Inertial manipulation and transfer of microparticles across laminar fluid streams

Experiment study on focusing pattern prediction of particles in asymmetric contraction-expansion array channel

Contraction-expansion array (CEA) microchannel is a typical structure applied on particle/cell manipulation. The prediction of the particle focusing pattern in CEA microchannel is worthwhile to be investigate deeply. Here, we demonstrated a virtual boundary method by flow field analysis and theoretical derivation. The calculating method of the virtual boundary location, related to the Reynolds number (Re) and the structure parameter RW, was proposed. Combining the approximate Poiseuille flow pattern based on the virtual boundary method with the simulation results of Dean flow, the main line pattern and the main/lateral lines pattern were predicted and validated in experiments. The transformation from the main line pattern to the main/lateral lines pattern can be facilitated by increasing Re, decreasing RW , and decreasing α. An empirical formula was derived to characterize the critical condition of the transformation. The virtual boundary method can provide a guidance for asymmetric CEA channel design and contribute to the widespread application of microfluidic particle focusing.

Thomson-Einstein's Tea Leaf Paradox Revisited: Aggregation in Rings

A distinct particle focusing spot occurs in the center of a rotating fluid, presenting an apparent paradox given the presence of particle inertia. It is recognized, however, that the presence of a secondary flow with a radial component drives this particle aggregation. In this study, we expand on the examination of this "Thomson-Einstein's tea leaf paradox" phenomenon, where we use a combined experimental and computational approach to investigate particle aggregation dynamics. We show that not only the rotational velocity, but also the vessel shape, have a significant influence on a particle's equilibrium position. We accordingly demonstrate the formation of a single focusing spot in a vessel center, as has been conclusively demonstrated elsewhere, but also the repeatable formation of stable ring-shaped particle arrangements.

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.

High-throughput multi-gate microfluidic resistive pulse sensing for biological nanoparticle detection

A microfluidic resistive pulse sensing technique offers a simple method for detecting and analysing microparticles in various fields, yet it has challenges such as the noise during detection and low throughput as the signal obtained from a small single sensing aperture and particle position is nonuniform. This study presents a microfluidic chip with multiple detection gates in the main channel to enhance the throughput while maintaining a simple operational system. A hydrodynamic sheathless particle focusing on a detection gate by modulation of the channel structure and measurement circuit with a reference gate to minimize the noise during detection is used for detecting resistive pulses. The proposed microfluidic chip can analyse the physical properties of 200 nm polystyrene particles and exosomes from MDA-MB-231 with high sensitivity with an error of <10% and high-throughput screening of more than 200 000 exosomes per seconds. The proposed microfluidic chip can analyse the physical properties with high sensitivity, so that it can be potentially used for exosome detection in biological and in vitro clinical applications.

Investigation of particle manipulation mechanism and size sorting strategy in a double-layered microchannel

Traditionally, comprehensive laboratorial experiments on newly proposed microfluidic devices are necessary for theoretical validation, technological design, methodological calibration and optimization. Multiple parameters and characteristics, such as the flow rate, particle size, microchannel dimensions, etc., should be studied by controlled trials, which could inevitably result in extensive experiments and a heavy burden on researchers. In this work, a novel numerical model was introduced to simulate particle migration within a complicated double-layered microchannel. Using the hybrid meshing method, the proposed model achieved a significant improvement in meshing quality, and remarkably reduced the required calculation resources at the same time. The robust, efficient and resource-saving numerical model was calibrated and validated with experimental results. Based on this model, 1) the mechanism of microparticle manipulation within the microchannel was revealed; 2) the primary reason for the microparticle focusing failure was investigated; and 3) the optimal microparticle sorting strategy at different flow rates was analyzed. In experiments, the obtained optimal strategy could approach a good sorting performance with a high recovery rate and high concentration ratio in a high-throughput manner. The proposed numerical model shows great potential in mechanism investigation and functional prediction for microfluidic technologies using unconventional designs.

Efficient bioparticle extraction using a miniaturized inertial microfluidic centrifuge

Conventional bioparticle extraction requires labor-intensive operation, and expensive and bulky centrifuges. Herein, we report a miniaturized centrifuge by cascading four paralleled inertial spiral channels with a two-stage serpentine channel, allowing for the efficient washing and acquisition of concentrated bioparticles from background fluids. First, the effects of channel size and flow rate on particle focusing dynamics and solution exchange performances are explored to enable the optimization and wide application of our device. Then, the integrated device is fabricated and tested experimentally. The results indicate that 10-20 μm particles can be washed from the original samples with increased concentrations and with recovery efficiencies of >93%. Finally, to verify its versatility, we use our miniaturized centrifuge to successfully change the culture medium for cultured MCF-7 breast cancer cells, extract A549 lung cancer cells from a calcein-AM staining solution, purify white blood cells (WBCs) from lysed whole blood, and extract target cells from an unbonded magnetic microbead background. Compared with conventional centrifuges, our device has the advantages of simple fabrication, easy operation, and small footprint. More importantly, it offers outstanding capability for extracting bioparticles from various background fluids, and avoids bioparticle damage that may be caused by high-speed centrifugation. Therefore, we envision that our miniaturized centrifuge could be a promising alternative to traditional centrifuges in many applications.

Advancing microfluidic diagnostic chips into clinical use: a review of current challenges and opportunities

Microfluidic diagnostic (μDX) technologies miniaturize sensors and actuators to the length-scales that are relevant to biology: the micrometer scale to interact with cells and the nanometer scale to interrogate biology's molecular machinery. This miniaturization allows measurements of biomarkers of disease (cells, nanoscale vesicles, molecules) in clinical samples that are not detectable using conventional technologies. There has been steady progress in the field over the last three decades, and a recent burst of activity catalyzed by the COVID-19 pandemic. In this time, an impressive and ever-growing set of technologies have been successfully validated in their ability to measure biomarkers in clinical samples, such as blood and urine, with sensitivity and specificity not possible using conventional tests. Despite our field's many accomplishments to date, very few of these technologies have been successfully commercialized and brought to clinical use where they can fulfill their promise to improve medical care. In this paper, we identify three major technological trends in our field that we believe will allow the next generation of μDx to have a major impact on the practice of medicine, and which present major opportunities for those entering the field from outside disciplines: 1. the combination of next generation, highly multiplexed μDx technologies with machine learning to allow complex patterns of multiple biomarkers to be decoded to inform clinical decision points, for which conventional biomarkers do not necessarily exist. 2. The use of micro/nano devices to overcome the limits of binding affinity in complex backgrounds in both the detection of sparse soluble proteins and nucleic acids in blood and rare circulating extracellular vesicles. 3. A suite of recent technologies that obviate the manual pre-processing and post-processing of samples before they are measured on a μDX chip. Additionally, we discuss economic and regulatory challenges that have stymied μDx translation to the clinic, and highlight strategies for successfully navigating this challenging space.

Inertial focusing of particles and cells in the microfluidic labyrinth device: Role of sharp turns

Inertial, size-based focusing was investigated in the microfluidic labyrinth device consisting of several U-shaped turns along with circular loops. Turns are associated with tight curvature and, therefore, induce strong Dean forces for separating particles; however, systematic studies exploring this possibility do not exist. We characterized the focusing dynamics of different-sized rigid particles, cancer cells, and white blood cells over a range of fluid Reynolds numbers R e f . Streak widths of the focused particle streams at all the turns showed intermittent fluctuations that were substantial for smaller particles and at higher R e f . In contrast, cell streaks were less prone to fluctuations. Computational fluid dynamics simulations revealed the existence of strong turn-induced Dean vortices, which help explain the intermittent fluctuations seen in particle focusing. Next, we developed a measure of pairwise separability to evaluate the quality of separation between focused streams of two different particle sizes. Using this, we assessed the impact of a single sharp turn on separation. In general, the separability was found to vary significantly as particles traversed the tight-curvature U-turn. Comparing the separability at the entry and exit sections, we found that turns either improved or reduced separation between different-sized particles depending on R e f . Finally, we evaluated the separability at the downstream expansion section to quantify the performance of the labyrinth device in terms of achieving size-based enrichment of particles and cells. Overall, our results show that turns are better for cell focusing and separation given that they are more immune to curvature-driven fluctuations in comparison to rigid particles.

On‐chip microfluidic buffer swap of biological samples in‐line with downstream dielectrophoresis

Microfluidic cell enrichment by dielectrophoresis, based on biophysical and electrophysiology phenotypes, requires that cells be resuspended from their physiological media into a lower conductivity buffer for enhancing force fields and enabling the dielectric contrast needed for separation. To ensure that sensitive cells are not subject to centrifugation for resuspension and spend minimal time outside of their culture media, we present an on‐chip microfluidic strategy for swapping cells into media tailored for dielectrophoresis. This strategy transfers cells from physiological media into a 100‐fold lower conductivity media by using tangential flows of low media conductivity at 200‐fold higher flow rate versus sample flow to promote ion diffusion over the length of a straight channel architecture that maintains laminarity of the flow‐focused sample and minimizes cell dispersion across streamlines. Serpentine channels are used downstream from the flow‐focusing region to modulate hydrodynamic resistance of the central sample outlet versus flanking outlets that remove excess buffer, so that cell streamlines are collected in the exchanged buffer with minimal dilution in cell numbers and at flow rates that support dielectrophoresis. We envision integration of this on‐chip sample preparation platform prior to or post‐dielectrophoresis, in‐line with on‐chip monitoring of the outlet sample for metrics of media conductivity, cell velocity, cell viability, cell position, and collected cell numbers, so that the cell flow rate and streamlines can be tailored for enabling dielectrophoretic separations from heterogeneous samples.

Multiphysics microfluidics for cell manipulation and separation: a review

Multiphysics microfluidics, which combines multiple functional physical processes in a microfluidics platform, is an emerging research area that has attracted increasing interest for diverse biomedical applications. Multiphysics microfluidics is expected to overcome the limitations of individual physical phenomena through combining their advantages. Furthermore, multiphysics microfluidics is superior for cell manipulation due to its high precision, better sensitivity, real-time tunability, and multi-target sorting capabilities. These exciting features motivate us to review this state-of-the-art field and reassess the feasibility of coupling multiple physical processes. To confine the scope of this paper, we mainly focus on five common forces in microfluidics: inertial lift, elastic, dielectrophoresis (DEP), magnetophoresis (MP), and acoustic forces. This review first explains the working mechanisms of single physical phenomena. Next, we classify multiphysics techniques in terms of cascaded connections and physical coupling, and we elaborate on combinations of designs and working mechanisms in systems reported in the literature to date. Finally, we discuss the possibility of combining multiple physical processes and associated design schemes and propose several promising future directions.

A needle tip CCEA microfluidic device based on enhanced Dean flow for cell washing

Particle/cell washing is an essential technique in biological and clinical manipulations. Herein, we propose a novel circular contraction-expansion array (CCEA) microdevice. It can be directly connected to a needle tip without connection tubes. Its small size and centrosymmetric structure are beneficial to low sample consumption, high connection stability, and a wide application range. Computational fluid dynamics (CFD) simulation results show that the CCEA structure can produce a stronger Dean flow and lead to faster particle/cell focusing than the circle structure and CEA structure with the same length. Experimentally, an optimal flow rate ratio of 1:3 and an optimal total flow rate of 120 μL/min were found to ensure a stable fluid distribution. Under these conditions, rapid focusing of 10-20 μm particles with high efficiencies was achieved. Compared with a normal CEA device using tubes, the particle loss rate could be reduced from 64 to 7% when washing 500 μL of a rare sample. Cell suspensions with concentrations from 3 × 105/mL to 1 × 103/mL were tested. The high cell collection efficiency (>85% for three cell lines) and stable waste removal efficiency (>80%) reflected the universality of the CCEA microfluidic device. After the washing, the cell activities of H1299 cells and MCF-7 cells were calculated to be 93.8 and 97.5%, respectively. This needle-tip CCEA microfluidic device showed potential in basic medical research and clinical diagnosis.

Inertial microfluidics for high-throughput cell analysis and detection: a review

Since it was first proposed in 2007, inertial microfluidics has been extensively studied in terms of theory, design, fabrication, and application. In recent years, with the rapid development of microfabrication technologies, a variety of channel structures that can focus, concentrate, separate, and capture bioparticles or fluids have been designed and manufactured to extend the range of potential biomedical applications of inertial microfluidics. Due to the advantages of high throughput, simplicity, and low device cost, inertial microfluidics is a promising candidate for rapid sample processing, especially for large-volume samples with low-abundance targets. As an approach to cellular sample pretreatment, inertial microfluidics has been widely employed to ensure downstream cell analysis and detection. In this review, a comprehensive summary of the application of inertial microfluidics for high-throughput cell analysis and detection is presented. According to application areas, the recent advances can be sorted into label-free cell mechanical phenotyping, sheathless flow cytometric counting, electrical impedance cytometer, high-throughput cellular image analysis, and other methods. Finally, the challenges and prospects of inertial microfluidics for cell analysis and detection are summarized.

The Yin and Yang of exosome isolation methods: conventional practice, microfluidics, and commercial kits

Exosomes are a subset of extracellular vesicles released from various cells, and they can be found in different bodily fluids. Exosomes are used as biomarkers to diagnose many diseases and to monitor therapy efficiency as they represent the status and origin of the cell, which they are released from. Considering that they co-exist in bodily fluids with other types of particles, their isolation still remains challenging since conventional separation methods are time-consuming, user-dependent, and result in low isolation yield. This review summarizes the conventional strategies and microfluidic-based methods for exosome isolation along with their strengths and limitations. Microfluidic devices emerge as a promising approach to overcome the limitations of the conventional methods due to their inherent characteristics, such as the need for minute sample volume and rapid operation, in order to isolate exosomes with a high yield and a high purity in a short amount of time, which make them unprecedented tools for molecular biology and clinical applications. This review elaborates on the existing microfluidic-based exosome isolation methods and denotes their benefits and drawbacks. Herein, we also introduce various commercially available platforms and kits for exosome isolation along with their working principles.

Machine learning assisted fast prediction of inertial lift in microchannels

Inertial effect has been extensively used in manipulating both engineered particles and biocolloids in microfluidic platforms. The design of inertial microfluidic devices largely relies on precise prediction of particle migration that is determined by the inertial lift acting on the particle. In spite of being the only means to accurately obtain the lift forces, direct numerical simulation (DNS) often consumes high computational cost and even becomes impractical when applied to microchannels with complex geometries. Herein, we proposed a fast numerical algorithm in conjunction with machine learning techniques for the analysis and design of inertial microfluidic devices. A database of inertial lift forces was first generated by conducting DNS over a wide range of operating parameters in straight microchannels with three types of cross-sectional shapes, including rectangular, triangular and semicircular shapes. A machine learning assisted model was then developed to gain the inertial lift distribution, by simply specifying the cross-sectional shape, Reynolds number and particle blockage ratio. The resultant inertial lift was integrated into the Lagrangian tracking method to quickly predict the particle trajectories in two types of microchannels in practical devices and yield good agreement with experimental observations. Our database and the associated codes allow researchers to expedite the development of the inertial microfluidic devices for particle manipulation.

Deep Learning-Enabled Label-Free On-Chip Detection and Selective Extraction of Cell Aggregate-Laden Hydrogel Microcapsules

Microfluidic encapsulation of cells/tissues in hydrogel microcapsules has attracted tremendous attention in the burgeoning field of cell-based medicine. However, when encapsulating rare cells and tissues (e.g., pancreatic islets and ovarian follicles), the majority of the resultant hydrogel microcapsules are empty and should be excluded from the sample. Furthermore, the cell-laden hydrogel microcapsules are usually suspended in an oil phase after microfluidic generation, while the microencapsulated cells require an aqueous phase for further culture/transplantation and long-term suspension in oil may compromise the cells/tissues. Thus, real-time on-chip selective extraction of cell-laden hydrogel microcapsules from oil into aqueous phase is crucial to the further use of the microencapsulated cells/tissues. Contemporary extraction methods either require labeling of cells for their identification along with an expensive detection system or have a low extraction purity (<≈30%). Here, a deep learning-enabled approach for label-free detection and selective extraction of cell-laden microcapsules with high efficiency of detection (≈100%) and extraction (≈97%), high purity of extraction (≈90%), and high cell viability (>95%) is reported. The utilization of deep learning to dynamically analyze images in real time for label-free detection and on-chip selective extraction of cell-laden hydrogel microcapsules is unique and may be valuable to advance the emerging cell-based medicine.

Inertial focusing of microparticles, bacteria, and blood in serpentine glass channels

Early detection of pathogenic microorganisms is pivotal to diagnosis and prevention of health and safety crises. Standard methods for pathogen detection often rely on lengthy culturing procedures, confirmed by biochemical assays, leading to >24 h for a diagnosis. The main challenge for pathogen detection is their low concentration within complex matrices. Detection of blood-borne pathogens via techniques such as PCR requires an initial positive blood culture and removal of inhibitory blood components, reducing its potential as a diagnostic tool. Among different label-free microfluidic techniques, inertial focusing on microscale channels holds great promise for automation, parallelization, and passive continuous separation of particles and cells. This work presents inertial microfluidic manipulation of small particles and cells (1-10 μm) in curved serpentine glass channels etched at different depths (deep and shallow designs) that can be exploited for (1) bacteria preconcentration from biological samples and (2) bacteria-blood cell separation. In our shallow device, the ability to focus Escherichia coli into the channel side streams with high recovery (89% at 2.2× preconcentration factor) could be applied for bacteria preconcentration in urine for diagnosis of urinary tract infections. Relying on differential equilibrium positions of red blood cells and E. coli inside the deep device, 97% red blood cells were depleted from 1:50 diluted blood with 54% E. coli recovered at a throughput of 0.7 mL/min. Parallelization of such devices could process relevant volumes of 7 mL whole blood in 10 min, allowing faster sample preparation for downstream molecular diagnostics of bacteria present in bloodstream.

Integration of microfluidic sample preparation with PCR detection to investigate the effects of simultaneous DNA-Inhibitor separation and DNA solution exchange

In this paper, we applied a curved-channel microfluidic device to separate DNA from PCR-inhibitor-containing water and simultaneously wash them into clean water for detection using a portable PCR thermocycler. Environmental DNA (eDNA) sampling has become an effective surveying approach for detecting rare organisms. However, low concentration eDNA molecules may be masked by PCR inhibitors during amplification and detection, increasing the risk of false negatives. Therefore, technologies for on-site DNA separation and washing are urgently needed. Our device consisted of a half-circle microchannel with a DNA-inhibitor sample inlet, a clean buffer inlet, and multiple outlets. By using the flow-induced inertial forces, 10 μm DNA-conjugated microparticles were focused at the inner-wall of the curved microchannel while separation from 1 μm inhibitor-conjugated microparticles and DNA washing were achieved simultaneously with the Dean flow. We achieved singleplex focusing, isolation and washing of 10 μm particles at an efficiency of 94.5 ± 2.0%. In duplex experiments with 1 μm and 10 μm particles, larger particles were washed with an efficiency of 92.1 ± 1.6% and a purity of 79 ± 2%. By surface-functionalizing the microparticles with affinity groups against Atlantic salmon DNA and humic acid (HA), and processing samples of various concentrations in our device, we achieved an effective purification and detection of DNA molecules using the portable PCR thermocycler. Our method significantly decreased PCR quantitation cycles from Cq > 38 to Cq = 30.35 ± 0.5, which confirmed enhancement of PCR amplification. The proposed device takes a promising step forward in sample preparation towards an integrated device that can be used for simultaneous purification and solution exchange of DNA in point-of-need environmental monitoring applications.

Inertial Microfluidics Enabling Clinical Research

Fast and accurate interrogation of complex samples containing diseased cells or pathogens is important to make informed decisions on clinical and public health issues. Inertial microfluidics has been increasingly employed for such investigations to isolate target bioparticles from liquid samples with size and/or deformability-based manipulation. This phenomenon is especially useful for the clinic, owing to its rapid, label-free nature of target enrichment that enables further downstream assays. Inertial microfluidics leverages the principle of inertial focusing, which relies on the balance of inertial and viscous forces on particles to align them into size-dependent laminar streamlines. Several distinct microfluidic channel geometries (e.g., straight, curved, spiral, contraction-expansion array) have been optimized to achieve inertial focusing for a variety of purposes, including particle purification and enrichment, solution exchange, and particle alignment for on-chip assays. In this review, we will discuss how inertial microfluidics technology has contributed to improving accuracy of various assays to provide clinically relevant information. This comprehensive review expands upon studies examining both endogenous and exogenous targets from real-world samples, highlights notable hybrid devices with dual functions, and comments on the evolving outlook of the field.

The Microfluidic Toolbox for Analyzing Exosome Biomarkers of Aging

As the fields of aging and neurological disease expand to liquid biopsies, there is a need to identify informative biomarkers for the diagnosis of neurodegeneration and other age-related disorders such as cancers. A means of high-throughput screening of biomolecules relevant to aging can facilitate this discovery in complex biofluids, such as blood. Exosomes, the smallest of extracellular vesicles, are found in many biofluids and, in recent years, have been found to be excellent candidates as liquid biopsy biomarkers due to their participation in intercellular communication and various pathologies such as cancer metastasis. Recently, exosomes have emerged as novel biomarkers for age-related diseases. Hence, the study of exosomes, their protein and genetic cargo can serve as early biomarkers for age-associated pathologies, especially neurodegenerative diseases. However, a disadvantage of exosome studies includes a lack in standardization of isolating, detecting, and profiling exosomes for downstream analysis. In this review, we will address current techniques for high-throughput isolation and detection of exosomes through various microfluidic and biosensing strategies and how they may be adapted for the detection of biomarkers of age-associated disorders.

Resolving dynamics of inertial migration in straight and curved microchannels by direct cross-sectional imaging

The explosive development of inertial microfluidic systems for label-free sorting and isolation of cells demands improved understanding of the underlying physics that dictate the intriguing phenomenon of size-dependent migration in microchannels. Despite recent advances in the physics underlying inertial migration, migration dynamics in 3D is not fully understood. These investigations are hampered by the lack of easy access to the channel cross section. In this work, we report on a simple method of direct imaging of the channel cross section that is orthogonal to the flow direction using a common inverted microscope, providing vital information on the 3D cross-sectional migration dynamics. We use this approach to revisit particle migration in both straight and curved microchannels. In the rectangular channel, the high-resolution cross-sectional images unambiguously confirm the two-stage migration model proposed earlier. In the curved channel, we found two vertical equilibrium positions and elucidate the size-dependent vertical and horizontal migration dynamics. Based on these results, we propose a critical ratio of blockage ratio (β) to Dean number (De) where no net lateral migration occurs (β/De ∼ 0.01). This dimensionless number (β/De) predicts the direction of lateral migration (inward or outward) in curved and spiral channels, and thus serves as a guideline in design of such channels for particle and cell separation applications. Ultimately, the new approach to direct imaging of the channel cross section enables a wealth of previously unavailable information on the dynamics of inertial migration, which serves to improve our understanding of the underlying physics.

Inertial migration of a deformable capsule in an oscillatory flow in a microchannel

Dynamics of a deformable capsule in an oscillatory flow of a Newtonian fluid in a microchannel has been studied numerically. The effects of oscillation frequency, capsule deformability, and channel flow rate have been explored by simulating the capsule within a microchannel. In addition, the simulation captures the effect of the type of imposed pressure oscillations on the migration pattern of the capsule. An oscillatory channel flow enables the focusing of extremely small biological particles by eliminating the need to design impractically long channels. The presented results show that the equilibrium position of the capsule changes not only by the addition of an oscillatory component to the pressure gradient but it also is influenced by the capsule deformability and channel flow rate. Furthermore, it has been shown that the amplitude of oscillation of capsules decreases as the channel flow rate and the rigidity of the capsule increases.

Inertial microfluidics: Recent advances

Inertial microfluidics has attracted significant attentions in last decade due to its superior advantages of high throughput, label- and external field-free operation, simplicity, and low cost. A wide variety of channel geometry designs were demonstrated for focusing, concentrating, isolating, or separating of various bioparticles such as blood components, circulating tumor cells, bacteria, and microalgae. In this review, we first briefly introduce the physics of inertial migration and Dean flow for allowing the readers with diverse backgrounds to have a better understanding of the fundamental mechanisms of inertial microfluidics. Then, we present a comprehensive review of the recent advances and applications of inertial microfluidic devices according to different channel geometries ranging from straight channels, curved channels to contraction-expansion-array channels. Finally, the challenges and future perspective of inertial microfluidics are discussed. Owing to its superior benefit for particle manipulation, the inertial microfluidics will play a more important role in biology and medicine applications.

Construction of core-shell microcapsules via focused surface acoustic wave microfluidics

The ability to construct core-shell microcapsules has the potential to shift the paradigm in the development of new delivery systems for nutrients, cosmetics, and drugs. In this work, we demonstrate an application of focused surface acoustic wave (FSAW) microfluidics to produce microcapsules with a core-shell structure using one or two focused interdigital transducers (FIDTs) on the microfluidic device. Solid particles or liquid microdroplets without any special modification in multiphase laminar flow are driven by the acoustic radiation force arising from the FSAW, and cross the oil/water interface back and forth, which is not only suitable for generation of core-shell microcapsules with solid cores but also used for coating an aqueous microdroplet core with an oil shell. On this basis, more FIDTs can be added to the device to manufacture more layers of microcapsules if needed. Single-layer, two-layer, or even multi-layer microcapsules can be selectively fabricated. This work provides a promising and robust platform to construct core-shell microcapsules via FSAW microfluidics, which are suitable for a wide range of applications.

Optimal Control of Colloidal Trajectories in Inertial Microfluidics Using the Saffman Effect

In inertial microfluidics colloidal particles in a Poiseuille flow experience the Segré-Silberberg lift force, which drives them to specific positions in the channel cross section. An external force applied along the microchannel induces a cross-streamline migration to a new equilibrium position because of the Saffman effect. We apply optimal control theory to design the time protocol of the axial control force in order to steer a single particle as precisely as possible from a channel inlet to an outlet at a chosen target position. We discuss the influence of particle radius and channel length and show that optimal steering is cheaper than using a constant control force. Using a single optimized control-force protocol, we demonstrate that even a pulse of particles spread along the channel axis can be steered to a target and that particles of different radii can be separarted most efficiently.

A review on microfluidics manipulation of the extracellular chemical microenvironment and its emerging application to cell analysis

Spatiotemporal manipulation of extracellular chemical environments with simultaneous monitoring of cellular responses plays an essential role in exploring fundamental biological processes and expands our understanding of underlying mechanisms. Despite the rapid progress and promising successes in manipulation strategies, many challenges remain due to the small size of cells and the rapid diffusion of chemical molecules. Fortunately, emerging microfluidic technology has become a powerful approach for precisely controlling the extracellular chemical microenvironment, which benefits from its integration capacity, automation, and high-throughput capability, as well as its high resolution down to submicron. Here, we summarize recent advances in microfluidics manipulation of the extracellular chemical microenvironment, including the following aspects: i) Spatial manipulation of chemical microenvironments realized by convection flow-, diffusion-, and droplet-based microfluidics, and surface chemical modification; ii) Temporal manipulation of chemical microenvironments enabled by flow switching/shifting, moving/flowing cells across laminar flows, integrated microvalves/pumps, and droplet manipulation; iii) Spatiotemporal manipulation of chemical microenvironments implemented by a coupling strategy and open-space microfluidics; and iv) High-throughput manipulation of chemical microenvironments. Finally, we briefly present typical applications of the above-mentioned technical advances in cell-based analyses including cell migration, cell signaling, cell differentiation, multicellular analysis, and drug screening. We further discuss the future improvement of microfluidics manipulation of extracellular chemical microenvironments to fulfill the needs of biological and biomedical research and applications.

Inertial focusing in triangular microchannels with various apex angles

We consider inertial focusing of particles in channels with triangular cross sections. The number and the location of inertial focusing positions in isosceles triangular channels can change with varying blockage ratios (a/H) and Reynolds numbers (Re). In triangular channels, asymmetric velocity gradient induced by the sloped sidewalls leads to changes in the direction and the strength of the inertial lift forces. Therefore, varying the configuration (specifically, angle) of the triangular cross section is expected to lead to a better understanding of the nature of the inertial lift forces. We fabricated triangular microchannels with various apex angles using channel molds that were shaped by a planing process, which provides precise apex angles and sharp corners. The focusing position shift was found to be affected by the channel cross section, as expected. It was determined that the direction of the focusing position shift can be reversed depending on whether the vertex is acute or obtuse. More interestingly, corner focusing modes and splitting of the corner focusing were observed with increasing Re, which could explain the origin of the inertial focusing position changes in triangular channels. We conducted fluid dynamic simulations to create force maps under various conditions. These force maps were analyzed to identify the basins of attraction of various attractors and pinpoint focusing locations using linear stability analysis. Calculating the relative sizes of the basins of attractions and exhaustively identifying the focusing positions, which are very difficult to investigate experimentally, provided us a better understanding of trends in the focusing mechanism.

Laborless, Automated Microfluidic Tandem Cell Processor for Visualizing Intracellular Molecules of Mammalian Cells

Visualization and quantification of intracellular molecules of mammalian cells are crucial steps in clinical diagnosis, drug development, and basic biological research. However, conventional methods rely mostly on labor-intensive, centrifugation-based manual operations for exchanging the cell carrier medium and have limited reproducibility and recovery efficiency. Here we present a microfluidic cell processor that can perform four-step exchange of carrier medium, simply by introducing a cell suspension and fluid reagents into the device. The reaction time period for each reaction step, including fixation, membrane permeabilization, and staining, was tunable in the range of 2 to 15 min by adjusting the volume of the reaction tube connecting the neighboring exchanger modules. We double-stained the cell nucleus and cytoskeleton (F-actin) using the presented device with an overall reaction period of ∼30 min, achieving a high recovery ratio and high staining efficiency. Additionally, intracellular cytokine (IL-2) was visualized for T cells to demonstrate the feasibility of the device as a pretreatment system for downstream flow-cytometric analysis. The presented approach would facilitate the development of laborless, automated microfluidic systems that integrate cell processing and analysis operations and would pave a new path to high-throughput biological experiments.

Inertial microfluidic cube for automatic and fast extraction of white blood cells from whole blood

We report here a novel inertial microfluidic (IM) cube integrated with lysis, storage and extraction modules for extracting white blood cells (WBCs) from whole blood automatically, harmlessly and quickly. Lysis, storage, and extraction modules are designed to realize the purposes of complete mixing of whole blood and lysing buffer, thorough lysis of red blood cells (RBCs), and automatic extraction of WBCs from the lysed background RBCs, respectively. After demonstrating its conceptual design, we characterize the performances of the lysis and extraction modules. The results show that a high mixing efficiency of 94.2% can be achieved using our lysis modules for complete mixing of whole blood and lysing buffer. In the extraction module, an extraction efficiency of 88.1% can be achieved for the extraction of WBCs. Finally, we successfully apply our IM cube for the high throughput extraction of WBCs from human whole blood with an extraction efficiency of 83.9% and a cell viability of 96.6%, which are comparable to those using centrifugation and even better in some aspects. Our IM cube is based on passive secondary-flow mixing and inertial sorting, offers the advantages of small footprint, high stability and simple fabrication, and is a promising alternative method for extracting WBCs from human blood.

Influence of non-Newtonian power law rheology on inertial migration of particles in channel flow

In this paper, the inertial migration of particles in the channel flow of power-law fluid is numerically investigated. The effects of the power-law index (n), Reynolds number (Re), blockage ratio (k), and channel aspect ratio (AR) on the inertial migration of particles and equilibrium position are explored. The results show that there exist two stages of particle migration and four stable equilibrium positions for particles in the cross section of a square channel. The particle equilibrium positions in a rectangular channel are much different from those in a square channel. In shear-thinning fluids, the long channel face equilibrium position and two kinds of particle trajectories are found at low Re. With increasing Re, the short channel face equilibrium position turns to be stable, multiequilibrium positions, and three kinds of particle trajectories along the long wall start to form. Only two stable equilibrium positions exist in shear-thickening fluids. The equilibrium positions are getting closer to the channel centerline with increasing n and k and with decreasing Re. The inertial focusing length L ₂ in the second stage of particle migration is much longer than inertial focusing length L ₁ in the first stage. In the square channel, L ₂ is decreased with increasing Re and k and with decreasing n. In the rectangular channel, L ₂ is the shortest in the shear-thinning fluid.

Programmable hydraulic resistor for microfluidic chips using electrogate arrays

Flow rates play an important role in microfluidic devices because they affect the transport of chemicals and determine where and when (bio)chemical reactions occur in these devices. Flow rates can conveniently be determined using external peripherals in active microfluidics. However, setting specific flow rates in passive microfluidics is a significant challenge because they are encoded on a design and fabrication level, leaving little freedom to users for adjusting flow rates for specific applications. Here, we present a programmable hydraulic resistor where an array of "electrogates" routes an incoming liquid through a set of resistors to modulate flow rates in microfluidic chips post-fabrication. This approach combines a battery-powered peripheral device with passive capillary-driven microfluidic chips for advanced flow rate control and measurement. We specifically show a programmable hydraulic resistor composed of 7 parallel resistors and 14 electrogates. A peripheral and smartphone application allow a user to activate selected electrogates and resistors, providing 127 (27-1) flow resistance combinations with values spanning on a 500 fold range. The electrogates feature a capillary pinning site (i.e. trench across the flow path) to stop a solution and an electrode, which can be activated in a few ms using a 3 V bias to resume flow based on electrowetting. The hydraulic resistor and microfluidic chip shown here enable flow rates from ~0.09 nL.s-1 up to ~5.66 nL.s-1 with the resistor occupying a footprint of only 15.8 mm2 on a 1 × 2 cm2 microfluidic chip fabricated in silicon. We illustrate how a programmable hydraulic resistor can be used to set flow rate conditions for laminar co-flow of 2 liquids and the enzymatic conversion of a substrate by stationary enzymes (alkaline phosphatase) downstream of the programmable hydraulic resistor.

Designing Microfluidic Devices to Sort Haematopoietic Stem Cells Based on Their Mechanical Properties

Aim

Few haematopoietic stem cells (HSCs) injected systemically for therapeutic purposes actually reach sites of injury as the vast majority become entrapped within pulmonary capillaries. One promising approach to maintain circulating HSC numbers would be to separate subpopulations with smaller size and/or greater deformability from a heterogeneous population. This study tested whether this could be achieved using label-free microfluidic devices.

Methods

2 straight (A-B) and 3 spiral (C-E) devices were fabricated with different dimensions. Cell sorting was performed at different flow rates after which cell diameter and stiffness were determined using micromanipulation. Cells isolated using the most efficient device were tested intravitally for their ability to home to the mouse injured gut.

Results

Only straight Device B at a high flow rate separated HSCs with different mechanical properties. Side outlets collected mostly deformable cells (nominal rupture stress/σ_R = 6.81 kPa; coefficient of variation/CV = 0.31) at a throughput of 2.3 × 10⁵ cells/min. All spiral devices at high flow rates separated HSCs with different stiffness and size. Inner outlets collected mostly deformable cells in Devices C (σ_R = 25.06 kPa; CV = 0.26), D (σ_R = 22.21 kPa; CV = 0.41), and E (σ_R = 29.26 kPa; CV = 0.27) at throughputs of 2.3 × 10⁵ cells/min, 1.5 × 10⁵ cells/min, and 1.6 × 10⁵ cells/min, respectively. Since Device C separated cells with higher efficiency and throughput, it was utilized to test the homing ability of separated cells in vivo. Significantly more deformable cells were observed trafficking through the injured gut—interestingly, increased retention was not observed.

Conclusion

This study applied microfluidics to separate subpopulations from one stem cell type based on their intrinsic mechanical heterogeneity. Fluid dynamics within curved devices most effectively separated HSCs. Such devices may benefit cellular therapy.

Inertial microfluidics: A method for fast prediction of focusing pattern of particles in the cross section of the channel

Inertial microfluidics is utilized as a powerful passive method for particle and cell manipulation, which uses the hydrodynamic forces of the fluid in the channel to focus particles in specific equilibrium positions in the cross section of the channel. To achieve high performance manipulation, knowledge of focusing pattern of particles in the cross section of channel is essential. In this paper, we propose a method to address this important issue. To this end, firstly inertial microfluidics is analyzed in rectangular cross section channels. The results indicate that fluid flow velocity and channel's cross-sectional profiles have great impacts on the forces exerted on particles. Next, these results are utilized to propose a method to predict equilibrium positions in non-rectangular cross section channels through some simple calculations. This method is based on approximating the velocity profile of a non-rectangular cross section channel by utilizing portions of velocity profiles of different rectangular cross section channels. To analyze the method's performance, results obtained from the proposed method are compared with Direct Numerical Simulation (DNS) and experimental studies of seven non-rectangular channels. It is observed that the proposed approach accurately predicts particles trajectories and their equilibrium positions in the cross section of channels.

Micro-Particle Operations Using Asymmetric Traps

Micro-particle operations in many lab-on-a-chip devices require active-type techniques that are accompanied by complex fabrication and operation. The present study describes an alternative method using a passive microfluidic scheme that allows for simpler operation and, therefore, potentially less expensive devices. We present three practical micro-particle operations using our previously developed passive mechanical trap, the asymmetric trap, in a non-acoustic oscillatory flow field. First, we demonstrate size-based segregation of both binary and ternary micro-particle mixtures using size-dependent trap-particle interactions to induce different transport speeds for each particle type. The degree of segregation, yield, and purity of the binary segregations are 0.97 ± 0.02, 0.96 ± 0.06, and 0.95 ± 0.05, respectively. Next, we perform a solution exchange by displacing particles from one solution into another in a trap array. Lastly, we focus and split groups of micro-particles by exploiting the transport polarity of asymmetric traps. These operations can be implemented in any closed fluidic circuit containing asymmetric traps using non-acoustic oscillatory flow, and they open new opportunities to flexibly control micro-particles in integrated lab-on-a-chip platforms with minimal external equipment.

Fabrication of 3D PDMS Microchannels of Adjustable Cross-Sections via Versatile Gel Templates

Flexible gel fibers with high stretchability were synthesized from physically cross-linked agar and covalently cross-linked polyacrylamide networks. Such gel material can withstand the temperature required for thermal curing of polydimethylsiloxane (PDMS), when the water in the gel was partially replaced with ethylene glycol. This gel template supported thermal replica molding of PDMS to produce high quality microchannels. Microchannels with different cross sections and representative 3D structures, including bifurcating junction, helical and weave networks, were smoothly fabricated, based on the versatile manipulation of gel templates. This gel material was confirmed as a flexible and reliable template in fabricating 3D microfluidic channels for potential devices.

A Dean-flow-coupled interfacial viscoelastic fluid for microparticle separation applied in a cell smear method

An interfacial microfluidic device realizing cell separation and washing simultaneously and efficiently.

Microfluidics in structured multimaterial fibers

Traditional fabrication techniques for microfluidic devices utilize a planar chip format that possesses limited control over the geometry of and materials placement around microchannel cross-sections. This imposes restrictions on the design of flow fields and external forces (electric, magnetic, piezoelectric, etc.) that can be imposed onto fluids and particles. Here we report a method of fabricating microfluidic channels with complex cross-sections. A scaled-up version of a microchannel is dimensionally reduced through a thermal drawing process, enabling the fabrication of meters-long microfluidic fibers with nonrectangular cross-sectional shapes, such as crosses, five-pointed stars, and crescents. In addition, by codrawing compatible materials, conductive domains can be integrated at arbitrary locations along channel walls. We validate this technology by studying unexplored regimes in hydrodynamic flow and by designing a high-throughput cell separation device. By enabling these degrees of freedom in microfluidic device design, fiber microfluidics provides a method to create microchannel designs that are inaccessible using planar techniques.