Direct Current Electrokinetic Particle Trapping in Insulator-Based Microfluidics: Theory and Experiments

A stepwise multi-stage continuous dielectrophoresis separation microfluidic chip with microfilter structures

The separation of target microparticles using microfluidic systems owns extensive applications in biomedical, chemical, and materials science fields. Integration of microfluidic sorting systems employing dielectrophoresis (DEP) technology has been widely investigated. However, enhancing separation efficiency, purity, stability, and integration remains a pressing issue. This study proposes a stepwise multi-stage continuous DEP separation microfluidic chip with a microfilter structure. By leveraging a stepwise electrode configuration, a gradient electric field is generated to drive target microparticles along the electric field gradient, thereby enhancing separation efficiency. Innovative integration of a microfilter structure facilitates simultaneous filtration and improves flow field distribution, thus enhancing system stability. Through the synergistic effect of stepwise electrodes and the microfilter structure, superior coupling of electric and flow fields is achieved, consequently improving the sorting purity, separation efficiency, and system stability of the DEP-based microfluidic sorting system. Validation through simulation and separation of polystyrene microspheres demonstrates the excellent particle separation performance of the proposed system. It evidently shows potential for seamless extension to various biological microparticle sorting applications, harboring significant prospects in the biomedical domain field.

Separating the Living from the Dead: An Electrophoretic Approach

Cell viability studies are essential in numerous applications, including drug development, clinical analysis, bioanalytical assessments, food safety, and environmental monitoring. Microfluidic electrokinetic (EK) devices have been proven to be effective platforms to discriminate microorganisms by their viability status. Two decades ago, live and dead Escherichia coli (E. coli) cells were trapped at distinct locations in an insulator-based EK (iEK) device with cylindrical insulating posts. At that time, the discrimination between live and dead cells was attributed to dielectrophoretic effects. This study presents the continuous separation between the live and dead E. coli cells, which was achieved primarily by combining linear and nonlinear electrophoretic effects in an iEK device. First, live and dead E. coli cells were characterized in terms of their electrophoretic migration, and then the properties of both live and dead E. coli cells were input into a mathematical model built using COMSOL Multiphysics software to identify appropriate voltages for performing an iEK separation in a T-cross iEK channel. Subsequently, live and dead cells were successfully separated experimentally in the form of an electropherogram, achieving a separation resolution of 1.87. This study demonstrated that linear and nonlinear electrophoresis phenomena are responsible for the discrimination between live and dead cells under DC electric fields in iEK devices. Continuous electrophoretic assessments, such as the one presented here, can be used to discriminate between distinct types of microorganisms including live and dead cell assessments.

Enabling the characterization of the nonlinear electrokinetic properties of particles using low voltage

Insulator-based electrokinetically driven microfluidic devices stimulated with direct current (DC) voltages are an attractive solution for particle separation, concentration, or isolation. However, to design successful particle manipulation protocols, it is mandatory to know the mobilities of electroosmosis, and linear and nonlinear electrophoresis of the microchannel/liquid/particle system. Several techniques exist to characterize the mobilities of electroosmosis and linear electrophoresis. However, only one method to characterize the mobility of nonlinear electrophoresis has been thoroughly assessed, which generally requires DC voltages larger than 1000 V and measuring particle velocity in a straight microchannel. Under such conditions, Joule heating, electrolysis, and the DC power source cost become a concern. Also, measuring particle velocity at high voltages is noisy, limiting characterization quality. Here we present a protocol-tested on 2 μm polystyrene particles-for characterizing the mobility of nonlinear electrophoresis of the liquid/particle system using a DC voltage of only 30 V and visual inspection of particle dynamics in a microchannel featuring insulating obstacles. Multiphysics numerical modelling was used to guide microchannel design and to correlate particle location during an experiment with electric field intensity. The method was validated against the conventional characterization protocol, exhibiting excellent agreement while significantly reducing measurement noise and experimental complexity.

Nonlinear Electrokinetic Methods of Particles and Cells

Nonlinear electrokinetic phenomena offer label-free, portable, and robust approaches for particle and cell assessment, including selective enrichment, separation, sorting, and characterization. The field of electrokinetics has evolved substantially since the first separation reports by Arne Tiselius in the 1930s. The last century witnessed major advances in the understanding of the weak-field theory, which supported developments in the use of linear electrophoresis and its adoption as a routine analytical technique. More recently, an improved understanding of the strong-field theory enabled the development of nonlinear electrokinetic techniques such as electrorotation, dielectrophoresis, and nonlinear electrophoresis. This review discusses the operating principles and recent applications of these three nonlinear electrokinetic phenomena for the analysis and manipulation of particles and cells and provides an overview of some of the latest developments in the field of nonlinear electrokinetics.

On the behavior of sub-micrometer polystyrene particles subjected to AC insulator-based dielectrophoresis

Polymer beads, especially polystyrene particles, have been extensively used as model species in insulator-based dielectrophoresis (iDEP) studies. Their use in alternating current iDEP (AC-iDEP) is less explored; however, an assessment in the low-frequency regime (≤10 kHz) allows to link surface conduction effects with the surface properties of polymer particles. Here, we provide a case study for various experimental conditions assessing sub-micrometer polystyrene particles with AC-iDEP and link to accepted surface conduction theory to predict and experimentally verify the observed AC-iDEP trapping behavior based on apparent zeta potential and solution conductivity. We find excellent agreement with the theoretical predictions, but also the occurrence of concentration polarization electroosmotic flow under the studied conditions, which have the potential to confound acting dielectrophoresis conditions. Furthermore, we study a case relevant to the assessment of microplastics in human and animal body fluids by mimicking the protein adsorption of high abundant proteins in blood by coating polystyrene beads with bovine serum albumin, a highly abundant protein in blood. Theoretical predictions and experimental observations confirm a difference in observed AC-iDEP behavior between coated and non-coated particles, which might be exploited for future studies of microplastics in blood to assess their exposure to humans and animals.

Capillary Electrophoresis as a Complementary Analytical Tool for the Separation and Detection of Nanoplastic Particles

Capillary electrophoresis (CE) is presented as a technique for the separation of polystyrene nanoparticles (NPs, particle diameters ranging from 30 to 300 nm) through a bare fused silica capillary and ultraviolet detection. The proposed strategy was also assessed for other types of nanoplastics, finding that stronger alkaline conditions, with an ammonium hydroxide buffer (7.5%, pH = 11.9), enabled the separation of poly(methyl methacrylate), polypropylene, and polyethylene NP for the first time by means of CE for particle diameters below 200 nm. Particle behavior has been investigated in terms of its effective electrophoretic mobility, showing an increasing absolute value of effective electrophoretic mobility from the smaller to the larger sizes. On the other hand, the absolute value of surface charge density decreased with increasing size of NPs. It was demonstrated and quantified that the separation mechanism was a combination of linear and nonlinear electrophoretic effects. This work is the first report on the quantification of nonlinear electrophoretic effects on nanoplastic particles in a CE system.

Development of a DC-Biased AC-Stimulated Microfluidic Device for the Electrokinetic Separation of Bacterial and Yeast Cells

Electrokinetic (EK) microsystems, which are capable of performing separations without the need for labeling analytes, are a rapidly growing area in microfluidics. The present work demonstrated three distinct binary microbial separations, computationally modeled and experimentally performed, in an insulator-based EK (iEK) system stimulated by DC-biased AC potentials. The separations had an increasing order of difficulty. First, a separation between cells of two distinct domains (Escherichia coli and Saccharomyces cerevisiae) was demonstrated. The second separation was for cells from the same domain but different species (Bacillus subtilis and Bacillus cereus). The last separation included cells from two closely related microbial strains of the same domain and the same species (two distinct S. cerevisiae strains). For each separation, a novel computational model, employing a continuous spatial and temporal function for predicting the particle velocity, was used to predict the retention time (tR,p) of each cell type, which aided the experimentation. All three cases resulted in separation resolution values Rs>1.5, indicating complete separation between the two cell species, with good reproducibility between the experimental repetitions (deviations < 6%) and good agreement (deviations < 18%) between the predicted tR,p and experimental (tR,e) retention time values. This study demonstrated the potential of DC-biased AC iEK systems for performing challenging microbial separations.

Manipulating the insulating post arrangement in DC-biased AC-iEK devices to improve microparticle separations

There is a growing interest in the advancement of microscale electrokinetic (EK) systems for biomedical and clinical applications, as these systems offer attractive characteristics such as portability, robustness, low sample requirements and short response time. The present work is focused on manipulating the characteristics of the insulating post arrangement in insulator-based EK (iEK) systems for separating a binary mixture of spherical microparticles with same diameter (5.1 μm), same shape, made from the same substrate material and only differing in their zeta potential by ∼14 mV. This study presents a combination of mathematical modeling and experimental separations performed by applying a low-frequency alternating current (AC) voltage in iEK systems with 12 distinct post arrangements. These iEK devices were used to systematically study the effect of three spatial characteristics of the insulating post array on particle separations: the horizontal separation and the vertical separation between posts, and introducing an offset to the posts arrangement. Through normalization of the spatial separation between the insulating posts with respect to particle diameter, guidelines to improve separation resolution for different particle mixtures possessing similar characteristics were successfully identified. The results indicated that by carefully designing the spatial arrangement of the post array, separation resolution values in the range of 1.4-2.8 can be obtained, illustrating the importance and effect of the arrangement of insulating posts on improving particle separations. This study demonstrates that iEK devices, with effectively designed spatial arrangement of the insulating post arrays, have the capabilities to perform discriminatory separations of microparticles of similar characteristics.

Nonlinear electrophoresis of nonspherical particles in a rectangular microchannel

Nonlinear electrophoresis offers advantageous prospects in microfluidic manipulation of particles over linear electrophoresis. Existing theories established for this phenomenon are entirely based on spherical particle models, some of which have been experimentally verified. However, there is no knowledge on if and how the particle shape may affect the nonlinear electrophoretic behavior. This work presents an experimental study of the nonlinear electrophoretic velocities of rigid peanut- and pear-shaped particles in a rectangular microchannel, which are compared with rigid spherical particles of similar diameter and surface charge in terms of the particle slenderness. We observe a decrease in the nonlinear electrophoretic mobility, whereas an increase in the nonlinear index of electric field when the particle slenderness increases from the peanut- to pear-shaped and spherical particles. The values of the nonlinear index for the nonspherical particles are, however, still within the theoretically predicted range for spherical particles. We also observe an enhanced nonlinear electrophoretic behavior in a lower concentration buffer solution regardless of the particle shape.

Improving device design in insulator-based electrokinetic tertiary separations

This study presents a methodology for designing effective insulator-based electrokinetic (iEK) systems for separating tertiary microparticle samples, which can be extended to more complex samples. First, 144 distinct iEK microchannel designs were built considering different shapes and arrangements of the insulating posts. Second, a mathematical model was developed with COMSOL software to predict the retention time of each particle type in the microchannel, this allowed identifying the best channel designs for two distinct types of separations: charge-based and sized-based. Third, the experimental charge-based and size-based separations of the tertiary microparticle mixtures were performed employing the improved designs identified with COMSOL modeling. The experimental results demonstrated successful separation in terms of separation resolution and good agreement with COMSOL predictions. The findings from this study show that the proposed method for device design, which combines mathematical modeling with varying post shape and post arrangement is an effective approach for identifying iEK systems capable of separating complex microparticle samples.

Effect of cell shape on nonlinear electrophoresis migration

This study contributes to the renewed interest in the study of nonlinear electrophoresis of colloidal particles. In this work the influence of cell shape on electrophoretic migration under the nonlinear regimes of moderate and strong field regimes was assessed. Four types of bacterial and yeast cells (one spherical, three non-spherical) were studied and their electrophoretic mobilities for the moderate and strong electric field magnitude regimes were estimated experimentally. The parameter of sphericity was employed to assess the effect cell shape on the nonlinear electrophoresis migration velocity and corresponding mobility under the two electric field magnitude regimes studied. As particle migration under nonlinear electrophoresis depends on particle size and shape, the results in terms of mobilities of nonlinear electrophoresis were presented as function of cell hydrodynamic diameter and sphericity. The results indicated that the magnitude of the mobilities of nonlinear electrophoresis for cells increase with increasing cell size and increase with increasing deviations from spherical shape, which is indicated by lower sphericity values. The results presented here are the very first assessment of the two types of mobilities of nonlinear electrophoresis of cells as a function of size and shape.

Particle Size-Dependent Electrophoresis in Polymer Solutions

It has long been known that the electrophoretic velocity of a charged particle is independent of its size under the thin-Debye-layer limit. This so-called Smoluchowski velocity is, however, valid only for Newtonian fluids. A couple of recent theoretical studies predict the rheology-induced particle size dependence of electrophoresis in non-Newtonian fluids. This work presents the first experimental demonstration of such dependence in viscoelastic poly(ethylene oxide) (PEO) solutions. Three different-sized particles are observed to travel at the same electrophoretic velocity in a Newtonian buffer through a rectangular microchannel. In contrast, their measured electrophoretic velocities in the PEO solution exhibit an increasing trend for larger particles, which is consistent with theoretical prediction. This particle size dependence is found to grow with an increasing concentration or length of the PEO polymer. Both trends are attributed to enhanced fluid elasticity, as characterized by the increasing elasticity number.

High-Throughput Continuous Free-Flow Dielectrophoretic Trapping of Micron-Scale Particles and Cells in Paper Using Localized Nonuniform Pore-Scale-Generated Paper-Based Electric Field Gradients

Dielectrophoresis (DEP) utilizes a spatially varying nonuniform electrical field to induce forces on suspended polarizable soft matter including particles and cells. Such nonuniformities are conventionally created using 2D or 3D micrometer-scale electrode arrays. Alternatively, insulator-based dielectrophoresis (iDEP) uses small micrometer-scale insulating structures to spatially distort and generate regions of localized field gradients to selectively trap, isolate, and concentrate bioparticles, including bacteria, viruses, red blood cells, and cancer cells from a suspending electrolyte solution. Despite significant advances in the microfabrication technology, the commercial adoption of DEP devices for soft matter manipulation remains elusive. One reason for low market penetration is a lack of low-cost and scalable fabrication methods to quickly microfabricate field-deforming structures to generate localized DEP-inducing electric field gradients. We propose here that paper-based devices can offer a low-cost and easy-to-use alternative to traditional iDEP devices. In this article, we demonstrate for the first time the ability to perform iDEP-style particle trapping using the naturally occurring micrometer-scale insulating porous structures of paper. In particular, we use polymeric laminated nonwoven fiberglass paper channels as a source of insulating structures for iDEP. We apply a flow of polarizable microparticles directly within the nonwoven channel and simultaneously drop an electric field perpendicular to the flow direction to induce DEP. We show the ability to readily trap and concentrate particles in paper by DEP with an applied voltage as low as 2 V using two different flow mechanisms: a constant fluid flow rate using an external pump and passive fluid flow by capillary wicking. Using a combination of micro computed tomography and finite element analysis, we then present a computational model to probe the microscale DEP force formation dynamics within the paper structure. This new paper-based iDEP platform enables the development of robust, low-cost, and portable next-generation iDEP systems for a wide variety of sample purification and liquid handling applications.

Zeta potential characterization using commercial microfluidic chips

Surface charge is a critical feature of microbes that affects their interactions with other cells and their environment. Because bacterial surface charge is difficult to measure directly, it is typically indirectly inferred through zeta potential measurements. Existing tools to perform such characterization are either costly and ill-suited for non-spherical samples or rely on microfluidic techniques requiring expensive fabrication equipment or specialized facilities. Here, we report the application of commercially available PMMA microfluidic chips and open-source data analysis workflows for facile electrokinetic characterization of particles and cells after prior zeta potential measurement with a Zetasizer for calibration. Our workflows eliminate the need for microchannel fabrication, increase measurement reproducibility, and make zeta potential measurements more accessible. This novel methodology was tested with functionalized 1 μm and 2 μm polystyrene beads as well as Escherichia coli MG1655 strain. Measured zeta potentials for these samples were in agreement with literature values obtained by conventional measurement methods. Taken together, our data demonstrate the power of this workflow to broadly enable critical measurements of particle and bacterial zeta potential for numerous applications.

Trends and challenges in microfluidic methods for protein manipulation-A review

Proteins are important molecules involved in an immensely large number of biological processes. Being capable of manipulating proteins is critical for developing reliable and affordable techniques to analyze and/or detect them. Such techniques would enable the production of therapeutic agents for the treatment of diseases or other biotechnological applications (e.g., bioreactors or biocatalysis). Microfluidic technology represents a potential solution to protein manipulation challenges because of the diverse phenomena that can be exploited to achieve micro- and nanoparticle manipulation. In this review, we discuss recent contributions made in the field of protein manipulation in microfluidic systems using different physicochemical principles and techniques, some of which are miniaturized versions of already established macro-scale techniques.

A Novel Micromixer That Exploits Electrokinetic Vortices Generated on a Janus Droplet Surface

Micromixers play a crucial role as essential components in microfluidic analysis systems. This paper introduces a novel micromixer designed by harnessing electrokinetic vortices arising on the surface of a Janus droplet within a microchannel. The Janus droplet is characterized by different polarities of charges on its two sides (upstream part and downstream part). In the presence of a direct current electric field, the droplet's surface generates electroosmotic flows in opposite directions, resulting in the formation of vortices and facilitating solution mixing. Results from numerical simulations suggest that a better mixing performance of the micromixer is associated with both a higher absolute value of the zeta potential ratio between the downstream and upstream surfaces of the Janus droplet and a larger downstream surface area. Additionally, this study reveals that microchannel dimensions significantly influence the performance of the micromixer. Smaller microchannel widths and heights correspond to a larger mixing index for the micromixer. The micromixer presented in this study features a simple structure, easy fabrication, and holds promising application potential.

Assessing the Discriminatory Capabilities of iEK Devices under DC and DC-Biased AC Stimulation Potentials

There is a rising need for rapid and reliable analytical methods for separating microorganisms in clinical and biomedical applications. Microscale-insulator-based electrokinetic (iEK) systems have proven to be robust platforms for assessing a wide variety of microorganisms. Traditionally, iEK systems are usually stimulated with direct-current (DC) potentials. This work presents a comparison between using DC potentials and using DC-biased alternating-current (AC) potentials in iEK systems for the separation of microorganisms. The present study, which includes mathematical modeling and experimentation, compares the separation of bacterial and yeast cells in two distinct modes by using DC and DC-biased AC potentials. The quality of both separations, assessed in terms of separation resolution (Rs), showed a complete separation (Rs = 1.51) with the application of a DC-biased low-frequency AC signal but an incomplete separation (Rs = 0.55) with the application of an RMS-equivalent DC signal. Good reproducibility between experimental repetitions (<10%) was obtained, and good agreement (~18% deviation) was observed between modeling and experimental retention times. The present study demonstrates the potential of extending the limits of iEK systems by employing DC-biased AC potentials to perform discriminatory separations of microorganisms that are difficult to separate with the application of DC potentials.

Electrokinetic Effect of a Two-Liquid Interface within a Slit Microchannel

This paper presents an investigation of the electrokinetic effect at a two-liquid (immiscible liquid-aqueous solution) interface within a slit microchannel using a three-dimensional (3D) numerical model, with a particular focus on the impact of the surface ζ-potential and liquid phase height on the interface electrokinetic velocity. The findings indicate that the direction of the interface movement depends on the ζ-potentials at the two-liquid interface and the microchannel wall. When the absolute value of the negative ζ-potential at the interface is smaller than that at the wall, the interface moves toward the negative pole of the applied direct current (DC) electric field; conversely, it moves in the opposite direction. The velocity of interface motion decreases as the height of the aqueous phase and the dynamic viscosity ratio between the immiscible liquid and the aqueous solution increase. Conversely, the velocity increases with an elevation in the height of the immiscible liquid phase and the DC electric field intensity. This study holds significant importance in elucidating the patterns of change in fluid interface electrokinetic effects and their potential applications in manipulating and separating particulate pollutants within water systems.

Fluid Elasticity-Enhanced Insulator-Based Dielectrophoresis for Sheath-Free Particle Focusing in Very Dilute Polymer Solutions

Focusing particles into a narrow stream is usually a necessary step in microfluidic flow cytometry and particle sorting. We demonstrate that the addition of a small amount of poly(ethylene oxide) (PEO) polymer into a buffer solution can reduce by almost 1 order of magnitude the threshold DC electric field for single-line dielectrophoretic focusing of particles in a constricted microchannel. The particle focusing effectiveness of this fluid elasticity-enhanced insulator-based dielectrophoresis (E-iDEP) in very dilute PEO solutions gets enhanced with the increase of the PEO molecular weight and particle size. These two trends are consistent with a theoretical analysis that accounts for the fluid elasticity effects on the electrokinetic and dielectrophoretic particle motions. Surprisingly, the particle-focusing effectiveness of E-iDEP is observed to first increase and then decrease with an increase in the PEO concentration.

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.

Leveraging microfluidic dielectrophoresis to distinguish compositional variations of lipopolysaccharide in E. coli

Lipopolysaccharide (LPS) is the unique feature that composes the outer leaflet of the Gram-negative bacterial cell envelope. Variations in LPS structures affect a number of physiological processes, including outer membrane permeability, antimicrobial resistance, recognition by the host immune system, biofilm formation, and interbacterial competition. Rapid characterization of LPS properties is crucial for studying the relationship between these LPS structural changes and bacterial physiology. However, current assessments of LPS structures require LPS extraction and purification followed by cumbersome proteomic analysis. This paper demonstrates one of the first high-throughput and non-invasive strategies to directly distinguish Escherichia coli with different LPS structures. Using a combination of three-dimensional insulator-based dielectrophoresis (3DiDEP) and cell tracking in a linear electrokinetics assay, we elucidate the effect of structural changes in E. coli LPS oligosaccharides on electrokinetic mobility and polarizability. We show that our platform is sufficiently sensitive to detect LPS structural variations at the molecular level. To correlate electrokinetic properties of LPS with the outer membrane permeability, we further examined effects of LPS structural variations on bacterial susceptibility to colistin, an antibiotic known to disrupt the outer membrane by targeting LPS. Our results suggest that microfluidic electrokinetic platforms employing 3DiDEP can be a useful tool for isolating and selecting bacteria based on their LPS glycoforms. Future iterations of these platforms could be leveraged for rapid profiling of pathogens based on their surface LPS structural identity.

Label-free microfluidic device reveals single cell phagocytic activity and screens plant medicine rapidly

Phagocytic activity is an extremely important indicator that evaluates medicinal effects related to the immune system and functions to investigate the mechanism of how a drug works under conditions such as immunological regulation, immune tolerance, inflammation, cancer, etc. Current techniques based on flow cytometry, fluorescence imaging or numbering CFUs after cell lysis for detecting phagocytosis suffer from long terms of bacteria culturing and complex preparation steps for fluorescent labeling or require a large amount of cell samples to be tested. This study aims at developing a simple and fast method for testing the phagocytic activity of unlabeled and native cells, taking advantage of very high-resolution direct current insulator-based dielectrophoresis (DC-iDEP). The properties of cells are characterized by native whole cell biophysical properties. This strategy not only eliminates the time-consuming bacterial culture work after cell lysis, but also lowers the expenses of bacteria labeling. The introduction of microfluidics reduces the sample volume or reagent needed. The analysis of the biophysical property distributions of native cells and medicine treated cells may lead to a less expensive and rapid tool for evaluating medicinal effects. Furthermore, berberine was investigated for decreasing the phagocytic activity of macrophages and used for comparison of activities. This study works on establishing a label-free, unbiased, and non-destructive method to determine cell phagocytic activity and investigate its use in evaluating medicinal effects on phagocytosis in a single step within a short time.

Electromagnetic Forces and Torques: From Dielectrophoresis to Optical Tweezers

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

Separation of Cells and Microparticles in Insulator-Based Electrokinetic Systems

Presented here is the first continuous separation of microparticles and cells of similar characteristics employing linear and nonlinear electrokinetic phenomena in an insulator-based electrokinetic (iEK) system. By utilizing devices with insulating features, which distort the electric field distribution, it is possible to combine linear and nonlinear EK phenomena, resulting in highly effective separation schemes that leverage the new advancements in nonlinear electrophoresis. This work combines mathematical modeling and experimentation to separate four distinct binary mixtures of particles and cells. A computational model with COMSOL Multiphysics was used to predict the retention times (t_R,p) of the particles and cells in iEK devices. Then, the experimental separations were carried out using the conditions identified with the model, where the experimental retention time (t_R,e) of the particles and cells was measured. A total of four distinct separations of binary mixtures were performed by increasing the level of difficulty. For the first separation, two types of polystyrene microparticles, selected to mimic Escherichia coli and Saccharomyces cerevisiae cells, were separated. By leveraging the knowledge gathered from the first separation, a mixture of cells of distinct domains and significant size differences, E. coli and S. cerevisiae, was successfully separated. The third separation also featured cells of different domains but closer in size: Bacillus cereus versus S. cerevisiae. The last separation included cells in the same domain and genus, B. cereus versus Bacillus subtilis. Separation results were evaluated in terms of number of plates (N) and separation resolution (R_s), where R_s values for all separations were above 1.5, illustrating complete separations. Experimental results were in agreement with modeling results in terms of retention times, with deviations in the 6-27% range, while the variation between repetitions was between 2 and 18%, demonstrating good reproducibility. This report is the first prediction of the retention time of cells in iEK systems.

Recent advances and challenges in temperature monitoring and control in microfluidic devices

Temperature is a critical-yet sometimes overlooked-parameter in microfluidics. Microfluidic devices can experience heating inside their channels during operation due to underlying physicochemical phenomena occurring therein. Such heating, whether required or not, must be monitored to ensure adequate device operation. Therefore, different techniques have been developed to measure and control temperature in microfluidic devices. In this contribution, the operating principles and applications of these techniques are reviewed. Temperature-monitoring instruments revised herein include thermocouples, thermistors, and custom-built temperature sensors. Of these, thermocouples exhibit the widest operating range; thermistors feature the highest accuracy; and custom-built temperature sensors demonstrate the best transduction. On the other hand, temperature control methods can be classified as external- or integrated-methods. Within the external methods, microheaters are shown to be the most adequate when working with biological samples, whereas Peltier elements are most useful in applications that require the development of temperature gradients. In contrast, integrated methods are based on chemical and physical properties, structural arrangements, which are characterized by their low fabrication cost and a wide range of applications. The potential integration of these platforms with the Internet of Things technology is discussed as a potential new trend in the field.

Nonlinear electrophoresis of dielectric particles in Newtonian fluids

In classical electrokinetics, the electrophoretic velocity of a dielectric particle is a linear function of the applied electric field. Theoretical studies have predicted the onset of nonlinear electrophoresis at high electric fields because of the nonuniform surface conduction over the curved particle. However, experimental studies have been left behind and are insufficient for a fundamental understanding of the parametric effects on nonlinear electrophoresis. We present in this work a systematic experimental study of the effects of buffer concentration, particle size, and particle zeta potential on the electrophoretic velocity of polystyrene particles in a straight rectangular microchannel for electric fields of up to 3 kV/cm. The measured nonlinear electrophoretic particle velocity is found to exhibit a 2(±0.5)-order dependence on the applied electric field, which appears to be within the theoretically predicted 3- and 3/2-order dependences for low and high electric fields, respectively. Moreover, the obtained nonlinear electrophoretic particle mobility increases with decreasing buffer concentration (for the same particle) and particle size (for particles with similar zeta potentials) or increasing particle zeta potential (for particles with similar sizes). These observations are all consistent with the theoretical predictions for high electric fields.

Dielectrophoresis: An Approach to Increase Sensitivity, Reduce Response Time and to Suppress Nonspecific Binding in Biosensors?

The performance of receptor-based biosensors is often limited by either diffusion of the analyte causing unreasonable long assay times or a lack of specificity limiting the sensitivity due to the noise of nonspecific binding. Alternating current (AC) electrokinetics and its effect on biosensing is an increasing field of research dedicated to address this issue and can improve mass transfer of the analyte by electrothermal effects, electroosmosis, or dielectrophoresis (DEP). Accordingly, several works have shown improved sensitivity and lowered assay times by order of magnitude thanks to the improved mass transfer with these techniques. To realize high sensitivity in real samples with realistic sample matrix avoiding nonspecific binding is critical and the improved mass transfer should ideally be specific to the target analyte. In this paper we cover recent approaches to combine biosensors with DEP, which is the AC kinetic approach with the highest selectivity. We conclude that while associated with many challenges, for several applications the approach could be beneficial, especially if more work is dedicated to minimizing nonspecific bindings, for which DEP offers interesting perspectives.

Dielectrophoresis-Based Universal Membrane Antifouling Strategy toward Colloidal Foulants

Membrane fouling compromises the benefits of membrane technology, leading to its performance deterioration and incremental cost. Coupling with an electric field has been attractive but is limited by the electrical dependence of the electrophoresis (EP) mechanism and undesired faradic reactions. This study reports a universal dielectrophoresis-based (DEP) membrane antifouling strategy for electronegative, electropositive, and neutral colloidal foulants, which depends on the particle polarizability rather than its charge. The porous Ni@PVDF model electroconductive membrane was fabricated to construct a nonuniform electric field inducing DEP, while applying a low voltage avoided side electrochemical reactions. For electronegative SiO₂(-) and electropositive Al₂O₃(+) particles with a lower relative permittivity than the medium water (78), the membrane permeability all remarkably increased by 90.1% under AC/DC (±1.0 V) fields. By contrast, serious membrane fouling occurred for the BaTiO₃ colloids with a higher relative permittivity (∼2000). Notably, the permittivity of nearly all colloids in wastewater treatment is much less than that of water, which makes the dielectrophoresis-based antifouling strategy universal. The theoretical simulation systematically analyzed the forces on particles including DEP, EP, and others, indicating that the formed protected area on the membrane pore wall by DEP forces prevented the irreversible membrane blockage of colloids and facilitated loose cake layer formation for alleviating membrane fouling. In brief, this work reported a hopeful concept for dielectrophoresis-based membrane antifouling and verified its antifouling mechanism.

High-Resolution Charge-Based Electrokinetic Separation of Almost Identical Microparticles

Well-established techniques, e.g., chromatography and capillary electrophoresis, are available for separating nanosized particles, such as proteins. However, similar techniques for separating micron-sized particles are still needed. Insulator-based electrokinetic (iEK) systems can achieve efficient microparticle separations by combining linear and nonlinear EK phenomena. Of particular interest are charge-based separations, which could be employed for separating similar microorganisms, such as bacterial cells of the same size, same genus, or same strain. Several groups have reported charge-based separations of microparticles where a zeta potential difference of at least 40 mV between the microparticles was required. The present work pushes the limit of the discriminatory capabilities of iEK systems by reporting the charged-based separation of two microparticles of the same size (5.1 μm), same shape, same substrate material, and with a small difference in particle zeta potentials of only 3.6 mV, which is less than 10% of the difference in previous studies. By building an accurate COMSOL Multiphysics model, which correctly accounts for dielectrophoresis and electrophoresis of the second kind, it was possible to identify the conditions to achieve this challenging separation. Furthermore, the COMSOL model allowed predicting particle retention times (t_R,p) which were compared with experimental values (t_R,e). The separations results had excellent reproducibility in terms of t_R,e with variations of only 9% and 11% between repetitions. These findings demonstrate that, by following a robust protocol that involves modeling and experimental work, it is possible to discriminate between highly similar particles, with much smaller differences in electrical charge than previously reported.

3D Numerical Study of the Electrokinetic Motion of a Microparticle Adsorbed at a Horizontal Oil/Water Interface in an Infinite Domain

This work builds a three-dimensional (3D) simulation model and studies the electrokinetic velocity of a microparticle adsorbed at a horizontal oil/water interface in an infinite domain. The effects of the interface zeta potentials, the electric field, the oil dynamic viscosity, and the contact angle between the particle and the oil/water interface are investigated in detail. The results show that in an infinite oil/water interface system, both the negatively charged mobile oil/water interface and the negatively charged particle adsorbed to it move toward the positive electrode of the DC electric field, and the particle velocity increases along with the contact angle, the electric field strength, and the absolute values of negative zeta potential of both the particle and the oil/water interface. When the oil/water interface is positively charged with a relatively small zeta potential, the negatively charged microparticle also moves in the opposite direction of the electric field. The larger the oil dynamic viscosity, the smaller the electrokinetic velocity of the microparticle at the interface. Additionally, the numerical simulation results are compared with the reported experiment results under the same conditions, and they have good agreement.

Protein Dielectrophoresis: A Tale of Two Clausius-Mossottis-Or Something Else?

Standard DEP theory, based on the Clausius-Mossotti (CM) factor derived from solving the boundary-value problem of macroscopic electrostatics, fails to describe the dielectrophoresis (DEP) data obtained for 22 different globular proteins over the past three decades. The calculated DEP force appears far too small to overcome the dispersive forces associated with Brownian motion. An empirical theory, employing the equivalent of a molecular version of the macroscopic CM-factor, predicts a protein's DEP response from the magnitude of the dielectric β-dispersion produced by its relaxing permanent dipole moment. A new theory, supported by molecular dynamics simulations, replaces the macroscopic boundary-value problem with calculation of the cross-correlation between the protein and water dipoles of its hydration shell. The empirical and formal theory predicts a positive DEP response for protein molecules up to MHz frequencies, a result consistently reported by electrode-based (eDEP) experiments. However, insulator-based (iDEP) experiments have reported negative DEP responses. This could result from crystallization or aggregation of the proteins (for which standard DEP theory predicts negative DEP) or the dominating influences of electrothermal and other electrokinetic (some non-linear) forces now being considered in iDEP theory.

Multiphysics microfluidics for cell manipulation and separation: a review

Multiphysics microfluidics, which combines multiple functional physical processes in a microfluidics platform, is an emerging research area that has attracted increasing interest for diverse biomedical applications. Multiphysics microfluidics is expected to overcome the limitations of individual physical phenomena through combining their advantages. Furthermore, multiphysics microfluidics is superior for cell manipulation due to its high precision, better sensitivity, real-time tunability, and multi-target sorting capabilities. These exciting features motivate us to review this state-of-the-art field and reassess the feasibility of coupling multiple physical processes. To confine the scope of this paper, we mainly focus on five common forces in microfluidics: inertial lift, elastic, dielectrophoresis (DEP), magnetophoresis (MP), and acoustic forces. This review first explains the working mechanisms of single physical phenomena. Next, we classify multiphysics techniques in terms of cascaded connections and physical coupling, and we elaborate on combinations of designs and working mechanisms in systems reported in the literature to date. Finally, we discuss the possibility of combining multiple physical processes and associated design schemes and propose several promising future directions.

Amplification factor in DC insulator-based electrokinetic devices: a theoretical, numerical, and experimental approach to operation voltage reduction for particle trapping

Insulator-based microfluidic devices are attractive for handling biological samples due to their simple fabrication, low-cost, and efficiency in particle manipulation. However, their widespread application is limited by the high operation voltages required to achieve particle trapping. We present a theoretical, numerical, and experimental study that demonstrates these voltages can be significantly reduced (to sub-100 V) in direct-current insulator-based electrokinetic (DC-iEK) devices for micron-sized particles. To achieve this, we introduce the concept of the amplification factor-the fold-increase in electric field magnitude due to the presence of an insulator constriction-and use it to compare the performance of different microchannel designs and to direct our design optimization process. To illustrate the effect of using constrictions with smooth and sharp features on the amplification factor, geometries with circular posts and semi-triangular posts were used. These were theoretically approximated in two different systems of coordinates (bipolar and elliptic), allowing us to provide, for the first time, explicit electric field amplification scaling laws. Finite element simulations were performed to approximate the 3D insulator geometries and provide a parametric study of the effect of changing different geometrical features. These simulations were used to predict particle trapping voltages for four different single-layer microfluidic devices using two particle suspensions (2 and 6.8 μm in size). The general agreement between our models demonstrates the feasibility of using the amplification factor, in combination with nonlinear electrokinetic theory, to meet the prerequisites for the development of portable DC-iEK microfluidic systems.

Microscale electrokinetic-based analysis of intact cells and viruses

Miniaturized electrokinetic methods have proven to be robust platforms for the analysis and assessment of intact microorganisms, offering short response times and higher integration than their bench-scale counterparts. The present review article discusses three types of electrokinetic-based methodologies: electromigration or motion-based techniques, electrode-based electrokinetics, and insulator-based electrokinetics. The fundamentals of each type of methodology are discussed and relevant examples from recent reports are examined, to provide the reader with an overview of the state-of-the-art on the latest advancements on the analysis of intact cells and viruses with microscale electrokinetic techniques. The concluding remarks discuss the potential applications and future directions.

Electrically driven microfluidic platforms for exosome manipulation and characterization

Exosomes are small extracellular vesicles that can be obtained from several body fluids such as blood and urine. Since these vesicles can carry biomarkers and other cargo, they have application in healthcare diagnostics and therapeutics, such as liquid biopsies and drug delivery. Yet, their identification and separation from a sample remain challenging due to their high degree of heterogeneity and their co-existence with other bioparticles. In this contribution, we review the state-of-the-art on electrical techniques and methods to displace, selectively trap/isolate, and detect/characterize exosomes in microfluidic devices. Although there are many reviews focused on exosome separation using benchtop equipment, such as ultracentrifugation, there are limited reviews focusing on the use of electrical phenomena in microfluidic devices for exosome manipulation and detection. Here, we highlight contributions published during the past decade and present perspectives for this research field for the near future, outlining challenges to address in years to come.

The latest advances on nonlinear insulator-based electrokinetic microsystems under direct current and low-frequency alternating current fields: a review

This review article presents an overview of the evolution of the field of insulator-based dielectrophoresis (iDEP); in particular, it focuses on insulator-based electrokinetic (iEK) systems stimulated with direct current and low-frequency(< 1 kHz) AC electric fields. The article covers the surge of iDEP as a research field where many different device designs were developed, from microchannels with arrays of insulating posts to devices with curved walls and nano- and micropipettes. All of these systems allowed for the manipulation and separation of a wide array of particles, ranging from macromolecules to microorganisms, including clinical and biomedical applications. Recent experimental reports, supported by important theoretical studies in the field of physics and colloids, brought attention to the effects of electrophoresis of the second kind in these systems. These recent findings suggest that DEP is not the main force behind particle trapping, as it was believed for the last two decades. This new research suggests that particle trapping, under DC and low-frequency AC potentials, mainly results from a balance between electroosmotic and electrophoretic effects (linear and nonlinear); although DEP is present in these systems, it is not a dominant force. Considering these recent studies, it is proposed to rename this field from DC-iDEP to DC-iEK (and low-frequency AC-iDEP to low-frequency AC-iEK). Whereas much research is still needed, this is an exciting time in the field of microscale EK systems, as these new findings seem to explain the challenges with modeling particle migration and trapping in iEK devices, and provide perhaps a better understanding of the mechanisms behind particle trapping.

A review: Dielectrophoresis for characterizing and separating similar cell subpopulations based on bioelectric property changes due to disease progression and therapy assessment

This paper reviews the use of dielectrophoresis for high-fidelity separations and characterizations of subpopulations to highlight the recent advances in the electrokinetic field as well as provide insight into its progress toward commercialization. The role of cell subpopulations in heterogeneous clinical samples has been studied to deduce their role in disease progression and therapy resistance for instances such as cancer, tissue regeneration, and bacterial infection. Dielectrophoresis (DEP), a label-free electrokinetic technique, has been used to characterize and separate target subpopulations from mixed samples to determine disease severity, cell stemness, and drug efficacy. Despite its high sensitivity to characterize similar or related cells based on their differing bioelectric signatures, DEP has been slowly adopted both commercially and clinically. This review addresses the use of dielectrophoresis for the identification of target cell subtypes in stem cells, cancer cells, blood cells, and bacterial cells dependent on cell state and therapy exposure and addresses commercialization efforts in light of its sensitivity and future perspectives of the technology, both commercially and academically.

Dielectrophoresis applications in biomedical field and future perspectives in biomedical technology

Dielectrophoresis (DEP) is a technique to manipulate trajectories of polarisable particles in non-uniform electric fields by utilising unique dielectric properties. The manipulation of a cell using DEP has been demonstrated in various modes, thereby indicating potential applications in the biomedical field. In this review, recent DEP applications in the biomedical field are discussed. This review is intended to highlight research work that shows significant approach related to dielectrophoresis application in biomedical field reported between 2016 and 2020. Firstly, single-shell model and multiple-shell model of cells are introduced. Current device structures and recently introduced electrode patterns for DEP applications are discussed. Secondly, the biomedical uses of DEP in liquid biopsies, stem cell therapies, and diagnosis of infectious diseases due to bacteria and viruses are presented. Finally, the challenges in DEP research are discussed, and the reported solutions are explained. DEP's potential research directions are mentioned. This article is protected by copyright. All rights reserved.

Measurement of the amplitude and phase of the electrophoretic and electroosmotic mobility based on fast single-particle tracking

The electrophoretic mobility of micron-scale particles is of crucial importance in applications related to pharmacy, electronic ink displays, printing, and food technology as well as in fundamental studies in these fields. Particle mobility measurements are often limited in accuracy because they are based on ensemble averages and because a correction for electroosmosis needs to be made based on a model. Single-particle approaches are better suited for examining polydisperse samples, but existing implementations either require multiple measurements to take the effect of electroosmosis into account or are limited in accuracy by short measurement times. In this work, accurate characterization of monodisperse and polydisperse samples is achieved by measuring the electrophoretic mobility on a particle-to-particle basis while suppressing electroosmosis. Electroosmosis can be suppressed by measuring in the middle of a microchannel while applying an AC voltage with a sufficiently high frequency. An accurate measurement of the electrophoretic mobility is obtained by analyzing the oscillating particle motion for 1.5 s per particle with a high-speed camera measuring at 850 Hz , synchronized to the applied electric field. Attention is paid to take into account the effect of the rolling shutter and the non-uniform sampling in order to obtain the accurate amplitude and phase of the electrophoretic mobility. The accuracy of method is experimentally verified and compared with a commercial apparatus for polystyrene microspheres in water. The method is further demonstrated on a range of particle materials and particle sizes and for a mixture of positively and negatively charged particles.

Particle trapping in electrically driven insulator-based microfluidics: Dielectrophoresis and induced-charge electrokinetics

Electrokinetically driven insulator‐based microfluidic devices represent an attractive option to manipulate particle suspensions. These devices can filtrate, concentrate, separate, or characterize micro and nanoparticles of interest. Two decades ago, inspired by electrode‐based dielectrophoresis, the concept of insulator‐based dielectrophoresis (iDEP) was born. In these microfluidic devices, insulating structures (i.e., posts, membranes, obstacles, or constrictions) built within the channel are used to deform the spatial distribution of an externally generated electric field. As a result, particles suspended in solution experience dielectrophoresis (DEP). Since then, it has been assumed that DEP is responsible for particle trapping in these devices, regardless of the type of voltage being applied to generate the electric field—direct current (DC) or alternating current. Recent findings challenge this assumption by demonstrating particle trapping and even particle flow reversal in devices that prevent DEP from occurring (i.e., unobstructed long straight channels stimulated with a DC voltage and featuring a uniform electric field). The theory introduced to explain those unexpected observations was then applied to conventional “DC‐iDEP” devices, demonstrating better prediction accuracy than that achieved with the conventional DEP‐centered theory. This contribution summarizes contributions made during the last two decades, comparing both theories to explain particle trapping and highlighting challenges to address in the near future.

Fine-Tuning Electrokinetic Injections Considering Nonlinear Electrokinetic Effects in Insulator-Based Devices

The manner of sample injection is critical in microscale electrokinetic (EK) separations, as the resolution of a separation greatly depends on sample quality and how the sample is introduced into the system. There is a significant wealth of knowledge on the development of EK injection methodologies that range from simple and straightforward approaches to sophisticated schemes. The present study focused on the development of optimized EK sample injection schemes for direct current insulator-based EK (DC-iEK) systems. These are microchannels that contain arrays of insulating structures; the presence of these structures creates a nonuniform electric field distribution when a potential is applied, resulting in enhanced nonlinear EK effects. Recently, it was reported that the nonlinear EK effect of electrophoresis of the second kind plays a major role in particle migration in DC-iEK systems. This study presents a methodology for designing EK sample injection schemes that consider the nonlinear EK effects exerted on the particles being injected. Mathematical modeling with COMSOL Multiphysics was employed to identify proper voltages to be used during the EK injection process. Then, a T-microchannel with insulating posts was employed to experimentally perform EK injection and separate a sample containing two types of similar polystyrene particles. The quality of the EK injections was assessed by comparing the resolution (Rs) and number of plates (N) of the experimental particle separations. The findings of this study establish the importance of considering nonlinear EK effects when planning for successful EK injection schemes.

Review of nonlinear electrokinetic flows in insulator-based dielectrophoresis: From induced charge to Joule heating effects

Insulator-based dielectrophoresis (iDEP) has been increasingly used for particle manipulation in various microfluidic applications. It exploits insulating structures to constrict and/or curve electric field lines to generate field gradients for particle dielectrophoresis. However, the presence of these insulators, especially those with sharp edges, causes two nonlinear electrokinetic flows, which, if sufficiently strong, may disturb the otherwise linear electrokinetic motion of particles and affect the iDEP performance. One is induced charge electroosmotic (ICEO) flow because of the polarization of the insulators, and the other is electrothermal flow because of the amplified Joule heating in the fluid around the insulators. Both flows vary nonlinearly with the applied electric field (either DC or AC) and exhibit in the form of fluid vortices, which have been utilized to promote some applications while being suppressed in others. The effectiveness of iDEP benefits from a comprehensive understanding of the nonlinear electrokinetic flows, which is complicated by the involvement of the entire iDEP device into electric polarization and thermal diffusion. This article is aimed to review the works on both the fundamentals and applications of ICEO and electrothermal flows in iDEP microdevices. A personal perspective of some future research directions in the field is also given.

On the potential of microscale electrokinetic cascade devices

Phages used for phage therapy of multidrug resistant bacteria must be highly purified prior to use. There are limited purification approaches that are broadly applicable to many phage types. Electrokinetics has shown great potential to manipulate phages, but obstructions from the cell debris produced during phage propagation can severely diminish the capacity of an electrokinetic device to concentrate and purify phage samples. A multipart insulator-based electrokinetic device is proposed here to remove the larger, undesirable components of mixtures from phage preparations while transferring the freshly purified and concentrated sample to a second stage for downstream analysis. By combining the large debris prescreen and analysis stages in a streamlined system, this approach simultaneously reduces the impact of clogging and minimizes the sample loss observed during manual transferring of purified samples. Polystyrene particles were used to demonstrate a diminished sample loss of approximately one order of magnitude when using the cascade device as opposed to a manual transfer scheme. The purification and concentration of three different phage samples were demonstrated using the first stage of the cascade device as a prescreen. This design provides a simple method of purifying and concentrating valuable samples from a complex mixture that might impede separation capacity in a single channel.

Dynamically controlled dielectrophoresis using resonant tuning

Electrically polarizable micro- and nanoparticles and droplets can be trapped using the gradient electric field of electrodes. But the spatial profile of the resultant dielectrophoretic force is fixed once the electrode structure is defined. To change the force profile, entire complex lab-on-a-chip systems must be re-fabricated with modified electrode structures. To overcome this problem, we propose an approach for the dynamic control of the spatial profile of the dielectrophoretic force by interfacing the trap electrodes with a resistor and an inductor to form a resonant resistor-inductor-capacitor (RLC) circuit. Using a dielectrophoretically trapped water droplet suspended in silicone oil, we show that the resonator amplitude, detuning, and linewidth can be continuously varied by changing the supply voltage, supply frequency, and the circuit resistance to obtain the desired trap depth, range, and stiffness. We show that by proper tuning of the resonator, the trap range can be extended without increasing the supply voltage, thus preventing sensitive samples from exposure to high electric fields at the stable trapping position. Such unprecedented dynamic control of dielectrophoretic forces opens avenues for the tunable active manipulation of sensitive biological and biochemical specimen in droplet microfluidic devices used for single-cell and biochemical reaction analysis.

Bacterial inactivation via microfluidic electroporation device with insulating micropillars

Electroporation is a promising method to inactivate cells and it has wide applications in medical science, biology and environmental health. Here, we investigate the bacteria inactivation performance of two different microfluidic electroporation devices with rhombus and circular micropillars used for generating locally enhanced electric field strength. Experiments are carried out to characterize the inactivation performance (i.e., the log removal efficiency) of two types of bacteria: Escherichia coli (E. coli, gram-negative) and Enterococcus faecalis (E. faecalis, gram-positive) in these two microfluidic devices. We find that under the same applied electric field, the device with rhombus micropillars performs better than the device with circular micropillars for both E. coli and E. faecalis. Numerical simulations show that due to the corner-induced singularity effect, the maximum electric field enhancement is higher in the device with rhombus micropillars than that in the device with circular micropillars. We also study the effects of DC and AC electric fields and flowrate. Our experiments demonstrate that the use of the DC field achieves higher log removal efficiencies than the use of AC field.

On the design, functions, and biomedical applications of high-throughput dielectrophoretic micro-/nanoplatforms: a review

As an efficient, rapid and label-free micro-/nanoparticle separation technique, dielectrophoresis (DEP) has attracted widespread attention in recent years, especially in the field of biomedicine, which exhibits huge potential in biomedically relevant applications such as disease diagnosis, cancer cell screening, biosensing, and others. DEP technology has been greatly developed recently from the low-flux laboratory level to high-throughput practical applications. In this review, we summarize the recent progress of DEP technology in biomedical applications, including firstly the design of various types and materials of DEP electrode and flow channel, design of input signals, and other improved designs. Then, functional tailoring of DEP systems with endowed specific functions including separation, purification, capture, enrichment and connection of biosamples, as well as the integration of multifunctions, are demonstrated. After that, representative DEP biomedical application examples in aspects of disease detection, drug synthesis and screening, biosensing and cell positioning are presented. Finally, limitations of existing DEP platforms on biomedical application are discussed, in which emphasis is given to the impact of other electrodynamic effects such as electrophoresis (EP), electroosmosis (EO) and electrothermal (ET) effects on DEP efficiency. This article aims to provide new ideas for the design of novel DEP micro-/nanoplatforms with desirable high throughput toward application in the biomedical community.

Microscale nonlinear electrokinetics for the analysis of cellular materials in clinical applications: a review

This review article presents a discussion of some of the latest advancements in the field of microscale electrokinetics for the analysis of cells and subcellular materials in clinical applications. The introduction presents an overview on the use of electric fields, i.e., electrokinetics, in microfluidics devices and discusses the potential of electrokinetic-based methods for the analysis of liquid biopsies in clinical and point-of-care applications. This is followed by four comprehensive sections that present some of the newest findings on the analysis of circulating tumor cells, blood (red blood cells, white blood cells, and platelets), stem cells, and subcellular particles (extracellular vesicles and mitochondria). The valuable contributions discussed here (with 131 references) were mainly published during the last 3 to 4 years, providing the reader with an overview of the state-of-the-art in the use of microscale electrokinetic methods in clinical analysis. Finally, the conclusions summarize the main advancements and discuss the future prospects.