Fluid dynamics characterization of biomedical implantable devices: experimental measurements and numerical simulation

Mechanical heart valves, specifically bileaflet valves, are widely applied to take place of diseased native valves. These prosthetic valve replacements exhibit good long-term mechanical durability. However, mechanical loading of blood is associated to complications (hemolysis and thrombogenicity), which represent important safety requirements for this kind of valves. In this thesis work, Particle Image Velocimetry (PIV) was applied as experimental tool for evaluating fluid dynamics characterization of biomedical implantable devices, especially for bileaflet mechanical heart valves (BMHVs). The procedures of PIV measurement of same target were repeatedly executed for multiple experimental validations of numerical data. The reproducibility of experimental data from PIV multiple measurements was proved reliable with the coefficients of variation for hemodynamic properties below 5%. These hemodynamic parameters such as velocity, turbulence shear stress (TSS) and hemolysis index (HI) did not only supply a good research basis for numerical simulation, but also directly analyze the potential risks for blood damages due to the artificial implantations. The maximum TSS captured in leakage jets of valve type 1 was around 40 Pa and that of valve type 2 was 20 Pa. The simulations using computational fluid dynamics (CFD) were carried out based on the physical parameters of experimental conditions. Good agreements between PIV and CFD were observed according to the comparisons in near-hinge regions. Furthermore, CFD data were used to investigate the hemodynamic properties inside of hinge region, where PIV measurements cannot be carried out due to optical inaccessibility. The experimental work was carried out entirely at the Istituto Superiore di Sanita (ISS) in Rome, Italy, whereas the simulation work was based on the cooperation between ISS and the Technical University of Cluj-Napoca (TuCN) in Cluj-Napoca, Romania. The final object of our cooperation is to take use of experimental and simulation methodologies to define a standard procedure for obtaining sufficient reliability in risk evaluation and mitigation of prosthetic heart valves

Fluid dynamics of aortic root dilation in Marfan syndrome

Aortic root dilation and propensity to dissection are typical manifestations of the Marfan Syndrome (MS), a genetic defect leading to the degeneration of the elastic fibres. Dilation affects the structure of the flow and, in turn, altered flow may play a role in vessel dilation, generation of aneurysms, and dissection. The aim of the present work is the investigation in-vitro of the fluid dynamic modifications occurring as a consequence of the morphological changes typically induced in the aortic root by MS. A mock-loop reproducing the left ventricle outflow tract and the aortic root was used to measure time resolved velocity maps on a longitudinal symmetry plane of the aortic root. Two dilated model aortas, designed to resemble morphological characteristics typically observed in MS patients, have been compared to a reference, healthy geometry. The aortic model was designed to quantitatively reproduce the change of aortic distensibility caused by MS. Results demonstrate that vorticity released from the valve leaflets, and possibly accumulating in the root, plays a fundamental role in redirecting the systolic jet issued from the aortic valve. The altered systolic flow also determines a different residual flow during the diastole.Comment: Accepted versio

Simulation Of Blood Flow Through Mechanical Heart Valve Using Ansys Fluid Structure Interaction

A successful mechanical prosthetic heart valve design is the bileaflet valve, which has been implanted for the first time more than 20 years ago. A key feature of bileaflet valves is the geometry of the two leaflets, which can be very important in determining the flow field. The aim of this research is to observe the flow pattern of the blood through the heart valve with different leaflets curvatures. In addition, the structural analysis of the leaflets was investigated. The simulation is carried out using ANSYS Fluid Structural Interaction(FSI). Particle image velocimetry experiment will also be conducted to validate the simulation results. Finally, the results such as velocity contour and vorticity are compared, revealing great similarity in leaflet motion and flow fields between the numerical simulation and the experimental test. Also, in this study, the leaflets which curved inwards with greater degrees are found to be the best configuration as it allows the greatest blood flow dynamic. Moreover, the maximum von mises stress is found to be at the hinge region. These results will serve as a basis for valve design to improve the hemodynamic of the heart

Numerical and Experimental Investigations of Pulsatile Blood Flow through a Dysfunctional Mechanical Heart Valve

Despite the marked improvement in prosthetic heart valve design and functionality, thromboembolism, structural failure, endocarditis and hemolysis are still possible complications. In such cases, native heart valve disease is replaced with “prosthetic heart valve disease”. Bileaflet Mechanical Heart Valve (BMHV) dysfunction can cause serious and potentially fatal complications. In vivo, in vitro, and Computational Fluid Dynamics (CFD) studies were conducted on dysfunctional BMHVs in order to: (1) investigate the relationship between blood flow patterns downstream of the dysfunctional BMHV and the levels of hemolysis and/or thrombus formation; (2) to evaluate the limitations of the hemodynamic parameters and cutoff values suggested by the American Society of Echocardiography (ASE) guidelines; and (3) to improve the accuracy of the current diagnosis methods using the same clinical modalities and settings. Pulsatile two-dimensional and two phase flow numerical simulations revealed that the flow upstream and downstream of a dysfunctional mechanical heart valve was highly influenced by dysfunction severity and this resulted in discrepancies between Doppler echocardiography and numerically derived transvalvular pressure gradients. Moreover, the flow downstream of the dysfunctional valve was characterized by abnormally elevated shear stress and large-scale vortices. These flow characteristics can predispose to blood components damage. Three-dimensional Fluid-Structure Interaction (FSI) numerical modeling showed that the flow nature is three-dimensional and time dependent, especially with the existence of valve dysfunction. A pulsatile 3-D FSI numerical model should be used when the evolution of the vortical structure downstream of the BMHV is the objective of the study. Only flow characteristics through the central orifice are measured by the current diagnosis methods. Therefore, revisiting the assumptions and the theory behind the current clinical method is critical in order to include the flow through the two lateral orifices. A practical mathematical model was proposed for predicting the normal reference values of Doppler-derived parameters for BMHVs. The new theoretical model overcomes the shortcomings of the parameters suggested by the ASE guidelines by taking into account flow conditions (Left Ventricle Outflow Tract (LVOT) measurements), valve size and valve type. The accuracy of diagnosis significantly improved using the new theoretical parameters compared to those suggested by the ASE. Finally, the new method improved the way to evaluate of the performance of BMHVs, not only after implantation, but also early during the stage of design and manufacturing

Blood Damage Analysis using Computational Fluid Dynamics of Blood Flow through a Functioning and Malfunctioning Bileaflet Artificial Heart Valve

Artificial heart valves are an invaluable tool to treat heart defects and diseases. However, these prosthetic devices may expose the blood to turbulent flow conditions leading to unnaturally high stress that can damage blood cells. The purpose of this research is to simulate blood flow in both a functioning and malfunctioning bi-leaflet artificial heart valve and predict the damage caused to red blood cells (RBCs), specifically hemolysis, from the magnitude of the stress and exposure time as determined by analysis of the turbulent flow eddies. Using the computational fluid dynamics (CFD) software ANSYS DesignModeler, two prosthetic heart valve models were constructed: one with both leaflets open and functioning and one with one leaflet mostly closed. Blood flow simulations were done using ANSYS Fluent and validated with experimental findings available in the literature. Results from the CFD simulations provided the spatial distribution of Kolmogorov length scales (KLS) that were used to find the spatial and size distributions of eddies in the flow field. This CFD-based research utilized the number and surface area of eddies in the blood as a way to predict the amount of hemolysis experienced by RBCs. The analysis is centered on the hypothesis that only some of the turbulent flow eddies – those with sizes comparable to or smaller than the size of RBCs – are the ones that contribute to cell damage. Results indicated that hemolysis levels are low, suggesting the need for further study of subhemolytic damage. The hemolysis predictions did allow for a comparative analysis of the heart valve simulations, which showed that more damage is expected at a higher flowrate, and that at the same flowrate, more damage is expected in the malfunctioning valve when compared to the functioning valve

Smartphone-based particle image velocimetry for cardiovascular flows applications: A focus on coronary arteries

An experimental set-up is presented for the in vitro characterization of the fluid dynamics in personalized phantoms of healthy and stenosed coronary arteries. The proposed set-up was fine-tuned with the aim of obtaining a compact, flexible, low-cost test-bench for biomedical applications. Technically, velocity vector fields were measured adopting a so-called smart-PIV approach, consisting of a smartphone camera and a low-power continuous laser (30 mW). Experiments were conducted in realistic healthy and stenosed 3D-printed phantoms of left anterior descending coronary artery reconstructed from angiographic images. Time resolved image acquisition was made possible by the combination of the image acquisition frame rate of last generation commercial smartphones and the flow regimes characterizing coronary hemodynamics (velocities in the order of 10 cm/s). Different flow regimes (Reynolds numbers ranging from 20 to 200) were analyzed. The smart-PIV approach was able to provide both qualitative flow visualizations and quantitative results. A comparison between smart-PIV and conventional PIV (i.e., the gold-standard experimental technique for bioflows characterization) measurements showed a good agreement in the measured velocity vector fields for both the healthy and the stenosed coronary phantoms. Displacement errors and uncertainties, estimated by applying the particle disparity method, confirmed the soundness of the proposed smart-PIV approach, as their values fell within the same range for both smart and conventional PIV measured data (≈5% for the normalized estimated displacement error and below 1.2 pixels for displacement uncertainty). In conclusion, smart-PIV represents an easy-to-implement, low-cost methodology for obtaining an adequately robust experimental characterization of cardiovascular flows. The proposed approach, to be intended as a proof of concept, candidates to become an easy-to-handle test bench suitable for use also outside of research labs, e.g., for educational or industrial purposes, or as first-line investigation to direct and guide subsequent conventional PIV measurements

A BIOMIMETIC APPROACH FOR THE RESTORATION OF THE PHYSIOLOGICAL LEFT VENTRICULAR VORTEX FORMATION

Ph.DDOCTOR OF PHILOSOPH