Lutz Helmut Pauli

Transient stress‐based and strain‐based hemolysis estimation in a simplified blood pump

We compare two approaches to numerical estimation of mechanical hemolysis in a simplified blood pump model. The stress-based model relies on the instantaneous shear stress in the blood flow, whereas the strain-based model uses an additional tensor equation to relate distortion of red blood cells to a shear stress measure. We use the newly proposed least-squares finite element method (LSFEM) to prevent negative concentration fields and show a stable and volume preserving LSFEM for the tensor equation. Application of both models to a simplified centrifugal blood pump at three different operating conditions shows that the stress-based model overestimates the rate of hemolysis. The strain-based model is found to deliver lower hemolysis rates because it incorporates a more detailed description of biophysical phenomena into the simulation process.

Strain-based blood damage estimation for computational design of ventricular assist devices

Aims Computational fluid dynamics (CFD) is used to predict damage of red blood cells (RBCs) in ventricular assist devices (VADs). The damage is measured by the hemoglobin ratio in the blood plasma. Methods A power law is used to relate the hemoglobin ratio to shear stress and exposure time. For the shear stress measure, the common stress-based model is compared to a strain-based model, which predicts the deformation of RBCs in the VAD. For both models an Eulerian approach is used. In this study, new parameters are determined for the power law of the strain-based model. Hereby, the power law is fitted to data of experiments performed at the University of Maryland, Baltimore. Results As an example, blood damage in a benchmark blood pump of the U.S. Food and Drug Administration (FDA) is computed with a stress-based and a strain-based model using the new parameter set as well as parameter sets that were obtained in previous studies. Conclusions Critical locations in the pump, as identified with the stress-based and the strain-based model, differ significantly between the two models.

Strain-Based Blood Damage Modeling : Current Status and Future Perspectives

Abstracts from the 44th ESAO and 7th IFAO Congress, 6-9 September 2017, Vienna, Austria.Background: Computational Fluid Dynamics is a useful tool for developing Ventricular Assist Devices (VADs). However, the results are not necessarily trusted, and validation studies are essential to increase confidence. Validation studies usually require expensive, time consuming, for example Particle Image Velocimetry (PIV). Simpler validation methods, which could be incorporated more naturally into the design process, are therefore desirable. Aim: The aim of this work was to investigate the extent to which design changes in the computational domain produced measureable effects on the experimental pressure-flow characteristics, with a view to using rapid prototyping of early design iterations to increase confidence in CFD. Methods: A small pump, similar to a VAD, was designed using CAD. The geometry was meshed and CFD calculated using ANSYS CFX. Mesh studies were conducted, and several turbulence methods were investigated, to assess errors. Transient simulations were performed to estimate the steady flow pressure- flow curves for a range of speeds. Based on examining the results a series of manual design changes were made and the simulation results were updated for each design iteration. A physical prototype of the pump was created from 3D printed parts; these fitted together allowing replacement of individual components. The pump was driven with an external motor and shaft. The pump is currently being tested in a custom designed rig. Results: For the original design the operating speed to reach the design point (100 mmHg at 5 l/min) was 10,500 rpm. At this speed the design iterations resulted in changes to the pressure head of between 10 and 200 mmHg; alternatively speed changes of 600 to 5000 rpm were required to produce the design point. Conclusions: These pressure differences are greater than both CFD and transducer measurement errors, meaning the design changes should produce measurable effects. However, rapid prototyping also has inherent errors. Good agreement between CFD and experimental pressure-flow curves in early design iterations could be extrapolated to assume good agreement at the later design stages.

On the Significance of Exposure Time in Computational Blood Damage Estimation

The reliability of common stress-based power law models for hemolysis estimations in blood pumps is still not satisfying. Stress-based models are based on an instantaneous shear stress measure. Therefore, such models implicitly assume that red blood cells deform immediately due to the action of forces. In contrast, a strain-based model considers the entire deformation history of the cells. By applying a viscoelastic tensor equation for the stress computation, the effect of exposure time is represented as a biophysical phenomenon. Comparisons of stress-based and strain-based hemolysis models in a centrifugal blood pump show very significant differences. Stress peaks with short exposure time contribute to the overall hemolysis in the stress-based model, whereas regions with increased shear and long exposure time are responsible for damage in the strain-based model.