Large-Scale Single Particle and Cell Trapping based on Rotating Electric Field Induced-Charge Electroosmosis

Analysis of In Situ Electroporation Utilizing Induced Electric Field at a Wireless Janus Microelectrode

In situ electroporation, a non-invasive technique for enhancing the permeability of cell membranes, has emerged as a powerful tool for intracellular delivery and manipulation. This method allows for the precise introduction of therapeutic agents, such as nucleic acids, drugs, and proteins, directly into target cells within their native tissue environment. Herein, we introduce an innovative electroporation strategy that employs a Janus particle (JP)-based microelectrode to generate a localized and controllable electric field within a microfluidic chip. The microfluidic device is engineered with an indium tin oxide (ITO)-sandwiched microchannel, where the electric field is applied, and suspended JP microelectrodes that induce a stronger localized electric field. The corresponding simulation model is developed to better understand the dynamic electroporation process. Numerical simulations for both single-cell and chain-assembled cell electroporation have been successfully conducted. The effects of various parameters, including pulse voltage, duration medium conductivity, and radius of Janus microelectrode, on cell membrane permeabilization are systematically investigated. Our findings indicate that the enhanced electric intensity near the poles of the JP microelectrode significantly contributes to the electroporation process. In addition, the distribution for both transmembrane voltage and the resultant nanopores can be altered by conveniently adjusting the relative position of the JP microelectrode, demonstrating a selective and in situ electroporation technique for spatial control over the delivery area. Moreover, the obtained differences in the distribution of electroporation between chain cells can offer insightful directives for the electroporation of tissues or cell populations, enabling the precise and targeted modulation of specific cell populations. As a proof of concept, this work can provide a robust alternative technique for the study of complex and personalized cellular processes.

Numerical and experimental investigation of the deviation of microparticles inside the microchannel using the vortices caused by the ICEK phenomenon

One field of study in microfluidics is the control, trapping, and separation of microparticles suspended in fluid. Some of its applications are related to cell handling, virus detection, and so on. One of the new methods in this field is using ICEK phenomena and dielectrophoresis forces. In the present study, considering the ICEK phenomena, the microparticles inside the fluid are deviated in the desired ratio using a novel ICEK microchip. The deviation is such that after the microparticles reach the floating electrode, they are trapped in the ICEK flow vortex and deviated through a secondary channel that was placed crosswise and noncoplanar above the main channel. For simulation verification, an experimental test is done. The method used for making two noncoplanar channels and separating the particles in the desired ratio with a simple ICEK microchip is an innovation of the present study. Moreover, the adjustment of the percentage of separation of microparticles by adjusting the parameters of the applied voltage and fluid inlet velocity is one of the other innovations of the present experimental study. We observed that for input velocities of 150-1200 µm/s with applied voltages of 10-33 V, 100% of the particles can be directed toward the secondary-channel.

Bipolar Electrode-based Sheath-Less Focusing and Continuous Acoustic Sorting of Particles and Cells in an Integrated Microfluidic Device

Sheath-less focusing and sorting of cells or particles is an important preprocessing step in a variety of biochemical applications. Most of the previous sorting methods depend on the use of sheath flows to realize efficient cell focusing. The sheath flow dilutes the sample and requires precise flow control via additional channels. We, for the first time, reported a method of bipolar electrode (BPE)-based sheath-less focusing, switching, and tilted-angle standing surface acoustic wave-based sorting of cells and particles in continuous flow. The device consists of a piezoelectric substrate with a pair of BPEs for focusing and switching, and a pair of interdigitated transducers for cell sorting. Smaller cells experience a weak acoustic force and reach the lower outlet, whereas larger cells are subjected to a strong acoustic force such that they are propelled toward the upper outlet. We first validate the device functionality by sorting 5 and 8 μm PS beads with a high sorting efficiency. The working and deflection regions were increased by propelling the particle beam toward the bottom edge of BPE via changing the applied voltage of BPE, further improving the sorting performance with high efficiency (94%) and purity (92%). We then conducted a verification for sorting THP-1 and yeast cells, and the efficiency and purity reached 90.7 and 91.5%, respectively. This integrated device eliminates the requirement of balancing the flow of several sheath inlets and provides a robust and unique approach for cell sorting applications, showing immense promise in various applications, such as medical diagnosis, drug delivery, and personalized medicine.

Bidirectional and Stepwise Rotation of Cells and Particles Using Induced Charge Electroosmosis Vortexes

The rotation of cells is of significant importance in various applications including bioimaging, biophysical analysis and microsurgery. Current methods usually require complicated fabrication processes. Herein, we proposed an induced charged electroosmosis (ICEO) based on a chip manipulation method for rotating cells. Under an AC electric field, symmetric ICEO flow microvortexes formed above the electrode surface can be used to trap and rotate cells. We have discussed the impact of ICEO and dielectrophoresis (DEP) under the experimental conditions. The capabilities of our method have been tested by investigating the precise rotation of yeast cells and K562 cells in a controllable manner. By adjusting the position of cells, the rotation direction can be changed based on the asymmetric ICEO microvortexes via applying a gate voltage to the gate electrode. Additionally, by applying a pulsed signal instead of a continuous signal, we can also precisely and flexibly rotate cells in a stepwise way. Our ICEO-based rotational manipulation method is an easy to use, biocompatible and low-cost technique, allowing rotation regardless of optical, magnetic or acoustic properties of the sample.

Three-dimensional rotation of deformable cells at a bipolar electrode array using a rotating electric field

Three-dimensional rotation of cells is imperative in a variety of applications such as biology, medicine, and chemistry. We report for the first time a versatile approach for executing controllable 3D rotation of cells or particles at a bipolar electrode (BPE) array using a rotating electric field. The versatility of this method is demonstrated by 3D rotating various cells including yeast cells and K562 cells and the cells can be rotated to a desired orientation and immobilized for further operations. Our results demonstrate how electrorotation torque, induced charge electroosmosis (ICEO) flow and dielectrophoresis can be exerted on certain cells for modulating the rotation axis, speed, and direction. ICEO-based out-of-plane rotation is capable of rotating various cells in a vertical plane regardless of their shape and size. It can realize cell orientation by rotating cells toward a specific angle and enable cell rotation by steadily rotating multiple cells at a controllable speed. The rotation spectrum for in-plane rotation is further used to extract the cellular dielectric properties. This work offers a flexible method for controllable, contactless and precise rotation of different cells or particles, offering a rapid, high-throughput, and nondestructive rotation method for cell analysis and drug discovery.

A Novel Electrokinetic-Based Technique for the Isolation of Circulating Tumor Cells

The separation of rare cells from complex biofluids has attracted attention in biological research and clinical applications, especially for cancer detection and treatment. In particular, various technologies and methods have been developed for the isolation of circulating tumor cells (CTCs) in the blood. Among them, the induced-charge electrokinetic (ICEK) flow method has shown its high efficacy for cell manipulation where micro-vortices (MVs), generated as a result of induced charges on a polarizable surface, can effectively manipulate particles and cells in complex fluids. While the majority of MVs have been induced by AC electric fields, these vortices have also been observed under a DC electric field generated around a polarizable hurdle. In the present numerical work, the capability of MVs for the manipulation of CTCs and their entrapment in the DC electric field is investigated. First, the numerical results are verified against the available data in the literature. Then, various hurdle geometries are employed to find the most effective geometry for MV-based particle entrapment. The effects of electric field strength (EFS), wall zeta potential magnitude, and the particles' diameter on the trapping efficacy are further investigated. The results demonstrated that the MVs generated around only the rectangular hurdle are capable of trapping particles as large as the size of CTCs. An EFS of about 75 V/cm was shown to be effective for the entrapment of above 90% of CTCs in the MVs. In addition, an EFS of 85 V/cm demonstrated a capability for isolating particles larger than 8 µm from a suspension of particles/cells 1-25 µm in diameter, useful for the enrichment of cancer cells and potentially for the real-time and non-invasive monitoring of drug effectiveness on circulating cancer cells in blood circulation.

Droplet Rolling Transport on Hydrophobic Surfaces Under Rotating Electric Fields: A Molecular Dynamics Study

Driving droplets by electric fields is usually achieved by controlling their wettability, and realizing a flexible operation requires complex electrode designs. Here, we show by molecular dynamics methods the droplet transport on hydrophobic surfaces in a rolling manner under a rotating electric field, which provides a simpler and promising way to manipulate droplets. The droplet internal velocity field shows the rolling mode. When the contact angle on the solid surface is 144.4°, the droplet can be transported steadily at a high velocity under the rotating electric field (E = 0.5 V nm-1, ω = π/20 ps-1). The droplet center-of-mass velocities and trajectories, deformation degrees, dynamic contact angles, and surface energies were analyzed regarding the electric field strength and rotational angular frequency. Droplet transport with a complex trajectory on a two-dimensional surface is achieved by setting the electric field, which reflects the programmability of the driving method. Nonuniform wettability stripes can assist in controlling droplet trajectories. The droplet transport on the three-dimensional surface is studied, and the critical conditions for the droplet passing through the surface corners and the motion law on the curved surface are obtained. Droplet coalescence has been achieved by surface designs.

Label free and high-throughput discrimination of cells at a bipolar electrode array using the AC electrodynamics

BACKGROUND

Cell characterization and manipulation play an important role in biological and medical applications. Cell viability evaluation is of significant importance for cell toxicology assay, dose test of anticancer drugs, and other biochemical stimulations. The electrical properties of cells change when cells transform from healthy to a pathological state. Current methods for evaluating cell viability usually requires a complicated chip and the throughput is limited.

RESULTS

In this paper, a bipolar electrode (BPE) array based microfluidic device for assessing cell viability is exploited using AC electrodynamics. The viability of various cells including yeast cells and K562 cells, can be evaluated by analyzing the electro-rotation (ROT) speed and direction of cells, as well as the dielectrophoresis (DEP) responses of cells. Firstly, the cell viability can be identified by the position of the cell captured on the BPE electrode in terms of DEP force. Besides, cell viability can also be evaluated based on both the cell rotation speed and direction using ROT. Under the action of travelling wave dielectric electrophoresis force, the cell viability can also be distinguished by the rotational motion of cells on bipolar electrode edges.

SIGNIFICANCE

This study demonstrates the utility of BPEs to enable scalable and high-throughput AC electrodynamics platforms by imparting a flexibility in chip design that is unparalleled by using traditional electrodes. By using BPEs, our proposed new technique owns wide application for cell characterization and viability assessment in situ detection and analysis.

Portable general microfluidic device with complex electric field regulation functions for electrokinetic experiments

Electrokinetic sample manipulation is a key step for many kinds of microfluidic chips to achieve various functions, such as particle focusing and separation, fluid pumping and material synthesis. But these microfluidic experiments usually rely on large-scale signal generators for power supply, microscopes for imaging and other instruments for analysis, which hampers the portable process of microfluidic technology. Inspired by this situation, we herein designed a portable general microfluidic device (PGMD) with complex electric field regulation functions, which can accurately regulate static or continuous fluid samples. Through the graphical user interface (GUI) and modular design, the PGMD can generate multiple different electrical signals, and the micro-flow of fluid can be pumped through the built-in micropump, which can meet the requirements of most microfluidic experiments. Photos or videos of the microfluidic chip captured by the built-in microscope are received and displayed by a smartphone. We carried out a variety of microfluidic experiments such as induced-charge electroosmosis (ICEO), particle beam exit switching, thermal buoyancy flow and dielectrophoresis (DEP) on the PGMD. In addition, the PGMD can perform rapid microalgae concentration estimation in an outdoor environment, which can be used to guide microalgae cultivation, further demonstrating the development potential of this device in the field of microbial applications. Numerous results show that the PGMD has a high degree of integration and strong reliability, which expands the application of microfluidic electrokinetic experiments and provides technical support for the integration and portability of microfluidic experimental devices.

Self-assembled microtubular electrodes for on-chip low-voltage electrophoretic manipulation of charged particles and macromolecules

On-chip manipulation of charged particles using electrophoresis or electroosmosis is widely used for many applications, including optofluidic sensing, bioanalysis and macromolecular data storage. We hereby demonstrate a technique for the capture, localization, and release of charged particles and DNA molecules in an aqueous solution using tubular structures enabled by a strain-induced self-rolled-up nanomembrane (S-RuM) platform. Cuffed-in 3D electrodes that are embedded in cylindrical S-RuM structures and biased by a constant DC voltage are used to provide a uniform electrical field inside the microtubular devices. Efficient charged-particle manipulation is achieved at a bias voltage of <2-4 V, which is ~3 orders of magnitude lower than the required potential in traditional DC electrophoretic devices. Furthermore, Poisson-Boltzmann multiphysics simulation validates the feasibility and advantage of our microtubular charge manipulation devices over planar and other 3D variations of microfluidic devices. This work lays the foundation for on-chip DNA manipulation for data storage applications.

Lab-on-Chip Microsystems for Ex Vivo Network of Neurons Studies: A Review

Increasing population is suffering from neurological disorders nowadays, with no effective therapy available to treat them. Explicit knowledge of network of neurons (NoN) in the human brain is key to understanding the pathology of neurological diseases. Research in NoN developed slower than expected due to the complexity of the human brain and the ethical considerations for in vivo studies. However, advances in nanomaterials and micro-/nano-microfabrication have opened up the chances for a deeper understanding of NoN ex vivo, one step closer to in vivo studies. This review therefore summarizes the latest advances in lab-on-chip microsystems for ex vivo NoN studies by focusing on the advanced materials, techniques, and models for ex vivo NoN studies. The essential methods for constructing lab-on-chip models are microfluidics and microelectrode arrays. Through combination with functional biomaterials and biocompatible materials, the microfluidics and microelectrode arrays enable the development of various models for ex vivo NoN studies. This review also includes the state-of-the-art brain slide and organoid-on-chip models. The end of this review discusses the previous issues and future perspectives for NoN studies.

Enhancement of Binding Kinetics on Affinity Substrates Using Asymmetric Electroosmotic Flow on a Sinusoidal Bipolar Electrode

In the context of the COVID-19 epidemic, enhancing the transport of analyte to a sensor surface is crucial for rapid detection of biomolecules since common conditions, including low diffusion coefficients, cause inordinately long detection times. Integrated microfluidic immunoassay chips are receiving increasing attention for their low sample volume and fast response time. We herein take advantage of asymmetric ICEO flow at a bipolar sinusoidal electrode to improve the rate of antibody binding to the reaction surface based on finite element modeling. Three different microfluidic cavities are proposed by changing the positions of the surface reaction area. We further investigate the relationship between binding enhancement and reaction surface positions, Damkohler number, and the voltage and frequency of the AC signal applied to the driving electrodes. Furthermore, the influence of the AC signal applied to the sinusoidal bipolar electrode on antigen–antibody-binding performance is studied in detail. Above all, the simulation results demonstrate that the microfluidic immune-sensor with a sinusoidal bipolar electrode could not only significantly improve the heterogeneous immunoassays but also enable efficient enhancement of assays in a selected reaction region within the micro-cavity, providing a promising approach to a variety of immunoassay applications, such as medical diagnostics and environmental and food monitoring.

Generation of droplets with adjustable chemical concentrations based on fixed potential induced-charge electro-osmosis

The effective control of the sample concentration within droplets is essential in a broad range of assays in chemistry and biochemistry. Here we provide an electrical method for producing batches of aqueous droplets with various chemical concentrations by exploiting fixed-potential induced-charge electroosmosis (ICEO) flow around a bipolar electrode. By applying an AC electric signal to the bipolar electrode and changing the zeta potential on it, the bipolar electrode acts as a gate electrode for generating asymmetric ICEO flow. The ICEO flow induced transverse vortexes interact with two parallel laminar streams with different chemical compositions. Controlled mixing of the aqueous solutions can be achieved by adjusting the shape and size of the asymmetric vortexes via altering the electric signal applied to the gate electrode. The mixed streams are split at a bifurcation, and one of the streams with a desired controlled concentration is pumped into a flow-focusing geometry to generate droplets with adjustable chemical concentrations. The in-droplet concentration increases in the range of 0.412-1.404 mM, as the applied voltage increases in the range of 0-70 mV at 15 kHz. This approach offers a promising method for on-chip control of chemical concentrations within droplets without labor-intensive dilutions while minimizing the sample consumption, showing great potential for next generation droplet-based applications.

Controlled generation of droplets using an electric field in a flow-focusing paper-based device

Droplet-based microfluidics is a modular platform in high-throughput single-cell and small sample analyses. However, this droplet microfluidic system was widely fabricated using soft lithography or glass capillaries, which is expensive and technically demanding for various applications, limiting use in resource-poor settings. Besides, the variation in droplet size is also restricted due to the limitations on the operating forces that the paper-based platform is able to withstand. Herein, we develop a fully integrated paper-based droplet microfluidic platform for conducting droplet generation and cell encapsulation in independent aqueous droplets dispersed in a carrier oil by incorporating electric fields. Through imposing an electric field, the droplet size would decrease with increasing the electric field and smaller droplets can be produced at high applied voltage. The droplet diameter can be adjusted by the ratio of inner and outer flow velocities as well as the applied electric field. We also demonstrated the proof of concept encapsulation application of our paper device by encapsulating yeast cells under an electric field. Using a simple wax printing method, carbon electrodes can be integrated on the paper. The integrated paper-based microfluidic platform can be fabricated easily and conducted outside of centralized laboratories. This microfluidic system shows great potential in drug and cell investigations by encapsulating cells in resource-limited environments.

Label-Free Multitarget Separation of Particles and Cells under Flow Using Acoustic, Electrophoretic, and Hydrodynamic Forces

Multiplex separation of mixed biological samples is essential in a considerable portion of biomedical research and clinical applications. An automated and operator-independent process for the separation of samples is highly sought after. There is a significant unmet need for methods that can perform fractionation of small volumes of multicomponent mixtures. Herein, we design an integrated chip that combines acoustic and electric fields to enable efficient and label-free separation of multiple different cells and particles under flow. To facilitate the connection of multiple sorting mechanisms in tandem, we investigate the electroosmosis (EO)-induced deterministic lateral displacement (DLD) separation in a combined pressure- and DC field-driven flow and exploit the combination of the bipolar electrode (BPE) focusing and surface acoustic wave (SAW) sorting modules. We successfully integrate four sequential microfluidic modules for multitarget separation within a single platform: (i) sorting particles and cells relying on the size and surface charge by adjusting the flow rate and electric field using a DLD array; (ii) alignment of cells or particles within a microfluidic channel by a bipolar electrode; (iii) separation of particles based on compressibility and density by the acoustic force; and (iv) separation of viable and nonviable cells using dielectric properties via the dielectrophoresis (DEP) force. As a proof of principle, we demonstrate the sorting of multiple cell and particle types (polystyrene (PS) particles, oil droplets, and viable and nonviable yeast cells) with high efficiency. This integrated microfluidic platform combines multiple functional components and, with its ability to noninvasively sort multiple targeted cells in a label-free manner relying on different properties, is compatible with high-definition imaging, showing great potential in diverse diagnostic and analysis applications.

Characterization of Particle Movement and High-Resolution Separation of Microalgal Cells via Induced-Charge Electroosmotic Advective Spiral Flow

Microalgae are renewable, sustainable, and economical sources of biofuels and are capable of addressing pressing global demand for energy security. However, two challenging issues to produce high-level biofuels are to separate promising algal strains and protect biofuels from contamination of undesired bacteria, which rely on an economical and high-resolution separation technology. Separation technology based on induced-charge electroosmotic (ICEO) vortices offers excellent promise in economical microalga separation for producing biofuels because of its reconfigurable and flexible profiles and sensitive and precise selectivity. In this work, a practical ICEO vortex device is developed to facilitate high-resolution isolation of rich-lipid microalgae for the first time. We investigate electrokinetic equilibrium states of particles and particle-fluid ICEO effect in binary-particle manipulation. Nanoparticle separation is performed to demonstrate the feasibility and resolution of this device, yielding clear separation. Afterward, we leverage this technology in isolation of Chlorella vulgaris from heterogeneous microalgae with the purity exceeding 96.4%. Besides, this platform is successfully engineered for the extraction of single-cell Oocystis sp., obtaining the purity surpassing 95.2%. Moreover, with modulating parameters, we isolate desired-cell-number Oocystis sp. enabling us to investigate proliferation mode and carry out transcriptome analyses of Oocystis sp. for high-quality neutral lipids. This platform can be extended directly to economically separate other biological micro/nanosamples to address pressing issues, involving energy security, environmental monitoring, and disease diagnosis.

Area cooling enables thermal positioning and manipulation of single cells

Contactless particle manipulation based on a thermal field has shown great potential for biological, medical, and materials science applications. However, thermal diffusion from a high-temperature area causes thermal damage to bio-samples. Besides, the permanent bonding of a sample chamber onto microheater substrates requires that the thermal field devices be non-disposable. These limitations impede use of the thermal manipulation approach. Here, a novel manipulation platform is proposed that combines microheaters and an area cooling system to produce enough force to steer sedimentary particles or cells and to limit the thermal diffusion. It uses the one-time fabricated motherboard and an exchangeable sample chamber that provides disposable use. Sedimentary objects can be steered to the bottom center of the thermal field by combined thermal convection and thermophoresis. Single particle or cell manipulation is realized by applying multiple microheaters in the platform. Results of a cell viability test confirmed the method's compatibility in biology fields. With its advantages of biocompatibility for live cells, operability for different sizes of particles and flexibility of platform fabrication, this novel manipulation platform has a high potential to become a powerful tool for biology research.

A novel automatic segmentation and tracking method to measure cellular dielectrophoretic mobility from individual cell trajectories for high throughput assay

BACKGROUND AND OBJECTIVE

The dielectrophoresis (DEP) technique is increasingly being recognised as a potentially valuable tool for non-contact manipulation of numerous cells as well as for biological single cell analysis with non-invasive characterisation of a cell's electrical properties. Several studies have attempted to track multiple cells to characterise their cellular DEP mobility. However, they encountered difficulties in simultaneously tracking the movement of a large number of individual cells in a bright-field image sequence because of interference from the background electrode pattern. Consequently, this present study aims to develop an automatic system for imaging-based characterisation of cellular DEP mobility, which enables the simultaneous tracking of several hundred of cells inside a microfluidic device.

METHODS

The proposed method for segmentation and tracking of cells consists of two main stages: pre-processing and particle centre localisation. In the pre-processing stage, background subtraction and contrast enhancement were performed to distinguish the cell region from the background image. In the particle centre localisation stage, the unmarked cell was automatically detected via graph-cut algorithm-based K-means clustering.

RESULTS

Our algorithm enabled segmentation and tracking of numerous Michigan Cancer Foundation-7 (MCF-7) cell trajectories while the DEP force was oscillated between positive and negative. The cell tracking accuracy and cell count capability was at least 90% of the total number of cells with the newly developed algorithm. In addition, the cross-over frequency was measured by analysing the segmented and tracked trajectory data of the cellular movements caused by the positive and negative DEP force. The measured cross-over frequency was compared with previous results. The multi-cellular movements investigation based on the measured cross-over frequency was repeated until the viability of cells was unchanged in the same environment as in a microfluidic device. The results were statistically consistent, indicating that the developed algorithm was reliable for the investigation of DEP cellular mobility.

CONCLUSION

This study developed a powerful platform to simultaneously measure the DEP-induced trajectories of numerous cells, and to investigate in a robust, efficient, and accurate manner the DEP properties at both the single cell and cell ensemble level.

Recent advancement in induced-charge electrokinetic phenomena and their micro- and nano-fluidic applications

Induced-charge electrokinetics (ICEK) remains a hot topic due to its promising applications in micro- and nano-fluidics. Over the past decade, researchers have made a great advancement in both fundamental studies and application developments. They captured (I) a flow reversal in induced-charge electroosmosis (ICEO) and attributed it to the phase delay effect of ions, (II) a chaotic ICEO and attributed it to the concentration polarization in the bulk solution, (III) a non-quadratic correlation for ICEO of non-Newtonian fluids and attributed it to the power-law viscosity, (IV) an induced-charge electrophoretic (ICEP) rotation of Janus doublets, etc. Furthermore, various ICEK-based micro- and nano-fluidic devices have been developed, namely, micropumps, particle focusers, trappers, sorters, and nanopore ion diodes. The present article provides a comprehensive review on the recent advancement of ICEK. Firstly, the fundamental studies of ICEK are introduced; then the micro- and nano-fluidic applications based on ICEK are presented; lastly, promising future directions for both fundamental and applications are discussed. This review presents the basic framework of ICEK, and can facilitate the development of ICEK-based applications.

Induced charge electro-osmotic particle separation

Vortex-based separation is a promising method in particle-particle separation and has only been demonstrated theoretically some years ago. To date, a continuous-flow separation device based on vortices has not been conceived because many known vortices were either unstable or controlling them lacked precision. Electro-convection from induced charge electro-osmosis (ICEO) has advantages, such as adjustable flow profiles, long-range actuation, and long-lived vortices, and offers an alternative means of particle separation. We found though a different ICEO focusing behaviour of particles whereby particles were trapped and concentrated in two vortex cores. Encouraged by these features of ICEO vortices, we proposed a direct method for particle separation in continuous flow. In various experiments, we first characterized the ICEO-induced focusing performances of various kinds of particle samples in a straight channel embedded with an individual central bipolar electrode, presenting a justifiable explanation. Second, the combined dependences of ICEO particle separation on the sample size and mass density were investigated. Third, an application to cell purification was performed in which we obtained a purity surpassing 98%. Finally, we investigated the ICEO characteristics of nanoparticles, exploiting our method in isolating nanoscale objects by separating 500 nm and 5 μm polystyrene beads, gaining clear separation. Certain features of this method, such as having ease of operation, simple structure, and continuous flow, and being prefocusing free and physical property-based, indicate its good potential in tackling environmental monitoring, cell sorting, chemical analysis, isolation of uniform-sized graphene and transesterification of micro-algal lipids to biodiesel.

Three-dimensional paper based platform for automatically running multiple assays in a single step

Paper based assays are paving the way to automated, simplified, robust and cost-effective point of care testing (POCT). We propose a method for fabricating three dimensional (3D) microfluidic paper based analytical devices (μPADs) via combining thin adhesive films and paper folding, which avoids the use of cellulose powders and the complex folding sequence and simultaneously permits assays in several layers. To demonstrate the effectiveness of this approach, a 3DμPADs was designed to conduct more assays on a small footprint, allowing dual colorimetric and electrochemical detections. More importantly, we further developed a 3D platform for implementing automated and multiplexed ELISA in parallel, since ELISA, a routine and standard laboratory method, has rarely been used in practical analyses outside of the laboratory. In this configuration, complex and multistep diagnostic assays can be carried out with the addition of the sample and buffer in a simple fashion. Using Troponin I as model, the device showed a broad dynamic range of detection with a detection limit of 0.35 ng/mL. Thus, the developed platforms allow for various assays to be cost-effectively carried out on a single 3D device, showing great potential in an academic setting and point of care testing under resource-poor conditions.

Microfluidic Single-Cell Manipulation and Analysis: Methods and Applications

In a forest of a hundred thousand trees, no two leaves are alike. Similarly, no two cells in a genetically identical group are the same. This heterogeneity at the single-cell level has been recognized to be vital for the correct interpretation of diagnostic and therapeutic results of diseases, but has been masked for a long time by studying average responses from a population. To comprehensively understand cell heterogeneity, diverse manipulation and comprehensive analysis of cells at the single-cell level are demanded. However, using traditional biological tools, such as petri-dishes and well-plates, is technically challengeable for manipulating and analyzing single-cells with small size and low concentration of target biomolecules. With the development of microfluidics, which is a technology of manipulating and controlling fluids in the range of micro- to pico-liters in networks of channels with dimensions from tens to hundreds of microns, single-cell study has been blooming for almost two decades. Comparing to conventional petri-dish or well-plate experiments, microfluidic single-cell analysis offers advantages of higher throughput, smaller sample volume, automatic sample processing, and lower contamination risk, etc., which made microfluidics an ideal technology for conducting statically meaningful single-cell research. In this review, we will summarize the advances of microfluidics for single-cell manipulation and analysis from the aspects of methods and applications. First, various methods, such as hydrodynamic and electrical approaches, for microfluidic single-cell manipulation will be summarized. Second, single-cell analysis ranging from cellular to genetic level by using microfluidic technology is summarized. Last, we will also discuss the advantages and disadvantages of various microfluidic methods for single-cell manipulation, and then outlook the trend of microfluidic single-cell analysis.

Asymmetrical Induced Charge Electroosmotic Flow on a Herringbone Floating Electrode and Its Application in a Micromixer

Enhancing mixing is of significant importance in microfluidic devices characterized by laminar flows and low Reynolds numbers. An asymmetrical induced charge electroosmotic (ICEO) vortex pair generated on the herringbone floating electrode can disturb the interface of two-phase fluids and deliver the fluid transversely, which could be exploited to accomplish fluid mixing between two neighbouring fluids in a microscale system. Herein we present a micromixer based on an asymmetrical ICEO flow induced above the herringbone floating electrode array surface. We investigate the average transverse ICEO slip velocity on the Ridge/Vee/herringbone floating electrode and find that the microvortex generated on the herringbone electrode surface has good potential for mixing the miscible liquids in microfluidic systems. In addition, we explore the effect of applied frequencies and bulk conductivity on the slip velocity above the herringbone floating electrode surface. The high dependence of mixing performance on the floating electrode pair numbers is analysed simultaneously. Finally, we investigate systematically voltage intensity, applied frequencies, inlet fluid velocity and liquid conductivity on the mixing performance of the proposed device. The microfluidic micromixer put forward herein offers great opportunity for fluid mixing in the field of micro total analysis systems.

Current developments and applications of microfluidic technology toward clinical translation of nanomedicines

Nanoparticulate drug delivery systems hold great potential for the therapy of many diseases, especially cancer. However, the translation of nanoparticulate drug delivery systems from academic research to industrial and clinical practice has been slow. This slow translation can be ascribed to the high batch-to-batch variations and insufficient production rate of the conventional preparation methods, and the lack of technologies for rapid screening of nanoparticulate drug delivery systems with high correlation to the in vivo tests. These issues can be addressed by the microfluidic technologies. For example, microfluidics can not only produce nanoparticles in a well-controlled, reproducible, and high-throughput manner, but also create 3D environments with continuous flow to mimic the physiological and/or pathological processes. This review provides an overview of the microfluidic devices developed to prepare nanoparticulate drug delivery systems, including drug nanosuspensions, polymer nanoparticles, polyplexes, structured nanoparticles and theranostic nanoparticles. We also highlight the recent advances of microfluidic systems in fabricating the increasingly realistic models of the in vivo milieu for rapid screening of nanoparticles. Overall, the microfluidic technologies offer a promise approach to accelerate the clinical translation of nanoparticulate drug delivery systems.

Continuous-flow trapping and localized enrichment of micro- and nano-particles using induced-charge electrokinetics

In this work, we report an effective microfluidic technique for continuous-flow trapping and localized enrichment of micro- and nano-particles by using induced-charge electrokinetic (ICEK) phenomena. The proposed technique utilizes a simple microfluidic device that consists of a straight microchannel and a conducting strip attached to the bottom wall of the microchannel. Upon application of the electric field along the microchannel, the conducting strip becomes polarized to introduce two types of ICEK phenomena, the ICEK flow vortex and particle dielectrophoresis, and they are identified by a theoretical model formulated in this study to be jointly responsible for the trapping of particles over the edge of the conducting strip. Our experiments showed that successful trapping requires an AC/DC combined electric field: the DC component is mainly to induce electroosmotic flow for transporting particles to the trapping location; the AC component induces ICEK phenomena over the edge of the conducting strip for particle trapping. The performance of the technique is examined with respect to the applied electric voltage, AC frequency and the particle size. We observed that the trapped particles form a narrow band (nearly a straight line) defined by the edge of the conducting strip, thereby allowing localized particle enrichment. For instance, we found that under certain conditions a high particle enrichment ratio of 200 was achieved within 30 seconds. We also demonstrated that the proposed technique was able to trap particles from several microns down to several tens of nanometer. We believe that the proposed ICEK trapping would have great flexibility that the trapping location can be readily varied by controlling the location of the patterned conducting strip and multiple-location trapping can be expected with the use of multiple conducting strips.

Simulation Analysis of Improving Microfluidic Heterogeneous Immunoassay Using Induced Charge Electroosmosis on a Floating Gate

On-chip immuno-sensors are a hot topic in the microfluidic community, which is usually limited by slow diffusion-dominated transport of analytes in confined microchannels. Specifically, the antigen-antibody binding reaction at a functionalized area cannot be provided with enough antigen source near the reaction surface, since a small diffusion flux cannot match with the quick rate of surface reaction, which influences the response time and sensitivity of on-chip heterogeneous immunoassay. In this work, we propose a method to enhance the transportation of biomolecules to the surface of an antibody-immobilized electrode with induce charge electroosmotic (ICEO) convection in a low concentration suspension, so as to improve the binding efficiency of microfluidic heterogeneous immunoassays. The circular stirring fluid motion of ICEO on the surface of a floating gate electrode at the channel bottom accelerates the transport of freely suspended antigen towards the wall-immobilized antibodies. We investigate the dependence of binding efficiency on voltage magnitude and field frequency of the applied alternate current (AC) electrical field. The binding rate yields a factor of 5.4 higher binding for an applied voltage of 4 V at 10 Hz when the Damkohler number is 1000. The proposed microfluidic immuno-sensor technology of a simple electrode structure using ICEO convective fluid flow around floating conductors could offer exciting opportunities for diffusion-limited on-chip bio-microfluidic sensors.