Tracking strain in ventricular assist devices

Implantable ventricular assist devices offer hope for many heart attack victims waiting for donor hearts. These autonomous devices are intended as a medium-term bridge to transplant, or, if enough progress is made, even as a permanent clinical solution. The design challenges are: limiting the shear stress, avoiding thrombus formation, and maintaining pump efficiency. High shear stresses are particularly evident in mechanical biomedical devices; they cause primarily hemolysis of red blood cells, depending on dose and time of exposure. The distribution of the shear stresses in the complex flow in a rotary blood pump, as well as the measure of the blood cells' exposure to these pathological conditions, are difficult to obtain. Device designers often must decide the details of pump configuration guided only by the global, time- and space-averaged, indicators of the shear stress inside the pump, such as hemoglobin release measurements made on the exiting blood stream. Here we compute the three-dimensional, unsteady blood flow in a detailed model of the implantable centrifugal blood pumps being developed at the Baylor College of Medicine by treating blood as a Newtonian liquid. The scalar stress measures currently used for analyzing computational results do not distinguish tensile and compressive stresses, and do not differentiate persistent straining-which is expected to yield hemolysis-from cyclic straining-which should affect blood cells to a much lower extent. We are examining tensor measures of accumulated strain experienced by typical individual blood cells and we are correlating such measures with available hemolysis data, with the goal of deriving better blood damage indicators from the computed flow field.