106 research outputs found

    Immersed boundary-finite element model of fluid-structure interaction in the aortic root

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    It has long been recognized that aortic root elasticity helps to ensure efficient aortic valve closure, but our understanding of the functional importance of the elasticity and geometry of the aortic root continues to evolve as increasingly detailed in vivo imaging data become available. Herein, we describe fluid-structure interaction models of the aortic root, including the aortic valve leaflets, the sinuses of Valsalva, the aortic annulus, and the sinotubular junction, that employ a version of Peskin's immersed boundary (IB) method with a finite element (FE) description of the structural elasticity. We develop both an idealized model of the root with three-fold symmetry of the aortic sinuses and valve leaflets, and a more realistic model that accounts for the differences in the sizes of the left, right, and noncoronary sinuses and corresponding valve cusps. As in earlier work, we use fiber-based models of the valve leaflets, but this study extends earlier IB models of the aortic root by employing incompressible hyperelastic models of the mechanics of the sinuses and ascending aorta using a constitutive law fit to experimental data from human aortic root tissue. In vivo pressure loading is accounted for by a backwards displacement method that determines the unloaded configurations of the root models. Our models yield realistic cardiac output at physiological pressures, with low transvalvular pressure differences during forward flow, minimal regurgitation during valve closure, and realistic pressure loads when the valve is closed during diastole. Further, results from high-resolution computations demonstrate that IB models of the aortic valve are able to produce essentially grid-converged dynamics at practical grid spacings for the high-Reynolds number flows of the aortic root

    Hemodynamic and thrombogenic analysis of a trileaflet polymeric valve using a fluid-structure interaction approach

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    Surgical valve replacement in patients with severe calcific aortic valve disease using either bioprosthetic or mechanical heart valves is still limited by structural valve deterioration for the former and thrombosis risk mandating anticoagulant therapy for the latter. Prosthetic polymeric heart valves have the potential to overcome the inherent material and design limitations of these valves, but their development is still ongoing. The aim of this study was to characterize the hemodynamics and thrombogenic potential of the Polynova polymeric trileaflet valve prototype using a fluid-structure interaction (FSI) approach. The FSI model replicated experimental conditions of the valve as tested in a left heart simulator. Hemodynamic parameters (transvalvular pressure gradient, flow rate, maximum velocity, and effective orifice area) were compared to assess the validity of the FSI model. The thrombogenic footprint of the polymeric valve was evaluated using a Lagrangian approach to calculate the stress accumulation (SA) values along multiple platelet trajectories and their statistical distribution. In the commissural regions, platelets were exposed to the highest SA values because of highest stress levels combined with local reverse flow patterns and vortices. Stress-loading waveforms from representative trajectories in regions of interest were emulated in our Hemodynamic Shearing Device (HSD). Platelet activity was measured using our platelet activation state (PAS) assay and the results confirmed the higher thrombogenic potential of the commissural hotspots. In conclusion, the proposed method provides an in depth analysis of the hemodynamic and thrombogenic performance of the polymer valve prototype and identifies locations for further design optimization

    INFLUENCE of LEAFLET'S MATRIX STIFFNESS and FIBER ORIENTATION on the OPENING DYNAMICS of A PROSTHETIC TRILEAFLET HEART VALVE

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    Biological valves are employed for aortic valve substitution since a long time but there is a growing effort toward the development of new engineered tissues, in which the complex mechanical response of native leaflets is replicated using composite materials consisting of a soft matrix with embedded reinforcing fibers. The main goal of the present study is to investigate the influence that variations on fiber orientation and matrix stiffness may have on valve dynamics. To this aim a Fluid-Structure Interaction (FSI) model of a trileaflet valve was implemented in which the opening phase was simulated and leaflet matrix stiffness and fiber orientation were varied in the framework of an anisotropic hyperelastic strain energy function. Results show that both parameters may affect significantly transvalvular pressure gradient and effective orifice area (EOA). For the opening phase of the valve examined less favourable flow conditions were found when preferred fiber orientation is circumferential, due to lower maximum EOA achievable. Such configuration in combination with stiffer matrix may result in significant degradation of valve performances. Overall fiber orientation can potentially be taylored to optimize valve dynamics, provided also structural aspects that may be prominent in the closure phase, are considered

    Quantification of leaflet flutter in bioprosthetic heart valves using fluid-structure interaction analysis

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    Many studies have indicated that leaflet fluttering and associated bending in biopros-thetic heart valves (BHVs) is an important criterion in determining the durability of BHVimplants. In this thesis work, a computational methodology for the flutter quantificationof BHV leaflets is presented using an immersogeometric fluid–structure interaction (FSI)framework. The proposed approach is based upon displacement tracking of the BHV leafletfree edges. Integrating over the discrete Fourier transform of free edge displacement data,the energy spectral density is computed for a measure of leaflet flutter. This methodologyseeks to improve approaches used in experimental flutter quantification through utiliza-tion of highly accurate simulation solutions and visualizations to capture a measurement ofleaflet flutter. A set of sampling cases with varying valve material thickness are generatedand FSI-based flutter quantification is performed to investigate the effect of leaflet materialthickness on the presence of flutter and bending in BHVs

    Development of a Biaxial Stretch Bioreactor and Finite Element Models for Mechanobiological Study of Aortic Valve Leaflets

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    Aortic heart valve disease is a significant cause of mortality worldwide; and replacement surgery is necessary in 70% of cases. Tissue engineered heart valves (TEHVs) are biocompatible and biodegradable, with ability to grow with the patient. However, to date, TEHVs mostly lack ability to withstand native mechanical forces since they are unable to mimic the heterogeneous and anisotropic structure of extracellular matrix (ECM) in native valves. Cyclic stretch is known to modulate ECM fiber synthesis and alignment. However, little tools are available for studying the interaction between aortic tissues and stretch condition. Finite element method is a powerful tool to simulate the complex structure of aortic valve, however, most current simulations modeled the leaflet as a homogenous material, and none of them took the distinctions between two surface layers into account, which were critical for the proper function of the aortic valve.To study the effects of cyclic stretch on extracellular matrix remodeling, the heterogeneous properties of the aortic leaflet, and the effects of heterogeneity on the function of valve, we have 1) Designed, fabricated and validated a biaxial stretch bioreactor; 2) Analyzed train patterns of native aortic leaflets using digital image correlation method; 3) Designed and validated an anisotropic and heterogeneous finite element (FE) model for leaflets. These studies provided insights into the interaction between aortic valve tissue and the mechanical environment, anisotropy and heterogeneity of aortic leaflets ECM due to the distribution of collagen fibers, and detailed distinct strain patterns in fibrosa vs. ventricularis sides and 3 aortic leaflets. Our novel biaxial stretch bioreactor and refined FE model of aortic leaflet will pave path for other scientists to study mechanobiology, design and condition engineered tissues and simulate engineered aortic valve grafts or pathology of calcium deposition

    Review of patient-specific simulations of transcatheter aortic valve implantation

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    International audienceTranscatheter Aortic Valve Implantation (TAVI) accounts for one of the most promising new cardiovascular procedures. This minimally invasive technique is still at its early stage and is constantly developing thanks to imaging techniques, computer science, biomechanics and technologies of prosthesis and delivery tools. As a result, patient-specific simulation can find an exciting playground in TAVI. It canexpress its potential by providing the clinicians with powerful decision support, offering great assistance in their workflow. Through a review of the current scientific field, we try to identify the challenges and future evolutions of patient-specific simulation for TAVI. This review article is an attempt to summarize and coordinate data scattered across the literature about patient-specific biomechanical simulation for TAVI

    Development of a patient-specific finite element model of the transcatheter aortic valve implantation (TAVI) procedure

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    Transcatheter Aortic Valve Implantation (TAVI) is a procedure developed for replacing the defective aortic valve of a patient as an alternative to open heart Surgical Aortic Valve Replacement (SAVR). In the TAVI procedure a prosthetic valve, which is assembled on to a stent, is crimped and delivered to the patient's aortic root site through several available percutaneous means. The percutaneous nature of TAVI, which is its core advantage in comparison to other SAVR procedures, can however also be its main disadvantage. This is due to lack of direct access to the calcified leaflets, and hence reliance on the host tissue for the proper positioning and anchorage of the deployed prosthetic valve. Therefore, it is desired to have a preoperative quantitative understanding of patient-specific biomechanical interaction of the stent and the native valve to be able to maximise the chance of success of the procedure. The aim of this study was to develop a patient-specific Finite Element (FE) model of the Transcatheter Aortic Valve Implantation (TAVI) procedure for two patients, using a model of the 23 mm percutaneous prosthetic aortic valve developed by Strait Access Technologies (SAT), for the purpose of its post-operative performance. In this regard, the image processing software ScanIP was used to extract the 3D models of the patient-specific aortic roots and leaflets from the provided Multi-Slice Computer Tomography (MSCT) images of the patients. An anisotropic hyperelastic material model was implemented for the roots and leaflets, using two and one families of collagen fibres for their tissues respectively. The stent is made of a cobalt-chromium alloy and its mechanical response was modelled as an isotropic elastoplastic material, with a linear elastic initial response, followed by plastic behaviour with isotropic hardening. The prosthetic leaflets are made of polymer and were modelled as an isotropic hyperelastic material, using the provided experimental test data. The results for the first patient showed that the stent maintained its structural integrity after deployment, and successfully pushed the native leaflets back to keep the aortic root clear of all impediments. No obstruction of the coronary ostia was observed, and prosthetic leaflets were seen to function normally. The stent radial recoil was calculated to be between 2 to 4.28 % after deployments. Its foreshortening was calculated to be approximately 20%. The stent was observed to move back and forth by approximately 3 mm in the last simulation step in which cardiac cycle pressure were applied to the aortic root and prosthetic leaflets. Also, two openings were observed between the stent and aortic root wall during this simulation step, which indicates the possibility of paravalvular leakage. From the second patient simulation, it was observed that the 23 mm stent was not a good choice for this patient, and will cause severe damage or tissue tearing. The maximum principal stress in the aortic root and valve tissues were observed to follow approximately the defined collagen fibre directions
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