Low Shear Stress Levels Research Articles

The fluid dynamic and shear environment in the NASA/JSC rotating-wall perfused-vessel bioreactor.

The rotating-wall perfused-vessel (RWPV) bioreactor, used for both microgravity and Earth-based cell science experiments, is characterized in terms of the fluid dynamic and fluid shear stress environment. A numerical model of the flow field is developed and verified with laser Doppler velocimeter measurements. The effects of changes in operating conditions, including rotation rates and fluid perfusion rates, are investigated with the numerical model. The operating conditions typically used for ground-based experiments (equal rotation of the inner and outer cylinders) leads to flow patterns with relatively poor mass distribution characteristics. Approximately 50% of the inlet-perfused fluid bypasses the bulk of the fluid volume and flows to the perfusion exit. For operating conditions typical in microgravity, small differential rotation rates between the inner and outer cylinders lead to greatly improved flow distribution patterns and very low fluid shear stress levels over a large percentage of the fluid volume. Differences in flow patterns for the different operating conditions are explored. Large differences in the hydrodynamic environments for operating conditions typical of true microgravity and ground-based "microgravity simulations" are demonstrated.

Non-newtonian plastic flow of a Ni-Si-B metallic glass at low stresses

The problem of the rheological behavior of metallic glasses (MGs) is quite important both from theoretical and practical viewpoints. Early experiments carried out on MGs at temperatures T > 300 K using low shear stress levels revealed plastic flow to be Newtonian while measurements at relative high shear stresses (more than 200 to 400 MPa, depending on temperature, thermal prehistory of samples and chemical composition) indicated a non-linear behavior with 1 < m < 12. Numerous investigations performed later both on as-cast and relaxed MGs of various chemical compositions using a number of testing methods (tensile creep, tensile and bend stress relaxation) showed that a transition from Newtonian behavior at low stresses to a non-linear flow at high stresses was observed. At present, such a situation is considered to be generally accepted. The authors performed precise creep measurements of a Ni-Si-B metallic glass. The results obtained indicate that plastic flow in this case at low tensile stress (12 {le} {sigma} {le} 307 MPa) is clearly non-Newtonian and, consequently, the viscosity is stress dependent.

Role and Initiation Mechanism of the Interaction of Glycoprotein Ib With Surface-Immobilized von Willebrand Factor in a Solid-Phase Platelet Cohesion Process

To know the role and initiation mechanism of the interaction of glycoprotein (GP) Ib with surface-immobilized von Willebrand factor (vWF), we examined the effect of shear stress levels on platelet binding to vWF-coated plates using a cone-and-plate type viscometer capable of loading various levels of shear stress. The extent of platelet binding to immobilized vWF reached a plateau at the shortest period tested (20 seconds) under high shear stress (90 dyne/cm2), whereas 9 to 12 minutes was necessary for saturable platelet binding under static conditions. This shear effect, which was found to be dependent on the vWF-GP Ib interaction, was observed even under the lowest shear stress (1.5 dyne/cm2) examined. In contrast with the high shear effect previously reported to initiate the interaction of GP Ib with soluble vWF, these results indicate that relatively low levels of shear stress can promote the interaction of GP Ib with surface-immobilized vWF. This effect of shear stress was observed regardless of the manner in which vWF was immobilized, suggesting that immobilization itself and not, as previously hypothesized, a conformational change in vWF induced by direct adsorption to the surface is responsible for the enhanced GPIb binding. Thus, the present findings suggest that the vWF-GP Ib interaction contributes optimally to rapid platelet cohesion on a thrombogenic surface when vWF is in a static state and when platelets are moved by an appropriate rheological force such as low shear stress.

Hemodynamic modulation of monocytic cell adherence to vascular endothelium

Hemodynamic shear stress is hypothesized to contribute to the localization of atherosclerotic plaques to certain arterial sites. Monocyte recruitment to these sites is an early event in artherogenesis. To determine the possible mechanisms by which shear stress modulates monocyte adhesion in vivo, studies of human monocytic cell adherence to endothelium were conducted under different shear conditions in a parallel-plate flow chamber. The number of monocytes capable of developing firm adhesive contacts with endothelium decreased as shear stress-induced drag forces increased over the range of 0.5 to 30 dynes/cm2. The number of adherent monocytic cells at a given shear stress was highly dependent on the activation state of the endothelium. To test the direct effect of shear stress on endothelial cell adhesivity, endothelial cells were presheared for 2 to 6 hr at 2, 10, or 30 dynes/cm2, and monocytic cell adherence was quantified at 1 dyne/cm2. Adherence increased 330% or 370% when endothelial cells were presheared for 2 hr at 2 or 10 dynes/cm2, respectively, as compared to unsheared endothelium. In contrast, when endothelial cells were presheared at 30 dynes/cm2, monocytic cell adherence at 1 dyne/cm2 was not significantly different from unsheared controls. Increased monocytic cell adherence to presheared endothelium was via a vascular cell adhesion molecule 1 (VCAM-1)/alpha(4) beta(1) mechanism, and enzyme-linked immunosorbent assay studies showed that preshearing at 2 dynes/cm2 for 2 hr increased endothelial VCAM-1 expression by 38%. These data demonstrate that low levels of shear stress induce endothelial VCAM-1 expression and increase monocytic cell adherence via a VCAM-1/alpha 4 beta 1 mechanism. Thus, shear stress can modulate monocyte adherence to vascular endothelium through drag forces that affect the establishment and maintenance of adhesive bonds and by directly modulating the expression of endothelial VCAM-1. This dual effect of shear stress produces the most favorable conditions for adhesion at low-shear regions, where drag forces are low and induction of VCAM-1 is likely. The preferential adherence of monocytes to these regions may contribute to the localization of atherosclerotic plaques to low-shear regions of the arterial circulation in vivo.

Endothelial Cell Function, Including Tissue Factor Expression, Under Flow Conditions

Vascular endothelium remains a dynamic interface between blood and the vessel wall. New developments make clear that many antithrombotic and prothrombotic responses of the endothelium depend on flow conditions in an adaptive manner: duration of a certain level of shear stress matters as well as level of shear. In general, over a time course of several hours, endothelium appears to be more actively antithrombotic under moderate shear conditions (e.g., 15 dynes/cm2), and more fibrinolytic under high (e.g., 30 dynes/cm2). Pulsatile flow and cyclic wall stress further modify these responses. Special consideration, moreover, must be given to branch points and regions of irregular geometry (i.e., stenoses, aneurysms) in the circulation. In such locations, predilection sites for thrombosis, lipid uptake, and atherosclerosis, low levels of fluid shear stress (e.g., 0.5 dynes/cm2), large gradients in fluid shear stress, and vessel wall bending stresses all become important. Preliminary work suggests that endothelial cells in such regions can become prothrombotic, leading to localized platelet adhesion/aggregation and fibrin formation on subendothelium and perhaps deeper structures following vessel injury.

Fluid Shear Stress Induces Synthesis and Nuclear Localization of c-fos in Cultured Human Endothelial Cells

Quiescent cultured primary human umbilical vein endothelial cells were exposed to low levels (4 dynes/cm 2) or arterial levels (25 dynes/cm 2) of steady laminar shear stress for one hour. Intracellular c-fos protein was measured by immunocytochemistry and quantitative fluorescence video microscopy. The nuclear-localized c-fos protein level was 5.4 ± 2.0 fold higher (p<0.01) in the cells exposed to arterial levels of shear stress as compared to the cells maintained in stationary culture. In contrast, the cells exposed to low levels of shear stress showed diffuse but slightly elevated levels of c-fos (2.4 ± 0.73 times higher than control; p<0.01) without preferential nuclear localization. The protein kinase C inhibitor, H7 (10 μM) significantly attenuated the induction of c-fos by 50 % in cells exposed to arterial shear stress for 1 hour.

Modelling of air-lift fluidized bed: Optimization of mass transfer with respect to design and operational parameters

Biological three-phase reactions are conventionally carried out in bubble columns, stirred contactors or fluidized beds. New bioreactors and fermenters used for production or waste water treatment include ICI deep shaft aerators or internal draft-tube air-lift loop reactors and draft-tube fluidized beds. The fluidized beds have higher productivity than the continuous stirred type and there is no wash out or stratification of biomass. The three-phase fluidized bed has high power requirements and adequate data are not available for design of large-scale equipment. It is proposed to use an external loop air-lift reactor as an air-lift fluidized bed (ALFB), by fluidizing the light particles in the downcomer. The proposed ALFB is expected to have lower shear stress levels and has a high degree of design flexibility. The effect of design and operational parameters were simulated for an air-water system for an ALFB of 100 m 3. Recommendations have been made for optimum mass transfer performance and suitable scale up criteria have been developed. The proposed ALFB has been shown to be more promising than the three-phase fluidized bed.

A viscous pump bioreactor

The fluid dynamic design and characterization of a low mechanical stress agitator and bioreactor vessel for large-scale bioprocessing using anchorage dependent mammalian cells is considered. The complex and fragile nature of mammalian cells puts stringent constraints on the design of agitators for stirred tank bioreactors. Traditional agitators have difficulty meeting the competing requirements of fluidization and mixing while maintaining low enough mechanical stress levels to avoid cell damage. A rotating disc agitator design is proposed. Flow visualization and laser Doppler velocimeter measurements reveal fluidization of microcarriers with a gentle (low turbulence level), highly three-dimensional flow characterized by good mixing at low hydrodynamic shear stress levels.

Strain Tensor and Deformation for Granular Material

A micromechanical analysis is carried out to introduce a tensor measure for the strain of granular media. This strain tensor, being different from that of the conventional definition, includes explicit descriptions for the spin of particles in granular media. A stress‐strain relationship is then derived based on the new definition of strain tensor. Due to the assumption of a linear displacement field, the stress‐strain model is intended for estimating mechanical properties of a packing under conditions of low shear stress levels. The Hertz‐Mindlin theory of nonlinear contact is used in the mathematical model to predict the mechanical behavior of rod assemblies with various packing structures. The theoretical predictions are compared with the measured results obtained from experimental tests. Results of this comparison are used to evaluate the applicability of the stress‐strain model. The laboratory device, test samples, and loading conditions are described and the comparisons between measured and theoretical results are discussed.

The behaviour of reinforced concrete beams under cyclic loading

The behaviour of beams in which plastic hinges are formed under cyclic loading is examined. The results are reported of five beam tests, in which the shear stress level was varied. It is shown that even relatively low shear stress levels have a significant influence on beam behaviour. Two main effects of shear are to reduce the ability of the hinge to dissipate energy and to reduce the stiffness of the beams at low load levels.. The degradation in shear under cyclic loading is accompanied by an appreciable growth in length of the beam.

Creep of a clay during shear and its rheological model

A succession of soil structures is formed and destroyed during the creep of a clay during shear; they are characterized by a mutual rearrangement of contacts and by the forces acting at the contacts. Transition from one structure to the other proceeds in a jump-like way. The creep process consists of two phases: mobilization and rupture. In the mobilization phase the strain rate decreases directly with time; in the rupture phase it increases. Therefore the creep curves in the mobilization phase consist of logarithmic curves whilst, in the rupture phase, of parabolas. The following features are characteristic of soil behaviour: creep limit, static viscosity and moduli of mobilization and stabilization. These quantities are linearly related to the normal pressure. The strain rate at low and moderate shear stress levels decreases following the stabilization limit, whilst at high stress levels the process ends in rupture. A theory of the creep of a clay during shear is suggested which is supported by experiments of long duration; the rheological model of the soil is developed and equations describing the mechanical behaviour are derived. Dans le processus du0 fluage pendant le cisaillement d'une argile une succession des structures du sol se forme et se détruit. Elles sont caractérisées par l'arrangement mutuel des contacts et par les forces aux contacts. Le passage d'une structure à l'autre s'accomplit par à-coups. Le processus du fluage est composé de deux phases—mobilisation et rupture. Dans la phase de la mobilisation la vitesse du fluage diminue proportionnellement au temps; dans la phase de rupture elle augmente. C'est pourquoi les courbes du fluage en phase de mobilisation sont compos&amp;s de différents segments de courbes logarithmiques, et dans la phase de rupture, de différents segments des paraboles. Quatre grandeurs caractérisent le comportement du sol: la limite du fluage, la viscosité statique, les modules de mobilisation et de stabilisation; ces grandeurs sont liées linéairement à la pression normale. Après la limite de stabilisation la vitesse du fluage pour les contraintes de cisaillement basse et moyenne diminue, mais pour les contraintes élevées le processus se termine par la rupture. On propose une théorie dufluage de l'argilependant le cisaillement, vérifiée par une experimentation de longue durée; le modèle rhéologique est établi et les équations décrivant le comportement mécanique en sont déduites.