Numerical Study of the Time-Periodic Electroosmotic Flow of Viscoelastic Fluid through a Short Constriction Microchannel.

Electroosmotic flow (EOF) is of utmost significance due to its numerous practical uses in controlling flow at micro/nanoscales. In the present study, the time-periodic EOF of a viscoelastic fluid is statistically analyzed using a short 10:1 constriction microfluidic channel joining two reservoirs on either side. The flow is modeled using the Oldroyd-B (OB) model and the Poisson-Boltzmann model. The EOF of a highly concentrated polyacrylamide (PAA) aqueous solution is investigated under the combined effects of an alternating current (AC) electric field and a direct current (DC) electric field. Power-law degradation is visible in the energy spectra of the velocity fluctuations over a wide frequency range, pointing to the presence of elastic instabilities in the EOF. The energy-spectra curves of the velocity fluctuations under a DC electric field exhibit peaks primarily beneath 20 Hz, with the greatest peak being observed close to 6 Hz. When under both DC and AC electric fields, the energy spectra of the velocity fluctuations exhibit a peak at the same frequency as the AC electric field, and the highest peak is obtained when the frequency of the AC electric field is near 6 Hz. Additionally, the frequency of the AC electric field affects how quickly the viscoelastic EOF flows. Higher flow rates are obtained at relatively low frequencies compared to under the DC electric field, and the greatest flow rate is found close to 6 Hz. But as the frequency rises further, the flow rate falls. The flow rate falls to a level below the DC electric field when the frequency is sufficiently high.

An automated microfluidic system with one-dimensional beads array for multiplexed torch detection at point-of-care testing.

An automated microfluidic system with functionalized beads has been developed for multiplexed TORCH detection at point-of-care testing. A concise microfluidic chip consisting of a one-dimensional beads array is developed to simultaneously detect TOX, RUB, CMV, HSV-I and HSV-II respectively with five functionalized beads. A compact liquid handling module has been developed to automate the sandwiched chemiluminescence immunoassay within the one-dimensional beads array of the microfluidic chip. A precise ram pump is adopted to not only add reagent into the microfluidic chip from outside, but also facilitate elaborate fluid control inside the microfluidic chip for improved performance. A large-size waste chamber with a liquid-absorbing sponge holds the waste reagent within the microfluidic chip to prevent backflow. The one-dimensional beads array is heated from double-sides at 37 ℃ for sensitive detection with reduced time. A sensitive CMOS camera is adopted to take chemiluminescence image from the one-dimensional beads array, and a custom processing algorithm is adopted to analyze the image. For each serum sample, five different infections can be simultaneously detected with the automated microfluidic system. Experimental results show that efficient, sensitive, and accurate multiplexed TORCH detection can be conveniently achieved with the integrated microfluidic system.

Editorial for the Special Issue on Micromachines for Non-Newtonian Microfluidics.

Microfluidics has seen a remarkable growth over the past few decades, with its extensive applications in engineering, medicine, biology, chemistry, etc [...].

Electroosmotic Flow of Viscoelastic Fluid through a Constriction Microchannel.

Electroosmotic flow (EOF) has been widely used in various biochemical microfluidic applications, many of which use viscoelastic non-Newtonian fluid. This study numerically investigates the EOF of viscoelastic fluid through a 10:1 constriction microfluidic channel connecting two reservoirs on either side. The flow is modelled by the Oldroyd-B (OB) model coupled with the Poisson-Boltzmann model. EOF of polyacrylamide (PAA) solution is studied as a function of the PAA concentration and the applied electric field. In contrast to steady EOF of Newtonian fluid, the EOF of PAA solution becomes unstable when the applied electric field (PAA concentration) exceeds a critical value for a fixed PAA concentration (electric field), and vortices form at the upstream of the constriction. EOF velocity of viscoelastic fluid becomes spatially and temporally dependent, and the velocity at the exit of the constriction microchannel is much higher than that at its entrance, which is in qualitative agreement with experimental observation from the literature. Under the same apparent viscosity, the time-averaged velocity of the viscoelastic fluid is lower than that of the Newtonian fluid.

Inertial migration of aerosol particles in three-dimensional microfluidic channels.

In recent years, manipulation of particles by inertial microfluidics has attracted significant attention. However, most studies focused on inertial focusing of particles suspended within liquid phase, in which the ratio of the density of the particle to that of the medium is O(1). The investigation on manipulation of aerosol particles in an inertial microfluidics is very limited. In this study, we numerically investigate the aerosol particle's motion in a 3D straight microchannel with rectangular cross section by fully resolved simulation of the particle-air flow. The air flow is modeled by the Navier-Stokes equations. The particle's motions, including translation and rotation, are governed, respectively, by the Newton's second law and the Euler equations without using any approximation models for the lift and drag forces. The coupled mathematical model is numerically solved by combining immersed boundary with lattice Boltzmann method (IB-LBM). We find that the Reynolds number (Re), the particle's initial position, particle's density and diameter are the influential parameters in this process. The equilibrium positions and their stabilities of aerosols are different from those suspended in liquid.

Editorial for the Special Issue on Micro/Nano-Chip Electrokinetics, Volume III.

With the support from contributors and the help from peer reviewers, the Special Issue on Micro/Nano-Chip Electrokinetics (Volume III) published fourteen regular research articles and one review article [...].

Free convective PCR: From principle study to commercial applications-A critical review.

Polymerase chain reaction (PCR) is an extremely important tool for molecular diagnosis, as it can specifically amplify nucleic acid templates for sensitive detection. As another division of PCR, free convective PCR was invented in 2001, which can be performed in a capillary tube pseudo-isothermally within a significantly short time. Convective PCR thermal cycling is implemented by inducing thermal convection inside the capillary tube, which stratifies the reaction into spatially separate and stable melting, annealing, and extension zones created by the temperature gradient. Convective PCR is a promising tool that can be used for nucleic acid diagnosis as a point-of-care test (POCT) due to the significantly simplified heating strategy, reduced cost, and shortened detection time without sacrificing sensitivity and accuracy. Here, we review the history of free convective PCR from its invention to development and its commercial applications.

Tunable-Focus Liquid Lens through Charge Injection.

Liquid lenses are the simplest and cheapest optical lenses, and various studies have been conducted to develop tunable-focus liquid lenses. In this study, a simple and easily implemented method for achieving tunable-focus liquid lenses was proposed and experimentally validated. In this method, charges induced by a corona discharge in the air were injected into dielectric liquid, resulting in "electropressure" at the interface between the air and the liquid. Through a 3D-printed U-tube structure, a tunable-focus liquid lens was fabricated and tested. Depending on the voltage, the focus of the liquid lens can be adjusted in large ranges (-∞ to -9 mm and 13.11 mm to ∞). The results will inspire various new liquid-lens applications.

Biomimetic metal-organic nanoparticles prepared with a 3D-printed microfluidic device as a novel formulation for disulfiram-based therapy against breast cancer.

Disulfiram (DSF) is currently tested in several clinical trials for cancer treatment in combination with copper (Cu) ions. Usually, DSF and Cu are administered in two separate formulations. In the body, DSF and Cu ions form diethyldithiocarbamate copper complex [Cu(DDC)2] which has potent antitumor activities. However, the "two formulation" approach often achieved low Cu(DDC)2 concentration at tumor regions and resulted in compromised anticancer efficacy. Therefore, preformed Cu(DDC)2 complex administered in a single formulation will have better anticancer efficacy. However, the poor aqueous solubility of Cu(DDC)2 is a significant challenge for its clinical use. In this work, a biomimetic nanoparticle formulation of Cu(DDC)2 was produced with a novel SMILE ( Stabilized Metal Ion Ligand complex) method developed in our laboratory to address the drug delivery challenges. The Metal-organic Nanoparticle (MON) is composed of Cu(DDC)2 metal-organic complex core and surface decorated bovine serum albumin (BSA). Importantly, we designed a 3D-printed microfluidic device to further improve the fabrication of BSA/Cu(DDC)2 MONs. This method could precisely control the MON preparation process and also has great potential for large scale production of Cu(DDC)2 MON formulations. We also used a computational modeling approach to simulate the MON formation process in the microfluidic device. The optimized BSA/Cu(DDC)2 MONs demonstrated good physicochemical properties. The MONs also showed potent antitumor activities in the breast cancer cell monolayers as well as the 3D-cultured tumor spheroids. The BSA/Cu(DDC)2 MONs also effectively inhibited the growth of tumors in an orthotopic 4T1 breast tumor model. This current study provided a novel method to prepare a biomimetic MON formulation for DSF/Cu cancer therapy.

Electroosmotic Flow of Viscoelastic Fluid in a Nanochannel Connecting Two Reservoirs.

: Electroosmotic flow (EOF) of viscoelastic fluid with Linear Phan-Thien-Tanner (LPTT) constitutive model in a nanochannel connecting two reservoirs is numerically studied. For the first time, the influence of viscoelasticity on the EOF and the ionic conductance in the micro-nanofluidic interconnect system, with consideration of the electrical double layers (EDLs), is investigated. Regardless of the bulk salt concentration, significant enhancement of the flow rate is observed for viscoelastic fluid compared to the Newtonian fluid, due to the shear thinning effect. An increase in the ionic conductance of the nanochannel occurs for the viscoelastic fluid. The enhancement of the ionic conductance is significant under the overlapping EDLs condition.

Mechanical response of an epithelial island subject to uniaxial stretch on a hybrid silicone substrate.

INTRODUCTION: The mechanical response of large multi-cellular collectives to external stretch has remained largely unexplored, despite its relevance to normal function and to external challenges faced by some tissues. Here, we introduced a simple hybrid silicone substrate to enable external stretch while providing a physiologically relevant physical micro-environment for cells. METHODS: We micropatterned epithelial islands on the substrate using a stencil to allow for a circular island shape without restraining island edges. We then used traction force microscopy to determine the strain energy and the inter-cellular sheet tension within the island as a function of time after stretch. RESULTS: While the strain energy stored in the substrate for unstretched cell islands stayed constant over time, a uniaxial 10% stretch resulted in an abrupt increase, followed by sustained increase in the strain energy of the islands over tens of minutes, indicating slower dynamics than for single cells reported previously. The sheet tension at the island mid-line perpendicular to the stretch direction also more than doubled compared to unstretched islands. Interestingly, the sheet tension at the island mid-line parallel to the stretch direction also reached similar levels over tens of minutes indicating the tendency of the island to homogenize its internal stress. CONCLUSIONS: We found that the sheet tension within large epithelial islands depends on its direction relative to that of the stretch initially, but not at longer times. We suggest that the hybrid silicone substrate provides for an accessible substrate for studying the mechanobiology of large epithelial cell islands.

Tuning Ion Transport through a Nanopore by Self-Oscillating Chemical Reactions.

Ion transport in nanofluidic devices and biological ion channels are highly dependent on the local environmental conditions in the electrolyte solution. Many life processes in living systems are in dynamic electrolyte solutions, and many of them are self-oscillated. Tuning ion transport through a nanofluidic diode by the self-oscillating chemical reactions is demonstrated by modeling the electrokinetic ion transport process with a validated continuum model, which includes the time-dependent Poisson-Nernst-Planck equations for the ionic mass transport of multiple ionic species with both volumetric and surface chemical reactions, and Stokes equations for the flow field. A pH oscillator caused by oscillating chemical reactions (i.e., bromate-sulfite-ferrocyanide system) is added at the tip side of the nanopore to periodically change its surface charge properties, consequently tuning the ion selectivity and ion transport through the nanopore. Results show that both the surface charge density of the nanopore and the electrokinetic ion transport phenomena oscillate simultaneously with the pH oscillation generated by the self-oscillating chemical reactions. The numerical results obtained by our model qualitatively agree with the published experimental observations.

Preparation of giant lipid vesicles with controllable sizes by a modified hydrophilic polydimethylsiloxane microarray chip.

This paper presents an accessible method to prepare giant lipid vesicles (GLVs) with controllable sizes based on the quick formation of patterned lipid films. Lipid solutions naturally penetrate into arrayed micro-apertures on a modified hydrophilic Polydimethylsiloxane (PDMS) chip, and excess lipid films on the surface are removed by a glass slide. Three main factors, the depth and diameter of the micro-apertures and concentration of the lipid solution, were investigated to obtain an optimal preparation condition. Based on this condition, the formed GLVs have a controllable size and narrow size distribution (the standard deviation < 5 µm). By controlling the diameter of the micro-aperture and concentration of the lipid solution, GLVs with various sizes (23, 48, 66 and 82 µm) can be formed.

Electroosmotic Flow of Viscoelastic Fluid in a Nanoslit.

The electroosmotic flow (EOF) of viscoelastic fluid in a long nanoslit is numerically studied to investigate the rheological property effect of Linear Phan-Thien-Tanner (LPTT) fluid on the fully developed EOF. The non-linear Poisson-Nernst-Planck equations governing the electric potential and the ionic concentration distribution within the channel are adopted to take into account the effect of the electrical double layer (EDL), including the EDL overlap. When the EDL is not overlapped, the velocity profiles for both Newtonian and viscoelastic fluids are plug-like and increase sharply near the charged wall. The velocity profile resembles that of pressure-driven flow when the EDL is overlapped. Regardless of the EDL thickness, apparent increase of velocity is obtained for viscoelastic fluid of larger Weissenberg number compared to the Newtonian fluid, indicating the shear thinning behavior of the LPTT fluid. The effect of the Weissenberg number on the velocity distribution is less significant as the degree of EDL overlapping increases, due to the overall decrease of the shear rate. The increase (decrease) of polymer extensibility (viscosity ratio) also enhances the EOF of viscoelastic fluid.

Stiffness Measurement of Soft Silicone Substrates for Mechanobiology Studies Using a Widefield Fluorescence Microscope.

Soft tissues in the human body typically have stiffness in the kilopascal (kPa) range. Accordingly, silicone and hydrogel flexible substrates have been proven to be useful substrates for culturing cells in a physical microenvironment that partially mimics in vivo conditions. Here, we present a simple protocol for characterizing the Young's moduli of isotropic linear elastic substrates typically used for mechanobiology studies. The protocol consists of preparing a soft silicone substrate on a Petri dish or stiff silicone, coating the top surface of the silicone substrate with fluorescent beads, using a millimeter-scale sphere to indent the top surface (by gravity), imaging the fluorescent beads on the indented silicone surface using a fluorescence microscope, and analyzing the resultant images to calculate the Young's modulus of the silicone substrate. Coupling the substrate's top surface with a moduli extracellular matrix protein (in addition to the fluorescent beads) allows the silicone substrate to be readily used for cell plating and subsequent studies using traction force microscopy experiments. The use of stiff silicone, instead of a Petri dish, as the base of the soft silicone, enables the use of mechanobiology studies involving external stretch. A specific advantage of this protocol is that a widefield fluorescence microscope, which is commonly available in many labs, is the major equipment necessary for this procedure. We demonstrate this protocol by measuring the Young's modulus of soft silicone substrates of different elastic moduli.

Effect of pharmacological modulation of actin and myosin on collective cell electrotaxis.

Electrotaxis-the directional migration of cells in response to an electric field-is most evident in multicellular collectives and plays an important role in physiological contexts. While most cell types respond to applied electric fields of the order of a Volt per centimeter, our knowledge of the factors influencing this response is limited. This is especially true for collective cell electrotaxis, in which the subcellular migration response within a cell has to be coordinated with coupled neighboring cells. Here, we investigated the effect of the level of actin cytoskeleton polymerization and myosin activity on collective cell electrotaxis of Madin-Darby Canine Kidney (MDCK) cells in response to a weak electric field of physiologically relevant magnitude. We modulated the polymerization state of the actin cytoskeleton using the depolymerizing agent cytochalasin D or the polymerizing agent jasplakinolide. We also modulated the contractility of the cell using the myosin motor inhibitor blebbistatin or the phosphatase inhibitor calyculin A. While all the above pharmacological treatments altered cell speed to various extents, we found that only increasing the contractility and a high level of increase/stabilization of polymerized actin had a strong inhibitory effect specifically on the directedness of collective cell electrotaxis. On the other hand, even as the effect of the actin modulators on collective cell migration was varied, most conditions of actin and myosin pharmacological modulation-except for high level of actin polymerization/stabilization-resulted in cell speeds that were similar in the absence or presence of the electric field. Our results led us to speculate that the applied electric field may largely impact the cellular apparatus specifying the polarity of collective cell migration, rather than the functioning of the migratory apparatus. Bioelectromagnetics. 39:289-298, 2018. © 2018 Wiley Periodicals, Inc.

Dynamic control of particle separation in deterministic lateral displacement separator with viscoelastic fluids.

We proposed an innovative method to achieve dynamic control of particle separation by employing viscoelastic fluids in deterministic lateral displacement (DLD) arrays. The effects of shear-thinning and elasticity of working fluids on the critical separation size in DLD arrays are investigated. It is observed that each effect can lead to the variation of the critical separation size by approximately 40%. Since the elasticity strength of the fluid is related to the shear rate, the dynamic control can for the first time be easily realized through tuning the flow rate in microchannels.

Rapid enumeration of CD4 + T lymphocytes using an integrated microfluidic system based on Chemiluminescence image detection at point-of-care testing.

An integrated microfluidic system has been developed for rapid enumeration of CD4 + T lymphocytes at point-of-care (POC) settings. A concise microfluidic chip, which consists of three separate chambers, respectively, for reaction, detection and waste storage, is developed to automate CD4 detection. To simplify CD4 + T lymphocyte enumeration, a single polycarbonate bead immobilized with CD4 antibody is adopted by the microfluidic chip to capture the CD4 antigen in the lysed testing sample. Desired performance is achieved by actuating the single bead for efficient mixing, as well as transferring it between different reaction chambers to reduce non-specific reaction. A controllable external magnetic field is applied to drive the single bead with a built-in ferrous core for different purposes. Chemiluminescence reaction is implemented in an independent chamber to reduce non-specific binding of enzyme. A simple flow control strategy is adopted to conveniently release the waste reagent into the waste storage chamber by just opening the vent hole without actively pumping. A sensitive CCD camera is used to collect the reaction signal by taking picture from the single bead, and then the signal intensity is further analyzed for CD4 + T lymphocyte enumeration. Experimental results show that rapid, convenient, accurate and low-cost CD4 + T lymphocyte enumeration can be obtained with the developed microfluidic system at POC test.

Energy dissipation mapping of cancer cells.

The purpose of this study is to map the energy dissipation of Jurkat cells using a single 60 nanosecond pulse electric field (NsPEF), primarily through atomic force microscopy (AFM). The phase shift is generated by the sample elements that do not have a heterogeneous surface. Monitoring and manipulating the phase shift is a powerful way for determining the dissipated energy and plotting the topography. The dissipated energy is a relative value, so the silica wafer and cover slip are given a set reference while the transmission of energy between the tip of the cantilever and cell surfaces is measured. The most important finding is that the magnitude and the number of variations in the dissipated energy change with the strength of NsPEF applied. Utilizing a single low field strength NsPEF (15kV/cm), minor changes in dissipated energy were found. The application of a single high field strength NsPEF (60kV/cm) to Jurkat cells resulted in a higher dissipated energy change versus that of in the low field strength condition. Thus, the dissipated energy from the Jurkat cells changes with the strength of NsPEF. By analyzing the forces via investigation in the tapping mode of the AFM, the stabilization of the cytoskeleton and membrane of the cell are related to the strength of NsPEF applied. Furthermore, the strength of NsPEF indicates a meaningful relationship to the survival of the Jurkat cells.