Measurement of the mechanical properties of alginate beads using ultrasounds

Alginate beads are one of the most common vehicles for encapsulation. When designing bioartificial organs, they are used as matrix for cell culture. Cell survival and activity depend essentially on the bead physical and mechanical properties. In this study, the mechanical properties of millimetric beads are evaluated using an ultrasound technique. We study the effects that the concentration in alginate has on the elastic properties of the beads. We show that density and stiffness coefficient of the bead are not independent. A measurement of the density can therefore provide a good estimation of the stiffness coefficient

Coupling of boundary integral and finite element methods to model the deformation of a microcapsule in flow

We introduce a novel method to study the fluid-structure interaction between a microcapsule and an external flow. A finite element method (for the membrane) is coupled with a boundary integral method (for the internal and external flows) to model the large deformation of the capsule. We find good agreement with previous results obtained with a solid solver based on the local equilibrium of the membrane

Can sonication increase the release from alginate capsules?

The objective is to investigate the use of sonication to enhance the release from loaded capsules. Alginate capsules filled with blue dextran and suspended in an aqueous solution were sonicated. The external concentration in blue dextran is found to increase due to sonication, the release step being proportional to the duration and intensity of sonication. Sonication eventually leads to the capsule destruction for longer durations. To conclude on the release mechanism induced by sonication, we have also studied its influence on the capsule properties (geometry, porosity, mechanical properties

Fully Eulerian models for the numerical simulation of capsules with an elastic bulk nucleus

In this paper, we present a computational framework based on fully Eulerian models for fluid-structure interaction for the numerical simulation of biological capsules. The flexibility of such models, given by the Eulerian treatment of the interface and deformations, allows us to easily deal with the large deformations experienced by the capsule. The modeling of the membrane is based on the full membrane elasticity model introduced in (Milcent, T., Maitre, E. (2016)) that is capable of capturing both area and shear variations thanks to the so-called backward characteristics. In the validation section several test cases are presented with the goal of comparing our results to others present in the literature. In this part, the comparisons are done with different well-known configurations (capsule in shear flow and square-section channel), and by deepening the effect of the elastic constitutive law and capillary number on the membrane dynamics. Finally, to show the potential of this framework we introduce a new test case that describes the relaxation of a capsule in an opening channel. In order to increase the challenges of this test we study the influence of an internal nucleus, modeled as a hyperelastic solid, on the membrane evolution. Several numerical simulations are presented to deeply study its influence by modifying the characteristic parameters of the nucleus (size and elastic parameter)

Simulation numérique des interactions fluide-structure dans une fistule artério-veineuse sténosée et des effets de traitements endovasculaires

Une fistule artérioveineuse (FAV) est un accès vasculaire permanent créé par voie chirurgicale en connectant une veine et une artère chez le patient en hémodialyse. Cet accès vasculaire permet de mettre en place une circulation extracorporelle partielle afin de remplacer les fonctions exocrines des reins. En France, environ 36000 patients sont atteint d insuffisance rénale chronique en phase terminale, stade de la maladie le plus grave qui nécessite la mise en place d un traitement de suppléance des reins : l hémodialyse. La création et présence de la FAV modifient significativement l hémodynamique dans les vaisseaux sanguins, au niveau local et systémique ainsi qu à court et à plus long terme. Ces modifications de l hémodynamiques peuvent induire différents pathologies vasculaires, comme la formation d anévrysmes et de sténoses. L objectif de cette étude est de mieux comprendre le comportement mécanique et l hémodynamique dans les vaisseaux de la FAV. Nous avons étudié numériquement les interactions fluide-structure (IFS) au sein d une FAV patient-spécifique, dont la géométrie a été reconstruite à partir d images médicales acquises lors d un précédent doctorat. Cette FAV a été créée chez le patient en connectant la veine céphalique du patient à l artère radiale et présente une sténose artérielle réduisant de 80% la lumière du vaisseau. Nous avons imposé le profil de vitesse mesuré sur le patient comme conditions aux limites en entrée et un modèle de Windkessel au niveau des sorties artérielle et veineuse. Nous avons considéré des propriétés mécaniques différentes pour l artère et la veine et pris en compte le comportement non-Newtonien du sang. Les simulations IFS permettent de calculer l évolution temporelle des contraintes hémodynamiques et des contraintes internes à la paroi des vaisseaux. Nous nous sommes demandées aussi si des simulations non couplées des équations fluides et solides permettaient d obtenir des résultats suffisamment précis tout en réduisant significativement le temps de calcul, afin d envisager son utilisation par les chirurgiens. Dans la deuxième partie de l étude, nous nous sommes intéressés à l effet de la présence d une sténose artérielle sur l hémodynamique et en particulier à ses traitements endovasculaires. Nous avons dans un premier temps simulé numériquement le traitement de la sténose par angioplastie. En clinique, les sténoses résiduelles après angioplastie sont considérées comme acceptables si elles obstruent moins de 30% de la lumière du vaisseau. Nous avons donc gonflé le ballonnet pour angioplastie avec différentes pressions de manière à obtenir des degrés de sténoses résiduelles compris entre 0 et 30%. Une autre possibilité pour traiter la sténose est de placer un stent après l angioplastie. Nous avons donc dans un deuxième temps simulé ce traitement numériquement et résolu le problème d IFS dans la fistule après la pose du stent. Dans ces simulations, la présence du stent a été prise en compte en imposant les propriétés mécaniques équivalentes du vaisseau après la pose du stent à une portion de l artère. Dans la dernière partie de l étude nous avons mis en place un dispositif de mesure par PIV (Particle Image Velocimetry). Un moule rigide et transparent de la géométrie a été obtenu par prototypage rapide. Les résultats expérimentaux ont été validés par comparaison avec les résultats des simulations numériques.An arteriovenous fistula (AVF) is a permanent vascular access created surgically connecting a vein onto an artery. It enables to circulate blood extra-corporeally in order to clean it from metabolic waste products and excess of water for patients with end-stage renal disease undergoing hemodialysis. The hemodynamics results to be significantly altered within the arteriovenous fistula compared to the physiological situation. Several studies have been carried out in order to better understand the consequences of AVF creation, maturation and frequent use, but many clinical questions still lie unanswered. The aim of the present study is to better understand the hemodynamics within the AVF, when the compliance of the vascularwall is taken into account. We also propose to quantify the effect of a stenosis at the afferent artery, the incidence of which has been underestimated for many years. The fluid-structure interactions (FSI) within a patient-specific radio-cephalic arteriovenous fistula are investigated numerically. The considered AVF presents an 80% stenosis at the afferent artery. The patient-specific velocity profile is imposed at the boundary inlet, and a Windkessel model is set at the arterial and venous outlets. The mechanical properties of the vein and the artery are differentiated. The non-Newtonian blood behavior has been taken into account. The FSI simulation advantageously provides the time-evolution of both the hemodynamic and structural stresses, and guarantees the equilibrium of the solution at the interface between the fluid and solid domains. The FSI results show the presence of large zones of blood flow recirculation within the cephalic vein, which might promote neointima formation. Large internal stresses are also observed at the venous wall, which may lead to wall remodeling. The fully-coupled FSI simulation results to be costly in computational time, which can so far limit its clinical use. We have investigated whether uncoupled fluid and structure simulations can provide accurate results and significantly reduce the computational time. The uncoupled simulations have the advantage to run 5 times faster than the fully-coupled FSI. We show that an uncoupled fluid simulation provides informative qualitative maps of the hemodynamic conditions in the AVF. Quantitatively, the maximum error on the hemodynamic parameters is 20%. The uncoupled structural simulation with non-uniform wall properties along the vasculature provides the accurate distribution of internal wall stresses, but only at one instant of time within the cardiac cycle. Although partially inaccurate or incomplete, the results of the uncoupled simulations could still be informative enough to guide clinicians in their decision-making. In the second part of the study we have investigated the effects of the arterial stenosis on the hemodynamics, and simulated its treatment by balloon-angioplasty. Clinically, balloon-angioplasty rarely corrects the stenosis fully and a degree of stenosis remains after treatment. Residual degrees of stenosis below 30% are considered as successful. We have inflated the balloon with different pressures to simulate residual stenoses ranging from 0 to 30%. The arterial stenosis has little impact on the blood flow distribution: the venous flow rate remains unchanged before and after the treatment and thus permits hemodialysis. But an increase in the pressure difference across the stenosis is observed, which could cause the heart work load to increase. To guarantee a pressure drop below 5 mmHg, which is considered as the threshold stenosis pressure difference clinically, we find that the residual stenosis degree must be 20% maximum.COMPIEGNE-BU (601592101) / SudocSudocFranceF

Space-Time-Parameter PCA for Data-Driven Modeling with Application to Bioengineering

Principal component analysis is a recognized powerful and practical method in statistics and data science. It can also be used in modeling as a dimensionality reduction tool to achieve low-order models of complex multiphysics or engineering systems. Model-order reduction (MOR) methodologies today are an important topic for engineering design and analysis. Design space exploration or accelerated numerical optimization for example are made easier by the use of reduced-order models. In this chapter, we will talk about the use of higher-order singular value decompositions (HOSVD) applied to spatiotemporal problems that are parameterized by a set of design variables or physical parameters. Here we consider a data-driven reduced order modeling based on a design of computer experiment: from high-dimensional computational results returned by high-fidelity solvers (e.g. finite element ones), the HOSVD allows us to determine spatial, time and parameters principal components. The dynamics of the system can then be retrieved by identifying the low-order discrete dynamical system. As application, we will consider the dynamics of deformable capsules flowing into microchannels. The study of such fluid-structure interaction problems is motivated by the use of microcapsules as innovative drug delivery carriers through blood vessels

Motion of a spherical capsule in simple shear flow: influence of the bending resistance

National audienceWe simulate the motion of an initially spherical capsule in a simple shear flow in order to determine the influence of the bending resistance on wrinkle formation on the membrane. We use a numerical method coupling a nonlinear shell finite element method for the capsule wall mechanics with a boundary integral method to solve the Stokes equation. The capsule wall is discretized with MITC linear triangular shell finite elements. We find that, at low flow strength, buckling occurs in the central region of the capsule. The number of wrinkles on the membrane decreases with the bending stiffness and above a critical value, wrinkles no longer form. For thickness to radius ratios below 5%, the bending stiffness does not have any significant effect on the overall capsule motion and deformation. The mean capsule shape is identical whether the wall is modeled as a shell or a two-dimensional membrane, which shows that the dynamics of thin capsules is mainly governed by shear elasticity and membrane effects

Coupling boundary integral and shell finite element methods to study the fluid structure interactions of a microcapsule in a simple shear flow

International audienceWe simulate the motion of an initially spherical capsule in a simple shear flow in order to determine the influence of the bending resistance on the formation of wrinkles on the membrane. The fluid structure interactions are obtained numerically coupling a boundary integral method to solve for the Stokes equation with a nonlinear finite element method for the capsule wall mechanics. The capsule wall is discretized with MITC linear triangular shell finite elements. We find that, at low flow strength, buckling occurs in the central region of the capsule. The number of wrinkles on the membrane decreases with the bending stiffness and, above a critical value, wrinkles no longer form. For thickness to radius ratios below 5%, the bending stiffness does not have any significant effect on the overall capsule motion and deformation. The mean capsule shape is identical whether the wall is modeled as a shell or a two-dimensional membrane, which shows that the dynamics of thin capsules is mainly governed by shear elasticity and membrane effects

Numerical study of the influence of wall compliance on the hemodynamics in a patient-specific arteriovenous fistula

An arteriovenous fistula (AVF) is a vessel connection created to provide adequate blood access for hemodialysis. The local hemodynamics was investigated in a patient-specific AVF using a computational fluid structure interaction simulation. The fluid and solid governing equations were solved using ANSYS (coupling CFX and ANSYS Structural) imposing physiological boundary conditions and vessel mechanical properties. An accurate map of unsteady velocity profiles and wall shear stresses was drawn up. It was compared to rigid wall simulations to quantify the effect of the wall compliance