Biorheology

The mechanical stimulus of shear stress has to date been neglected when studying the adhesion of cancer cells to the endothelium. Confluent monolayers of endothelial cells were subjected to either 4 or 15 hours of arterial shear stress. Adhesion of nonmetastatic (MCF‐7) and highly metastatic (MDA‐MB‐435) human breast cancer cells was then quantified using a detachment assay carried out inside the parallel plate flow chamber. Four hours of shear stress exposure had no effect on adhesion. However, 15 hours of shear stress exposure led to marked changes in the ability of the endothelial monolayer to bind human breast cancer cells. An increase in adhesive strength was observed for nonmetastatic MCF‐7 cells, while a decrease in adhesive strength was observed for highly metastatic MDA‐MB‐435 cells. Hence, endothelial shear stress stimulation does influence the adhesion of cancer cells to the endothelium and can have different effects on the adhesion of cancer cells with different metastatic potentials. Furthermore, adhesion of nonmetastatic and highly metastatic human breast cancer cells may be controlled by two different endothelial cell adhesion molecules that are differentially regulated by shear stress. Immunohistochemistry confirmed that shear stress did in fact differentially regulate endothelial cell adhesion molecule expression.

Fluid mechanics associated with blood flows induced by the so‐called myocardial bridge (MB) has been studied systematically using a computational fluid dynamic modeling of the Newtonian, incompressible, two‐dimensional, unsteady flow in a channel with a time‐dependently flushing in/out indentation. During each cycle, a train of vortex wave flow was observed downstream of the phasic stenosis and both upper and lower walls suffer severely from consistently high, oscillating wall shear stresses (WSS). Extensive studies were conducted on the influence of the Reynolds number, the geometry and the Strouhal number of the MB movement on the nature of the vortex flow and the time‐dependent wall shear stress distribution.

Special attention was drawn to the relationship between the vortex wave and the pressure distribution. It was found that the pressure gradient changed markedly during one cycle, which was apparently dominated by the dynamics of the indentation. A steep, adverse pressure gradient was observed when the indentation was flushing out, which corresponded to the existence of the most developing vortices. It implies the possibility that the MB in a coronary artery can produce an extremely low pressure region immediately downstream of the phasic stenosis, where elastic choking or collapse of the coronary artery might occur.

This study is designed to better understand the mode of lymph transport, particularly through the extrinsic pumping by external compression of the lymph vessel. The pressure‐diameter relationship of lymphatic segments isolated from the canine thoracic duct was examined using a laser optical micrometer measurement system. Results revealed that the thoracic duct displayed a high extensibility or compliance in the physiological pressure range, yet became progressively less so with increasing internal pressure. The calculated incremental circumferential modulus of the thoracic duct under physiological pressure (range of 2 to 6 cm H $_{2}$ O) showed values ranging from 1.2 × 10 $^{4 }$ to 3.61 × 10 $^{5}$ dyn/cm $^{2}$ . At a pressure of 35 cm H $_{2}$ O, the modulus reached a limiting value of approximately 6.0 ×10 $^{6}$ dyn/cm $^{2}$ . In the physiological pressure range, the relative wall thickness ( $h/R_\mathrm{o}$ ) of the canine thoracic duct was approximately 3.5%, which was much lower than that reported for canine arterial segments and similar in value to that of the canine jugular vein. In conclusion, the pressure‐diameter curve of the canine thoracic duct was shown to resemble that of venous vessels. However, the circumferential elastic modulus of the thoracic duct wall was lower than the moduli of veins, proving that lymphatics are more compliant than veins. This suggests lymph flow in the thoracic duct may be better promoted by external compression of the lymphatic vessel.

Swelling and flow properties of tubular poly(vinyl alcohol) (PVA) hydrogels prepared with the cooling method were investigated using an inflation testing method. When the tubular hydrogel in liquid paraffin was inflated by using liquid paraffin as a pressure transmitting medium, namely in the case that the liquids inside and outside the gel are both liquid paraffin (P/P combination), the gel showed a slight volume change determined by Poisson’s ratio of the gel. When the gel in water was inflated by liquid paraffin (P/W combination), the gel swelled to large extent compared with the case of P/P. The hydrogel in W/W combination, namely in the situation that the gel was immersed in water and also inflated by water, showed a very large volume change if the comparison was done at the same pressure. The origin of the volume change in P/P, P/W and W/W combinations is discussed. The volume change in P/P was governed by the Poisson ratio as a material constant (µ $_{0}$ ) of the PVA gels, and the gels swelled by the change in the application of pressure (or deformation) in P/W. The volume change in W/W was closely related to the flow of solvent in the gel.

A method based on dielectric properties of dispersed systems was developed to investigate red blood cell (RBC) aggregation in blood and RBC suspensions. Measurements of capacitance and resistance were made in a rectangular channel at low (0.2 MHz) and high (14 MHz) frequencies relative to the mid‐point of the β‐dispersion range. Compared to capacitance, minimal post‐shearing changes of resistance were observed; capacitance changes at 0.2 MHz were two orders of magnitude larger than those at 14 MHz and hence subsequent measurements were carried out at the lower frequency. It is shown that post‐shearing changes in the capacitance are affected by the recovery of RBC shape and relaxation processes at the electrode‐suspension interface. However, the dominant factor contributing to time‐dependent changes in the capacitance is the dynamic process of RBC aggregation. It is experimentally shown that the time record of the capacitance at 0.2 MHz quantitatively reflects the aggregation process in RBC‐plasma suspensions with hematocrit up to 0.56 (v/v) and in suspensions of RBCs in artificial aggregating media. It is concluded that a dielectric approach to the study of RBC aggregation in whole blood offers great potential for basic studies and for diagnostic use.