Dielectrophoretic manipulation of cells with spiral electrodes

Interactions between electromagnetic radiation and biological systems

Even though the bioeffects of electromagnetic radiation (EMR) have been extensively investigated during the past several decades, our understandings of the bioeffects of EMR and the mechanisms of the interactions between the biological systems and the EMRs are still far from satisfactory. In this article, we introduce and summarize the consensus, controversy, limitations, and unsolved issues. The published works have investigated the EMR effects on different biological systems including humans, animals, cells, and biochemical reactions. Alternative methodologies also include dielectric spectroscopy, detection of bioelectromagnetic emissions, and theoretical predictions. In many studies, the thermal effects of the EMR are not properly controlled or considered. The frequency of the EMR investigated is limited to the commonly used bands, particularly the frequencies of the power line and the wireless communications; far fewer studies were performed for other EMR frequencies. In addition, the bioeffects of the complex EM environment were rarely discussed. In summary, our understanding of the bioeffects of the EMR is quite restrictive and further investigations are needed to answer the unsolved questions.

A di-electrophoretic simulation procedure of iron-oxide micro-particle drug attachment system for leukemia treatment using COMSOL software: a potential treatment reference for LMICs

Background

Leukemia encompasses various subtypes, each with unique characteristics and treatment approaches. The challenge lies in developing targeted therapies that can effectively address the specific genetic mutations or abnormalities associated with each subtype. Some leukemia cases may become resistant to existing treatments over time making them less susceptible to chemotherapy or other standard therapies.

Objective

Developing new treatment strategies to overcome resistance is an ongoing challenge particularly in Low and Middle Income Countries (LMICs). Computational studies using COMSOL software could provide an economical, fast and resourceful approach to the treatment of complicated cancers like leukemia.

Methods

Using COMSOL Multiphysics software, a continuous flow microfluidic device capable of delivering anti-leukemia drugs to early-stage leukemia cells has been computationally modeled using dielectrophoresis (DEP).

Results

The cell size difference enabled the micro-particle drug attachment to the leukemia cells using hydrodynamic focusing from the dielectrophoretic force. This point of care application produced a low voltage from numerically calculated electrical field and flow speed simulations.

Conclusion

Therefore, such a dielectrophoretic low voltage application model can be used as a computational treatment reference for early-stage leukemia cells with an approximate size of 5 μm.

Dielectrophoresis-Based SERS Sensors for the Detection of Cancer Cells in Microfluidic Chips

The detection of freely circulating cancer cells (CTCs) is one of the greatest challenges of modern medical diagnostics. For several years, there has been increased attention on the use of surface-enhanced Raman spectroscopy (SERS) for the detection of CTCs. SERS is a non-destructive, accurate and precise technique, and the use of special SERS platforms even enables the amplification of weak signals from biological objects. In the current study, we demonstrate the unique arrangement of the SERS technique combined with the deposition of CTCs cells on the surface of the SERS platform via a dielectrophoretic effect. The appropriate frequencies of an alternating electric field and a selected shape of the electric field can result in the efficient deposition of CTCs on the SERS platform. The geometry of the microfluidic chip, the type of the cancer cells and the positive dielectrophoretic phenomenon resulted in the trapping of CTCs on the surface of the SERS platform. We presented results for two type of breast cancer cells, MCF-7 and MDA-MB-231, deposited from the 0.1 PBS solution. The limit of detection (LOD) is 20 cells/mL, which reflects the clinical potential and usefulness of the developed approach. We also provide a proof-of-concept for these CTCs deposited on the SERS platform from blood plasma.

Continuous-Flow Nanoparticle Trapping Driven by Hybrid Electrokinetics in Microfluidics

We introduce herein an efficient microfluidic approach for continuous transport and localized collection of nanoparticles via hybrid electrokinetics, which delicately combines linear and nonlinear electrokinetics driven by a composite DC-biased AC voltage signal. The proposed technique utilizes a simple geometrical structure, in which one or a series of metal strips serving as floating electrode (FE) are attached to the substrate surface and arranged in parallel between a pair of coplanar driving electrodes (DE) in a straight microchannel. On application of a DC-biased AC electric field across the channel, nanoparticles can be transported continuously by DC bulk electroosmotic flow, and then trapped selectively onto the metal strips due to AC-field induced-charge electrokinetic (ICEK) phenomenon, which behaves as counter-rotating micro-vortices around the ideally polarizable surfaces of FE. Finite-element simulation is carried out by coupling the dual-frequency electric field, flow field and sample mass transfer in sequence, for guiding a practical design of the microfluidic nanoparticle concentrator. With the optimal device geometry, the actual performance of the technique is investigated with respect to DC bias, AC voltage amplitude, and field frequency by using both latex nanospheres (∼500 nm) and BSA molecules (∼10 nm). Our experimental observation indicates nanoparticles are always enriched into a narrow bright band on the surface of each FE, and a horizontal concentration gradient even emerges in the presence of multiple metal strips, which therefore permits localized analyte enrichment. The proposed trapping method is supposed to guide an elaborate design of flexible electrokinetic frameworks embedding FE for continuous-flow analyte manipulation in modern microfluidic systems.

Single Cell Level Dielectrophoretic Responses & Dielectrophoretic Deformations of Monocytes to Quantify Population Heterogeneity

Single-cell dielectrophoretic movement and dielectrophoretic deformation of monocyte cells were interrogated applying 20 V_pp, 50 kHz to 1 MHz signal in the 3D carbon electrode array. Heterogeneity of the monocyte population is shown in terms of the crossover frequencies, translational movement, and deformation index of the cells. The results presented that crossover range for monocytes was 100 kHz - 200 kHz, the translational movement of the cells was rapidly altered when the initial positions of the cells were in the negative dielectrophoretic region. Finally, the deformation index of the monocyte population varied from 0.5 to 1.5.

Effects of surface treatments on trapping with DC insulator-based dielectrophoresis

A central challenge in measuring the biophysical properties of cells with electrokinetic approaches is the assignment of these biophysical properties to specific biological characteristics. Changes in the electrokinetic behavior of cells may come from mutations, altered gene expression levels, post-translation modifications, or environmental effects. Here we assess the electrokinetic behavior of chemically surface-modified bacterial cells in order to gain insight into the biophysical properties that are specifically affected by changes in surface chemistry. Using E. coli as a scaffold, an amine coupling reaction was used to covalently attach glycine, spermine, bovine serum albumin (protein), or 7-amino-4-methyl-3-coumarinylacetic acid (fluorescent dye) to the free carboxylic acid groups on the surface of the cells. These populations, along with unlabeled control cells, were subject to electrokinetic and dielectrophoretic measurements to quantify any changes in the biophysical properties upon alteration. The properties associated with each electrokinetic force are discussed relative to the specific reactant used. We conclude that relatively modest and superficial changes to cell surfaces can cause measurable changes in their biophysical properties.

Sonoporation of Cells by a Parallel Stable Cavitation Microbubble Array

Sonoporation is a targeted drug delivery technique that employs cavitation microbubbles to generate transient pores in the cell membrane, allowing foreign substances to enter cells by passing through the pores. Due to the broad size distribution of microbubbles, cavitation events appear to be a random process, making it difficult to achieve controllable and efficient sonoporation. In this work a technique is reported using a microfluidic device that enables in parallel modulation of membrane permeability by an oscillating microbubble array. Multirectangular channels of uniform size are created at the sidewall to generate an array of monodispersed microbubbles, which oscillate with almost the same amplitude and resonant frequency, ensuring homogeneous sonoporation with high efficacy. Stable harmonic and high harmonic signals emitted by individual oscillating microbubbles are detected by a laser Doppler vibrometer, which indicates stable cavitation occurred. Under the influence of the acoustic radiation forces induced by the oscillating microbubble, single cells can be trapped at an oscillating microbubble surface. The sonoporation of single cells is directly influenced by the individual oscillating microbubble. The parallel sonoporation of multiple cells is achieved with an efficiency of 96.6 ± 1.74% at an acoustic pressure as low as 41.7 kPa.

Sonoporation of Cells by a Parallel Stable Cavitation Microbubble Array

Sonoporation is a targeted drug delivery technique that employs cavitation microbubbles to generate transient pores in the cell membrane, allowing foreign substances to enter cells by passing through the pores. Due to the broad size distribution of microbubbles, cavitation events appear to be a random process, making it difficult to achieve controllable and efficient sonoporation. In this work a technique is reported using a microfluidic device that enables in parallel modulation of membrane permeability by an oscillating microbubble array. Multirectangular channels of uniform size are created at the sidewall to generate an array of monodispersed microbubbles, which oscillate with almost the same amplitude and resonant frequency, ensuring homogeneous sonoporation with high efficacy. Stable harmonic and high harmonic signals emitted by individual oscillating microbubbles are detected by a laser Doppler vibrometer, which indicates stable cavitation occurred. Under the influence of the acoustic radiation forces induced by the oscillating microbubble, single cells can be trapped at an oscillating microbubble surface. The sonoporation of single cells is directly influenced by the individual oscillating microbubble. The parallel sonoporation of multiple cells is achieved with an efficiency of 96.6 ± 1.74% at an acoustic pressure as low as 41.7 kPa.

Two-dimensional computational method for generating planar electrode patterns with enhanced volumetric electric fields and its application to continuous dielectrophoretic bacterial capture

An array of microfabricated interdigitated electrodes (IDEs) is one of the most commonly used forms of electrode geometry for dielectrophoretic manipulation of biological particles in microfluidic biochips owing to simplicity of fabrication and ease of analysis. However, the dielectrophoretic force dramatically reduces as the distance from the electrode surface increases; therefore, the effective region is usually close to the electrode surface for a given electric potential difference. Here, we present a novel two-dimensional computational method for generating planar electrode patterns with enhanced volumetric electric fields, which we call the "microelectrode discretization (MED)" method. It involves discretization and reconstruction of planar electrodes followed by selection of the electrode pattern that maximizes a novel objective function, factor S, which is determined by the electric potentials on the electrode surface alone. In this study, IDEs were used as test planar electrodes. Two arrays of IDEs and respective MED-optimized electrodes were implemented in microfluidic devices for the selective capture of Escherichia coli against 1 μm-diameter polystyrene beads, and we experimentally observed that 1.4 to 35.8 times more bacteria were captured using the MED-optimized electrodes than the IDEs (p < 0.0016), with a bacterial purity against the beads of more than 99.8%. This simple design method offered simplicity of fabrication, highly enhanced electric field, and uniformity of particle capture, and can be used for many dielectrophoresis-based sensors and microfluidic systems.

Single-neuronal cell culture and monitoring platform using a fully transparent microfluidic DEP device

Dielectrophoresis using multi-electrode arrays allows a non-invasive interface with biological cells for long-term monitoring of electrophysiological parameters as well as a label-free and non-destructive technique for neuronal cell manipulation. However, experiments for neuronal cell manipulation utilizing dielectrophoresis have been constrained because dielectrophoresis devices generally function outside of the controlled environment (i.e. incubator) during the cell manipulation process, which is problematic because neurons are highly susceptible to the properties of the physiochemical environment. Furthermore, the conventional multi-electrode arrays designed to generate dielectrophoretic force are often fabricated with non-transparent materials that confound live-cell imaging. Here we present an advanced single-neuronal cell culture and monitoring platform using a fully transparent microfluidic dielectrophoresis device for the unabated monitoring of neuronal cell development and function. The device is mounted inside a sealed incubation chamber to ensure improved homeostatic conditions and reduced contamination risk. Consequently, we successfully trap and culture single neurons on a desired location and monitor their growth process over a week. The proposed single-neuronal cell culture and monitoring platform not only has significant potential to realize an in vitro ordered neuronal network, but also offers a useful tool for a wide range of neurological research and electrophysiological studies of neuronal networks.

Energy landscapes for ellipsoids in non-uniform AC electric fields

We report a closed-form analytical model for energy landscapes of ellipsoidal particles in non-uniform high-frequency AC electric fields to identify all possible particle positions and orientations. Three-dimensional equilibrium positions and orientations of prolate (r_x = r_y < r_z), oblate (r_x = r_z > r_y), and scalene (r_x≠r_y≠r_z) ellipsoids are reported vs. field frequency and amplitude, which are determined from energy landscape minima. For ellipsoids within non-uniform electric fields between co-planar parallel electrodes, the number of configurations of position and orientation is 6 for prolate, 5 for oblate, and 9 for scalene ellipsoids. In addition, for coplanar electrodes, conditions are identified when particles can be treated using a quasi-2D analysis in the plane of their most probable elevation near an underlying surface. The reported expressions are valid for time-averaged interactions of ellipsoid particles in arbitrary AC electric field configurations, such that our results are applicable to electromagnetic tweezers interacting with particles having an appropriate material property contrast with the medium in the frequency range of interest.

Biomedical microelectromechanical systems (BioMEMS): Revolution in drug delivery and analytical techniques

Advancement in microelectromechanical system has facilitated the microfabrication of polymeric substrates and the development of the novel class of controlled drug delivery devices. These vehicles have specifically tailored three dimensional physical and chemical features which together, provide the capacity to target cell, stimulate unidirectional controlled release of therapeutics and augment permeation across the barriers. Apart from drug delivery devices microfabrication technology’s offer exciting prospects to generate biomimetic gastrointestinal tract models. BioMEMS are capable of analysing biochemical liquid sample like solution of metabolites, macromolecules, proteins, nucleic acid, cells and viruses. This review summarized multidisciplinary application of biomedical microelectromechanical systems in drug delivery and its potential in analytical procedures.

Dielectrophoretic separation of micron and submicron particles: a review

This paper provides an overview on separation of micron and submicron sized biological (cells, yeast, virus, bacteria, etc.) and nonbiological particles (latex, polystyrene, CNTs, metals, etc.) by dielectrophoresis (DEP), which finds wide applications in the field of medical and environmental science. Mathematical models to predict the electric field, flow profile, and concentration profiles of the particles under the influence of DEP force have also been covered in this review. In addition, advancements made primarily in the last decade, in the area of electrode design (shape and arrangement), new materials for electrode (carbon, silicon, polymers), and geometry of the microdevice, for efficient DEP separation of particles have been highlighted.

Dielectrophoretic manipulation and separation of microparticles using microarray dot electrodes

This paper introduces a dielectrophoretic system for the manipulation and separation of microparticles. The system is composed of five layers and utilizes microarray dot electrodes. We validated our system by conducting size-dependent manipulation and separation experiments on 1, 5 and 15 μm polystyrene particles. Our findings confirm the capability of the proposed device to rapidly and efficiently manipulate and separate microparticles of various dimensions, utilizing positive and negative dielectrophoresis (DEP) effects. Larger size particles were repelled and concentrated in the center of the dot by negative DEP, while the smaller sizes were attracted and collected by the edge of the dot by positive DEP.

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.

Detecting particles flowing through interdigitated 3D microelectrodes

Counting cells in a large microchannel remains challenging and is particularly critical for in vitro assays, such as cell adhesion assays. This paper addresses this issue, by presenting the development of interdigitated three-dimensional electrodes, which are fabricated around passivated pillarshaped silicon microstructures, to detect particles in a flow. The arrays of micropillars occupy the entire channel height and detect the passage of the particle through their gaps by monitoring changes in the electrical resistance. Impedance measurements were employed in order to characterize the electrical equivalent model of the system and to detect the passage of particles in real-time. Three different geometrical micropillar configurations were evaluated and numerical simulations that supported the experimental activity were used to characterize the sensitive volume in the channel. Moreover, the signal-to-noise-ratio related to the passage of a single particle through an array was plotted as a function of the dimension and number of micropillars.

Microarray dot electrodes utilizing dielectrophoresis for cell characterization

During the last three decades; dielectrophoresis (DEP) has become a vital tool for cell manipulation and characterization due to its non-invasiveness. It is very useful in the trend towards point-of-care systems. Currently, most efforts are focused on using DEP in biomedical applications, such as the spatial manipulation of cells, the selective separation or enrichment of target cells, high-throughput molecular screening, biosensors and immunoassays. A significant amount of research on DEP has produced a wide range of microelectrode configurations. In this paper; we describe the microarray dot electrode, a promising electrode geometry to characterize and manipulate cells via DEP. The advantages offered by this type of microelectrode are also reviewed. The protocol for fabricating planar microelectrodes using photolithography is documented to demonstrate the fast and cost-effective fabrication process. Additionally; different state-of-the-art Lab-on-a-Chip (LOC) devices that have been proposed for DEP applications in the literature are reviewed. We also present our recently designed LOC device, which uses an improved microarray dot electrode configuration to address the challenges facing other devices. This type of LOC system has the capability to boost the implementation of DEP technology in practical settings such as clinical cell sorting, infection diagnosis, and enrichment of particle populations for drug development.

Counterflow dielectrophoresis for trypanosome enrichment and detection in blood

Human African trypanosomiasis or sleeping sickness is a deadly disease endemic in sub-Saharan Africa, caused by single-celled protozoan parasites. Although it has been targeted for elimination by 2020, this will only be realized if diagnosis can be improved to enable identification and treatment of afflicted patients. Existing techniques of detection are restricted by their limited field-applicability, sensitivity and capacity for automation. Microfluidic-based technologies offer the potential for highly sensitive automated devices that could achieve detection at the lowest levels of parasitemia and consequently help in the elimination programme. In this work we implement an electrokinetic technique for the separation of trypanosomes from both mouse and human blood. This technique utilises differences in polarisability between the blood cells and trypanosomes to achieve separation through opposed bi-directional movement (cell counterflow). We combine this enrichment technique with an automated image analysis detection algorithm, negating the need for a human operator.

Interdigitated comb-like electrodes for continuous separation of malignant cells from blood using dielectrophoresis

In this paper, a method for continuous flow separation of circulating malignant cells from blood in a microfluidic device using dielectrophoresis is discussed. Separation of MDA231 breast cancer cells after mixing with normal blood cells was achieved with a level of accuracy that enabled precise counting of the malignant cells, separation and eventually, sub-culturing. MDA231 cells were separated from the blood to a daughter channel using two pairs of interdigitated activated comb-like electrode structures. All experiments are performed with conductivity adjusted medium samples. The electrode pairs were positioned divergent and convergent with respect to the flow. The AC signals used in the separation are 20 V peak-to-peak with frequencies of 10-50 kHz. The separation is based on balance of magnitude of the dielectrophoretic force and hydrodynamic force. The difference in response between circulating malignant cells and normal cells at a certain band of alternating current frequencies was used for rapid separation of cancer cells from blood. The significance of these experimental results is discussed in this paper, with detailed reporting on the suspension medium, preparation of cells, flow condition and the fabrication process of the microfluidic chip. The present technique could potentially be applied to identify incident cancer at a stage and size that is not yet detectable by standard diagnostic techniques (imaging and biochemical testing). Alternatively, it may also be used to detect cancer recurrences.

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.

Large-scale arrays of picolitre chambers for single-cell analysis of large cell populations

We present a new method to analyze the cytoplasmic contents of single cells in large cell populations. This new method consists of an array of microchambers in which individual cells are collected, enclosed, and lysed to create a reaction mixture of the cytoplasm with extracellular detection agents. This approach was tested for the analysis of red blood cells in 10,000 microchambers in parallel. Single cells were routinely collected in more than 60% of microchambers, the collected cells were robustly (up to 99%) lysed by electric fields, and the cytoplasm enclosed in each microchamber was analyzed with fluorescence microscopy. Using a heterogeneous cell mixture, we verified that the new method could distinguish individual cells by cytoplasmic composition and the analysis compared well with conventional flow-cytometric evaluation of mixed cell populations. In contrast to flow-cytometry, the new method monitored single cells over time, thus characterizing the distributions of caspase activities of 5000 individual cells. This approach should be interesting for a variety of applications that would benefit from the ability to measure the distribution of cytoplasmic compounds in complex cell populations, including hematology, oncology, and immunology.

A continuous high-throughput bioparticle sorter based on 3D traveling-wave dielectrophoresis

We present a high throughput (maximum flow rate approximately 10 microl/min or linear velocity approximately 3 mm/s) continuous bio-particle sorter based on 3D traveling-wave dielectrophoresis (twDEP) at an optimum AC frequency of 500 kHz. The high throughput sorting is achieved with a sustained twDEP particle force normal to the continuous through-flow, which is applied over the entire chip by a single 3D electrode array. The design allows continuous fractionation of micron-sized particles into different downstream sub-channels based on differences in their twDEP mobility on both sides of the cross-over. Conventional DEP is integrated upstream to focus the particles into a single levitated queue to allow twDEP sorting by mobility difference and to minimize sedimentation and field-induced lysis. The 3D electrode array design minimizes the offsetting effect of nDEP (negative DEP with particle force towards regions with weak fields) on twDEP such that both forces increase monotonically with voltage to further increase the throughput. Effective focusing and separation of red blood cells from debris-filled heterogeneous samples are demonstrated, as well as size-based separation of poly-dispersed liposome suspensions into two distinct bands at 2.3 to 4.6 microm and 1.5 to 2.7 microm, at the highest throughput recorded in hand-held chips of 6 microl/min.

Fabrication and evaluation of a ratchet type dielectrophoretic device for particle analysis

Dielectrophoresis is an electrokinetic phenomenon that utilizes an asymmetric electric field to separate analytes based on differences in their polarizabilities relative to that of the suspending medium. One dielectrophoretic device architecture that offers interesting possibilities for particle transport without the use of external flow is the ratchet geometry. This paper describes the fabrication and evaluation of a novel dielectrophoretic ratchet device using a series of fine particles as test probes. The asymmetrical electric field required to selectively transport target analytes was produced using electroformed electrodes which offer the possibility of reducing convective heating and which can be used to construct a device in which all particles located within the fluidic channel are exposed to the applied field. Initial tests of this device were conducted using magnetite and polystyrene fine particles to demonstrate selective particle collection and a separation based on differences in the electrical properties of the analytes employed.

Dielectric and dielectrophoretic properties of DNA

The physical properties of DNA are quite important for molecular genetics as well as for its nanotechnological applications. Studying the interactions of alternating current (AC) electric fields with deoxyribonucleic acid (DNA) allows one to draw conclusions about these properties. These interactions are usually investigated in two different ways. In dielectric spectroscopy, a DNA solution is placed in a homogeneous AC field and electronic parameters are measured over several frequency decades in the Hz to GHz range. These electronic data are then interpreted on the basis of physico-chemical models as a result of certain phenomena on the molecular level. In dielectrophoretic studies, a DNA solution is exposed to an inhomogeneous AC field and the spatial response of few or single molecules is monitored by optical or scanning force microscopy. This response can involve translation, elongation and orientation of the molecular strings. In this review, a survey is given of the literature dealing with the dielectric and dielectrophoretic properties of DNA as well as with applications of DNA dielectrophoresis.

Dielectrophoresis for manipulation of micro/nano particles in microfluidic systems

Dielectrophoretic (DEP) force is exerted when a neutral particle is polarized in a non-uniform electric field, and depends on the dielectric properties of the particle and the suspending medium. The integration of DEP and microfluidic systems offers numerous applications for the separation, trapping, assembling, transportation, and characterization of micro/nano particles. This article reviews the applications of DEP forces in microfluidic systems. It presents the theory of dielectrophoresis, different configurations, and the applications of such systems for particle manipulation and device fabrication.

Micropatterning of poly(dimethylsiloxane) using a photoresist lift-off technique for selective electrical insulation of microelectrode arrays

A poly(dimethylsiloxane) (PDMS) patterning method based on a photoresist lift-off technique to make an electrical insulation layer with selective openings is presented. The method enables creating PDMS patterns with small features and various thicknesses without any limitation in the designs and without the need for complicated processes or expensive equipments. Patterned PDMS layers were created by spin-coating liquid phase PDMS on top of a substrate having sacrificial photoresist patterns, followed by a photoresist lift-off process. The thickness of the patterned PDMS layers could be accurately controlled (6.5-24 µm) by adjusting processing parameters such as PDMS spin-coating speeds, PDMS dilution ratios, and sacrificial photoresist thicknesses. PDMS features as small as 15 µm were successfully patterned and the effects of each processing parameter on the final patterns were investigated. Electrical resistance tests between adjacent electrodes with and without the insulation layer showed that the patterned PDMS layer functions properly as an electrical insulation layer. Biocompatibility of the patterned PDMS layer was confirmed by culturing primary neuron cells on top of the layer for up to two weeks. An extensive neuronal network was successfully formed, showing that this PDMS patterning method can be applied to various biosensing microdevices. The utility of this fabrication method was further demonstrated by successfully creating a patterned electrical insulation layer on flexible substrates containing multi-electrode arrays.

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.

Dielectrophoresis switching with vertical sidewall electrodes for microfluidic flow cytometry

A novel dielectrophoresis switching with vertical electrodes in the sidewall of microchannels for multiplexed switching of objects has been designed, fabricated and tested. With appropriate electrode design, lateral DEP force can be generated so that one can dynamically position particulates along the width of the channel. A set of interdigitated electrodes in the sidewall of the microchannels is used for the generation of non-uniform electrical fields to generate negative DEP forces that repel beads/cells from the sidewalls. A countering DEP force is generated from another set of electrodes patterned on the opposing sidewall. These lateral negative DEP forces can be adjusted by the voltage and frequency applied. By manipulating the coupled DEP forces, the particles flowing through the microchannel can be positioned at different equilibrium points along the width direction and continue to flow into different outlet channels. Experimental results for switching biological cells and polystyrene microbeads to multiple outlets (up to 5) have been achieved. This novel particle switching technique can be integrated with other particle detection components to enable microfluidic flow cytometry systems.

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.

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.

Droplet-based chemistry on a programmable micro-chip

We describe the manipulation of aqueous droplets in an immiscible, low-permittivity suspending medium. Such droplets may serve as carriers for not only air- and water-borne samples, contaminants, chemical reagents, viral and gene products, and cells, but also the reagents to process and characterise these samples. We present proofs-of-concept for droplet manipulation through dielectrophoresis by: (1). moving droplets on a two-dimensional array of electrodes, (2). achieving dielectrically-activated droplet injection, (3). fusing and reacting droplets, and (4). conducting a basic biological assay through a combination of these steps. A long-term goal of this research is to provide a platform fluidic processor technology that can form the core of versatile, automated, micro-scale devices to perform chemical and biological assays at or near the point of care, which will increase the availability of modern medicine to people who do not have ready access to modern medical institutions, and decrease the cost and delays associated with that lack of access.

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.

Dielectrophoretic dynamic light-scattering (DDLS) spectroscopy

Dielectrophoretic dynamic light-scattering (DDLS) spectroscopy is presented. DDLS identifies macromolecules based on their dielectric, or polarizability, properties. DDLS measurements are carried out in an oscillating, nonuniform electric field. The field induces macromolecules to undergo dielectrophoretic motion, which is detected by the modulation in the dynamic light-scattering autocorrelation function. The DDLS experimental setup, data analysis, and data on latex particles and yeast cells are presented.

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.

Dielectrically Addressable Microspheres Engineered Using Self-Assembled Monolayers

We have used self-assembled monolayer techniques to produce a new class of microspheres with specifically engineered dielectric properties to enable their dielectrophoretic manipulation and identification in microsystems. Dielectrophoresis is an electrokinetic phenomenon that exploits frequency-dependent polarizability differences between a particle and its suspending medium to drive the movement of the particle toward or away from the high-field regions of an inhomogeneous electric field. While dielectrophoretic methods have been used extensively for cell manipulation, separation, and identification, we wished to extend the applicability of dielectrophoresis to molecular analysis by developing a panel of dielectric microspheres or "handles". Dielectric shell theory was used to model the dielectrophoretic response for a biomimetic particle composed of a thin insulating shell over a conductive interior. We specifically sought to modulate the specific capacitance, and thereby the dielectric properties, of the particle by controlling the thickness of the insulating layer. Such a structure was fabricated by covering a gold-coated polystyrene core particle with self-assembled monolayers of alkanethiol and phospholipid. To test the prediction that the carbon chain length of these layers should dictate the dielectric properties of the particles, we constructed a panel of six microsphere types with shell compositions ranging from a C(9) alkanethiol monolayer to a C(32) hybrid bilayer membrane. These microsphere populations were distinguishable and manipulatable by dielectrophoresis in a characteristic, frequency-dependent manner as predicted by theory. Experimentally derived specific membrane capacitance values were inversely related to the insulating shell thickness and agreed with published capacitance values for planar layers of similar thicknesses. These proof of principle studies are the first to demonstrate that the dielectric properties of particles can be specifically engineered to allow their dielectrophoretic manipulation and are a first step toward the development of bead-based dielectrophoretic microsystems for multiplexed molecular separation and analysis.

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.