Eric Bertrand

Validation of a numerical 3-D fluid―structure interaction model for a prosthetic valve based on experimental PIV measurements

A numerical 3-D fluid-structure interaction (FSI) model of a prosthetic aortic valve was developed, based on a commercial computational fluid dynamics (CFD) software program using an Arbitrary Eulerian Lagrangian (ALE) formulation. To make sure of the validity of this numerical model, an equivalent experimental model accounting for both the geometrical features and the hydrodynamic conditions was also developed. The leaflet and the flow behaviours around the bileaflet valve were investigated numerically and experimentally by performing particle image velocimetry (PIV) measurements. Through quantitative and qualitative comparisons, it was shown that the leaflet behaviour and the velocity fields were similar in both models. The present study allows the validation of a fully coupled 3-D FSI numerical model. The promising numerical tool could be therefore used to investigate clinical issues involving the aortic valve.

New insights into the understanding of flow dynamics in an in vitro model for abdominal aortic aneurysms

An in vitro dynamics set-up of the flow in a compliant abdominal aortic aneurysm (AAA) model with an anterior posterior asymmetry, aorto-iliac bifurcation, and physiological inlet flow rate and outlet pressure waveforms was developed. The aims were first to show that the structural mechanical behavior of the used material to mimic the AAA wall was similar to this of patients with AAA and then to study the influence of the aorto-iliac bifurcation presence and to study the influence of the imbalanced flow rate in the iliac branches on the AAA flow field. 3D visualizations, never performed in the literature, have clearly put into evidence the development of a vortex ring generated at the AAA proximal neck during the decelerating phase of flow rate, which detaches and progresses downstream during the cardiac cycle, impinges on the anterior wall in the distal AAA region, breaks up, and separates into two vortices of which one rolls on upstream along the anterior wall. 2D particle image velocimetry measurements, swirling strength and enstrophy calculations allowed quantification of the vorticity, vortex trajectory and energy for the different geometrical and hydrodynamical conditions. The main results show that the instant and the intensity of the vortex ring impingement depend on the presence of the aorto-iliac bifurcation, with higher intensity, by about 90%, for an AAA without bifurcation. The imbalance of the flow rates into the iliac branches induces different propagation velocities of the vortex ring and lowers the intensity of the vortex impact by about 60%. The potential influence of the AAA dynamics is discussed in terms of AAA remodeling and rupture.

Flow of a blood analogue fluid in a compliant abdominal aortic aneurysm model: experimental modelling.

The aim of this work is to develop a unique in vitro set-up in order to analyse the influence of the shear thinning fluid-properties on the flow dynamics within the bulge of an abdominal aortic aneurysm (AAA). From an experimental point of view, the goals are to elaborate an analogue shear thinning fluid mimicking the macroscopic blood behaviour, to characterise its rheology at low shear rates and to propose an experimental device able to manage such an analogue fluid without altering its feature while reproducing physiological flow rate and pressure, through compliant AAA. Once these experimental prerequisites achieved, the results obtained in the present work show that the flow dynamics is highly dependent on the fluid rheology. The main results point out that the propagation of the vortex ring, generated in the AAA bulge, is slower for shear thinning fluids inducing a smaller travelled distance by the vortex ring so that it never impacts the anterior wall in the distal region, in opposition to Newtonian fluids. Moreover, scalar shear rate values are globally lower for shear thinning fluids inducing higher maximum stress values than those for the Newtonian fluids. Consequently, this work highlights that a Newtonian fluid model is finally inadequate to obtain a reliable prediction of the flow dynamics within AAA.

Stereoscopically Observed Deformations of a Compliant Abdominal Aortic Aneurysm Model

A new experimental setup has been implemented to precisely measure the deformations of an entire model abdominal aortic aneurysm (AAA). This setup addresses a gap between the computational and experimental models of AAA that have aimed at improving the limited understanding of aneurysm development and rupture. The experimental validation of the deformations from computational approaches has been limited by a lack of consideration of the large and varied deformations that AAAs undergo in response to physiologic flow and pressure. To address the issue of experimentally validating these calculated deformations, a stereoscopic imaging system utilizing two cameras was constructed to measure model aneurysm displacement in response to pressurization. The three model shapes, consisting of a healthy aorta, an AAA with bifurcation, and an AAA without bifurcation, were also evaluated with computational solid mechanical modeling using finite elements to assess the impact of differences between material properties and for comparison against the experimental inflations. The device demonstrated adequate accuracy (surface points were located to within 0.07 mm) for capturing local variation while allowing the full length of the aneurysm sac to be observed at once. The experimental model AAA demonstrated realistic aneurysm behavior by having cyclic strains consistent with reported clinical observations between pressures 80 and 120 mm Hg. These strains are 1-2%, and the local spatial variations in experimental strain were less than predicted by the computational models. The three different models demonstrated that the asymmetric bifurcation creates displacement differences but not cyclic strain differences within the aneurysm sac. The technique and device captured regional variations of strain that are unobservable with diameter measures alone. It also allowed the calculation of local strain and removed rigid body motion effects on the strain calculation. The results of the computations show that an asymmetric aortic bifurcation created displacement differences but not cyclic strain differences within the aneurysm sac.

Particle Imaging Velocimetry Measurements in a Heart Simulator

In vitro experiments are often unable to reproduce all the complexities of biological flows observed in vivo. The in vitro models are often rigid, use Newtonian fluids, and/or some ideal geometry tested under ideal physiological parameters. The study presented in this article describes the in vitro assessment of mitral prosthetic heart valves in a setup able to simulate the pulsatile blood flow in a model of the left heart with moving walls. The specific laboratory mockup built for these experiments consists in a Dual Activation Simulator (DAS) that provides a realistic simulation of the atrial and ventricular flow in anatomically shaped silicone models cavities. This mockup, initially designed for ultrasonic velocity measurements took recently advantage of the use of particle image velocimetry. We present here some aspects of flow visualization and phase averaged two-dimensional PIV measurements which can provide new insight in the interaction between the flow dynamics and the heart valves.

In-plane mechanics of soft architectured fibre-reinforced silicone rubber membranes.

Silicone rubber membranes reinforced with architectured fibre networks were processed with a dedicated apparatus, allowing a control of the fibre content and orientation. The membranes were subjected to tensile loadings combined with continuous and discrete kinematical field measurements (DIC and particle tracking). These tests show that the mechanical behaviour of the membranes is hyperelastic at the first order. They highlight the influence of the fibre content and orientation on both the membrane in-plane deformation and stress levels. They also prove that for the considered fibrous architectures and mechanical loadings, the motion and deformation of fibres is an affine function of the macroscale transformation. These trends are fairly well described by the micromechanical model proposed recently in Bailly et al. (JMBBM, 2012). This result proves that these materials are very good candidates for new biomimetic membranes, e.g. to improve aortic analogues used for in vitro experiments, or existing textiles used for vascular (endo)prostheses.

Biaxial tensile tests of the porcine ascending aorta

One of the aims of this work is to develop an original custom built biaxial set-up to assess mechanical behavior of soft tissues. Stretch controlled biaxial tensile tests are performed and stereoscopic digital image correlation (SDIC) is implemented to measure the 3D components of the generated displacements. Using this experimental device, the main goal is to investigate the mechanical behavior of porcine ascending aorta in the more general context of human ascending aorta pathologies. The results highlight that (i) SDIC arrangement allows accurate assessment of displacements and so stress strain curves, (ii) porcine ascending aorta has a nearly linear and anisotropic mechanical behavior until 30% of strain, (iii) porcine ascending aorta is stiffer in the circumferential direction than in the longitudinal one, (iv) the material coefficient representing the interaction between the two loading directions is thickness dependent, (v) taking into account the variability of the samples the stress values are independent of the stretch rate in the range of values from 10(-3) to 10(-1)s(-1) and finally, (vi) unlike other segments of the aorta, 4-month-old pigs ascending aorta is definitely not a relevant model to investigate the mechanical behavior of the human ascending aorta.

Flow dynamics characterisation of a novel perfusion-type bioreactor for bone tissue engineering

The successful in vitro engineering of living tissues relies on numerous parameters that can be grouped in four main families: cells, scaffolds, medium and bioreactor. In the case of regenerative medicine, one principal target is to be able to produce implantable grafts obtained by combining biocompatible, biomimetic 3D resorbable scaffolds, preferably with autologous stem cells, primarily cultured in vitro and properly induced in the right lineage in an ergonomic dynamic perfusion like bioreactor, preferably using a serum-free culture media (Bodle et al. 2011; Rauh et al. 2011). Thepresentwork focuses on the design and experimental PIV characterisation of a versatile dynamic bioreactor that seeks to reach such a goal, particularly in the case of bone tissue engineering (BTE).While able to host various types of cells and media, the bioreactor relies on the use of macroporous hydrogel types of scaffolds in which adiposederived stem cells (ASC) are seeded, cultured and mechanically induced into an osteogenic lineage without any inductive biochemical additives.

FULL INTRAVENTRICULAR FLOW MAPPING BY CONVENTIONAL COLOR-DOPPLER ECHOCARDIOGRAPHY

Accurate quantification of left ventricular function is primordial in clinical decision making and follow-up assessment in patients with cardiovascular disease. Its ability to derive real-time noninvasive information makes Doppler ultrasound the cornerstone technique for cardiovascular diagnosis [Thomas, 2006]. Current color-Doppler ultrasound, however, is limited to the only components of the actual three-dimensional velocity field projected along the direction of the ultrasound beam. Computational [Pedrizzetti, 2005] and in vitro [Kheradvar, 2007] studies have suggested that more complete clinical diagnostic information could be derived from the full-field characterization of intraventricular flow. To date, the detailed visualization of full-flow in the left ventricle is only accessible with magnetic resonance velocimetry. The objective of our study was thus to develop and validate an original algorithm to construct a twodimensional time-resolved (2D+T) left ventricular flow field from conventional echocardiographic acquisitions.

Asymmetric flows in an anatomical-shaped left atrium by 2C-3D+T PIV measurements

Spatial characterisation of intra-ventricular flows have already been extensively investigated either computationally, in vitro or in vivo. Concerning the left atrium, relatively few studies are faced with the visualisation of the flow dynamics across the pulmonary veins, the atrium and the mitral valve (MV). From in vivo magnetic resonance phase-velocity mapping, Kilner et al. (2000) suggested that eccentric alignments of the pulmonary veins with respect to the atrium predispose to asymmetry of the intra-atrial flow and avoid instabilities by allowing swirling flows without collision. Fyrenius et al. (2001) also supported the idea that the left pulmonary venous flows recirculated in vortices at the center of the atrium whereas the right pulmonary venous flows passed along the vortex periphery with minimal entrainment. However, these ideas have not yet been validated in vitro, mainly because of non-realistic and/or oversimplified experimental left atrial geometry. Therefore, we used our new pulsed mock circulatory system (Tanné et al. 2007) to explore the flow dynamics inside the left atrium by multiplanes multi-phases two components particle image velocimetry (PIV). The aim was to verify that both left atrial realistic geometry and pressure–volume loop are mandatory to simulate coherent flow structures similar to in vivo data.