dielectrophoretic levitation and separation of latex beads in microchips.

A linear travelling wave dielectrophoretic (twDEP) microchip was fabricated and used to investigate both the levitation and the twDEP motion of latex beads as a function of applied potential and frequency, suspending medium conductivity, bead size, and surface characteristics. The surface conductance of the latex beads was characterised by measurement of the dielectrophoretic (DEP) crossover frequency. Collection of sample prior to initiation of twDEP was achieved using positive DEP forces generated by an integrated pair of parallel electrodes positioned in front of the twDEP array within the microfluidic channel. The principle of linear twDEP separation is shown using latex beads and rabbit heart cells.     

Multiphase electropatterning of cells and biomaterials

Tissues formed by cells encapsulated in hydrogels have uses in biotechnology, cell-based assays, and tissue engineering. We have previously presented a 3D micropatterning technique that rapidly localizes live cells within hydrogels using dielectrophoretic (DEP) forces, and have demonstrated the ability to modulate tissue function through the control of microscale cell architecture. A limitation of this method is the requirement that a single biomaterial must simultaneously harbor biological properties that support cell survival and function and material properties that permit efficient dielectrophoretic patterning. Here, we resolve this issue by forming multiphase tissues consisting of microscale tissue sub-units in a 'local phase' biomaterial, which, in turn, are organized by DEP forces in a separate, mechanically supportive 'bulk phase' material. We first define the effects of medium conductivity on the speed and quality of DEP cell patterning. As a case study, we then produce multiphase tissues with microscale architecture that combine high local hydrogel conductivity for enhanced survival of sensitive liver progenitor cells with low bulk conductivity required for efficient DEP micropatterning. This approach enables an expanded range of studies examining the influence of 3D cellular architecture on diverse cell types, and in the future may improve the biological function of inhomogeneous tissues assembled from a variety of modular tissue sub-units.     

阵列式对电极介电电泳芯片及其用于细胞分离富集研究

基于介电电泳原理, 设计并制作了一种新型的能够用于细胞分离和富集的微流控介电电泳芯片. 该芯片由沉积有金电极的石英基片和带有微管道的聚二甲基硅氧烷(PDMS)盖片组成. 通过在管道底部布置间距不同的对电极阵列, 增大了正介电电泳力在管道中的有效作用范围, 能够在降低施加电压的同时, 实现对流动体系中细胞样品的捕获. 在3 V和3 MHz条件下, 该DEP芯片对人血红细胞的捕获效率达到83%; 进一步通过将肝癌细胞捕获在芯片电极上可实现对红细胞和肝癌细胞混合样品的分离, 在5 V和400 kHz条件下对肝癌细胞的捕获效率达到86%.     

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.     

Dielectrophoresis induced clustering regimes of viable yeast cells

We experimentally study the transient clustering behavior of viable yeast cells in a dilute suspension suddenly subjected to a nonuniform alternating current (AC) electric field of a microelectrode device. The frequency of the applied electric field is varied to identify two distinct regimes of positive dielectrophoresis. In both regimes, the yeast cells eventually cluster at electrodes' edges, but their transient behavior as well as their final arrangement is quite different. Specifically, when the frequency is much smaller than the cross-over frequency, the nearby yeast cells quickly rearrange in well-defined chains which then move toward the electrodes' edges and remain aligned as elongated chains at their final location. However, when the frequency is close to the cross-over frequency, cells move individually toward the regions of collection and simply agglomerate along the electrodes' edges. Our analysis shows that in the first regime both the dielectrophoretic (DEP) force and the mutual DEP force, which arises due to the electrostatic particle-particle interactions, are important. In the second regime, on the other hand, the DEP force dominates.     

Dielectrophoretic investigation of plant virus particles: Cow Pea Mosaic Virus and Tobacco Mosaic Virus

This paper reports experimental results on the dielectrophoretic (DEP) behaviour on two nonenveloped plant viruses of different geometrical shapes, namely Cow Pea Mosaic Virus (CPMV) and Tobacco Mosaic Virus (TMV). The DEP properties of carboxy-modified latex beads of the same size are also reported. The DEP properties of single particles were obtained from measurement of the frequency at which the DEP force on a particle goes to zero (the crossover frequency). The DEP behaviour of particle ensembles was also measured using image processing. The dielectric properties of the particles were evaluated from the DEP data. The surface conductance was found to be 0.3 nS for CPMV, 0.38 nS for TMV, and 0.52 nS for 27 nm diameter carboxy-latex beads. Data analysis has shown that the optimal condition for separation of TMV and CPMV is a low-conductivity suspending medium - below 1 mS/m.     

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.     

Lateral dielectrophoretic microseparators to measure the size distribution of blood cells

Lateral displacement of blood cells occurred when they were passed over a planar interdigitated electrode array placed at an angle to the direction of flow, and was determined to be a function of cell size. A simplified line charge model was used to estimate numerically the lateral displacement. Based on the size-specific lateral displacement, a lateral dielectrophoretic (DEP) microseparator was developed to measure the size distribution of blood cells using fluorescence microscopy. To determine whether the lateral DEP microseparator was useful, it was used to detect acute leukemia by measuring the size distribution of blood cells. The lateral DEP microseparator provided a practical method for continuously and simultaneously separating multi-cell populations by size from a heterogeneous cell population. In the future, sensitivity of the lateral DEP microseparator could be improved and it could be automated by integrating subsequent advanced detection technologies in a micro-format.     

Continuous dielectrophoretic cell separation microfluidic device

We present a prototype microfluidic device developed for the continuous dielectrophoretic (DEP) fractionation and purification of sample suspensions of biological cells. The device integrates three fully functional and distinct units consisting of an injector, a fractionation region, and two outlets. In the sheath and sample injection ports, the cell sample are hydrodynamically focused into a stream of controlled width; in the DEP fractionation region, a specially shaped nonuniform (isomotive) electric field is synthesized and employed to facilitate the separation, and the sorted cells are then delivered to two sample collection ports. The microfluidic behavior of the injector region was simulated and then experimentally verified. The operation and performance of the device was evaluated using yeast cells as model biological particles. Issues relating to the fabrication and operation of the device are discussed in detail. Such a device takes a significant step towards an integrated lab-on-a-chip device, which could interface/integrate to a number of other on-chip components for the device to undertake the whole laboratory procedure.     

Label-free Sorting and Counting of Yeast Cells for Viability Studies

This paper reports the new combination of cell sorting and counting capabilities on a single device. Most state-of-the-art devices combining these technologies use optical techniques requiring complicate experimental setups and labeled samples. The use of a label-free, electrical device significantly decreases the system complexity and makes it more appropriate for use in point-of-care diagnostics.Living and dead yeast cells are separated by dielectrophoretic forces and counted using coulter counters. The combination of these two methods allows the determination of the percentage of living and dead cells for viability studies of cell samples. It could further be used for sorting and counting of blood cells in applications such as diagnosis of insufficient cell concentrations, identification of cell deficiencies or bacterial contamination. The use of dielectrophoresis (DEP) as sorting principle allows to separate cells based on their dielectric properties in place of size-based separation, enabling sorting of large panels of cells and separation of infected and non-infected cells of the same type.     

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.     

Lateral-driven continuous dielectrophoretic microseparators for blood cells suspended in a highly conductive medium

This paper presents lateral-driven continuous dielectrophoretic (DEP) microseparators for separating red and white blood cells suspended in highly conductive dilute whole blood. The continuous microseparators enable the separation of blood cells based on the lateral DEP force generated by a planar interdigitated electrode array placed at an angle to the direction of flow. The simplified line charge model that we developed for the theoretical analysis was verified by comparing it with simulated and measured results. Experimental results showed that the divergent type of microseparator can continuously separate out 87.0% of the red blood cells (RBCs) and 92.1% of the white blood cells (WBCs) from dilute whole blood within 5 min simply by using a 2 MHz, 3 Vp-p AC voltage to create a gradient electric field in a medium that conducts at 17 mS cm(-1). Under the same conditions, the convergent type of microseparator could separate out 93.6% of the RBCs and 76.9% of the WBCs. We have shown that our lateral-driven continuous DEP microseparator design is practical for the continuous separation of blood cells without the need to control the conductivity of the suspension medium, overcoming critical drawbacks of DEP microseparators.     

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.     

Dielectrophoretic characterization and separation of monocytes and macrophages using 3D carbon‐electrodes

Monocyte heterogeneity and its prevalence are revealed as indicator of several human diseases ranking from cardiovascular diseases to rheumatoid arthritis, chronic kidney diseases, autoimmune multiple sclerosis, and stroke injuries. When monocytes and macrophages are characterized and isolated with preserved genetic, phenotypic and functional properties, they can be used as label‐free biomarkers for precise diagnostics and treatment of various diseases. Here, the dielectrophoretic responses of the monocytes and macrophages were examined. We present 3D carbon‐electrode dielectrophoresis (carbon‐DEP) as a separation tool for U937 monocytes and U937 monocyte‐differentiated macrophages. The carbon‐electrodes advanced the usability and throughput of DEP separation, presented wider electrochemical stability. Using the 3D carbon‐DEP chip, we first identified the selective positive and negative DEP responses and specific crossover frequencies of monocytes and macrophages as their signatures for separation. The crossover frequency of monocytes and macrophages was 17 and 30 kHz, respectively. Next, we separated monocyte and macrophage subpopulations using their specific dielectrophoretic responses. Afterward, we used a fluorescence‐activated cell sorter to confirm our results. Finally, we enriched 70% of monocyte cells from the mixed cell population, in other words, concentration of monocyte cells to macrophage cells was five times increased, using the 30‐kHz, 10‐Vpp electric field and 1 μL/min flow rate.