Biorheology - Volume 35, issue 4-5

Authors: Frame, Mary D.S. | Chapman, Gary B. | Makino, Yoshia | Sarelius, Ingrid H.

Article Type: Research Article

Abstract: Our purpose was to test a scale model of the microcirculation by measuring the shear forces to which endothelial cells were exposed, and comparing this to computer simulations. In vitro experiments were performed to measure the 2‐dimensional projected velocity profile along endothelial cell lined microchannels (D‐shaped, 10–30 \mu m radius, n=15 ), or in microchannels without endothelial cells (n=18 ). Microchannels were perfused with fluorescently labeled microspheres (0.5 \mu m dia., < 1%) suspended in cell culture media. The velocity of individual microspheres was obtained off‐line (videorecording), using an interactive software program; velocity was determined …as the distance traveled in one video field (1/60 s). Mass balance was verified in the microchannels by comparing the microsphere velocities to the perfusion pump rate. In confluent endothelial cell lined microchannels, a velocity profile was obtained as microspheres passed an endothelial cell nucleus (identified by fluorescent dye), and again, for a paired region 100 \mu m away without nuclei (cytoplasm region). The velocity profile was significantly shifted and sharpened by the endothelial cell nucleus, as anticipated. Over the nucleus, data are consistent with a normal sized nucleus extending into the lumen, further confirming that this scale model can be used to determine the wall shear stress to which endothelial cells are exposed. Using the experimental bulk phase fluid parameters as boundary conditions, we used computational fluid dynamics (CFD) to predict the expected wall shear stress gradient along an endothelial cell lined D‐shaped tube. The wall shear stress gradient over the nucleus was 2‐fold greater in the radial versus axial directions, and was sensitive to lateral versus midline positioned nuclei. Show more

Keywords: Blood flow, computational fluid dynamics, shear gradient

Citation: Biorheology, vol. 35, no. 4-5, pp. 245-261, 1998

Authors: Gijsen, F.J.H. | van de Vosse, F.N. | Janssen, J.D.

Article Type: Research Article

Abstract: An experimental investigation of the wall shear stress distribution downstream of a backward‐facing step is carried out. Flow in this geometry is considered to be representative of flow in large and medium sized curved arteries and bifurcations. The wall shear stress distribution was determined by measuring the deformation of a gel layer, attached to the wall downstream of the step. Speckle pattern interferometry was applied to measure the deformation of the gel layer. The measured deformation, combined with the properties of the gel layer, served as an input for a finite element solid mechanics computation to determine the stress distribution …in the gel layer. The wall shear stress, required to generate the measured deformation of the gel layer, was determined from these computations. A Newtonian buffer solution and a non‐Newtonian red blood cell suspension were used as measuring fluids. The deformation of the gel layer was determined for a Newtonian buffer solution to evaluate the method and to obtain the properties of the gel layer. Subsequently, the wall shear stress distribution for the non‐Newtonian red blood cell suspension was determined for three different flow rates. The inelastic non‐Newtonian Carreau–Yasuda model served as constitutive model for the red blood cell suspension. Using this model, the velocity and wall shear stress distribution were computed by means of a finite element fluid mechanics computation. From the comparison between the numerical and the experimental results, it can be concluded that wall shear stresses, induced by the red blood cell suspension, can be modeled accurately by employing a Carreau–Yasuda model. Show more

Keywords: Wall shear stress, red blood cell suspension, backwards‐facing step flow, speckle interferometry, FEM simulation

Citation: Biorheology, vol. 35, no. 4-5, pp. 263-279, 1998

Authors: Ghalichi, Farzan | Ghalichi, Farzan | Deng, Xiaoyan | De Champlain, Alain | Douville, Yvan | King, Martin | Guidoin, Robert

Article Type: Research Article

Abstract: Moderate and severe arterial stenoses can produce highly disturbed flow regions with transitional and or turbulent flow characteristics. Neither laminar flow modeling nor standard two‐equation models such as the k ‐\varepsilon turbulence ones are suitable for this kind of blood flow. In order to analyze the transitional or turbulent flow distal to an arterial stenosis, authors of this study have used the Wilcox low‐\mathit{Re} turbulence model. Flow simulations were carried out on stenoses with 50, 75 and 86% reductions in cross‐sectional area over a range of physiologically relevant Reynolds numbers. The results obtained with this …low‐\mathit{Re} turbulence model were compared with experimental measurements and with the results obtained by the standard k ‐\varepsilon model in terms of velocity profile, vortex length, wall shear stress, wall static pressure, and turbulence intensity. The comparisons show that results predicted by the low‐\mathit{Re} model are in good agreement with the experimental measurements. This model accurately predicts the critical Reynolds number at which blood flow becomes transitional or turbulent distal an arterial stenosis. Most interestingly, over the \mathit{Re} range of laminar flow, the vortex length calculated with the low‐\mathit{Re} model also closely matches the vortex length predicted by laminar flow modeling. In conclusion, the study strongly suggests that the proposed model is suitable for blood flow studies in certain areas of the arterial tree where both laminar and transitional/turbulent flows coexist. Show more

Keywords: Turbulent flow, numerical modeling, hemodynamics, wall shear stress

Citation: Biorheology, vol. 35, no. 4-5, pp. 281-294, 1998

Authors: Fulford, Glenn R. | Katz, David F. | Powell, Robert L.

Article Type: Research Article

Abstract: A modified resistive force theory is developed for a spermatozoon swimming in a general linear viscoelastic fluid. The theory is based on a Fourier decomposition of the flagellar velocity, which leads to solving the Stokes flow equations with a complex viscosity. We use a model spermatozoon with a spherical head which propagates small amplitude sinusoidal waves along its flagellum. Results are obtained for the velocity of propulsion and the rate of working for a free swimming spermatozoon and the thrust on a fixed spermatozoon. There is no change in propulsive velocity for a viscoelastic fluid compared to a Newtonian fluid. …The rate of working does change however, decreasing with increasing elasticity of the fluid, for a Maxwell fluid. Thus the theory predicts that a spermatozoon can swim faster in a Maxwell fluid with the same expenditure of energy for a Newtonian fluid. Show more

Keywords: Sperm motility, viscoelastic fluid, flagellar, spermatazoa

Citation: Biorheology, vol. 35, no. 4-5, pp. 295-309, 1998

Authors: Pawłowski, P. | Fikus, M.

Article Type: Research Article

Abstract: The influence of a prolonged and recurrent shear stress created by a periodic electric field on the mechanical properties of Neurospora crassa cells was investigated. Conditions were found under which modifications of cellular structures responding to stress become irreversible, and plastic flow of the viscoelastic structural elements is observed. The symmetry of the response of the cell under stress application and relaxation was lost, when compared to the reference conditions. To interpret the results a general rheological model was proposed. As previously described (Pawłowski et al., 1997), the existence of the three hypothetical supramolecular regions of the membrane (F, S …and 0) was suggested. Rheological parameters for the above regions were calculated. Theoretical functions were satisfactorily fitted to the experimental results. Show more

Keywords: Cellular rheology, periodic electric field, shear stress, membrane skeleton, irreversible modifications, rheological models

Citation: Biorheology, vol. 35, no. 4-5, pp. 311-324, 1998

Authors: Riquelme, B.D. | Valverde, J. | Rasia, R.J.

Article Type: Research Article

Abstract: A new method to find directly complex viscoelastic parameters (CVP) of red blood cells (RBC) is presented in this paper. Experimental determinations were carried out in an Erythrodeformeter (Rasia et al., 1986) operating in oscillating mode (0.5 to 3.5 Hz). The Erythrodeformeter performs direct determination of CVP of erythrocytes undergoing sinusoidal shear stresses by laser diffractometry. The measurements lead to the determination of mean values of the four components of erythrocyte complex viscoelasticity. The influence of the alterations induced on erythrocyte membrane by vegetable lectins (Ulex europaeus, wheat germ agglutinin and Enterolobium contorticilicum seeds) was analyzed to verify the sensitivity …of this method. Differences observed between the CVP parameters of treated cells and the ones corresponding to control samples (non treated cells) are analyzed. Results obtained from cells treated with wheat germ agglutinin agree with observations published by Smith and Hochmuth (1982). Determinations of RBC complex viscoelasticity carried out by laser diffractometry could become an important tool to understand the influence of the factors associated with alterations of the rheologic properties of RBC membrane, which can affect the in vivo blood flow. Show more

Keywords: Erythrocyte rheology, viscoelasticity, Erythrodeformeter, lectins, laser diffractometry

Citation: Biorheology, vol. 35, no. 4-5, pp. 325-334, 1998

Authors: Picart, C. | Picart, C. | Piau, J.M. | Galliard, H. | Carpentier, P.

Article Type: Research Article

Abstract: Red blood cell (RBC) aggregation is of prime importance in vivo and in vitro for low flow rates. It may be estimated by rheometrical measurements at low shear rates, but these are perturbed by slip and migrational effects which have already been highlighted in the past. These effects lead to a torque decay with time so that the true value of the stress at low shear rates may be greatly underestimated. Elevated aggregation being associated with different diseases, pathological blood samples show more pronounced perturbing effects and a strong time dependency in low shear rate rheometry. To test the …dependence of slip and migrational effects on RBC aggregation, and particularly to determine the way in which they depend upon fibrinogen concentration ([Fb]), a home‐made measuring system with roughened internal and external walls (170 \mu m roughness) was used to study low shear rate rheometry for RBC suspensions in PBS buffer containing albumin (at 50 g/l) and fibrinogen at various concentrations. The influences of hematocrit, shear rate, and fibrinogen concentration were investigated. Particular attention was paid to data acquisition at low shear rates (10^{-3} s^{-1} to 3\times10^{-2} s^{-1} ). The combined influence of hematocrit and fibrinogen was investigated by adjusting hematocrit to 44 or 57% and fibrinogen concentration ([Fb]) to 3.0–4.5–6.5 g/l. Microscopic observations of the blood samples at rest were performed. They showed that different structures were formed according to fibrinogen concentration. The rheometrical measurements indicated that torque decay with shearing duration was strongly dependent on fibrinogen concentration and on shear rate at fixed hematocrit. Migrational and slip effects were more pronounced as shear rate decreased, fibrinogen concentration was raised, and hematocrit was lowered. The results have been explained on the basis of the expected microstructure of flowing blood in relation to the microscopic observations at rest. Show more

Keywords: Hemorheology, rheometry, RBC aggregation, fibrinogen, microstructure

Citation: Biorheology, vol. 35, no. 4-5, pp. 335-353, 1998

Authors: Huang, Kun | Zeng, Yanjun | Xia, Hetao | Liu, Changgui

Article Type: Short Communication

Abstract: The aim of this study was to investigate the effects of a surgical limb lengthening procedure on the biorheological features of some lengthened soft tissues. In this procedure external fixators were applied to goats’ right radius to stretch the tissues. The right forelegs of goats were lengthened by 2, 4 cm, respectively. After lengthening ceased, the goats were examined after different periods of time. The lengthened median nerves, arteries and veins were harvested and used to study their biorheological features. Tensile strength of lengthened and control specimens were measured and their stress relaxation features and stress‐strain relationships were studied. Results …showed that at the beginning of recovery, the stress‐strain curves, relaxation curves and tensile strengths of the lengthened specimens began to deviate from those of their controls. However, with increasing recovery time, the curves and tensile strength of the lengthened specimens reverted to those of their controls. All the tissues studied exhibited the same behavior. Show more

Keywords: Limb lengthening, biorheological features, median nerves, arteries, veins, stress relaxation, stress‐strain, tensile strength

Citation: Biorheology, vol. 35, no. 4-5, pp. 355-363, 1998