Straight Microfluidic Channel Research Articles

Blood component separation in straight microfluidic channels.

Separation of blood components is required in many diagnostic applications and blood processes. In laboratories, blood is usually fractionated by manual operation involving a bulk centrifugation equipment, which significantly increases logistic burden. Blood sample processing in the field and resource-limited settings cannot be readily implemented without the use of microfluidic technology. In this study, we developed a small footprint, rapid, and passive microfluidic channel device that relied on margination and inertial focusing effects for blood component separation. No blood dilution, lysis, or labeling step was needed as to preserve sample integrity. One main innovation of this work was the insertion of fluidic restrictors at outlet ports to divert the separation interface into designated outlet channels. Thus, separation efficiency was significantly improved in comparison to previous works. We demonstrated different operation modes ranging from platelet or plasma extraction from human whole blood to platelet concentration from platelet-rich plasma through the manipulation of outlet port fluidic resistance. Using straight microfluidic channels with a high aspect ratio rectangular cross section, we demonstrated 95.4% platelet purity extracted from human whole blood. In plasma extraction, 99.9% RBC removal rate was achieved. We also demonstrated 2.6× concentration of platelet-rich plasma solution to produce platelet concentrate. The extraction efficiency and throughput rate are scalable with continuous and clog-free recirculation operation, in contrast to other blood fractionation approaches using filtration membranes or affinity-based purification methods. Our microfluidic blood separation method is highly tunable and versatile, and easy to be integrated into multi-step blood processing and advanced sample preparation workflows.

Inertial migration of non-spherical particles in straight microfluidic channels

The paper reports on simulations of particulate flows in square ducts with oblate and prolate particles at a bulk Reynolds number of 100 and dilute particle concentration. Inertial migration leads to focusing of particles in specific regions of the cross section. It is observed that these positions are different for the non-spherical particles compared to those obtained with spherical ones. Prolate particles exhibit Jeffery-type orbits, while oblate particles rotate around their axis of symmetry. As a result, the rotation-induced migration of prolates is much slower than for spheres and oblates. An analysis of the surrounding flow is used to show differences in the velocity field. In a second set of simulations, the particle concentration was increased by a factor of 4 with the same domain size, so that neighboring particles influence each other. The duration until focusing is achieved is substantially increased. The focusing position moves slightly to the wall, and further effects are generated. Steady particle oscillations in position are seen for spheres, and the formation of particle trains is observed in all cases. The interaction of prolate particles is particularly complex and addressed in substantial detail.

Periodic switching of acoustic radiation force with beat created by multitone field

Acoustic radiation force plays a key role in microfluidic systems for particle and cell manipulation. In this study, we investigate the acoustic radiation force resulting from synthesized ultrasounds that are emitted from multiple sound sources with slightly different oscillation frequencies. Due to the synthesized field, the acoustic radiation force is expressed as the sum of a dc component and harmonics of fundamental frequencies of a few hertz. This induces the beat of the acoustic radiation force. We demonstrate that the synthesized field provides the periodic on/off switching of the acoustic radiation force associated with the one denominational planar standing wave in a straight microfluidic channel. Consequently, our system can temporally manipulate acoustic radiation force without active controls.

Femtosecond Laser Assisted 3D Etching Using Inorganic-Organic Etchant.

Selective laser etching (SLE) is a technique that allows the fabrication of arbitrarily shaped glass micro-objects. In this work, we show how the capabilities of this technology can be improved in terms of selectivity and etch rate by applying an etchant solution based on a Potassium Hydroxide, water, and isopropanol mixture. By varying the concentrations of these constituents, the wetting properties, as well as the chemical reaction of fused silica etching, can be changed, allowing us to achieve etching rates in modified fused silica up to 820 μm/h and selectivity up to ∼3000. This is used to produce a high aspect ratio (up to 1:1000), straight and spiral microfluidic channels which are embedded inside a volume of glass. Complex 3D glass micro-structures are also demonstrated.

Directional reorientation of migrating neutrophils is limited by suppression of receptor input signaling at the cell rear through myosin II activity

To migrate efficiently to target locations, cells must integrate receptor inputs while maintaining polarity: a distinct front that leads and a rear that follows. Here we investigate what is necessary to overwrite pre-existing front-rear polarity in neutrophil-like HL60 cells migrating inside straight microfluidic channels. Using subcellular optogenetic receptor activation, we show that receptor inputs can reorient weakly polarized cells, but the rear of strongly polarized cells is refractory to new inputs. Transient stimulation reveals a multi-step repolarization process, confirming that cell rear sensitivity to receptor input is the primary determinant of large-scale directional reversal. We demonstrate that the RhoA/ROCK/myosin II pathway limits the ability of receptor inputs to signal to Cdc42 and reorient migrating neutrophils. We discover that by tuning the phosphorylation of myosin regulatory light chain we can modulate the activity and localization of myosin II and thus the amenability of the cell rear to ‘listen’ to receptor inputs and respond to directional reprogramming.

Influence of lift forces on particle capture on a functionalized surface

Numerous situations involve the capture of particles onto a functionalized surface in a laminar flow, such as classical biomedical assays, lab on a chip devices or even biological research protocols. Being able to control this capture is thus an important issue that we address in this paper. We focus on a simple and widely used geometry, the straight microfluidic channel, in which particles undergo two weak effects: diffusion towards the functionalized surface and lift forces expelling them away from it. We show that the competition between these two weak mechanisms yields strongly different capture behavior whose occurrence depends on the value of a new lifto-diffusive dimensionless number $${\mathcal {N}}_{\text {LD}}$$ . We show that tuning the flow rate and the channel dimension to get proper values of this number allow to trigger, via a pure hydrodynamic effect, the capture or non-capture of particles on surfaces. For example, we show that, under certain conditions, doubling the flow rate reduces the capture rate by four orders of magnitude. Additionally, we provide the particle distribution in the liquid along the channel, resulting from this competition, for different $${\mathcal {N}}_{\text {LD}}$$ values. We believe that this work opens new perspectives for analysis and biotechnology applications. More precisely, the proposed model should extend to any transverse force that can be written in the form of a potential energy.

Experimental determination and computational prediction of the mixing efficiency of a simple, continuous, serpentine-channel microdevice

Micromixing devices often utilize complex architectures to mix miscible liquid streams and can be complex and expensive to fabricate. Here, we developed, built, experimentally tested, and computationally analyzed a serpentine micromixer that can be fabricated using simple tools and supplies available in non-microdevice dedicated laboratories. Fluorescence imaging was used to quantify its mixing effectiveness experimentally. A Computational Fluid Dynamics (CFD) software package (COMSOL) was used to model the micromixing process. The predictions were in excellent agreement with the experimental data. The serpentine micromixer can achieve significant levels of mixing efficiency. CFD predictions for a straight microfluidic channel of the same length as the serpentine favorably compared with previous theoretical predictions, indicating that the serpentine's mixing efficiency was vastly superior. Finally, CFD predictions were conducted for different and possibly improved designs of the basic serpentine. In all cases, the mixing efficiency was primarily associated with the number of 90o elbows in the device rather than the straight sections' length, with the first serpentine bend playing a significant role. Future design improvements should focus on incorporating as many elbows as possible in the device to maximize mixing efficiency and reduce the device size.

Acoustofluidic microdevice for precise control of pressure nodal positions

Acoustic wave-based manipulation of cells and particles in microfluidic channels has gained wide popularity in the past decade since it provides label-free and contact-less manipulation of them in a microfluidic environment using a very simple microfluidic structure and experimental setup. In bulk acoustofluidics, an acoustic resonance field that generates an acoustic standing wave within a microfluidic channel creates acoustic pressure nodes and anti-nodes, to which particles migrate to or migrate away from. However, in a given straight microfluidic channel, the position of the acoustic pressure nodes and anti-nodes are fixed and cannot be changed along the channel, limiting more diverse capabilities in moving particles and cells to a desired location within a microfluidic channel. Here, an acoustic echo-channel where its width changes along the flow direction was created right next to the main flow channel separated by a thin wall that minimizes the disturbance of the acoustic wave. This allows the location of the acoustic pressure nodes and anti-nodes to be controlled in the main flow channel depending on the width of the echo-channel, hence providing more flexibility in manipulating particles and cells to a certain position within a given microfluidic channel. The capability to more freely manipulate particles and cells within a microfluidic channel further expands the application areas of bulk acoustofluidics.

Viscoelastic focusing of polydisperse particle suspensions in a straight circular microchannel

Flow cytometry is a technique for the analysis of cells and particles by coupling a sheath flow assisted focusing microfluidic device to an optical or electrical reader. In this context, polymer solutions can be employed to drive particle and cell focusing on the centreline of simple straight microfluidic channels. However, the change of the focusing efficiency due to polydispersity, which is a rule rather than the exception across cell populations, has not been extensively studied yet. In this work, the effect of particle polydispersity on the viscoelastic focusing in a straight cylindrical microchannel was studied by preparing two particle suspensions, containing different concentrations of particles with average diameters of 10,upmu hbox {m}, 15,upmu hbox {m} and 20,upmu hbox {m}, suspended in three solutions of polyethylene oxide with mass concentrations of 0.1 wt%, 0.25 wt% and 0.5 wt%. When the fluid explored the constant-viscosity region of the rheological curve, up to 95% of the particles were aligned on the channel centreline. When the fluid explored the shear-thinning region of the rheological curve, centreline focusing efficiency decreased, with the highest value of aligned particles being of 60%. For both mixtures, it was also observed that the fraction of aligned particles in a polydisperse system was not equivalent to that derived from the estimate of independent experiments with monodisperse particles.

Responses of Cellular Adhesion Strength and Stiffness to Fluid Shear Stress during Tumor Cell Rolling Motion.

Biochemical and physical factors affect the rolling of tumor cells across the blood vessel. The biochemical factors have been well studied, while the influence of physical factors such as fluid shear stress (FSS) remains poorly understood. Here, human glioma cells (U87 cells) in a straight microfluidic channel were exposed to FSS (0.12, 1.2, and 1.8 dyn/cm2); and their locomotion behaviors from crawling-to-rolling and changes in cellular morphology (concave, elongated, less elongated, and round) were observed. The adhesion strength and stiffness of the cells of different morphologies were analyzed using a live single-cell extractor and atomic force microscopy, respectively. In general, the FSS stimulated cells showed stronger adhesion strength than the cells not exposed to FSS. The cell not exposed to FSS always exhibited greater nuclear stiffness than cortex stiffness, while after FSS treatment the cortex hardened and nucleus softened, where the round-shaped cell had a cortex that was more rigid than its nucleus. These results indicated that FSS influenced the biomechanics of circulating tumor cells, and elucidation of the mechanical responses to FSS might provide a deeper insight for cancer metastasis.

Towards an Optimal Pressure Tap Design for Fluid-Flow Characterisation at Microscales.

Measuring fluid pressure in microchannels is difficult and constitutes a challenge to even the most experienced of experimentalists. Currently, to the best of the authors’ knowledge, no optimal solution are being used for the design of pressure taps, nor guidelines concerning their shape and its relation with the accuracy of the readings. In an attempt to address this issue, a parametric study was devised to evaluate the performance of different pressure tap designs, 18 in total. These were obtained by combining three shape parameters: sub-channel width (w) and sub-channel–tap radius (R) or angle (), while having the sub-channel length kept constant. For each configuration, pressure drop measurements were carried out along several lengths of a straight microfluidic rectangular channel and later compared to an analytical solution. The microchannels were fabricated out of PDMS using standard soft-lithography techniques, pressure drop was measured with differential pressure sensors, the test fluid was DI water and the flow conditions varied from creeping flow up to ∼100. Pressure taps, having smooth contours (characterised by the radius R) and a sub-channel width (w) of , performed the best with results from that of radius only falling short of the theory by a mere .

Fluid Viscoelasticity Drives Self-Assembly of Particle Trains in a Straight Microfluidic Channel

Hop on board: The authors present experimental evidence of particle trains in viscoelastic polymer solutions. It turns out that such lines of particles in viscoelastic liquids are easy to produce, prompting future applications in biomedical engineering and materials science---for example, in optimized flow cytometry, deterministic cell encapsulation and microfluidic fabrication of materials with enhanced localized properties. Also, the simple setup here can be coupled with other microfluidic techniques, to clarify the hydrodynamic interactions between particles.

In-situ shear-banding quantification of surfactant solutions in straight microfluidic channels

Surfactant molecules aggregate into various micellar morphologies, depending on temperature, concentration, formulation, and flow. Micellar solutions are known to undergo shear-banding when subjected to strong shear rates, as the fluid spontaneously divides itself into bands of high and low-shear rate, both under the same applied shear stress. This phenomenon occurs because of the complex structure of micellar solutions, which undergo phase transitions upon applied flow, changing the viscosity accordingly. Here, we study shear-banding of micellar solutions in one of the simplest microfluidic geometries, a straight planar channel with rectangular cross section. Four solutions with similar zero-shear viscosity and nonlinear rheological response, but different structures are compared to investigate the flow-structure relation and its impact on shear-banding. Micellar solutions are prepared by adding different amounts of the same organic salt, sodium salicylate, to surfactant molecules with different headgroups, i.e., cetyltrimethylammonium bromide and cetylpyridinium chloride. From spatially resolved microparticle image velocimetry and flow-induced birefringence measurements, the shear rate and shear stress profiles developed on the xy-plane of a planar microchannel are obtained from a series of volumetric flow rates. Based on these profiles, in-situ rheological parameters, such as the local viscosity, are calculated by applying the stress-optical rule. The local response in a microfluidic channel is compared to the bulk rheological response in a rotational rheometer and clear correlations are found especially for the stress plateau region, the fingerprint of shear-banding. Based on the local rheological characterization of these micellar solutions, the development and growth of shear-bands is observed and quantified. The role of salt concentration and surfactant headgroup on the resulting micellar morphology is discussed, as well as its impact on the development of shear-banding.

Effect of reservoir geometry on vortex trapping of cancer cells

Vortex-aided particle separation is a powerful method to efficiently isolate circulating tumor cells from blood, since it allows high throughput and continuous sample separation, with no need for time-consuming sample preprocessing. With this approach, only the larger particles from a heterogeneous sample will be stably trapped in reservoirs that expand from a straight microfluidic channel, allowing for efficient particle sorting along with simultaneous concentration. A possible limitation is related to the loss of particles from vortex traps due to particle–particle interactions that limit the final cellularity of the enriched solution. It is fundamental to minimize this issue considering that a scant number of target cells are diluted in highly cellular blood. In this work, we present a device for size-based particle separation, which exploits the well-consolidated vortex-aided sorting, but new reservoir layouts are presented and investigated in order to increase the trapping efficiency of the chip. Through simulations and experimental validations, we have been able to optimize the device design to increase the maximum number of particles that can be stably trapped in each reservoir and therefore the total efficiency of the chip.

Internal flow in droplets within a concentrated emulsion flowing in a microchannel

Droplet microfluidics has enabled a wide variety of high-throughput biotechnical applications through the use of monodisperse micro-droplets as bioreactors. Previous fluid dynamics studies of droplet microfluidics have focused on single droplets or emulsions at low volume fractions. The study of concentrated emulsions at high volume fractions is important for further increasing the throughput of droplet microfluidics, but the fluid dynamics of such emulsions in confined microchannels is not well understood. This paper describes the use of microscopic particle image velocimetry to quantify the flow inside individual droplets within a concentrated emulsion having volume fraction φ ∼ 85% flowing as a monolayer in a straight microfluidic channel. The effects of confinement (namely, the number of rows of droplets across the width of the channel) and viscosity ratio on the internal flow patterns inside the drops at a fixed capillary number of 10−3 and a Reynolds number of 10−2 to 10−1 are studied. The results show that rotational structures inside the droplets always exist and are independent of viscosity ratio for the conditions tested. The structures depend on droplet mobility, the ratio of the velocity of the droplet to the velocity of the continuous phase. These values, in turn, depend on the confinement of the emulsion and the location of the droplets in the channel. Although this work presents two-dimensional measurements at the mid-height of the microchannel only, the results reveal flow patterns that are never described before in single drops or dilute emulsions.

Multi-train elasto-inertial particle focusing in straight microfluidic channels

Herein, we reported a multi-train focusing in viscoelastic flows where the fluid inertia effect is comparable with the elastic effect. Different from their behaviors in inertia- or elasticity-dominating flows, particles in the current elasto-inertial flow were found to equilibrate at two symmetrical positions in the vertical center plane under the balance of the shear-induced inertial lift force and the elastic force. With further increasing flow rate, a third focusing position was observed at the channel center due to the fast-increasing elastic force. Based on these findings, we experimentally explored the particle focusing behaviors over various influential factors to better understand the underlying mechanics. The sheathless and high-throughput focusing of particles into multiplex deterministic trains at the same plane provides a potential method for image-based particle/cell detection.

Shear-induced tail breakup of droplets (bubbles) flowing in a straight microfluidic channel

Dispersed droplets and bubbles were generated in a microfluidic flow-focusing device to investigate the morphological developments of the droplets (bubbles) flowing in the downstream straight micro-channel by a high speed camera. The present study is mainly aimed at the influences of both the dispersed droplet (bubble) size and two-phase average flow velocity on the formation of tip stream in the rear of a droplet (bubble). It was found that the particle deformation increased with the droplet (bubble) length or the capillary number. There exists a critical droplet (bubble) length dependent on capillary number, beyond which the tip stream ejecting tiny daughter droplets will take place. The generated tiny droplets are usually three orders of magnitude smaller than primary droplet. This study provides the possibility to avoid a high polydispersity index in microchannels.

Numerical study of thermoviscous effects in ultrasound-induced acoustic streaming in microchannels.

We present a numerical study of thermoviscous effects on the acoustic streaming flow generated by an ultrasound standing-wave resonance in a long straight microfluidic channel containing a Newtonian fluid. These effects enter primarily through the temperature and density dependence of the fluid viscosity. The resulting magnitude of the streaming flow is calculated and characterized numerically, and we find that even for thin acoustic boundary layers, the channel height affects the magnitude of the streaming flow. For the special case of a sufficiently large channel height, we have successfully validated our numerics with analytical results from 2011 by Rednikov and Sadhal for a single planar wall. We analyzed the time-averaged energy transport in the system and the time-averaged second-order temperature perturbation of the fluid. Finally, we have made three main changes in our previously published numerical scheme to improve the numerical performance: (i) The time-averaged products of first-order variables in the time-averaged second-order equations have been recast as flux densities instead of as body forces. (ii) The order of the finite-element basis functions has been increased in an optimal manner. (iii) Based on the International Association for the Properties of Water and Steam (IAPWS 1995, 2008, and 2011), we provide accurate polynomial fits in temperature for all relevant thermodynamic and transport parameters of water in the temperature range from 10 to 50 °C.

Microfluidic Western blotting of low-molecular-mass proteins.

We describe a microfluidic Western blot assay (μWestern) using a Tris tricine discontinuous buffer system suitable for analyses of a wide molecular mass range (6.5–116 kDa). The Tris tricine μWestern is completed in an enclosed, straight glass microfluidic channel housing a photopatterned polyacrylamide gel that incorporates a photoactive benzophenone methacrylamide monomer. Upon brief ultraviolet (UV) light exposure, the hydrogel toggles from molecular sieving for size-based separation to a covalent immobilization scaffold for in situ antibody probing. Electrophoresis controls all assay stages, affording purely electronic operation with no pumps or valves needed for fluid control. Electrophoretic introduction of antibody into and along the molecular sieving gel requires that the probe must traverse through (i) a discontinuous gel interface central to the transient isotachophoresis used to achieve high-performance separations and (ii) the full axial length of the separation gel. In-channel antibody probing of small molecular mass species is especially challenging, since the gel must effectively sieve small proteins while permitting effective probing with large-molecular-mass antibodies. To create a well-controlled gel interface, we introduce a fabrication method that relies on a hydrostatic pressure mismatch between the buffer and polymer precursor solution to eliminate the interfacial pore-size control issues that arise when a polymerizing polymer abuts a nonpolymerizing polymer solution. Combined with a new swept antibody probe plug delivery scheme, the Tris tricine μWestern blot enables 40% higher separation resolution as compared to a Tris glycine system, destacking of proteins down to 6.5 kDa, and a 100-fold better signal-to-noise ratio (SNR) for small pore gels, expanding the range of applicable biological targets.

Flow-through cell electroporation microchip integrating dielectrophoretic viable cell sorting.

Microfluidics based continuous cell electroporation is an appealing approach for high-throughput cell transfection, but cell viability of existing methods is usually compromised by adverse electrical or hydrodynamic effects. Here we present the validation of a flow-through cell electroporation microchip, in which dielectrophoretic force was employed to sort viable cells. By integrating parallel electroporation electrodes and dielectrophoresis sorting electrodes together in a simple straight microfluidic channel, sufficient electrical pulses were applied for efficient electroporation, and a proper sinusoidal electrical field was subsequently utilized to exclude damaged cells by dielectrophoresis. Thus, the difficulties for seeking the fine balance between electrotransfection efficiency and cell viability were steered clear. After careful investigation and optimization of the DEP behaviors of electroporated cells, efficient electrotransfection of plasmid DNA was demonstrated in vulnerable neuron cells and several hard-to-transfect primary cell types with excellent cell viability. This microchip constitutes a novel way of continuous cell transfection to significantly improve the cell viability of existing methodologies.