Elastic-inertial separation of microparticle in a gradually contracted microchannel

Separation of microparticle in viscoelastic fluid is highly required in the field of biology and clinical medicine. For instance, the separation of the target cell from blood is an important prerequisite step for the drug screening and design. The microfluidic device is an efficient way to achieve the separation of the microparticle in the viscoelastic fluid. However, the existing microfluidic methods often have some limitations, including the requirement of the long channel length, the labeling process, and the low throughput. In this work, based on the elastic-inertial effect in the viscoelastic fluid, a new separation method is proposed where a gradually contracted microchannel is designed to efficiently adjust the forces exerted on the particle, eventually achieving the high-efficiency separation of different sized particles in a short channel length and at a high throughput. In addition, the separation of WBCs and RBCs is also validated in the present device. The effect of the flow rate, the fluid property, and the channel geometry on the particle separation is systematically investigated by the experiment. With the advantage of small footprint, simple structure, high throughput, and high efficiency, the present microfluidic device could be utilized in the biological and clinical fields, such as the cell analysis and disease diagnosis.     

Electrokinetically driven micro flow cytometers with integrated fiber optics for on-line cell/particle detection

This paper presents an innovative micro flow cytometer which is capable of counting and sorting cells or particles. This compact device employs electrokinetic forces rather than the more conventional hydrodynamic forces technique for flow focusing and sample switching, and incorporates buried optical fibers for the on-line detection of cells or particles. This design approach results in a compact microfluidic system and an easier integration process. The proposed cytometer integrates several critical modules, namely electrokinetic-focusing devices, built-in control electrodes, buried optical fibers for on-line detection, and electrokinetic flow switches for bio-particle collection. A linear relationship exists between the focused stream width (d) and the focusing ratio (F/φ), which is estimated to be D≈134.5−53.8F/φ. The relationship between the particle velocity (U) and the applied voltage (V) is also investigated. Numerical and experimental data confirm the effectiveness of the device when applied to the counting and sorting of 10 μm diameter particles and red blood cells.     

Double spiral microchannel for label-free tumor cell separation and enrichment

This work reports on a passive double spiral microfluidic device allowing rapid and label-free tumor cell separation and enrichment from diluted peripheral whole blood, by exploiting the size-dependent hydrodynamic forces. A numerical model is developed to simulate the Dean flow inside the curved geometry and to track the particle/cell trajectories, which is validated against the experimental observations and serves as a theoretical foundation for optimizing the operating conditions. Results from separating tumor cells (MCF-7 and Hela) spiked into whole blood indicate that 92.28% of blood cells and 96.77% of tumor cells are collected at the inner and the middle outlet, respectively, with 88.5% tumor recovery rate at a throughput of 3.33 × 10(7) cells min(-1). We expect that this label-free microfluidic platform, driven by purely hydrodynamic forces, would have an impact on fundamental and clinical studies of circulating tumor cells.     

Mass diffusion-based separation of sugars in a microfluidic contactor with nanofiltration membranes

Processes such as chromatographic separation and nanofiltration can remove low molecular weight sugars from liquid mixtures of oligosaccharides. As an alternative for the separation of such liquid mixtures, we studied mass diffusion separation of such sugars in a microfluidic device with incorporated nanofiltration membranes. This separation method is based on differences between diffusivities of components and does not require high transmembrane pressures. The effects of channel depth and flow rate were studied in experiments. The key parameters selectivity and rejection increased with increasing channel depth due to increased external mass transfer limitations. Among the studied membranes, the obtained selectivities and rejections correlated to the specified retention values by the manufacturers. Compared to more conventional nanofiltration where high pressure forces solutes through membranes, we obtained corresponding selectivities and fluxes of only an order of magnitude smaller. Simulated results indicated that with optimized microchannel and membrane dimensions, the presented separation process can compete with currently available separation technologies.     

Behavior of double emulsions in a cross-type optical separation system

The behavior of double emulsions in a cross-type optical particle separation system was studied for different combinations of refractive indices and different inner and outer layer radii. The radii and refractive indices of the double emulsions were easily adjusted by taking advantage of the coflowing geometry of a cross-type optical particle separation device. An analytical expression of the optical forces on a pair of concentric spheres was derived using the photon stream method in the ray optics regime. The predicted trajectories of the double emulsions by the optical force agreed well with the experimental data. This work has potential uses in cell separation by morphometry, drug delivery vehicle, and emulsion-based biomedical applications.     

Improvement of size-based particle separation throughput in slanted spiral microchannel by modifying outlet geometry

The inertial microfluidic technique, as a powerful new tool for accurate cell/particle separation based on the hydrodynamic phenomenon, has drawn considerable interest in recent years. Despite numerous microfluidic techniques of particle separation, there are few articles in the literature on separation techniques addressing external outlet geometry to increase the throughput efficiency and purity. In this work, we report on a spiral inertial microfluidic device with high efficiency (>98%). Herein, we demonstrate how changing the outlet geometry can improve the particle separation throughput. We present a complete separation of 4 and 6 μm from 10 μm particles potentially applicable to separate microalgae (Tetraselmis suecica from Phaeodactylum tricornutum). Two spiral microchannels with the same cross section dimension but different outlet geometry were considered and tested to investigate the particle focusing behavior and separation efficiency. As compared with particle focusing observed in channels with a simple outlet, the particle focusing in a modified outlet geometry appears in a more successful focusing manner with complete separation. This simple approach of particle separation makes it attractive for lab-on-a-chip devices for continuous extraction and filtration of a wide range of cell/particle sizes.     

Continuous dielectrophoretic particle separation via isomotive dielectrophoresis with bifurcating stagnation flow

We present a novel technique for continuous label‐free separation of particles based on their dielectrophoretic crossover frequencies. Our technique relies on our unique microfluidic geometry which performs hydrodynamic focusing, generates a stagnation flow with two outlets, and simultaneously produces an isomotive dielectrophoretic field via wall‐situated electrodes. To perform particle separation, we hydrodynamically focus particles onto stagnation streamlines and use isomotive dielectrophoretic force to nudge the particles off these streamlines and direct them into appropriate outlets. Focusing particles onto stagnation streamlines obviates the need for large forces to be applied to the particles and therefore increases system throughput. The use of isomotive (spatially uniform) dielectrophoretic force increases system reliability. To guide designers, we develop and describe a simple scaling model for the particle separation dynamics of our technique. The model predicts the range of particle sizes that can be separated as well as the processing rate that can be achieved as a function of system design parameters: channel size, flow rate, and applied potential. Finally, as a proof‐of‐principle, we use this technique to separate polystyrene bead and cell mixtures of the same diameters as well as mixtures of both particles with varying diameters.     

A water-activated pump for portable microfluidic applications

An on-chip micropump for portable microfluidic applications was investigated using mathematical modeling and experimental testing. This micropump is activated by the addition of water, via a dropper, to ionic polymer particles that swell due to osmotic effects when wetted. The resulting particle volume increase deflects a membrane, forcing a separate fluid from an adjacent reservoir. The micropump components, along with the microfluidic components, are fabricated using the contact liquid photolithographic polymerization (CLiPP) method. The maximum flow rate achieved with this pump is 17 microL per minute per mg of dry polymer particles of 355-425 microm in diameter. The pump flow rate may be controlled by adjusting the particle size and amount, the membrane properties, and the channel dimensions. The experimental results demonstrate good agreement with an analytical model describing the particle swelling and its coupling with resistive forces from the bending membrane, viscous flow in the microchannel, and interfacial effects. Key features of this micropump are that it can be placed directly on a microdevice, and that it requires only a small amount of water and no external power supply to function. Therefore, this pumping system is useful for applications in which a highly portable device is required.     

Sodium silicate based sol–gel structures for generating pressure-driven flow in microfluidic channels

In this article, we report the design of a microchip based hydraulic pump that employs a sodium silicate derived sol–gel structure for generating pressure-driven flow within a microfluidic network. The reported sol–gel structure was fabricated in a chosen location of our device by selectively retaining sodium silicate solution within a sub-micrometer deep segment via capillary forces, and then providing the precursor material appropriate thermal treatment. It was shown that while the molecular weight cut-off for these membranes is at least an order of magnitude smaller than their photo-polymerized counterparts, their electrical conductance is significant. Moreover, unlike their polymeric counterparts these structures were found to be capable of blocking electroosmotic flow, thereby generating a pressure-gradient around their interface with an open microchannel upon application of an electric field across the microchannel–membrane junction. In this work, a fraction of the resulting hydrodynamic flow was successfully guided to an electric field-free analysis channel to implement a pressure-driven assay. Our experiments show that the pressure-driven velocity produced in the analysis channel of our device varied linearly with the voltage applied across the sol–gel membrane and was nearly independent of the cross-sectional dimensions of the membrane and the microfluidic channels. With our current design pressure-driven velocities up to 1.7 mm/s were generated for an applied voltage of 2 kV, which easily covers the range of flow speeds that can minimize the plate height in most microfluidic separations. Finally, the functionality of our device was demonstrated by implementing a reverse phase chromatographic separation in the analysis channel of our device using the pressure-driven flow generated on-chip.     

Microfluidic device for concentration and SERS-based detection of bacteria in drinking water

There is a constant need for the development of easy-to-operate systems for the rapid and unambiguous identification of bacterial pathogens in drinking water without the requirement for time-consuming culture processes. In this study, we present a disposable and low-cost lab-on-a-chip device utilizing a nanoporous membrane, which connects two stacked perpendicular microfluidic channels. Whereas one of the channels supplies the sample, the second one attracts it by potential-driven forces. Surface-enhanced Raman spectrometry (SERS) is employed as a reliable detection method for bacteria identification. To gain the effect of surface enhancement, silver nanoparticles were added to the sample. The pores of the membrane act as a filter trapping the bodies of microorganisms as well as clusters of nanoparticles creating suitable conditions for sensitive SERS detection. Therein, we focused on the construction and characterization of the device performance. To demonstrate the functionality of the microfluidic chip, we analyzed common pathogens (Escherichia coli DH5α and Pseudomonas taiwanensis VLB120) from spiked tap water using the optimized experimental parameters. The obtained results confirmed our system to be promising for the construction of a disposable optical platform for reliable and rapid pathogen detection which couples their electrokinetic concentration on the integrated nanoporous membrane with SERS detection.     

Image‐based sorting and negative dielectrophoresis for high purity cell and particle separation

Microelectrode arrays are used to sort single fluorescently labeled cells and particles as they flow through a microfluidic channel using dielectrophoresis. Negative dielectrophoresis is used to create a “Dielectrophoretic virtual channel” that runs along the center of the microfluidic channel. By switching the polarity of the electrodes, the virtual channel can be dynamically reconfigured to direct particles along a different path. This is demonstrated by sorting particles into two microfluidic outlets, controlled by an automated system that interprets video data from a color camera and makes complex sorting decisions based on color, intensity, size, and shape. This enables the rejection of particle aggregates and other impurities, and the system is optimized to isolate high purity populations from a heterogeneous sample. Green beads are isolated from an excess of red beads with 100% purity at a rate of up to 0.9 particles per second, in addition application to the sorting of osteosarcoma and human bone marrow cells is evidenced. The extension of Dielectrophoretic Virtual Channels to an arbitrary number of sorting outputs is examined, with design, simulation, and experimental verification of two alternate geometries presented and compared.     

An optically driven pump for microfluidics

We demonstrate a method for generating flow within a microfluidic channel using an optically driven pump. The pump consists of two counter rotating birefringent vaterite particles trapped within a microfluidic channel and driven using optical tweezers. The transfer of spin angular momentum from a circularly polarised laser beam rotates the particles at up to 10 Hz. We show that the pump is able to displace fluid in microchannels, with flow rates of up to 200 microm(3) s(-1) (200 fL s(-1)). The direction of fluid pumping can be reversed by altering the sense of the rotation of the vaterite beads. We also incorporate a novel optical sensing method, based upon an additional probe particle, trapped within separate optical tweezers, enabling us to map the magnitude and direction of fluid flow within the channel. The techniques described in the paper have potential to be extended to drive an integrated lab-on-chip device, where pumping, flow measurement and optical sensing could all be achieved by structuring a single laser beam.     

Theoretical and experimental analysis of negative dielectrophoresis-induced particle trajectories

Many biomedical analysis applications require trapping and manipulating single cells and cell clusters within microfluidic devices. Dielectrophoresis (DEP) is a label-free technique that can achieve flexible cell trapping, without physical barriers, using electric field gradients created in the device by an electrode microarray. Little is known about how fluid flow forces created by the electrodes, such as thermally driven convection and electroosmosis, affect DEP-based cell capture under high conductance media conditions that simulate physiologically relevant fluids such as blood or plasma. Here, we compare theoretical trajectories of particles under the influence of negative DEP (nDEP) with observed trajectories of real particles in a high conductance buffer. We used 10-µm diameter polystyrene beads as model cells and tracked their trajectories in the DEP microfluidic chip. The theoretical nDEP trajectories were in close agreement with the observed particle behavior. This agreement indicates that the movement of the particles was highly dominated by the DEP force and that contributions from thermal- and electroosmotic-driven flows were negligible under these experimental conditions. The analysis protocol developed here offers a strategy that can be applied to future studies with different applied voltages, frequencies, conductivities, and polarization properties of the targeted particles and surrounding medium. These findings motivate further DEP device development to manipulate particle trajectories for trapping applications.     

Acceleration of absolute negative mobility

Recently, the counter intuitive migration phenomenon of absolute negative mobility (ANM) has been demonstrated to occur for colloidal particles in a suitably arranged post array within a microfluidic device [1]. This effect is based on the interplay of Brownian motion, nonlinear dynamics induced through microstructuring, and nonequilibrium driving, and results in a particle movement opposite to an applied static force. Simultaneously, the migration of a different particle species along the direction of the static force is possible [19], thus providing a new tool for particle sorting in microfluidic device format. The so far demonstrated maximum velocities for micrometer-sized spheres are slow, i. e., in the order of 10 nm per second. Here, we investigate numerically, how maximum ANM velocities can be significantly accelerated by a careful adjustment of the post size and shape. Based on this numerical analysis, a post design is developed and tested in a microfluidic device made of PDMS. The experiment reveals an order of magnitude increase in velocity.     

Acoustofluidics 11: Affinity specific extraction and sample decomplexing using continuous flow acoustophoresis

Acoustic standing wave technology combined with ligand complexed microbeads offers a means for affinity specific selection of target analytes from complex samples. When realized in a microfluidic format we can capitalize on laminar flow and acoustic forces that can drive cells or microbeads across fluid interfaces. Given this, we have the ability to perform carrier fluid (suspending medium) exchange operations in continuous flow in microfluidic chips based solely on acoustofluidic properties. A key issue here is to ensure that a minimum of the original carrier fluid follows the cells/particles across the fluid interface. Simple processing protocols can be achieved that may outperform macroscale magnetic bead-based sample extraction or centrifugation steps, which can also be straightforwardly integrated with downstream analytical instrumentation. This tutorial outlines some basic fluidic configurations for acoustophoresis based sample decomplexing and details the different system parameters that will impact the outcome of an acoustophoresis based affinity extraction experiment or a cell medium exchange step. Examples are given of both targeted extraction of microbes and selective elusion of molecular species.     

Fluorescence correlation spectroscopy for flow rate imaging and monitoring--optimization, limitations and artifacts

We recently demonstrated a new method for mapping fluid velocities in 3 dimensions and with exceptionally high spatial resolution for the characterization of flow in microfluidic devices. In the method, a colloidal suspension containing fluorescent tracer particles, dye doped polymer spheres, is pumped through a microchannel and confocal microscopy combined with fluorescence correlation spectroscopy is used to measure fluid velocities. In this report, we further characterize the technique and report on optimizations that allow a 5-fold increase in speed of single point velocity measurements. This increase in measurement speed will yield a 25 fold reduction in the time needed to collect a complete velocity image. The precision of measured velocities was characterized as a function of tracer particle concentration, measurement time, and fluid velocity. In addition, we confirm the linearity of the measurement method (velocity vs. applied pressure) over a range of velocities spanning four orders of magnitude. Furthermore, we demonstrate that an artifact in velocity measurements using fluorescence correlation spectroscopy (FCS) that was interpreted by others as being caused by optical trapping forces is actually an artifact caused by detector saturation and can be avoided by careful choice of experimental conditions.     

Flexible planar microfluidic chip employing a light emitting diode and a PIN-photodiode for portable flow cytometers

Detection of fluorescence particles is a key method of flow cytometry. We evaluate the performance of a design for a microfluidic fluorescence particle detection device. Due to the planar design with low layer thicknesses, we avoid optical components such as lenses or dichroic mirrors and substitute them with a shadow mask and colored film filters. A commercially available LED is used as the light source and a PIN-photodiode as detector. This design approach reduces component cost and power consumption and enables supplying the device with power from a standard USB port. From evaluation of this design, we obtain a maximum particle detection frequency of up to 600 particles per second at a sensitivity of better than 4.7 × 10(5) MESF (molecules of equivalent soluble fluorochrome) measured with particles for FITC sensitivity calibration. Lowering the flow rate increases the instrument sensitivity by an order of magnitude enabling the detection of particles with 4.5 × 10(4) MESF.     

On-chip electrochemical detection of CdS quantum dots using normal and multiple recycling flow through modes

A flexible hybrid polydimethylsiloxane (PDMS)-polycarbonate (PC) microfluidic chip with integrated screen printed electrodes (SPE) was fabricated and applied for electrochemical quantum dots (QDs) detection. The developed device combines the advantages of flexible microfluidic chips, such as their low cost, the possibility to be disposable and amenable to mass production, with the advantages of electrochemistry for its facility of integration and the possibility to miniaturize the analytical device. Due to the interest in biosensing applications in general and particularly the great demand for labelling alternatives in affinity biosensors, the electrochemistry of cadmium sulfide quantum dots (CdS QDs) is evaluated. Square wave anodic stripping voltammetry (SWASV) is the technique used due to its sensitivity and low detection limits that can be achieved. The electrochemical as well as the microfluidic parameters of the developed system are optimized. The detection of CdS QDs in the range between 50 to 8000 ng mL(-1) with a sensitivity of 0.0009 μA/(ng mL(-1)) has been achieved. In addition to the single in-chip flow through measurements, the design of a recirculation system with the aim of achieving lower detection limits using reduced volumes (25 μL) of sample was proposed as a proof-of-concept.     

Microfluidic immunosensor with integrated liquid core waveguides for sensitive Mie scattering detection of avian influenza antigens in a real biological matrix

This work presents the use of integrated, liquid core, optical waveguides for measuring immunoagglutination-induced light scattering in a microfluidic device, towards rapid and sensitive detection of avian influenza (AI) viral antigens in a real biological matrix (chicken feces). Mie scattering simulations were performed and tested to optimize the scattering efficiency of the device through proper scatter angle waveguide geometry. The detection limit is demonstrated to be 1 pg mL⁻¹ in both clean buffer and real biological matrix. This low detection limit is made possible through on-chip diffusional mixing of AI target antigens and high acid content microparticle assay reagents, coupled with real-time monitoring of immunoagglutination-induced forward Mie scattering via high refractive index liquid core optical waveguides in close proximity (100 μm) to the sample chamber. The detection time for the assay is <2 min. This device could easily be modified to detect trace levels of any biological molecules that antibodies are available for, moving towards a robust platform for point-of-care disease diagnostics.     

Off-chip passivated-electrode, insulator-based dielectrophoresis (OπDEP)

In this study, we report the first off-chip passivated-electrode, insulator-based dielectrophoresis microchip (OπDEP). This technique combines the sensitivity of electrode-based dielectrophoresis (eDEP) with the high-throughput and inexpensive device characteristics of insulator-based dielectrophoresis (iDEP). The device is composed of a permanent, reusable set of electrodes and a disposable, polymer microfluidic chip with microposts embedded in the microchannel. The device operates by capacitively coupling the electric fields into the microchannel; thus, no physical connections are made between the electrodes and the microfluidic device. During operation, the polydimethylsiloxan (PDMS) microfluidic chip fits onto the electrode substrate as a disposable cartridge. OπDEP uses insulting structures within the channel as well as parallel electrodes to create DEP forces by the same working principle that iDEP devices use. The resulting devices create DEP forces which are larger by two orders of magnitude for the same applied voltage when compared to off-chip eDEP designs from literature, which rely on parallel electrodes alone to produce the DEP forces. The larger DEP forces allow the OπDEP device to operate at high flow rates exceeding 1 mL/h. In order to demonstrate this technology, Escherichia coli (E. coli), a known waterborne pathogen, was trapped from water samples. Trapping efficiencies of 100 % were obtained at flow rates as high as 400 μL/h and 60 % at flow rates as high as 1200 μL/h. Additionally, bacteria were selectively concentrated from a suspension of polystyrene beads.