Controlled Shear Stress Research Articles

A new fixed-film mobile bed bioreactor for denitrification of wastewaters

An innovative fixed-film mobile bed bioreactor for high-rate denitrification of wastewaters has been developed and tested. This reactor, called the Mixazur® enables a high denitrification efficiency due to high kinetics. These properties are combined with a very simple system, particularly due to the choice of a suitable mineral granular support for bacterial growth, enabling a very low filling ratio. This low filling ratio, usually smaller than 5% vol., offers a sufficient surface area for fixed-film development, easily moved by a simple mechanical agitation with low energy consumption (< 30 W/m3 of reactor). The experimental study showed that this reactor can achieve an eliminated loading rate of 0.6 kg NOx-N/m3 of reactor per day at a removal efficiency more than 80% and at water temperatures between 13 and 20°C. The separation of a denitrifying biomass in a specific pre-denitrifying bioreactor led to a development of a very active biofilm with denitrification rates as high as 10 mg NOx-N/g MLVSS.h at 18°C. The scale-up of this system does not need any sophisticated wastewater inlet distribution system, any additional material/biomass separation device and any recirculation loop for maintaining a constant fluidization velocity. Moreover, the reactor is characterized by its low-cost equipment, continuous operation and by auto-regulation of both biofilm thickness and activity due to a controlled shear stress and by an overall easy operation. The first full-scale facility, for 1200 p.e., using this system has been operating since 1996.

Leukocyte-biomaterial interactions in the presence of Staphylococcus epidermidis: flow cytometric evaluation of leukocyte activation.

The adhesion of bacteria on a biomaterial surface is believed to be the first step in the development of biomaterial-related infection. The goal of this study was to investigate the mechanisms that permit adherent bacteria to persist on the surface of an implanted cardiovascular biomaterial. We hypothesized that circulating leukocytes are unable to adhere to the biomaterial surface under physiologic shear stress conditions, and this prevents them from interacting with adherent bacteria. To address this hypothesis, we investigated the adhesion profiles of Staphylococcus epidermidis and polymorphonuclear leukocytes (PMN), incubated under controlled shear stress conditions with the test biomaterial. We found that bacteria could adhere on the biomaterial surface, even when their concentration in the test medium was as low as 10(3) cfu/mL. At this concentration, the bacteria did not induce significant complement activation. PMN adhesion on the biomaterial surface was sensitive to shear stress and minimal at shear stress > 10 dynes/cm2. Low concentrations of bacteria could induce a significant increase in the expression of PMN adhesion molecules CD11b and CD11c. We conclude that the presence of bacteria induces PMN activation but does not increase PMN adhesion on biomaterial surfaces under physiologic shear stress conditions. This could be a major mechanism that protects adherent bacteria from PMN antibacterial activity.

Rheologic study of the dissociation reaction of erythrocyte immunologic agglutinates: application to subgroup A erythrocytes

Red blood cell Agglutinates can be dissociated by providing enough energy which may be supplied by shear stress in a viscometer. In this paper a new technique is proposed to determine dissociation parameters of erythrocyte immunological agglutinates by using a laser backscattering method. Backscattered light intensity increases during the process of agglutinate dissociation when a controlled shear stress in a transparent Couette viscometer is applied. The obtained curves of dissociation can be fitted by an exponential function. A numerical integration of the dissociation curves allows us to obtain a parameter associated to the energy used in the dissociation process. A satisfactory differentiation of erythrocyte sub-groups Al, Az, A,B and Am has been carried out using this technique.

Experimental studies on the suitability of human mesothelial cells for seeding vascular prostheses: shear stress resistance in vitro

This investigation forms part of a study on the suitability of human omentum mesothelial cells (HOMES) as an alternative to endothelial cells (EC) for seeding vascular grafts. Isolated HOMES were grown in primary culture and characterized by their morphology (light microscopy and scanning electron microscopy (SEM)), as well as by fluorescence-activated cell sorting (FACS) and immunocytochemistry. The latter two methods showed cells which were positive for smooth muscle-type actin and cytokeratin, but negative for factor VIII-related antigen. HOMES were grown to confluence on glass with or without a fibronectin coating. Controlled shear stress was applied for up to 30 min using a plate and cone rheometer at 20 dynes/cm2. These dynamic culture conditions led to loss of only occasional cells. The most marked alterations seen on SEM were some cell elongation, marked raising of the nucleus and loss of luminal cytoplasmic microvilli. Time-lapse video microscopy revealed that shear stress also increased the spreading capacity of some cells. Similar experiments with venous endothelial cells gave a shearing off of a confluent monolayer. This investigation shows the marked shear-stress resistance of HOMES, a pre-requisite for their use to seed vascular prostheses.

Differential Attachment of Lymphocyte Subpopulations on Poly(2-hydroxyethyl methacrylate)–Graft–Polyamine Copolymers with Different Degrees of Quaternization

The interaction of rat lymphocyte subpopulations with the surfaces of poly(2-hydroxyethyl methacrylate)–graft–polyamine (HA) copolymer and poly(2-hydroxyethyl methacrylate)–graft–partially quaternized polyamine (HQA) copolymer was evaluated under a controlled shear stress by using a plane-parallel-type channel equipped with a polymer-coated bottom wall [hybrid field-flow fractionation/adsorption chromatography (FFF/AC) technique]. Estimation of the recovery profile of lymphocytes from HA and HQA channels under varying shear stress indicated that the contribution of electrostatic interaction to cellular attachment was different among lymphocyte subpopulations, having a good correlation with their apparent isoelectric points. Extended settling time caused a progressive increase in the attachment force between the lymphocyte and the copolymer surface for the HA channel, resulting in a considerable decrease in the recovery of both B and T cells from the channel. On the contrary, recovery of T cells from the HQA channel showed only a slight decrease with a settling time, allowing attainment of a T-cell population as the effluent in good yield even under the condition of prolonged settling time. The time-dependent increase in the cell/surface interaction observed for the HA channel might be due to a progressive contribution of the hydrophobic interaction caused by an increase in the hydrophobicity of the HA copolymer through the deprotonation of ethylenediamine units in polyamine grafts at the cellular interface. This assumption was supported by the fact that polyamine macromonomer was quantitatively extracted inton-octanol from water through deprotonation at the critical pH, where the evaluation of cell/surface interaction was carried out. These results indicated that lymphocyte attachment is able to be controlled by changing the amount and nature (fixed or unfixed) of the charges in polyamine–graft copolymers.

YIELD STRESS OF FOOD DISPERSIONS WITH THE VANE METHOD AT CONTROLLED SHEAR RATE AND SHEAR STRESS

ABSTRACTThe vane method was used to measure yield stresses of 15 commercial food dispersions under controlled shear stress (Cσ) and controlled shear rate (C) operating conditions. Magnitudes of yield stress (σσ) at Cσ were higher than those of yield stress (σ) at C on tomato products and baby foods. There were good linear correlations (R2= 0.96, 0.86) between γ and σσ for food dispersions with undisturbed structure (UDS) and with broken down structure (BDS). Data obtained under Cσ were also used to calculate shear moduli of foods.

Dihydroergocryptine maintains erythrocyte fluidity in acidotic and hyperosmolar buffer solutions modelling hypoxic and ischemic microcirculation

Received 20,09.1994; accepted 21.12.1994) The positive effects of Ergopeptide Alkaloids, particularly Dihydroergotoxine with Dihydroergocryptine (DHEC) as one of the components, on cerebral functions has been known for about three decades. DHEC in particular has multiple biological effects: It is an a-ad reno receptor blocking agent and an agonist of dopamine receptors and is known as a peripheral and cerebral vasodilator. An anti-aggregative activity has been shown on platelet aggregation induced by adrenaline and collagen as well as on erythrocyte aggregation induced by dextran. It also is a metabolic enhancer, protecting against metabolic stress due to hypoxia and ischemia and it increases the uptake of deoxyglucose and oxygen utilisation in the brain. We studied the protective effect of DHEC (concentration range 10 nM to 100 /lM) on human red blood cell (RBC) rheology in acidotic and hyperosmolar solutions (AHS) modelling the hypoxic and ischemic microcirculation. The microscopic photometric monolayer technique was used to quantify alterations of RBC resting shape, stiffness and relaxation time automatically and time dependently (Elias-c-analyser). The technique is based on changes in light transmission through a RBC monolayer in a flow chamber at controlled shear stresses. A HEPES buffer solution was used as control buffer and an acidotic/hyperosmotic buffer solution (pH 6.8, 380 mosmoVkg) to simulate the metabolic challenge in ischemia. AHS itself hardly altered the RBC discocytic shape. However, it significantly increased RBC stiffness and relaxation time (p<O.OOI). Both RBC in control buffer and RBC pre-stressed in AHS for 20 minutes were incubated with DHEC at different concentrations within the flow chamber. DHEC-induced stomatocytes appeared dose-dependently in both, control buffer and AHS, within less than 2 minutes. The shape change was 1

EFFECT OF UNIMODAL PARTICLE SIZE AND PULP CONTENT ON RHEOLOGICAL PROPERTIES OF TOMATO PUREE

ABSTRACTThe vane method in controlled shear stress mode was used to determine the yield stress and the shear rate—shear stress data of tomato purees containing 10–35% pulp of two different average particle sizes: 0.34 and 0.71 mm. Consistency index and apparent viscosity increased significantly with pulp content and decreased with average particle size. The effect of pulp weight fraction (P) on relative viscosity (ηr) could be described by the single parameter equation: ηr= [1 – (P/A)]−2, while the effect of particle diameter on ηr could be described using Peclet number. Magnitudes of yield stress determined directly by the vane method were higher than those obtained by using the Casson model, and were proportional to the square of pulp content. Reduced Casson yield stress—P data on purees of both particle sizes followd a single curve. Effects of pulp content and particle size on vane yield stress and apparent viscosity were evident from the correlation forms with high values of R2.

Differential Affinity of Lymphocyte Subpopulations toward PHEMA Surface Derivatized with a Small Amount of Amino Groups—Evaluation under the Regulated Shear Stress

The interaction of rat lymphocyte subpopulations with the surface of poly[ N-methyl- N-(4-vinylphenethyl)ethylenediamine- co-2-hydroxyethyl methacrylate] (HAV copolymer) was evaluated under controlled shear stress by using a flat ribbon-like chamber with a plane-parallel type channel equipped with a polymer-coated bottom wall (hybrid field-flow fractionation/adsorption chromatography (FFF/AC) technique). Due to this simple geometry, it was able to control the hydrodynamic shear stress at the channel wall resulting from the parabolic flow in the channel. The cells in the channel were able to be detached from the polymer surface by increasing shear stress to exceed the strength of cell/materials interaction. Judging from the value of critical shear stress for lymphocyte take-off, the interaction force of lymph node lymphocytes was concluded to be minimized on the HAV copolymer containing 3 mol% of N-methyl- N-(4-vinylphenethyl)ethylenediamine (HAV3). This anomalous minimum of lymphocyte attachment by the introduction of a small amount of cationic sites on the surface is considered to be due to the drastic increase in free water content of polymers triggered by incorporated amino groups, which was demonstrated through DSC measurement. Further, by varying the settling time of lymphocytes in the chamber as well as by changing shear stress through the different profiles of applied flow, differential take-off of two major subpopulations of lymphocytes, B cells and T cells, was achieved on the HAV3 surface due to the differential contribution of the electrostatic interaction with HAV3 between B cells and T cells. This result indicated that the separation of B cells from T cells can be done by FFF/AC technique with a HAV3-coated chamber.

Shear stress effects on growth and activity of Lactobacillus delbrueckii subsp. bulgaricus

Cells are frequently submitted to shear stresses during industrial processes. Shear stress can be either beneficial or detrimental to the cells depending on the organism and on the level of intensity. The present work was designed to study the effect of shear stress on cell activity of a widely used lactic acid bacterium, Lactobacillus delbrueckii subsp. bulgaricus ( L. bulgaricus). A constant shear stress bioreactor, based on Couette device, was developed and used to control shear stress from 0 to 72 Pa during a 4-h cultivation of a supplemented whey permeate medium with L. bulgaricus at 42°C. In order to reach high shear stresses and to perform experiments within the laminar flow range where hydrodynamic conditions are accurately defined, the medium was thickened with carboxymethylcellulose (CMC). pH, cell counts, optical density, lactose and lactic acid concentrations were monitored during culture, and cell activity was evaluated after culture and after a freezing treatment at −80°C, using a standardized activity test based on optical density measurement. Cell metabolism was significantly improved by intermediate shear stress levels (36 and 54 Pa) during culture. Furthermore, biomass concentration, evaluated by optical density, was clearly higher at 36 Pa. Cell lengthening was observed, which was mainly related to the presence of CMC and partly to shear stress level, especially at 36 Pa. Hydrodynamic conditions during culture could affect the membrane permeability of the cell and its resistance to freezing. Cells cultivated at 72 Pa were certainly weakened by shearing forces, and these cultures exhibited lag times twice as long after freezing as those grown at 36 Pa. Furthermore, after freezing, cultures grown at 36 Pa had shorter lag times than at 0 Pa (1.1 and 1.3 h, respectively) and higher specific growth rates (1.24 and 0.99 h −1, respectively).

A Survey of Rheological Properties of One-Component Epoxy Adhesives

Abstract Flow characteristics of seven commercially available one-component epoxy adhesive pastes were measured using a controlled shear stress rheometer and a controlled shear rate rheometer over a temperature range from 5°C to 60°C. Combining data obtained from both controlled rate and controlled stress experiments over a wide range of shear rates, we observed Newtonian flow (shear stress proportional to shear rate) at very low shear rates, a plateau “shear thinning” region at intermediate shear rates, and a second region of linear dependence of shear stress on shear rate at high shear rates. The adhesive pastes exhibited a very broad range of rheological behavior. Two flow parameters important to adhesive application technology, the plastic viscosity and the apparent yield stress, were measured for each adhesive. The plastic viscosity ranged from 11.6 to 329.5 Pa. s; the apparent yield stress ranged from 56.2 to 413 Pa. The temperature dependence of the rheological parameters of the epoxy adhesive pastes was also determined. The results are reported as the activation energies, Eη and Eσ , of plastic viscosity and apparent yield stress, respectively. The apparent yield stress of each adhesive paste was much less sensitive to changes in temperature than was the plastic viscosity. This suggests that the processing characteristics are likely to show qualitative as well as quantitative changes with temperature.

Control of shear stress in the epicardial coronary arteries of humans: Impairment by atherosclerosis

Altered arterial wall shear stress may adversely affect vascular endothelium and contribute to atherogenesis. This study examined the hypothesis that, in humans, dilation of normal coronary arteries with increased flow limits increases in shear stress and that loss of flow-mediated dilation in atherosclerosis results in failure to control shear stress. Coronary blood flow was increased by infusing adenosine (0.022 to 2.2 mg/min) through a 2.5F Doppler flow catheter positioned in the middlesegment of the left anterior descending coronary artery in 8 patients with mild atherosclerosis but no flow-limiting stenosis and in 10 patients with entirely smooth coronary arteries. Quantitative angiography and coronary flow velocity were used to estimate shear stress in a proximalsegment of the left anterior descending artery exposed to increased flow, but not to adenosine. The peak increase in blood flow was the same in smooth (371 ± 65%) and irregular (377 ± 50%) arteries. However, at peak flow, dilation was greater in smooth segments (16.3 ± 2.7%) than in irregular segments (2.0 ± 1.5%) (p < 0.001). In each patient, smooth segments dilated with increasing shear stress (slope 7.4 ± 0.9%), whereas irregular segments dilated less (slope 0.9 ± 0.6%) and showed greater increases in shear stress (p < 0.01). The peak increase in shear stress was less in smooth (189 ± 23%) than in irregular (365 ± 52%) segments (p < 0.01). These results suggest a control mechanism in normal coronary arteries whereby increases in shear stress stimulate vasodilation and thus limit further increases in this force at the endothelial surface. In atherosclerosis, failure of this dilator mechanism results in greater changes in shear stress, which might adversely affect the vessel wall.

Control of shear stress in the epicardial coronary arteries of humans: Impairment by atherosclerosis

Altered arterial wall shear stress may adversely affect vascular endothelium and contribute to atherogenesis. This study examined the hypothesis that, in humans, dilation of normal coronary arteries with increased flow limits increases in shear stress and that loss of flow-mediated dilation in atherosclerosis results in failure to control shear stress. Coronary blood flow was increased by infusing adenosine (0.022 to 2.2 mg/min) through a 2.5F Doppler flow catheter positioned in the middlesegment of the left anterior descending coronary artery in 8 patients with mild atherosclerosis but no flow-limiting stenosis and in 10 patients with entirely smooth coronary arteries. Quantitative angiography and coronary flow velocity were used to estimate shear stress in a proximalsegment of the left anterior descending artery exposed to increased flow, but not to adenosine.The peak increase in blood flow was the same in smooth (371 ± 65%) and irregular (377 ± 50%) arteries. However, at peak flow, dilation was greater in smooth segments (16.3 ± 2.7%) than in irregular segments (2.0 ± 1.5%) (p < 0.001). In each patient, smooth segments dilated with increasing shear stress (slope 7.4 ± 0.9%), whereas irregular segments dilated less (slope 0.9 ± 0.6%) and showed greater increases in shear stress (p < 0.01). The peak increase in shear stress was less in smooth (189 ± 23%) than in irregular (365 ± 52%) segments (p < 0.01).These results suggest a control mechanism in normal coronary arteries whereby increases in shear stress stimulate vasodilation and thus limit further increases in this force at the endothelial surface. In atherosclerosis, failure of this dilator mechanism results in greater changes in shear stress, which might adversely affect the vessel wall.

A cone and plate viscometer for the continuous measurement of blood platelet activation.

A cone and plate viscometer was modified to permit the continuous study of platelet response during shear stress exposure times on the order of one second to 180 seconds. Platelets may be stimulated by uniform, controlled shear stress alone or with the addition of chemical platelet agonists. The time course of platelet aggregation is interpreted from alterations in the apparent optical density of the platelet suspension. The rate and extent of platelet dense granule release is estimated from the intensity of the luminescent reaction of platelet released ATP with firefly luciferase and luciferin. Intracellular calcium ion concentration is determined as a function of shear exposure time through the fluorescence intensity of indo-1(5-), a membrane-permeant pentacarboxylate calcium ion chelator.

Laser diffraction ellipsometry of erythrocytes under controlled shear stress using a rotational viscosimeter

An ektacytometric extension for the Contraves LS-30 viscosimeter is described, as well as the procedure to measure erythrocyte deformability and rouleau formation with this combination. The method error (coefficient of variation) for the measurements of the erythrocyte elongation index appeared to be < 1%, while both intra- and interindividual variation were around 2%. Preliminary clinical experiments performed on blood from different patient groups (i.e., diabetes, uraemia, sickle cell anaemia) clearly demonstrated more rigid erythrocytes than normal. It can be concluded that it is possible now to analyse with 1 instrument: plasma viscosity, whole blood viscosity at shear rates from 0.01 to 236 s −1, erythrocyte deformability and rouleau formation.

EFFECT OF SHEAR STRESS ON PLATELET-PMN LEUKOCYTE INTERACTIONS

A cone and plate viscometer, Coulter counter, and spectrophotometer were used to study platelet modulation of polymorphonuclear leukocyte (PMNL) function caused by controlled shear stress. Aggregation and release of the enzymes β-glucuronidase and lactate dehydrogenase (LDH) were observed. PMNL suspensions in buffer (1.0 × 107 cells/ml, final concentration) did not show any aggregate formation or enzyme release at shear stresses below 150 dynes/cm2 for an exposure time of 1 minute. However, supernatant plasma from sheared platelets initiated PMNL aggregation and β-glucuronidase release at moderate shear stresses (150 dynes/cm2 for 1 minute). These stress-induced alterations were compared with chemotactic factor and calcium ionophore A23187 induced functional changes. LDH release was less than 3% in all the experiments indicating little overt lysis. The shear-induced enzyme release was enhanced by cytochalasin B and heparinized samples showed greater effects than citrated samples. The use of a cyclooxygena...

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Platelet modulation of polymorphonuclear leukocyte shear induced aggregation

A cone and plate viscometer and Coulter Counter were used to study platelet modulation of polymorphonuclear leukocyte (PMNL) aggregation caused by controlled shear stress. As an index of aggregation, the large-particle percentage (LPP) was calculated. This represents the ratio of aggregated cell count to total cell count. PMNL suspensions in buffer (1.0 X 10(7) cells per milliliter, final concentration) did not show any aggregate formation at shear stresses below 150 dynes/cm2 for one minute exposure time (LPP less than 3%). However, there was PMNL aggregation in mixed PMNL and platelet-rich plasma suspensions in this shear stress range. Supernatant plasma from sheared platelets initiated PMNL aggregation at moderate shear stress (150 dynes/cm2 for one minute; LPP, 20.3% +/- 2.5%). In contrast, platelet release factors, such as adenosine diphosphate (2 mumol/L) and serotonin (2 mumol/L) did not cause PMNL aggregation (LPP, 2.9% +/- 1.2% and 3.3% +/- 0.8%, respectively). The use of a cyclo-oxygenase inhibitor (acetylsalicylic acid, 50 mumol/L) did not suppress the aggregation of PMNLs after shear (LPP, 20.1% +/- 2.4%). However, preincubation with nordihydroguaiaretic acid (10 mumol/L), an inhibitor of C-5 and C-12 lipoxygenase, and 6,9- deepoxy-6,9-(phenylimino)-6,8-prostaglandin I1 (U-60257, 10 mumol/L), an inhibitor of C-5 lipoxygenase in human leukocytes, suppressed this aggregation (LPP, 9.1% +/- 2.5% and 10.4% +/- 3.2%, respectively). Also, the formation of lipoxygenase products (5-HETE, 12-HETE, 15-HETE, and LTB4) activated by shear stress was documented by reversed phase- high-performance liquid chromatography (RP-HPLC). These data support the possibility of a cooperation between platelets and leukocytes in shear-induced PMNL aggregation that is dependent on C-12 or C-5 lipoxygenase activity, or both.

Platelet Modulation of Polymorphonuclear Leukocyte Shear Induced Aggregation

A cone and plate viscometer and Coulter Counter were used to study platelet modulation of polymorphonuclear leukocyte (PMNL) aggregation caused by controlled shear stress. As an index of aggregation, the large-particle percentage (LPP) was calculated. This represents the ratio of aggregated cell count to total cell count. PMNL suspensions in buffer (1.0 X 10(7) cells per milliliter, final concentration) did not show any aggregate formation at shear stresses below 150 dynes/cm2 for one minute exposure time (LPP less than 3%). However, there was PMNL aggregation in mixed PMNL and platelet-rich plasma suspensions in this shear stress range. Supernatant plasma from sheared platelets initiated PMNL aggregation at moderate shear stress (150 dynes/cm2 for one minute; LPP, 20.3% +/- 2.5%). In contrast, platelet release factors, such as adenosine diphosphate (2 mumol/L) and serotonin (2 mumol/L) did not cause PMNL aggregation (LPP, 2.9% +/- 1.2% and 3.3% +/- 0.8%, respectively). The use of a cyclo-oxygenase inhibitor (acetylsalicylic acid, 50 mumol/L) did not suppress the aggregation of PMNLs after shear (LPP, 20.1% +/- 2.4%). However, preincubation with nordihydroguaiaretic acid (10 mumol/L), an inhibitor of C-5 and C-12 lipoxygenase, and 6,9-deepoxy-6,9-(phenylimino)-6,8-prostaglandin I1 (U-60257, 10 mumol/L), an inhibitor of C-5 lipoxygenase in human leukocytes, suppressed this aggregation (LPP, 9.1% +/- 2.5% and 10.4% +/- 3.2%, respectively). Also, the formation of lipoxygenase products (5-HETE, 12-HETE, 15-HETE, and LTB4) activated by shear stress was documented by reversed phase-high-performance liquid chromatography (RP-HPLC). These data support the possibility of a cooperation between platelets and leukocytes in shear-induced PMNL aggregation that is dependent on C-12 or C-5 lipoxygenase activity, or both.

Integrity of arterial endothelium following acute exposure to high shear stress.

Shear-induced injury or denudation of arterial endothelium has been implicated in atherogenesis. This study reports on an in vitro technique for imposing high, controlled shear stresses on endothelium. Samples of dog aorta were mounted in a chamber so that the endothelium was 1 mm +/- 0.03 mm from a rotating disc. The chamber was filled with a high viscosity solution (10% polyvinyl pyrrolidone in Tyrode's solution, viscosity = 1.97 +/- .07 Poise) which was sheared over the endothelium by the disc. A servo amplifier drove the motor that rotated the disc, so that motor RPM (therefore shear stress) could be made to follow either steady or pulsatile signals played into the amplifier. Acute (10 min-1 hr) exposure to steady shear stresses of up to 2000 dyne/cm2 did not cause gross endothelial injury or denudation. Exposure of endothelium to pulsatile shear stresses that followed a tape recording of physiological flow waveforms (electromagnetic flowmeter) did not cause gross injury or denudation even when peak shear exceeded 1500 dyne/cm2. Furthermore exposure to high shear stress did not degrade the nonthrombogenic nature of the endothelium because subsequent platelet adhesion was poorly and negatively correlated with shear stress.