Biorheology - Volume 43, issue 2

Authors: Chien, Shu

Article Type: Research Article

Abstract: Vascular endothelial cells (EC) play significant roles in regulating circulatory functions. Shear stress and stretch can modulate EC functions by activating mechano-sensors, signaling pathways, and gene and protein expressions. Laminar shear stress with a significant forward direction causes transient activations of monocyte chemotactic protein-1 (MCP-1), sterol response element binding protein (SREBP), and proliferative genes. Sustained laminar shear stress down-regulates these genes and up-regulates genes that inhibit EC growth. In EC subjected to complex flow patterns with little forward direction, activations of MCP-1, SREBP, and proliferation genes become sustained, and mitosis and apoptosis are enhanced. Cyclic uniaxial stretch causes actin stress …fibers to orient perpendicular to stretch direction, with a consequent reduction of intracellular stress, transient JNK activation, and protection of EC against apoptosis. Cyclic biaxial stretch without a preferred direction has opposite effects. In the straight part of arterial tree, laminar shear stress with a net forward direction and uniaxial strain in the circumferential direction have anti-atherogenic effects. At vascular branch points, the complex shear flow and mechanical strain with little net direction are atherogenic. Therefore, the direction of stress has important influences on the biorheological effects of flow and deformation on vascular endothelium in health and disease. Show more

Keywords: Endothelial cells, gene expression, mechanotransduction, shear stress, signal transduction, strain

Citation: Biorheology, vol. 43, no. 2, pp. 95-116, 2006

Authors: Iyo, Toshiya | Sasaki, Naoki | Maki, Yasuyuki | Nakata, Mitsuo

Article Type: Research Article

Abstract: In 1993 we proposed an empirical formula for describing the relaxation modulus of cortical bone based on the results of stress relaxation experiments performed for 1×105 sec: \[E(t)=E_{0}\bigl\{A\exp \bigl[-(t/\tau_{1})^{\beta}\bigr]+(1-A)\exp (-t/\tau_{2})\bigr\},\quad (0<A,\ \beta <1\ \mbox{and}\ \tau_{1}\ll\tau_{2}),\] where E0 is the initial value of the relaxation modulus, A is the portion of the first term, τ1 and τ2 are characteristic relaxation times, and β is a shape factor [Sasaki et al., J. Biomechanics 26 (1993), 1369]. Although the relaxation properties of bone under various external conditions were described well by the above equation, recent experimental …results have indicated some limitations in its application. In order to construct an empirical formula for the relaxation modulus of cortical bone that has a high degree of completeness, stress relaxation experiments were performed for 6×105 seconds. The second term in the equation was determined as an apparently linear portion in a log E(t) vs t plot at t>1×104 sec. The same plot for experiments performed for 6×105 seconds revealed that the linear portion corresponding to the second term was in fact a curve with a large radius of curvature. On the basis of this fact, we proposed a second improved empirical equation \[E(t)=E_{0}\bigl\{A\exp \bigl[-(t/\tau_{1})^{\beta}\bigr]+(1-A)\exp \bigl[-(t/\tau_{2})^{\gamma}\bigr]\bigr\},\quad (0<A,\ \beta,\ \gamma <1)\] to describe the stress relaxation of cortical bone. The early stage of the stress relaxation process, which could not be expressed by the first, is well described by the second equation. Show more

Keywords: Bone, collagen, viscoelasticity, relaxation modulus, KWW function

Citation: Biorheology, vol. 43, no. 2, pp. 117-132, 2006

Authors: Mark, M. | Häusler, K. | Dual, J. | Reinhart, W.H.

Article Type: Research Article

Abstract: A viscometer for bedside blood measurements was developed, consisting of an oscillating resonator probe mounted directly into a disposable vacutainer tube for blood withdrawal. It was tested in vitro on blood samples with variable hematocrits (20–60%), increasing fibrinogen concentrations (0–20 g/l), increasing concentrations of an admixed radiographic contrast medium and erythrocyte suspensions in dextran 40 and dextran 70. Results were compared with those obtained with a conventional Couette viscometer. Oscillating viscometry yielded generally higher values than Couette viscometry, and had a good sensitivity for changes in hematocrit with a good correlation between the two methods (r=0.96, p<0.0001). Oscillating viscosity depended …on the resonator frequency, it was higher at 3900 Hz than at 215 Hz, suggesting a viscoelastic behavior of blood. Erythrocyte aggregation, induced by increasing fibrinogen concentrations or dextran 70, affected oscillating viscometry. At a high frequency, i.e. a smaller penetration depth of the shear wave, oscillating viscosity tended to decrease, which suggests a depletion of the boundary layer from erythrocytes when they aggregate. At low frequency with a deeper shear wave penetration (about 50 μm), erythrocyte aggregation increased oscillating viscosity. Bedside tests in 17 patients with coronary heart disease and 10 controls confirmed the easy practicability of the test and showed lower oscillating viscosity in these patients despite higher fibrinogen concentrations presumably due to increased erythrocyte aggregation. We conclude that oscillating viscometry is an interesting bedside test, which is capable of providing new information on the biorheology of the erythrocyte-poor boundary layer near the vessel wall. Show more

Keywords: Aggregation, blood, dextran, erythrocyte, fibrinogen, viscosity

Citation: Biorheology, vol. 43, no. 2, pp. 133-146, 2006

Authors: Faivre, Magalie | Abkarian, Manouk | Bickraj, Kimberly | Stone, Howard A.

Article Type: Research Article

Abstract: It is well known that when a suspension of cells flows in small vessels (arterioles or venules), there exists a cell-free layer of a few microns adjacent to the vascular walls. Using an in vitro model, we show experimentally that for a fixed flow rate a geometrical constriction in the flow can artificially enhance the cell-free layer. Also, we show that rapid variation of the geometry coupled to the deformability of the cells can dramatically modify their spatial distribution in the channel. The effects of the constriction geometry, flow rate, suspending fluid viscosity, cell concentration, and cell deformability are studied …and the results are interpreted in terms of a model of the hydrodynamic drift of an ellipsoidal cell in a shear flow. We propose a microfluidic application of this focusing effect for separation of the red blood cells from the suspending plasma. Show more

Keywords: Lift force, cell-free layer, red blood cells, hemodilution

Citation: Biorheology, vol. 43, no. 2, pp. 147-159, 2006

Authors: Ulker, Pinar | Alexy, Tamas | Meiselman, Herbert J. | Başkurt, Oguz K.

Article Type: Research Article

Citation: Biorheology, vol. 43, no. 2, pp. 161-166, 2006