Cell separation on microfabricated electrodes using dielectrophoretic/gravitational field-flow fractionation

A Novel Microfluidic Strategy for Efficient Exosome Separation via Thermally Oxidized Non-Uniform Deterministic Lateral Displacement (DLD) Arrays and Dielectrophoresis (DEP) Synergy

Exosomes, with diameters ranging from 30 to 150 nm, are saucer-shaped extracellular vesicles (EVs) secreted by various type of human cells. They are present in virtually all bodily fluids. Owing to their abundant nucleic acid and protein content, exosomes have emerged as promising biomarkers for noninvasive molecular diagnostics. However, the need for exosome separation purification presents tremendous technical challenges due to their minuscule size. In recent years, microfluidic technology has garnered substantial interest as a promising alternative capable of excellent separation performance, reduced reagent consumption, and lower overall device and operation costs. In this context, we hereby propose a novel microfluidic strategy based on thermally oxidized deterministic lateral displacement (DLD) arrays with tapered shapes to enhance separation performance. We have achieved more than 90% purity in both polystyrene nanoparticle and exosome experiments. The use of thermal oxidation also significantly reduces fabrication complexity by avoiding the use of high-precision lithography. Furthermore, in a simulation model, we attempt to integrate the use of dielectrophoresis (DEP) to overcome the size-based nature of DLD and distinguish particles that are close in size but differ in biochemical compositions (e.g., lipoproteins, exomeres, retroviruses). We believe the proposed strategy heralds a versatile and innovative platform poised to enhance exosome analysis across a spectrum of biochemical applications.

Field-Flow Fractionation in Molecular Biology and Biotechnology

Field-flow fractionation (FFF) is a family of single-phase separative techniques exploited to gently separate and characterize nano- and microsystems in suspension. These techniques cover an extremely wide dynamic range and are able to separate analytes in an interval between a few nm to 100 µm size-wise (over 15 orders of magnitude mass-wise). They are flexible in terms of mobile phase and can separate the analytes in native conditions, preserving their original structures/properties as much as possible. Molecular biology is the branch of biology that studies the molecular basis of biological activity, while biotechnology deals with the technological applications of biology. The areas where biotechnologies are required include industrial, agri-food, environmental, and pharmaceutical. Many species of biological interest belong to the operational range of FFF techniques, and their application to the analysis of such samples has steadily grown in the last 30 years. This work aims to summarize the main features, milestones, and results provided by the application of FFF in the field of molecular biology and biotechnology, with a focus on the years from 2000 to 2022. After a theoretical background overview of FFF and its methodologies, the results are reported based on the nature of the samples analyzed.

Ion Bunching in Square-Wave-Driven Mobility Capillary Electrophoresis-Mass Spectrometry

The ion-bunching effect was typically produced for ion beams in the gas phase, such as in ion accelerators. In this work, ion bunching was generated for ions in a liquid channel, specifically in a mobility capillary electrophoresis-mass spectrometry (MCE-MS) setup. MCE was recently developed and coupled with MS for ion separation and the precise measurements of ion hydrodynamic radius and effective charge in solution. In conventional MCE, a DC high voltage is applied, which serves as the separation voltage. In this study, square waves were employed to replace this DC voltage, and the ion-bunching phenomenon was observed and characterized in both simulations and experiments. After applying a high voltage square wave, cations and anions would be bunched and concentrated at the positive and negative half cycle of the square wave, respectively. Accordingly, ion signal intensities detected by the following mass spectrometer could be increased by up to ∼50 folds for the aspartic acid anion. This square wave could also dissociate metal adduct cations from nucleic acid anions, which results in stronger nucleic acid ion intensities (up to ∼10 folds) with cleaner backgrounds.

Selective Single-Cell Sorting Using a Multisectorial Electroactive Nanowell Platform

Current approaches in targeted patient treatments often require the rapid isolation of specific rare target cells. Stream-based dielectrophoresis (DEP) based cell sorters have the limitation that the maximum number of sortable cell types is equivalent to the number of output channels, which makes upscaling to a higher number of different cell types technically challenging. Here, we present a microfluidic platform for selective single-cell sorting that bypasses this limitation. The platform consists of 10 000 nanoliter wells which are placed on top of interdigitated electrodes (IDEs) that facilitate dielectrophoresis-driven capture of cells. By use of a multisectorial design formed by 10 individually addressable IDE structures, our platform can capture a large number of different cell types. The sectorial approach allows for fast and straightforward modification to sort complex samples as different cell types are captured in different sectors and therefore removes the need for individual output channels per cell type. Experimental results obtained with a mixed sample of benign (MCF-10A) and malignant (MDA-MB-231) breast cells showed a target to nontarget sorting accuracy of over 95%. We envision that the high accuracy of our platform, in addition to its versatility and simplicity, will aid clinical environments where reliable sorting of varying complex samples is essential.

Optoelectronic tweezers: a versatile toolbox for nano-/micro-manipulation

This review covers the fundamentals, recent progress and state-of-the-art applications of optoelectronic tweezers technology, and demonstrates that optoelectronic tweezers technology is a versatile and powerful toolbox for nano-/micro-manipulation.

Label-free enrichment of rare unconventional circulating neoplastic cells using a microfluidic dielectrophoretic sorting device

Cellular circulating biomarkers from the primary tumor such as circulating tumor cells (CTCs) and circulating hybrid cells (CHCs) have been described to harbor tumor-like phenotype and genotype. CHCs are present in higher numbers than CTCs supporting their translational potential. Methods for isolation of CHCs do not exist and are restricted to low-throughput, time consuming, and biased methodologies. We report the development of a label-free dielectrophoretic microfluidic platform facilitating enrichment of CHCs in a high-throughput and rapid fashion by depleting healthy peripheral blood mononuclear cells (PBMCs). We demonstrated up to 96.5% depletion of PBMCs resulting in 18.6-fold enrichment of cancer cells. In PBMCs from pancreatic adenocarcinoma patients, the platform enriched neoplastic cells identified by their KRAS mutant status using droplet digital PCR with one hour of processing. Enrichment was achieved in 75% of the clinical samples analyzed, establishing this approach as a promising way to non-invasively analyze tumor cells from patients.

Effect of geometry on dielectrophoretic trap stiffness in microparticle trapping

Dielectrophoresis, an electrokinetic technique, can be used for contactless manipulation of micro- and nano-size particles suspended in a fluid. We present a 3-D microfluidic DEP device with an orthogonal electrode configuration that uses negative dielectrophoresis to trap spherical polystyrene micro-particles. Traps with three different basic geometric shapes, i.e. triangular, square, and circular, and a fixed trap area of around 900 μm² were investigated to determine the effect of trap shape on dynamics and strength of particle trapping. Effects of trap geometry were quantitatively investigated by means of trap stiffness, with applied electric potentials from 6 V_P-P to 10 V_P-P at 1 MHz. Analyzing the trap stiffness with a trapped 4.42 μm spherical particle showed that the triangular trap is the strongest, while the square shape trap is the weakest. The trap stiffness grew more than eight times in triangular traps and six times in both square and circular traps when the potential of the applied electric field was increased from 6 V_P-P to 10 V_P-P at 1 MHz. With the maximum applied potential, i.e. 10 V_P-P at 1 MHz, the stiffness of the triangular trap was 60% and 26% stronger than the square and circular trap, respectively. A finite element model of the microfluidic DEP device was developed to numerically compute the DEP force for these trap shapes. The findings from the numerical computation demonstrate good agreement with the experimental analysis. The analysis of three different trap shapes provides important insights to predict trapping location, strength of the trapping zone, and optimized geometry for high throughput particle trapping.

Dielectrophoresis-field flow fractionation for separation of particles: A critical review

Dielectrophoresis-field flow fractionation (DEP-FFF) has emerged as an efficient in-vitro, non-invasive, and label-free mechanism to manipulate a variety of nano- and micro-scaled particles in a continuous-flow manner. The technique is mainly used to fractionate particles/cells based on differences in their sizes and/or dielectric properties by employing dielectrophoretic force as an external force field applied perpendicular to the flow direction. The dielectrophoretic force is the result of a spatially non-uniform electric field in the microchannel that can be generated either by exploiting microchannel geometry or using special arrangements of microelectrode arrays. Several two-dimensional (e.g., coplanar interdigitated, castellated) and three-dimensional (e.g., top-bottom, side-wall) microelectrode designs have been successfully utilized to perform fractionation of heterogeneous samples. Although originally introduced as a separation technique, DEP-FFF has attracted increasing interest in performing other important operations such as switching, focusing, dipping, and surface functionalization of target particles. Nonetheless, the technique still suffers from limitations such as low throughput and joule heating. By comparatively analyzing recent developments that address these shortcomings, this work is a step forward towards realizing the full potential of DEP-FFF as an ideal candidate for point-of-care (POC) devices with diverse applications in the fields of biomedical, chemical, and environmental engineering.

High-Sensitivity in Dielectrophoresis Separations

The applications of dielectrophoretic (DEP) techniques for the manipulation of cells in a label-free fashion within microfluidic systems continue to grow. However, a limited number of methods exist for making highly sensitive separations that can isolate subtle phenotypic differences within a population of cells. This paper explores efforts to leverage that most compelling aspect of DEP—an actuation force that depends on particle electrical properties—in the background of phenotypic variations in cell size. Several promising approaches, centering around the application of multiple electric fields with spatially mapped magnitude and/or frequencies, are expanding the capability of DEP cell separation.

Sequential Cell-Processing System by Integrating Hydrodynamic Purification and Dielectrophoretic Trapping for Analyses of Suspended Cancer Cells

Microfluidic devices employing dielectrophoresis (DEP) have been widely studied and applied in the manipulation and analysis of single cells. However, several pre-processing steps, such as the preparation of purified target samples and buffer exchanges, are necessary to utilize DEP forces for suspended cell samples. In this paper, a sequential cell-processing device, which is composed of pre-processing modules that employ deterministic lateral displacement (DLD) and a single-cell trapping device employing an electroactive microwell array (EMA), is proposed to perform the medium exchange followed by arraying single cells sequentially using DEP. Two original microfluidic devices were efficiently integrated by using the interconnecting substrate containing rubber gaskets that tightly connect the inlet and outlet of each device. Prostate cancer cells (PC3) suspended in phosphate-buffered saline buffer mixed with microbeads were separated and then resuspended into the DEP buffer in the integrated system. Thereafter, purified PC3 cells were trapped in a microwell array by using the positive DEP force. The achieved separation and trapping efficiencies exceeded 94% and 93%, respectively, when using the integrated processing system. This study demonstrates an integrated microfluidic device by processing suspended cell samples, without the requirement of complex preparation steps.

A review of sorting, separation and isolation of cells and microbeads for biomedical applications: microfluidic approaches

We have reviewed the microfluidic approaches for cell/particle isolation and sorting, and extensively explained the mechanism behind each method.

In vitro Models and On-Chip Systems: Biomaterial Interaction Studies With Tissues Generated Using Lung Epithelial and Liver Metabolic Cell Lines

In vitro models are very important in medicine and biology, because they provide an insight into cells' and microorganisms' behavior. Since these cells and microorganisms are isolated from their natural environment, these models may not completely or precisely predict the effects on the entire organism. Improvement in this area is secured by organ-on-a-chip development. The organ-on-a-chip assumes cells cultured in a microfluidic chip. The chip simulates bioactivities, mechanics and physiological behavior of organs or organ systems, generating artificial organs in that way. There are several cell lines used so far for each tested artificial organ. For lungs, mostly used cell lines are 16HBE, A549, Calu-3, NHBE, while mostly used cell lines for liver are HepG2, Hep 3B, TPH1, etc. In this paper, state of the art for lung and liver organ-on-a-chip is presented, together with the established in vitro testing on lung and liver cell lines, with the emphasis on Calu-3 (for lung cell lines) and Hep-G2 (for liver cell lines). Primary focus in this review is to discuss different researches on the topics of lung and liver cell line models, approaches in determining fate and transport, cell partitioning, cell growth and division, as well as cell dynamics, meaning toxicity and effects. The review is finalized with current research gaps and problems, stating potential future developments in the field.

Modeling erythrocyte electrodeformation in response to amplitude modulated electric waveforms

We present a comprehensive theoretical-experimental framework for quantitative, high-throughput study of cell biomechanics. An improved electrodeformation method has been developed by combing dielectrophoresis and amplitude shift keying, a form of amplitude modulation. This method offers a potential to fully control the magnitude and rate of deformation in cell membranes. In healthy human red blood cells, nonlinear viscoelasticity of cell membranes is obtained through variable amplitude load testing. A mathematical model to predict cellular deformations is validated using the experimental results of healthy human red blood cells subjected to various types of loading. These results demonstrate new capabilities of the electrodeformation technique and the validated mathematical model to explore the effects of different loading configurations on the cellular mechanical behavior. This gives it more advantages over existing methods and can be further developed to study the effects of strain rate and loading waveform on the mechanical properties of biological cells in health and disease.

A Simplified Microfluidic Device for Particle Separation with Two Consecutive Steps: Induced Charge Electro-osmotic Prefocusing and Dielectrophoretic Separation

Continuous dielectrophoretic separation is recognized as a powerful technique for a large number of applications including early stage cancer diagnosis, water quality analysis, and stem-cell-based therapy. Generally, the prefocusing of a particle mixture into a stream is an essential process to ensure all particles are subjected to the same electric field geometry in the separation region. However, accomplishing this focusing process either requires hydrodynamic squeezing, which requires an encumbering peripheral system and a complicated operation to drive and control the fluid motion, or depends on dielectrophoretic forces, which are highly sensitive to the dielectric characterization of particles. An alternative focusing technique, induced charge electro-osmosis (ICEO), has been demonstrated to be effective in focusing an incoming mixture into a particle stream as well as nonselective regarding the particles of interest. Encouraged by these aspects, we propose a hybrid method for microparticle separation based on a delicate combination of ICEO focusing and dielectrophoretic deflection. This method involves two steps: focusing the mixture into a thin particle stream via ICEO vortex flow and separating the particles of differing dielectic properties through dielectrophoresis. To demonstrate the feasibility of the method proposed, we designed and fabricated a microfluidic chip and separated a mixture consisting of yeast cells and silica particles with an efficiency exceeding 96%. This method has good potential for flexible integration into other microfluidic chips in the future.

Fabrication of large-area polymer microfilter membranes and their application for particle and cell enrichment

A vacuum assisted UV micro-molding (VAUM) process is proposed for the fabrication of freestanding and defect-free polymer membranes based on a UV-curable methacrylate polymer (MD 700). VAUM is a highly flexible and powerful method for fabricating low cost, robust, large-area membranes over 9 × 9 cm² with pore sizes from 8 to 20 μm in diameter, 20 to 100 μm in thickness, high aspect ratio (the thickness of the polymer over the diameter of the hole is up to 15 : 1), high porosity, and a wide variety of geometrical characteristics. The fabricated freestanding membranes are flexible while mechanically robust enough for post manipulation and handling, which allows them to be cut and integrated as a plastic cartridge onto thermoplastic 3D microfluidic devices with single or double filtration stages. Very high particle capture efficiencies (≈98%) have been demonstrated in the microfluidic devices integrated with polymer membranes, even when the size of the beads is very close to the size of the pores of the microfilter. About 85% of the capture efficiency has been achieved in cancer cell trapping experiments, in which a breast cancer cell line (MDA-MB-231) spiked with phosphate-buffered saline buffer when the pore size of the filter is 8 μm and the device is operated at a flow rate of 0.1 mL min^-1.

Review: Microbial analysis in dielectrophoretic microfluidic systems

Infections caused by various known and emerging pathogenic microorganisms, including antibiotic-resistant strains, are a major threat to global health and well-being. This highlights the urgent need for detection systems for microbial identification, quantification and characterization towards assessing infections, prescribing therapies and understanding the dynamic cellular modifications. Current state-of-the-art microbial detection systems exhibit a trade-off between sensitivity and assay time, which could be alleviated by selective and label-free microbial capture onto the sensor surface from dilute samples. AC electrokinetic methods, such as dielectrophoresis, enable frequency-selective capture of viable microbial cells and spores due to polarization based on their distinguishing size, shape and sub-cellular compositional characteristics, for downstream coupling to various detection modalities. Following elucidation of the polarization mechanisms that distinguish bacterial cells from each other, as well as from mammalian cells, this review compares the microfluidic platforms for dielectrophoretic manipulation of microbials and their coupling to various detection modalities, including immuno-capture, impedance measurement, Raman spectroscopy and nucleic acid amplification methods, as well as for phenotypic assessment of microbial viability and antibiotic susceptibility. Based on the urgent need within point-of-care diagnostics towards reducing assay times and enhancing capture of the target organism, as well as the emerging interest in isolating intact microbials based on their phenotype and subcellular features, we envision widespread adoption of these label-free and selective electrokinetic techniques.

Automated electrorotation shows electrokinetic separation of pancreatic cancer cells is robust to acquired chemotherapy resistance, serum starvation, and EMT

We used automated electrorotation to measure the cytoplasmic permittivity, cytoplasmic conductivity, and specific membrane capacitance of pancreatic cancer cells under environmental perturbation to evaluate the effects of serum starvation, epithelial-to-mesenchymal transition, and evolution of chemotherapy resistance which may be associated with the development and dissemination of cancer. First, we compared gemcitabine-resistant BxPC3 subclones with gemcitabine-naive parental cells. Second, we serum-starved BxPC3 and PANC-1 cells and compared them to untreated counterparts. Third, we induced the epithelial-to-mesenchymal transition in PANC-1 cells and compared them to untreated PANC-1 cells. We also measured the electrorotation spectra of white blood cells isolated from a healthy donor. The properties from fit electrorotation spectra were used to compute dielectrophoresis (DEP) spectra and crossover frequencies. For all three experiments, the median crossover frequency for both treated and untreated pancreatic cancer cells remained significantly lower than the median crossover frequency for white blood cells. The robustness of the crossover frequency to these treatments indicates that DEP is a promising technique for enhancing capture of circulating cancer cells.

Dielectrophoretic applications for disease diagnostics using lab-on-a-chip platforms

Dielectrophoresis is a powerful technique used to distinguish distinct cellular identities in heterogeneous cell populations and to monitor changes in the cell state without the need for biochemical tags, including live and dead cells. Recent studies in the past decade have indicated that dielectrophoresis can be used to discriminate the disease state of cells by exploring the differences in the dielectric polarizabilities of the cells. Factors controlling the dielectric polarizability are dependent on the conductivity and permittivity of the cell and the suspending medium, the cell morphology, the internal structure, and the electric double layer effects associated with the charges on the cell surface. Diseased cells, such as those associated with malaria, cancer, dengue, anthrax and human African trypanosomiasis, could be spatially trapped by positive dielectrophoresis or spatially separated from other healthy cells by negative dielectrophoretic forces. The aim of this review was to provide a better and deeper understanding on how dielectrophoresis can be utilized to manipulate diseased cells. This review compiles and compares the significant findings obtained by researchers in manipulating abnormal or unhealthy cells.

Clinical-scale purification of pluripotent stem cell derivatives for cell-based therapies

Human pluripotent stem cells (hPSCs) have the potential to revolutionize cell-replacement therapies because of their ability to self renew and differentiate into nearly every cell type in the body. However, safety concerns have delayed the clinical translation of this technology. One cause for this is the capacity that hPSCs have to generate tumors after transplantation. Because of the challenges associated with achieving complete differentiation into clinically relevant cell types, the development of safe and efficient strategies for purifying committed cells is essential for advancing hPSC-based therapies. Several purification strategies have now succeeded in generating non-tumorigenic and homogeneous cell-populations. These techniques typically enrich for cells by either depleting early committed populations from teratoma-initiating hPSCs or by positively selecting cells after differentiation. Here we review the working principles behind separation methods that have facilitated the safe and controlled application of hPSC-derived cells in laboratory settings and pre-clinical research. We underscore the need for improving and integrating purification strategies within differentiation protocols in order to unlock the therapeutic potential of hPSCs.

Insulator-based dielectrophoresis with β-galactosidase in nanostructured devices

Insulator-based dielectrophoresis (iDEP) has been explored as a powerful analytical technique in recent years. Unlike with larger entities such as cells, bacteria or organelles, the mechanism of iDEP transport of proteins remains little explored. In this work, we extended the pool of proteins investigated with iDEP in nanostructured devices with β-galactosidase. Our work indicates that β-galactosidase shows concentration due to negative DEP which we compare to DEP response of immunoglobulin G (IgG) encapsulated in micelles also showing negative DEP. Experimental observations are further compared with numerical simulations to elucidate the influence of electrokinetic transport and the magnitude of DEP mobility. Numerical simulations suggest that the DEP mobility calculated using the classical model underestimates the actual contribution of DEP on the experimentally monitored concentration effect of proteins. Moreover, we observed a unique voltage dependent β-galactosidase concentration which we attribute to an additional factor influencing the protein concentration at the nanoconstrictions, namely ion concentration polarization. Our work aids in understanding factors influencing protein iDEP transport which is required for the future development of protein preconcentration or separation methods based on iDEP.

Quantitative Biomechanics of Healthy and Diseased Human Red Blood Cells using Dielectrophoresis in a Microfluidic System

We present an experimental method to quantitatively characterize the mechanical properties of a large number of biological cells by introducing controlled deformation through dielectrophoresis in a microfluidic device. We demonstrate the capability of this technique by determining the force versus deformation characteristics of healthy human red blood cells (RBCs) and RBCs infected in vitro with Plasmodium falciparum malaria parasites. These experiments clearly distinguish uninfected and healthy RBCs from infected ones, and the mechanical signatures extracted from these tests are in agreement with data from other independent methods. The method developed here thus provides a potentially helpful tool to characterize quickly and effectively the isolated biomechanical response of cells in a large population, for probing the pathological states of cells, disease diagnostics, and drug efficacy assays.

Cell pairing using microwell array electrodes based on dielectrophoresis

We report a simple device with an array of 10,000 (100 × 100) microwells for producing vertical pairs of cells in individual microwells with a rapid manipulation based on positive dielectrophoresis (p-DEP). The areas encircled with micropoles which fabricated from an electrical insulating photosensitive polymer were used as microwells. The width (14 μm) and depth (25 μm) of the individual microwells restricted the size to two vertically aligned cells. The DEP device for the manipulation of cells consisted of a microfluidic channel with an upper indium tin oxide (ITO) electrode and a lower microwell array electrode fabricated on an ITO substrate. Mouse myeloma cells stained in green were trapped within 1 s in the microwells by p-DEP by applying an alternating current voltage between the upper ITO and the lower microwell array electrode. The cells were retained inside the wells even after switching off the voltage and washing with a fluidic flow. Other myeloma cells stained in blue were then trapped in the microwells occupied by the cells stained in green to form the vertical cell pairing in the microwells. Cells stained in different colors were paired within only 1 min and a pairing efficiency of over 50% was achieved.

Microfluidic dielectrophoretic sorter using gel vertical electrodes

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

Rapid and label-free separation of Burkitt's lymphoma cells from red blood cells by optically-induced electrokinetics

Early stage detection of lymphoma cells is invaluable for providing reliable prognosis to patients. However, the purity of lymphoma cells in extracted samples from human patients' marrow is typically low. To address this issue, we report here our work on using optically-induced dielectrophoresis (ODEP) force to rapidly purify Raji cells' (a type of Burkitt's lymphoma cell) sample from red blood cells (RBCs) with a label-free process. This method utilizes dynamically moving virtual electrodes to induce negative ODEP force of varying magnitudes on the Raji cells and RBCs in an optically-induced electrokinetics (OEK) chip. Polarization models for the two types of cells that reflect their discriminate electrical properties were established. Then, the cells' differential velocities caused by a specific ODEP force field were obtained by a finite element simulation model, thereby established the theoretical basis that the two types of cells could be separated using an ODEP force field. To ensure that the ODEP force dominated the separation process, a comparison of the ODEP force with other significant electrokinetics forces was conducted using numerical results. Furthermore, the performance of the ODEP-based approach for separating Raji cells from RBCs was experimentally investigated. The results showed that these two types of cells, with different concentration ratios, could be separated rapidly using externally-applied electrical field at a driven frequency of 50 kHz at 20 V_pp. In addition, we have found that in order to facilitate ODEP-based cell separation, Raji cells' adhesion to the OEK chip's substrate should be minimized. This paper also presents our experimental results of finding the appropriate bovine serum albumin concentration in an isotonic solution to reduce cell adhesion, while maintaining suitable medium conductivity for electrokinetics-based cell separation. In short, we have demonstrated that OEK technology could be a promising tool for efficient and effective purification of Raji cells from RBCs.

Multiscale immunomagnetic enrichment of circulating tumor cells: from tubes to microchips

We review the rare cancer cell sorting technologies, with a focus on multiscale immunomagnetic approaches. Starting from the conventional magnetic activated cell sorting system, we derive the scaling laws of immunomagnetic assay and justify the recent trend of using downscaled systems for CTC studies. Furthermore, we introduce recent work on combining the immunomagnetic assay with microfluidic technology for enhanced separation. We summarize different types of in-channel micro-magnetic structures that can further increase the local magnetic field without lowering the system throughput. Related design concepts, principles, and microfabrication techniques are presented and evaluated.

Technologies for label-free separation of circulating tumor cells: from historical foundations to recent developments

Circulating tumor cells (CTCs) are malignant cells shed into the bloodstream from a tumor that have the potential to establish metastases in different anatomical sites. The separation and subsequent characterization of these cells is emerging as an important tool for both biomarker discovery and the elucidation of mechanisms of metastasis. Established methods for separating CTCs rely on biochemical markers of epithelial cells that are known to be unreliable because of epithelial-to-mesenchymal transition, which reduces expression for epithelial markers. Emerging label-free separation methods based on the biophysical and biomechanical properties of CTCs have the potential to address this key shortcoming and present greater flexibility in the subsequent characterization of these cells. In this review we first present what is known about the biophysical and biomechanical properties of CTCs from historical studies and recent research. We then review biophysical label-free technologies that have been developed for CTC separation, including techniques based on filtration, hydrodynamic chromatography, and dielectrophoresis. Finally, we evaluate these separation methods and discuss requirements for subsequent characterization of CTCs.

Detection and isolation of circulating tumor cells: principles and methods

Efforts to improve the clinical management of several cancers include finding better methods for the quantitative and qualitative analysis of circulating tumor cells (CTCs). However, detection and isolation of CTCs from the blood circulation is not a trivial task given their scarcity and the lack of reliable markers to identify these cells. With a variety of emerging technologies, a thorough review of the exploited principles and techniques as well as the trends observed in the development of these technologies can assist researchers to recognize the potential improvements and alternative approaches. To help better understand the related biological concepts, a simplified framework explaining cancer formation and its spread to other organs as well as how CTCs contribute to this process has been presented first. Then, based on their basic working-principles, the existing methods for detection and isolation of CTCs have been classified and reviewed as nucleic acid-based, physical properties-based and antibody-based methods. The review of literature suggests that antibody-based methods, particularly in conjunction with a microfluidic lab-on-a-chip setting, offer the highest overall performance for detection and isolation of CTCs. Further biological and engineering-related research is required to improve the existing methods. These include finding more specific markers for CTCs as well as enhancing the throughput, sensitivity, and analytic functionality of current devices.

Microfluidic devices for the isolation of circulating rare cells: a focus on affinity-based, dielectrophoresis, and hydrophoresis

Circulating rare cells have attracted interest because they can be good indicators of various types of diseases. For example, enumeration of circulating tumor cells is used for cancer diagnosis and prognosis, while DNA analysis or enumeration of nucleated red blood cells is useful for prenatal diagnosis or hypoxic anemia, and that of circulating stem cells to diagnose cancer metastasis. Isolation of these cells and their downstream analyses can provide significant information such as the origin and characteristics of a disease. Novel approaches based on microfluidics have many advantages, including the continuous process and integration with other components for analysis. For these reasons, a variety of microfluidic devices have been developed to isolate and characterize rare cells. In this article, we review several microfluidic devices, with a focus on affinity-based isolation (e.g. antigen-antibody reaction) and label-free separation (DEP and hydrophoresis).

Electrokinetic focusing and separation of mammalian cells in conductive biological fluids

Active manipulation of cells, such as trapping, focusing, and isolation, is essential for various bioanalytical applications. Herein, we report a hybrid electrokinetic technique for manipulating mammalian cells in physiological fluids. This technique applies a combination of negative dielectrophoretic force and hydrodynamic drag force induced by electrohydrodynamics, which is effective in conductive biological fluids. With a three-electrode configuration, the stable equilibrium positions of cells can be adjusted for separation and focusing applications. Cancer cells and white blood cells can be positioned and isolated into specific locations in the microchannel under both static and dynamic flow conditions. To investigate the sensitivity of the hybrid electrokinetic process, AC voltage, frequency, and bias dependences of the cell velocity were studied systematically. The applicability of the hybrid electrokinetic technique for manipulating cells in physiological samples is demonstrated by continuous focusing of human breast adenocarcinoma spiked in urine, buffy coats, and processed blood samples with 98% capture efficiency.

A micro device for separation of erythrocytes and leukocytes in human blood

We report a MEMS device for continuous and binary separation of leukocytes (white blood cells, WBCs) and erythrocytes (red blood cells, RBCs) by their sizes. The system consists of PDMS cylindrical pillars inside fluidic chamber on glass slides. A 420 μm lateral separation was achieved for 5 μm and 10 μm beads. The critical particle size for separation was successful predicted by simulation and experimentally determined by calibration with polystyrene beads. The separation function (lateral displacement versus particle size) was also measured experimentally. Diluted whole blood and blood fraction of concentrated leukocytes were tested with the devices with erythrocyte to leukocyte ratio in agreement with blood count results. Experiments with fluorescent labeled leukocytes confirm the separation of leukocytes from erythrocytes and measured the separation efficiency.