Role of peroxide in AC electrical field exposure effects on friend murine erythroleukemia cells during dielectrophoretic manipulations

A correlation of conductivity medium and bioparticle viability on dielectrophoresis-based biomedical applications

Dielectrophoresis (DEP) bioparticle research has progressed from micro to nano levels. It has proven to be a promising and powerful cell manipulation method with an accurate, quick, inexpensive, and label-free technique for therapeutic purposes. DEP, an electrokinetic phenomenon, induces particle movement as a result of polarization effects in a nonuniform electrical field. This review focuses on current research in the biomedical field that demonstrates a practical approach to DEP in terms of cell separation, trapping, discrimination, and enrichment under the influence of the conductive medium in correlation with bioparticle viability. The current review aims to provide readers with an in-depth knowledge of the fundamental theory and principles of the DEP technique, which is influenced by conductive medium and to identify and demonstrate the biomedical application areas. The high conductivity of physiological fluids presents obstacles and opportunities, followed by bioparticle viability in an electric field elaborated in detail. Finally, the drawbacks of DEP-based systems and the outlook for the future are addressed. This article will aid in advancing technology by bridging the gap between bioscience and engineering. We hope the insights presented in this review will improve cell suspension medium and promote DEP-viable bioparticle manipulation for health-care diagnostics and therapeutics.

Signal-Based Methods in Dielectrophoresis for Cell and Particle Separation

Separation and detection of cells and particles in a suspension are essential for various applications, including biomedical investigations and clinical diagnostics. Microfluidics realizes the miniaturization of analytical devices by controlling the motion of a small volume of fluids in microchannels and microchambers. Accordingly, microfluidic devices have been widely used in particle/cell manipulation processes. Different microfluidic methods for particle separation include dielectrophoretic, magnetic, optical, acoustic, hydrodynamic, and chemical techniques. Dielectrophoresis (DEP) is a method for manipulating polarizable particles' trajectories in non-uniform electric fields using unique dielectric characteristics. It provides several advantages for dealing with neutral bioparticles owing to its sensitivity, selectivity, and noninvasive nature. This review provides a detailed study on the signal-based DEP methods that use the applied signal parameters, including frequency, amplitude, phase, and shape for cell/particle separation and manipulation. Rather than employing complex channels or time-consuming fabrication procedures, these methods realize sorting and detecting the cells/particles by modifying the signal parameters while using a relatively simple device. In addition, these methods can significantly impact clinical diagnostics by making low-cost and rapid separation possible. We conclude the review by discussing the technical and biological challenges of DEP techniques and providing future perspectives in this field.

The Role of Dielectrophoresis for Cancer Diagnosis and Prognosis

Liquid biopsy is emerging as a potential diagnostic tool for prostate cancer (PC) prognosis and diagnosis. Unfortunately, most circulating tumor cells (CTC) technologies, such as AdnaTest or Cellsearch®, critically rely on the epithelial cell adhesion molecule (EpCAM) marker, limiting the possibility of detecting cancer stem-like cells (CSCs) and mesenchymal-like cells (EMT-CTCs) that are present during PC progression. In this context, dielectrophoresis (DEP) is an epCAM independent, label-free enrichment system that separates rare cells simply on the basis of their specific electrical properties. As compared to other technologies, DEP may represent a superior technique in terms of running costs, cell yield and specificity. However, because of its higher complexity, it still requires further technical as well as clinical development. DEP can be improved by the use of microfluid, nanostructured materials and fluoro-imaging to increase its potential applications. In the context of cancer, the usefulness of DEP lies in its capacity to detect CTCs in the bloodstream in their epithelial, mesenchymal, or epithelial-mesenchymal phenotype forms, which should be taken into account when choosing CTC enrichment and analysis methods for PC prognosis and diagnosis.

The effect of dielectrophoresis on living cells: crossover frequencies and deregulation in gene expression

This article documents the effect of dielectrophoresis on living cells.

Membrane capacitance of thousands of single white blood cells

As label-free biomarkers, the electrical properties of single cells are widely used for cell type classification and cellular status evaluation. However, as intrinsic cellular electrical markers, previously reported membrane capacitances (e.g. specific membrane capacitance C_spec and total membrane capacitance C_mem) of white blood cells were derived from tens of single cells, lacking statistical significance due to low cell numbers. In this study, white blood cells were first separated into granulocytes and lymphocytes by density gradient centrifugation and were then aspirated through a microfluidic constriction channel to characterize both C_spec and C_mem Thousands of granulocytes (n_cell = 3327) and lymphocytes (n_cell = 3302) from 10 healthy blood donors were characterized, resulting in C_spec values of 1.95 ± 0.22 µF cm^-2 versus 2.39 ± 0.39 µF cm^-2 and C_mem values of 6.81 ± 1.09 pF versus 4.63 ± 0.57 pF. Statistically significant differences between granulocytes and lymphocytes were located for both C_spec and C_mem In addition, neural network-based pattern recognition was used to classify white blood cells, producing successful classification rates of 78.1% for C_spec and 91.3% for C_mem, respectively. These results indicate that as intrinsic bioelectrical markers, membrane capacitances may contribute to the classification of white blood cells.

Electrokinetics and Rare-Cell Detection

Lab-on-a-chip devices perform functions which are not feasible or difficult to achieve with macroscale devices. Importantly, isolating and enriching rare cells is key in health and environmental applications, such as detecting circulating tumor cells from body fluid biopsies, or pathogens from water. Within a microdevice, the dominant mechanical force on a suspended particle is the drag force as it flows through the fluid. Electrokinetic forces such as dielectrophoresis - the motion of a particle due to its polarization in the presence of a non-uniform electric field - may also be applied to manipulate particles. For instance, separation of particles can be achieved using a combination of drag and dielectrophoretic forces to precisely manipulate a particle. Understanding the interaction of electrokinetic forces, particles, and fluid flow is critical for engineering novel microsystems used for cell sorting. Determining this interaction is even more complicated when dealing with bioparticles, especially cells, due to their intrinsic complex biological properties which influence their electrical and mechanical behaviors. In order to design novel and more practical microdevices for medical, biological, and chemical applications, it is essential to have a comprehensive understanding of the mechanics of particle-fluid interaction and the dynamics of particle movement. This chapter will describe the role of electrokinetic techniques in rare cell detection and the behavior of electrokinetic microsystems.

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.

Dielectrophoresis: an assessment of its potential to aid the research and practice of drug discovery and delivery

Dielectrophoresis (DEP) is an electrokinetic technique with proven ability to discriminate and selectively manipulate cells based on their phenotype and physiological state, without the need for biological tags and markers. The DEP response of a cell is predominantly determined by the physico-chemical properties of the plasma membrane, subtle changes of which can be detected from two so-called 'cross-over' frequencies, f(xo1) and f(xo2). Membrane capacitance and structural changes can be monitored by measurement of f(xo1) at sub-megahertz frequencies, and current indications suggest that f(xo2), located above 100 MHz, is sensitive to changes of trans-membrane ion fluxes. DEP lends itself to integration in microfluidic devices and can also operate at the nanoscale to manipulate nanoparticles. Apart from measurements of f(xo1) and f(xo2), other examples where DEP could contribute to drug discovery and delivery include its ability to: enrich stem cells according to their differentiation potential, and to engineer artificial cell structures and nano-structures.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Advancing practical usage of microtechnology: a study of the functional consequences of dielectrophoresis on neural stem cells

The integration of microscale engineering, microfluidics, and AC electrokinetics such as dielectrophoresis has generated novel microsystems that enable quantitative analysis of cellular phenotype, function, and physiology. These systems are increasingly being used to assess diverse cell types, such as stem cells, so it becomes critical to thoroughly evaluate whether the systems themselves impact cell function. For example, engineered microsystems have been utilized to investigate neural stem/progenitor cells (NSPCs), which are of interest due to their potential to treat CNS disease and injury. Analysis by dielectrophoresis (DEP) microsystems determined that unlabeled NSPCs with distinct fate potential have previously unrecognized distinguishing electrophysiological characteristics, suggesting that NSPCs could be isolated by DEP microsystems without the use of cell type specific labels. To gauge the potential impact of DEP sorting on NSPCs, we investigated whether electric field exposure of varying times affected survival, proliferation, or fate potential of NSPCs in suspension. We found short-term DEP exposure (1 min or less) had no effect on NSPC survival, proliferation, or fate potential revealed by differentiation. Moreover, NSPC proliferation (measured by DNA synthesis and cell cycle kinetics) and fate potential were not altered by any length of DEP exposure (up to 30 min). However, lengthy exposure (>5 min) to frequencies near the crossover frequency (50-100 kHz) led to decreased survival of NSPCs (maximum ∼30% cell loss after 30 min). Based on experimental observations and mathematical simulations of cells in suspension, we find that frequencies near the crossover frequency generate an induced transmembrane potential that results in cell swelling and rupture. This is in contrast to the case for adherent cells since negative DEP frequencies lower than the crossover frequency generate the highest induced transmembrane potential and damage for these cells. We clarify contrasting effects of DEP on adherent and suspended cells, which are related to the cell position within the electric field and the strength of the electric field at specific distances from the electrodes. Modeling of electrode configurations predicts optimal designs to induce cell movement by DEP while limiting the induced transmembrane potential. We find DEP electric fields are not harmful to stem cells in suspension at short exposure times, thus providing a basis for developing DEP-based applications for stem cells.

Antibody-independent isolation of circulating tumor cells by continuous-flow dielectrophoresis

Circulating tumor cells (CTCs) are prognostic markers for the recurrence of cancer and may carry molecular information relevant to cancer diagnosis. Dielectrophoresis (DEP) has been proposed as a molecular marker-independent approach for isolating CTCs from blood and has been shown to be broadly applicable to different types of cancers. However, existing batch-mode microfluidic DEP methods have been unable to process 10 ml clinical blood specimens rapidly enough. To achieve the required processing rates of 10(6) nucleated cells/min, we describe a continuous flow microfluidic processing chamber into which the peripheral blood mononuclear cell fraction of a clinical specimen is slowly injected, deionized by diffusion, and then subjected to a balance of DEP, sedimentation and hydrodynamic lift forces. These forces cause tumor cells to be transported close to the floor of the chamber, while blood cells are carried about three cell diameters above them. The tumor cells are isolated by skimming them from the bottom of the chamber while the blood cells flow to waste. The principles, design, and modeling of the continuous-flow system are presented. To illustrate operation of the technology, we demonstrate the isolation of circulating colon tumor cells from clinical specimens and verify the tumor origin of these cells by molecular analysis.

Spatial manipulation of cells and organelles using single electrode dielectrophoresis

The selection, isolation, and accurate positioning of single cells in three dimensions are increasingly desirable in many areas of cell biology and tissue engineering. We describe the application of a simple and low cost dielectrophoretic device for picking out and relocating single target cells. The device consists of a single metal electrode and an AC signal generator. It does not require microfabrication technologies or sophisticated electronics. The dielectrophoretic manipulator also discriminates between live and dead cells and is capable of redistributing intracellular organelles.

ApoStream(™), a new dielectrophoretic device for antibody independent isolation and recovery of viable cancer cells from blood

Isolation and enumeration of circulating tumor cells (CTCs) are used to monitor metastatic disease progression and guide cancer therapy. However, currently available technologies are limited to cells expressing specific cell surface markers, such as epithelial cell adhesion molecule (EpCAM) or have limited specificity because they are based on cell size alone. We developed a device, ApoStream(™) that overcomes these limitations by exploiting differences in the biophysical characteristics between cancer cells and normal, healthy blood cells to capture CTCs using dielectrophoretic technology in a microfluidic flow chamber. Further, the system overcomes throughput limitations by operating in continuous mode for efficient isolation and enrichment of CTCs from blood. The performance of the device was optimized using a design of experiment approach for key operating parameters such as frequency, voltage and flow rates, and buffer formulations. Cell spiking studies were conducted using SKOV3 or MDA-MB-231 cell lines that have a high and low expression level of EpCAM, respectively, to demonstrate linearity and precision of recovery independent of EpCAM receptor levels. The average recovery of SKOV3 and MDA-MB-231 cancer cells spiked into approximately 12 × 10(6) peripheral blood mononuclear cells obtained from 7.5 ml normal human donor blood was 75.4% ± 3.1% (n = 12) and 71.2% ± 1.6% (n = 6), respectively. The intra-day and inter-day precision coefficients of variation of the device were both less than 3%. Linear regression analysis yielded a correlation coefficient (R(2)) of more than 0.99 for a spiking range of 4-2600 cells. The viability of MDA-MB-231 cancer cells captured with ApoStream was greater than 97.1% and there was no difference in cell growth up to 7 days in culture compared to controls. The ApoStream device demonstrated high precision and linearity of recovery of viable cancer cells independent of their EpCAM expression level. Isolation and enrichment of viable cancer cells from ApoStream enables molecular characterization of CTCs from a wide range of cancer types.

Biophysical characteristics reveal neural stem cell differentiation potential

Background

Distinguishing human neural stem/progenitor cell (huNSPC) populations that will predominantly generate neurons from those that produce glia is currently hampered by a lack of sufficient cell type-specific surface markers predictive of fate potential. This limits investigation of lineage-biased progenitors and their potential use as therapeutic agents. A live-cell biophysical and label-free measure of fate potential would solve this problem by obviating the need for specific cell surface markers.

Methodology/Principal Findings

We used dielectrophoresis (DEP) to analyze the biophysical, specifically electrophysiological, properties of cortical human and mouse NSPCs that vary in differentiation potential. Our data demonstrate that the electrophysiological property membrane capacitance inversely correlates with the neurogenic potential of NSPCs. Furthermore, as huNSPCs are continually passaged they decrease neuron generation and increase membrane capacitance, confirming that this parameter dynamically predicts and negatively correlates with neurogenic potential. In contrast, differences in membrane conductance between NSPCs do not consistently correlate with the ability of the cells to generate neurons. DEP crossover frequency, which is a quantitative measure of cell behavior in DEP, directly correlates with neuron generation of NSPCs, indicating a potential mechanism to separate stem cells biased to particular differentiated cell fates.

Conclusions/Significance

We show here that whole cell membrane capacitance, but not membrane conductance, reflects and predicts the neurogenic potential of human and mouse NSPCs. Stem cell biophysical characteristics therefore provide a completely novel and quantitative measure of stem cell fate potential and a label-free means to identify neuron- or glial-biased progenitors.

Dynamic physical properties of dissociated tumor cells revealed by dielectrophoretic field-flow fractionation

Metastatic disease results from the shedding of cancer cells from a solid primary tumor, their transport through the cardiovascular system as circulating tumor cells (CTCs) and their engraftment and growth at distant sites. Little is known about the properties and fate of tumor cells as they leave their growth site and travel as single cells. We applied analytical dielectrophoretic field-flow fractionation (dFFF) to study the membrane capacitance, density and hydrodynamic properties together with the size and morphology of cultured tumor cells after they were harvested and placed into single cell suspensions. After detachment, the tumor cells exhibited biophysical properties that changed with time through a process of cytoplasmic shedding whereby membrane and cytoplasm were lost. This process appeared to be distinct from the cell death mechanisms of apoptosis, anoikis and necrosis and it may explain why multiple phenotypes are seen among CTCs isolated from patients and among the tumor cells obtained from ascitic fluid of patients. The implications of dynamic biophysical properties and cytoplasmic loss for CTC migration into small blood vessels in the circulatory system, survival and gene expression are discussed. Because the total capacitance of tumor cells remained higher than blood cells even after they had shed cytoplasm, dFFF offers a compelling, antibody-independent technology for isolating viable CTCs from blood even when they are no larger than peripheral blood mononuclear cells.

Rare Cell Capture in Microfluidic Devices

This article reviews existing methods for the isolation, fractionation, or capture of rare cells in microfluidic devices. Rare cell capture devices face the challenge of maintaining the efficiency standard of traditional bulk separation methods such as flow cytometers and immunomagnetic separators while requiring very high purity of the target cell population, which is typically already at very low starting concentrations. Two major classifications of rare cell capture approaches are covered: (1) non-electrokinetic methods (e.g., immobilization via antibody or aptamer chemistry, size-based sorting, and sheath flow and streamline sorting) are discussed for applications using blood cells, cancer cells, and other mammalian cells, and (2) electrokinetic (primarily dielectrophoretic) methods using both electrode-based and insulative geometries are presented with a view towards pathogen detection, blood fractionation, and cancer cell isolation. The included methods were evaluated based on performance criteria including cell type modeled and used, number of steps/stages, cell viability, and enrichment, efficiency, and/or purity. Major areas for improvement are increasing viability and capture efficiency/purity of directly processed biological samples, as a majority of current studies only process spiked cell lines or pre-diluted/lysed samples. Despite these current challenges, multiple advances have been made in the development of devices for rare cell capture and the subsequent elucidation of new biological phenomena; this article serves to highlight this progress as well as the electrokinetic and non-electrokinetic methods that can potentially be combined to improve performance in future studies.

Cell-based sensors for quantifying the physiological impact of microsystems

Microsystems are increasingly used in the manipulation, patterning and sorting of cells. Critical to the widespread adoption of these new technologies is development of an understanding of their impact on cellular physiology. Here we show the integration of a cell-based sensor, a microfabricated electrical screening platform, and quantitative imaging to enable the first large-scale physiological screens of the impact of microsystems on cells. To perform physiological screening, we developed a cell-based sensor that reports on stress-mediated transcription (via Heat Shock Factor 1 induced expression of GFP). This cell-based sensor was quantitatively characterized using automated imaging. The integration of this quantitative physiological sensor with a microfabricated system enabled the execution of multiplexed screens across electric field strength, frequency, and application duration. Voltage sweeps indicate increasing physiological stress with increasing voltage due to Joule heating, while frequency sweeps indicate increased stress at lower frequencies (<500 kHz) compared with higher frequencies (>1 MHz) due to generation of reactive species at lower frequencies. Combined voltage and frequency sweeps enable the generation of complex maps of physiological state.

Axon repair: surgical application at a subcellular scale

Injury to the nervous system is a common occurrence after trauma. Severe cases of injury exact a tremendous personal cost and place a significant healthcare burden on society. Unlike some tissues in the body that exhibit self healing, nerve cells that are injured, particularly those in the brain and spinal cord, are incapable of regenerating circuits by themselves to restore neurological function. In recent years, researchers have begun to explore whether micro/nanoscale tools and materials can be used to address this major challenge in neuromedicine. Efforts in this area have proceeded along two lines. One is the development of new nanoscale tissue scaffold materials to act as conduits and stimulate axon regeneration. The other is the use of novel cellular-scale surgical micro/nanodevices designed to perform surgical microsplicing and the functional repair of severed axons. We discuss results generated by these two approaches and hurdles confronting both strategies.

Label-free cell separation and sorting in microfluidic systems

Cell separation and sorting are essential steps in cell biology research and in many diagnostic and therapeutic methods. Recently, there has been interest in methods which avoid the use of biochemical labels; numerous intrinsic biomarkers have been explored to identify cells including size, electrical polarizability, and hydrodynamic properties. This review highlights microfluidic techniques used for label-free discrimination and fractionation of cell populations. Microfluidic systems have been adopted to precisely handle single cells and interface with other tools for biochemical analysis. We analyzed many of these techniques, detailing their mode of separation, while concentrating on recent developments and evaluating their prospects for application. Furthermore, this was done from a perspective where inertial effects are considered important and general performance metrics were proposed which would ease comparison of reported technologies. Lastly, we assess the current state of these technologies and suggest directions which may make them more accessible.

Figure

AC-electrokinetic applications in a biological setting

Dielectrophoresis is a phenomenon which can be exploited to provide significant quantitative electrophysiological data in a range of biochemical setting, from oncology to drug discovery. This chapter seeks to elucidate those applications and the electrophysiological phenomena underpinning those applications.

Correlating short-term Ca(2+) responses with long-term protein expression after activation of single T cells

In order to elucidate the dynamics of cellular processes that are induced in context with intercellular communication, defined events along the signal transduction cascade and subsequent activation steps have to be analyzed on the level of individual cells and correlated with each other. Here we present an approach that allows the initiation of cell-cell or cell-particle interactions and the analysis of cellular reactions within various regimes while the identity of each individual cell is preserved. It utilizes dielectrophoresis (DEP) and microfluidics in a lab-on-chip system. With high spatial and temporal precision we contacted single T cells with functionalized microbeads and monitored their immediate cytosolic Ca(2+) response. After this, the cells were released from the chip system and cultivated further. Expression of the activation marker molecule CD69 was analyzed the next day and correlated with the previously recorded Ca(2+) signal for each individual cell. We found a significant difference in the patterns of Ca(2+) traces between activated and non-activated cells, which shows that Ca(2+) signals in T cells can provide early information about a later reaction of the cell. Although T cells are non-excitable cells, we also observed irregular Ca(2+) transients upon exposure to the DEP field only. These Ca(2+) signals depended on exposure time, electric field strength and field frequency. By minimizing their occurrence rate, we could identify experimental conditions that caused the least interference with the physiology of the cell.

Dielectric cell separation of fine needle aspirates from tumor xenografts

As an approach to isolating tumor cells from fine needle biopsy specimens, we investigated a dielectric cell preparation method using an in vivo xenographic tumor model. Cultured human MDA-MB-435 tumor cells were grown as solid tumors in nude mice and fine needle aspiration biopsies were conducted. Biopsied cells were suspended in sucrose medium and collected on slides patterned with microelectrode arrays (electrosmears) energized by electrical signals in the range 10 to 960 kHz. The unlabeled cells adhered to characteristic regions of the slides in accordance with their morphology as a result of dielectric forces. Tumor cells were trapped between 40 and 60 kHz and were separated according to whether they were mitotic, large and complex, or small. Damaged tumor cells were captured at between 60 and 120 kHz; granulocytes between 70 and 90 kHz; lymphocytes between 85 and 105 kHz; healthy erythrocytes between 140 and 180 kHz, and damaged erythrocytes above 180 kHz. Using intrinsic cell characteristics, the electrosmear presented cell subpopulations from fine needle aspiration biopsy specimens in a manner that is compatible with automated slide-based analysis systems. The approach has the potential to facilitate the analysis of the role of cell subpopulations in disease.

Dielectrophoretic field-flow fractionation system for detection of aquatic toxicants

Dielectrophoretic field-flow fractionation (dFFF) was applied as a contact-free way to sense changes in the plasma membrane capacitances and conductivities of cultured human HL-60 cells in response to toxicant exposure. A micropatterned electrode imposed electric forces on cells in suspension in a parabolic flow profile as they moved through a thin chamber. Relative changes in the dFFF peak elution time, reflecting changes in cell membrane area and ion permeability, were measured as indices of response during the first 150 min of exposure to eight toxicants having different single or mixed modes of action (acrylonitrile, actinomycin D, carbon tetrachloride, endosulfan, N-nitroso- N-methylurea (NMU), paraquat dichloride, puromycin, and styrene oxide). The dFFF method was compared with the cell viability assay for all toxicants and with the mitochondrial potentiometric dye assay or DNA alkaline comet assay according to the mode of action of the specific agents. Except for low doses of nucleic acid-targeting agents (actinomycin D and NMU), the dFFF method detected all toxicants more sensitively than other assays, in some cases up to 10 (5) times more sensitively than the viability approach. The results suggest the dFFF method merits additional study for possible applicability in toxicology.

Enrichment of putative stem cells from adipose tissue using dielectrophoretic field-flow fractionation

We have applied the microfluidic cell separation method of dielectrophoretic field-flow fractionation (DEP-FFF) to the enrichment of a putative stem cell population from an enzyme-digested adipose tissue derived cell suspension. A DEP-FFF separator device was constructed using a novel microfluidic-microelectronic hybrid flex-circuit fabrication approach that is scaleable and anticipates future low-cost volume manufacturing. We report the separation of a nucleated cell fraction from cell debris and the bulk of the erythrocyte population, with the relatively rare (<2% starting concentration) NG2-positive cell population (pericytes and/or putative progenitor cells) being enriched up to 14-fold. This work demonstrates a potential clinical application for DEP-FFF and further establishes the utility of the method for achieving label-free fractionation of cell subpopulations.

Effects of Dielectrophoresis on Growth, Viability and Immuno-reactivity of Listeria monocytogenes

Dielectrophoresis (DEP) has been regarded as a useful tool for manipulating biological cells prior to the detection of cells. Since DEP uses high AC electrical fields, it is important to examine whether these electrical fields in any way damage cells or affect their characteristics in subsequent analytical procedures. In this study, we investigated the effects of DEP manipulation on the characteristics of Listeria monocytogenes cells, including the immuno-reactivity to several Listeria-specific antibodies, the cell growth profile in liquid medium, and the cell viability on selective agar plates. It was found that a 1-h DEP treatment increased the cell immuno-reactivity to the commercial Listeria species-specific polyclonal antibodies (from KPL) by ~31.8% and to the C11E9 monoclonal antibodies by ~82.9%, whereas no significant changes were observed with either anti-InlB or anti-ActA antibodies. A 1-h DEP treatment did not cause any change in the growth profile of Listeria in the low conductive growth medium (LCGM); however, prolonged treatments (4 h or greater) caused significant delays in cell growth. The results of plating methods showed that a 4-h DEP treatment (5 MHz, 20 Vpp) reduced the viable cell numbers by 56.8–89.7 %. These results indicated that DEP manipulation may or may not affect the final detection signal in immuno-based detection depending on the type of antigen-antibody reaction involved. However, prolonged DEP treatment for manipulating bacterial cells could produce negative effects on the cell detection by growth-based methods. Careful selection of DEP operation conditions could avoid or minimize negative effects on subsequent cell detection performance.

Early detection of oral cancer – Is dielectrophoresis the answer?

The early detection of oral squamous cell carcinoma by non-invasive methods has the potential to hasten diagnosis and thus lessen the morbidity associated with tumour therapy. Dielectrophoresis (DEP) can non-invasively determine electrophysiological parameters such as conductivity and permittivity of cellular cytoplasm and membrane. The present study demonstrates that DEP can be utilised to characterise H357 and UP cells and reveals that there are significant differences in these parameters between malignant and more normal epithelial cell lines. The present results suggest that DEP has potential for the early detection of cancerous from non-cancerous cells in a clinical setting.

2-Dimensional MEMS dielectrophoresis device for osteoblast cell stimulation

A fixed microelectrode device for cell stimulation has been designed and fabricated using micro-electro-mechanical systems (MEMS) technology. Dielectrophoretic forces obtained from non-uniform electric fields were used for manipulating and positioning osteoblasts. The experiments show that the osteoblasts experience positive dielectrophoresis (p-DEP) when suspended in iso-osmotic culture medium and exposed to AC fields at 5 MHz frequency. Negative dielectrophoresis (n-DEP) is obtained at 0.1 MHz. The viability of osteoblasts under dielectrophoresis has been investigated. The viability values for cells exposed to DEP are nearly three times higher than the control values, indicating that dielectrophoresis may have an anabolic effect on osteoblasts.

Biophysical characterization of MDR breast cancer cell lines reveals the cytoplasm is critical in determining drug sensitivity

Dielectrophoresis (DEP) was used to examine a panel of MCF-7 cell lines comprising parental MCF-7 cells and MDR derivatives: MCF-7TaxR (paclitaxel-resistant, P-glycoprotein (P-gp) positive), MCF-7DoxR (doxorubicin-resistant MRP2 positive) plus MCF-7MDR1 (MDR1 transfected, P-gp positive). MCF-7DoxR and MCF-7MDR1 were broadly cross-resistant to natural product anticancer agents, whereas MCF-7TaxR cells were not, contrary to P-gp expression. Whilst DEP revealed modest membrane changes in MDR sub-lines, we saw significant changes in their cytoplasmic conductivity: MCF-7TaxR<MCF-7<MCF-7MDR1<MCF-7DoxR (range 0.14-0.40 S/m). Cytoplasmic conductivity is affected by the movement of molecules e.g. as in intracellular trafficking MCF-7TaxR showed a reduced membrane potential, whereas MCF-7DoxR and MCF-7MDR1 showed an increase. Thus, altered membrane potential is associated with an MDR phenotype, but in a complex manner. DEP data suggest a model whereby relative increases in cytoplasmic conductivity are correlated with MDR, whilst relative decreases equate with a sensitised phenotype e.g. MCF-7TaxR. Moreover, extent of anthracycline accumulation was inversely related to cytoplasmic conductivity. These data are representative of a model where drug sensitivity is associated with low ionic conductance (reduced cellular trafficking and ion transport) and substantial anthracycline accumulation. For classical MDR i.e. MCF-7MDR1, we saw the reverse picture. Thus, the drug resistance phenotypes of this panel of MCF-7 lines can be delineated by assessment of cytoplasmic biophysical properties using DEP.

Electrical forces for microscale cell manipulation

Electrical forces for manipulating cells at the microscale include electrophoresis and dielectrophoresis. Electrophoretic forces arise from the interaction of a cell's charge and an electric field, whereas dielectrophoresis arises from a cell's polarizability. Both forces can be used to create microsystems that separate cell mixtures into its component cell types or act as electrical "handles" to transport cells or place them in specific locations. This review explores the use of these two forces for microscale cell manipulation. We first examine the forces and electrodes used to create them, then address potential impacts on cell health, followed by examples of devices for both separating cells and handling them.

Nanoscale dielectrophoretic spectroscopy of individual immobilized mammalian blood cells

Dielectrophoretic force microscopy (DEPFM) and spectroscopy have been performed on individual intact surface-immobilized mammalian red blood cells. Dielectrophoretic force spectra were obtained in situ in approximately 125 ms and could be acquired over a region comparable in dimension to the effective diameter of a scanning probe microscopy tip. Good agreement was observed between the measured dielectrophoretic spectra and predictions using a single-shell cell model. In addition to allowing for highly localized dielectric characterization, DEPFM provided a simple means for noncontact imaging of mammalian blood cells under aqueous conditions. These studies demonstrate the feasibility of using DEPFM to monitor localized changes in membrane capacitance in real time with high spatial resolution on immobilized cells, complementing previous studies of mobile whole cells and cell suspensions.

Dielectrophoresis based-cell patterning for tissue engineering

Engineering functional tissues and organs in vitro is considered integral to regenerative medicine. Many recent cell patterning technique developments position cells at a pre-designated pattern to improve tissue engineering efficiency and quality and to facilitate 3-D cell-cell interaction exploration. Among these techniques, dielectrophoresis (DEP)-based cell patterning advantageously offers speed, ease of operation, low degree of cell damage, and precision. This article reviews recent advances in DEP-based patterning techniques, including electrode design, suitable buffer and hydrogel, effects of the electric current to cells, combination potential with other techniques, as well as efforts to generate 3-D tissues.

Rapid assessment of early biophysical changes in K562 cells during apoptosis determined using dielectrophoresis

Apoptosis, or programmed cell death, is a vital cellular process responsible for causing cells to self-terminate at the end of their useful life. Abrogation of this process is commonly linked to cancer, and rapid detection of apoptosis in vitro is vital to the discovery of new anti-cancer drugs. In this paper, we describe the application of the electrical phenomenon dielectrophoresis for detecting apoptosis at very early stages after drug induction, on the basis of changes in electrophysiological properties. Our studies have revealed that K562 (human myelogenous leukemia) cells show a persistent elevation in the cytoplasmic conductivity occurring as early as 30 minutes following exposure to staurosporine. This method therefore allows a far more rapid detection method than existing biochemical marker methods.

Rapid heterogeneous liver-cell on-chip patterning via the enhanced field-induced dielectrophoresis trap

Biomimetic heterogeneous patterning of hepatic and endothelial cells, which start from randomly distributed cells inside the microfluidic chamber, via the chip design of enhanced field-induced dielectrophoresis (DEP) trap is demonstrated and reported in this paper. The concentric-stellate-tip electrode array design in this chip generates radial-pattern electric fields for the DEP manipulation of the live liver cells. By constructing the geometric shape and the distribution of stellate tips, the DEP electrodes enhance the desired spatial electric-field gradients to guide and snare individual cells to form the desired biomimetic pattern. With this proposed microfluidic chip design, the original randomly distributed hepatocytes inside the microfluidic chamber can be manipulated in parallel and align into the desired pearl-chain array pattern. This radial pattern mimics the lobular morphology of real liver tissue. The endothelial cells, then, are snared into the additional pearl-chain array and settle at the space in-between the previous hepatic pearl-chain array. By this cell-lab chip, we demonstrate the in vitro reconstruction of the heterogeneous lobule-mimetic radial pattern with good cell viability after cell patterning. This work reports the rapid in-parallel patterning of the dual types of live liver cells via the enhanced DEP trap inside the microfluidic chip.

Harnessing dielectric forces for separations of cells, fine particles and macromolecules

A review of conventional dielectrophoresis on a microchip platform is presented. The benefits of miniaturization, some device geometries used to accomplish on-chip separations, and applications of these devices are discussed.

Dielectrophoretic segregation of different human cell types on microscope slides

A new method for preparing cells for microscopic examination is presented in which cell mixtures are fractionated by dielectrophoretic forces and simultaneously collected into characteristic zones on slides. The method traps cells directly from the suspending medium onto the slide, reducing cell loss. Furthermore, it exploits differences in the dielectric properties of the cells, which sensitively reflect their morphology. Because different cell types are trapped in characteristic zones on the slide, the technique represents an advance over existing methods for slide preparation, such as centrifugation and smears where cells are randomly distributed. In particular, the new method should aid in the detection of rare and anomalous cell subpopulations that might otherwise go unnoticed against a high background of normal cells. As well as being suitable for traditional microscopic examination and automated slide scanning approaches, it is compatible with histochemical and immunochemical techniques, as well as emerging molecular and proteomic methods. This paper describes the rationale and design of this so-called electrosmear instrumentation and shows experimental results that verify the theory and applicability of the method with model cell lines and normal peripheral blood subpopulations.

Dielectrophoretic traps for single-particle patterning

We present a novel microfabricated dielectrophoretic trap designed to pattern large arrays of single cells. Because flowing away untrapped cells is often the rate-limiting step during cell patterning, we designed the trap to be strong enough to hold particles against practical flow rates. We experimentally validated the trap strength by measuring the maximum flow rate that polystyrene beads could withstand while remaining trapped. These bead experiments have shown excellent agreement with our model predictions, without the use of fitting parameters. The model was able to provide us with a fundamental understanding of how the traps work, and additionally allowed us to establish a set of design rules for optimizing the traps for a wide range of cell sizes. We provide the foundations for an enabling technology that can be used to pattern cells in unique ways, allowing us to do novel cell biology experiments at the microscale.

Dielectrophoresis-Based Sample Handling in General-Purpose Programmable Diagnostic Instruments

As the molecular origins of disease are better understood, the need for affordable, rapid, and automated technologies that enable microscale molecular diagnostics has become apparent. Widespread use of microsystems that perform sample preparation and molecular analysis could ensure that the benefits of new biomedical discoveries are realized by a maximum number of people, even those in environments lacking any infrastructure. While progress has been made in developing miniaturized diagnostic systems, samples are generally processed off-device using labor-intensive and time-consuming traditional sample preparation methods. We present the concept of an integrated programmable general-purpose sample analysis processor (GSAP) architecture where raw samples are routed to separation and analysis functional blocks contained within a single device. Several dielectrophoresis-based methods that could serve as the foundation for building GSAP functional blocks are reviewed including methods for cell and particle sorting, cell focusing, cell ac impedance analysis, cell lysis, and the manipulation of molecules and reagent droplets.

Assessment of multidrug resistance reversal using dielectrophoresis and flow cytometry

In cancer, multidrug resistance (MDR) is the simultaneous resistance of tumor cells to different natural product anticancer drugs that have no common structure. This is an impediment to the successful treatment of many human cancers. A common correlate of MDR is the overexpression of a membrane protein, P-glycoprotein. Many studies have shown that MDR can be reversed after the use of substrate analogs, called MDR modulators. However, our understanding of MDR modulation is incomplete. In this article, we examine the electrical properties of the human leukemic cells (K562) and its MDR counterpart (K562AR) using dielectrophoresis and flow cytometry (with a membrane potential sensitive dye, DIOC5), both before and after treatment with XR9576 (a P-glycoprotein-specific MDR-reversal agent). The results show significant differences in the cytoplasmic conductivity between the cell lines themselves, but indicate no significant changes after modulation therapy. We conclude that the process of MDR modulation is not associated with changes in the electrical properties of cancer cells. Moreover, the results demonstrate that using the flow cytometry method alone, with MDR cells, may produce artifactual results--whereas in combination with dielectrophoresis, the results show the role of MDR modulators in preventing drug efflux in MDR cells.

Applications to cancer research of "lab-on-a-chip" devices based on dielectrophoresis (DEP)

The recent development of advanced analytical and bioseparation methodologies based on microarrays and biosensors is one of the strategic objectives of the so-called post-genomic. In this field, the development of microfabricated devices could bring new opportunities in several application fields, such as predictive oncology, diagnostics and anti-tumor drug research. The so called "Laboratory-on-a-chip technology", involving miniaturisation of analytical procedures, is expected to enable highly complex laboratory testing to move from the central laboratory into non-laboratory settings. The main advantages of Lab-on-a-chip devices are integration of multiple steps of different analytical procedures, large variety of applications, sub-microliter consumption of reagents and samples, and portability. One of the requirement for new generation Lab-on-a-chip devices is the possibility to be independent from additional preparative/analytical instruments. Ideally, Lab-on-a-chip devices should be able to perform with high efficiency and reproducibility both actuating and sensing procedures. In this review, we discuss applications of dielectrophoretic(DEP)-based Lab-on-a-chip devices to cancer research. The theory of dielectrophoresis as well as the description of several devices, based on spiral-shaped, parallel and arrayed electrodes are here presented. In addition, in this review we describe manipulation of cancer cells using advanced DEP-based Lab-on-a-chip devices in the absence of fluid flow and with the integration of both actuating and sensing procedures.

Membrane dielectric changes indicate induced apoptosis in HL-60 cells more sensitively than surface phosphatidylserine expression or DNA fragmentation

The specific membrane capacitance and conductivity of mammalian cells, which reflect their surface morphological complexities and membrane barrier functions, respectively, have been shown to respond to cell physiologic and pathologic changes. Here, the effects of induced apoptosis on these membrane properties of cultured human promyelocytic HL-60 cells are reported. Changes in membrane capacitance and conductivity were deduced from measurements of cellular dielectrophoretic crossover frequencies following treatment with genistein (GEN). The apparent specific cell membrane capacitance of HL-60 cells fell from an initial value of 17.6+/-0.9 to 9.1+/-0.5 mF/m(2) 4 h after treatment. Changes began within minutes of treatment and preceded both the externalization of phosphatidylserine (PS), as gauged by the Annexin V assay, and the appearance of a sub-G1 cell subpopulation, as determined through ethidium bromide staining of DNA. Treatment by the broad spectrum caspase inhibitor N-benzyloxycarbony-Val-Ala-Asp(O-methyl)-fluoromethyketone (zVAD-fmk) did not prevent these early cell membrane dielectric responses, suggesting that the caspase system was not involved. Although membrane conductivity did not alter during the first 4 h of GEN treatment, it rose significantly and progressively thereafter. Finally, as the barrier function failed and the cells became necrotic, it increased by many orders of magnitude. The effective membrane capacitance and conductivity findings serve to focus attention on the membrane as a site for early participation in apoptosis. In conjunction with our prior reports of the use of dielectric methods for cell manipulation and separation, these results demonstrate that dielectrophoretic technologies should be applicable to the rapid detection, separation, and quantification of normal, apoptotic, and necrotic cells from cell mixtures.

Particle separation by dielectrophoresis

The application of dielectrophoresis to particle discrimination, separation, and fractionation is reviewed, some advantages and disadvantages of currently available approaches are considered, and some caveats are noted.

Microsample preparation by dielectrophoresis: isolation of malaria

An important enabling factor for realising integrated micro fluidic analysis instruments for medical diagnostics purposes is front-end sample preparation. Dielectrophoresis is a method that offers great potential for cell discrimination and isolation for sample processing, and here we have applied it to the problem of isolating malaria-infected cells from blood. During development of the malarial pathogen, Plasmodium falciparum, increases occur in the ionic permeability of the plasma membrane of infected erythrocytes. When challenged by suspension in a low conductivity medium, infected cells lose internal ions while uninfected cells retain them. The resultant dielectric differences between infected and uninfected cells were exploited by dielectrophoretic manipulation in spatially inhomogeneous, travelling electrical fields produced by two types of microelectrode arrays. Parasitised cells of ring form or later stage from cultures and clinical specimens were isolated by steric dielectric field-flow-fractionation, focused at the centre of a spiral electrode array, and identified and counted. The dielectrophoretic methods require only a few micro litres of blood, and should be applicable to the production of small, low-cost automated devices for assessing parasite concentrations with potential applicability to drug sensitivity studies and the diagnosis of malaria. By simple adjustment of the electrical field parameters, other cell subpopulations that characterise disease, such as residual cancer cells in blood, can be similarly isolated and analysed.

The removal of human breast cancer cells from hematopoietic CD34+ stem cells by dielectrophoretic field-flow-fractionation

Dielectrophoretic field-flow-fractionation (DEP-FFF) was used to purge human breast cancer MDA-435 cells from hematopoietic CD34+ stem cells. An array of interdigitated microelectrodes lining the bottom surface of a thin chamber was used to generate dielectrophoretic forces that levitated the cell mixture in a fluid flow profile. CD34+ stem cells were levitated higher, were carried faster by the fluid flow, and exited the separation chamber earlier than the cancer cells. Using on-line flow cytometry, efficient separation of the cell mixture was observed in less than 12 min, and CD34+ stem cell fractions with a purity >99.2% were obtained. The method of DEP-FFF is potentially applicable to many biomedical cell separation problems, including microfluidic-scale diagnosis and preparative-scale purification of cell subpopulations.