Holding forces of single-particle dielectrophoretic traps

Analysis of Cell Dielectrophoretic Properties Using Isomotive Creek-Gap Electrode Device

Various types of dielectrophoresis (DEP) cell separation devices using AC electric fields have been proposed and developed. However, its capability is still limited by a lack of quantitative characterization of the relationship between frequency and force. In the present study, this limitation was addressed by developing a method capable of fast and accurate quantification of the dielectric properties of biological cells. A newly designed Creek-gap electrode device can induce constant DEP forces on cells, realizing the isomotive movement of cells suitable for DEP analysis. The real number part of the Clausius-Mossotti (CM) factor of cells, Re(β), was obtained by simple cell velocimetry together with the numerical three-dimensional (3D) electric field analysis. Human mammary cells, MCF10A, and its cancer cells, MCF7 and MDAMB231, were used as model cells to evaluate the capability of the proposed device. The estimation of Re(β) using the Creek-gap electrode device showed good agreement with previously reported values. Furthermore, the thermal behavior of the Creek-gap electrode device, which is crucial to cell viability, was investigated by adopting micro laser-induced fluorescence (LIF) thermometry using Rhodamine B. The temperature rise in the device was found to be approximately several degrees Celsius at most. The results demonstrate that the proposed method could be a powerful tool for fast and accurate noninvasive measurement of the DEP spectrum and the determination of the dielectric properties of biological cells.

Dielectrophoretic separation and purification: From colloid and biological particles to droplets

It is indispensable to realize the high level of purification and separation, so that objective particles, such as malignant cells, harmful bacteria, and special proteins or biological molecules, could satisfy the high precise measurement in the pharmaceutical analysis, clinical diagnosis, targeted therapy, and food defense. In addition, this could reveal the intrinsic nature and evolution mechanisms of individual biological variations. Consequently, many techniques related to optical tweezers, microfluidics, acoustophoresis, and electrokinetics can be broadly used to achieve micro- and nano-scale particle separations. Dielectrophoresis (DEP) has been used for various manipulation, concentration, transport, and separation processes of biological particles owing to its early development, mature theory, low cost, and high throughput. Although numerous reviews have discussed the biological applications of DEP techniques, comprehensive descriptions of micro- and nano-scale particle separations feature less frequently in the literature. Therefore, this review summarizes the current state of particle separation attention to relevant technological developments and innovation, including theoretical simulation, microchannel structure, electrode material, pattern and its layout. Moreover, a brief overview of separation applications using DEP in combination with other technologies is also provided. Finally, conclusions, future guidelines, and suggestions for potential promotion are highlighted.

Numerical investigation of ternary particle separation in a microchannel with a wall-mounted obstacle using dielectrophoresis

In recent years, microfluidic-based particle/cell manipulation techniques have catalyzed significant advances in several fields of science. As an efficient, precise, and label-free particle/cell manipulation technique, dielectrophoresis (DEP) has recently attracted widespread attention. This paper presents the design and investigation of a straight sheathless 3D microchannel with a wall-mounted trapezoidal obstacle for continuous-flow separation of three different populations of polystyrene (PS) particles (5, 10 and 20 µm) using DEP. An OpenFOAM code is developed to simulate and investigate the movement of particles in the microchannel. Then, the code is validated by performing various experimental tests using a microdevice previously fabricated in our lab. By comparing the numerical simulation results with the experimental tests, it can be claimed that the newly developed solver is highly accurate, and its results agree well with experimental tests. Next, the effect of various operational and geometrical parameters such as obstacle height, applied voltage, electrode pairs angle, and flow rate on the efficient focusing and separation of particles are numerically investigated. The results showed that efficient particle separation could only be achieved for obstacle heights of more than 350 µm. Furthermore, the appropriate voltage range for efficient particle separation is increased by decreasing the electrode angle as well as increasing the flow rate. Moreover, the results showed that by employing the appropriate channel design and operational conditions, at a maximum applied voltage of 10V, a sample flow rate of 2.5μL/min could be processed. The proposed design can be beneficial for integrating with lab-on-a-chip and clinical diagnosis applications due to advantages, such as simple design, no need for sheath flow, the simultaneous ternary separation of particles, and providing precise particle separation.

Enhancing the Accuracy of Measuring DEP Force Applied on Cells by Considering the Friction Effect

The Dielectrophoresis (DEP) phenomenon has been widely used for cell separation in recent years. The experimental measurement of the DEP force is one of the concerns of scientists. This research presents a novel method for more accurately measuring the DEP force. The innovation of this method is considered the friction effect, which has been neglected in previous studies. For this purpose, first, the direction of the microchannel was aligned with the electrodes. As there was no DEP force in this direction, the release force of the cells caused by the fluid flow equaled the friction force between the cells and the substrate. Then, the microchannel was aligned perpendicular to the direction of the electrodes, and the release force was measured. The net DEP force was obtained by the difference between the release forces of these two alignments. In the experimental tests, the DEP force, when applied to the sperm and white blood cell (WBC), was measured. The WBC was used to validate the presented method. The experimental results showed that the forces applied by DEP to WBC and human sperm were 42 pN and 3 pN, respectively. On the other hand, with the conventional method, these figures were as high as 72 pN and 4 pN due to neglecting the friction force. The compression between the simulation results in COMSOL Multiphysics and the experiments determined the new approach to be valid and capable of use in any cell, such as sperm.

Electromagnetic Forces and Torques: From Dielectrophoresis to Optical Tweezers

Electromagnetic forces and torques enable many key technologies, including optical tweezers or dielectrophoresis. Interestingly, both techniques rely on the same physical process: the interaction of an oscillating electric field with a particle of matter. This work provides a unified framework to understand this interaction both when considering fields oscillating at low frequencies─dielectrophoresis─and high frequencies─optical tweezers. We draw useful parallels between these two techniques, discuss the different and often unstated assumptions they are based upon, and illustrate key applications in the fields of physical and analytical chemistry, biosensing, and colloidal science.

Dielectrophoretic Traps for Efficient Bead and Cell Trapping and Formation of Aggregates of Controlled Size and Composition

We present a microfluidic dielectrophoretic-actuated system designed to trap chosen single-cell and form controlled cell aggregates. A novel method is proposed to characterize the efficiency of the dielectrophoretic trapping, considering the flow speed but also the heat generated by the traps as limiting criteria in cell-safe manipulation. Two original designs with different manufacturing processes are experimentally compared. The most efficient design is selected and the cell membrane integrity is monitored by fluorescence imaging to guarantee a safe-cell trapping. Design rules are suggested to adapt the traps to multiple-cells trapping and are experimentally validated as we formed aggregates of controlled size and composition with two different types of cells. We provide hereby a simple manufactured tool allowing the controlled manipulation of particles for the composition of multicellular assemblies.

Particle-Induced Electrostatic Repulsion within an Electric Curtain Operating below the Paschen Limit

The electric curtain is a platform developed to lift and transport charged particles in air. Its premise is the manipulation of charged particles; however, fewer investigations isolate dielectric forces that are observed at lower voltages (i.e., less than the Paschen limit). This work focuses on observations of simultaneous dielectrophoretic and electrostatic forces. The electric curtain was a printed circuit board with interdigitated electrodes (0.020 inch width and spacing) coated with a layer of polypropylene, where a standing wave or travelling wave AC signal was applied (50 Hz) to produce an electric field below the Paschen limit. Soda lime glass beads (180–212 µm) demonstrated oscillatory rolling via dielectrophoretic forces. In addition, several particles simultaneously experienced rapid projectile repulsion, a behavior consistent with electrostatic phenomena. This second result is discussed as a particle-induced local increase in the electric field, with simulations demonstrating that a particle in close proximity to the curtain’s surface produces a local field enhancement of over 2.5 times. The significance of this is that individual particles themselves can trigger electrostatic repulsion in an otherwise dielectric system. These results could be used for advanced applications where particles themselves provided triggered responses, perhaps for selective sorting of micrometer particles in air.

Measuring Nanoparticle Polarizability Using Fluorescence Microscopy

Using a novel method developed to quantify the polarizability of photoluminescent nanoparticles in water, we present experimental observations of the extraordinary polarizability exhibited by nanoparticles of commensurate size with the Debye screening length, confirming previously reported theory. Semiconductor quantum dots (QDs) are ideal model nanoparticles to demonstrate this assay, due to their tunable size and bright photoluminescence. This assay is based upon microfluidic chambers with microelectrodes that generate trapping potentials that are weaker than thermal energy. By comparing the local electric field strength and variations in QD concentration, their polarizability was computed and found to agree with estimates based upon the hydrodynamic diameter found using light scattering. Strikingly, the polarizability of the nanoparticles increased 30-fold in low salt conditions compared to high salt conditions due to the increased thickness of the Debye layer relative to the particle radius. In addition to providing evidence that corroborates theoretical work studying direct solutions to the Poisson-Nernst-Planck equations, these observations provide an explanation for the previously observed conductivity dependence of biomolecule polarizability. As the polarizability of nanoparticles is of high importance to the electrically directed assembly of particles, as well as their interactions with other materials in complex environments, we anticipate that these results will be highly relevant to ongoing efforts in materials by design and nanomedicine.

Lipid coated liquid crystal droplets for the on-chip detection of antimicrobial peptides

We describe a novel biosensor based on phospholipid-coated nematic liquid crystal (LC) droplets and demonstrate the detection of Smp43, a model antimicrobial peptide (AMP) from the venom of North African scorpion Scorpio maurus palmatus. Mono-disperse lipid-coated LC droplets of diameter 16.7 ± 0.2 μm were generated using PDMS microfluidic devices with a flow-focusing configuration and were the target for AMPs. The droplets were trapped in a bespoke microfluidic trap structure and were simultaneously treated with Smp43 at gradient concentrations in six different chambers. The disruption of the lipid monolayer by the Smp43 was detected (<6 μM) at concentrations well within its biologically active range, indicated by a dramatic change in the appearance of the droplets associated with the transition from a typical radial configuration to a bipolar configuration, which is readily observed by polarizing microscopy. This suggests the system has feasibility as a drug-discovery screening tool. Further, compared to previously reported LC droplet biosensors, this LC droplet biosensor with a lipid coating is more biologically relevant and its ease of use in detecting membrane-related biological processes and interactions has the potential for development as a reliable, low-cost and disposable point of care diagnostic tool.

Dielectrophoresis-assisted creation of cell aggregates under flow conditions using planar electrodes

We present a microfluidic platform allowing dielectrophoresis-assisted formation of cell aggregates of controlled size and composition under flow conditions. When specific experimental conditions are met, negative dielectrophoresis allows efficient concentration of cells towards electric field minima and subsequent aggregation. This bottom-up assembly strategy offers several advantages with respect to the targeted application: first, dielectrophoresis offers precise control of spatial cell organization, which can be adjusted by optimizing electrode design. Then, it could contribute to accelerate the establishment of cell-cell interactions by favoring close contact between neighboring cells. The trapping geometry of our chip is composed of eight electrodes arranged in a circle. Several parameters have been tested in simulations to find the best configurations for trapping in flow. Those configurations have been tested experimentally with both polystyrene beads and human embryonic kidney cells. The final design and experimental setup have been optimized to trap cells and release the created aggregates on demand.

A Handy Liquid Metal Based Non-Invasive Electrophoretic Particle Microtrap

A handy liquid metal based non-invasive particle microtrap was proposed and demonstrated in this work. This kind of microtrap can be easily designed and fabricated at any location of a microfluidic chip to perform precise particle trapping and releasing without disturbing the microchannel itself. The microsystem demonstrated in this work utilized silicon oil as the continuous phase and fluorescent particles (PE-Cy5, SPHERO^TM Fluorescent Particles, BioLegend, San Diego, CA, USA, 10.5 μm) as the target particles. To perform the particle trapping, the micro system utilized liquid-metal-filled microchannels as noncontact electrodes to generate different patterns of electric field inside the fluid channel. According to the experimental results, the target particle can be selectively trapped and released by switching the electric field patterns. For a better understanding the control mechanism, a numerical simulation of the electric field was performed to explain the trapping mechanism. In order to verify the model, additional experiments were performed and are discussed.

Stagnation point flows in analytical chemistry and life sciences

Isolated microfluidic stagnation points – formed within microfluidic interfaces – have come a long way as a tool for characterizing materials, manipulating micro particles, and generating confined flows and localized chemistries.

Self-concentrating buoyant glass microbubbles for high sensitivity immunoassays

Here, we report the novel application of a material with self-concentrating properties for enhancing the sensitivity of immunoassays. Termed as glass microbubbles, they are antibody functionalized buoyant hollow glass microspheres that simultaneously float and concentrate into a dense monolayer when dispensed in a liquid droplet. This self-concentrating charactaristic of the microbubbles allow for autonomous signal localization, which translates to a higher sensitivity compared to other microparticle-based immunoassays. We then demonstrated a "microbubble array" platform consisting of the glass microbubbles floating in a microfluidic liquid hemisphere array for performing multiplex immunoassays.

Characterization of the Stiffness of Multiple Particles Trapped by Dielectrophoretic Tweezers in a Microfluidic Device

Characterization of the stiffness of multiple particles trapped by tweezers-based force spectroscopy is a key step in building simple, high-throughput, and robust systems that can investigate the molecular interactions in a biological process, but the technology to characterize it in a given environment simultaneously is still lacking. We first characterized the stiffness of multiple particles trapped by dielectrophoretic (DEP) tweezers inside a microfluidic device. In this characterization, we developed a method to measure the thermal fluctuations of the trapped multiple particles with DEP tweezers by varying the heights of the particles in the given environment at the same time. Using the data measured in this controlled environment, we extracted the stiffness of the trapped particles and calculated their force. This study not only provides a simple and high-throughput method to measure the trap stiffness of multiple particles inside a microfluidic device using DEP tweezers but also inspires the application of the trapped multiple particles to investigate the dynamics in molecular interactions.

Optimization of coil geometries for bone fracture healing via dielectrophoretic force stimulation - a simulation study

In this paper we propose a novel technique for shortening fracture healing times based on the use of dielectrophoretic forces (DEPFs). If a non-uniform electromagnetic field is applied around a fracture site, red blood cells within the blood will be polarized; creating electrical dipoles. The dielectrophoretic forces resulting from the interaction of these dipoles and the electromagnetic field, can be used to manipulate blood flow at a fracture site, promote vascularization, increase transmembrane signaling, increase supply of nutrients, necessary hormones and growth factors at the fracture site and thus may help bone healing. For the generation of non-uniform fields we considered three different coil designs (linear, parabolic and square root) and using Mathcad numerically studied the dielectrophoretic forces for a long bone fracture where the main arteries are vertically-oriented and the blood flow is downward. The gravitational force and the drag force on the red blood cells determine the steady state blood flow. The dielectrophoretic force added to the force balance is functional in increasing the blood flow. The ratio of the velocity in the presence of dielectrophoresis to the velocity without dielectrophoresis (called here as the Dielectrophoretic Force Factor, K(DEpF)) is a good measure of the performance of the dielectrophoresis, since it indicates the increase in blood flow. It was found that the dielectorophoretic force reaches peak levels at a frequency range between 5-15 Hz. At 5 Hz, the average value of dielectrophoretic force factor is 1.90, 2.51 and 1.61 for the linear, parabolic and the square root coils, respectively. The parabolic coil results in the best DEPF and therefore would be the configuration to use in an experimental study to determine if DEPF is useful for bone healing.

Digital microfluidics for time-resolved cytotoxicity studies on single non-adherent yeast cells

Single cell analysis (SCA) has gained increased popularity for elucidating cellular heterogeneity at genomic, proteomic and cellular levels. Flow cytometry is considered as one of the most widely used techniques to characterize single cell responses; however, its inability to analyse cells with spatio-temporal resolution poses a major drawback. Here, we introduce a digital microfluidic (DMF) platform as a useful tool for conducting studies on isolated yeast cells in a high-throughput fashion. The reported system exhibits (i) a microwell array for trapping single non-adherent cells by shuttling a cell-containing droplet over the array, and allows (ii) implementation of high-throughput cytotoxicity assays with enhanced spatio-temporal resolution. The system was tested for five different concentrations of the antifungal drug Amphotericin B, and the cell responses were monitored over time by time lapse fluorescence microscopy. The DMF platform was validated by bulk experiments, which mimicked the DMF experimental design. A correlation analysis revealed that the results obtained on the DMF platform are not significantly different from those obtained in bulk; hence, the DMF platform can be used as a tool to perform SCA on non-adherent cells, with spatio-temporal resolution. In addition, no external forces, other than the physical forces generated by moving the droplet, were used to capture single cells, thereby avoiding cell damage. As such, the information on cellular behaviour during treatment could be obtained for every single cell over time making this platform noteworthy in the field of SCA.

Towards high-throughput microfluidic Raman-activated cell sorting

Raman-activated cell sorting (RACS) is a promising single-cell analysis technology that is able to identify and isolate individual cells of targeted type, state or environment from an isogenic population or complex consortium of cells, in a label-free and non-invasive manner.

Dielectrophoresis for bioparticle manipulation

As an ideal method to manipulate biological particles, the dielectrophoresis (DEP) technique has been widely used in clinical diagnosis, disease treatment, drug development, immunoassays, cell sorting, etc. This review summarizes the research in the field of bioparticle manipulation based on DEP techniques. Firstly, the basic principle of DEP and its classical theories are introduced in brief; Secondly, a detailed introduction on the DEP technique used for bioparticle manipulation is presented, in which the applications are classified into five fields: capturing bioparticles to specific regions, focusing bioparticles in the sample, characterizing biomolecular interaction and detecting microorganism, pairing cells for electrofusion and separating different kinds of bioparticles; Thirdly, the effect of DEP on bioparticle viability is analyzed; Finally, the DEP techniques are summarized and future trends in bioparticle manipulation are suggested.

Microfluidic dielectrophoretic sorter using gel vertical electrodes

We report the development and results of a two-step method for sorting cells and small particles in a microfluidic device. This approach uses a single microfluidic channel that has (1) a microfabricated sieve which efficiently focuses particles into a thin stream, followed by (2) a dielectrophoresis (DEP) section consisting of electrodes along the channel walls for efficient continuous sorting based on dielectric properties of the particles. For our demonstration, the device was constructed of polydimethylsiloxane, bonded to a glass surface, and conductive agarose gel electrodes. Gold traces were used to make electrical connections to the conductive gel. The device had several novel features that aided performance of the sorting. These included a sieving structure that performed continuous displacement of particles into a single stream within the microfluidic channel (improving the performance of downstream DEP, and avoiding the need for additional focusing flow inlets), and DEP electrodes that were the full height of the microfluidic walls ("vertical electrodes"), allowing for improved formation and control of electric field gradients in the microfluidic device. The device was used to sort polymer particles and HeLa cells, demonstrating that this unique combination provides improved capability for continuous DEP sorting of particles in a microfluidic device.

Isolation of circulating tumor cells by dielectrophoresis

Dielectrophoresis (DEP) is an electrokinetic method that allows intrinsic dielectric properties of suspended cells to be exploited for discrimination and separation. It has emerged as a promising method for isolating circulation tumor cells (CTCs) from blood. DEP-isolation of CTCs is independent of cell surface markers. Furthermore, isolated CTCs are viable and can be maintained in culture, suggesting that DEP methods should be more generally applicable than antibody-based approaches. The aim of this article is to review and synthesize for both oncologists and biomedical engineers interested in CTC isolation the pertinent characteristics of DEP and CTCs. The aim is to promote an understanding of the factors involved in realizing DEP-based instruments having both sufficient discrimination and throughput to allow routine analysis of CTCs in clinical practice. The article brings together: (a) the principles of DEP; (b) the biological basis for the dielectric differences between CTCs and blood cells; (c) why such differences are expected to be present for all types of tumors; and (d) instrumentation requirements to process 10 mL blood specimens in less than 1 h to enable routine clinical analysis. The force equilibrium method of dielectrophoretic field-flow fractionation (DEP-FFF) is shown to offer higher discrimination and throughput than earlier DEP trapping methods and to be applicable to clinical studies.

Depletion of cells and abundant proteins from biological samples by enhanced dielectrophoresis

Platforms that are sensitive and specific enough to assay low-abundance protein biomarkers, in a high throughput multiplex format, within a complex biological fluid specimen, are necessary to enable protein biomarker based diagnostics for diseases such as cancer. The signal from an assay for a low-abundance protein biomarker in a biological fluid sample like blood is typically buried in a background that arises from the presence of blood cells and from high-abundance proteins that make up 90% of the assayed protein mass. We present an automated on-chip platform for the depletion of cells and highly abundant serum proteins in blood. Our platform consists of two components, the first of which is a microfluidic mixer that mixes beads containing antibodies against the highly abundant proteins in the whole blood. This complex mixture (consisting of beads, cells, and serum proteins) is then injected into the second component of our microfluidic platform, which comprises a filter trench to capture all the cells and the beads. The size-based trapping of the cells and beads into the filter trench is significantly enhanced by leveraging additional negative dielectrophoretic forces to push the micron sized particles (cells and beads which have captured the highly abundant proteins) down into the trench, allowing the serum proteins of lower abundance to flow through. In general, dielectrophoresis using bare electrodes is incapable of producing forces beyond the low piconewton range that tend to be insufficient for separation applications. However, by using electrodes passivated with atomic layer deposition, we demonstrate the application of enhanced negative DEP electrodes together with size-based flltration induced by the filter trench, to deplete 100% of the micron sized particles in the mixture.

Programmable interactions of functionalized single bioparticles in a dielectrophoresis-based microarray chip

Manipulating single biological objects is a major unmet challenge of biomedicine. Herein, we describe a lab-on-a-chip platform based on dielectrophoresis (DEP). The DEParray is a prototypal version consisting of 320 × 320 arrayed electrodes generating >10,000 spherical DEP cages. It allows the capture and software-guided movement to predetermined spatial coordinates of single biological objects. With the DEParray we demonstrate (a) forced interaction between a single, preselected target cell and a programmable number of either microspheres or natural killer (NK) cells, (b) on-chip immunophenotypic discrimination of individual cells based on differential rosetting with microspheres functionalized with monoclonal antibodies to an inhibitory NK cell ligand (HLA-G), (c) on-chip, real-time (few minutes) assessment of immune lysis by either visual inspection or semiautomated, time-lapse reading of a fluorescent dye released from NK cell-sensitive targets, and (d) manipulation and immunophenotyping with limiting amounts (about 500) cells. To our knowledge, this is the first report describing a DEP-based lab-on-a-chip platform for the quick, arrayed, software-guided binding of individually moved biological objects, the targeting of single cells with microspheres, and the real-time characterization of immunophenotypes. The DEParray candidates as a discovery tool for novel cell:cell interactions with no prior (immuno)phenotypic knowledge.

Numerical modelling and measurement of cell trajectories in 3-D under the influence of dielectrophoretic and hydrodynamic forces

This research is part of a program aiming at the development of a fluidic microsystem for in vitro drug testing. For this purpose, primary cells need to be assembled to form cellular aggregates in such a way as to resemble the basic functional units of organs. By providing for in vivo-like cellular contacts, proper extracellular matrix interaction and medium perfusion it is expected that cells will retain their phenotype over prolonged periods of time. In this way, in vitro test systems exhibiting in vivo type predictivity in drug testing are envisioned. Towards this goal a 3-D microstructure micro-milled in a cyclic olefin copolymer (COC) was designed in such a way as to assemble liver cells via insulator-based dielectrophoresis (iDEP) in a sinusoid-type fashion. First, numeric modelling and simulation of dielectrophoretic and hydrodynamic forces acting on cells in this microsystem was performed. In particular, the problem of the discontinuity of the electric field at the interface between the fluid media in the system and the polymer materials it consists of was addressed. It was shown that in certain cases, the material of the microsystem may be neglected altogether without introducing considerable error into the numerical solution. This simplification enabled the simulation of 3-D cell trajectories in complex chip geometries. Secondly, the assembly of HepG2 cells by insulator-based dielectrophoresis in this device is demonstrated. Finally, theoretical results were validated by recording 3-D cell trajectories and the Clausius-Mossotti factor of liver cells was determined by combining results obtained from both simulation and experiment.

Metabolic viability of Escherichia coli trapped by dielectrophoresis in microfluidics

The spatial and temporal control of biological species is essential in complex microfluidic biosystems. In addition, if the biological species is a cell, microfluidic handling must ensure that the cell's metabolic viability is maintained. The use of DEP for cell manipulation in microfluidics has many advantages because it is remote and fast, and the voltages required for cell trapping scale well with miniaturization. In this paper, the conditions for bacterial cell (Escherichia coli) trapping using a quadrupole electrode configuration in a PDMS microfluidic channel were developed both for stagnant and for in-flow fluidic situations. The effect of the electrical conductivity of the fluid, the applied electric field and frequency, and the fluid-flow velocity were studied. A dynamic exchange between captured and free-flowing cells during DEP trapping was demonstrated. The metabolic activity of trapped cells was confirmed by using E. coli cells genetically engineered to express green fluorescent protein under the control of an inducible promoter. Noninduced cells trapped by negative DEP and positive DEP were able to express green fluorescent protein minutes after the inducer was inserted in the microchannel system immediately after DEP trapping. Longer times of trapping prior to exposure to the inducer indicated first a degradation of the cell metabolic activity and finally cell death.

Smart surface for elution of protein-protein bound particles: nanonewton dielectrophoretic forces using atomic layer deposited oxides

By increasing the strength of the negative dielectrophoresis force, we demonstrated a significantly improved electrokinetic actuation and switching microsystem that can be used to elute specifically bound beads from the surface. In this work using atomic layer deposition we deposited a pinhole free nanometer-scale thin film oxide as a protective layer to prevent electrodes from corrosion, when applying high voltages (>20 V(pp)) at the electrodes. Then, by exciting the electrodes at high frequency, we capacitively coupled the electrodes to the buffer in order to avoid electric field degradation and, hence, reduction in dielectrophoresis force due to the presence of the insulating oxide layer. To illustrate the functionality of our system, we demonstrated 100% detachment of anti-IgG and IgG bound beads (which is on the same order of magnitude in strength as typical antibody-antigen interactions) from the surface, upon applying the improved negative dielectrophoresis force. The significantly enhanced switching performance presented in this work shows orders of magnitude of improvement in on-to-off ratio and switching response time, without any need for chemical eluting agents, as compared to the previous work. The promising results from this work vindicates that the functionality of this singleplexed platform can be extended to perform a multiplexed bead-based assay where in a single channel an array of proteins are patterned each targeting a different antigen or protein.

Use of negative dielectrophoresis for selective elution of protein-bound particles

In this paper with the aid of negative dielectrophoresis force in conjunction with shear force and at an optimal sodium hydroxide concentration we demonstrated a switchlike functionality to elute specifically bound beads from the surface. At an optimal flow rate and sodium hydroxide concentration, negative dielectrophoresis turned on results in bead detachment, whereas when negative dielectrophoresis is off, the beads remain attached. This platform offers the potential for performing a bead-based multiplexed assay where in a single channel various regions are immobilized with a different antibody, each targeting a different antigen. To develop the proof of concept and to demonstrate the switchlike functionality in eluting specifically bound beads from the surface we looked at two different protein interactions. We chose interactions that were in the same order of magnitude in strength as typical antibody-antigen interactions. The first was protein G-IgG interaction, and the second was the interaction between anti-IgG and IgG.

Direct assembling methodologies for high-throughput bioscreening

Over the last few decades, high-throughput (HT) bioscreening, a technique that allows rapid screening of biochemical compound libraries against biological targets, has been widely used in drug discovery, stem cell research, development of new biomaterials, and genomics research. To achieve these ambitions, scaffold-free (or direct) assembly of biological entities of interest has become critical. Appropriate assembling methodologies are required to build an efficient HT bioscreening platform. The development of contact and non-contact assembling systems as a practical solution has been driven by a variety of essential attributes of the bioscreening system, such as miniaturization, high throughput, and high precision. The present article reviews recent progress on these assembling technologies utilized for the construction of HT bioscreening platforms.

Non-vanishing ponderomotive AC electrophoretic effect for particle trapping

We present here a study on overlooked aspects of alternating current (AC) electrokinetics-AC electrophoretic (ACEP) phenomena. The dynamics of a particle with both polarizability and net charges in a non-uniform AC electric trapping field is investigated. It is found that either electrophoretic (EP) or dielectrophoretic (DEP) effects can dominate the trapping dynamics, depending on experimental conditions. A dimensionless parameter γ is developed to predict the relative strength of EP and DEP effects in a quadrupole AC field. An ACEP trap is feasible for charged particles in 'salt-free' or low salt concentration solutions. In contrast to DEP traps, an ACEP trap favors the downscaling of the particle size.

Negative dielectrophoretic capture of bacterial spores in food matrices

A microfluidic device with planar square electrodes is developed for capturing particles from high conductivity media using negative dielectrophoresis (n-DEP). Specifically, Bacillus subtilis and Clostridium sporogenes spores, and polystyrene particles are tested in NaCl solution (0.05 and 0.225 S∕m), apple juice (0.225 S∕m), and milk (0.525 S∕m). Depending on the conductivity of the medium, the Joule heating produces electrothermal flow (ETF), which continuously circulates and transports the particles to the DEP capture sites. Combination of the ETF and n-DEP results in different particle capture efficiencies as a function of the conductivity. Utilizing 20 μm height DEP chambers, "almost complete" and rapid particle capture from lower conductivity (0.05 S∕m) medium is observed. Using DEP chambers above 150 μm in height, the onset of a global fluid motion for high conductivity media is observed. This motion enhances particle capture on the electrodes at the center of the DEP chamber. The n-DEP electrodes are designed to have well defined electric field minima, enabling sample concentration at 1000 distinct locations within the chip. The electrode design also facilitates integration of immunoassay and other surface sensors onto the particle capture sites for rapid detection of target micro-organisms in the future.

Quantitative measurements of absolute dielectrophoretic forces using optical tweezers

Optical tweezers were used for quantitative measurement of the absolute dielectrophoresis (DEP) forces acting on polystyrene microparticles. The electrodes and tweezers were configured to create one-dimensional DEP forces acting perpendicular to the tweezers' beam. The influences of various external factors, such as applied voltage frequency, conductivity of the medium, and particle size on the measurement were estimated. By accounting for these factors, actual measurements were in close agreement with theoretical predictions. Our results show that the optical tweezers may serve as a unique tool for the measurement of DEP forces in various applications.

Rotational maneuver of ferromagnetic nanowires for cell manipulation

1-D magnetic nanowires provide a powerful tool for investigating biological systems because such nanomaterials possess unique magnetic properties, which allow effective manipulation of cellular and subcellular objects. In this study, we report the rotational maneuver of ferromagnetic nanowires and their applications in cell manipulation. The rotational maneuver is studied under two different suspension conditions. The rotation of nanowires in the fluid is analyzed using Stokes flow assumption. Experimental results show that when the nanowires develop contacts with the bottom surfaces, the rotational maneuver under a modest external magnetic field can generate rapid lateral motion. The floating nanowires, on the other hand, do not exhibit substantial lateral displacements. Cell manipulation using skeletal myoblasts C2C12 shows that living cells can be manipulated efficiently on the bottom surface by the rotational maneuver of the attached nanowires. We also demonstrate the use of rotational maneuver of nanowires for creating 3-D nanowire clusters and multicellular clusters. This study is expected to add to the knowledge of nanowire-based cell manipulation and contribute to a full spectrum of control strategies for efficient use of nanowires for micro-total-analysis. It may also facilitate mechanobiological studies at cellular level, and provide useful insights for development of 3-D in vivo-like multicellular models for various applications in tissue engineering.

Anti-Brownian traps for studies on single molecules

Until recently, Brownian motion was seen as an immutable feature of small particles in room-temperature liquids. Molecules, viruses, organelles, and small cells jiggle incessantly due to countless collisions with thermally agitated molecules of solvent. Einstein showed in 1905 that this motion is intimately linked to the tendency of every system to relax toward thermal equilibrium. In recent years, we and others have realized that Brownian motion is not as inescapable as one might think. By tracking the motion of a small particle and applying correction forces to the particle or to the measurement apparatus, one can largely suppress the Brownian motion of particles as small as a few nanometers in diameter, in aqueous solution at room temperature. This new ability to stabilize single molecules has led to a host of studies on topics ranging from the conformational dynamics of DNA to the optical properties of metal nanoparticles. In this review, we outline the physical principles behind suppression of Brownian motion. We discuss the relative merits of several systems that have been implemented. We give examples of studies performed with our anti-Brownian Electrokinetic trap (ABEL trap) as well as other anti-Brownian traps, and we discuss prospects for future research.

A microwave interferometric system for simultaneous actuation and detection of single biological cells

In biomedical applications ranging from the study of pathogen invasion to drug efficacy assays, there is a growing need to develop minimally invasive techniques for single-cell analysis. This has inspired researchers to develop optical, electrical, microelectromechanical and microfluidic devices for exploring phenomena at the single-cell level. In this work, we demonstrate an electrical approach for single-cell analysis wherein a 1.6 GHz microwave interferometer detects the capacitance changes (DeltaC) produced by single cells flowing past a coplanar interdigitated electrode pair. The experimental and simulated capacitance changes generated by yeast cells are in close agreement. By using the capacitance changes of uniform polystyrene spheres (diameter = 5.7 microm) for calibration purposes, we demonstrate a 0.65 aF sensitivity in a 10 ms response time. Using an RC circuit, a low frequency sinusoidal potential is simultaneously superimposed on the electrode pair to generate a dielectrophoretic force that translates cells. Specifically, when yeast cells suspended in a solution of 90 ppm NaCl in deionized water are exposed to 10 kHz and 3 MHz potentials (ranging from 1-3 V(pp)), they experience negative and positive dielectrophoresis, respectively. The corresponding changes in cell elevation above the interdigitated electrodes are detected using the asymmetry of the capacitance signature produced by the cell. Cell elevation changes can be detected in less than 80 ms. The minimum detectable change in elevation is estimated to be 0.22 microm. This approach will have applications in rapid single-cell dielectrophoretic analysis, and may also prove useful in conjunction with impedance spectroscopy.

Separation of particles by pulsed dielectrophoresis

In this paper, we introduce a dielectrophoresis (DEP)-based separation method that allows for tunable multiplex separation of particles. In traditional DEP separations where the field is applied continuously, size-based separation of particles uses the cubic dependence of the DEP force on particle radius, causing large particles to be retained while small particles are released. Here we show that by pulsing the DEP force in time, we are able to reverse the order of separation (eluting the large particles while retaining the small ones), and even extract mid-size particles from a heterogeneous population in one step. The operation is reminiscent of prior dielectrophoretic ratchets which made use of DEP and Brownian motion, but we have applied the asymmetric forces in time rather than in a spatial arrangement of electrodes, thus simplifying the system. We present an analytical model to study the dynamic behavior of particles under pulsed DEP and to understand the different modes of separation. Results from the model and the experimental observations are shown to be in agreement.

Pillar array microtraps with negative dielectrophoresis

We present a microfluidic particle-trap array that utilizes negative dielectrophoresis (nDEP) force and hydrodynamic force. The traps are located at the stagnation points of cylindrical pillars arranged in a regular array, and they can function as both single-particle traps (capable of discriminating particles based on size) and multiparticle traps (capable of controlling the number of particles trapped). By adjusting the relative strength of the nDEP and hydrodynamic forces, we are able to control the number of trapped particles accurately. We have used 5 microm polystyrene beads to validate and demonstrate the capability of this new particle-trap design. Pulsed nDEP was used to increase the selectivity and stability. Good correlation between simulation and the experimental results was obtained.

High-throughput cell and particle characterization using isodielectric separation

Separations can be broadly categorized as preparative, where the objective is to extract purified quantities of a sample from a complex mixture, or analytic, where the goal is to determine and quantify the contents of the original mixture. Here we demonstrate the application of a new microfluidic separation method, isodielectric separation (IDS), to a range of analytic separations involving cells and particles spanning several orders of magnitude in volume and electrical conductivity. In IDS, cells are dielectrophoretically concentrated to the region along an electrical conductivity gradient where their polarizability vanishes; by measuring this position--the isodielectric point (IDP)--as operating conditions such as the frequency and voltage of the applied electric field are varied, we are able to sort cells or particles with distinct IDPs while simultaneously characterizing their electrical properties. We apply this technique to measure the electrical properties of polystyrene microspheres, viable and nonviable cells of the budding yeast Saccharomyces cerevisiae , and murine pro B cells, including how these electrical properties vary with the electrical conductivity of the surrounding solvent.

Separation of malignant human breast cancer epithelial cells from healthy epithelial cells using an advanced dielectrophoresis-activated cell sorter (DACS)

In this paper, we successfully separated malignant human breast cancer epithelial cells (MCF 7) from healthy breast cells (MCF 10A) and analyzed the main parameters that influence the separation efficiency with an advanced dielectrophoresis (DEP)-activated cell sorter (DACS). Using the efficient DACS, the malignant cancer cells (MCF 7) were isolated successfully by noninvasive methods from normal cells with similar cell size distributions (MCF 10A), depending on differences between their material properties such as conductivity and permittivity, because our system was able to discern the subtle differences in the properties by generating continuously changed electrical field gradients. In order to evaluate the separation performance without considering size variations, the cells collected from each outlet were divided into size-dependent groups and counted statistically. Following that, the quantitative relative ratio of numbers between MCF 7 and MCF 10A cells in each size-dependent group separated by the DEP were compared according to applied frequencies in the range 48, 51, and 53 MHz with an applied amplitude of 8 V(pp). Finally, under the applied voltage of 48 MHz-8 V(pp) and a flow rate of 290 microm/s, MCF 7 and MCF 10A cells were separated with a maximum efficiency of 86.67% and 98.73% respectively. Therefore, our suggested system shows it can be used for detection and separation of cancerous epithelial cells from noncancerous cells in clinical applications.

Increasing the trapping efficiency of particles in microfluidic planar platforms by means of negative dielectrophoresis

We present a novel planar electrode geometry in which particles (typically 10 microm in diameter) are focused near a defined surface before being trapped using negative dielectrophoresis. The focusing element can deflect particles having speeds up to hundreds of micrometers per second. This trapping configuration results in improved trapping yields and a decrease in overall reagent consumption. Particles are trapped dynamically while flowing in a microfluidic channel.

Analysis of pairwise cell interactions using an integrated dielectrophoretic-microfluidic system

Blood vessel formation, during either normal vascular reconstruction or pathogenic tumour formation, relies upon highly organized cell–cell interactions. Isolating the function of any particular component of this cell–cell communication is often difficult, given the vast complexity of communication networks in multicellular systems. One way to address this problem is to analyse cell–cell communication on the most elementary scale—cell pairs. Here, we describe an integrated dielectrophoretic (DEP)-microfluidic device allowing for such analysis. Single cancer and endothelial cells (ECs) and cell pairs were patterned using DEP force and cultured within a minimally stressful microfluidic channel network. Controlling both the initial cell positions and extracellular environment, we investigated cell motility in homo- and heterotypic cell pairs under diverse conditions. We found that secreted collagen IV and soluble vascular endothelial growth factor have considerable guidance effect on ECs at the level of two interacting cells. Cell interaction rules extracted from the experiments of cell pairs were used to mathematically predict branching patterns characteristic of developing multicellular blood vessels. This integrative analysis method can be extended to other systems involving complex multicellular interactions.