Filter-less submicron hydrodynamic size sorting

Viro-fluidics: Real-time analysis of virus production kinetics at the single-cell level

Real-time visualization and quantification of viruses released by a cell are crucial to further decipher infection processes. Kinetics studies at the single-cell level will circumvent the limitations of bulk assays with asynchronous virus replication. We have implemented a "viro-fluidic" method, which combines microfluidics and virology at single-cell and single-virus resolutions. As an experimental model, we used standard cell lines producing fluorescent HIV-like particles (VLPs). First, to scale the strategy to the single-cell level, we validated a sensitive flow virometry system to detect VLPs in low concentration samples (≥10⁴ VLPs/mL). Then, this system was coupled to a single-cell trapping device to monitor in real-time the VLPs released, one at a time, from single cells under cell culture conditions. Our results revealed an average production rate of 50 VLPs/h/cell similar to the rate estimated for the same cells grown in population. Thus, the virus-producing capacities of the trapped cells were preserved and its real-time monitoring was accurate. Moreover, single-cell analysis revealed a release of VLPs with stochastic bursts with typical time intervals of few minutes, revealing the existence of limiting step(s) in the virus biogenesis process. Our tools can be applied to other pathogens or to extracellular vesicles to elucidate the dissemination mechanisms of these biological nanoparticles.

Massively Multiplexed Submicron Particle Patterning in Acoustically Driven Oscillating Nanocavities

Nanoacoustic fields are a promising method for particle actuation at the nanoscale, though THz frequencies are typically required to create nanoscale wavelengths. In this work, the generation of robust nanoscale force gradients is demonstrated using MHz driving frequencies via acoustic-structure interactions. A structured elastic layer at the interface between a microfluidic channel and a traveling surface acoustic wave (SAW) device results in submicron acoustic traps, each of which can trap individual submicron particles. The acoustically driven deformation of nanocavities gives rise to time-averaged acoustic fields which direct suspended particles toward, and trap them within, the nanocavities. The use of SAWs permits massively multiplexed particle manipulation with deterministic patterning at the single-particle level. In this work, 300 nm diameter particles are acoustically trapped in 500 nm diameter cavities using traveling SAWs with wavelengths in the range of 20-80 µm with one particle per cavity. On-demand generation of nanoscale acoustic force gradients has wide applications in nanoparticle manipulation, including bioparticle enrichment and enhanced catalytic reactions for industrial applications.

Two-phase flow in microfluidic-chip design of hydrodynamic filtration for cell particle sorting

As one of the flow-based passive sorting, the hydrodynamic filtration using a microfluidic-chip has shown to effectively separate into different sizes of subpopulations from cell or particle suspensions. Its model framework involving two-phase Newtonian or generalized Newtonian fluid (GNF) was developed, by performing the complete analysis of laminar flow and complicated networks of main and multiple branch channels. To predict rigorously what occurs in flow fields, we estimated pressure drop, velocity profile, and the ratio of the flow fraction at each branch point, in which the analytical model was validated with numerical flow simulations. As a model fluid of the GNF, polysaccharide solution based on Carreau type was examined. The objective parameters aiming practical channel design include the number of the branches and the length of narrow section of each branch for arbitrary conditions. The flow fraction and the number of branches are distinctly affected by the viscosity ratio between feed and side flows. As the side flow becomes more viscous, the flow fraction increases but the number of branches decreases, which enables a compact chip designed with fewer branches being operated under the same throughput. Hence, our rational design analysis indicates the significance of constitutive properties of each stream.

Label-free microfluidic sorting of microparticles

Massive growth of the microfluidics field has triggered numerous advances in focusing, separating, ordering, concentrating, and mixing of microparticles. Microfluidic systems capable of performing these functions are rapidly finding applications in industrial, environmental, and biomedical fields. Passive and label-free methods are one of the major categories of such systems that have received enormous attention owing to device operational simplicity and low costs. With new platforms continuously being proposed, our aim here is to provide an updated overview of the state of the art for passive label-free microparticle separation, with emphasis on performance and operational conditions. In addition to the now common separation approaches using Newtonian flows, such as deterministic lateral displacement, pinched flow fractionation, cross-flow filtration, hydrodynamic filtration, and inertial microfluidics, we also discuss separation approaches using non-Newtonian, viscoelastic flow. We then highlight the newly emerging approach based on shear-induced diffusion, which enables direct processing of complex samples such as untreated whole blood. Finally, we hope that an improved understanding of label-free passive sorting approaches can lead to sophisticated and useful platforms toward automation in industrial, environmental, and biomedical fields.

Microfluidic technologies in cell isolation and analysis for biomedical applications

Efficient platforms for cell isolation and analysis play an important role in applied and fundamental biomedical studies. As cells commonly have a size of around 10 microns, conventional handling approaches at a large scale are still challenged in precise control and efficient recognition of cells for further performance of isolation and analysis. Microfluidic technologies have become more prominent in highly efficient cell isolation for circulating tumor cells (CTCs) detection, single-cell analysis and stem cell separation, since microfabricated devices allow for the spatial and temporal control of complex biochemistries and geometries by matching cell morphology and hydrodynamic traps in a fluidic network, as well as enabling specific recognition with functional biomolecules in the microchannels. In addition, the fabrication of nano-interfaces in the microchannels has been increasingly emerging as a very powerful strategy for enhancing the capability of cell capture by improving cell-interface interactions. In this review, we focus on highlighting recent advances in microfluidic technologies for cell isolation and analysis. We also describe the general biomedical applications of microfluidic cell isolation and analysis, and finally make a prospective for future studies.

New advances in microfluidic flow cytometry

In recent years, researchers are paying the increasing attention to the development of portable microfluidic diagnostic devices including microfluidic flow cytometry for the point-of-care testing. Microfluidic flow cytometry, where microfluidics and flow cytometry work together to realize novel functionalities on the microchip, provides a powerful tool for measuring the multiple characteristics of biological samples. The development of a portable, low-cost, and compact flow cytometer can benefit the health care in underserved areas such as Africa or Asia. In this article, we review recent advancements of microfluidics including sample pumping, focusing and sorting, novel detection approaches, and data analysis in the field of flow cytometry. The challenge of microfluidic flow cytometry is also examined briefly.

Extracellular vesicles: lipids as key components of their biogenesis and functions

Intercellular communication has been known for decades to involve either direct contact between cells or to operate via circulating molecules, such as cytokines, growth factors, or lipid mediators. During the last decade, we have begun to appreciate the increasing importance of intercellular communication mediated by extracellular vesicles released by viable cells either from plasma membrane shedding (microvesicles, also named microparticles) or from an intracellular compartment (exosomes). Exosomes and microvesicles circulate in all biological fluids and can trigger biological responses at a distance. Their effects include a large variety of biological processes, such as immune surveillance, modification of tumor microenvironment, or regulation of inflammation. Extracellular vesicles can carry a large array of active molecules, including lipid mediators, such as eicosanoids, proteins, and nucleic acids, able to modify the phenotype of receiving cells. This review will highlight the role of the various lipidic pathways involved in the biogenesis and functions of microvesicles and exosomes.

Surface acoustic wave diffraction driven mechanisms in microfluidic systems

Acoustic forces arising from high-frequency surface acoustic waves (SAW) underpin an exciting range of promising techniques for non-contact manipulation of fluid and objects at micron scale. Despite increasing significance of SAW-driven technologies in microfluidics, the understanding of a broad range of phenomena occurring within an individual SAW system is limited. Acoustic effects including streaming and radiation force fields are often assumed to result from wave propagation in a simple planar fashion. The propagation patterns of a single SAW emanating from a finite-width source, however, cause a far richer range of physical effects. In this work, we seek a better understanding of the various effects arising from the incidence of a finite-width SAW beam propagating into a quiescent fluid. Through numerical and experimental verification, we present five distinct mechanisms within an individual system. These cause fluid swirling in two orthogonal planes, and particle trapping in two directions, as well as migration of particles in the direction of wave propagation. For a range of IDT aperture and channel dimensions, the relative importance of these mechanisms is evaluated.

Transport of nano-objects in narrow channels: influence of Brownian diffusion, confinement and particle nature

This paper presents experimental results about transport of dilute suspensions of nano-objects in silicon-glass micrometric and sub-micrometric channels. Two kinds of objects are used: solid, rigid latex beads and spherical capsule-shaped, soft polymersomes. They are tracked using fluorescence microscopy. Three aspects are studied: confinement (ratio between particle diameter and channel depth), Brownian diffusion and particle nature. The aim of this work is to understand how these different aspects affect the transport of suspensions in narrow channels and to understand the different mechanisms at play. Concerning the solid beads we observe the appearance of two regimes, one where the experimental mean velocity is close to the expected one and another where this velocity is lower. This is directly related to a competition between confinement, Brownian diffusion and advection. These two regimes are shown to be linked to the inhomogeneity of particles distribution in the channel depth, which we experimentally deduce from velocity distributions. This inhomogeneity appears during the entrance process into the sub-micrometric channels, as for hydrodynamic separation or deterministic lateral displacement. Concerning the nature of the particles we observed a shift of transition towards the second regime likely due to the relationships between shear stress and polymersomes mechanical properties which could reduce the inhomogeneity imposed by the geometry of our device.

High-Throughput Inertial Focusing of Micrometer- and Sub-Micrometer-Sized Particles Separation

The ability to study individual bacteria or subcellular organelles using inertial microfluidics is still nascent. This is due, in no small part, to the significant challenges associated with concentrating and separating specific sizes of micrometer and sub-micrometer bioparticles in a microfluidic format. In this study, using a rigid polymeric microfluidic network with optimized microchannel geometry dimensions, it is demonstrated that 2 µm, and even sub-micrometer, particles can be continuously and accurately focused to stable equilibrium positions. Suspensions have been processed at flow rates up to 1400 µL min-1 in an ultrashort 4 mm working channel length. A wide range of suspension concentrations-from 0.01 to 1 v/v%-have been systematically investigated, with yields greater than 97%, demonstrating the potential of this technology for large-scale implementation. Additionally, the ability of this chip to separate micrometer- and sub-micrometer-sized particles and to focus bioparticles (cyanobacteria) has been demonstrated. This study pushes the microfluidic inertial focusing particle range down to sub-micrometer length scales, enabling novel routes for investigation of individual microorganisms and subcellular organelles.

Slanted, asymmetric microfluidic lattices as size-selective sieves for continuous particle/cell sorting

Hydrodynamic microfluidic platforms have been proven to be useful and versatile for precisely sorting particles/cells based on their physicochemical properties. In this study, we demonstrate that a simple lattice-shaped microfluidic pattern can work as a virtual sieve for size-dependent continuous particle sorting. The lattice is composed of two types of microchannels ("main channels" and "separation channels"). These channels cross each other in a perpendicular fashion, and are slanted against the macroscopic flow direction. The difference in the densities of these channels generates an asymmetric flow distribution at each intersection. Smaller particles flow along the streamline, whereas larger particles are filtered and gradually separated from the stream, resulting in continuous particle sorting. We successfully sorted microparticles based on size with high accuracy, and clearly showed that geometric parameters, including the channel density and the slant angle, critically affect the sorting behaviors of particles. Leukocyte sorting and monocyte purification directly from diluted blood samples have been demonstrated as biomedical applications. The presented system for particle/cell sorting would become a simple but versatile unit operation in microfluidic apparatus for chemical/biological experiments and manipulations.

Continuous micro-vortex-based nanoparticle manipulation via focused surface acoustic waves

Despite increasing demand in the manipulation of nanoscale objects for next generation biological and industrial processes, there is a lack of methods for reliable separation, concentration and purification of nanoscale objects. Acoustic methods have proven their utility in contactless manipulation of microscale objects mainly relying on the acoustic radiation effect, though the influence of acoustic streaming has typically prevented manipulation at smaller length scales. In this work, however, we explicitly take advantage of the strong acoustic streaming in the vicinity of a highly focused, high frequency surface acoustic wave (SAW) beam emanating from a series of focused 6 μm substrate wavelength interdigital transducers patterned on a piezoelectric lithium niobate substrate and actuated with a 633 MHz sinusoidal signal. This streaming field serves to focus fluid streamlines such that incoming particles interact with the acoustic field similarly regardless of their initial starting positions, and results in particle displacements that would not be possible with a travelling acoustic wave force alone. This streaming-induced manipulation of nanoscale particles is maximized with the formation of micro-vortices that extend the width of the microfluidic channel even with the imposition of a lateral flow, occurring when the streaming-induced flow velocities are an order of magnitude larger than the lateral one. We make use of this acoustic streaming to demonstrate the continuous and differential focusing of 100 nm, 300 nm and 500 nm particles.