Negligible Shear Stress Research Articles

Bone Spheroid Development Under Flow Conditions with Mesenchymal Stem Cells and Human Umbilical Vein Endothelial Cells in a 3D Porous Hydrogel Supplemented with Hydroxyapatite.

Understanding the niche interactions between blood and bone through the in vitro co-culture of osteo-competent cells and endothelial cells is a key factor in unraveling therapeutic potentials in bone regeneration. This can be additionally supported by employing numerical simulation techniques to assess local physical factors, such as oxygen concentration, and mechanical stimuli, such as shear stress, that can mediate cellular communication. In this study, we developed a Mesenchymal Stem Cell line (MSC) and a Human Umbilical Vein Endothelial Cell line (HUVEC), which were co-cultured under flow conditions in a three-dimensional, porous, natural pullulan/dextran scaffold that was supplemented with hydroxyapatite crystals that allowed for the spontaneous formation of spheroids. After 2 weeks, their viability was higher under the dynamic conditions (>94%) than the static conditions (<75%), with dead cells central in the spheroids. Mineralization and collagen IV production increased under the dynamic conditions, correlating with osteogenesis and vasculogenesis. The endothelial cells clustered at the spheroidal core by day 7. Proliferation doubled in the dynamic conditions, especially at the scaffold peripheries. Lattice Boltzmann simulations showed negligible wall shear stress in the hydrogel pores but highlighted highly oxygenated zones coinciding with cell proliferation. A strong oxygen gradient likely influenced endothelial migration and cell distribution. Hypoxia was minimal, explaining high viability and spheroid maturation in the dynamic conditions.

User-friendly microfluidic system reveals native-like morphological and transcriptomic phenotypes induced by shear stress in proximal tubule epithelium.

Drug-induced nephrotoxicity is a leading cause of drug attrition, partly due to the limited relevance of pre-clinical models of the proximal tubule. Culturing proximal tubule epithelial cells (PTECs) under fluid flow to mimic physiological shear stress has been shown to improve select phenotypes, but existing flow systems are expensive and difficult to implement by non-experts in microfluidics. Here, we designed and fabricated an accessible and modular flow system for culturing PTECs under physiological shear stress, which induced native-like cuboidal morphology, downregulated pathways associated with hypoxia, stress, and injury, and upregulated xenobiotic metabolism pathways. We also compared the expression profiles of shear-dependent genes in our in vitro PTEC tissues to that of ex vivo proximal tubules and observed stronger clustering between ex vivo proximal tubules and PTECs under physiological shear stress relative to PTECs under negligible shear stress. Together, these data illustrate the utility of our user-friendly flow system and highlight the role of shear stress in promoting native-like morphological and transcriptomic phenotypes in PTECs in vitro, which is critical for developing more relevant pre-clinical models of the proximal tubule for drug screening or disease modeling.

Changes in Energy Metabolism and Cell Damage in RPTEC‐TERT1 Cells Exposed to Hypoxia or Ischemia after Shear Stress Conditioning

Study Objective Renal proximal tubules are exposed in vivo to physiological levels of shear stress. We aim to determine how preconditioning human proximal tubule cells with shear stress affects their sensibility to hypoxia or ischemia. Hypothesis Physiological levels of shear stress would induce further differentiation and metabolic specialization resulting in changes in their response to low oxygen or nutrient conditions. Methods Immortalized human proximal tubule cells (RPTEC-TERT1 from Evercyte) were grown to confluence in Ibidi µSlides and then exposed for three days to two flow regimes providing negligible (0.01 dyn/cm2) or physiological shear stress (0.3 dyn/cm2). Hypoxia or ischemic conditions were applied for 16h, by keeping cells under 1%O2 alone or together with glucose deprivation. Results RPTEC-TERT1 cells were first adapted to a culturing regime under 5 mM glucose culture medium, instead of 17 mM glucose present in the medium recommended by this cell line vendor. Proliferation rates and expression of phenotypic markers were unchanged. RPTEC-TERT1 cells switched from proliferative-glycolytic metabolism to non-proliferative-oxidative metabolism upon reaching confluency, approximately in 6-7 days. Seven days confluent, not-perfused RPTEC-TERT1 cells exposed to hypoxia or ischemia for 16 hours exhibited 58% and 75% mortality, respectively. In cells perfused for 3 days, preconditioning with shear stress reduced their sensitivity to hypoxia (mortality = 51% no-SS vs. 35% SS) and ischemia (mortality = 83% no-SS vs. 75% SS). Shear stress conditioning increased expression of genes related to energy metabolism and cells stress (AMPK alpha-2, HIF-1alpha, KIM-1) Conclusions Preconditioning of RPTEC-TERT1 by exposing them to physiological shear-stress modifies expression of genes related to mechanisms used to cope with cellular stress and this may explain the reduction in cell mortality when exposed to hypoxia or oxygen-glucose deprivation.

A new model of shoaling and breaking waves: one-dimensional solitary wave on a mild sloping beach

We present a new approach to model coastal waves in the shoaling and surf zones. The model can be described as a depth-averaged large-eddy simulation model with a cutoff in the inertial subrange. The large-scale turbulence is explicitly resolved through an extra variable called enstrophy while the small-scale turbulence is modelled with a turbulent-viscosity hypothesis. The equations are derived by averaging the mass, momentum and kinetic energy equations assuming a shallow-water flow, a negligible bottom shear stress and a weakly turbulent flow assumption which is not restrictive in practice. The model is fully nonlinear and has the same dispersive properties as the Green–Naghdi equations. It is validated by numerical tests and by comparison with experimental results of the literature on the propagation of a one-dimensional solitary wave over a mild sloping beach. The wave breaking is characterized by a sudden increase of the enstrophy which allows us to propose a breaking criterion based on the new concept of virtual enstrophy. The model features three empirical parameters. The first one governs the turbulent dissipation and was found to be a constant. The eddy viscosity is determined by a turbulent Reynolds number depending only on the bottom slope. The third parameter defines the breaking criterion and depends only on the wave initial nonlinearity. These dependences give a predictive character to the model which is suitable for further developments.

Large-scale microfluidic gradient arrays reveal axon guidance behaviors in hippocampal neurons

High-throughput quantitative approaches to study axon growth behaviors have remained a challenge. We have developed a 1024-chamber microfluidic gradient generator array that enables large-scale investigations of axon guidance and growth dynamics from individual primary mammalian neurons, which are exposed to gradients of diffusible molecules. Our microfluidic method (a) generates statistically rich data sets, (b) produces a stable, reproducible gradient with negligible shear stresses on the culture surface, (c) is amenable to the long-term culture of primary neurons without any unconventional protocol, and (d) eliminates the confounding influence of cell-secreted factors. Using this platform, we demonstrate that hippocampal axon guidance in response to a netrin-1 gradient is concentration-dependent—attractive at higher concentrations and repulsive at lower concentrations. We also show that the turning of the growth cone depends on the angle of incidence of the gradient. Our study highlights the potential of microfluidic devices in producing large amounts of data from morphogen and chemokine gradients that play essential roles not only in axonal navigation but also in stem cell differentiation, cell migration, and immune response.

Microparticle trajectories in a high-throughput channel for contact-free fractionation by dielectrophoresis

Continuous, contact-free fractionation of sensitive microparticles at high throughput is a challenge. For this purpose, we developed a sheath flow assisted dielectrophoretic (DEP) field-flow separator with a tailored arrangement of cylindrical interdigitated electrodes (cIDE) and observed size-dependent trajectories of dispersed particles. Using a voltage input of 200Veff at a frequency of 200kHz, polystyrene particles (45, 25, and 11µm in diameter) levitated to different heights along the channel length due to a negative DEP force. Experimental observations agree well with simulated particle trajectories that were obtained by a modified Lagrangian particle tracking model in combination with Laplace's and Navier–Stokes equations. By exploiting the size-dependent levitation height difference the desired particle size fraction can be collected at a specific channel length. The required channel length of the proposed cIDE separator increases with decreasing particle size to be separated. The quality of theoretical fractionation, which we quantify by resolution, improves strongly with reduced collector width, reduced volume flow rate and increased voltage input. The sensitivity of these dependencies increases with decreasing particle size. We calculated a theoretical throughput of up to 47mLmin−1 when trading-off design and operation parameters, allowing for contact-free fractionation of sensitive microparticles with negligible shear stress.

Mixing in an enclosed microfluidic chamber through moving boundary motions

Mixing in enclosed culture micro-chambers is an important criterion on achieving the long-term cell growth with more consistent characteristics spatially distributed in the microfluidic environments. Here, we report a microfluidic mixer composed of multiple deformable membranes to drive circulation flows within the culture region in a peristaltic manner. This mechanically driven mixing scheme has the advantages over many others existing mixing schemes by inducing negligible shear stress over cells without side effects to the viability and growth characteristics. The membrane movements can induce moving boundary motions of the liquid volume contained in the culture chamber. The flow characteristics such as the velocity profile and shear stress are investigated by lumped-element modeling and computational simulations. Both experiments and simulations are performed to show the effectiveness of the mixing of both soluble substances and sub-microscale particles, including bacteria. The mixing performance under different operation parameters (e.g., membrane size and membrane switching time) is also investigated for the optimized operation. Further, the dental bacteria Streptococcus mutans are cultured for 2 days to demonstrate that the reported mixing scheme can generate a more even distribution of growth, which may be further applied for a uniform dental biofilm development in vitro for the related biofilm research. Additionally, this micro-mixer is highly compatible with the widely used soft lithography technique, and hence, it can be directly integrated with general microfluidic devices for extended biomedical diagnosis and bio-sample processing applications.

DPDE-based mesoscale simulations of shock response of HE composites

The dissipative particle dynamics with energy conservation (DPDE) method is extended to simulate the shock response of high energetic (HE) materials at micron length scales. The symmetrical impact of an RDX impactor and target plates with 1μm diameter spheres is simulated at planar impact velocities of 208 m/s and 876 m/s with a Lennard-Jones-like potential, dissipative, and random forces, and artificial viscosity force between particles. The in situ shock quantities were obtained using Hardy's averaging. In situ longitudinal stresses from simulations were 0.84 and 3.82 GPa. Values from the literature are 0.81 and 2.92 GPa at the two impact velocities, respectively. The uniaxial strain condition was predicted with equal lateral stresses and negligible shear stresses. The higher stress value at 876 m/s may be due to lack of inelasticity in the interparticle potential. The temperature increases of 5.5 0K and 93.7 0K, respectively, were predicted assuming dissipation of a fraction of the potential energy. It is concluded that the DPDE method holds promise for a unified computational framework for multi-scale simulations of HE.

Unsteady Two-Dimensional Stagnation-Point Flow and Heat Transfer of a Viscous, Compressible Fluid on an Accelerated Flat Plate

The general formulation and exact solution of the Navier–Stokes and energy equations regarding the problem of steady and unsteady two-dimensional stagnation-point flow and heat transfer is investigated in the vicinity of a flat plate. The plate is moving at time-dependent or constant velocity towards the main low Mach number free stream or away from it. The main stream impinges along z-direction on the flat plate with strain rate a and produces two-dimensional flow. The fluid is assumed to be viscous and compressible. The density of the fluid is affected by the existing temperature difference between the plate and potential far field flow. Suitably introduced similarity transformations are used to reduce the governing equations to a coupled system of ordinary differential equations. Finite Difference Scheme is used to solve these non-linear ordinary differential equations. The obtained results are presented over a wide range of parameters characterizing the problem. It is revealed that the significance of the increase of thermal expansion coefficient, β, and wall temperature on velocity and temperature distributions is much more noticeable for a plate moving away from impinging flow. Moreover, negligible shear stress and heat transfer is reported between the plate and fluid viscous layer close to the plate for a wide range of β coefficient when the plate moves away from incoming far field flow.

Epithelial‐to‐mesenchymal transition of human lung alveolar epithelial cells in a microfluidic gradient device

Epithelial-to-mesenchymal transition (EMT), a process in which epithelial cells undergo phenotypic transitions to fibrotic cells, is induced by stimulants including transforming growth factor-beta1 (TGF-β1). In the present study, we developed a microfluidic gradient device to reproduce EMT in A549 human lung alveolar epithelial cells in response to TGF-β1 gradients. The device was directly mounted on the cells that had grown in cell culture plates and produced a stable concentration gradient of TGF-β1 with negligible shear stress, thereby providing a favorable environment for the anchorage-dependent cells. A549 cells elongated with the characteristic spindle-shaped morphological changes with upregulation of alpha-smooth muscle actin, a mesenchyme marker, in a gradient-dependent manner, suggestive of EMT progression. We observed that at higher TGF-β1 concentrations ranging from 5 to 10 ng/mL, the cultures in the microfluidic device allowed to quantitatively pick up subtle differences in the EMT cellular response as compared with plate cultures. These results suggest that the microfluidic gradient device would accurately determine the optimal concentrations of TGF-β1, given that epithelial cells of different tissue origins greatly vary their responses to TGF-β1. Therefore, this microfluidic device could be a powerful tool to monitor EMT induced by a variety of environmental stresses including cigarette smoke with high sensitivity.

The effect of oscillations on the flow patterns near a simulated bed

This work investigates the effect of oscillations on the flow near a simulated bed and their impact on erosion. Those oscillations were generated by using wavy lids at the top of a small toroidal channel. Particular attention is dedicated to their influence on the velocity field and turbulence near the bed. The experiments were made in a mini circular flume using a 2D LDA system. It was concluded that such oscillations induce non negligible shear stresses in the flow main stream, becoming more important to the dispersion of entrained matter into the flow than to the initiation of the erosion process.

A neuron-benign microfluidic gradient generator for studying the response of mammalian neurons towards axon guidance factors.

Investigation of biochemical cues in isolation or in combinations in cell culture systems is crucial for unraveling the mechanisms that govern neural development and repair. The most widely used experimental paradigms that elicit axon guidance in vitro utilize as the source of the gradient a pulsatile pipette, transfected cells, or a loaded gel, producing time-varying gradients of poor reproducibility which are not well suited for studying slow-growing mammalian cells. Although microfluidic device design have allowed for generating stable, complex gradients of diffusible molecules, the flow-induced shear forces in a microchannel has made it impossible to maintain viable mammalian neuronal cultures for sufficiently long times. In this paper, we describe axonal responses of mouse cortical neurons in a "neuron-benign" gradient-generator device based on an open chamber that can establish highly stable gradients of diffusible molecules for at least 6 h with negligible shear stress, and also allows the neurons to thrive for at least 2 weeks. Except for the period when the gradient is on, the cells in the gradient are under the same conditions as the cells on the control surfaces, which ensure a consistent set of micro-environmental variables. The gradient stability and uniformity over the cell culture surface achieved by the device, together with our software platform for acquiring, post-processing and quantitatively analyzing the large number of images allowed us to extract valuable information even from small datasets. We report a directed response of primary mammalian neurons (from E14 embryonic mice cortex) to a diffusible gradient of netrin in vitro. We infer from our studies that a large majority (∼73%) of the neurons that extend axons during the gradient application grow towards the netrin source, and our data analysis also indicates that netrin acts as a growth factor for this same population of neurons.

Hydro-Thermo-Mechanical Model for Highly Deformable Product during Convective Drying

The aim of this work was to simulate in 1D, the spatio-temporal evolution of the moisture content, the temperature and the mechanical stress within a highly deformable and saturated product during convective drying. The hydro-thermal model, written in a fixed coordinate system, consisted of solid mass balance equation, moisture transfer diffusion/advection equation and heat transfer conduction/advection equation. These equations were coupled by the solid phase velocity terms due to hydric shrinkage. Convective boundary conditions completed this set of equations. The hydro-thermal model had been merged with a static mechanical model which was based on the hypothesis of an elastic behavior, of a plane deformation and of an ideal shrinkage. The hydro-thermo-mechanical model had been applied to a parallelepipedical potato sample. Its thickness was small compared to the other dimensions, in order to observe an unidirectional drying and a negligible shear stress case. The transport and equilibrium properties of the product required for the modelling were determined from previous experiments which were independent of the drying trials.

Evaluating the strength and deformation characteristics of a soil bag pile from full-scale laboratory tests

The compressive strength of a soil bag pile when compressed vertically and the shear strength when sheared laterally were evaluated by performing a series of full-scale loading tests. The effects of soil bag material, backfill soil type, number of soil bags in a pile and associated effects of the end restraint at the top and bottom ends of a soil bag pile were evaluated. The results of vertical compression tests showed that initial compaction of the backfill in the soil bags is effective in increasing the initial stiffness at small deformation, and that preloading is even more effective in increasing the overall stiffness and decreasing the creep deformation. The shear strength of a soil bag pile subjected to lateral shear is substantially smaller as compared with a high compressive strength when compressed vertically. It was found that the shear strength of well-graded granular backfill was partially mobilised in vertical compression tests but the mobilisation was even lower in the lateral shear tests. The soil bag pile showed highly anisotropic strength characteristics, resulting from no or negligible complementary shear stresses acting at the vertical interface between the horizontally adjacent soil bags and highly anisotropic reinforcing effects of geosynthetic cover sheets of soil bags.

Solidification and epitaxial regrowth in surface nanostructuring with laser-assisted scanning tunneling microscope

In this work, parallel molecular-dynamics simulation is conducted to study the long-time (up to 2ns) behavior of argon crystal in surface nanostructuring with a laser-assisted scanning tunneling microscope. A large system consisting of more than 1×108at. is explored. The study is focused on the solidification procedure after laser irradiation, which is driven by heat conduction in the material. Epitaxial regrowth is observed in the solidification. Atomic dislocation due to thermal strain-induced structural damages is observed as well in epitaxial regrowth. During solidification, the liquid is featured with decaying normal compressive stresses and negligible shear stresses. Two functions are designed to capture the structure and distinguish the solid and liquid regions. These functions work well in terms of reflecting the crystallinity of the material and identifying the atomic dislocations. The study of the movement of the solid-liquid interface reveals an accelerating moving speed in the order of 3–5m∕s. The spatial distribution of the moving speed at the solid-liquid interface indicates a nonuniform epitaxial regrowth in space. The bottom of the liquid solidifies slower than that at the edge.

Stagnant-cap bubbles with both diffusion and adsorption rate-determining

When a gas bubble rises in an impure liquid, its surface often has an upper spherical cap with negligible shear stress, a lower spherical cap with negligible tangential velocity, and a very small transition region between the two caps. We give the diffusion boundary-layer theory for the distribution of surfactant around a stagnant-cap bubble, allowing for slowness of both adsorption and diffusion. The resulting singular Volterra integro-differential equations are solved numerically for creeping flow (small Reynolds number)

Fluvial sediment transport and deposition following the 1991 eruption of Mount Pinatubo

The 1991 eruption of Mount Pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows. Bedload sampling in the Pasig–Potrero River, one of the most heavily impacted rivers, revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth, fine-grained bed. Dimensionless bedload transport rates in the Pasig–Potrero River differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress, but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments. The similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply; in arid rivers, this is attributed to a flashy hydrograph, whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery. This work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced.

Brownian dynamics simulations of the stress and molecular configuration of polymers in exponential and linearly-ramped shear flow

The rheological and optical properties of dilute polymer solutions during the startup and subsequent relaxation in exponential shear flow are studied using Brownian dynamics simulations of freely draining, flexible bead-rod chains. We find that exponential shear flow is capable of effecting large molecular deformation for finite shear rates and strains, and furthermore, the polymer undergoes an initial stress and index of refraction relaxation that is much faster than a single exponential decay upon cessation of flow. When we compare the first normal stress difference ( τ 11– τ 22) to the corresponding components of the index of refraction tensor, n 11– n 22, we find that there is hysteresis when plotted over the course of a full startup and relaxation experiment. This is very similar to the hysteresis originally discovered by Doyle et al. [J. Non-Newtonian Fluid Mech. 76 (1998) 79–110] for uniaxial extensional flow. Moreover, the shear components of the stress and index of refraction tensors ( τ 12 and n 12) also exhibit hysteretic behavior. However, whereas the hysteresis loop for the 11–22 component traverses up the left branch during startup and relaxes on the right branch, that for the 12 component is in the completely opposite direction, i.e. the right branch corresponds to startup and the left branch relaxation. The presence of stress-index of refraction hysteresis clearly has important implications for the stress-optic rule. We calculate the stress-optic coefficient for the startup process, and find that the normal and shear components give the same value, but that it is strongly dependent on Wi. This result is consistent with that found by Doyle et al. during the startup of steady shear flow [Doyle and Shaqfeh, Dynamic simulation of freely-draining, flexible bead-rod chains: startup of extensional and shear flow, J. Non-Newtonian Fluid Mech. 76 (1998) 43–78]. We also simulated shear flows with a linearly ramped shear rate to compare to our results for exponential shear. We find that for comparable final shear rate and time, both exponential and linear ramping shear produce very similar hysteresis effects in both the shear and normal components suggesting that there may be many unsteady shear flow capable of creating large polymer stretch and stress-birefringence hysteresis. Finally, we compare our bead-rod chain simulations to the FENE dumbbell model, and we find that while the latter can qualitatively capture the normal stress hysteresis, it predicts negligible shear stress hysteresis at equivalent shear rates and shear strains.

Sediment deformation and basal dynamics beneath a glacier surge front: Bakaninbreen, Svalbard

Bakaninbreen is a 17 km long surge-type glacier in southern Spitsbergen, Svalbard, which began surging between the springs of 1985 and 1986, forming a surge front where fast-moving surge ice meets non-surging ice. This surge front has propagated 6 km down-glacier over the period to 1995. Instruments known as “ploughmeters” were installed into the deformable sedimentary bed close to the surge front to assess mechanical conditions year-round. Forces experienced by ploughmeters located down-glacier of the surge front are generally lower than those recorded by a ploughmeter up-glacier of the surge front. Ploughmeters installed at the bed down-glacier of the surge front show initially low applied forces, followed by increasing applied forces. We interpret this increase in applied forcing as a late-active-phase motion event. Analysis of ploughmeter data allows calculation of the yield strength of basal sediments. Yield-strength estimates at Bakaninbreen are in the range 16.6–87.5 kPa. Comparison with estimates of basal shear stress suggests that sediments up-glacier of the surge front will be actively deforming, whereas there will be only limited deformation down-glacier of the surge front. Immediately down-glacier of the surge front, calculations indicate negligible basal shear stresses. Together with the deformation of sediment from up-glacier, this implies a build-up of sediment at the surge front, offering a potential explanation for the sediment-filled thrust faults outcropping on the surge front.

Sediment deformation and basal dynamics beneath a glacier surge front: Bakaninbreen, Svalbard

Bakaninbreen is a 17 km long surge-type glacier in southern Spitsbergen, Svalbard, which began surging between the springs of 1985 and 1986, forming a surge front where fast-moving surge ice meets non-surging ice. This surge front has propagated 6 km down-glacier over the period to 1995. Instruments known as “ploughmeters” were installed into the deformable sedimentary bed close to the surge front to assess mechanical conditions year-round. Forces experienced by ploughmeters located down-glacier of the surge front are generally lower than those recorded by a ploughmeter up-glacier of the surge front. Ploughmeters installed at the bed down-glacier of the surge front show initially low applied forces, followed by increasing applied forces. We interpret this increase in applied forcing as a late-active-phase motion event. Analysis of ploughmeter data allows calculation of the yield strength of basal sediments. Yield-strength estimates at Bakaninbreen are in the range 16.6–87.5 kPa. Comparison with estimates of basal shear stress suggests that sediments up-glacier of the surge front will be actively deforming, whereas there will be only limited deformation down-glacier of the surge front. Immediately down-glacier of the surge front, calculations indicate negligible basal shear stresses. Together with the deformation of sediment from up-glacier, this implies a build-up of sediment at the surge front, offering a potential explanation for the sediment-filled thrust faults outcropping on the surge front.