Lucie Bailly

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.

Towards a biomimetism of abdominal healthy and aneurysmal arterial tissues.

The aim of this work is to develop a new hyperelastic and anisotropic material mimicking histological and mechanical features of healthy and aneurysmal arterial tissues. The material is constituted by rhombic periodic lattices of hyperelastic fibres embedded into a soft elastomer membrane. To fit bi-axial experimental data obtained from the literature, with normal or pathologic human abdominal aortic tissues, the microstructure of the periodic lattices (fibre length, angle between fibres) together with the mechanical behaviour of the fibres (fibre tension-elongation curve) were optimised by using theoretical results arising from a multi-scale homogenisation process. It is shown that (i) a material constituted by only one periodic lattice of fibres is clearly not sufficient to describe all the experimental data set, (ii) a quantitative agreement between measurements and theoretical predictions is obtained by using a material with two fibre lattices, (iii) the optimised microstructures and mechanical properties of the fibrous lattices are strongly different for the abdominal healthy and aneurysmal arterial tissues, (iv) the anisotropic mechanical behaviour of the optimised material is described by only five parameters and (v) the optimal angles between fibres in the case of the healthy aorta are consistent with histological data. Several technical solutions of fibres can be considered as relevant candidates: this is illustrated in the particular cases of straight and wavy fibres.

Realistic glottal motion and airflow rate during human breathing

The glottal geometry is a key factor in the aerosol delivery efficiency for treatment of lung diseases. However, while glottal vibrations were extensively studied during human phonation, the realistic glottal motion during breathing is poorly understood. Therefore, most current studies assume an idealized steady glottis in the context of respiratory dynamics, and thus neglect the flow unsteadiness related to this motion. This is particularly important to assess the aerosol transport mechanisms in upper airways. This article presents a clinical study conducted on 20 volunteers, to examine the realistic glottal motion during several breathing tasks. Nasofibroscopy was used to investigate the glottal geometrical variations simultaneously with accurate airflow rate measurements. In total, 144 breathing sequences of 30s were recorded. Regarding the whole database, two cases of glottal time-variations were found: static or dynamic ones. Typically, the peak value of glottal area during slow breathing narrowed from 217 ± 54 mm(2) (mean ± STD) during inspiration, to 178 ± 35 mm(2) during expiration. Considering flow unsteadiness, it is shown that the harmonic approximation of the airflow rate underevaluates the inertial effects as compared to realistic patterns, especially at the onset of the breathing cycle. These measurements provide input data to conduct realistic numerical simulations of laryngeal airflow and particle deposition.

Glottal motion and its impact on the respiratory flow

The aim of this study was (i) to characterise the glottal dynamics during human breathing in vivo using laryngofiberscopy and synchronised airflow recordings and (ii) to quantify the effects of a mobile glottis and unsteady flow conditions on laryngeal jet-flow dynamics using CFD modelling. The in vivo study showed that the glottis can be extremely variable during breathing and hence influence airflow characteristics. A glottal area widening was quantified during inspiration, with a typical ratio of 3:1 as compared to expiration. Airflow rate variations differ from harmonic signal during eupnea as well as tachypnea. The correlation between flow-rate and glottal area will be discussed and compared to previous clinical investigations. Preliminary 2D CFD simulations of the glottal jet were carried out based on the measured flow-rate and glottal changes during eupnea. Impact of unsteady flow conditions on the jet development is demonstrated.

Ventricular-Fold Dynamics in Human Phonation

PURPOSEnIn this study, the authors aimed (a) to provide a classification of the ventricular-fold dynamics during voicing, (b) to study the aerodynamic impact of these motions on vocal-fold vibrations, and (c) to assess whether ventricular-fold oscillations could be sustained by aerodynamic coupling with the vocal folds.nnnMETHODnA 72-sample database of vocal gestures accompanying different acoustical events comprised high-speed cinematographic, audio, and electroglottographic recordings of 5 subjects. Combining the physiological correlates with a theoretical model of phonation, the vocal-ventricular aerodynamic interactions were investigated.nnnRESULTSnA ventricular-fold motion is found during (de)crescendos, shout, throat singing, yodel, growls, and glides with transitions between registers. Three main types of dynamics are identified: slow nonoscillatory motion and fast oscillatory motion with aperiodical or periodical vibrations. These patterns accompany a change in voice quality, pitch, and/or intensity. Alterations of glottal-oscillatory amplitude, frequency, and contact were predicted. It is shown that a ventricular oscillation can be initiated and sustained by aerodynamic coupling with the vocal folds.nnnCONCLUSIONSnVocal-ventricular aerodynamic interactions can alter, enhance, or suppress vocal-fold vibrations or leave them unchanged, depending on the ventricular-fold dynamics involved. Depending on its variation in time, a similar level of ventricular-fold adduction impacts the glottal vibratory magnitude and contact much differently.

New experimental protocols for tensile testing of abdominal aortic analogues

This work proposes an in vitro tensile testing protocol that is able to characterize abdominal aortic (AA) analogues under physiologically inspired mechanical loadings. Kinematic parameters are defined in agreement with in vivo measurements of aortic dynamics. A specific focus is given to the choice of the applied loading rates, deriving from the knowledge of aortic Peterson modulus and blood pressure variations from diastolic to systolic instants. The influence of physiological elongation rates has been tested on both porcine AAs and a thermoplastic polyurethane (TPU) material used to elaborate AA analogues. The diastolic and systolic elongation rates estimates vary between orders of magnitude O(10(-2)) and O(10(-1))s(-1). Negligible differences are obtained when comparing stress-elongation responses between both physiological elongation rates. In contrast, a noticeable stiffening of the TPU mechanical response is observed compared to that obtained under the common low traction rate of O(10(-3))s(-1). This work shows how relevant physiological elongation rates can be evaluated as a function of age, gender and pathological context.

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.

Comparison between the mechanical behaviour of the human healthy AA and commercial prostheses under various mechanical loadings

Standard chirurgical treatment of abdominal aortic aneurysm (AAA) involves the placement of tubular synthetic aortic prostheses. Most of these implants are made up of polyester textiles or porous expanded polytetrafluoroethylene. Normalized tests are dedicated to their assessment (ISO7198:1998). However, such experiments are not sufficient to characterize the complete mechanical performance of these implants (Le Magnen et al., 2001) and to ensure their mechanical compatibility with the host artery. Thus, the design of mechanically compatible vascular prostheses still remains a challenge. Within this context, a full comparison of the mechanical behavior of the human healthy abdominal aorta (AA) with commercial prostheses is proposed. An original numerical database on the mechanical behavior of human AA subjected to various mechanical loadings is first built and then compared with experimental data obtained from mechanical tests performed on prostheses.

Micromechanical modelling of the arterial wall: influence of mechanical heterogeneities on the wall stress distribution and the peak wall stress.

Previous tensile tests have shown the high anisotropy and heterogeneity of the arterial wall mechanical properties as a function of age, pathology (e.g. abdominal aortic aneurysm, AAA) and location (Vande Geest et al. 2006). The issues of this anisotropic behaviour and the AAA geometrical shapes have been tackled in many numerical studies. However, the wall mechanical heterogeneity has been very sparsely considered (Tierney et al. 2012). This work aims to study the effects of such heterogeneity on the stress distribution and the peak wall stress during a static pressurisation. Thence, a micromechani- cal-based model was used for the wall, and finite element analyses (FEA) were carried out on idealised AAAs. Unlike many previous phenomenological models, the current constitutive model depends on five material parameters only, which also allow us to control the wall microarchitecture.