Size-based hydrodynamic rare tumor cell separation in curved microfluidic channels

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.

Flow study of Dean's instability in high aspect ratio microchannels

Dean's flow and Dean's instability have always been important concepts in the inertial microfluidics. Curved channels are widely used for applications like mixing and sorting but are limited to Dean's flow only. This work first reports the Dean's instability flow in high aspect ratio channels on the deka-microns level for [Formula: see text]. A new channel geometry (the tortuous channel), which creates a rolled-up velocity profile, is presented and studied experimentally and numerically along with other three typical channel geometries at Dean's flow condition and Dean's instability condition. The tortuous channel generates a higher De environment at the same Re compared to the other channels and allows easier Dean's instability creation. We further demonstrate the use of multiple vortexes for mixing. The mixing efficiency is considered among different channel patterns and the tortuous channel outperforms the others. This work offers more understanding of the creation of Dean's instability at high aspect ratio channels and the effect of channel geometry on it. Ultimately, it demonstrates the potential for applications like mixing and cell sorting.

Analytical Validation of a Spiral Microfluidic Chip with Hydrofoil-Shaped Pillars for the Enrichment of Circulating Tumor Cells

The isolation of circulating tumor cells (CTCs) from peripheral blood with high efficiency remains a challenge hindering the utilization of CTC enrichment methods in clinical practice. Here, we propose a microfluidic channel design for the size-based hydrodynamic enrichment of CTCs from blood in an epitope-independent and high-throughput manner. The microfluidic channel comprises a spiral-shaped part followed by a widening part, incorporating successive streamlined pillars, that improves the enrichment efficiency. The design was tested against two benchmark designs, a spiral microfluidic channel and a spiral microfluidic channel followed by a widening channel without the hydrofoils, by processing 5 mL of healthy blood samples spiked with 100 MCF-7 cells. The results proved that the design with hydrofoil-shaped pillars perform significantly better in terms of recovery (recovery rate of 67.9% compared to 23.6% in spiral and 56.7% in spiral with widening section), at a cost of slightly lower white blood cell (WBC) depletion (depletion rate of 94.2% compared to 98.6% in spiral and 94.2% in spiral with widening section), at 1500 µL/min flow rate. For analytical validation, the design was further tested with A549, SKOV-3, and BT-474 cell lines, yielding recovery rates of 62.3 ± 8.4%, 71.0 ± 6.5%, and 82.9 ± 9.9%, respectively. The results are consistent with the size and deformability variation in the respective cell lines, where the increasing size and decreasing deformability affect the recovery rate in a positive manner. The analysis before and after the microfluidic chip process showed that the process does not affect cell viability.

Cell damage evaluation by intelligent imaging flow cytometry

Essential thrombocythemia (ET) is an uncommon situation in which the body produces too many platelets. This can cause blood clots anywhere in the body and results in various symptoms and even strokes or heart attacks. Removing excessive platelets using acoustofluidic methods receives extensive attention due to their high efficiency and high yield. While the damage to the remaining cells, such as erythrocytes and leukocytes is yet evaluated. Existing cell damage evaluation methods usually require cell staining, which are time-consuming and labor-intensive. In this paper, we investigate cell damage by optical time-stretch (OTS) imaging flow cytometry with high throughput and in a label-free manner. Specifically, we first image the erythrocytes and leukocytes sorted by acoustofluidic sorting chip with different acoustic wave powers and flowing speed using OTS imaging flow cytometry at a flowing speed up to 1 m/s. Then, we employ machine learning algorithms to extract biophysical phenotypic features from the cellular images, as well as to cluster and identify images. The results show that both the errors of the biophysical phenotypic features and the proportion of abnormal cells are within 10% in the undamaged cell groups, while the errors are much greater than 10% in the damaged cell groups, indicating that acoustofluidic sorting causes little damage to the cells within the appropriate acoustic power, agreeing well with clinical assays. Our method provides a novel approach for high-throughput and label-free cell damage evaluation in scientific research and clinical settings.

Poly(ethylene oxide) Concentration Gradient-Based Microfluidic Isolation of Circulating Tumor Cells

Circulating tumor cells (CTCs) have emerged as promising circulating biomarkers for non-invasive cancer diagnosis and management. Isolation and detection of CTCs in clinical samples are challenging due to the extreme rarity and high heterogeneity of CTCs. Here, we describe a poly(ethylene oxide) (PEO) concentration gradient-based microfluidic method for rapid, label-free, highly efficient isolation of CTCs directly from whole blood samples. Stable concentration gradients of PEO were formed within the microchannel by co-injecting the side fluid (blood sample spiked with 0.025% PEO) and center fluid (0.075% PEO solution). The competition between the elastic lift force and the inertial lift force enabled size-based separation of large CTCs and small blood cells based on their distinct migration patterns. The microfluidic device could process 1 mL of blood sample in 30 min, with a separation efficiency of >90% and an enrichment ratio of >700 for tumor cells. The isolated CTCs from blood samples were enumerated by immunofluorescence staining, allowing for discrimination of breast cancer patients from healthy donors with an accuracy of 84.2%. The concentration gradient-based microfluidic separation provides a powerful tool for label-free isolation of CTCs for a wide range of clinical applications.

Inertial microfluidics: current status, challenges, and future opportunities

Inertial microfluidics uses the hydrodynamic effects induced at finite Reynolds numbers to achieve passive manipulation of particles, cells, or fluids and offers the advantages of high-throughput processing, simple channel geometry, and label-free and external field-free operation. Since its proposal in 2007, inertial microfluidics has attracted increasing interest and is currently widely employed as an important sample preparation protocol for single-cell detection and analysis. Although great success has been achieved in the inertial microfluidics field, its performance and outcome can be further improved. From this perspective, herein, we reviewed the current status, challenges, and opportunities of inertial microfluidics concerning the underlying physical mechanisms, available simulation tools, channel innovation, multistage, multiplexing, or multifunction integration, rapid prototyping, and commercial instrument development. With an improved understanding of the physical mechanisms and the development of novel channels, integration strategies, and commercial instruments, improved inertial microfluidic platforms may represent a new foundation for advancing biomedical research and disease diagnosis.

Electro-osmotic peristaltic flow and heat transfer in an ionic viscoelastic fluid through a curved micro-channel with viscous dissipation

Emerging systems in microfluidics are embracing bio-inspired designs in which boundaries are flexible and mimic peristaltic propulsion mechanisms encountered in nature. These devices utilize electro-kinetic body forces to manipulate very precisely ionic biofluids for a range of medical applications including. Motivated by exploring in more detail electro-hemorheological micro-pumping, in the current article, a mathematical model is developed for peristalsis propulsion of a viscoelastic biofluid in a curved microchannel with electro-osmotic effect and thermal transport under static axial electrical field and with viscous heating. The third grade Reiner-Rivlin model is deployed for blood rheology. The novelty of the current work is therefore the simultaneous consideration of electrokinetics, viscoelastic behavior with the third grade Reiner-Rivlin model and coupled flow and heat transport with viscous dissipation in peristaltic pumping in a curved micro-channel. A Poisson-Boltzmann formulation is adopted to simulate the charge number density associated with the electrical potential. Asymmetric zeta potential (25 mV) is prescribed and mobilizes an electric double layer (EDL). The governing conservation equations for mass, energy, momentum and electrical potential with associated boundary conditions are simplified using lubrication approximations and rendered dimensionless via appropriate scaling transformations. Analytical solutions are derived in the form of Bessel functions and numerical evaluations are conducted via the ND solver command in MATHEMATICA symbolic software. The simulations show that with stronger viscoelastic effect, boluses are eliminated and there is relaxation in streamlines in the core and peripheral regions of the micro-channel. Increasing Brinkman number (dissipation parameter) elevates temperatures. An increase in electrical double layer thickness initially produces a contraction in the upper bolus and an expansion (lateral) in the lower bolus in the micro-channel. With modification in zeta potential ratio parameter from positive to negative values, in the lower half of the micro-channel, axial flow deceleration is generated.

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.

Recent progress of inertial microfluidic-based cell separation

Cell separation has consistently been a pivotal technology of sample preparation in biomedical research. Compared with conventional bulky cell separation technologies applied in the clinic, cell separation based on microfluidics can accurately manipulate the displacement of liquid or cells at the microscale, which has great potential in point-of-care testing (POCT) applications due to small device size, low cost, low sample consumption, and high operating accuracy. Among various microfluidic cell separation technologies, inertial microfluidics has attracted great attention due to its simple structure and high throughput. In recent years, many researchers have explored the principles and applications of inertial microfluidics and developed different channel structures, including straight channels, curved channels, and multistage channels. However, the recently developed multistage channels have not been discussed and classified in detail compared with more widely discussed straight and curved channels. Therefore, in this review, a comprehensive and detailed review of recent progress in the multistage channel is presented. According to the channel structure, the inertial microfluidic separation technology is divided into (i) straight channel, (ii) curved channel, (iii) composite channel, and (iv) integrated device. The structural development of straight and curved channels is discussed in detail. And based on straight and curved channels, the multistage cell separation structures are reviewed, with a special focus on a variety of latest structures and related innovations of composite and integrated channels. Finally, the future prospects for the existing challenges in the development of inertial microfluidic cell separation technology are presented.

Separation of circulating tumor cells from blood using dielectrophoretic DLD manipulation

Circulating Tumor Cells (CTCs) play a prominent role in early cancer detection. Emerging label-free techniques can be promising to CTC detection due to advantages in preserving cell integrity and minimal sample consumption. Deterministic Lateral Displacement (DLD) is a size-based label-free technique employing laminar flow for continuous sorting of suspended cells. However, separation based solely on size is challenging as the size distributions of CTCs tend to overlap with blood cells. Moreover, the rarity of CTCs in blood requires high throughput processing of samples for clinical utility. In this work, a dielectrophoretic DLD technique is presented to segregate CTCs from blood. This technique utilizes the cell size and dielectric properties as well as particle movement caused by polarization effect to accomplish continuous separation at high flow rates. A numerical model is developed and validated to investigate the effects of various parameters related to the fluid flow, micro-post array, and electric field. It is demonstrated that the dielectrophoretic DLD with specific post arrangement can continuously separate A549 lung CTCs from WBCs by applying a field frequency close to the crossover frequency of CTCs. The analysis further indicates that such a device can perform well despite uncertainties of CTC crossover frequencies. Additionally, efficient separation with minimum clogging can be achieved by setting the electric field perpendicular to fluid flow. The presented platform offers distinct advantages and can be potentially combined with techniques such as antibody-based immune-binding methods for rapid detection of CTCs.

Dean migration of unfocused micron sized particles in low aspect ratio spiral microchannels

We present an analysis of the microfluidic Dean migration of 2.5 µm particles, which do not meet focus criterion, in tall and low aspect ratio microchannels. We demonstrate the use of such low aspect ratio and tall spirals (h > 50 µm) for isolating high concentration (> 10⁶ particles or cells/mL) micron sized particles without an initial off-chip dilution step. We specifically show the need for a sheath fluid for isolation and systematically analyze the particle stream profile (i.e. thickness and distance from the channel wall) as a function of downstream channel length and curvature ratio, with changes in the fluid velocity and the flow rate ratio of particles to sheath fluid (FRR). We also show that the width of the particle stream can control the particle migration and that a threshold stream width and Dean drag is necessary to initiate the particle stream migration from the channel wall. We then propose a design guide based on the selection of optimum curvatures, flow velocities and the FRRs required for achieving a narrow particle stream through a particular outlet. Finally, we use the design guide to demonstrate the isolation of bacteria from bladder epithelial cells.

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.

Inertial-Assisted Immunomagnetic Bioplatform towards Efficient Enrichment of Circulating Tumor Cells

Serving as an effective biomarker in liquid biopsy, circulating tumor cells (CTCs) can provide an accessible source for cancer biology study. For the in-depth evaluation of CTCs in cancer analysis, their efficient enrichment is essential, owing to their low abundance in peripheral blood. In this paper, self-assembled immunomagnetic beads were developed to isolate CTCs from the ordered bundles of cells under the assistance of the spiral inertial effect. Parametric numerical simulations were performed to explore the velocity distribution in the cross section. Based on this chip, rare CTCs could be recovered under the throughput of 500 μL/min, making this device a valuable supplement in cancer analysis, diagnostics, and therapeutics.

Three step flow focusing enables image-based discrimination and sorting of late stage 1 Haematococcus pluvialis cells

Label-free and gentle separation of cell stages with desired target properties from mixed stage populations are a major research task in modern biotechnological cultivation process and optimization of micro algae. The reported microfluidic sorter system (MSS) allows the subsequent investigation of separated subpopulations. The implementation of a viability preserving MSS is shown for separation of late stage 1 Haematococcus pluvialis (HP) cells form a mixed stage population. The MSS combines a three-step flow focusing unit for aligning the cells in single file transportation mode at the center of the microfluidic channel with a pure hydrodynamic sorter structure for cell sorting. Lateral displacement of the cells into one of the two outlet channels is generated by piezo-actuated pump chambers. In-line decision making for sorting is based on a user-definable set of image features and properties. The reported MSS significantly increased the purity of target cells in the sorted population (94%) in comparison to the initial mixed stage population (19%).

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.

Recent advances in microfluidic technologies for circulating tumor cells: enrichment, single-cell analysis, and liquid biopsy for clinical applications

Circulating tumor cells (CTCs) detach from primary or metastatic lesions and circulate in the peripheral blood, which is considered to be the cause of distant metastases. CTC analysis in the form of liquid biopsy, enumeration and molecular analysis provide significant clinical information for cancer diagnosis, prognosis and therapeutic strategies. Despite the great clinical value, CTC analysis has not yet entered routine clinical practice due to lack of efficient technologies to perform CTC isolation and single-cell analysis. Taking the rarity and inherent heterogeneity of CTCs into account, reliable methods for CTC isolation and detection are in urgent demand for obtaining valuable information on cancer metastasis and progression from CTCs. Microfluidic technology, featuring microfabricated structures, can precisely control fluids and cells at the micrometer scale, thus making itself a particularly suitable method for rare CTC manipulation. Besides the enrichment function, microfluidic chips can also realize the analysis function by integrating multiple detection technologies. In this review, we have summarized the recent progress in CTC isolation and detection using microfluidic technologies, with special attention to emerging direct enrichment and enumeration in vivo. Further, few insights into single CTC molecular analysis are also demonstrated. We have provided a review of potential clinical applications of CTCs, ranging from early screening and diagnosis, tumor progression and prognosis, treatment and resistance monitoring, to therapeutic evaluation. Through this review, we conclude that the clinical utility of CTCs will be expanded as the isolation and analysis techniques are constantly improving.

A Novel Microfluidic Method Utilizing a Hydrofoil Structure to Improve Circulating Tumor Cell Enrichment: Design and Analytical Validation

Being one of the major pillars of liquid biopsy, isolation and characterization of circulating tumor cells (CTCs) during cancer management provides critical information on the evolution of cancer and has great potential to increase the success of therapies. In this article, we define a novel strategy to effectively enrich CTCs from whole blood based on size, utilizing a spiral microfluidic channel embedded with a hydrofoil structure at the downstream of the spiral channel. The hydrofoil increases the distance between the streams of CTCs and peripheral blood cells, which are already distributed about two focal axes by the spiral channel, thereby improving the resolution of the separation. Analytical validation of the system has been carried out using Michigan Cancer Foundation-7 (MCF7) breast cancer cell lines spiked into blood samples from healthy donors, and the performance of the system in terms of white blood cell (WBC) depletion, CTC recovery rate and cell viability has been shown in single or two-step process: by passing the sample once or twice through the microfluidic chip. Single step process yielded high recovery (77.1%), viable (84.7%) CTCs. When the collected cell suspension is re-processed by the same chip, recovery decreases to 65.5%, while the WBC depletion increases to 88.3%, improving the purity. Cell viability of >80% was preserved after two-step process. The novel microfluidic chip is a good candidate for CTC isolation applications requiring high recovery rate and viability, including functional downstream analyses for variety of cancer types.

Channel innovations for inertial microfluidics

Inertial microfluidics has gained significant attention since first being proposed in 2007 owing to the advantages of simplicity, high throughput, precise manipulation, and freedom from an external field. Superior performance in particle focusing, filtering, concentrating, and separating has been demonstrated. As a passive technology, inertial microfluidics technology relies on the unconventional use of fluid inertia in an intermediate Reynolds number range to induce inertial migration and secondary flow, which depend directly on the channel structure, leading to particle migration to the lateral equilibrium position or trapping in a specific cavity. With the advances in micromachining technology, many channel structures have been designed and fabricated in the past decade to explore the fundamentals and applications of inertial microfluidics. However, the channel innovations for inertial microfluidics have not been discussed comprehensively. In this review, the inertial particle manipulations and underlying physics in conventional channels, including straight, spiral, sinusoidal, and expansion-contraction channels, are briefly described. Then, recent innovations in channel structure for inertial microfluidics, especially channel pattern modification and unconventional cross-sectional shape, are reviewed. Finally, the prospects for future channel innovations in inertial microfluidic chips are also discussed. The purpose of this review is to provide guidance for the continued study of innovative channel designs to improve further the accuracy and throughput of inertial microfluidics.

Scalable Signature-Based Molecular Diagnostics Through On-chip Biomarker Profiling Coupled with Machine Learning

Molecular diagnostics have traditionally relied on discrete biological substances as diagnostic markers. In recent years however, advances in on-chip biomarker screening technologies and data analytics have enabled signature-based diagnostics. Such diagnostics aim to utilize unique combinations of multiple biomarkers or diagnostic 'fingerprints' rather than discrete analyte measurements. This approach has shown to improve both diagnostic accuracy and diagnostic specificity. In this review, signature-based diagnostics enabled by microfluidic and micro-/nano- technologies will be reviewed with a focus on device design and data analysis pipelines and methodologies. With increasing amounts of data available from microfluidic biomarker screening, isolation, and detection platforms, advanced data handling and analytics approaches can be employed. Thus, current data analysis approaches including machine learning and recent advances with image processing, along with potential future directions will be explored. Lastly, the needs and gaps in current literature will be elucidated to inform future efforts towards development of molecular diagnostics and biomarker screening technologies.

Fully-automated and field-deployable blood leukocyte separation platform using multi-dimensional double spiral (MDDS) inertial microfluidics

A fully-automated and portable leukocyte separation platform was developed based on a new type of inertial microfluidic device, multi-dimensional double spiral (MDDS) device, as an alternative to centrifugation. By combining key innovations in inertial microfluidic device designs and check-valve-based recirculation processes, highly purified and concentrated WBCs (up to >99.99% RBC removal, ∼80% WBC recovery, >85% WBC purity, and ∼12-fold concentrated WBCs compared to the input sample) were achieved in less than 5 minutes, with high reliability and repeatability (coefficient of variation, CV < 5%). Using this, one can harvest up to 0.4 million of intact WBCs from 50 μL of human peripheral blood (50 μL), without any cell damage or phenotypic changes in a fully-automated operation. Alternatively, hand-powered operation is demonstrated with comparable separation efficiency and speed, which eliminates the need for electricity altogether for truly field-friendly sample preparation. The proposed platform is therefore highly deployable for various point-of-care applications, including bedside assessment of the host immune response and blood sample processing in resource-limited environments.

High-speed particle detection and tracking in microfluidic devices using event-based sensing

Visualising fluids and particles within channels is a key element of microfluidic work. Current imaging methods for particle image velocimetry often require expensive high-speed cameras with powerful illuminating sources, thus potentially limiting accessibility. This study explores for the first time the potential of an event-based camera for particle and fluid behaviour characterisation in a microfluidic system. Event-based cameras have the unique capacity to detect light intensity changes asynchronously and to record spatial and temporal information with low latency, low power and high dynamic range. Event-based cameras could consequently be relevant for detecting light intensity changes due to moving particles, chemical reactions or intake of fluorescent dyes by cells to mention a few. As a proof-of-principle, event-based sensing was tested in this work to detect 1 μm and 10 μm diameter particles flowing in a microfluidic channel for average fluid velocities of up to 1.54 m s^-1. Importantly, experiments were performed by directly connecting the camera to a standard fluorescence microscope, only relying on the microscope arc lamp for illumination. We present a data processing strategy that allows particle detection and tracking in both bright-field and fluorescence imaging. Detection was achieved up to a fluid velocity of 1.54 m s^-1 and tracking up to 0.4 m s^-1 suggesting that event-based cameras could be a new paradigm shift in microscopic imaging.

Enhancement of inflection point focusing and rare-cell separations from untreated whole blood

Inertial microfluidic systems have been widely used for particle or cell separation applications, especially for rare-cell enrichment and separation from blood due to the high throughput and simplicity of the systems. However, most of the separation techniques using inertial microfluidic systems require dilution of blood samples or RBC lysis to achieve a high separation efficiency, which can adversely affect the throughput and/or analysis of the collected sample. We developed a cell separation technique compatible with untreated whole blood by inflection point focusing, that is, an inertial focusing phenomenon toward inflection points of the velocity profile. We generated a strong shear-gradient lift force by varying the velocity profile in a channel cross-section with the combined effect of the channel geometry and the co-flows of two liquids with different viscosities. The natural viscosity of blood leads to a lagging flow region in the middle of the channel where larger particles and cells can be extracted to the side flow region, which enables a highly efficient separation scheme with an unprecedented high throughput.

The mechanical responses of advecting cells in confined flow

Fluid dynamics have long influenced cells in suspension. Red blood cells and white blood cells are advected through biological microchannels in both the cardiovascular and lymphatic systems and, as a result, are subject to a wide variety of complex fluidic forces as they pass through. In vivo, microfluidic forces influence different biological processes such as the spreading of infection, cancer metastasis, and cell viability, highlighting the importance of fluid dynamics in the blood and lymphatic vessels. This suggests that in vitro devices carrying cell suspensions may influence the viability and functionality of cells. Lab-on-a-chip, flow cytometry, and cell therapies involve cell suspensions flowing through microchannels of approximately 100-800  μ m. This review begins by examining the current fundamental theories and techniques behind the fluidic forces and inertial focusing acting on cells in suspension, before exploring studies that have investigated how these fluidic forces affect the reactions of suspended cells. In light of these studies' findings, both in vivo and in vitro fluidic cell microenvironments shall also be discussed before concluding with recommendations for the field.

Detection of Circulating Tumor Cells by Fluorescence Microspheres-Mediated Amplification

Here we describe a fluorescent microspheres-based separation and analysis that enables the isolation of circulating tumor cells (CTCs) from whole blood of patients with metastatic cancer and the identification of isolated CTCs in situ without immunostaining. This approach uses antibody-functionalized fluorescent polystyrene (PS) microspheres that can selectively bind to CTCs. The binding of CTCs and fluorescent PS microspheres leads to the formation of complexes of CTCs and fluorescent PS microspheres, thereby the CTCs are size-amplified and labeled simultaneously. A pyramidal microcavity array (PMCA) is fabricated using microfabrication technology to create a precise microfilter structure with a high aspect ratio. The PMCA filter device can effectively isolate microspheres-labeled CTCs, while allow hematologic cells to deform and pass through. Using this approach, CTCs are isolated and identified in 15 of 18 patients with metastatic colorectal cancer. This approach will open new possibilities for CTCs isolation and identification and can serve a versatile platform to facilitate CTCs analysis in diverse biomedical applications.

A Review of Secondary Flow in Inertial Microfluidics

Inertial microfluidic technology, which can manipulate the target particle entirely relying on the microchannel characteristic geometry and intrinsic hydrodynamic effect, has attracted great attention due to its fascinating advantages of high throughput, simplicity, high resolution and low cost. As a passive microfluidic technology, inertial microfluidics can precisely focus, separate, mix or trap target particles in a continuous and high-flow-speed manner without any extra external force field. Therefore, it is promising and has great potential for a wide range of industrial, biomedical and clinical applications. In the regime of inertial microfluidics, particle migration due to inertial effects forms multiple equilibrium positions in straight channels. However, this is not promising for particle detection and separation. Secondary flow, which is a relatively minor flow perpendicular to the primary flow, may reduce the number of equilibrium positions as well as modify the location of particles focusing within channel cross sections by applying an additional hydrodynamic drag. For secondary flow, the pattern and magnitude can be controlled by the well-designed channel structure, such as curvature or disturbance obstacle. The magnitude and form of generated secondary flow are greatly dependent on the disturbing microstructure. Therefore, many inventive and delicate applications of secondary flow in inertial microfluidics have been reported. In this review, we comprehensively summarize the usage of the secondary flow in inertial microfluidics.

The in vitro inertial positions and viability of cells in suspension under different in vivo flow conditions

The influence of Poiseuille flow on cell viability has applications in the areas of cancer metastasis, lab-on-a-chip devices and flow cytometry. Indeed, retaining cell viability is important in the emerging field of cell therapy as cells need to be returned to patients’ bodies. Despite this, it is unclear how this fundamental fluid regime affects cell viability. This study investigated the influence that varying flow rate, and the corresponding wall shear stress (τ_w) has on the viability and inertial positions of circulating cells in laminar pipe flow. The viability of two representative cell lines under different shear stresses in two different systems were investigated while particle streak imaging was used to determine their inertial positions. It was found that peristaltic pumps have a negative effect on cell viability in comparison to syringe pumps. Increasing shear stress in a cone and plate above 3 Pa caused an increase in cell death, however, τ_w as high as 10 Pa in circulation has little to no effect on cell viability. Inertial lift forces that move cells towards the centre of the channel protect them from experiencing detrimental levels of τ_w, indicating that τ_w in Poiseuille flow is not a good predictor of cell viability during advection.

Sheathless Inertial Focusing Chip Combining a Spiral Channel with Periodic Expansion Structures for Efficient and Stable Particle Sorting

Efficient and reliable manipulation of biological particles is crucial in medical diagnosis and chemical synthesis. Inertial microfluidic devices utilizing passive hydrodynamic forces in the secondary flow have drawn considerable attention for their high throughputs, low costs, and harmless particle manipulation. However, as the dominant mechanism, the inertial lift force is difficult to quantitatively analyze because of the uncertainties of its magnitude and direction. The equilibrium position of particles varies along the migration process, thus inducing the instabilities of particle separation. Herein, we present a designable inertial microfluidic chip combining a spiral channel with periodic expansion structures for the sheathless separation of particles with different sizes. The stable vortex-induced lift force arising from the periodic expansion and the Dean drag force significantly enhanced the focusing process and determined the final equilibrium position. The experimental results showed that over 99% of target particles could be isolated with the high target sample purity of 86.12%. In the biological experiment, 93.5% of the MCF-7, 89.5% of the Hela, and 88.6% of the A549 cells were steadily recovered with excellent viabilities to verify the potential of the device in dealing with biological particles over a broad range of throughputs. The device presented in this study can further serve as a lab-on-chip platform for liquid biopsy and diagnostic analysis.

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.

TiO2 nanopillar arrays coated with gelatin film for efficient capture and undamaged release of circulating tumor cells

Circulating tumor cells (CTCs) are important for the detection and treatment of cancer. Nevertheless, a low density of circulating tumor cells makes the capture and release of CTCs an obstacle. In this work, TiO₂ nanopillar arrays coated with gelatin film were synthesized for efficient capture and undamaged release of circulating tumor cells. The scanning electron microscope and atomic force microscope images demonstrate that the substrate has a certain roughness. The interaction between the cell membrane and the nanostructure substrate contributes to the efficient capture of CTC (capture efficiency up to 94.98%). The gelatin layer has excellent biocompatibility and can be rapidly digested by matrix metalloproteinase (MMP9), which realizes the non-destructive release of CTCs (0.1 mg ml^-1, 5 min, nearly 100% release efficiency, activity 100%). Therefore, by our strategy, the CTCs can be efficiently captured and released undamaged, which is important for subsequent analysis.

Label-free isolation of rare tumor cells from untreated whole blood by interfacial viscoelastic microfluidics

Label-free, high-throughput, and efficient separation and enrichment of rare tumor cells, such as circulating tumor cells (CTCs), from untreated whole blood is a challenging task, owing to extremely rare events of CTCs and an enormous amount of blood cells. Current strategies for CTC separation always require pre-processing steps including lysis of blood or labeling of CTCs, leading to loss or damage of CTCs. Here, we report an interfacial viscoelastic microfluidic system for size-selective separation of tumor cells directly from whole blood, without the need of cell labeling and other treatments. The sharp flow interfaces between the sample flow and viscoelastic flow (0.05% PEO solutions) in the straight microchannel allow for the penetration of large tumor cells while blocking small blood cells, through exploiting the competition between the inertial lift forces and interfacial elastic lift forces. The microfluidic paradigm does not involve external force fields or complicated fabrication procedures, while achieving 95.1% separation efficiency and 77.5% recovery rate for isolating as few as 50 tumor cells in 1 mL whole blood. The viability of tumor cells after separation is ∼100%, and normal proliferation of separated tumor cells is observed. The interfacial viscoelastic microfluidics holds great promise to facilitate the fundamental and clinical studies of CTCs.

Assessment of Lagrangian Modeling of Particle Motion in a Spiral Microchannel for Inertial Microfluidics

Inertial microfluidics is a promising tool for a label-free particle manipulation for microfluidics technology. It can be utilized for particle separation based on size and shape, as well as focusing of particles. Prediction of particles’ trajectories is essential for the design of inertial microfluidic devices. At this point, numerical modeling is an important tool to understand the underlying physics and assess the performance of devices. A Monte Carlo-type computational model based on a Lagrangian discrete phase model is developed to simulate the particle trajectories in a spiral microchannel for inertial microfluidics. The continuous phase (flow field) is solved without the presence of a discrete phase (particles) using COMSOL Multi-physics. Once the flow field is obtained, the trajectory of particles is determined in the post-processing step via the COMSOL-MATLAB interface. To resemble the operation condition of the device, the random inlet position of the particles, many particles are simulated with random initial locations from the inlet of the microchannel. The applicability of different models for the inertial forces is discussed. The computational model is verified with experimental results from the literature. Different cases in a spiral channel with aspect ratios of 2.0 and 9.0 are simulated. The simulation results for the spiral channel with an aspect ratio of 9.0 are compared against the experimental data. The results reveal that despite certain limitations of our model, the current computational model satisfactorily predicts the location and the width of the focusing streams.

Progress of Inertial Microfluidics in Principle and Application

Inertial microfluidics has become a popular topic in microfluidics research for its good performance in particle manipulation and its advantages of simple structure, high throughput, and freedom from an external field. Compared with traditional microfluidic devices, the flow field in inertial microfluidics is between Stokes state and turbulence, whereas the flow is still regarded as laminar. However, many mechanical effects induced by the inertial effect are difficult to observe in traditional microfluidics, making particle motion analysis in inertial microfluidics more complicated. In recent years, the inertial migration effect in straight and curved channels has been explored theoretically and experimentally to realize on-chip manipulation with extensive applications from the ordinary manipulation of particles to biochemical analysis. In this review, the latest theoretical achievements and force analyses of inertial microfluidics and its development process are introduced, and its applications in circulating tumor cells, exosomes, DNA, and other biological particles are summarized. Finally, the future development of inertial microfluidics is discussed. Owing to its special advantages in particle manipulation, inertial microfluidics will play a more important role in integrated biochips and biomolecule analysis.

A Microfluidic Chip with Double-Slit Arrays for Enhanced Capture of Single Cells

The application of microfluidic technology to manipulate cells or biological particles is becoming one of the rapidly growing areas, and various microarray trapping devices have recently been designed for high throughput single-cell analysis and manipulation. In this paper, we design a double-slit microfluidic chip for hydrodynamic cell trapping at the single-cell level, which maintains a high capture ability. The geometric effects on flow behaviour are investigated in detail for optimizing chip architecture, including the flow velocity, the fluid pressure, and the equivalent stress of cells. Based on the geometrical parameters optimized, the double-slit chip enhances the capture of HeLa cells and the drug experiment verifies the feasibility of the drug delivery.

Parametric study on mixing process in an in-plane spiral micromixer utilizing chaotic advection

Recent advances in the field of microfabrication have made the application of high-throughput microfluidics feasible. Mixing which is an essential part of any miniaturized standalone system remains the key challenge. This paper proposes a geometrically simple micromixer for efficient mixing for high-throughput microfluidic devices. The proposed micromixer utilizes a curved microchannel (spiral microchannel) to induce chaotic advection and enhance the mixing process. It is shown that the spiral microchannel is more efficient in comparison to a straight microchannel, mixing wise. The pressure drop in the spiral microchannel is only slightly higher than that in the straight microchannel. It is found that the mixing process in the spiral microchannel enhances with increasing the inlet velocity, unlike what happens in the straight microchannel. It is also realized that the initial radius of the spiral microchannel plays a prominent role in enhancing the mixing process. Studying different cross sections, it is gathered that the square cross section yields a higher mixing quality.

High-Throughput Isolation of Circulating Tumor Cells Using Cascaded Inertial Focusing Microfluidic Channel

Circulating tumor cells (CTCs) are rare cells that detach from a primary or metastasis tumor and flow into the bloodstream. Intact and viable tumor cells are needed for genetic characterization of CTCs, new drug development, and other research. Although separation of CTCs using spiral channel with two outlets has been reported, few literature demonstrated simultaneous isolation of different types of CTCs from human blood using cascaded inertial focusing microfluidic channel. Herein, we introduce a cascaded microfluidic device consisting of two spiral channels and one zigzag channel designed with different fluid fields, including lift force, Dean drag force, and centrifugal force. Both red blood cells (RBCs)-lysed human blood spiked with CTCs and 1:50 diluted human whole blood spiked with CTCs were tested on the presented chip. This chip successfully separated RBCs, white blood cells (WBCs), and two different types of tumor cells (human lung cancer cells (A549) and human breast cancer cells (MCF-7)) simultaneously based on their physical properties. A total of 80.75% of A549 and 73.75% of MCF-7 were faithfully separated from human whole blood. Furthermore, CTCs gathered from outlets could propagate and remained intact. The cell viability of A549 and MCF-7 were 95% and 98%, respectively. The entire separating process for CTCs from blood cells could be finished within 20 min. The cascaded microfluidic device introduced in this study serves as a novel platform for simultaneous isolation of multiple types of CTCs from patient blood.

Detection of Circulating Tumor Cells Using Microfluidics

Metastasis is the main cause of death in cancer patients worldwide. During metastasis, cancer cells detach from the primary tumor and invade distant tissue. The cells that undergo this process are called circulating tumor cells (CTCs). Studies show that the number of CTCs in the peripheral blood can predict progression-free survival and overall survival and can be informative concerning the efficacy of treatment. Research is now concentrated on developing devices that can detect CTCs in the blood of cancer patients with improved sensitivity and specificity that can lead to improved clinical evaluation. This review focuses on devices that detect and capture CTCs using different cell properties (surface markers, size, deformability, electrical properties, etc.). We also discuss the process of tumor cell dissemination, the biology of CTCs, epithelial-mesenchymal transition (EMT), and several challenges and clinical applications of CTC detection.

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.

Field-Free Isolation of Exosomes from Extracellular Vesicles by Microfluidic Viscoelastic Flows

Exosomes, molecular cargos secreted by almost all mammalian cells, are considered as promising biomarkers to identify many diseases including cancers. However, the small size of exosomes (30-200 nm) poses serious challenges in their isolation from complex media containing a variety of extracellular vesicles (EVs) of different sizes, especially in small sample volumes. Here we present a viscoelasticity-based microfluidic system to directly separate exosomes from cell culture media or serum in a continuous, size-dependent, and label-free manner. Using a small amount of biocompatible polymer as the additive in the media to control the viscoelastic forces exerted on EVs, we are able to achieve a high separation purity (>90%) and recovery (>80%) of exosomes. The proposed technique may serve as a versatile platform to facilitate exosome analyses in diverse biochemical applications.

Towards the prediction of flow-induced shear stress distributions experienced by breast cancer cells in the lymphatics

Tumour metastasis in the lymphatics is a crucial step in the progression of breast cancer. The dynamics by which breast cancer cells (BCCs) travel in the lymphatics remains poorly understood. The goal of this work is to develop a model capable of predicting the shear stresses metastasising BCCs experience using numerical and experimental techniques. This paper models the fluidic transport of large particles ([Formula: see text] where [Formula: see text] is the particle diameter and W is the channel width) subjected to lymphatic flow conditions ([Formula: see text]), in a [Formula: see text] microchannel. The feasibility of using the dynamic fluid body interaction (DFBI) method to predict particle motion was assessed, and particle tracking experiments were performed. The experiments found that particle translational velocity decreased from the undisturbed fluid velocity with increasing particle size (5-14% velocity lag for [Formula: see text]). DFBI simulations were found to better predict particle behaviour than theoretical predictions; however, mesh restrictions in the near-wall region ([Formula: see text]) result in computationally expensive models. The simulations were in good agreement with the experiments ([Formula: see text] difference) across the channel ([Formula: see text]), with differences up to 25% in the near-wall region. Particles experience a range of shear stresses (0.002-0.12 Pa) and spatial shear gradients ([Formula: see text]) depending on their size and radial position. The predicted shear gradients are far in excess of values associated with BCC apoptosis ([Formula: see text]). Increasing our understanding of the shear stress magnitudes and gradients experienced by BCCs could be leveraged to elucidate whether a particular BCC size or location exists that encourages metastasis within the lymphatics.

Analysis of Hydrodynamic Mechanism on Particles Focusing in Micro-Channel Flows

In this paper, the hydrodynamic mechanism of moving particles in laminar micro-channel flows was numerically investigated. A hydrodynamic criterion was proposed to determine whether particles in channel flows can form a focusing pattern or not. A simple formula was derived to demonstrate how the focusing position varies with Reynolds number and particle size. Based on this proposed criterion, a possible hydrodynamic mechanism was discussed as to why the particles would not be focused if their sizes were too small or the channel Reynolds number was too low. The Re-λ curve (Re, λ respectively represents the channel-based Reynolds number and the particle’s diameter scaled by the channel) was obtained using the data fitting with a least square method so as to obtain a parameter range of the focusing pattern. In addition, the importance of the particle rotation to the numerical modeling for the focusing of particles was discussed in view of the hydrodynamics. This research is expected to deepen the understanding of the particle transport phenomena in bounded flow, either in micro or macro fluidic scope.

The advection of microparticles, MCF-7 and MDA-MB-231 breast cancer cells in response to very low Reynolds numbers

The lymphatic system is an extensive vascular network that serves as the primary route for the metastatic spread of breast cancer cells (BCCs). The dynamics by which BCCs travel in the lymphatics to distant sites, and eventually establish metastatic tumors, remain poorly understood. Particle tracking techniques were employed to analyze the behavior of MCF-7 and MDA-MB-231 BCCs which were exposed to lymphatic flow conditions in a 100 μm square microchannel. The behavior of the BCCs was compared to rigid particles of various diameters (η = d_p/H= 0.05-0.32) that have been used to simulate cell flow in lymph. Parabolic velocity profiles were recorded for all particle sizes. All particles were found to lag the fluid velocity, the larger the particle the slower its velocity relative to the local flow (5%-15% velocity lag recorded). A distinct difference between the behavior of BCCs and particles was recorded. The BCCs travelled approximately 40% slower than the undisturbed flow, indicating that morphology and size affects their response to lymphatic flow conditions (Re < 1). BCCs adhered together, forming aggregates whose behavior was irregular. At lymphatic flow rates, MCF-7s were distributed uniformly across the channel in comparison to the MDA-MB-231 cells which travelled in the central region (88% of cells found within 0.35 ≤ W ≤ 0.64), indicating that metastatic MDA-MB-231 cells are subjected to a lower range of shear stresses in vivo. This suggests that both size and deformability need to be considered when modelling BCC behavior in the lymphatics. This finding will inform the development of in vitro lymphatic flow and metastasis models.