Experimental and computational studies were performed to elucidate the role of turbulent stresses in mechanical blood damage (hemolysis). A suspension of bovine red blood cells (RBC) was driven through a closed circulating loop by a centrifugal pump. A small capillary tube (inner diameter 1 mm and length 70 mm) was incorporated into the circulating loop via tapered connectors. The suspension of RBCs was diluted with saline to achieve an asymptotic apparent viscosity of 2.0 +/- 0.1 cP at 23 degrees C to produce turbulent flow at nominal flow rate and pressure. To study laminar flow at the identical wall shear stresses in the same capillary tube, the apparent viscosity of the RBC suspension was increased to 6.3 +/- 0.1 cP (at 23 degrees C) by addition of Dextran-40. Using various combinations of driving pressure and Dextran mediated adjustments in dynamic viscosity Reynolds numbers ranging from 300-5,000 were generated, and rates of hemolysis were measured. Pilot studies were performed to verify that the suspension media did not affect mechanical fragility of the RBCs. The results of these bench studies demonstrated that, at the same wall shear stress in a capillary tube, the level of hemolysis was significantly greater (p < 0.05) for turbulent flow as compared with laminar flow. This confirmed that turbulent stresses contribute strongly to blood mechanical trauma. Numerical predictions of hemolysis obtained by computational fluid dynamic modeling were in good agreement with these experimental data.

Mechanical blood trauma is still one of the major obstacles in the development of cardiovascular devices. The mechanisms of this blood damage are heterogeneous and are not completely identified. Expe&ental and compurational studies were performed to elucidate the role of turbulent sueses in hemolysis. For the experimental study suspensions of bovine red blood cells (RBCs) in saline or in dextran solution were driven through a closed circulating loop by a centrifugal pump. A small capillary tube with the inner diameter of 1 mm and the length of 50 mm was incorporated into loop with tapered connectors. It was shown that, at the same wall shear sees, the level of hemolysis is significantly higher under turbulent flow conditions than laminar flow conditions. This demonstrated that turbulent stresses contribute strongly to blood trauma. These results concurred with predicted hemolysis by computational fluid dynamics (CFD) modeling of the same blood flow conditions. Jntmductioa The development of heart-assist devices and prosthetic hem valvesbas focused increasing attention to trauma of red blood cells (RBCs). Despite many investigations of blood trauma in heart-assist devices, our understanding about the mechanisms and criteria of RBC damage remains quite limited. The efforts to predict clinical blood trauma analytically using mathematical models have not bsen very successful thus far. JJI fact, determining the mechanisms of blood trauma in mechanically assisted circulation is essential for all of the artificial organ efforts. In artificial organs, abnormal rates of hemolysis are. known to be directly related to the shear stresses and exposure times experienced by red blood cells, however, the contribution of turbulent stresses to blood damage is not fully elucidated. Methods. For the experimental studies blood was drivenh u g h a closed circulating loop by a centrifugal pump. Smell capillary tube with the inner diameters of 1 mm and the length of 50 mm was incorporated into loop with tapend connectors. Suspension of bovine RBCs in d i n e with viscosity of 1.56 CP was used for a study of turbulent flow with Reynolds Numbers ranged from 4500 to 6ooo and wall shear stresses ranged from 200 to 500 P a The same suspension was driven at the laminar flow condition with Reynolds numbcr of 2500 and wall shear stress of 100 Pa. To provide. laminar flow at the same wall shear s~~esses in the capillary be, RBCs were suspended in Dextran-40 solution at the same hematocrit and with the resulting viscosity of 5.2 cP. llerefore, the obtained range of Reynolds numbers was from 300 to 1600. Each test was performed for 90 min, during which blood samples for measurement of plasma free hemoglobin were withdrawn every 30 min. To control the ability of dextran to protect blood cells from mechanical damage, the mechanical &agility of RBCs suspended in each suspension medium was measured by using a standard fragility test (1).For the theoretical analysis we have utilized computational fluid dynamics (CFD) to quantitatively provide us with t...