Effect of Shear on Hemolysis in a Centrifugal Blood Pump

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In order to reduce hemolysis in centrifugal blood pumps, it is necessary to understand the mechanism of pump internal flow and at the same time the mechanism of mechanical damage of red blood cells. In this study, using a cone and a plate rheoscope, hemolysis was investigated in a steady shear flow, and the relationship between shear rate and hemolysis was obtained. Deformation of red blood cells was observed in a cone and a plate steady shear flow and in parallel plates unsteady shear flow. Influence of steady and unsteady shear on the deformation of red blood cells was investigated. The results indicated that the cells showed an increase in their surface area for the shear rate higher than 100Pa, which was corresponding a yield point. We carried out unsteady flow experiment up to 600Hz, and obtained the same results as the steady flow. Our time scale of 1 ms order is shorter than 10 ms reported by the past researcher.We visualized the pump internal flow by a streak line method and a path line method. The radial reversal flow was observed in the pump. The shear stress distribution on the pump casing cover was measured by means of a cyclic conditional sampling method and a flush-mounted hot-film probe. The shear stress increased with pump rotational speed, and for the constant rotational speed the higher shear stress was measured at the impeller passage rather than the center between the impellers. This high shear region is limited to very thin boundary layer. However, when red blood cells pass through this boundary layer, the cells suffer very high deformation, which would produce hemolysis. Pump design that makes this high shear layer are narrower is important to reduce hemolysis.