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An emerging class of models has been developed in recent years to predict cardiac growth and remodeling (G&R). We recently developed a cardiac G&R constitutive model that predicts remodeling in response to elevated hemodynamics loading, and a subsequent reversal of the remodeling process when the loading is reduced. Here, we describe the integration of this G&R model to an existing strongly coupled electromechanical model of the heart. A separation of timescale between growth deformation and elastic deformation was invoked in this integrated electromechanical-growth heart model. To test our model, we applied the G&R scheme to simulate the effects of myocardial infarction in a realistic left ventricular (LV) geometry using the finite element method. We also simulate the effects of a novel therapy that is based on alteration of the infarct mechanical properties. We show that our proposed model is able to predict key features that are consistent with experiments. Specifically, we show that the presence of a non-contractile infarct leads to a dilation of the left ventricle that results in a rightward shift of the pressure volume loop. Our model also predicts that G&R is attenuated by a reduction in LV dilation when the infarct stiffness is increased.

Myocardial microstructure has been challenging to probe in vivo. Spin echo-based diffusion-weighted sequences allow for single-shot acquisitions but are highly sensitive to cardiac motion. In this study, the use of second-order motion-compensated diffusion encoding was compared with first-order motion-compensated diffusion-weighted imaging during systolic contraction of the heart.First- and second-order motion-compensated diffusion encoding gradients were incorporated into a triggered single-shot spin echo sequence. The effect of contractile motion on the apparent diffusion coefficients and tensor orientations was investigated in vivo from basal to apical level of the heart.Second-order motion compensation was found to increase the range of systolic trigger delays from 30%-55% to 15%-77% peak systole at the apex and from 25%-50% to 15%-79% peak systole at the base. Diffusion tensor analysis yielded more physiological transmural distributions when using second-order motion-compensated diffusion tensor imaging.Higher-order motion-compensated diffusion encoding decreases the sensitivity to cardiac motion, thereby enabling cardiac DTI over a wider range of time points during systolic contraction of the heart.

International audience; This paper presents an approach to predict the strength distribution of quasi-brittle materials across multiple length-scales, with emphasis on Nature-inspired ceramic structures. It permits the computation of the failure probability of any structure under any mechanical load, solely based on considerations of the microstructure and its failure properties by naturally incorporating the statistical and size-dependent aspects of failure. We overcome the intrinsic limitations of single periodic unit-based approaches by computing the successive failures of the material components and associated stress redistributions on arbitrary numbers of periodic units. For large size samples, the microscopic cells are replaced by a homogenized continuum with equivalent stochastic and damaged constitutive behavior. After establishing the predictive capabilities of the method, and illustrating its potential relevance to several engineering problems, we employ it in the study of the shape and scaling of strength distributions across differing length-scales for a particular quasi-brittle system. We find that the strength distributions display a Weibull form for samples of size approaching the periodic unit; however, these distributions become closer to normal with further increase in sample size before finally reverting to a Weibull form for macroscopic sized samples. In terms of scaling, we find that the weakest link scaling applies only to microscopic, and not macroscopic scale, samples. These findings are discussed in relation to failure patterns computed at different size-scales.

International audience; Ventricular growth is widely considered to be an important feature in the adverse progression of heart diseases, whereas reverse ventricular growth (or reverse remodeling) is often considered to be a favorable response to clinical intervention. In recent years, a number of theoretical models have been proposed to model the process of ventricular growth while little has been done to model its reverse. Based on the framework of volumetric strain-driven finite growth with a homeostatic equilibrium range for the elastic myofiber stretch, we propose here a reversible growth model capable of describing both ventricular growth and its reversal. We used this model to construct a semi-analytical solution based on an idealized cylindrical tube model, as well as numerical solutions based on a truncated ellipsoidal model and a human left ventricular model that was reconstructed from magnetic resonance images. We show that our model is able to predict key features in the end-diastolic pressure–volume relationship that were observed experimentally and clinically during ventricular growth and reverse growth. We also show that the residual stress fields generated as a result of differential growth in the cylindrical tube model are similar to those in other nonidentical models utilizing the same geometry.

Although computational modeling is common in many areas of science and engineering, only recently have advances in experimental techniques and medical imaging allowed this tool to be applied in cardiac surgery. Despite its infancy in cardiac surgery, computational modeling has been useful in calculating the effects of clinical devices and surgical procedures. In this review, we present several examples that demonstrate the capabilities of computational cardiac modeling in cardiac surgery. Specifically, we demonstrate its ability to simulate surgery, predict myofiber stress and pump function, and quantify changes to regional myocardial material properties. In addition, issues that would need to be resolved in order for computational modeling to play a greater role in cardiac surgery are discussed.

Standard electroanatomic mapping systems use a single catheter to perform left ventricular substrate mapping. A new mapping system uses a 64-electrode mini-basket catheter to perform rapid automated acquisition of chamber geometry, voltage, and activation.The aim of this study was to compare the accuracy of electroanatomic mapping using the basket catheter with that of mapping using a standard linear catheter in a swine model of chronic myocardial infarction.Ten swine underwent left anterior descending coronary artery occlusion to create an anteroseptal myocardial infarction. Animals underwent delayed-enhancement magnetic resonance imaging (MRI) and then detailed left ventricular voltage mapping with both the basket and the linear catheter. Map characteristics and scar area were compared between the basket catheter, linear catheter, and MRI. Induced ventricular tachycardia (VT) was mapped with the basket catheter.More points were acquired with the basket catheter than with the standard catheter (8762 ± 3164 vs 1712 ± 551; P.001). The fifth percentile of normal bipolar voltage distribution with the basket catheter was 1.54 mV. Using a bipolar voltage cutoff of 1.5 mV, the total infarct areas measured using the basket catheter, linear catheter, and MRI were similar (17.8 cm(2) vs 20.9 cm(2) vs 17.5 cm(2); P = .69); however, the correlation between MRI and catheter scar area measurement was best for the basket catheter (basket vs linear: r = .76 vs r = .71). In 3 animals, sustained poorly tolerated VT was initiated and the circuit mapped successfully with the basket catheter in5 minutes.Rapid substrate and activation mapping using a 64-electrode mini-basket catheter allows detailed voltage and activation mapping in postinfarction cardiomyopathy. This system may be useful for substrate and VT mapping in human postinfarction cardiomyopathy.

International audience; Ceramic matrix composites (CMC) are good candidates for structural applications at high temperatures in oxidizing environments. A main question is then to be able to predict, that is, to compute the lifetime of engineering structures. Multiphysic models are needed to reproduce couplings between the mechanical and environmental fields.The chapter is first focused on the central and crucial question of highly anisotropic materials as CMCs: how does one handle mathematically what we call damage, that is, how does one reproduce the microcracking mechanisms? The answer described here is a general anisotropic damage theory that takes into account crack closure effects.The second part deals with the modeling of couplings with the environment characterized by temperature, oxygen partial pressure, and humidity. Fatigue of interface, matrix and fiber oxidation, and self-healing mechanisms are also taken into account. The idea, here, is to develop micro-multiphysic models that provide information for the computational macromodel. This strategy is illustrated on a self-healing CMC material.The last part concerns the use of the computational multiphysic model, which allows one to predict the macrocracks' initiation and their propagation. Classical numerical difficulties are displayed and answers are given. To end, an application to an engineering structure is shown.

International audience; This paper describes the formulation and numerical implementation of a family of anisotropic and unilateral damage models for the prediction of damage and final rupture in engineering structures. The damage can be load-oriented, microstructure-oriented or (for the first time within this modeling framework) softening. The local equations are solved using a combination of fixed-point and Newton-Raphson algorithms, whose efficiencies are drastically improved through Aitken's relaxation and BFGS approximation. A delay-effect method is used to control the localization of damage, which leads to an objective calculation of the final rupture of structures.

Hydroxyapatite/β-tricalcium phosphate (HA/β-TCP) composite scaffolds have shown great potential for bone-tissue engineering applications. In this work, ceramic scaffold with different HA/β-TCP compositions (pure HA, 60HA/40β-TCP, and 20HA/80β-TCP) were fabricated by a robotic-assisted deposition (robocasting) technique using water-based hydrogel inks. A systematic study was conducted to investigate the porosity, mechanical property, and degradation of the scaffolds. Our results indicate that, at a similar volume porosity, the mechanical strength of the sintered scaffolds increased with the decreasing rod diameter. The compressive strength of the fabricated scaffolds (porosity ≈ 25-80 vol %) varied between ∼3 and ∼50 MPa, a value equal or higher than that of human cancellous bone (2-12 MPa). Although there was a slight increase of Ca and P ions in water after 5 month, no noticeable degradation of the scaffolds in SBF or water was observed. Our findings from this work indicate that composite calcium phosphate scaffolds with customer-designed chemistry and architecture may be fabricated by a robotic-assisted deposition method.

International audience; Self-healing Ceramic Matrix Composites (CMCs) are good candidates for structural applications at high temperatures in oxidizing environments. These materials generate complex couplings between the thermal and mechanical fields. A multiphysics macroscopic model of both the mechanical behavior and the lifetime of CMC structures was proposed previously and was validated on the material's level. Here, its effectiveness in dealing with structural calculations with heterogeneous fields is analyzed and a non-local fracture criterion is proposed for high-gradient cases. All the simulations were carried out using Abaqus/Standard. The main interest of the model is its ability to predict the evolution of each of the material's mechanisms throughout the structure until final fracture. Another advantage is the ability to predict the fracture zone and the influence of indentations on the lifetime of the structure (damage tolerance analysis), both of which are very important for industrial developments.

International audience; A new modeling approach to subcritical crack propagation ( static fatigue) in ceramics is introduced and applied to the case of ceramic fibers. This approach relies on classical fracture mechanics, in which the zone near the crack's tip is modified by the environment (particularly through oxidation). Chemical reactions and diffusion are introduced naturally, which makes the model highly dependent on the environmental conditions, such as partial oxygen pressure. In some cases, the result comes down to the widely-used Paris-like subcritical crack propagation law. For the general case, a two-parameter model calibrated and validated up to 1,000 C for Hi-Nicalon fibers is presented.

International audience; A computationally-efficient numerical approach to treating matrix nonlinearity in ceramic matrix composite components has been developed and validated. The model employs a dual mesh comprising strings of line elements that represent the fiber tows and 3D effective medium elements that define the external geometry and embody the matrix-dominated properties. Validation addressed test data for unnotched and open-hole tension specimens. For these tests, the onset of nonlinearity and subsequent plasticity due to matrix microcracking and interfacial debonding and sliding are satisfactorily represented by a linear Drucker–Prager model for failure initiation in the effective medium along with a fully-associated flow rule with isotropic, perfectly-plastic flow. Composite failure is assumed to be correlated with the maximum local stress averaged over a gauge volume dictated by the fiber tow width. Using one set of specimens for calibration, very good predictions of the nonlinear stress–strain response and ultimate strength of other specimens are obtained.

International audience; The finite element method and dimensional analysis have been applied in the present paper to study a hydraulic impact, which is utilized in a non-explosive rock breaking technology in mining industry. The impact process of a high speed piston on liquid water, previously introduced in a borehole drilled in rock, is numerically simulated. The research is focused on the influences of all the parameters involved in the technology on the largest principal stress in the rock, which is considered as one of the key factors to break the rock. Our detailed parametric investigation reveals that the variation of the isotropic rock material properties, especially its density, has no significant influence on the largest principal stress. The influences of the depth of the hole and the depth of the water column are also very small. On the other hand, increasing the initial kinetic energy of the piston can dramatically increase the largest principal stress and the best way to increase the initial kinetic energy of the piston is to increase its initial velocity. Results from the current dimensional analysis can be applied to optimize this non-explosive rock breaking technology.

Background Computational cardiac modelling has been established as a valuable tool for simulating electrophysiology and electromechanics of the heart [1], with promising applications to “personalized medicine” [1]. Realistic computational models require a detailed description of left-ventricular cardiac fiber architecture. So far, nonpatient-specific fiber architectures have been obtained from histology or diffusion tensor imaging (DTI) of excised post-mortem hearts. However, compared to invivo, ex-vivo physiological conditions including ventricular pressure and residual contractile forces deviate significantly, hence potentially impacting measured fiber metrics. The objective of this work was to obtain and make available cardiac DTI data of the in-vivo human heart with full cardiac coverage in both peak systole and mid diastole including correction for myocardial strain.

Journal of Cardiovascular Magnetic Resonance, 17 (S1), ISSN:1097-6647, ISSN:1532-429X

Journal of Cardiovascular Magnetic Resonance, 17 (S1), ISSN:1097-6647, ISSN:1532-429X

The myocardium is a complex tissue, primarily made of cardiac cells and extra-cellular collagenous matrix, arranged in a hierarchical microstructure. This microstructure evolves with time, either physiologically or pathologically, altering the macroscopic mechanical properties of heart, hence its function. The objective of this PhD thesis is to improve our understanding of the structure-properties relationships in the cardiac tissue by focusing on the role of mesostructure, called the sheetlet, on the macroscopic mechanical properties. In a first part, a literature review of the existing literature on the role of sheetlet is performed. In the second chapter, we present an experimental setup we developed, combining a full-field Mueller polarimetric imager and an in-situ traction device with a local measure of the deformation during sample stretch. We observe a global cohesion of the tissue apart on separation lines at the mesoscale, that tend to open under traction. We hypothesize that these lines are the collageneous layers separating the sheetles, bundles of cardiomyocytes surrounded by layers of perimysial collagen. In the third chapter, we propose a simple optical method combined with a fast image analysis to extract the sheetlet local orientation on left ventricle slices, in order to construct a 3D mapping of the sheetlet organization in the heart. In the second part of the PhD thesis (chapter four), we propose a multiscale model of the myocardium aiming at determining the structural origin of the observed macroscale mechanical anisotropy. Indeed, the observed orthotropy can be due either to the rotation of the cell orientation, or to the presence of the mesoscale collageneous layers. We design three mesostructures made of different configurations of cardiomyocytes and collagen. The mechanical response of these mesostructures are homogenized with numerical periodic homogenization, and the obtained material parameters are used in a macroscopic simulation of a shear experiment. The comparison between our results and the experimental data show that the collagen layers are necessary to reproduce the material orthotropy. The final chapter of this thesis presents some perspectives of this work.; Le myocarde est un tissu complexe, principalement constitué de cellules cardiaques et d'une matrice collagénique extracellulaire, disposés selon une microstructure hiérarchique. Cette microstructure évolue au cours du temps, de manière physiologique ou pathologique, altérant les propriétés mécaniques macroscopiques du cœur, donc sa fonction. L'objectif de cette thèse est d'améliorer notre compréhension des relations structure-propriétés dans le tissu cardiaque en s'intéressant au rôle de la mésostructure, appelée feuillet, sur les propriétés mécaniques macroscopiques. Dans une première partie, une revue de la littérature existante sur le rôle du feuillet est réalisée. Dans le deuxième chapitre, nous présentons un montage expérimental que nous avons développé, combinant un imageur polarimétrique Mueller plein champ et un dispositif de traction in-situ avec une mesure locale de la déformation lors de l'étirement de l'échantillon. On observe une cohésion globale des tissus à part sur des lignes de séparation à mésoéchelle, qui ont tendance à s'ouvrir sous traction. Nous émettons l'hypothèse que ces lignes sont les couches collagéniques séparant les feuillets, des faisceaux de cardiomyocytes entourés de couches de collagène périmysial. Dans le troisième chapitre, nous proposons une méthode optique simple combinée à une analyse d'image rapide pour extraire l'orientation locale des feuillets sur des coupes de ventricule gauche, afin de construire une cartographie 3D de l'organisation des feuillets dans le cœur. Dans la deuxième partie de la thèse (chapitre quatre), nous proposons un modèle multi-échelle du myocarde visant à déterminer l'origine structurale de l'anisotropie mécanique macro-échelle observée. En effet, l'orthotropie observée peut être due soit à la rotation de l'orientation des cellules, soit à la présence des couches collagéniques méso-échelle. Nous concevons trois mésostructures constituées de différentes configurations de cardiomyocytes et de collagène. La réponse mécanique de ces mésostructures est homogénéisée avec une homogénéisation périodique numérique, et les paramètres de matériau obtenus sont utilisés dans une simulation macroscopique d'une expérience de cisaillement. La comparaison entre nos résultats et les données expérimentales montre que les couches de collagène sont nécessaires pour reproduire l'orthotropie du matériau. Le dernier chapitre de ce manuscrit présente quelques perspectives de ce travail.

Idiopathic pulmonary fibrosis (IPF), an interstitial lung disease, strongly impacts lung mechanics, which raises clinical issues. The objective of this work is to improve the understanding and diagnosis of IPF based on poromechanical modeling of the lung, personalized with clinical imaging data. In a first part, a literature review analyzes the state of the art of pulmonary physiology in relation to the existing mechanical models, insisting on the multi-scale, multi-phase and multi-physics characteristics of the organ. We then propose a poromechanical model of the lung at the organ spatial scale and breathing time scale, derived from a general poromechanical theory formulated recently in the MΞDISIM team. The constitutive law proposed reproduces mainly the pressure-volume behaviour of the lung as well as the quasi-incompressiblity of the solid phase. The boundary conditions take into account the lung environment (thoracic cage, diaphragm, pleura) and distinguish between free and ventilated breathing. The unloaded configuration, non observed during a breathing cycle, is estimated, with a special attention given to maintain a positive porosity. Various elements of the model are then personalized with standard clinical data, i.e., two 3DCT images acquired at end-exhalation and end-inhalation. In particular, an inverse problem is formulated to estimate the pulmonary compliance of the healthy and fibrotic regions, since the poromechanical formulation allows to distinguish the effect of the porosity from that of the absolute compliance of the interstitial tissue. Applied to three patients suffering from IPF, the personalized model allows to find the foreseen properties of IPF, i.e., the stiffening of the diseased region. Stress concentrations are also observed at the diseased region interface, supporting the hypothesis of a mechanical vicious circle governing the IPF progress, where fibrosis induces large stresses, which in turn favors fibrosis. This numerical tool could later be used for objective and quantitative diagnosis of IPF and, with longitudinal data, to study the mechanics-induced remodeling.; Les poumons réalisent leur fonction vitale d'échanges gazeux grâce notamment à leur élasticité et leur porosité. La fibrose pulmonaire idiopathique (FPI), une maladie pulmonaire interstitielle, impacte fortement la mécanique pulmonaire, soulevant des problématiques cliniques. L'objectif de ce travail est d'améliorer la compréhension et le diagnostic de la FPI en s'appuyant sur une modélisation poromécanique du poumon, personnalisée grâce à des données d'imagerie médicale. Dans une première partie, une analyse bibliographique fait l'état de l'art de la physiologie pulmonaire en relation avec les modélisations mécaniques existantes, en insistant sur les caractères multiéchelle, multiphasique et multiphysique de l'organe. Nous proposons ensuite un modèle poromécanique du poumon à l'échelle spatiale de l'organe et à l'échelle temporelle de la respiration, dérivé d'une théorie générale de poromécanique récemment formulée dans l'équipe MΞDISIM. La loi de comportement proposée reproduit notamment le comportement pression-volume du poumon ainsi que la quasi-incompressiblité de la phase solide. Les conditions aux limites prennent en compte l'environnement du poumon (cage thoracique, diaphragme, plèvre) et distinguent les cas de la respiration libre vs ventilée. La configuration déchargée, non observée au cours d'un cycle respiratoire, est estimée, avec une attention particulière apportée pour contraindre la positivité de la porosité. Plusieurs éléments du modèle sont ensuite personnalisés à partir de données cliniques standards, i.e., deux images 3DCT prises en début et en fin d'inspiration. Un problème inverse est notamment formulé pour estimer la compliance pulmonaire en zones saines et fibrosées, la formulation poromécanique permettant de distinguer l’effet de la porosité de celui de la compliance du tissu interstitiel. Appliqué sur trois patients atteints de FPI, le modèle personnalisé permet de retrouver les propriétés pressenties de la FPI, i.e., la rigidification des zones malades. Des surcontraintes sont aussi observées en bordure de la région malade, corroborant ainsi l'hypothèse d'un cercle vicieux mécanique régissant l'évolution de la FPI, où la fibrose entraînerait des contraintes importantes, qui à leur tour favoriseraient la fibrose. Cet outil numérique pourrait servir par la suite au diagnostic objectif et quantitatif de la FPI et, avec des données longitudinales, à l'étude de son remodelage induit par la mécanique.

Idiopathic pulmonary fibrosis (IPF), an interstitial lung disease, strongly impacts lung mechanics, which raises clinical issues. The objective of this work is to improve the understanding and diagnosis of IPF based on poromechanical modeling of the lung, personalized with clinical imaging data. In a first part, a literature review analyzes the state of the art of pulmonary physiology in relation to the existing mechanical models, insisting on the multi-scale, multi-phase and multi-physics characteristics of the organ. We then propose a poromechanical model of the lung at the organ spatial scale and breathing time scale, derived from a general poromechanical theory formulated recently in the MΞDISIM team. The constitutive law proposed reproduces mainly the pressure-volume behaviour of the lung as well as the quasi-incompressiblity of the solid phase. The boundary conditions take into account the lung environment (thoracic cage, diaphragm, pleura) and distinguish between free and ventilated breathing. The unloaded configuration, non observed during a breathing cycle, is estimated, with a special attention given to maintain a positive porosity. Various elements of the model are then personalized with standard clinical data, i.e., two 3DCT images acquired at end-exhalation and end-inhalation. In particular, an inverse problem is formulated to estimate the pulmonary compliance of the healthy and fibrotic regions, since the poromechanical formulation allows to distinguish the effect of the porosity from that of the absolute compliance of the interstitial tissue. Applied to three patients suffering from IPF, the personalized model allows to find the foreseen properties of IPF, i.e., the stiffening of the diseased region. Stress concentrations are also observed at the diseased region interface, supporting the hypothesis of a mechanical vicious circle governing the IPF progress, where fibrosis induces large stresses, which in turn favors fibrosis. This numerical tool could later be used for objective and quantitative diagnosis of IPF and, with longitudinal data, to study the mechanics-induced remodeling.; Les poumons réalisent leur fonction vitale d'échanges gazeux grâce notamment à leur élasticité et leur porosité. La fibrose pulmonaire idiopathique (FPI), une maladie pulmonaire interstitielle, impacte fortement la mécanique pulmonaire, soulevant des problématiques cliniques. L'objectif de ce travail est d'améliorer la compréhension et le diagnostic de la FPI en s'appuyant sur une modélisation poromécanique du poumon, personnalisée grâce à des données d'imagerie médicale. Dans une première partie, une analyse bibliographique fait l'état de l'art de la physiologie pulmonaire en relation avec les modélisations mécaniques existantes, en insistant sur les caractères multiéchelle, multiphasique et multiphysique de l'organe. Nous proposons ensuite un modèle poromécanique du poumon à l'échelle spatiale de l'organe et à l'échelle temporelle de la respiration, dérivé d'une théorie générale de poromécanique récemment formulée dans l'équipe MΞDISIM. La loi de comportement proposée reproduit notamment le comportement pression-volume du poumon ainsi que la quasi-incompressiblité de la phase solide. Les conditions aux limites prennent en compte l'environnement du poumon (cage thoracique, diaphragme, plèvre) et distinguent les cas de la respiration libre vs ventilée. La configuration déchargée, non observée au cours d'un cycle respiratoire, est estimée, avec une attention particulière apportée pour contraindre la positivité de la porosité. Plusieurs éléments du modèle sont ensuite personnalisés à partir de données cliniques standards, i.e., deux images 3DCT prises en début et en fin d'inspiration. Un problème inverse est notamment formulé pour estimer la compliance pulmonaire en zones saines et fibrosées, la formulation poromécanique permettant de distinguer l’effet de la porosité de celui de la compliance du tissu interstitiel. Appliqué sur trois patients atteints de FPI, le modèle personnalisé permet de retrouver les propriétés pressenties de la FPI, i.e., la rigidification des zones malades. Des surcontraintes sont aussi observées en bordure de la région malade, corroborant ainsi l'hypothèse d'un cercle vicieux mécanique régissant l'évolution de la FPI, où la fibrose entraînerait des contraintes importantes, qui à leur tour favoriseraient la fibrose. Cet outil numérique pourrait servir par la suite au diagnostic objectif et quantitatif de la FPI et, avec des données longitudinales, à l'étude de son remodelage induit par la mécanique.

International audience