Flow mechanotransduction regulates traction forces, intercellular forces, and adherens junctions

Endothelial cells respond to fluid shear stress through mechanotransduction responses that affect their cytoskeleton and cell-cell contacts. Here, endothelial cells were grown as monolayers on arrays of microposts and exposed to laminar or disturbed flow to examine the relationship among traction forces, intercellular forces, and cell-cell junctions. Cells under laminar flow had traction forces that were higher than those under static conditions, whereas cells under disturbed flow had lower traction forces. The response in adhesion junction assembly matched closely with changes in traction forces since adherens junctions were larger in size for laminar flow and smaller for disturbed flow. Treating the cells with calyculin-A to increase myosin phosphorylation and traction forces caused an increase in adherens junction size, whereas Y-27362 cause a decrease in their size. Since tugging forces across cell-cell junctions can promote junctional assembly, we developed a novel approach to measure intercellular forces and found that these forces were higher for laminar flow than for static or disturbed flow. The size of adherens junctions and tight junctions matched closely with intercellular forces for these flow conditions. These results indicate that laminar flow can increase cytoskeletal tension while disturbed flow decreases cytoskeletal tension. Consequently, we found that changes in cytoskeletal tension in response to shear flow conditions can affect intercellular tension, which in turn regulates the assembly of cell-cell junctions.     

A mathematical model for the dissolution of non-occlusive blood clots in fast tangential blood flow

Our aim was to study the effect of an axially directed blood plasma flow on the dissolution rate of cylindrical non-occlusive blood clots in an in vitro flow system and to derive a mathematical model for the process. The model was based on the hypothesis that clot dissolution dynamics is proportional not only to the biochemical proteolysis of fibrin but also to the power of the flowing blood plasma dissipated along the clot. The predicted rate of thrombolysis is then proportional to the square of the average blood plasma velocity for laminar flow and to the third power of the average velocity for turbulent flow. To verify the model, the time dependence of the clot cross-sectional area was measured by dynamic magnetic resonance microscopy during fast (turbulent) and slow (laminar) flow of plasma through an axially directed channel along the clot. The flowing plasma contained a magnetic resonance imaging contrast agent (Gd-DTPA) and a thrombolytic agent (recombinant tissue-type plasminogen activator). The experimental data fitted well to the model, and confirmed the predicted increase in the dissolution rate when blood flow changed from a laminar to a turbulent flow regime.     

Flutter in collapsible tubes: a theoretical model of wheezes

A mathematical analysis of flow through a flexible channel is examined as a model of flow-induced flutter oscillations that pertain to the production of wheezing breath sounds. The model provides predictions for the critical fluid speed that will initiate flutter waves of the wall, as well as their frequency and wavelength. The mathematical results are separated into linear theory (small oscillations) and nonlinear theory (larger oscillations). Linear theory determines the onset of the flutter, whereas nonlinear theory determines the relationships between the fluid speed and both the wave amplitudes and frequencies. The linear theory predictions correlate well with data taken at the onset of flutter and flow limitation during experiments of airflow in thick-walled collapsible tubes. The nonlinear theory predictions correlate well with data taken as these flows are forced to higher velocities while keeping the flow rate constant. Particular ranges of the parameters are selected to investigate and discuss the applications to airway flows. According to this theory, the mechanism of generation of wheezes is based in the interactions of fluid forces and friction and wall elastic-restoring forces and damping. In particular, a phase delay between the fluid pressure and wall motion is necessary. The wave speed theory of flow limitation is discussed with respect to the specific data and the flutter model.     

Orientation-Based Control of Microfluidics

Most microfluidic chips utilize off-chip hardware (syringe pumps, computer-controlled solenoid valves, pressure regulators, etc.) to control fluid flow on-chip. This expensive, bulky, and power-consuming hardware severely limits the utility of microfluidic instruments in resource-limited or point-of-care contexts, where the cost, size, and power consumption of the instrument must be limited. In this work, we present a technique for on-chip fluid control that requires no off-chip hardware. We accomplish this by using inert compounds to change the density of one fluid in the chip. If one fluid is made 2% more dense than a second fluid, when the fluids flow together under laminar flow the interface between the fluids quickly reorients to be orthogonal to Earth’s gravitational force. If the channel containing the fluids then splits into two channels, the amount of each fluid flowing into each channel is precisely determined by the angle of the channels relative to gravity. Thus, any fluid can be routed in any direction and mixed in any desired ratio on-chip simply by holding the chip at a certain angle. This approach allows for sophisticated control of on-chip fluids with no off-chip control hardware, significantly reducing the cost of microfluidic instruments in point-of-care or resource-limited settings.     

Development of a pH gradient within a biofilm is dependent upon the limiting nutrient

Monospecies Citrobacter sp. biofilms were grown in a laminar flow cell using a carbon-limiting medium. Microelectrode measurements showed no change in pH between the bulk fluid and biofilm when the flow cell was supplied with the carbon-limiting medium under static or flowing conditions. When the biofilm was supplied with a phosphate-limiting medium the biofilm became more acidic than the bulk fluid and developed a gradient within. The implications for metals-bioremediation processes are discussed.     

Fractionation of macromolecules in an alternating transverse electric field: simulation of the method

An electric field of alternating polarity applied in a direction transverse to the direction of solute transport is used as the basis of a method for the separation of biological macromolecules. The method derives directly from the ability of an electric field to induce movement of a charged macromolecule and from the physics of laminar fluid flow; no adsorptive immobile phase component is involved.

The method is simulated by computer for the case of solute molecules in a solvent flowing through a narrow chamber of recta generates an electric field orthogonal to the direction of solvent flow. Solute molecules repetitively traverse the solvent channel at rates determined by their electrophoretic mobility. During the transit across the channel, solute molecules are transported in the direction of solvent flow; at the channel wall, solvent velocity is negligible and solute transport is limited to that provided by transient diffusion into a mobile solvent zone. Molecules of different intrinsic electrophoretic mobility are separated.

The computer model was used to illustrate the process and to demonstrate the ‘tunability’ of the method as a function of the oscillation frequency and voltage wave form. Because of this tunability, a single instrument can function as the equivalent of several different chromatographic systems. Because fractionation is effected by direct physicochemical phenomena rather than via interaction with chromatographic sites, variations in fractionation results arising from formation of polymers for gel electrophoresis, packing of chromatography columns, or deterioration of columns with use are avoided. This method may be of particular use for the purification of nucleic acid fragments and for the analysis of protei: nucleic acid interactions.     

Fabrication,Operation and Flow Visualization in Surface-acoustic-wave-driven Acoustic-counterflow Microfluidics

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we present the fabrication protocol for a multilayered SAW acoustic counterflow device. The device is fabricated starting from a lithium niobate (LN) substrate onto which two interdigital transducers (IDTs) and appropriate markers are patterned. A polydimethylsiloxane (PDMS) channel cast on an SU8 master mold is finally bonded on the patterned substrate. Following the fabrication procedure, we show the techniques that allow the characterization and operation of the acoustic counterflow device in order to pump fluids through the PDMS channel grid. We finally present the procedure to visualize liquid flow in the channels. The protocol is used to show on-chip fluid pumping under different flow regimes such as laminar flow and more complicated dynamics characterized by vortices and particle accumulation domains.     

Cytoplasmic motion induced by cytoskeleton stretching and its effect on cell mechanics

Cytoplasmic motion assumed as a steady state laminar flow induced by cytoskeleton stretching in a cell is determined and its effect on the mechanical behavior of the cell under externally applied forces is demonstrated. Non-Newtonian fluid is assumed for the multiphase cytoplasmic fluid and the analytical velocity field around the macromolecular chain is obtained by solving the reduced nonlinear momentum equation using homotopy technique. The entropy generation by the fluid internal friction is calculated and incorporated into the entropic elasticity based 8-chain constitutive relations. Numerical examples showed strengthening behavior of cells in response to externally applied mechanical stimuli. The spatial distribution of the stresses within a cell under externally applied fluid flow forces were also studied.     

Fabrication and use of a transient contractional flow device to quantify the sensitivity of mammalian and insect cells to hydrodynamic forces

A microfluidic device was fabricated via photolithographic techniques which can create transient elongational and shear forces ranging over three orders of magnitude while still maintaining laminar flow conditions. The contractional fluid flow inside the microfluidic device was simulated with FLUENT (a computational fluid dynamics computer program) and the local deformation forces were characterized with the scalar quantity, local energy dissipation rate. The sensitivities of four cell lines (CHO, HB-24, Sf-9, and MCF7) were tested in the device. The results indicate that all four cell lines are able to withstand relatively intense energy dissipation rates (up to 10(4)-10(5) kW/m(3)), which is orders of magnitude higher than the maximum local energy dissipation rates generated by impellers in bioreactors, but comparable to that associated with small bursting bubbles. While the concept that suspended animal cells are relatively robust with respect to purely hydrodynamic forces in bioprocess equipment is well known, these results quantitatively demonstrate these observations.     

Theoretical analysis of molecular diffusion in pressure-driven laminar flow in microfluidic channels

The T-sensor is a microfluidic analytical device that operates at low Reynolds numbers to ensure entirely laminar flow. Diffusion of molecules between streams flowing side by side may be observed directly. The pressure-driven velocity profile in the duct-shaped device influences diffusive transport in ways that affect the use of the T-sensor to measure molecular properties. The primary effect is a position-dependent variation in the extent of diffusion that occurs due to the distribution of residence time among different fluid laminae. A more detailed characterization reveals that resultant secondary concentration gradients yield variations in the scaling behavior between diffusive displacement and elapsed time in different regions of the channel. In this study, the time-dependent evolution of analyte distribution has been quantified using a combination of one- and two-dimensional models. The results include an accurate portrayal of the shape of the interdiffusion region in a representative T-sensor assay, calculation of the diffusive scaling law across the width of the channel, and quantification of artifacts that occur when making diffusion coefficient measurements in the T-sensor.     

Introducing dielectrophoresis as a new force field for field-flow fractionation.

We present the principle of cell characterization and separation by dielectrophoretic field-flow fractionation and show preliminary experimental results. The operational device takes the form of a thin chamber in which the bottom wall supports an array of microelectrodes. By applying appropriate AC voltage signals to these electrodes, dielectrophoretic forces are generated to levitate cells suspended in the chamber and to affect their equilibrium heights. A laminar flow profile is established in the chamber so that fluid flows faster with increasing distance from the chamber walls. A cell carried in the flow stream will attain an equilibrium height, and a corresponding velocity, based on the balance of dielectrophoretic, gravitational, and hydrodynamic lift forces it experiences. We describe a theoretical model for this system and show that the cell velocity is a function of the mean fluid velocity, the voltage and frequency of the signals applied to the electrodes, and, most significantly, the cell dielectric properties. The validity of the model is demonstrated with human leukemia (HL-60) cells subjected to a parallel electrode array, and application of the device to separating HL-60 cells from peripheral blood mononuclear cells is shown.     

Three-dimensional computational model of left heart diastolic function with fluid-structure interaction

Aided by advancements in computer speed and modeling techniques, computational modeling of cardiac function has continued to develop over the past twenty years. The goal of the current study was to develop a computational model that provides blood-tissue interaction under physiologic flow conditions, and apply it to a thin-walled model of the left heart. To accomplish this goal, the Immersed Boundary Method was used to study the interaction of the tissue and blood in response to fluid forces and changes in tissue pathophysiology. The fluid mass and momentum conservation equations were solved using Patankar's Semi-Implicit Method for Pressure Linked Equations (SIMPLE). A left heart model was developed to examine diastolic function, and consisted of the left ventricle, left atrium, and pulmonary flow. The input functions for the model included the pulmonary driving pressure and time-dependent relationship for changes in chamber tissue properties during the simulation. The results obtained from the left heart model were compared to clinically observed diastolic flow conditions for validation. The inflow velocities through the mitral valve corresponded with clinical values (E-wave = 74.4 cm/s, A-wave = 43 cm/s, and E/A = 1.73). The pressure traces for the atrium and ventricle, and the appearance of the ventricular flow fields throughout filling, agreed with those observed in the heart. In addition, the atrial flow fields could be observed in this model and showed the conduit and pump functions that current theory suggests. The ability to examine atrial function in the present model is something not described previously in computational simulations of cardiac function.     

Three Dimensional Modeling of the Cerebrospinal Fluid Dynamics and Brain Interactions in the Aqueduct of Sylvius

A computational fluid dynamics (CFD) method is presented to investigate the flow of cerebro-spinal fluid (CSF) in the cerebral aqueduct. In addition to former approaches exhibiting a rigid geometry, we propose a model which includes a deformable membrane as the wall of this flow channel. An anatomical shape of the aqueduct was computed from magnetic resonance images (MRI) and the resulting meshing was immersed in a marker-and-cell (MAC) staggered grid for to take into account fluid–structure interactions. The time derivatives were digitized using the Crank–Nicolson scheme. The equation of continuity was modified by introducing an artificial compressibility and digitized by a finite difference scheme.

Calculations were validated with the simulation of laminar flow in a rigid tube. Then, comparisons were made between simulations of a rigid aqueduct and a deformable one. We found that the deformability of the walls has a strong influence on the pressure drop for a given flow.     

Direct visualizations of air flow in the human upper airway using in-vitro models

A better understanding of airflow characteristics in the upper airway(UA) is crucial in investigating obstructive sleep apnea(OSA), particle sedimentation, drug delivery, and many biomedical problems. Direct visualization of air flow patterns in in-vitro models with realistic anatomical structures is a big challenge. In this study, we constructed unique half-side transparent physical models of normal UA based on realistic anatomical structures. A smoke-wire method was developed to visualize the air flow in UA models directly. The results revealed that the airflow through the pharynx was laminar but not turbulent under normal inspiration, which suggested that compared with turbulent models, a laminar model should be more suitable in numerical simulations. The flow predicted numerically using the laminar model was consistent with the observations in the physical models. The comparison of the velocity fields predicted numerically using the half-side and complete models confirmed that it was reasonable to investigate the flow behaviors in UA using the half-side model. Using the laminar model, we simulated the flow and evaluated the effects of UA narrowing caused by rostral fluid shift on pharyngeal resistance. The results suggested that fluid shift could play an important role in the formation of hypopnea or OSA during sleep.     

On the mechanism of the formation of magnetohydrodynamic vortices in the solar plasma

Based on the magnetohydrodynamic (MHD) equations for an incompressible conductive viscous fluid, the possible mechanism of the formation of giant MHD vortices recently discovered in the solar atmosphere (chromosphere) is analyzed. It is assumed that these vortices arise in the regions of the solar surface (photosphere) with ascending flows of hot plasma that arrives from the inner regions of the Sun as a result of thermal convection and is accelerated upward under the action of the chromospheric plasma pressure gradient. It is shown that, under the assumption of plasma incompressibility and flow continuity, the ascending plasma flows induce converging radial plasma flows, which create the convective and Coriolis nonlinear hydrodynamic forces due to the nonzero initial vorticity of the chromospheric plasma caused by Sun’s rotation. The combined action of these two forces leads to the exponential acceleration of the solid-body rotation of plasma inside the ascending flow, thereby creating a vortex that generates an axial magnetic field, in agreement with astrophysical observations.     

Using a CFD model to understand the fluid dynamics promoting E. coli breakage in a high-pressure homogenizer

A Computational Fluid Dynamic (CFD) model of flow in a high-pressure homogenizing valve (APV Gaulin model 30CD) was developed with the Fluent software. The 2D model consists of an unstructured hexagonal mesh, dense in the regions of high gradients. The flow (single-phase) was modeled as laminar upstream of and in the channel (gap) and turbulent downstream of the channel exit. Applying a realizable kappa-epsilon turbulence model, the CFD model accurately predicted the effect of gap space on fluid dynamic conditions upstream (inlet pressure and pressure gradient) and downstream (impact pressure) of the channel for a valve with a standard (CD-0) impact distance (0.25 mm) and a 1 cP fluid. This CFD model was then used to estimate the magnitude of the fluid dynamic parameters (except cavitation effects) presumed to be responsible for cell breakage, as a function of gap space, impact distance and fluid viscosity. The CFD models predicted that for a given volumetric flowrate the upstream fluid conditions (inlet pressure gradient, maximum channel strain rate) and the maximum energy dissipation rate in the post-gap jet depend only on the gap space and the fluid viscosity and not on the impact distance. The impact pressure however depends on the gap spacing, the fluid viscosity and especially the impact distance. Experimental results indicate that higher inlet pressures are required to break cells, if the impact distance is increased. By conducting experiments to isolate individual cell breakage mechanisms for a single pass, threshold values were identified for breaking Escherichia coli cells: pressure gradient, 1.2 x 10(12) Pa/m; energy dissipation rate, 1.0 x 10(10) m(3)/s(2); and impact pressure, 160 psig. By isolating the wall impact as the sole mechanism responsible for breaking the E. coli cells between 3000 and 6000 psig inlet pressure, a relationship between E. coli cell breakage rate and maximum wall impact pressure was established (eq 5).     

Dynamics of rotating flows in plasma accelerator channels with a longitudinal magnetic field

Results are presented from numerical simulations of axisymmetric plasma flows that occur in a coaxial accelerator with a longitudinal magnetic field. The simulations were carried out based on a two-dimensional MHD plasma dynamic model for the general case of a three-component magnetic field. The steady plasma flows are calculated in solving the time-dependent MHD problem by the relaxation method. The results of simulations of steady transonic flows are compared with the solutions that were obtained in the smooth accelerator channel approximation. The main regular features of plasmodynamic processes are revealed. It is found that current sheets arise in the plasma flow in a comparatively strong longitudinal magnetic field.     

Three dimensional modeling of the cerebrospinal fluid dynamics and brain interactions in the aqueduct of sylvius

A computational fluid dynamics (CFD) method is presented to investigate the flow of cerebro-spinal fluid (CSF) in the cerebral aqueduct. In addition to former approaches exhibiting a rigid geometry, we propose a model which includes a deformable membrane as the wall of this flow channel. An anatomical shape of the aqueduct was computed from magnetic resonance images (MRI) and the resulting meshing was immersed in a marker-and-cell (MAC) staggered grid for to take into account fluid-structure interactions. The time derivatives were digitized using the Crank-Nicolson scheme. The equation of continuity was modified by introducing an artificial compressibility and digitized by a finite difference scheme. Calculations were validated with the simulation of laminar flow in a rigid tube. Then, comparisons were made between simulations of a rigid aqueduct and a deformable one. We found that the deformability of the walls has a strong influence on the pressure drop for a given flow.     

Quantitative determination of glucose transfer between cocurrent laminar water streams in a H‐shaped microchannel

To explore the applicability of a laminar fluid diffusion interface (LFDI) for the controlled feeding of microbioreactors, glucose diffusion experiments were carried out in a rounded H‐shaped microstructure etched in a glass substrate. The diffusion channel of the microstructure had a length of 4 mm and a depth of 50 μm with a trapezoidal cross section with a width of 100 μm at the bottom and 200 μm at the surface of the channel. The microchannel was operated at residence times of less than 1 s ensuring high‐mass‐transfer rates. It was confirmed, both by microscopic observations as well as computational fluid dynamics (CFD) studies that the flow characteristics in the microchannel were fully laminar. Special attention was paid to flow splitting at the end of the channel, because the CFD simulations indicated that the performance of the device was sensitive to unequal flow splitting. The difference in outflow volume of the two streams was measured to be small (1.25% ± 0.6%). The measured glucose concentration in both exit ports at a fixed residence time was found to be stable in time and reproducible in multiple experiments. CFD simulation was shown to be a powerful tool for estimating the mass transfer in the LFDI, even at very short residence times. The results obtained in this work show the applicability of LFDI for the controlled diffusive supply of a solute to a water stream, with as possible application substrate and/or precursor feeding to microreactors. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009