Lab on a chip for continuous-flow magnetic cell separation

Microfluidic sorting of intrinsically magnetic cells under visual control

Magnetic cell sorting provides a valuable complementary mechanism to fluorescent techniques, especially if its parameters can be fine-tuned. In addition, there has recently been growing interest in studying naturally occurring magnetic cells and genetic engineering of cells to render them magnetic in order to control molecular processes via magnetic fields. For such approaches, contamination-free magnetic separation is an essential capability. We here present a robust and tunable microfluidic sorting system in which magnetic gradients of up to 1700 T/m can be applied to cells flowing through a sorting channel by reversible magnetization of ferrofluids. Visual control of the sorting process allowed us to optimize sorting efficiencies for a large range of sizes and magnetic moments of cells. Using automated quantification based on imaging of fluorescent markers, we showed that macrophages containing phagocytosed magnetic nanoparticles, with cellular magnetic dipole moments on the order of 10 fAm², could be sorted with an efficiency of 90 ± 1%. Furthermore, we successfully sorted intrinsically magnetic magnetotactic bacteria with magnetic moments of 0.1 fAm². In distinction to column-based magnetic sorting devices, microfluidic systems can prevent sample contact with superparamagnetic material. This ensures contamination-free separation of naturally occurring or bioengineered magnetic cells and is essential for downstream characterization of their properties.

Size-tunable microvortex capture of rare cells

Inertial separation of particles and cells based on their size has advanced significantly over the last decade. However, size-based inertial separation methods require precise tuning of microfluidic device geometries to adjust the separation size of particles or cells. Here, we show a passive capture method that targets a wide size range of cells by controlling the flow conditions in a single device geometry. This multimodal capture device is designed to generate laminar vortices in lateral cavities that branch from long rectangular channels. Micro-vortices generated at lower Reynolds numbers capture and stabilize large particles in equilibrium orbits or limit cycles near the vortex core. Other smaller particles or cells orbit near the vortex boundaries and they are susceptible to exiting the cavity flow. In the same cavity, however, at higher Reynolds number, we observe small particles migrating inward. This evolution in limit cycle trajectories led to a corresponding evolution in the average size of captured particles, indicating that the outermost orbits are less stable. We identify three phases of capture as a function of Reynolds number that give rise to unique particle orbit trajectories. Flow-based switching overcomes a major engineering challenge to automate capture and release of polydisperse cell subpopulations. The approach can expand clinical applications of label free trapping in isolating and processing a larger subset of rare cells like circulating tumor cells (CTCs) from blood and other body fluids.

Recent Advances and Future Perspectives on Microfluidic Liquid Handling

The interdisciplinary research field of microfluidics has the potential to revolutionize current technologies that require the handling of a small amount of fluid, a fast response, low costs and automation. Microfluidic platforms that handle small amounts of liquid have been categorised as continuous-flow microfluidics and digital microfluidics. The first part of this paper discusses the recent advances of the two main and opposing applications of liquid handling in continuous-flow microfluidics: mixing and separation. Mixing and separation are essential steps in most lab-on-a-chip platforms, as sample preparation and detection are required for a variety of biological and chemical assays. The second part discusses the various digital microfluidic strategies, based on droplets and liquid marbles, for the manipulation of discrete microdroplets. More advanced digital microfluidic devices combining electrowetting with other techniques are also introduced. The applications of the emerging field of liquid-marble-based digital microfluidics are also highlighted. Finally, future perspectives on microfluidic liquid handling are discussed.

Simultaneous Separation and Washing of Nonmagnetic Particles in an Inertial Ferrofluid/Water Coflow

Magnetic fluids (e.g., paramagnetic solutions and ferrofluids) have been increasingly used for label-free separation of nonmagnetic particles in microfluidic devices. Their biocompatibility, however, becomes a concern in high-throughput or large-volume applications. One way to potentially resolve this issue is resuspending the particles that are separated in a magnetic fluid immediately into a biocompatible buffer. We demonstrate herein the proof-of-principle of the first integration of negative magnetophoresis and inertial focusing for a simultaneous separation and washing of nonmagnetic particles in coflowing ferrofluid and water streams. The two operations take place in parallel in a simple T-shaped rectangular microchannel with a nearby permanent magnet. We find that the larger and smaller particles' exiting positions (and hence their separation distance) in the sheath water and ferrofluid suspension, respectively, vary with the total flow rate or the flow rate ratio between the two streams.

In situ single cell detection via microfluidic magnetic bead assay

We present a single cell detection device based on magnetic bead assay and micro Coulter counters. This device consists of two successive micro Coulter counters, coupled with a high gradient magnetic field generated by an external magnet. The device can identify single cells in terms of the transit time difference of the cell through the two micro Coulter counters. Target cells are conjugated with magnetic beads via specific antibody and antigen binding. A target cell traveling through the two Coulter counters interacts with the magnetic field, and have a longer transit time at the 1st counter than that at the 2nd counter. In comparison, a non-target cell has no interaction with the magnetic field, and hence has nearly the same transit times through the two counters. Each cell passing through the two counters generates two consecutive voltage pulses one after the other; the pulse widths and magnitudes indicating the cell’s transit times through the counters and the cell’s size respectively. Thus, by measuring the pulse widths (transit times) of each cell through the two counters, each single target cell can be differentiated from non-target cells even if they have similar sizes. We experimentally proved that the target human umbilical vein endothelial cells (HUVECs) and non-target rat adipose-derived stem cells (rASCs) have significant different transit time distribution, from which we can determine the recognition regions for both cell groups quantitatively. We further demonstrated that within a mixed cell population of rASCs and HUVECs, HUVECs can be detected in situ and the measured HUVECs ratios agree well with the pre-set ratios. With the simple device structure and easy sample preparation, this method is expected to enable single cell detection in a continuous flow and can be applied to facilitate general cell detection applications such as stem cell identification and enumeration.

Broadening of analyte streams due to a transverse pressure gradient in free-flow isoelectric focusing

Pressure-driven cross-flows can arise in free-flow isoelectric focusing systems (FFIEF) due to a non-uniform electroosmotic flow velocity along the channel width induced by the pH gradient in this direction. In addition, variations in the channel cross-section as well as unwanted differences in hydrostatic heads at the buffer/sample inlet ports can also lead to such pressure-gradients which besides altering the equilibrium position of the sample zones have a tendency to substantially broaden their widths deteriorating the separations. In this situation, a thorough assessment of stream broadening due to transverse pressure-gradients in FFIEF devices is necessary in order to establish accurate design rules for the assay. The present article describes a mathematical framework to estimate the noted zone dispersion in FFIEF separations based on the method-of-moments approach under laminar flow conditions. A closed-form expression has been derived for the spatial variance of the analyte streams at their equilibrium positions as a function of the various operating parameters governing the assay performance. This expression predicts the normalized stream variance under the chosen conditions to be determined by two dimensionless Péclet numbers evaluated based on the transverse pressure-driven and electrophoretic solute velocities in the separation chamber, respectively. Moreover, the analysis shows that while the stream width can be expected to increase with an increase in the value of the first Péclet number, the opposite trend will be followed with respect to the latter. The noted results have been validated using Monte Carlo simulations that also establish a time/length scale over which the predicted equilibrium stream width is attained in the system.

A Rapid Magnetofluidic Micromixer Using Diluted Ferrofluid

Effective and rapid mixing is essential for various chemical and biological assays. The present work describes a simple and low-cost micromixer based on magnetofluidic actuation. The device takes advantage of magnetoconvective secondary flow, a bulk flow induced by an external magnetic field, for mixing. A superparamagnetic stream of diluted ferrofluid and a non-magnetic stream are introduced to a straight microchannel. A permanent magnet placed next to the microchannel induced a non-uniform magnetic field. The magnetic field gradient and the mismatch in magnetic susceptibility between the two streams create a body force, which leads to rapid and efficient mixing. The micromixer reported here could achieve a high throughput and a high mixing efficiency of 88% in a relatively short microchannel.

Inertial Focusing of Microparticles in Curvilinear Microchannels

A passive, continuous and size-dependent focusing technique enabled by “inertial microfluidics”, which takes advantage of hydrodynamic forces, is implemented in this study to focus microparticles. The objective is to analyse the decoupling effects of inertial forces and Dean drag forces on microparticles of different sizes in curvilinear microchannels with inner radius of 800 μm and curvature angle of 280°, which have not been considered in the literature related to inertial microfluidics. This fundamental approach gives insight into the underlying physics of particle dynamics and offers continuous, high-throughput, label-free and parallelizable size-based particle separation. Our design allows the same footprint to be occupied as straight channels, which makes parallelization possible with optical detection integration. This feature is also useful for ultrahigh-throughput applications such as flow cytometers with the advantages of reduced cost and size. The focusing behaviour of 20, 15 and 10 μm fluorescent polystyrene microparticles was examined for different channel Reynolds numbers. Lateral and vertical particle migrations and the equilibrium positions of these particles were investigated in detail, which may lead to the design of novel microfluidic devices with high efficiency and high throughput for particle separation, rapid detection and diagnosis of circulating tumour cells with reduced cost.

Turning Diamagnetic Microbes into Multinary Micro-Magnets: Magnetophoresis and Spatio-Temporal Manipulation of Individual Living Cells

Inspired by the biogenic magnetism found in certain organisms, such as magnetotactic bacteria, magnetic nanomaterials have been integrated into living cells for bioorthogonal, magnetic manipulation of the cells. However, magnetized cells have so far been reported to be only binary system (on/off) without any control of magnetization degree, limiting their applications typically to the simple accumulation or separation of cells as a whole. In this work, the magnetization degree is tightly controlled, leading to the generation of multiple subgroups of the magnetized cells, and each subgroup is manipulated independently from the other subgroups in the pool of heterogeneous cell-mixtures. This work will provide a strategic approach to tailor-made fabrication of magnetically functionalized living cells as micro-magnets, and open new vistas in biotechnological and biomedical applications, which highly demand the spatio-temporal manipulation of living cells.

Guided transport of nanoparticles by plasmonic nanowires

Herein, we report the optical trapping and directional transport of nanoparticles in an aqueous solution by plasmonic nanowires. A laser illuminated one end of a silver nanowire and excited the localized and propagating surface plasmons. Optical forces were induced by the surface plasmons, which could trap the nanoparticles in an aqueous solution. Interestingly, the trapped nanoparticles moved along the silver nanowires from the trapping site to the excitation spot of the laser. Such movements of nanoparticles were also observed on curved nanowires, in which the trajectories of the particles were explicitly determined by the shape of the nanowires. More importantly, for a V-shaped silver nanowire, the direction of the movement could be modulated by the polarization of the incident laser. The direction of the movement was opposite to the prediction by the scattering force due to the propagation of surface plasmons, and the driving force could involve the thermal convection of local fluid due to a heating effect. Our findings indicate a novel approach to transport nanoparticles by plasmonic waveguides in aqueous solution.

A New Microfluidic Device for Classification of Microalgae Cells Based on Simultaneous Analysis of Chlorophyll Fluorescence, Side Light Scattering, Resistance Pulse Sensing

Fast on-site monitoring of foreign microalgae species carried by ship ballast water has drawn more and more attention. In this paper, we presented a new method and a compact device of classification of microalgae cells by simultaneous detection of three kinds of signals of single microalgae cells in a disposable microfluidic chip. The microfluidic classification device has advantages of fast detection, low cost, and portability. The species of a single microalgae cell can be identified by simultaneous detection of three signals of chlorophyll fluorescence (CF), side light scattering (SLS), and resistance pulse sensing (RPS) of the microalgae cell. These three signals represent the different characteristics of a microalgae cell. A compact device was designed to detect these three signals of a microalgae cell simultaneously. In order to demonstrate the performance of the developed system, the comparison experiments of the mixed samples of three different species of microalgae cells between the developed system and a commercial flow cytometer were conducted. The results show that three kinds of microalgae cells can be distinguished clearly by our developed system and the commercial flow cytometer and both results have good agreement.

Magnetophoretic transistors in a tri-axial magnetic field

The ability to direct and sort individual biological and non-biological particles into spatially addressable locations is fundamentally important to the emerging field of single cell biology. Towards this goal, we demonstrate a new class of magnetophoretic transistors, which can switch single magnetically labeled cells and magnetic beads between different paths in a microfluidic chamber. Compared with prior work on magnetophoretic transistors driven by a two-dimensional in-plane rotating field, the addition of a vertical magnetic field bias provides significant advantages in preventing the formation of particle clumps and in better replicating the operating principles of circuits in general. However, the three-dimensional driving field requires a complete redesign of the magnetic track geometry and switching electrodes. We have solved this problem by developing several types of transistor geometries which can switch particles between two different tracks by either presenting a local energy barrier or by repelling magnetic objects away from a given track, hereby denoted as "barrier" and "repulsion" transistors, respectively. For both types of transistors, we observe complete switching of magnetic objects with currents of ∼40 mA, which is consistent over a range of particle sizes (8-15 μm). The switching efficiency was also tested at various magnetic field strengths (50-90 Oe) and driving frequencies (0.1-0.6 Hz); however, we again found that the device performance only weakly depended on these parameters. These findings support the use of these novel transistor geometries to form circuit architectures in which cells can be placed in defined locations and retrieved on demand.

A novel viscoelastic-based ferrofluid for continuous sheathless microfluidic separation of nonmagnetic microparticles

Separation of microparticles has found broad applications in biomedicine, industry and clinical diagnosis. In a conventional aqueous ferrofluid, separation of microparticles usually employs a sheath flow or two offset magnets to confine particle streams for downstream particle sorting. This complicates the fluid control, device fabrication, and dilutes the particle sample. In this work, we propose and develop a novel viscoelastic ferrofluid by replacing the Newtonian base medium of the conventional ferrofluid with non-Newtonian poly(ethylene oxide) (PEO) aqueous solution. The properties of both viscoelastic 3D focusing and negative magnetophoresis of the viscoelastic ferrofluid were verified and investigated. By employing the both properties in a serial manner, continuous and sheathless separation of nonmagnetic particles based on particle size has been demonstrated. This novel viscoelastic ferrofluid is expected to bring more flexibility and versatility to the design and functionality in microfluidic devices.

Emerging microfluidic devices for cancer cells/biomarkers manipulation and detection

Circulating tumour cells (CTCs) are active participants in the metastasis process and account for ∼90% of all cancer deaths. As CTCs are admixed with a very large amount of erythrocytes, leukocytes, and platelets in blood, CTCs are very rare, making their isolation, capture, and detection a major technological challenge. Microfluidic technologies have opened-up new opportunities for the screening of blood samples and the detection of CTCs or other important cancer biomarker-proteins. In this study, the authors have reviewed the most recent developments in microfluidic devices for cells/biomarkers manipulation and detection, focusing their attention on immunomagnetic-affinity-based devices, dielectrophoresis-based devices, surface-plasmon-resonance microfluidic sensors, and quantum-dots-based sensors.

Magnetic separation of acoustically focused cancer cells from blood for magnetographic templating and analysis

Liquid biopsies hold enormous promise for the next generation of medical diagnoses. At the forefront of this effort, many are seeking to capture, enumerate and analyze circulating tumor cells (CTCs) as a means to prognosticate and develop individualized treatments for cancer. Capturing these rare cells, however, represents a major engineering challenge due to their low abundance, morphology and heterogeneity. A variety of microfluidic tools have been developed to isolate CTCs from drawn blood samples; however, few of these approaches offer a means to separate and analyze cells in an integrated system. We have developed a microfluidic platform comprised of three modules that offers high throughput separation of cancer cells from blood and on-chip organization of those cells for streamlined analyses. The first module uses an acoustic standing wave to rapidly align cells in a contact-free manner. The second module then separates magnetically labeled cells from unlabeled cells, offering purities exceeding 85% for cells and 90% for binary mixtures of synthetic particles. Finally, the third module contains a spatially periodic array of microwells with underlying micromagnets to capture individual cells for on-chip analyses (e.g., staining, imaging and quantification). This array is capable of capturing with accuracies exceeding 80% for magnetically labeled cells and 95% for magnetic particles. Overall, by virtue of its holistic processing of complex biological samples, this system has promise for the isolation and evaluation of rare cancer cells and can be readily extended to address a variety of applications across single cell biology and immunology.

Virtual membrane for filtration of particles using surface acoustic waves (SAW)

Surface acoustic wave (SAW) based particle manipulation is contactless, versatile, non-invasive and biocompatible making it useful for biological studies and diagnostic technologies. In this work, we present a sensitive particle sorting system, termed the virtual membrane, in which a periodic acoustic field with a wavelength on the order of particle dimensions permits size-selective filtration. Polystyrene particles that are larger than approximately 0.3 times the acoustic half-wavelength experience a force repelling them from the acoustic field. If the particle size is such that, at a given acoustic power and flow velocity, this repulsive force is dominant over the drag force, these particles will be prohibited from progressing further downstream (i.e. filtered), while smaller particles will be able to pass through the force field along the pressure nodes (akin to a filter's pores). Using this mechanism, we demonstrate high size selectivity using a standing SAW generated by opposing sets of focused interdigital transducers (FIDTs). The use of FIDTs permits the generation of a highly localized standing wave field, here used for filtration in μl min(-1) order flow rates at 10s of mW of applied power. Specifically, we demonstrate the filtration of 8 μm particles from 5 μm particles and 10.36 μm particles from 7.0 μm and 5.0 μm particles, using high frequency SAW at 258 MHz, 192.5 MHz, and 129.5 MHz, respectively.

Magnetic fingerprints of rolling cells for quantitative flow cytometry in whole blood

Over the past 50 years, flow cytometry has had a profound impact on preclinical and clinical applications requiring single cell function information for counting, sub-typing and quantification of epitope expression. At the same time, the workflow complexity and high costs of such optical systems still limit flow cytometry applications to specialized laboratories. Here, we present a quantitative magnetic flow cytometer that incorporates in situ magnetophoretic cell focusing for highly accurate and reproducible rolling of the cellular targets over giant magnetoresistance sensing elements. Time-of-flight analysis is used to unveil quantitative single cell information contained in its magnetic fingerprint. Furthermore, we used erythrocytes as a biological model to validate our methodology with respect to precise analysis of the hydrodynamic cell diameter, quantification of binding capacity of immunomagnetic labels, and discrimination of cell morphology. The extracted time-of-flight information should enable point-of-care quantitative flow cytometry in whole blood for clinical applications, such as immunology and primary hemostasis.

Sheathless electrokinetic particle separation in a bifurcating microchannel

Particle separation has found practical applications in many areas from industry to academia. Current electrokinetic particle separation techniques primarily rely on dielectrophoresis, where the electric field gradients are generated by either active microelectrodes or inert micro-insulators. We develop herein a new type of electrokinetic method to continuously separate particles in a bifurcating microchannel. This sheath-free separation makes use of the inherent wall-induced electrical lift to focus particles towards the centerline of the main-branch and then deflect them to size-dependent flow paths in each side-branch. A theoretical model is also developed to understand such a size-based separation, which simulates the experimental observations with a good agreement. This electric field-driven sheathless separation can potentially be operated in a parallel or cascade mode to increase the particle throughput or resolution.

Translating microfluidics: Cell separation technologies and their barriers to commercialization

Advances in microfluidic cell sorting have revolutionized the ways in which cell-containing fluids are processed, now providing performances comparable to, or exceeding, traditional systems, but in a vastly miniaturized format. These technologies exploit a wide variety of physical phenomena to manipulate cells and fluid flow, such as magnetic traps, sound waves and flow-altering micropatterns, and they can evaluate single cells by immobilizing them onto surfaces for chemotherapeutic assessment, encapsulate cells into picoliter droplets for toxicity screenings and examine the interactions between pairs of cells in response to new, experimental drugs. However, despite the massive surge of innovation in these high-performance lab-on-a-chip devices, few have undergone successful commercialization, and no device has been translated to a widely distributed clinical commodity to date. Persistent challenges such as an increasingly saturated patent landscape as well as complex user interfaces are among several factors that may contribute to their slowed progress. In this article, we identify several of the leading microfluidic technologies for sorting cells that are poised for clinical translation; we examine the principal barriers preventing their routine clinical use; finally, we provide a prospectus to elucidate the key criteria that must be met to overcome those barriers. Once established, these tools may soon transform how clinical labs study various ailments and diseases by separating cells for downstream sequencing and enabling other forms of advanced cellular or sub-cellular analysis. © 2016 International Clinical Cytometry Society.

Magnetofluidic concentration and separation of non-magnetic particles using two magnet arrays

The present paper reports the use of diluted ferrofluid and two arrays of permanent magnets for the size-selective concentration of non-magnetic particles. The micro magnetofluidic device consists of a straight channels sandwiched between two arrays of permanent magnets. The permanent magnets create multiple capture zones with minimum magnetic field strength along the channel. The complex interaction between magnetic forces and hydrodynamic force allows the device to operate in different regimes suitable for concentration of non-magnetic particles with small difference in size. Our experimental results show that non-magnetic particles with diameters of 3.1 μm and 4.8 μm can be discriminated and separated with this method. The results from this study could be used as a guide for the design of size-sensitive separation devices for particle and cell based on negative magnetophoresis.

Magnetic micro/nanoparticle flocculation-based signal amplification for biosensing

We report a time and cost efficient signal amplification method for biosensors employing magnetic particles. In this method, magnetic particles in an applied external magnetic field form magnetic dipoles, interact with each other, and accumulate along the magnetic field lines. This magnetic interaction does not need any biomolecular coating for binding and can be controlled with the strength of the applied magnetic field. The accumulation can be used to amplify the corresponding pixel area that is obtained from an image of a single magnetic particle. An application of the method to the Escherichia coli 0157:H7 bacteria samples is demonstrated in order to show the potential of the approach. A minimum of threefold to a maximum of 60-fold amplification is reached from a single bacteria cell under a magnetic field of 20 mT.

Label-Free Microfluidic Manipulation of Particles and Cells in Magnetic Liquids

Manipulating particles and cells in magnetic liquids through so-called "negative magnetophoresis" is a new research field. It has resulted in label-free and low-cost manipulation techniques in microfluidic systems and many exciting applications. It is the goal of this review to introduce the fundamental principles of negative magnetophoresis and its recent applications in microfluidic manipulation of particles and cells. We will first discuss the theoretical background of three commonly used specificities of manipulation in magnetic liquids, which include the size, density and magnetic property of particles and cells. We will then review and compare the media used in negative magnetophoresis, which include paramagnetic salt solutions and ferrofluids. Afterwards, we will focus on reviewing existing microfluidic applications of negative magnetophoresis, including separation, focusing, trapping and concentration of particles and cells, determination of cell density, measurement of particles' magnetic susceptibility, and others. We will also examine the need for developing biocompatible magnetic liquids for live cell manipulation and analysis, and its recent progress. Finally, we will conclude this review with a brief outlook for this exciting research field.

Label-Free and Continuous-Flow Ferrohydrodynamic Separation of HeLa Cells and Blood Cells in Biocompatible Ferrofluids

In this study, a label-free, low-cost, and fast ferrohydrodynamic cell separation scheme is demonstrated using HeLa cells (an epithelial cell line) and red blood cells. The separation is based on cell size difference, and conducted in a custom-made biocompatible ferrofluid that retains the viability of cells during and after the assay for downstream analysis. The scheme offers moderate-throughput (≈106 cells h-1 for a single channel device) and extremely high recovery rate (>99%) without the use of any label. It is envisioned that this separation scheme will have clinical applications in settings where rapid cell enrichment and removal of contaminating blood will improve efficiency of screening and diagnosis such as cervical cancer screening based on mixed populations in exfoliated samples.

High gradient magnetic field microstructures for magnetophoretic cell separation

Microfluidics has advanced magnetic blood fractionation by making integrated miniature devices possible. A ferromagnetic microstructure array that is integrated with a microfluidic channel rearranges an applied magnetic field to create a high gradient magnetic field (HGMF). By leveraging the differential magnetic susceptibilities of cell types contained in a host medium, such as paramagnetic red blood cells (RBCs) and diamagnetic white blood cells (WBCs), the resulting HGMF can be used to continuously separate them without attaching additional labels, such as magnetic beads, to them. We describe the effect of these ferromagnetic microstructure geometries have on the blood separation efficacy by numerically simulating the influence of microstructure height and pitch on the HGMF characteristics and resulting RBC separation. Visualizations of RBC trajectories provide insight into how arrays can be optimized to best separate these cells from a host fluid. Periodic microstructures are shown to moderate the applied field due to magnetic interference between the adjacent teeth of an array. Since continuous microstructures do not similarly weaken the resultant HGMF, they facilitate significantly higher RBC separation. Nevertheless, periodic arrays are more appropriate for relatively deep microchannels since, unlike continuous microstructures, their separation effectiveness is independent of depth. The results are relevant to the design of microfluidic devices that leverage HGMFs to fractionate blood by separating RBCs and WBCs.

Quantitative Magnetic Separation of Particles and Cells Using Gradient Magnetic Ratcheting

Extraction of rare target cells from biosamples is enabling for life science research. Traditional rare cell separation techniques, such as magnetic activated cell sorting, are robust but perform coarse, qualitative separations based on surface antigen expression. A quantitative magnetic separation technology is reported using high-force magnetic ratcheting over arrays of magnetically soft micropillars with gradient spacing, and the system is used to separate and concentrate magnetic beads based on iron oxide content (IOC) and cells based on surface expression. The system consists of a microchip of permalloy micropillar arrays with increasing lateral pitch and a mechatronic device to generate a cycling magnetic field. Particles with higher IOC separate and equilibrate along the miropillar array at larger pitches. A semi-analytical model is developed that predicts behavior for particles and cells. Using the system, LNCaP cells are separated based on the bound quantity of 1 μm anti-epithelial cell adhesion molecule (EpCAM) particles as a metric for expression. The ratcheting cytometry system is able to resolve a ±13 bound particle differential, successfully distinguishing LNCaP from PC3 populations based on EpCAM expression, correlating with flow cytometry analysis. As a proof-of-concept, EpCAM-labeled cells from patient blood are isolated with 74% purity, demonstrating potential toward a quantitative magnetic separation instrument.

Continuum modeling of hydrodynamic particle-particle interactions in microfluidic high-concentration suspensions

A continuum model is established for numerical studies of hydrodynamic particle-particle interactions in microfluidic high-concentration suspensions. A suspension of microparticles placed in a microfluidic channel and influenced by an external force, is described by a continuous particle-concentration field coupled to the continuity and Navier-Stokes equation for the solution. The hydrodynamic interactions are accounted for through the concentration dependence of the suspension viscosity, of the single-particle mobility, and of the momentum transfer from the particles to the suspension. The model is applied on a magnetophoretic and an acoustophoretic system, respectively, and based on the results, we illustrate three main points: (1) for relative particle-to-fluid volume fractions greater than 0.01, the hydrodynamic interaction effects become important through a decreased particle mobility and an increased suspension viscosity. (2) At these high particle concentrations, particle-induced flow rolls occur, which can lead to significant deviations of the advective particle transport relative to that of dilute suspensions. (3) Which interaction mechanism that dominates, depends on the specific flow geometry and the specific external force acting on the particles.

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Highly focused high-frequency travelling surface acoustic waves (SAW) for rapid single-particle sorting

High-speed sorting is an essential process in a number of clinical and research applications, where single cells, droplets and particles are segregated based on their properties in a continuous flow. With recent developments in the field of microscale actuation, there is increasing interest in replicating the functions available to conventional fluorescence activated cell sorting (FACS) flow cytometry in integrated on-chip systems, which have substantial advantages in cost and portability. Surface acoustic wave (SAW) devices are ideal for many acoustofluidic applications, and have been used to perform such sorting at rates on the order of kHz. Essential to the accuracy of this sorting, however, is the dimensions of the region over which sorting occurs, where a smaller sorting region can largely avoid inaccurate sorting across a range of sample concentrations. Here we demonstrate the use of flow focusing and a highly focused SAW generated by a high-frequency (386 MHz), 10 μm wavelength set of focused interdigital transducers (FIDTs) on a piezoelectric lithium niobate substrate, yielding an effective sorting region only ~25 μm wide, with sub-millisecond pulses generated at up to kHz rates. Furthermore, because of the use of high frequencies, actuation of particles as small as 2 μm can be realized. Such devices represent a substantial step forward in the evolution of highly localized forces for lab-on-a-chip microfluidic applications.

Mass transport improvement in microscale using diluted ferrofluid and a non-uniform magnetic field

We investigate the mass transport enhancement of a non-magnetic fluorescent dye with the help of diluted ferrofluid and a non-uniform magnetic field.

A portable immunomagnetic cell capture system to accelerate culture diagnosis of bacterial infections

An affordable immuno-magnetic cell capture system for bacterial detection in 7 hours with 10 CFU ml⁻¹sensitivity.

Recent advances in nanotechnology-based detection and separation of circulating tumor cells

Although circulating tumor cells (CTCs) in blood have been widely investigated as a potential biomarker for diagnosis and prognosis of metastatic cancer, their inherent rarity and heterogeneity bring tremendous challenges to develop a CTC detection method with clinically significant specificity and sensitivity. With advances in nanotechnology, a series of new methods that are highly promising have emerged to enable or enhance detection and separation of CTCs from blood. In this review, we systematically categorize nanomaterials, such as gold nanoparticles, magnetic nanoparticles, quantum dots, graphenes/graphene oxides, and dendrimers and stimuli-responsive polymers, used in the newly developed CTC detection methods. This will provide a comprehensive overview of recent advances in the CTC detection achieved through application of nanotechnology as well as the challenges that these existing technologies must overcome to be directly impactful on human health.

Negative magnetophoresis in diluted ferrofluid flow

We report magnetic manipulation of non-magnetic particles suspended in diluted ferrofluid. Diamagnetic particles were introduced into a circular chamber to study the extent of their deflection under the effect of a non-uniform magnetic field of a permanent magnet. Since ferrofluid is a paramagnetic medium, it also experiences a bulk magnetic force that in turn induces a secondary flow opposing the main hydrodynamic flow. Sheath flow rate, particle size, and magnetic field strength were varied to examine this complex behaviour. The combined effect of negative magnetophoresis and magnetically induced secondary flow leads to various operation regimes, which can potentially find applications in separation, trapping and mixing of diamagnetic particles such as cells in a microfluidic system.

Simultaneous diamagnetic and magnetic particle trapping in ferrofluid microflows via a single permanent magnet

Trapping and preconcentrating particles and cells for enhanced detection and analysis are often essential in many chemical and biological applications. Existing methods for diamagnetic particle trapping require the placement of one or multiple pairs of magnets nearby the particle flowing channel. The strong attractive or repulsive force between the magnets makes it difficult to align and place them close enough to the channel, which not only complicates the device fabrication but also restricts the particle trapping performance. This work demonstrates for the first time the use of a single permanent magnet to simultaneously trap diamagnetic and magnetic particles in ferrofluid flows through a T-shaped microchannel. The two types of particles are preconcentrated to distinct locations of the T-junction due to the induced negative and positive magnetophoretic motions, respectively. Moreover, they can be sequentially released from their respective trapping spots by simply increasing the ferrofluid flow rate. In addition, a three-dimensional numerical model is developed, which predicts with a reasonable agreement the trajectories of diamagnetic and magnetic particles as well as the buildup of ferrofluid nanoparticles.

Dean-flow-coupled elasto-inertial three-dimensional particle focusing under viscoelastic flow in a straight channel with asymmetrical expansion-contraction cavity arrays

In this paper, 3D particle focusing in a straight channel with asymmetrical expansion-contraction cavity arrays (ECCA channel) is achieved by exploiting the dean-flow-coupled elasto-inertial effects. First, the mechanism of particle focusing in both Newtonian and non-Newtonian fluids was introduced. Then particle focusing was demonstrated experimentally in this channel with Newtonian and non-Newtonian fluids using three different sized particles (3.2 μm, 4.8 μm, and 13 μm), respectively. Also, the effects of dean flow (or secondary flow) induced by expansion-contraction cavity arrays were highlighted by comparing the particle distributions in a single straight rectangular channel with that in the ECCA channel. Finally, the influences of flow rates and distances from the inlet on focusing performance in the ECCA channel were studied. The results show that in the ECCA channel particles are focused on the cavity side in Newtonian fluid due to the synthesis effects of inertial and dean-drag force, whereas the particles are focused on the opposite cavity side in non-Newtonian fluid due to the addition of viscoelastic force. Compared with the focusing performance in Newtonian fluid, the particles are more easily and better focused in non-Newtonian fluid. Besides, the Dean flow in visco-elastic fluid in the ECCA channel improves the particle focusing performance compared with that in a straight channel. A further advantage is three-dimensional (3D) particle focusing that in non-Newtonian fluid is realized according to the lateral side view of the channel while only two-dimensional (2D) particle focusing can be achieved in Newtonian fluid. Conclusively, this novel Dean-flow-coupled elasto-inertial microfluidic device could offer a continuous, sheathless, and high throughput (>10 000 s(-1)) 3D focusing performance, which may be valuable in various applications from high speed flow cytometry to cell counting, sorting, and analysis.

Magnetophoresis 'meets' viscoelasticity: deterministic separation of magnetic particles in a modular microfluidic device

The deflection of magnetic beads in a microfluidic channel through magnetophoresis can be improved if the particles are somehow focused along the same streamline in the device. We design and fabricate a microfluidic device made of two modules, each one performing a unit operation. A suspension of magnetic beads in a viscoelastic medium is fed to the first module, which is a straight rectangular-shaped channel. Here, the magnetic particles are focused by exploiting fluid viscoelasticity. Such a channel is one inlet of the second module, which is a H-shaped channel, where a buffer stream is injected in the second inlet. A permanent magnet is used to displace the magnetic beads from the original to the buffer stream. Experiments with a Newtonian suspending fluid, where no focusing occurs, are carried out for comparison. When viscoelastic focusing and magnetophoresis are combined, magnetic particles can be deterministically separated from the original streamflow to the buffer, thus leading to a high deflection efficiency (up to ~96%) in a wide range of flow rates. The effect of the focusing length on the deflection of particles is also investigated. Finally, the proposed modular device is tested to separate magnetic and non-magnetic beads.

Diamagnetic repulsion of particles for multilaminar flow assays

A continuous multilaminar flow reaction was performed on functionalised polymer particlesviadiamagnetic repulsion forces, using a simple, inexpensive setup.