Viscoelastic Separation of Particles by Size in Straight Rectangular Microchannels: A Parametric Study for a Refined Understanding

Particle focusing mechanisms in λ-DNA solution flowing in a straight microchannel

Most biological fluids (such as blood, saliva, and lymph) in nature have certain viscoelasticity and are beginning to be used as the carrying fluids for viscoelastic microfluidics. However, the particle-focusing mechanisms in these new biological viscoelastic fluids are still unclear. In this work, the particle-focusing mechanisms in λ-DNA solutions were systematically explored. We first explored the particle focusing dynamics in a square cross-section under varied flow rates to uncover the effects of flow rate on particle focusing. Three focusing stages, from the classic five-position viscoelastic focusing to single-stream focusing and finally to multiplex-stream focusing, were clearly demonstrated. In addition, the particle focusing process along the channel length was demonstrated, and a first-fast-and-then-slow focusing process was clearly observed. Then, the effects of λ-DNA concentrations on particle focusing were explored and compared using the solutions with 0-25 ppm λ-DNA. Finally, we discussed the inferences of blockage ratio on particle focusing by changing the particle diameter and cross-sectional dimensions. Our work may provide a deeper understanding on the particle focusing mechanisms in biological viscoelastic fluids and lays a foundation for the subsequent particle counting and analysis and the development of low-cost portable flow cytometers.

Elasto-inertial focusing and particle migration in high aspect ratio microchannels for high-throughput separation

The combination of flow elasticity and inertia has emerged as a viable tool for focusing and manipulating particles using microfluidics. Although there is considerable interest in the field of elasto-inertial microfluidics owing to its potential applications, research on particle focusing has been mostly limited to low Reynolds numbers (Re<1), and particle migration toward equilibrium positions has not been extensively examined. In this work, we thoroughly studied particle focusing on the dynamic range of flow rates and particle migration using straight microchannels with a single inlet high aspect ratio. We initially explored several parameters that had an impact on particle focusing, such as the particle size, channel dimensions, concentration of viscoelastic fluid, and flow rate. Our experimental work covered a wide range of dimensionless numbers (0.05 < Reynolds number < 85, 1.5 < Weissenberg number < 3800, 5 < Elasticity number < 470) using 3, 5, 7, and 10 µm particles. Our results showed that the particle size played a dominant role, and by tuning the parameters, particle focusing could be achieved at Reynolds numbers ranging from 0.2 (1 µL/min) to 85 (250 µL/min). Furthermore, we numerically and experimentally studied particle migration and reported differential particle migration for high-resolution separations of 5 µm, 7 µm and 10 µm particles in a sheathless flow at a throughput of 150 µL/min. Our work elucidates the complex particle transport in elasto-inertial flows and has great potential for the development of high-throughput and high-resolution particle separation for biomedical and environmental applications.

Elasto-inertial particle focusing in sinusoidal microfluidic channels

Dean flow existing in sinusoidal channels could enhance the throughput and efficiency for elasto-inertial particle focusing. However, the fundamental mechanisms of elasto-inertial focusing in sinusoidal channels are still unclear. This work employs four microfluidic devices with symmetric and asymmetric sinusoidal channels to explore the elasto-inertial focusing mechanisms over a wide range of flow rates. The effects of rheological property, flow rate, sinusoidal channel curvature, particle size, and asymmetric geometry on particle focusing performance are investigated. It is intriguing to find that the Dean flow makes a substantial contribution to the particle elasto-inertial focusing. The results illustrate that a better particle focusing performance and a faster focusing process are obtained in the sinusoidal channel with a small curvature radius due to stronger Dean flow. In addition, the particle focusing performance is also related to particle diameter and rheological properties, the larger particles show a better focusing performance than smaller particles, and the smaller flow rate is required for particles to achieve stable focusing at the outlet in the higher concentration of polyvinylpyrrolidone solutions. Our work offers an increased knowledge of the mechanisms of elasto-inertial focusing in sinusoidal channels. Ultimately, these results provide supportive guidelines into the design and development of sinusoidal elasto-inertial microfluidic devices for high-performance focusing.

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

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

Continuous On-Chip Cell Washing Using Viscoelastic Microfluidics

Medium exchange of particles/cells to a clean buffer with a low background is essential for biological, chemical, and clinical research, which has been conventionally conducted using centrifugation. However, owing to critical limitations, such as possible cell loss and physical stimulation of cells, microfluidic techniques have been adopted for medium exchange. This study demonstrates a continuous on-chip washing process in a co-flow system using viscoelastic and Newtonian fluids. The co-flow system was constructed by adding a small amount of biocompatible polymer (xanthan gum, XG) to a sample containing particles or cells and introducing Newtonian fluids as sheath flows. Polymer concentration-dependent and particle size-dependent lateral migration of particles in the co-flow system were examined, and then the optimal concentration and the critical particle size for medium exchange were determined at the fixed total flow rate of 100 μL/min. For clinical applications, the continuous on-chip washing of white blood cells (WBCs) in lysed blood samples was demonstrated, and the washing performance was evaluated using a scanning spectrophotometer.

Formation of Droplets of Shear-Thinning Non-Newtonian Fluids in Asymmetrical Parallelized Microchannels

Fluids containing polymers are frequently utilized in the chemical industry and exhibit shear-thinning characteristics. The flow distribution of non-Newtonian fluids in parallelized microchannels is a key issue to be solved during numbering-up. Numbering-up means increasing the number of parallelized microchannels. In this study, a high-speed camera is used to explore the distribution of fluid flow as well as the uniformity and stability of droplets in conceptual asymmetrical parallelized microchannels. Cyclohexane and carboxymethylcellulose sodium (CMC) aqueous solutions are used as the continuous phase and dispersed phase, respectively. The effects of fluctuation of pressure difference around the T-junction, the hydrodynamic resistance in microchannels, and the shear-thinning property of fluids on flow distribution and droplet formation are revealed. The uniformity and stability of droplets in microdevices with various cavity settings are compared, and an optimal configuration is proposed. Finally, prediction models for the flow distribution of shear-thinning fluids in asymmetrical parallelized microchannels are established.

Elasto-Inertial Focusing Mechanisms of Particles in Shear-Thinning Viscoelastic Fluid in Rectangular Microchannels

Growth of the microfluidics field has triggered numerous advances in focusing and separating microparticles, with such systems rapidly finding applications in biomedical, chemical, and environmental fields. The use of shear-thinning viscoelastic fluids in microfluidic channels is leading to evolution of elasto-inertial focusing. Herein, we showed that the interplay between the elastic and shear-gradient lift forces, as well as the secondary flow transversal drag force that is caused by the non-zero second normal stress difference, lead to different particle focusing patterns in the elasto-inertial regime. Experiments and 3D simulations were performed to study the effects of flowrate, particle size, and the shear-thinning extent of the fluid on the focusing patterns. The Giesekus constitutive equation was used in the simulations to capture the shear-thinning and viscoelastic behaviors of the solution used in the experiments. At low flowrate, with Weissenberg number Wi ~ O(1), both the elastic force and secondary flow effects push particles towards the channel center. However, at a high flowrate, Wi ~ O(10), the elastic force direction is reversed in the central regions. This remarkable behavior of the elastic force, combined with the enhanced shear-gradient lift at the high flowrate, pushes particles away from the channel center. Additionally, a precise prediction of the focusing position can only be made when the shear-thinning extent of the fluid is correctly estimated in the modeling. The shear-thinning also gives rise to the unique behavior of the inertial forces near the channel walls which is linked with the ‘warped’ velocity profile in such fluids.

Multiple-Streams Focusing-Based Cell Separation in High Viscoelasticity Flow

Viscoelastic flow has been widely used in microfluidic particle separation processes, in which particles get focused on the channel center in diluted viscoelastic flow. In this paper, the transition from single-stream focusing to multiple-streams focusing (MSF) in high viscoelastic flow is observed, which is applied for cell separation processes. Particle focusing stream bifurcation is caused by the balance between elastic force and viscoelastic secondary flow drag force. The influence of cell physical properties, such as cell dimension, shape, and deformability, on the formation of multiple-streams focusing is studied in detail. Particle separation is realized utilizing different separation criteria. The size-based separation of red (RBC) and white (WBC) blood cells is demonstrated in which cells get focused in different streams based on their dimension difference. Cells with different deformabilities get stretched in the viscoelastic flow, leading to the change of focusing streams, and this property is harnessed to separate red blood cells infected with the malaria parasite, Plasmodium falciparum. The achieved results promote our understanding of particle movement in the high viscoelastic flow and enable new particle manipulation and separation processes for sample treatment in biofluids.

Effect of wall slip on the viscoelastic particle ordering in a microfluidic channel

The formation of a line of equally spaced particles at the centerline of a microchannel, referred as "particle ordering," is desired in several microfluidic applications. Recent experiments and simulations highlighted the capability of viscoelastic fluids to form a row of particles characterized by a preferential spacing. When dealing with non-Newtonian fluids in microfluidics, the adherence condition of the liquid at the channel wall may be violated and the liquid can slip over the surface, possibly affecting the ordering efficiency. In this work, we investigate the effect of wall slip on the ordering of particles suspended in a viscoelastic liquid by numerical simulations. The dynamics of a triplet of particles in an infinite cylindrical channel is first addressed by solving the fluid and particle governing equations. The relative velocities computed for the three-particle system are used to predict the dynamics of a train of particles flowing in a long microchannel. The distributions of the interparticle spacing evaluated at different slip coefficients, linear particle concentrations, and distances from the channel inlet show that wall slip slows down the self-assembly mechanism. For strong slipping surfaces, no significant change of the initial microstructure is observed at low particle concentrations, whereas strings of particles in contact form at higher concentrations. The detrimental effect of wall slip on viscoelastic ordering suggests care when designing microdevices, especially in case of hydrophobic surfaces that may enhance the slipping phenomenon.

Direct separation and enumeration of CTCs in viscous blood based on co-flow microchannel with tunable shear rate: a proof-of-principle study

Circulating tumor cells (CTCs), which have extremely low density in whole blood, are an important indicator of primary tumor metastasis. Isolation and enumeration of these cells are critical for clinical applications. Separation of CTCs from massive blood cells without labeling and addition of synthetic polymers is challenging. Herein, a novel well-defined co-flow microfluidic device is presented and used to separate CTCs in viscous blood by applying both inertial and viscoelastic forces. Diluted blood without any synthetic polymer and buffer solution were used as viscoelastic fluid and Newtonian fluid, respectively, and they were co-flowed in the designed chip to form a sheath flow. The co-flow system provides the function of particle pre-focusing and creates a tunable shear rate region at the interface to adjust the migration of particles or cells from the sample solution to the buffer solution. Successful separation of CTCs from viscous blood was demonstrated and enumeration was also conducted by image recognition after separation. The statistical results indicated that a recovery rate of cancer cells greater than 87% was obtained using the developed method, which proved that the direct separation of CTCs from diluted blood can be achieved without the addition of any synthetic polymer to prepare viscoelastic fluid. This method holds great promise for the separation of cells in viscous biological fluid without either complicated channel structures or the addition of synthetic polymers.

Microscale hydrodynamic confinements: shaping liquids across length scales as a toolbox in life sciences

Hydrodynamic phenomena can be leveraged to confine a range of biological and chemical species without needing physical walls. In this review, we list methods for the generation and manipulation of microfluidic hydrodynamic confinements in free-flowing liquids and near surfaces, and elucidate the associated underlying theory and discuss their utility in the emerging area of open space microfluidics applied to life-sciences. Microscale hydrodynamic confinements are already starting to transform approaches in fundamental and applied life-sciences research from precise separation and sorting of individual cells, allowing localized bio-printing to multiplexing for clinical diagnosis. Through the choice of specific flow regimes and geometrical boundary conditions, hydrodynamic confinements can confine species across different length scales from small molecules to large cells, and thus be applied to a wide range of functionalities. We here provide practical examples and implementations for the formation of these confinements in different boundary conditions - within closed channels, in between parallel plates and in an open liquid volume. Further, to enable non-microfluidics researchers to apply hydrodynamic flow confinements in their work, we provide simplified instructions pertaining to their design and modelling, as well as to the formation of hydrodynamic flow confinements in the form of step-by-step tutorials and analytical toolbox software. This review is written with the idea to lower the barrier towards the use of hydrodynamic flow confinements in life sciences research.

Separation of U87 glioblastoma cell-derived small and medium extracellular vesicles using elasto-inertial flow focusing (a spiral channel)

Nanoscale and microscale cell-derived extracellular vesicle types and subtypes are of significant interest to researchers in biology and medicine. Extracellular vesicles (EVs) have diagnostic and therapeutic potential in terms of biomarker and nanomedicine applications. To enable such applications, EVs must be isolated from biological fluids or separated from other EV types. Developing methods to fractionate EVs is of great importance to EV researchers. Our goal was to begin to develop a device that would separate medium EVs (mEVs, traditionally termed microvesicles or shedding vesicles) and small EVs (sEVs, traditionally termed exosomes) by elasto-inertial effect. We sought to develop a miniaturized technology that works similar to and provides the benefits of differential ultracentrifugation but is more suitable for EV-based microfluidic applications. The aim of this study was to determine whether we could use elasto-inertial focusing to re-isolate and recover U87 mEVs and sEVs from a mixture of mEVs and sEVs isolated initially by one round of differential ultracentrifugation. The studied spiral channel device can continuously process 5 ml of sample fluid per hour. Using the channel, sEVs and mEVs were recovered and re-isolated from a mixture of U87 glioma cell-derived mEVs and sEVs pre-isolated by one round of differential ultracentrifugation. Following two passes through the spiral channel, approximately 55% of sEVs were recovered with 6% contamination by mEVs (the recovered sEVs contained 6% of the total mEVs). In contrast, recovery of U87 mEVs and sEVs re-isolated using a typical second centrifugation wash step was only 8% and 53%, respectively. The spiral channel also performed similar to differential ultracentrifugation in reisolating sEVs while significantly improving mEV reisolation from a mixture of U87 sEVs and mEVs. Ultimately this technology can also be coupled to other microfluidic EV isolation methods in series and/or parallel to improve isolation and minimize loss of EV subtypes.

Viscoelastic Particle Focusing and Separation in a Spiral Channel

As one type of non-Newtonian fluid, viscoelastic fluids exhibit unique properties that contribute to particle lateral migration in confined microfluidic channels, leading to opportunities for particle manipulation and separation. In this paper, particle focusing in viscoelastic flow is studied in a wide range of polyethylene glycol (PEO) concentrations in aqueous solutions. Polystyrene beads with diameters from 3 to 20 μm are tested, and the variation of particle focusing position is explained by the coeffects of inertial flow, viscoelastic flow, and Dean flow. We showed that particle focusing position can be predicted by analyzing the force balance in the microchannel, and that particle separation resolution can be improved in viscoelastic flows.

Cascaded contraction-expansion channels for bacteria separation from RBCs using viscoelastic microfluidics

Implementation of viscoelasticity-based particle migration techniques has attracted significant interest thanks to its simplicity to achieve particle separation and enrichment at high sensitivity and accuracy for the last decade. Many methods have previously been developed for particle focusing and separation, but they all require long fluidic channels to enable the desired elastic force on particles. Here, a cascade contraction-expansion microfluidic system with a much shorter channel length is presented. Experimental results show that this system achieved continuous, sheathless particle separation in a viscoelastic fluid, and Enterococcus faecalis was successfully separated from red blood cells (RBCs). Thanks to its small size, the system provides extra advantage for its integration into small chips.

A weak shear stress microfluidic device based on Viscoelastic Stagnant Region (VSR) for biosensitive particle capture

Particle manipulation in microfluidic devices is of great significance in biological research. However, currently available inertial capture methods require relatively high flow rates, which will cause damage to biological particles, especially for single-celled organisms that are sensitive to environments. Herein, we demonstrate a label-free, size-based, low shear stress manipulation method using the Viscoelastic Stagnant Region (VSR) to capture sensitive bioparticles. This method uses the deformation of molecular chains in the polymer solution which can generate elastic stresses to form vortices which is called VSR because of the extremely low velocity in the contraction-expansion array (CEA) microchannel. Formation and evolution of VSR was observed experimentally using the Micro-PIV system in polyethylene oxide (PEO) solutions with different concentrations. On this basis, 20 μm and 5 μm polystyrene (PS) particles were confined to a certain area in the microchamber and the trajectory of particles motion in VSR was observed. Both the inertial lift force and the viscoelastic force are affected by the particle size, so the method also presented size selectivity. By quantitatively studying the velocity distribution of the particles on the orbit of motion, it was found that the maximum velocity of the particles in the VSR was about 0.02 m s^-1, which is only about 1/100 of that in inertial vortices. Moreover, the spiral motion of particles captured by VSR with variable trajectories was first observed, which is different from the equilibrium orbit in inertial vortices and can provide more motion paths for the particles. This method was further applied to the research of Crypthecodinium cohnii (C. cohnii), which is vulnerable but has high nutritious value. We found that the activity of the C. cohnii decreased slightly after being processed by VSR, but was lost after being processed by the inertial vortices. This research, as a simple and low shear stress particle manipulation method, will provide useful guidance for the manipulation, capture and separation of sensitive biological cells under higher biocompatibility, which is of great significance for the study of cellular and molecular biology.

A Rapid and Sensitive Salmonella Biosensor Based on Viscoelastic Inertial Microfluidics

Salmonella is a main cause of foodborne illnesses and rapid screening of Salmonella is the key to prevent Salmonella outbreaks, however available detection methods either require a long time, or need complex pretreatment, or have low sensitivity. In this study, a microfluidic biosensor was developed for Salmonella detection using viscoelastic inertial microfluidics for separating magnetic bacteria from unbound magnetic nanoparticles (MNPs) and enzyme catalytic colorimetry for amplifying biological signals. The polyclonal antibodies and horseradish peroxidase (HRP) modified MNPs were first used to specifically capture Salmonella to form magnetic HRP-bacteria. Both magnetic HRP-bacteria and unbound MNPs were magnetically separated from background and resuspended in viscoelastic polyvinylpyrrolidone solution as sample flow. When sample flow was injected with polyvinylpyrrolidone sheath flow into a T-shaped microchannel, larger-sized magnetic HRP-bacteria could penetrate the sample flow, however smaller-sized MNPs remained in the sample flow due to weaker inertial lift force and elastic lift force, resulting in continuous-flow separation of magnetic HRP-bacteria. Finally, magnetic HRP-bacteria were collected and concentrated to catalyze tetramethyl benzidine, and absorbance was measured to determine the bacteria. This biosensor was able to detect Salmonella as low as 30 CFU/mL in 1 h and featured the advantages of shorter time due to a one-step immunoreaction, easier extension due to only one antibody and one label, and lower cost due to less expensive materials.

Oscillatory Viscoelastic Microfluidics for Efficient Focusing and Separation of Nanoscale Species

The ability to precisely control particle migration within microfluidic systems is essential for focusing, separating, counting, and detecting a wide range of biological species. To date, viscoelastic microfluidic systems have primarily been applied to the focusing, separation, and isolation of micrometer-sized species, with their use in nanoparticle manipulations being underdeveloped and underexplored, due to issues related to nanoparticle diffusivity and a need for extended channel lengths. To overcome such issues, we herein present sheathless oscillatory viscoelastic microfluidics as a method for focusing and separating both micrometer and sub-micrometer species. To highlight the efficacy of our approach, we segment our study into three size regimes, namely, micrometer (where characteristic particle dimensions are above 1 μm), sub-micrometer (where characteristic dimensions are between 1 μm and 100 nm), and nano (where characteristic dimensions are below 100 nm) regimes. Based on the ability to successfully manipulate particles in all these regimes, we demonstrate the successful isolation of p-bodies from biofluids (in the micrometer regime), the focusing of λ-DNA (in the sub-micrometer regime), and the focusing of extracellular vesicles (in the nanoregime). Finally, we characterize the physics underlying viscoelastic microflows using a dimensionless number that relates the lateral velocity (due to elastic effects) to the diffusion constant of the species within the viscoelastic carrier fluid. Based on the ability to precisely manipulate species in all three regimes, we expect that sheathless oscillatory viscoelastic microfluidics may be used to good effect in a range of biological and life science applications.

Viscoelastic microfluidics: progress and challenges

The manipulation of cells and particles suspended in viscoelastic fluids in microchannels has drawn increasing attention, in part due to the ability for single-stream three-dimensional focusing in simple channel geometries. Improvement in the understanding of non-Newtonian effects on particle dynamics has led to expanding exploration of focusing and sorting particles and cells using viscoelastic microfluidics. Multiple factors, such as the driving forces arising from fluid elasticity and inertia, the effect of fluid rheology, the physical properties of particles and cells, and channel geometry, actively interact and compete together to govern the intricate migration behavior of particles and cells in microchannels. Here, we review the viscoelastic fluid physics and the hydrodynamic forces in such flows and identify three pairs of competing forces/effects that collectively govern viscoelastic migration. We discuss migration dynamics, focusing positions, numerical simulations, and recent progress in viscoelastic microfluidic applications as well as the remaining challenges. Finally, we hope that an improved understanding of viscoelastic flows in microfluidics can lead to increased sophistication of microfluidic platforms in clinical diagnostics and biomedical research.

Viscoelastic second normal stress difference dominated multiple-stream particle focusing in microfluidic channels

Particle focusing in viscoelastic fluid flow is a promising approach for inducing particle separations in microfluidic devices. The results from theoretical studies indicated that multiple stream particle focusing can be realized with a large magnitude of the elastic second normal stress difference (N₂). For dilute polymer solutions, theoretical and experimental studies show that the magnitude of N₂ is never large, no matter how large the polymer molecular weight nor how high the shear rate. However, for concentrated entangled polymer solutions, the magnitude of N₂ becomes large at high shear rates. Therefore, in order to test the hypothesis that N₂ can be used to induce multiple particle stream focusing behavior, we perform the systematic study of the effects of increasing carrier fluid polymer concentrations in a microchannel containing fluorescent particles. In a dilute polymer solution, multiple particle stream focusing is not observed, even at high shear rates and large dimensionless Weissenberg number values (Wi ≈ 30) at which the elastic first normal stress difference (N₁) and the viscosity shear-thinning should be very large, while in a concentrated entangled polymer solution, we observe that particle streams focused upon the channel centerline bifurcate to form two symmetric off-channel particle streams at higher shear rates. This particle focusing behavior is different from previous multiple-stream focusing phenomena, and that we attribute to the influence of the second normal stress difference N₂. This N₂ induced multiple stream focusing phenomenon provides a different approach for manipulating the particle trajectory and separation in a microchannel.

Viscoelastic second normal stress difference dominated multiple-stream particle focusing in microfluidic channels

Particle focusing in viscoelastic fluid flow is a promising approach for inducing particle separations in microfluidic devices. The results from theoretical studies indicated that multiple stream particle focusing can be realized with a large magnitude of the elastic second normal stress difference (N₂). For dilute polymer solutions, theoretical and experimental studies show that the magnitude of N₂ is never large, no matter how large the polymer molecular weight nor how high the shear rate. However, for concentrated entangled polymer solutions, the magnitude of N₂ becomes large at high shear rates. Therefore, in order to test the hypothesis that N₂ can be used to induce multiple particle stream focusing behavior, we perform the systematic study of the effects of increasing carrier fluid polymer concentrations in a microchannel containing fluorescent particles. In a dilute polymer solution, multiple particle stream focusing is not observed, even at high shear rates and large dimensionless Weissenberg number values (Wi ≈ 30) at which the elastic first normal stress difference (N₁) and the viscosity shear-thinning should be very large, while in a concentrated entangled polymer solution, we observe that particle streams focused upon the channel centerline bifurcate to form two symmetric off-channel particle streams at higher shear rates. This particle focusing behavior is different from previous multiple-stream focusing phenomena, and that we attribute to the influence of the second normal stress difference N₂. This N₂ induced multiple stream focusing phenomenon provides a different approach for manipulating the particle trajectory and separation in a microchannel.

Manipulation of micro- and nanoparticles in viscoelastic fluid flows within microfluid systems

Manipulation of micro- and nanoparticles in complex biofluids is highly demanded in most biological and biomedical applications. A significant number of microfluidic platforms have been developed for inexpensive, rapid, accurate, and efficient particle manipulation. Due to the enormous potential of viscoelastic fluids (VEFs) for particle manipulation, various emerging microfluidic-based VEFs techniques have been presented over the last decade. This review provides an intuitive understanding of VEF physics for particle separation in different microchannel geometries. Besides, active and passive VEF methods are critically reviewed, highlighting the potential and practical challenges of each technique for particle/cell focusing, sorting, and separation. The outcome of this study could enable recognizing deliverable VEF technology with the promising prospect in the manipulation of submicron biological samples (e.g., exosomes, DNA, and proteins).

Sheathless separation of microalgae from bacteria using a simple straight channel based on viscoelastic microfluidics

Microalgae cells have been recognized as a promising sustainable resource to meet worldwide growing demands for renewable energy, food, livestock feed, water, cosmetics, pharmaceuticals, and materials. In order to ensure high-efficiency and high-quality production of biomass, biofuel, or bio-based products, purification procedures prior to the storage and cultivation of the microalgae from contaminated bacteria are of great importance. The present work proposed and developed a simple, sheathless, and efficient method to separate microalgae Chlorella from bacteria Bacillus Subtilis in a straight channel using the viscoelasticity of the medium. Microalgae and bacteria migrate to different lateral positions closer to the channel centre and channel walls respectively. Fluorescent microparticles with 1 μm and 5 μm diameters were first used to mimic the behaviours of bacteria and microalgae to optimize the separating conditions. Subsequently, size-based separation in Newtonian fluid and in viscoelastic fluid in straight channels with different aspect ratios was compared and demonstrated. Under the optimal condition, the removal ratio for 1 μm microparticles and separation efficiency for 5 μm particles can reach up to 98.28% and 93.85% respectively. For bacteria and microalgae cells separation, the removal ratio for bacteria and separation efficiency for microalgae cells is 92.69% and 100% respectively. This work demonstrated the continuous and sheathless separation of microalgae from bacteria for the first time by viscoelastic microfluidics. This technique can also be applied as an efficient and user-friendly method to separate mammalian cells or other kinds of cells.

Sheathless High-Throughput Circulating Tumor Cell Separation Using Viscoelastic non-Newtonian Fluid

Circulating tumor cells (CTCs) have attracted increasing attention as important biomarkers for clinical and biological applications. Several microfluidic approaches have been demonstrated to separate CTCs using immunoaffinity or size difference from other blood cells. This study demonstrates a sheathless, high-throughput separation of CTCs from white blood cells (WBCs) using a viscoelastic fluid. To determine the fluid viscoelasticity and the flow rate for CTC separation, and to validate the device performance, flow characteristics of 6, 13, and 27 μm particles in viscoelastic fluids with various concentrations were estimated at different flow rates. Using 0.2% hyaluronic acid (HA) solution, MCF-7 (Michigan Cancer Foundation-7) cells mimicking CTCs in this study were successfully separated from WBCs at 500 μL/min with a separation efficiency of 94.8%. Small amounts of MCF-7 cells (~5.2%) were found at the center outlet due to the size overlap with WBCs.

Viscoelastic Separation and Concentration of Fungi from Blood for Highly Sensitive Molecular Diagnostics

Isolation and concentration of fungi in the blood improves sensitivity of the polymerase chain reaction (PCR) method to detect fungi in blood. This study demonstrates a sheathless, continuous separation and concentration method of candida cells using a viscoelastic fluid that enables rapid detection of rare candida cells by PCR analysis. To validate device performance using a viscoelastic fluid, flow characteristics of 2 μm particles were estimated at different flow rates. Additionally, a mixture of 2 μm and 13 μm particles was successfully separated based on size difference at 100 μl/min. Candida cells were successfully separated from the white blood cells (WBCs) with a separation efficiency of 99.1% and concentrated approximately 9.9-fold at the center outlet compared to the initial concentration (~2.5 × 10⁷ cells/ml). Sequential 1st and 2nd concentration processes were used to increase the final number of candida cells to ~2.3 × 10⁹ cells/ml, which was concentrated ~92-fold. Finally, despite the undetectable initial concentration of 10¹ CFU/ml, removal of WBCs and the additional buffer solution enabled the quantitative reverse transcription (RT)-PCR detection of candida cells after the 1st concentration (Ct = 31.43) and the 2nd concentration process (Ct = 29.30).

Impedance-based viscoelastic flow cytometry

Elastic nature of the viscoelastic fluids induces lateral migration of particles into a single streamline and can be used by microfluidic based flow cytometry devices. In this study, we investigated focusing efficiency of polyethylene oxide based viscoelastic solutions at varying ionic concentration to demonstrate their use in impedimetric particle characterization systems. Rheological properties of the viscoelastic fluid and particle focusing performance are not affected by ionic concentration. We investigated the viscoelastic focusing dynamics using polystyrene (PS) beads and human red blood cells (RBCs) suspended in the viscoelastic fluid. Elasto-inertial focusing of PS beads was achieved with the combination of inertial and viscoelastic effects. RBCs were aligned along the channel centerline in parachute shape which yielded consistent impedimetric signals. We compared our impedance-based microfluidic flow cytometry results for RBCs and PS beads by analyzing particle transit time and peak amplitude at varying viscoelastic focusing conditions obtained at different flow rates. We showed that single orientation, single train focusing of nonspherical RBCs can be achieved with polyethylene oxide based viscoelastic solution that has been shown to be a good candidate as a carrier fluid for impedance cytometry.

Microparticle separation using asymmetrical induced-charge electro-osmotic vortices on an arc-edge-based floating electrode

We developed a device for particle separation by designing an arc-edge-based floating electrode to alternately actuate opposite-direction asymmetrical induced-charge electro-osmotic vortices.

Splitting and separation of colloidal streams in sinusoidal microchannels

The control of the distribution of colloidal particles in microfluidic flows plays an important role in biomedical and industrial applications. A particular challenge is to induce cross-streamline migration in laminar flows, enabling the separation of colloidal particles according to their size, shape or elasticity. Here we show that viscoelastic fluids can mediate cross-streamline migration of deformable spherical and cylindrical colloidal particles in sinusoidal microchannels at low Reynolds numbers. For colloidal streams focused into the center of the channel entrance this leads to a symmetric stream-splitting and separation into four substreams. The degree of stream splitting and separation can be controlled via the flow rates, viscoelasticity of the focusing fluid, and the spatial microchannel modulation with an upper limit when reaching the microchannel walls. We demonstrate that this effect can be used to separate flexible particles of different size and shape. This methodology of cross-stream migration has thus great potential for the passive separation of colloids and cells in microfluidic channels.

Continuous sheath-free separation of drug-treated human fungal pathogen Cryptococcus neoformans by morphology in biocompatible polymer solutions

Cryptococcal meningitis caused by Cryptococcus neoformans is the leading cause of fungal central nervous system infections. Current antifungal treatments for cryptococcal infections are inadequate partly due to the occurrence of drug resistance. Recent studies indicate that the treatment of the azole drug fluconazole changes the morphology of C. neoformans to form enlarged "multimeras" that consist of three or more connected cells/buds. To analyze if these multimeric cells are a prerequisite for C. neoformans to acquire drug resistance, a tool capable of separating them from normal cells is critical. We extend our recently demonstrated sheath-free elasto-inertial particle separation technique to fractionate drug-treated C. neoformans cells by morphology in biocompatible polymer solutions. The separation performance is evaluated for both multimeric and normal cells in terms of three dimensionless metrics: efficiency, purity, and enrichment ratio. The effects of flow rate, polymer concentration, and microchannel height on cell separation are studied.

Recent progress of particle migration in viscoelastic fluids

Recently, research on particle migration in non-Newtonian viscoelastic fluids has gained considerable attention. In a viscoelastic fluid, three dimensional (3D) particle focusing can be easily realized in simple channels without the need for any external force fields or complex microchannel structures compared with that in a Newtonian fluid. Due to its promising properties for particle precise focusing and manipulation, this field has been developed rapidly, and research on the field has been shifted from fundamentals to applications. This review will elaborate the recent progress of particle migration in viscoelastic fluids, especially on the aspect of applications. The hydrodynamic forces on the micro/nano particles in viscoelastic fluids are discussed. Next, we elaborate the basic particle migration in viscoelasticity-dominant fluids and elasto-inertial fluids in straight channels. After that, a comprehensive review on the applications of viscoelasticity-induced particle migration (particle separation, cell deformability measurement and alignment, particle solution exchange, rheometry-on-a-chip and others) is presented; finally, we thrash out some perspectives on the future directions of particle migration in viscoelastic fluids.

Sheathless inertial cell focusing and sorting with serial reverse wavy channel structures

Inertial microfluidics utilizing passive hydrodynamic forces has been attracting significant attention in the field of precise microscale manipulation owing to its low cost, simplicity and high throughput. In this paper, we present a novel channel design with a series of reverse wavy channel structures for sheathless inertial particle focusing and cell sorting. A single wavy channel unit consists of four semicircular segments, which produce periodically reversed Dean secondary flow along the cross-section of the channel. The balance between the inertial lift force and the Dean drag force results in deterministic equilibrium focusing positions, which also depends on the size of the flow-through particles and cells. Six sizes of fluorescent microspheres (15, 10, 7, 5, 3 and 1 μm) were used to study the size-dependent inertial focusing behavior. Our novel design with sharp-turning subunits could effectively focus particles as small as 3 μm, the average size of platelets, enabling the sorting of cancer cells from whole blood without the use of sheath flows. Utilizing an optimized channel design, we demonstrated the size-based sorting of MCF-7 breast cancer cells spiked in diluted whole blood samples without using sheath flows. A single sorting process was able to recover 89.72% of MCF-7 cells from the original mixture and enrich MCF-7 cells from an original purity of 5.3% to 68.9% with excellent cell viability.

Sheathless Microflow Cytometry Using Viscoelastic Fluids

Microflow cytometry is a powerful technique for characterization of particles suspended in a solution. In this work, we present a microflow cytometer based on viscoelastic focusing. 3D single-line focusing of microparticles was achieved in a straight capillary using viscoelastic focusing which alleviated the need for sheath flow or any other actuation mechanism. Optical detection was performed by fiber coupled light source and photodetectors. Using this system, we present the detection of microparticles suspended in three different viscoelastic solutions. The rheological properties of the solutions were measured and used to assess the focusing performance both analytically and numerically. The results were verified experimentally, and it has been shown that polyethlyene oxide (PEO) and hyaluronic acid (HA) based sheathless microflow cytometer demonstrates similar performance to state-of-the art flow cytometers. The sheathless microflow cytometer was shown to present 780 particles/s throughput and 5.8% CV for the forward scatter signal for HA-based focusing. The presented system is composed of a single capillary to accommodate the fluid and optical fibers to couple the light to the fluid of interest. Thanks to its simplicity, the system has the potential to widen the applicability of microflow cytometers.

A Simplified Microfluidic Device for Particle Separation with Two Consecutive Steps: Induced Charge Electro-osmotic Prefocusing and Dielectrophoretic Separation

Continuous dielectrophoretic separation is recognized as a powerful technique for a large number of applications including early stage cancer diagnosis, water quality analysis, and stem-cell-based therapy. Generally, the prefocusing of a particle mixture into a stream is an essential process to ensure all particles are subjected to the same electric field geometry in the separation region. However, accomplishing this focusing process either requires hydrodynamic squeezing, which requires an encumbering peripheral system and a complicated operation to drive and control the fluid motion, or depends on dielectrophoretic forces, which are highly sensitive to the dielectric characterization of particles. An alternative focusing technique, induced charge electro-osmosis (ICEO), has been demonstrated to be effective in focusing an incoming mixture into a particle stream as well as nonselective regarding the particles of interest. Encouraged by these aspects, we propose a hybrid method for microparticle separation based on a delicate combination of ICEO focusing and dielectrophoretic deflection. This method involves two steps: focusing the mixture into a thin particle stream via ICEO vortex flow and separating the particles of differing dielectic properties through dielectrophoresis. To demonstrate the feasibility of the method proposed, we designed and fabricated a microfluidic chip and separated a mixture consisting of yeast cells and silica particles with an efficiency exceeding 96%. This method has good potential for flexible integration into other microfluidic chips in the future.