A review of polystyrene bead manipulation by dielectrophoresis

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.

Dielectrophoretic separation/classification/focusing of microparticles using electrified lab-on-a-disc platforms

BACKGROUND

Separation, classification, and focusing of microparticles are essential issues in microfluidic devices that can be implemented in two categories: using labeling and label-free methods. Label-free methods differentiate microparticles based on their inherent properties, including size, density, shape, electrical conductivity/permittivity, and magnetic susceptibility. Dielectrophoresis is an advantageous label-free technique for this objective. Besides, centrifugal microfluidic devices exploit centrifugal forces to move liquid and particles. The simultaneous combination of dielectrophoretic and centrifugal forces exerted on microparticles still needs to be scrutinized more to predict their trajectories in such devices.

RESULTS

An integrated system utilizing two categories in microfluidics is proposed: dielectrophoretic manipulation of microparticles and centrifugal-driven microfluidics, followed by a numerical analysis. In this regard, we assumed a rectangular microchannel with internal unilateral planar electrodes equipped with three equal-sized outlets placed radially on a centrifugal platform where microparticles flow toward the disc's outer edge. The effect of different coordinate-based parameters, including radial and lateral distances (X and Y offsets)/tilting angles toward the radius direction (α), on the particles' movement was investigated. Additionally, the effect of operational parameters, including applied voltage, the microchannel width, the number of enabled electrodes, the diameter of particles, and the configuration of electrodes, were analyzed, and the distributions of particles toward the outlets were monitored. It was found that enhanced particle focusing becomes possible at lower rotation speeds and higher electric field values. Furthermore, the proposed centrifugal-DEP system's efficiency for classifying red blood cells/platelets and Live/Dead yeast cells systems was evaluated.

SIGNIFICANCE

Our integrated system is introduced as a novel method for focusing and classifying various microparticles with no need for sheath flows, having the ability to conduct particles at desired routes and focusing width. Furthermore, the system effectively separates various bioparticles and offers ease of operation and high-efficiency throughput over conventional dielectrophoretic devices.

Combination of an Optically Induced Dielectrophoresis (ODEP) Mechanism and a Laminar Flow Pattern in a Microfluidic System for the Continuous Size-Based Sorting and Separation of Microparticles

Optically induced dielectrophoresis (ODEP)-based microparticle sorting and separation is regarded as promising. However, current methods normally lack the downstream process for the transportation and collection of separated microparticles, which could limit its applications. To address this issue, an ODEP microfluidic chip encompassing three microchannels that join only at the central part of the microchannels (i.e., the working zone) was designed. During operation, three laminar flows were generated in the zone, where two dynamic light bar arrays were designed to sort and separate PS (polystyrene) microbeads of different sizes in a continuous manner. The separated PS microbeads were then continuously transported in laminar flows in a partition manner for the final collection. The results revealed that the method was capable of sorting and separating PS microbeads in a high-purity manner (e.g., the microbead purity values were 89.9 ± 3.7, 88.0 ± 2.5, and 92.8 ± 6.5% for the 5.8, 10.8, and 15.8 μm microbeads harvested, respectively). Overall, this study demonstrated the use of laminar flow and ODEP to achieve size-based sorting, separation, and collection of microparticles in a continuous and high-performance manner. Apart from the demonstration, this method can also be utilized for size-based sorting and the separation of other biological or nonbiological microparticles.

Numerical studies of manipulation and separation of microparticles in ODEP-based microfluidic chips

A novel optical-induced dielectrophoresis (ODEP) method employing a pressure-driven flow for the continuous separation of microparticles is presented in this study. By applying alternate current electric field on conductive indium tin oxide substrate and projecting the light geometry into the photoconductive layer, an inhomogeneous electric field is locally induced. The particles experience the dielectrophoretic force when passing through the lighting area, where the strongest electrical field gradient exists. By optimizing the structure of the lighting pattern, a stronger nonuniform electric field gradient is generated which predicts the separation of 1 and 3 µm polystyrene particles. Moreover, the effects of key parameters, including the light pattern geometry, applied voltage, and flow rate, were investigated in this study, leading to the successful sorting of 700 nm and 1 µm particles. To further examine the separation sensitivity and practicability of the proposed ODEP microfluidic method, the isolation of two different types of circulating tumor cells from T-cells and red blood cells are demonstrated, providing a novel method for the manipulation and separation of microparticles and nanoparticles.

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.

Elasto-inertial microfluidic separation of microspheres with submicron resolution at high-throughput

Elasto-inertial microfluidic separation offers many advantages including high throughput and separation resolution. Even though the separation efficiency highly depends on precise control of the flow conditions, no concrete guidelines have been reported yet in elasto-inertial microfluidics. Here, we propose a dimensionless analysis for precise estimation of the microsphere behaviors across the interface of Newtonian and viscoelastic fluids. Reynolds number, modified Weissenberg number, and modified elastic number are used to investigate the balance between inertial and elastic lift forces. Based on the findings, we introduce a new dimensionless number defined as the width of the Newtonian fluid stream divided by microsphere diameter. The proposed dimensionless analysis allows us to predict whether the microspheres migrate across the co-flow interface. The theoretical estimation is found to be in good agreement with the experimental results using 2.1- and 3.2-μm-diameter polystyrene microspheres in a co-flow of water and polyethylene oxide solution. Based on the theoretical estimation, we also realize submicron separation of the microspheres with 2.1 and 2.5 μm in diameter at high throughput, high purity (>95%), and high recovery rate (>97%). The applicability of the proposed method was validated by separation of platelets from similar-sized Escherichia coli (E.coli).

Optimizing Optical Dielectrophoretic (ODEP) Performance: Position- and Size-Dependent Droplet Manipulation in an Open-Chamber Oil Medium

An optimization study is presented to enhance optical dielectrophoretic (ODEP) performance for effective manipulation of an oil-immersed droplet in the floating electrode optoelectronic tweezers (FEOET) device. This study focuses on understanding how the droplet's position and size, relative to light illumination, affect the maximum ODEP force. Numerical simulations identified the characteristic length (Lc) of the electric field as a pivotal factor, representing the location of peak field strength. Utilizing 3D finite element simulations, the ODEP force is calculated through the Maxwell stress tensor by integrating the electric field strength over the droplet's surface and then analyzed as a function of the droplet's position and size normalized to Lc. Our findings reveal that the optimal position is xopt= Lc+ r, (with r being the droplet radius), while the optimal droplet size is ropt = 5Lc, maximizing light-induced field perturbation around the droplet. Experimental validations involving the tracking of droplet dynamics corroborated these findings. Especially, a droplet sized at r = 5Lc demonstrated the greatest optical actuation by performing the longest travel distance of 13.5 mm with its highest moving speed of 6.15 mm/s, when it was initially positioned at x0= Lc+ r = 6Lc from the light's center. These results align well with our simulations, confirming the criticality of both the position (xopt) and size (ropt) for maximizing ODEP force. This study not only provides a deeper understanding of the position- and size-dependent parameters for effective droplet manipulation in FEOET systems, but also advances the development of low-cost, disposable, lab-on-a-chip (LOC) devices for multiplexed biological and biochemical analyses.

Dielectrophoresis-assisted removal of Cd and Cu heavy metal ions by using Chlorella microalgae

A novel dielectrophoresis (DEP)-assisted device for the bioremediation of heavy metal ions by using Chlorella microalgae is presented in this paper. To generate the DEP forces, pairs of electrode mesh were inserted in the DEP-assisted device. By applying DC electric field via the electrodes, the inhomogeneous electric field gradient is induced and the strongest non-uniform electric field exists near the mesh cross-corner. After the adsorption of Cd and Cu heavy metal ions by Chlorella, the Chlorella chain were trapped along the vicinity of the electrode mesh. Then, the effects of Chlorella concentration on the adsorption of heavy metal ions, and the applied voltage and electrode mesh size on the removal of Chlorella are conducted. In the co-existing Cd and Cu solutions, the individual adsorption ratio of Cd and Cu reaches as high as approximately 96% and 98%, respectively, showing excellent bioremediation capability of multiple heavy metal ions in wastewater. By adjusting the applied electric voltage and the mesh size, the Chlorella adsorbed with Cd and Cu are captured by negative DC-DEP effects and the removal ratio of Chlorella reach an average of 97%, providing a method for the removal of multiple heavy metal ions in wastewater by using Chlorella microalgae.

Microparticle electrical conductivity measurement using optoelectronic tweezers

When it comes to simulate or calculate an optoelectronic tweezer (OET) response for a microparticle suspended in a given medium, a precise electrical conductivity (later referred to as conductivity) value for the microparticle is critical. However, there are not well-established measurements or well-referenced values for microparticle conductivities in the OET realm. Thus, we report a method based on measuring the escape velocity of a microparticle with a standard OET system to calculate its conductivity. A widely used 6 μm polystyrene bead (PSB) is used for the study. The conductivity values are found to be invariant around 2×10-3 S/m across multiple different aqueous media, which helps clarify the ambiguity in the usage of PSB conductivity. Our convenient approach could principally be applied for the measurement of multiple unknown OET-relevant material properties of microparticle-medium systems with various OET responses, which can be beneficial to carry out more accurate characterization in relevant fields.

Microparticle separation using dielectrophoresis-assisted inertial microfluidics: A GPU-accelerated immersed boundary-lattice Boltzmann simulation

In this study, the migration of microparticles towards the inertial equilibrium positions in a straight microchannel with a square cross section in the presence of an inhomogeneous oscillating electric field was examined. The dynamics of microparticles were simulated using the immersed boundary-lattice Boltzmann method of fluid-structure interaction simulation. Moreover, the lattice Boltzmann Poisson solver was applied to calculate the electric field required for calculation of the dielectrophoretic force using the equivalent dipole moment approximation. These numerical methods were implemented on a single GPU coupled with the AA pattern of storing distribution functions in memory to speed up the computationally demanding simulation of microparticles dynamics. In the absence of an electric field, spherical polystyrene microparticles migrate to four symmetric stable equilibrium positions corresponding to the sidewalls of the square cross-sectional microchannel. The equilibrium distance from the sidewall was increased by increasing the particle size. The equilibrium positions near electrodes disappeared and particles migrated to the other equilibrium positions far from the electrodes by the application of the high-frequency oscillatory electric field at voltages beyond a threshold value. Finally, a two-step dielectrophoresis-assisted inertial microfluidics methodology was introduced for particle separation based on the crossover frequencies and the observed threshold voltages of different particles. The proposed method exploited the synergistic effect of dielectrophoresis and inertial microfluidics methods to remove their limitations, allowing the separation of a broad range of polydisperse particle mixtures with a single device in a short time.

Protein Dielectrophoresis with Gradient Array of Conductive Electrodes Sheds New Light on Empirical Theory

Dielectrophoresis (DEP) is a versatile tool for the precise microscale manipulation of a broad range of substances. To unleash the full potential of DEP for the manipulation of complex molecular-sized particulates such as proteins requires the development of appropriate theoretical models and their comprehensive experimental verification. Here, we construct an original DEP platform and test the Hölzel-Pethig empirical model for protein DEP. Three different proteins are studied: lysozyme, BSA, and lactoferrin. Their molecular Clausius-Mossotti function is obtained by detecting their trapping event via the measurement of the fluorescence intensity to identify the minimum electric field gradient required to overcome dispersive forces. We observe a significant discrepancy with published theoretical data and, after a very careful analysis to rule out experimental errors, conclude that more sophisticated theoretical models are required for the response of molecular entities in DEP fields. The developed experimental platform, which includes arrays of sawtooth metal electrode pairs with varying gaps and produces variations of the electric field gradient, provides a versatile tool that can broaden the utilization of DEP for molecular entities.

The Utilization of Tunable Transducer Elements Formed by the Manipulation of Magnetic Beads with Different Sizes via Optically Induced Dielectrophoresis (ODEP) for High Signal-to-Noise Ratios (SNRs) and Multiplex Fluorescence-Based Biosensing Applications

Magnetic beads improve biosensing performance by means of their small volume and controllability by magnetic force. In this study, a new technique composed of optically induced dielectrodphoresis (ODEP) manipulation and image processing was used to enhance the signal-to-noise ratio of the fluorescence for stained magnetic beads. According to natural advantages of size-dependent particle isolation by ODEP manipulation, biomarkers in clinical samples can be easily separated by different sizes of magnetic beads with corresponding captured antibodies, and rapidly distinguished by separated location of immunofluorescence. To verify the feasibility of the concept, magnetic beads with three different diameters, including 21.8, 8.7, and 4.2 μm, were easily separated and collected into specific patterns in the defined target zone treated as three dynamic transducer elements to evaluate fluorescence results. In magnetic beads with diameter of 4.2 μm, the lowest signal-to-noise ratio between stained and nonstained magnetic beads was 3.5. With the help of ODEP accumulation and detection threshold setting of 32, the signal-to-noise ratio was increased to 77.4, which makes this method more reliable. With the further optimization of specific antibodies immobilized on different-size magnetic beads in the future, this platform can be a potential candidate for a high-efficiency sensor array in clinical applications.

Emerging electrochemical techniques for identifying and removing micro/nanoplastics in urban waters

The ubiquitous micro/nanoplastics (MPs/NPs) in urban waters are priority pollutants due to their toxic effects on living organisms. Currently, great efforts have been made to realize a plastic-free urban water system, and the identification and removal of MPs/NPs are two primary issues. Among diverse methods, emerging electrochemical techniques have gained growing interests owing to their facile implementation, high efficiency, eco-compatibility, onsite operation, etc. Herein, recent progress in the electrochemical identification and removal of MPs/NPs in urban waters are comprehensively reviewed. The electrochemical sensing of MPs/NPs and their released pollutants (e.g., bisphenol A (BPA)) has been analyzed, and the sensing principles and the featured electrochemical devices/electrodes are examined. Afterwards, recent applications of electrochemical methods (i.e., electrocoagulation, electroadsorption, electrokinetic separation and electrochemical degradation) in MPs/NPs removal are discussed in detail. The influences of critical parameters (e.g., plastics' property, current density and electrolyte) in the electrochemical identification and removal of MPs/NPs are also analyzed. Finally, the current challenges and prospects in electrochemical sensing and removal of MPs/NPs in urban waters are elaborated. This review would advance efficient electrochemical technologies for future MPs/NPs pollutions management in urban waters.

Signal-Based Methods in Dielectrophoresis for Cell and Particle Separation

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

Implementation of flexible virtual microchannels based on optically induced dielectrophoresis

Micro-nano particle manipulation methods in liquid environments have been widely used in the fields such as medicine, biology and material science. Nevertheless, the methods usually rely on pre-prepared physical microfluidic channels. In this work, virtual electrodes based on the optically induced dielectrophoresis (ODEP) method were used as virtual microchannels instead of traditional physical microfluidic channels. Virtual microchannels with different shapes were implemented by the designs of projected light patterns, which made the virtual microchannels have great flexibility and controllability. The theory of ODEP was verified by simulation and analysis of electric field distributions. The relationship between the manipulation force and the alternating current (AC) voltage or the AC frequency exerted on the cells was assessed. The experimental results indicated that the manipulation force was increased with the increase of the AC voltage, and it was reduced with the increase of the AC frequency. Moreover, different virtual microchannels were designed to carry out the transportation, aggregation and sorting of yeast cells and rat basophilic leukemia cells (RBL-2H3 cells) and the survival rate of the cells was evaluated. This work shows that the virtual microchannels can be flexibly realized by ODEP in liquid environments.

Ultra-sensitive dielectrophoretic surface charge multiplex detection inside a micro-dielectrophoretic device

Label-free dielectrophoretic force-based surface charge detection has shown great potential for highly sensitive and selective sensing of metal ions and small biomolecules. However, this method suffers from a complex calibration process and measurement signal interference in simultaneous multi-analyte detection, thus creating difficulties in multiplex detection. We have developed a method to overcome these issues based on the optical discrimination of the dielectrophoretic behaviors of multiple microparticle probes considering the surface charge difference before and after self-assembling conjugation. In this report, we demonstrate and characterize this dielectrophoretic force-based surface charge detection method with particle probes functionalized by various biomolecules. This technique achieved an attomolar limit of detection (LOD) for Hg²⁺ in distilled water and a femtomolar LOD in drinking water using DNA aptamer-functionalized particle probes. More importantly, using two different DNA aptamer-functionalized particle probes for Hg²⁺ and Ag⁺, label-free dielectrophoretic multiplex detection of these species in drinking water with a femtomolar and a nanomolar LOD was achieved for the first time.

Artificial Intelligence Algorithms Enable Automated Characterization of the Positive and Negative Dielectrophoretic Ranges of Applied Frequency

The present work describes the phenomenological approach to automatically determine the frequency range for positive and negative dielectrophoresis (DEP)—an electrokinetic force that can be used for massively parallel micro- and nano-assembly. An experimental setup consists of the microfabricated chip with gold microelectrode array connected to a function generator capable of digitally controlling an AC signal of 1 V (peak-to-peak) and of various frequencies in the range between 10 kHz and 1 MHz. The suspension of latex microbeads (3-μm diameter) is either attracted or repelled from the microelectrodes under the influence of DEP force as a function of the applied frequency. The video of the bead movement is captured via a digital camera attached to the microscope. The OpenCV software package is used to digitally analyze the images and identify the beads. Positions of the identified beads are compared for successive frames via Artificial Intelligence (AI) algorithm that determines the cloud behavior of the microbeads and algorithmically determines if the beads experience attraction or repulsion from the electrodes. Based on the determined behavior of the beads, algorithm will either increase or decrease the applied frequency and implement the digital command of the function generator that is controlled by the computer. Thus, the operation of the study platform is fully automated. The AI-guided platform has determined that positive DEP (pDEP) is active below 500 kHz frequency, negative DEP (nDEP) is evidenced above 1 MHz frequency and the crossover frequency is between 500 kHz and 1 MHz. These results are in line with previously published experimentally determined frequency-dependent DEP behavior of the latex microbeads. The phenomenological approach assisted by live AI-guided feedback loop described in the present study will assist the active manipulation of the system towards the desired phenomenological outcome such as, for example, collection of the particles at the electrodes, even if, due to the complexity and plurality of the interactive forces, model-based predictions are not available.

Advances and enabling technologies for phase-specific cell cycle synchronisation

Cell cycle synchronisation is the process of isolating cell populations at specific phases of the cell cycle from heterogeneous, asynchronous cell cultures. The process has important implications in targeted gene-editing and drug efficacy of cells and in studying cell cycle events and regulatory mechanisms involved in the cell cycle progression of multiple cell species. Ideally, cell cycle synchrony techniques should be applicable for all cell types, maintain synchrony across multiple cell cycle events, maintain cell viability and be robust against metabolic and physiological perturbations. In this review, we categorize cell cycle synchronisation approaches and discuss their operational principles and performance efficiencies. We highlight the advances and technological development trends from conventional methods to the more recent microfluidics-based systems. Furthermore, we discuss the opportunities and challenges for implementing high throughput cell synchronisation and provide future perspectives on synchronisation platforms, specifically hybrid cell synchrony modalities, to allow the highest level of phase-specific synchrony possible with minimal alterations in diverse types of cell cultures.

Optical Dielectrophoretic (DEP) Manipulation of Oil-Immersed Aqueous Droplets on a Plasmonic-Enhanced Photoconductive Surface

We present a plasmonic-enhanced dielectrophoretic (DEP) phenomenon to improve optical DEP performance of a floating electrode optoelectronic tweezers (FEOET) device, where aqueous droplets can be effectively manipulated on a light-patterned photoconductive surface immersed in an oil medium. To offer device simplicity and cost-effectiveness, recent studies have utilized a polymer-based photoconductive material such as titanium oxide phthalocyanine (TiOPc). However, the TiOPc has much poorer photoconductivity than that of semiconductors like amorphous silicon (a-Si), significantly limiting optical DEP applications. The study herein focuses on the FEOET device for which optical DEP performance can be greatly enhanced by utilizing plasmonic nanoparticles as light scattering elements to improve light absorption of the low-quality TiOPc. Numerical simulation studies of both plasmonic light scattering and electric field enhancement were conducted to verify wide-angle scattering light rays and an approximately twofold increase in electric field gradient with the presence of nanoparticles. Similarly, a spectrophotometric study conducted on the absorption spectrum of the TiOPc has shown light absorption improvement (nearly twofold) of the TiOPc layer. Additionally, droplet dynamics study experimentally demonstrated a light-actuated droplet speed of 1.90 mm/s, a more than 11-fold improvement due to plasmonic light scattering. This plasmonic-enhanced FEOET technology can considerably improve optical DEP capability even with poor-quality photoconductive materials, thus providing low-cost, easy-fabrication solutions for various droplet-based microfluidic applications.

Continuous size-based DEP separation of particles using a bi-gap electrode pair

The design, fabrication, and characterization of an advanced microfluidic device containing a bi-gap electrode pair for the continuous separation of three different populations of particles based on their size using DEP are presented.

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.

Microsphere mediated exosome isolation and ultra-sensitive detection on a dielectrophoresis integrated microfluidic device

Tumor-derived exosomes have been recognized as potential biomarkers for cancer diagnosis because they are actively involved in cancer progression and metastasis. However, progress in practical exosome analysis is still slow due to the limitation in exosome isolation and detection. The development of microfluidic devices has provided a promising analytical platform compared with traditional methods. In this study, we develop an exosome isolation and detection method based on a microfluidic device (ExoDEP-chip), which realized microsphere mediated dielectrophoretic isolation and immunoaffinity detection. Exosomes were firstly isolated by binding to antibodies pre-immobilized on the polystyrene (PS) microsphere surface and were further detected using fluorescently labeled antibodies by fluorescence microscopy. Single microspheres were then trapped into single microwells under the DEP force in the ExoDEP-chip. A wide range from 1.4 × 10³ to 1.4 × 10⁸ exosomes per mL with a detection limit of 193 exosomes per mL was obtained. Through monitoring five proteins (CD81, CEA, EpCAM, CD147, and AFP) of exosomes from three different cell lines (A549, HEK293, and HepG2), a significant difference in marker expression levels was observed in different cell lines. Therefore, this method has good prospects in exosome-based tumor marker detection and cancer diagnosis.

Sheath-assisted versus sheathless dielectrophoretic particle separation

Lab-on-chip devices are widely being used for binary and ternary cell/particle separation applications. Among the lab-on-chip methods, dielectrophoresis (DEP) is a cost-effective and label-free method, with great capabilities for size-based separation of cells and particles, which is mostly performed in sheath-assisted forms. However, the elimination of the sheath flows offers advantages such as ease of operation and higher sample throughput. In this work, we present a comparison of sheath-assisted and sheathless DEP separation of three sizes of microparticles using tilted electrodes. The sheath-assisted design was capable of separating the 5, 10, and 15 μm particles with a separation efficiency as high as 98.0% for 15 μm particles. By adding a DEP focusing region, a sheathless DEP separator was proposed, which offered higher throughputs (up to 10 times) at the cost of lowering the separation efficiency (a reduction up to 10.3% for 15 μm) compared to the sheath-assisted design. To enhance the separation efficiency, a combination of the DEP focusing accompanied by weak sheath flows from both sides was proposed. This design achieved the highest sample separation yield in the outlets (as high as 98.7% for 15 μm) with a sample throughput of more than 4.2 μL/min. This study provides insights into the choice of an appropriate platform for any application in which the yield, purity, throughput, and portability must be considered.

Precise micro-particle and bubble manipulation by tunable ultrasonic bottle beams

This paper reports a method to generate tunable bottle beams using an ultrasonic lens, by which the bottle position can be precisely adjusted with the change of the acoustic frequency. Therefore, the position of a single particle or bubble in liquid can be manipulated without using phased array which is costly and huge with complex circuits. Furthermore, we introduced this method to multiple bubble manipulation using acoustic holography. The bottle properties against frequency are theoretically and experimentally analyzed. It is shown that the bottle position depends almost linearly on the operating frequency, which provides a basis for the precise manipulation of bubbles and particles. In addition, the relationship between the acoustic radiation force and the drag force under different incident acoustic pressures is considered, establishing a limit on the moving velocity of the trapped particles. The ultrasonic field observation is further demonstrated by Schlieren imaging system. The proposed method has potential biomedical applications, such as more flexible cell manipulation and targeted drug delivery in vivo, as well as potential applications in the study of chemical reactions between micro objects.

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.

Enhancing Raman signals from bacteria using dielectrophoretic force between conductive lensed fiber and black silicon

An osmium-coated lensed fiber (OLF) probe combined with a silver-coated black silicon (SBS) substrate was used to generate a dielectrophoretic (DEP) force that traps bacteria and enables Raman signal detection from bacteria. The lensed fiber coated with a 2-nm osmium layer was used as an electrode for the DEP force and also as a lens to excite Raman signals. The black silicon coated with a 150-nm silver layer was used both as the surface-enhanced Raman scattering (SERS) substrate and the counter electrode. The enhanced Raman signal was collected by the same OLF probe and further analyzed with a spectrometer. For Raman measurements, a drop of bacterial suspension was placed between the OLF probe and the SBS substrate. By controlling the frequency of an AC voltage on the OLF probe and SBS substrate, a DEP force at 1 MHz concentrated bacteria on the SBS surface and removed the unbound micro-objects in the solution at 1 kHz. A bacteria concentration of 6 × 10⁴ CFU/mL (colony forming units per mL) could be identified in less than 15 min, using a volume of only 1 μL, by recording the variation of the Raman peak at 740 cm^-1.

Active, Reactive, and Apparent Power in Dielectrophoresis: Force Corrections from the Capacitive Charging Work on Suspensions Described by Maxwell-Wagner's Mixing Equation

A new expression for the dielectrophoresis (DEP) force is derived from the electrical work in a charge-cycle model that allows the field-free transition of a single object between the centers of two adjacent cubic volumes in an inhomogeneous field. The charging work for the capacities of the volumes is calculated in the absence and in the presence of the object using the external permittivity and Maxwell-Wagner's mixing equation, respectively. The model provides additional terms for the Clausius-Mossotti factor, which vanish for the mathematical boundary transition toward zero volume fraction, but which can be interesting for narrow microfluidic systems. The comparison with the classical solution provides a new perspective on the notorious problem of electrostatic modeling of AC electrokinetic effects in lossy media and gives insight into the relationships between active, reactive, and apparent power in DEP force generation. DEP moves more highly polarizable media to locations with a higher field, making a DEP-related increase in the overall polarizability of suspensions intuitive. Calculations of the passage of single objects through a chain of cubic volumes show increased overall effective polarizability in the system for both positive and negative DEP. Therefore, it is proposed that DEP be considered a conditioned polarization mechanism, even if it is slow with respect to the field oscillation. The DEP-induced changes in permittivity and conductivity describe the increase in the overall energy dissipation in the DEP systems consistent with the law of maximum entropy production. Thermodynamics can help explain DEP accumulation of small objects below the limits of Brownian motion.

Field-Portable Microplastic Sensing in Aqueous Environments: A Perspective on Emerging Techniques

Microplastics (MPs) have been found in aqueous environments ranging from rural ponds and lakes to the deep ocean. Despite the ubiquity of MPs, our ability to characterize MPs in the environment is limited by the lack of technologies for rapidly and accurately identifying and quantifying MPs. Although standards exist for MP sample collection and preparation, methods of MP analysis vary considerably and produce data with a broad range of data content and quality. The need for extensive analysis-specific sample preparation in current technology approaches has hindered the emergence of a single technique which can operate on aqueous samples in the field, rather than on dried laboratory preparations. In this perspective, we consider MP measurement technologies with a focus on both their eventual field-deployability and their respective data products (e.g., MP particle count, size, and/or polymer type). We present preliminary demonstrations of several prospective MP measurement techniques, with an eye towards developing a solution or solutions that can transition from the laboratory to the field. Specifically, experimental results are presented from multiple prototype systems that measure various physical properties of MPs: pyrolysis-differential mobility spectroscopy, short-wave infrared imaging, aqueous Nile Red labeling and counting, acoustophoresis, ultrasound, impedance spectroscopy, and dielectrophoresis.

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.

Dielectrophoresis Prototypic Polystyrene Particle Synchronization toward Alive Keratinocyte Cells for Rapid Chronic Wound Healing

Diabetes patients are at risk of having chronic wounds, which would take months to years to resolve naturally. Chronic wounds can be countered using the electrical stimulation technique (EST) by dielectrophoresis (DEP), which is label-free, highly sensitive, and selective for particle trajectory. In this study, we focus on the validation of polystyrene particles of 3.2 and 4.8 μm to predict the behavior of keratinocytes to estimate their crossover frequency (fXO) using the DEP force (FDEP) for particle manipulation. MyDEP is a piece of java-based stand-alone software used to consider the dielectric particle response to AC electric fields and analyzes the electrical properties of biological cells. The prototypic 3.2 and 4.8 μm polystyrene particles have fXO values from MyDEP of 425.02 and 275.37 kHz, respectively. Fibroblast cells were also subjected to numerical analysis because the interaction of keratinocytes and fibroblast cells is essential for wound healing. Consequently, the predicted fXO from the MyDEP plot for keratinocyte and fibroblast cells are 510.53 and 28.10 MHz, respectively. The finite element method (FEM) is utilized to compute the electric field intensity and particle trajectory based on DEP and drag forces. Moreover, the particle trajectories are quantified in a high and low conductive medium. To justify the simulation, further DEP experiments are carried out by applying a non-uniform electric field to a mixture of different sizes of polystyrene particles and keratinocyte cells, and these results are well agreed. The alive keratinocyte cells exhibit NDEP force in a highly conductive medium from 100 kHz to 25 MHz. 2D/3D motion analysis software (DIPP-MotionV) can also perform image analysis of keratinocyte cells and evaluate the average speed, acceleration, and trajectory position. The resultant NDEP force can align the keratinocyte cells in the wound site upon suitable applied frequency. Thus, MyDEP estimates the Clausius-Mossotti factors (CMF), FEM computes the cell trajectory, and the experimental results of prototypic polystyrene particles are well correlated and provide an optimistic response towards keratinocyte cells for rapid wound healing applications.

Dielectrophoresis assisted high-throughput detection system for multiplexed immunoassays

Digital ELISA is introduced as a novel platform with unique advantages for detecting multiple kinds of single-molecule in the sample. How to improve the sensitivity of detection is the direction of current related research. Here, we report an immunoassay method that applied electrokinetic effects to isolate the individual encoded beads and confine in micro-wells to improve the efficiency of cytokines detection simultaneously. The microfluidic design provided a non-uniform electric field to induce dielectrophoresis (DEP) force and to manipulate the beads. Two wavelengths of excitation light excited the encoded beads for simultaneous detection of reporters. The light was confined to the bottom slide via the principle of total internal reflection. Finally, the concentration of captured cytokines was obtained by picking up each bead from the image and then integrating the intensity of fluorescent light emitted from the reporters. The results demonstrated that the fill percentage of encoded beads was raised from 10-20% to 60-80% via DEP effect. By comparing the fluorescence color of the particle, itself and its surface, the concentration of four target cytokines, IL-2, IL-6, IL-10 and TNF-α, were calculated to the pg/ml level. The spike and recovery experiments verified the efficiency, more than 70% of the target molecules were captured. The reliability of our method was verified by flow cytometry as well. In conclusion, we expect the application of DEP can increase the sensitivity of digital ELISA for multiple rapid detection.

Analysis of microorganisms with nonlinear electrokinetic microsystems

Nonlinear electrokinetics (EK), specifically electrophoresis of the second kind, dielectrophoresis (DEP) and electrorotation (EROT), have gained significant interest recently for their flexibility and labeless discriminant manner of operation. The current applications of these technologies are a clear advancement from what they were when first discovered, but also still show strong signs of future growth. The present review article presents a discussion of the current uses of microscale nonlinear EK technologies as analytical, sensing, and purification tools for microorganisms. The discussion is focused on some of the latest discoveries with various nonlinear EK microfluidic techniques, such as DEP particle trapping and EROT for particle assessments, for the analysis of microorganisms ranging from viruses to parasites. Along the way, special focus was given to key research articles from within the past two years to provide the most up-to-date knowledge on the current state-of-the-art within the field of microscale EK, and from there, an outlook on where the future of the field is headed is also included.

Integrated microfluidic platform with electrohydrodynamic focusing and a carbon-nanotube-based field-effect transistor immunosensor for continuous, selective, and label-free quantification of bacteria

Electrokinetic technologies such as AC electro-osmosis (EO) and dielectrophoresis (DEP) have been used for effective manipulation of bacteria to enhance the sensitivity of an assay, and many previously reported electrokinetics-enhanced biosensors are based on stagnant fluids. An effective region for positive DEP for particle capture is usually too close to the electrode for the flowing particles to move toward the detection zone of a biosensor against the flow direction; this poses a technical challenge for electrokinetics-assisted biosensors implemented within pressure-driven flows, especially if the particles flow with high speed and if the detection zone is small. Here, we present a microfluidic single-walled carbon nanotube (SWCNT)-based field-effect transistor immunosensor with electrohydrodynamic (EHD) focusing and DEP concentration for continuous and label-free detection of flowing Staphylococcus aureus in a 0.01× phosphate buffered saline (PBS) solution. The EHD focusing involved AC EO and negative DEP to align the flowing particles along lines close to the bottom surface of a microfluidic channel for facilitating particle capture downstream at the detection zone. For feasibility, 380 nm-diameter fluorescent beads suspended in 0.001× PBS were tested, and 14.6 times more beads were observed to be concentrated in the detection area with EHD focusing. Moreover, label-free, continuous, and selective measurement of S. aureus in 0.01× PBS was demonstrated, showing good linearity between the relative changes in electrical conductance of the SWCNTs and logarithmic S. aureus concentrations, a capture/detection time of 35 min, and a limit of detection of 150 CFU mL-1, as well as high specificity through electrical manipulation and biological interaction.

Rapid Multianalyte Microfluidic Homogeneous Immunoassay on Electrokinetically Driven Beads

The simplicity of homogeneous immunoassays makes them suitable for diagnostics of acute conditions. Indeed, the absence of washing steps reduces the binding reaction duration and favors a rapid and compact device, a critical asset for patients experiencing life-threatening diseases. In order to maximize analytical performance, standard systems employed in clinical laboratories rely largely on the use of high surface-to-volume ratio suspended moieties, such as microbeads, which provide at the same time a fast and efficient collection of analytes from the sample and controlled aggregation of collected material for improved readout. Here, we introduce an integrated microfluidic system that can perform analyte detection on antibody-decorated beads and their accumulation in confined regions within 15 min. We employed the system to the concomitant analysis of clinical concentrations of Neutrophil Gelatinase-Associated Lipocalin (NGAL) and Cystatin C in serum, two acute kidney injury (AKI) biomarkers. To this end, high-aspect-ratio, three-dimensional electrodes were integrated within a microfluidic channel to impart a controlled trajectory to antibody-decorated microbeads through the application of dielectrophoretic (DEP) forces. Beads were efficiently retained against the fluid flow of reagents, granting an efficient on-chip analyte-to-bead binding. Electrokinetic forces specific to the beads' size were generated in the same channel, leading differently decorated beads to different readout regions of the chip. Therefore, this microfluidic multianalyte immunoassay was demonstrated as a powerful tool for the rapid detection of acute life-threatening conditions.

Insulator Based Dielectrophoresis: Micro, Nano, and Molecular Scale Biological Applications

Insulator based dielectrophoresis (iDEP) is becoming increasingly important in emerging biomolecular applications, including particle purification, fractionation, and separation. Compared to conventional electrode-based dielectrophoresis (eDEP) techniques, iDEP has been demonstrated to have a higher degree of selectivity of biological samples while also being less biologically intrusive. Over the past two decades, substantial technological advances have been made, enabling iDEP to be applied from micro, to nano and molecular scales. Soft particles, including cell organelles, viruses, proteins, and nucleic acids, have been manipulated using iDEP, enabling the exploration of subnanometer biological interactions. Recent investigations using this technique have demonstrated a wide range of applications, including biomarker screening, protein folding analysis, and molecular sensing. Here, we review current state-of-art research on iDEP systems and highlight potential future work.

The Combination of Immunomagnetic Bead-Based Cell Isolation and Optically Induced Dielectrophoresis (ODEP)-Based Microfluidic Device for the Negative Selection-Based Isolation of Circulating Tumor Cells (CTCs)

Negative selection-based circulating tumor cell (CTC) isolation is able to harvest viable, label-free, and clinically meaningful CTCs from the cancer patients' blood. Nevertheless, its main shortcoming is its inability to isolate high-purity CTCs, restricting subsequent CTC-related analysis. To address this issue, this study proposed a two-step optically-induced dielectrophoresis (ODEP) cell manipulation to process the cell sample harvested by negative selection-/immunomagnetic microbeads-based CTC isolation. The working mechanism is that the ODEP force acting on the cells with and without magnetic microbeads binding is different. Accordingly, the use of ODEP cell manipulation in a microfluidic system was designed to first separate and then isolate the cancer cells from other magnetic microbead-bound cells. Immunofluorescent microscopic observation and ODEP cell manipulation were then performed to refine the purity of the cancer cells. In this study, the optimum operating conditions for effective cell isolation were determined experimentally. The results revealed that the presented method was able to further refine the purity of cancer cell in the sample obtained after negative selection-based CTC isolation with high cell purity (81.6~86.1%). Overall, this study proposed the combination of immunomagnetic bead-based cell isolation and ODEP cell manipulation for the negative selection-based isolation of CTCs.

A review of dielectrophoretic separation and classification of non-biological particles

Dielectrophoresis (DEP) is a selective electrokinetic particle manipulation technology that is applied for almost 100 years and currently finds most applications in biomedical research using microfluidic devices operating at moderate to low throughput. This paper reviews DEP separators capable of high-throughput operation and research addressing separation and analysis of non-biological particle systems. Apart from discussing particle polarization mechanisms, this review summarizes the early applications of DEP for dielectric sorting of minerals and lists contemporary applications in solid/liquid, liquid/liquid, and solid/air separation, for example, DEP filtration or airborne fiber length classification; the review also summarizes developments in DEP fouling suppression, gives a brief overview of electrocoalescence and addresses current problems in high-throughput DEP separation. We aim to provide inspiration for DEP application schemes outside of the biomedical sector, for example, for the recovery of precious metal from scrap or for extraction of metal from low-grade ore.

Focusing of sub-micrometer particles in microfluidic devices

Sub-micrometer particles (0.10-1.0 μm) are of great significance to study, e.g., microvesicles and protein aggregates are targets for therapeutic intervention, and sub-micrometer fluorescent polystyrene (PS) particles are used as probes for diagnostic imaging. Focusing of sub-micrometer particles - precisely control over the position of sub-micrometer particles in a tightly focused stream - has a wide range of applications in the field of biology, chemistry and environment, by acting as a prerequisite step for downstream detection, manipulation and quantification. Microfluidic devices have been attracting great attention as desirable tools for sub-micrometer particle focusing, due to their small size, low reagent consumption, fast analysis and low cost. Recent advancements in fundamental knowledge and fabrication technologies have enabled microfluidic focusing of particles at sub-micrometer scale in a continuous, label-free and high-throughput manner. Microfluidic methods for the focusing of sub-micrometer particles can be classified into two main groups depending on whether an external field is applied: 1) passive methods, which utilize intrinsic fluidic properties without the need of external actuation, such as inertial, deterministic lateral displacement (DLD), viscoelastic and hydrophoretic focusing; and 2) active methods, where external fields are used, such as dielectrophoretic, thermophoretic, acoustophoretic and optical focusing. This article mainly reviews the studies on the focusing of sub-micrometer particles in microfluidic devices over the past 10 years. It aims to bridge the gap between the focusing of micrometer and nanometer scale (1.0-100 nm) particles and to improve the understanding of development progress, current advances and future prospects in microfluidic focusing techniques.

Study on non-bioparticles and Staphylococcus aureus by dielectrophoresis

This article demonstrated a chip device with alternating current (AC) dielectrophoresis (DEP) for separation of non-biological micro-particle and bacteria mixtures. The DEP separation was achieved by a pair of metal electrodes with the shape of radal-interdigital to generate a localized non-uniform AC electric field. The electric field and DEP force were firstly investigated by finite element methods (FEM). The mixed microparticles such as different scaled polystyrene (PS) beads, PS beads with inorganic micro-particles (e.g., ZnO and silica beads) and non-bioparticles with bacterial Staphylococcus aureus (S. aureus) were successfully separated at DEP-on-a-chip by an AC electric field of 20 kHz, 10 kHz and 1 MHz, respectively. The results indicated that DEP trapping can be considered as a potential candidate method for investigating the separation of biological mixtures, and may well prove to have a great impact on in situ monitoring of environmental and/or biological samples by DEP-on-a-chip.

This article demonstrated a chip device with alternating current (AC) dielectrophoresis (DEP) for separation of non-biological micro-particle and bacteria mixtures.

Plasmonic Tweezers towards Biomolecular and Biomedical Applications

With the capability of confining light into subwavelength scale, plasmonic tweezers have been used to trap and manipulate nanoscale particles. It has huge potential to be utilized in biomolecular research and practical biomedical applications. In this short review, plasmonic tweezers based on nano-aperture designs are discussed. A few challenges should be overcome for these plasmonic tweezers to reach a similar level of significance as the conventional optical tweezers.

Nanoscale integration of single cell biologics discovery processes using optofluidic manipulation and monitoring

The new and rapid advancement in the complexity of biologics drug discovery has been driven by a deeper understanding of biological systems combined with innovative new therapeutic modalities, paving the way to breakthrough therapies for previously intractable diseases. These exciting times in biomedical innovation require the development of novel technologies to facilitate the sophisticated, multifaceted, high-paced workflows necessary to support modern large molecule drug discovery. A high-level aspiration is a true integration of "lab-on-a-chip" methods that vastly miniaturize cellulmical experiments could transform the speed, cost, and success of multiple workstreams in biologics development. Several microscale bioprocess technologies have been established that incrementally address these needs, yet each is inflexibly designed for a very specific process thus limiting an integrated holistic application. A more fully integrated nanoscale approach that incorporates manipulation, culture, analytics, and traceable digital record keeping of thousands of single cells in a relevant nanoenvironment would be a transformative technology capable of keeping pace with today's rapid and complex drug discovery demands. The recent advent of optical manipulation of cells using light-induced electrokinetics with micro- and nanoscale cell culture is poised to revolutionize both fundamental and applied biological research. In this review, we summarize the current state of the art for optical manipulation techniques and discuss emerging biological applications of this technology. In particular, we focus on promising prospects for drug discovery workflows, including antibody discovery, bioassay development, antibody engineering, and cell line development, which are enabled by the automation and industrialization of an integrated optoelectronic single-cell manipulation and culture platform. Continued development of such platforms will be well positioned to overcome many of the challenges currently associated with fragmented, low-throughput bioprocess workflows in biopharma and life science research.

Low frequency cyclical potentials for fine tuning insulator-based dielectrophoretic separations

In this study, we demonstrate the use of cyclical low frequency signals with insulator-based dielectrophoresis (iDEP) devices for the separation of particles of similar characteristics and an experimental method for estimating particle DEP mobilities. A custom signal designer program was created using Matlab® and COMSOL Multiphysics® for the identification of specific low frequency signals aimed at separating particle mixtures by exploiting slight differences in surface charge (particle zeta potential) or particle size. For the separation by surface charge, a mixture of two types of 10 μm particles was analyzed and effectively separated employing both a custom step signal and a sawtooth left signal. Notably, these particles had the same shape, size, and surface functionalization as well as were made from the same substrate material. For the separation by size, a sample containing 2 μm and 5 μm particles was successfully separated using a custom step signal; these particles had the same shape, surface functionalization, were made from the same substrate materials, and had only a small difference in zeta potential (10 mV). Additionally, an experimental technique was developed to estimate the dielectrophoretic mobility of each particle type; this information was then utilized by the signal designer program. The technique developed in this study is readily applicable for designing signals capable of separating micron-sized particles of similar characteristics, such as microorganisms, where slight differences in cell size and the shape of surface charge could be effectively exploited. These findings open the possibility for applications in microbial screening using iDEP devices.