Inertial Focusing Research Articles

Size-based separation of fine mineral particles using inertial microfluidics: A case of jarosite and anglesite

Many metal phases in industrial hazardous waste coexist as fine mineral particles (FMPs, with particle sizes of 0.01 μm–10 μm) in a physical mixture, making conventional methods unable to achieve separation. Advanced microfluidics has demonstrated its potential in the separation of FMPs, as exemplified by jarosite and anglesite. Our study innovatively proposed an inertial microfluidic method for the separation of FMPs by force difference in curved microchannels, thus produced two distinct migration behaviors and subsequent separation. In addition, the computational fluid dynamics (CFD) simulation model was constructed to predict and analyze the migration behavior and separation trend of FMPs through flow field simulation and particle trajectory tracking. The results indicated that 2.1 μm and 3.6 μm small particles migrated to the outer outlet of the microchannel due to the dominant Dean vortex force, while 7.2 μm and 9.4 μm large particles migrated to the inner outlet relying on inertial focusing. At a flow rate of 1.2 mL/min, the separation efficiency of 3.6 μm jarosite and 9.4 μm anglesite has achieved a leap from 0 % to 60.7 %. This discovery provided feasibility and insights into the application of microfluidic technology for particle separation, and offered enormous potential for practical iron residue treatment.

Bifurcate migration of neutrally buoyant particles in unilateral slippery channel flows

As an important technique for manipulating particles in fluid–solid channel flows, inertial focusing encourages the design of the channel geometry to enhance particle radial aggregation. Traditional methods typically use exquisite sheathes or elbows to create constricted flows, which ultimately increase flow resistance and lower fluid–solid separation efficiency. This paper presents a slippery wall modification technique that, by regulating the channel flows, is expected to induce nontrivial particle lateral migrations. More specifically, interface-resolved simulations are performed using the lattice Boltzmann method. A slip boundary condition is applied to the redesigned hydrophobic bottom wall. It is observed that the typical bifurcate migration, i.e., particles moving divergently toward the upper and lower equilibrium positions around a crucial location (CL), does not occur along the channel centerline. The CL is always below the centerline, and it decreases consistently with an increase in Kn or Re. By increasing Re, particles are prone to approach the channel centerline. With larger Kn, particles in the higher equilibrium position are affected in the same way, but their lower counterparts are drawn to the bottom wall.

Spiral microchannels with concave cross-section for enhanced cancer cell inertial separation.

Inertial microfluidic technologies have proven effective for particle focusing and separation in many microchannels, typically the channels with the rectangular and trapezoidal shapes. To advance particle focusing in complex channels, we propose a spiral channel combining rectangular and concave cross-sections for high-resolution particle and cell focusing and separation. Numerical simulations were conducted to illustrate the effects of channel geometry on secondary flow distribution and particle focusing positions. The simulation shows the concave cross-section generates two asymmetrical Dean vortices skewing towards the inner and outer channel walls, resulting to stronger flow velocity magnitudes near the walls than the channel center. Consequently, larger particles focus near the inner wall, while smaller particles are trapped closer to the outer wall under the influence of the stronger velocity magnitude near the walls. A microfluidic chip with the proposed channelgeometry, along with a traditional rectangular channel, was fabricated by 3D printing and PDMS casting. Fluorescent microbeads were used to investigate inertial focusing and separation behaviors in the microfluidicchips. Experimental results show that the concave channel facilitates particle focusing or trapping much closer to the walls than the traditional rectangular channel, achieving better separation resolution. Finally, the proposed channel was applied to separate lung cancer A549 cells from human blood, achieving a cancer cell recovery rate of ~ 84.78% (enrichment ratio over 820-fold) and a blood cell rejection rate of ~ 99.88%. This innovative channel design in inertial microfluidics offers new insights for enhanced particle focusing and holds significant promise for cell manipulation with improved separation resolution.

Lab-on-Chip Systems for Cell Sorting: Main Features and Advantages of Inertial Focusing in Spiral Microchannels.

Inertial focusing-based Lab-on-Chip systems represent a promising technology for cell sorting in various applications, thanks to their alignment with the ASSURED criteria recommended by the World Health Organization: Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment-free, and Delivered. Inertial focusing techniques using spiral microchannels offer a rapid, portable, and easy-to-prototype solution for cell sorting. Various microfluidic devices have been investigated in the literature to understand how hydrodynamic forces influence particle focusing in spiral microchannels. This is crucial for the effective prototyping of devices that allow for high-throughput and efficient filtration of particles of different sizes. However, a clear, comprehensive, and organized overview of current research in this area is lacking. This review aims to fill this gap by offering a thorough summary of the existing literature, thereby guiding future experimentation and facilitating the selection of spiral geometries and materials for cell sorting in microchannels. To this end, we begin with a detailed theoretical introduction to the physical mechanisms underlying particle separation in spiral microfluidic channels. We also dedicate a section to the materials and prototyping techniques most commonly used for spiral microchannels, highlighting and discussing their respective advantages and disadvantages. Subsequently, we provide a critical examination of the key details of inertial focusing across various cross-sections (rectangular, trapezoidal, triangular, hybrid) in spiral devices as reported in the literature.

Simultaneous high-throughput particle-bacteria separation and solution exchange via in-plane and out-of-plane parallelization of microfluidic centrifuges

Inertial microfluidic devices have gained attention for point-of-need (PoN) sample preparation. Yet, devices capable of simultaneous particle-bacteria solution exchange and separation are low in throughput, hindering their applicability to PoN settings. This paper introduces a microfluidic centrifuge for high-throughput solution exchange and separation of microparticles, addressing the need for processing large sample volumes at elevated flow rates. The device integrates Dean flow recirculation and inertial focusing of microparticles within 24 curved microchannels assembled in a three-layer configuration via in-plane and out-of-plane parallelization. We studied solution exchange and particle migration using singleplex and duplex samples across devices with varying curve numbers (2-curve, 8-curve, and 24-curve). Processing 5 and 10 μm microparticles at flow rates up to 16.8 ml/min achieved a solution exchange efficiency of 96.69%. In singleplex solutions, 10 and 5 μm particles selectively migrated to inner and outer outlets, demonstrating separation efficiencies of 99.7% and 90.3%, respectively. With duplex samples, sample purity was measured to be 93.4% and 98.6% for 10 and 5 μm particles collected from the inner and the outer outlets, respectively. Application of our device in biological assays was shown by performing duplex experiments where 10 μm particles were isolated from Salmonella bacterial suspension with purity of 97.8% while increasing the state-of-the-art particle solution exchange and separation throughput by 16 folds. This parallelization enabled desirable combinations of high throughput, low-cost, and scalability, without compromising efficiency and purity, paving the way for sample preparation at the PoN in the future.

Efficient Microfluidic Particle Separation Using Inertial Pre-Focusing and Pre-Separation Structures Without Sheath Flow

Abstract This study introduces an innovative microfluidic particle separation technique that integrates inertial focusing with deterministic lateral displacement (DLD) on a single chip, significantly enhancing the efficiency of particle separation. This new method completes flow rate matching through a designed pre-focusing and pre-separation inertial structure, avoiding the use of sheath flow. The process involves a sequence of channels: a rectangular channel, a contraction-expansion array (CEA), a lateral separation channel, and another rectangular channel. This three-stage inertial method shortens the focusing channel length and reduces the pressure on subsequent separation stages, streamlining the separation process. It has been demonstrated to separate 20 μm particles from a mixture containing both 10 μm and 20 μm particles with remarkable precision. The technique achieves a 100% separation efficiency, ensuring all target particles are correctly isolated, and a 96.1% separation purity, indicating that the isolated particles are almost entirely free from contaminants. By eliminating the need for sheath flow, this method simplifies the apparatus and reduces operational complexity, offering significant advantages over traditional particle separation techniques. The high efficiency and purity levels achieved by this method highlight its potential for a wide range of applications in fields requiring precise particle separation, such as medical diagnostics, environmental monitoring, and industrial processing.

(Digital Presentation) Sorting of Circulating Tumor Cells Based on Phenotypic Separation in Using Magnetic Nanoparticle Labels

Representing a practical criterion for sorting captured circulating tumor cells (CTCs) based on the cancer progression stage is a new approach that recently received traction. High-throughput, rapid, and cost-effective microfluidic systems that allow for high-efficiency separation and continuous inertial focusing are needed. The goal of the present study is to design of such systems via computational modeling and experimental tests. We propose a new microfluidic device that uses magnetic nanoparticles to label surface biomarkers on CTCs to isolate, focus and sort CTCs based on their cancer progression stage. By eliminating sheath flow interference that may result in harming cell survival, the proposed microchip can successfully determine the progression of cancer by arranging cells in one step. Additionally, the proposed device allows for rapid, efficient separation of the blood cells (RBCs and WBCs) and CTCs in the shortest length of the channel.

Fabrication of heterocellular spheroids with controllable core-shell structure using inertial focusing effect for scaffold-free 3D cell culture models

Three-dimensional (3D) cell culture models capable of emulating the biological functions of natural tissues are pivotal in tissue engineering and regenerative medicine. Despite progress, the fabrication ofin vitroheterocellular models that mimic the intricate structures of natural tissues remains a significant challenge. In this study, we introduce a novel, scaffold-free approach leveraging the inertial focusing effect in rotating hanging droplets for the reliable production of heterocellular spheroids with controllable core-shell structures. Our method offers precise control over the core-shell spheroid's size and geometry by adjusting the cell suspension density and droplet morphology. We successfully applied this technique to create hair follicle organoids, integrating dermal papilla cells within the core and epidermal cells in the shell, thereby achieving markedly enhanced hair inducibility compared to mixed-structure models. Furthermore, we have developed melanoma tumor spheroids that accurately mimic the dynamic interactions between tumor and stromal cells, showing increased invasion capabilities and altered expressions of cellular adhesion molecules and proteolytic enzymes. These findings underscore the critical role of cellular spatial organization in replicating tissue functionalityin vitro. Our method represents a significant advancement towards generating heterocellular spheroids with well-defined architectures, offering broad implications for biological research and applications in tissue engineering.

Inertial Focusing Dynamics of Spherical Particles in Curved Microfluidic Ducts with a Trapezoidal Cross Section

Inertial Focusing Dynamics of Spherical Particles in Curved Microfluidic Ducts with a Trapezoidal Cross Section

Inertial particle focusing in fluid flow through spiral ducts: dynamics, tipping phenomena and particle separation

Small finite-size particles suspended in fluid flow through an enclosed curved duct can focus to points or periodic orbits in the two-dimensional duct cross-section. This particle focusing is due to a balance between inertial lift forces arising from axial flow and drag forces arising from cross-sectional vortices. The inertial particle focusing phenomenon has been exploited in various industrial and medical applications to passively separate particles by size using purely hydrodynamic effects. A fixed size particle in a circular duct with a uniform rectangular cross-section can have a variety of particle attractors, such as stable nodes/spirals or limit cycles, depending on the radius of curvature of the duct. Bifurcations occur at different radii of curvature, such as pitchfork, saddle-node and saddle-node infinite period (SNIPER), which result in variations in the location, number and nature of these particle attractors. By using a quasi-steady approximation, we extend the theoretical model of Harding et al. (J. Fluid Mech., vol. 875, 2019, pp. 1–43) developed for the particle dynamics in circular ducts to spiral duct geometries with slowly varying curvature, and numerically explore the particle dynamics within. Bifurcations of particle attractors with respect to radius of curvature can be traversed within spiral ducts and give rise to a rich nonlinear particle dynamics and various types of tipping phenomena, such as bifurcation-induced tipping (B-tipping), rate-induced tipping (R-tipping) and a combination of both, which we explore in detail. We discuss implications of these unsteady dynamical behaviours for particle separation and propose novel mechanisms to separate particles by size in a non-equilibrium manner.

Investigation on sheathless inertial focusing within low-aspect ratio spiral microchannel for cascaded microfluidic tumor cell separation

The high-precision and high-purity isolation of circulating tumor cells (CTCs) from whole blood is vital to early cancer detection. Cascaded microfluidic separation is highly efficient because it connects multiple-stage separations in series. Here, we numerically investigated sheathless tumor cell separation with size-dependent cascaded inertial and deterministic lateral displacement (DLD) microfluidic device. The inertial microfluidic is arranged in the first-stage unit for particle focusing and rough sorting, and the cascaded DLD microfluidic is arranged in the second stage for realizing further sorting and purification. A parametric study with flow rate range from 100–600 μl/min and aspect ratio range from 60:100 to 60:300 of the first stage was carried out to optimize channel structure for realizing high-efficiency separation. Then, the pre-separation mechanism within the spiral microchannel was analyzed. The purity of the obtained CTCs and the separation efficiency were further improved using a droplet-type microcolumn DLD microfluidic device as the second unit. The cascade eliminates the need for additional force fields and reduces device complexity while simplifying operation and reducing the chance of sample contamination.

Mixed particles separation based on a cylindrical microfluidic centrifuge

This paper presents a novel cylindrical centrifuge designed to separate microspheres and cells. The polyvinylidene-fluoride tubes with heat shrink characteristics were used to encapsulate helix microchannels. Numerical simulations were employed to analyze and design the channel helix structure (with five annular loops, 2 mm pitch and 20 mm cylinder diameter). Then, The inertial focusing of microspheres (with diameters of 10, 15 and 20 µm) and the freshwater microalgae (i.e. Haematococcus pluvialis) in the cylindrical centrifuge were studied experimentally. As the particle size increased, the focusing position moved toward the center line of the flow channel, and the focus degree tended to decrease. When the flow rate increased, the focus position barely changed, but the focus degree increased significantly. A quantitative study of the centrifuge efficiency revealed that when the initial concentration was 104 particles ml−1 and the flow rate was at its optimal 1.7 ml min−1, the centrifugal efficiency (CE) values of 10 µm, 15 µm, 20 µm microspheres, and H. pluvialis, were 98.8%, 87.8%, 70.8% and 64.6%, respectively. The CE is inversely proportional to the microsphere size and the initial concentrations. Compared with the other two centrifuges (the cavity-vortex and planar spiral), the cylindrical centrifuge design and manufacturing process have a simplicity that provides low cost, efficient sample handling, and effective separation of microspheres and biological cells.

Inertial focusing of small particles in oscillatory channel flows

This study leverages the immersed boundary-lattice Boltzmann method to investigate the inertial focusing dynamics of a small neutrally buoyant particle in oscillatory channel flows driven by two distinct pressure gradient (PG) waveforms. Through an in-depth analysis of particle behavior, this research delineates the effects of PG waveforms, particle Reynolds number (Rep), and Womersley number (Wo) on the inertial focusing processes, and contrasts these findings with conventional Poiseuille flow-induced inertial focusing. Our results reveal that oscillatory flows induce more intricate focusing patterns than those observed in Poiseuille flow. The streamwise PG oscillation alters the particle focusing positions and introduces minor lateral oscillations in the post-focusing phase. The square PG waveform positions particles closer to the channel wall compared to Poiseuille flow, whereas the sine PG waveform positions particles closer to the channel center. Moreover, an increase in Rep nudges the focusing positions towards the channel center, while a higher Wo pushes them towards the walls. Notably, at Wo ≥ 5, the flow's oscillatory effect dominates over its inertial effect (i.e. Rep) on particle focusing. The study also underscores the capability of oscillatory flows to reduce the required channel length for particle focusing, albeit at the cost of increased focusing time. Contrary to Poiseuille flow, where the focusing length shortens with an increased Rep, oscillatory flow demonstrates an inverse correlation, with an increase in Rep extending the focusing length. Furthermore, while a higher Rep is beneficial in Poiseuille flow, it adversely affects the focusing effect in oscillatory flow. The focusing efficiency peaks at Wo = 3.5 for both waveforms, with the sine PG waveform offering superior efficiency due to the lower lift forces generated. Consequently, for optimal focusing in similar conditions, it is recommended to set Rep and Wo to 0.075 and 3.5, respectively.

Numerical Simulation of a Sheathless Multi-CTC Separator Lab-on-a-Chip Using Inertial Focusing Method

Numerical Simulation of a Sheathless Multi-CTC Separator Lab-on-a-Chip Using Inertial Focusing Method

High-throughput enrichment of portal venous circulating tumor cells for highly sensitive diagnosis of CA19-9-negative pancreatic cancer patients using inertial microfluidics

The carbohydrate antigen 19-9 (CA19-9) is commonly used as a representative biomarker for pancreatic cancer (PC); however, it lacks sensitivity and specificity for early-stage PC diagnosis. Furthermore, some patients with PC are negative for CA19-9 (<37 U/mL), which introduces additional limitations to their accurate diagnosis and treatment. Hence, improved methods to accurately detect PC stages in CA19-9-negative patients are warranted. In this study, tumor-proximal liquid biopsy and inertial microfluidics were coupled to enable high-throughput enrichment of portal venous circulating tumor cells (CTCs) and support the effective diagnosis of patients with early-stage PC. The proposed inertial microfluidic system was shown to provide size-based enrichment of CTCs using inertial focusing and Dean flow effects in slanted spiral channels. Notably, portal venous blood samples were found to have twice the yield of CTCs (21.4 cells per 5 mL) compared with peripheral blood (10.9 CTCs per 5 mL). A combination of peripheral and portal CTC data along with CA19-9 results showed to greatly improve the average accuracy of CA19-9-negative PC patients from 47.1% with regular CA19-9 tests up to 87.1%. Hence, portal venous CTC-based microfluidic biopsy can be used with high sensitivity and specificity for the diagnosis of early-stage PC, particularly in CA19-9-negative patients.

Micropump integrated white blood cell separation platform for detection of chronic granulomatous disease.

White blood cells (WBCs) are robust defenders during antigenic challenges and prime immune cell functioning indicators. High-purity WBC separation is vital for various clinical assays and disease diagnosis. Red blood cells (RBCs) are a major hindrance in WBC separation, constituting 1000 times the WBC population. The study showcases a low-cost micropump integrated microfluidic platform to provide highly purified WBCs for point-of-care testing. An integrated user-friendly microfluidic platform was designed to separate WBCs from finger-prick blood (⁓5 μL), employing an inertial focusing technique. We achieved an efficient WBC separation with 86% WBC purity and 99.99% RBC removal rate in less than 1min. In addition, the microdevice allows lab-on-chip colorimetric evaluation of chronic granulomatous disease (CGD),a rare genetic disorder affecting globally. The assay duration, straight from separation to disease detection, requires only 20min. Hence, the proposed microfluidic platform can further be implemented to streamline various clinical procedures involving WBCs in healthcare industries.

Human leucocytes processed by fast-rate inertial microfluidics retain conventional functional characteristics.

The manufacturing of clinical cellular therapies is a complex process frequently requiring manipulation of cells, exchange of buffers and volume reduction. Current manufacturing processes rely on either low throughput open centrifugation-based devices, or expensive closed-process alternatives. Inertial focusing (IF) microfluidic devices offer the potential for high-throughput, inexpensive equipment which can be integrated into a closed system, but to date no IF devices have been approved for use in cell therapy manufacturing, and there is limited evidence for the effects that IF processing has on human cells. The IF device described in this study was designed to simultaneously separate leucocytes, perform buffer exchange and provide a volume reduction to the cell suspension, using high flow rates with high Reynolds numbers. The performance and effects of the IF device were characterized using peripheral blood mononuclear cells and isolated monocytes. Post-processing cell effects were investigated using multi-parameter flow cytometry to track cell viability, functional changes and fate. The IF device was highly efficient at separating CD14+ monocytes (approx. 97% to one outlet, approx. 60% buffer exchange, 15 ml min-1) and leucocyte processing was well tolerated with no significant differences in downstream viability, immunophenotype or metabolic activity when compared with leucocytes processed with conventional processing techniques. This detailed approach provides robust evidence that IF devices could offer significant benefits to clinical cell therapy manufacture.

High throughput generating stable spheroids with tip-refill wafer.

Three-dimensional (3D) cell cultures have garnered significant attention in biomedical research due to their ability to mimic the in vivo cellular environment more accurately. The formation of 3D cell spheroids using hanging drops has emerged as a cost-effective and crucial method for generating uniformly-sized spheroids. This study aimed to validate the potential of a tip-refill wafer (TrW), a disposable laboratory item used to hold pipette tips, in facilitating 3D cell culture. The TrW allows for easy generation of hanging drops by pipetting the solution into the holes of the wafer. The mechanical stability of the hanging drops is ensured by the surface wettability and thickness of the TrW. Hanging drops containing 60-µL of solution remained securely attached to the TrW even when subjected to orbital shaking at 210rpm. The exceptional resistance to mechanical shaking enabled the use of inertial focusing to facilitate spheroid formation. This was demonstrated through live/dead cell staining, quantitative polymerase chain reaction (qPCR) analysis, and cytoskeleton staining, which revealed that horizontal orbiting at 60rpm for 15 min promoted cell aggregation and ultimately led to the formation of 3D spheroids. The spheroid harvest rate is 96.1%±3.5% across three TrWs, each containing 60 hanging drops. In addition to generating mono-culture 3D spheroids, the TrW-based hanging drop platform also enables the formation of multicellular spheroids, and on-demand pairing and fusion of spheroids. The TrW is a disposable item that does not require any fabrication or surface modification procedures, further enhancing its application potential in conventional biological laboratories.

Twisted fiber microfluidics: a cutting-edge approach to 3D spiral devices.

The development of 3D spiral microfluidics has opened new avenues for leveraging inertial focusing to analyze small fluid volumes, thereby advancing research across chemical, physical, and biological disciplines. While traditional straight microchannels rely solely on inertial lift forces, the novel spiral geometry generates Dean drag forces, eliminating the necessity for external fields in fluid manipulation. Nevertheless, fabricating 3D spiral microfluidics remains a labor-intensive and costly endeavor, hindering its widespread adoption. Moreover, conventional lithographic methods primarily yield 2D planar devices, thereby limiting the selection of materials and geometrical configurations. To address these challenges, this work introduces a streamlined fabrication method for 3D spiral microfluidic devices, employing rotational force within a miniaturized thermal drawing process, termed as mini-rTDP. This innovation allows for rapid prototyping of twisted fiber-based microfluidics featuring versatility in material selection and heightened geometric intricacy. To validate the performance of these devices, we combined computational modeling with microtomographic particle image velocimetry (μTPIV) to comprehensively characterize the 3D flow dynamics. Our results corroborate the presence of a steady secondary flow, underscoring the effectiveness of our approach. Our 3D spiral microfluidics platform paves the way for exploring intricate microflow dynamics, with promising applications in areas such as drug delivery, diagnostics, and lab-on-a-chip systems.

Spiral Large-Dimension Microfluidic Channel for Flow-Rate- and Particle-Size-Insensitive Focusing by the Stabilization and Acceleration of Secondary Flow.

Inertial microfluidics has demonstrated its ability to focus particles in a passive and straightforward manner. However, achieving flow-rate- and particle-size-insensitive focusing in large-dimension channels with a simple design remains challenging. In this study, we developed a spiral microfluidic with a large-dimension channel to achieve inertial focusing. By designing a unique "big buffering area" and a "small buffering area" in the spiral microchannel, we observed the stabilization and acceleration of secondary flow. Our optimized design allowed for efficient (>99.9%) focusing of 15 μm particles within a wide range of flow rates (0.5-4.5 mL/min) during a long operation duration (0-60 min). Additionally, we achieved effective (>95%) focusing of different-sized particles (7, 10, 15, and 30 μm) and three types of tumor cells (K562, HeLa, and MCF-7) near the inner wall of the 1 mm wide outlet when applying different flow rates (1-3 mL/min). Finally, successful 3D cell focusing was achieved within an optimized device, with the cells positioned at a distance of 50 μm from the wall. Our strategy of stabilizing and accelerating Dean-like secondary flow through the unique configuration of a "big buffering area" and a "small buffering area" proved to be highly effective in achieving inertial focusing that is insensitive to the flow rate and particle size, particularly in large-dimension channels. Consequently, it shows great potential for use in hand-operated microfluidic tools for flow cytometry.