Hybrid finite difference/finite element immersed boundary method

The immersed boundary method is an approach to fluid-structure interaction that uses a Lagrangian description of the structural deformations, stresses, and forces along with an Eulerian description of the momentum, viscosity, and incompressibility of the fluid-structure system. The original immersed boundary methods described immersed elastic structures using systems of flexible fibers, and even now, most immersed boundary methods still require Lagrangian meshes that are finer than the Eulerian grid. This work introduces a coupling scheme for the immersed boundary method to link the Lagrangian and Eulerian variables that facilitates independent spatial discretizations for the structure and background grid. This approach employs a finite element discretization of the structure while retaining a finite difference scheme for the Eulerian variables. We apply this method to benchmark problems involving elastic, rigid, and actively contracting structures, including an idealized model of the left ventricle of the heart. Our tests include cases in which, for a fixed Eulerian grid spacing, coarser Lagrangian structural meshes yield discretization errors that are as much as several orders of magnitude smaller than errors obtained using finer structural meshes. The Lagrangian-Eulerian coupling approach developed in this work enables the effective use of these coarse structural meshes with the immersed boundary method. This work also contrasts two different weak forms of the equations, one of which is demonstrated to be more effective for the coarse structural discretizations facilitated by our coupling approach

In Silico Modeling, Simulation and Optimization of Human Cardiac Motion

Cardiac diseases are the number one reasons for death in the western world. Computational simulations provide the opportunity to conduct experiments and predictions that are not possible in humans due to ethical and other reasons. High performance computation allows the use of demanding coupled computational models of high complexity and a high level of detail, complying with a wide range of experimental data from the human heart. In this thesis, different aspects of computational heart modeling are covered: models describing passive tissue behavior, active contractile behavior, circulatory system modeling, influences of the pericardium and surrounding tissue on the heart as well as methods to obtain suitable parameters for these models. For each aspect, several modeling approaches are presented and compared. Finally, a scalability evaluation of the highly-parallelized implementation and an evaluation of the proper choice of mesh resolution for credible numerical results are covered. Concludingly, this thesis allows the reader to gain insights into the complexity of computational heart modeling and to make an appropriate choice of models and parameters suitable for specific applications

An Immersed Interface Method for Discrete Surfaces

Fluid-structure systems occur in a range of scientific and engineering applications. The immersed boundary(IB) method is a widely recognized and effective modeling paradigm for simulating fluid-structure interaction(FSI) in such systems, but a difficulty of the IB formulation is that the pressure and viscous stress are generally discontinuous at the interface. The conventional IB method regularizes these discontinuities, which typically yields low-order accuracy at these interfaces. The immersed interface method(IIM) is an IB-like approach to FSI that sharply imposes stress jump conditions, enabling higher-order accuracy, but prior applications of the IIM have been largely restricted to methods that rely on smooth representations of the interface geometry. This paper introduces an IIM that uses only a C0 representation of the interface,such as those provided by standard nodal Lagrangian FE methods. Verification examples for models with prescribed motion demonstrate that the method sharply resolves stress discontinuities along the IB while avoiding the need for analytic information of the interface geometry. We demonstrate that only the lowest-order jump conditions for the pressure and velocity gradient are required to realize global 2nd-order accuracy. Specifically,we show 2nd-order global convergence rate along with nearly 2nd-order local convergence in the Eulerian velocity, and between 1st-and 2nd-order global convergence rates along with 1st-order local convergence for the Eulerian pressure. We also show 2nd-order local convergence in the interfacial displacement and velocity along with 1st-order local convergence in the fluid traction. As a demonstration of the method's ability to tackle complex geometries,this approach is also used to simulate flow in an anatomical model of the inferior vena cava.Comment: - Added a non-axisymmetric example (flow within eccentric rotating cylinder in Sec. 4.3) - Added a more in-depth analysis and comparison with a body-fitted approach for the application in Sec. 4.

Characterization of Cardiac Electrogram Signals During Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia in United States. The most popular treatment for AF is a percutaneous procedure called catheter ablation. Current AF ablation procedures unfortunately have a poor success rate, primarily because the mechanisms involved in AF are incompletely understood even today. Intra-atrial electrograms have previously been shown to provide information on the mechanisms of AF. This thesis focuses on two such mechanisms – AF-sustaining sites known as sustained rotational activities (RotAs), and atrial tissue with unique electrical properties known as myocardial scars. Catheter ablation procedures today construct the 3D electroanatomic map of the left atrium (LA) by maneuvering a conventional Multipolar Diagnostic Catheter (MPDC) along the LA endocardial surface. These procedures are limited to pulmonary vein isolation and other linear ablation performed on various regions of the left atrium (such as roof and mitral isthmus) where the regions are decided based on the atrial anatomy. However, it remains unclear how to utilize the information provided by the MPDC to analyze and characterize the RotAs and scars. Previous electrogram characterization studies mainly use a single bipole rather than MPDCs to characterize the electrograms based on features such as cycle length or dominant frequency from the time or frequency domain. In this thesis we developed novel techniques for investigating the above mentioned mechanisms using signal analysis, mathematical modeling, numerical simulation and clinical experiments, all utilizing MPDC recordings. First, the variations in the total conduction delay (TCD) from MPDC electrograms as the MPDC moves towards a RotA source was investigated. Second, the maximum peak-to-peak amplitudes of MPDC electrograms recorded during AF and NSR were analyzed. This thesis provides insights into methods of characterization of cardiac electrograms and the findings of this thesis could address the current challenges in AF ablation

Immersed boundary simulations and tools for studying insect flight and other applications

All organisms must deal with fluid transport and interaction, whether it be internal, such as lungs moving air for the extraction of oxygen, or external, such as the expansion and contraction of a jellyfish bell for locomotion. Most organisms are highly deformable and their elastic deformations can be used to move fluid, move through fluid, and resist fluid forces. A particularly effective numerical method for biological fluid-structure interaction simulations is the immersed boundary (IB) method. An important feature of this method is that the fluid is discretized separately from the boundary interface, meaning that the two meshes do not need to conform with each other. This thesis covers the development of a new software tool for the semi-automated creation of finite difference meshes of complex 2D geometries for use with immersed boundary solvers IB2d and IBAMR, alongside two examples of locomotion - the flight of tiny insects and the metachronal paddling of brine shrimp. As mentioned, an advantage of the IB method is that complex geometries, e.g., internal or external morphology, can easily be handled without the need to generate matching grids for both the fluid and the structure. Consequently, the difficulty of modeling the structure lies often in discretizing the boundary of the complex geometry (morphology). Both commercial and open source mesh generators for finite element methods have long been established; however, the traditional immersed boundary method is based on a finite difference discretization of the structure. In chapter \ref{chap:meshmerizeme}, I present a software library called MeshmerizeMe for obtaining finite difference discretizations of boundaries for direct use in the 2D immersed boundary method. This library provides tools for extracting such boundaries as discrete mesh points from digital images. Several examples of how the method can be applied are given to demonstrate the effectiveness of the software, including passing flow through the veins of insect wings, within lymphatic capillaries, and around starfish using open-source immersed boundary software. As an example of insect flight, I present a 3D model of clap and fling. Of the smallest insects filmed in flight, most if not all clap their wings together at the end of the upstroke and fling them apart at the beginning of the downstroke. This motion increases the strength of the leading edge vortices generated during the downstroke and augments the lift. At the Reynolds numbers ($Re$) relevant to flight in these insects (roughly $4<Re<40$), the drag produced during the fling is substantial, although this can be reduced through the presence of wing bristles, chordwise wing flexibility, and more complex wingbeat kinematics. It is not clear how flexibility in the spanwise direction of the wings can alter the lift and drag generated. In chapter \ref{chap:clapfling}, a hybrid version of the immersed boundary method with finite elements is used to simulate a 3D idealized clap and fling motion across a range of wing flexibilities. I find that spanwise flexibility, in addition to three-dimensional spanwise flow, can reduce the drag forces produced during the fling while maintaining lift, especially at lower $Re$. While the drag required to fling 2D wings apart may be more than an order of magnitude higher than the force required to translate the wings, this effect is significantly reduced in 3D. Similar to previous studies, dimensionless drag increases dramatically for $Re<20$, and only moderate increases in lift are observed. Both lift and drag decrease with increasing wing flexibility, but below some threshold, lift decreases much faster. This study highlights the importance of flexibility in both the chordwise and spanwise directions for low $Re$ insect flight. The results also suggest that there is a large aerodynamic cost if insect wings are too flexible. My second application of locomotion pertains to a 2D model of swimming, specifically the method known as metachronal paddling. This method is used by a variety of organisms to propel themselves through a fluid. This mode of swimming is characterized by an array of appendages that beat out of phase, such as the swimmerets used by long-tailed crustaceans like crayfish and lobster. This form of locomotion is typically observed over a range of Reynolds numbers greater than 1 where the flow is dominated by inertia. The majority of experimental, modeling, and numerical work on metachronal paddling has been conducted on the higher Reynolds number regime (order 100). In this chapter, a simplified numerical model of one of the smaller metachronal swimmers, the brine shrimp, is constructed. Brine shrimp are particularly interesting since they swim at Reynolds numbers on the order of 10 and sprout additional paddling appendages as they grow. The immersed boundary method is used to numerically solve the fluid-structure interaction problem of multiple rigid paddles undergoing cycles of power and return strokes with a constant phase difference and spacing that are based on brine shrimp parameters. Using a phase difference of 8\%, the volumetric flux and efficiency per paddle as a function of the Reynolds number and the spacing between legs is quantified. I find that the time to reach periodic steady state for adult brine shrimp is large ($\approx 150$ stroke cycles) and decreases with decreasing Reynolds number. Both efficiency and average flux increase with Reynolds number. In terms of leg spacing, the average flux decreases with increased spacing while the efficiency is maximized for intermediate leg spacing.Doctor of Philosoph

A Novel Propeller Design for Micro-Swimming robot

The applications of a micro-swimming robot such as minimally invasive surgery, liquid pipeline robot etc. are widespread in recent years. The potential application fields are so inspiring, and it is becoming more and more achievable with the development of microbiology and Micro-Electro-Mechanical Systems (MEMS). The aim of this study is to improve the performance of micro-swimming robot through redesign the structure. To achieve the aim, this study reviewed all of the modelling methods of low Reynolds number flow including Resistive-force Theory (RFT), Slender Body Theory (SBT), and Immersed Boundary Method (IBM) etc. The swimming model with these methods has been analysed. Various aspects e.g. hydrodynamic interaction, design, development, optimisation and numerical methods from the previous researches have been studied. Based on the previous design of helix propeller for micro-swimmer, this study has proposed a novel propeller design for a micro-swimming robot which can improve the velocity with simplified propulsion structure. This design has adapted the coaxial symmetric double helix to improve the performance of propulsion and to increase stability. The central lines of two helical tails overlap completely to form a double helix structure, and its tail radial force is balanced with the same direction and can produce a stable axial motion. The verification of this design is conducted using two case studies. The first one is a pipe inspection robot which is in mm scale and swims in high viscosity flow that satisfies the low Reynolds number flow condition. Both simulation and experiment analysis are conducted for this case study. A cross-development method is adopted for the simulation analysis and prototype development. The experiment conditions are set up based on the simulation conditions. The conclusion from the analysis of simulation results gives suggestions to improve design and fabrication for the prototype. Some five revisions of simulation and four revisions of the prototype have been completed. The second case study is the human blood vessel robot. For the limitations of fabrication technology, only simulation is conducted, and the result is compared with previous researches. The results show that the proposed propeller design can improve velocity performance significantly. The main outcomes of this study are the design of a micro-swimming robot with higher velocity performance and the validation from both simulation and experiment

In-Vitro and In-Silico Investigations of Alternative Surgical Techniques for Single Ventricular Disease

Single ventricle (SV) anomalies account for one-fourth of all cases of congenital Heart disease. The conventional second and third stage i.e. Comprehensive stage II and Fontan procedure of the existing three-staged surgical approach serving as a palliative treatment for this anomaly, entails multiple complications and achieves a survival rate of 50%. Hence, to reduce the morbidity and mortality rate associated with the second and third stages of the existing palliative procedure, the novel alternative techniques called “Hybrid Comprehensive Stage II” (HCSII), and a “Self-powered Fontan circulation” have been proposed. The goal of this research is to conduct in-vitro investigations to validate computational and clinical findings on these proposed novel surgical techniques. The research involves the development of a benchtop study of HCSII and self-powered Fontan circulation

Mathematical modeling approaches for the diagnosis and treatment of reentrant atrial tachyarrhythmias

[EN] Atrial tachyarrhythmias present a high prevalence in the developed world, and several studies predict that in the coming decades it will be increased. Micro or macro-reentrant mechanisms of the electrical wavefronts that govern the mechanical behavior of the heart are one of the main responsibles for the maintenance of these arrhythmias. Atrial flutter is maintained by a macro-reentry around an anatomical or functional obstacle located in the atria. In the case of atrial fibrillation, the hypothesis which describes high frequency rotors as dominant sources of the fibrillation and responsible for the maintenance of the arrhythmia, has been gaining relevance in the last years. However, the therapies that target high frequency sources have a limited efficacy with current techniques. Radiofrequency ablation allows the destruction of parts of the cardiac tissue resulting in the interruption of the reentrant circuit in case of macro-reentries or the isolation of micro-reentrant circuits. The non-invasive location of reentrant circuits would increment the efficacy of these therapies and would shorten surgery interventions. In parallel, pharmacological therapies modify ionic expressions associated to the excitability and electrical refractoriness of the cardiac tissue with the objective of hindering the maintenance of reentrant behaviors. These therapies require a deep knowledge of the ionic mechanisms underlying the reentrant behavior and its properties in order to be effective. The research in these mechanisms allows the evaluation of new targets for the treatment and thus may improve the efficacy in atrial fibrillation termination. In this thesis, mathematical modeling is used to go forward in the minimization of the limitations associated to these treatments. Body surface potential mapping has been evaluated, both clinically and by means of mathematical simulations for the diagnosis and location of macro-reentrant circuits. The analysis of phase maps obtained from multiple lead electrocardiographic recordings distributed in the whole torso allowed the discrimination between different reentrant circuits. It is the reason why this technique is presented as a tool for the non-invasive location of macro and micro-reentrant circuits. A population of mathematical models designed in this thesis based on the action potentials recordings of atrial cardiomyocites from 149 patients, allowed the evaluation of the ionic mechanisms defining the properties of reentrant behaviors. This study has allowed us defining the blockade of ICaL as a target for the pharmacological treatment. The blockade of this current is associated with the increase of the movement in the core of the rotor which easies the collision of the rotor with other wavefronts or anatomical obstacles promoting the extinction of the reentry. The variability observed between patients modeled in our population has allowed showing and explaining the mechanisms promoting divergent results of a single treatment. This is why the introduction of populations of models will allow the prevention of side effects associated to inter-subject variability and to go forward in the development of individualized therapies. These works are built through a simulation platform of cardiac electrophysiology based in Graphic Processing Units (GPUs) and developed in this thesis. The platform allows the simulation of cellular models, tissues and organs with a realistic geometry and shows features comparable to that of the platforms used by the most relevant electrophysiology research groups at the moment.[ES] Las taquiarritmias auriculares tienen una alta prevalencia en el mundo desarrollado, además diversos estudios poblacionales indican que en las próximas décadas ésta se verá incrementada. Los mecanismos de micro o macro-reentrada de los frentes de onda eléctricos que rigen el comportamiento mecánico del corazón, se presentan como una de las principales causas del mantenimiento de estas arritmias. El flutter auricular es mantenido por un macro-reentrada alrededor de un obstáculo anatómico o funcional en las aurículas, mientras que en el caso de la fibrilación auricular la hipótesis que define a los rotores de alta frecuencia como elementos dominantes y responsables del mantenimiento de la arritmia se ha ido imponiendo al resto en los últimos años. Sin embargo, las terapias que tienen como objetivo finalizar o aislar estas reentradas tienen todavía una eficacia limitada. La ablación por radiofrecuencia permite eliminar zonas del tejido cardiaco resultando en la interrupción del circuito de reentrada en el caso de macro-reentradas o el aislamiento de comportamientos micro-reentrantes. La localización no invasiva de los circuitos reentrantes incrementaría la eficacia de estas terapias y reduciría la duración de las intervenciones quirúrgicas. Por otro lado, las terapias farmacológicas alteran las expresiones iónicas asociadas a la excitabilidad y la refractoriedad del tejido con el fin de dificultar el mantenimiento de comportamientos reentrantes. Este tipo de terapias exigen incrementar el conocimiento de los mecanismos subyacentes que explican el proceso de reentrada y sus propiedades, la investigación de estos mecanismos permite definir las dianas terapéuticas que mejoran la eficacia en la extinción de estos comportamientos. En esta tesis el modelado matemático se utiliza para dar un paso importante en la minimización de las limitaciones asociadas a estos tratamientos. La cartografía eléctrica de superficie ha sido testada, clínicamente y con simulaciones matemática,s como técnica de diagnóstico y localización de circuitos macro-reentrantes. El análisis de mapas de fase obtenidos a partir de los registros multicanal de derivaciones electrocardiográficas distribuidas en la superficie del torso permite diferenciar distintos circuitos de reentrada. Es por ello que esta técnica de registro y análisis se presenta como una herramienta para la localización no invasiva de circuitos macro y micro-reentrantes. Una población de modelos matemáticos, diseñada en esta tesis a partir de los registros de los potenciales de acción de 149 pacientes, ha permitido evaluar los mecanismos iónicos que definen las propiedades asociadas a los procesos de reentrada. Esto ha permitido apuntar al bloqueo de la corriente ICaL como diana terapéutica. Ésta se asocia al incremento del movimiento del núcleo que facilita el impacto del rotor con otros frentes de onda u obstáculos extinguiéndose así el comportamiento reentrante. La variabilidad entre pacientes reflejada en la población de modelos ha permitido además mostrar los mecanismos por los cuales un mismo tratamiento puede mostrar efectos divergentes, así el uso de poblaciones de modelos matemáticos permitirá prevenir efectos secundarios asociados a la variabilidad entre pacientes y profundizar en el desarrollo de terapias individualizadas. Estos trabajos se cimientan sobre una plataforma de simulación de electrofisiología cardiaca de basado en Unidades de Procesado Gráfico (GPUs) y desarrollada en esta tesis. La plataforma permite la simulación de modelos celulares cardiacos así como de tejidos u órganos con geometría realista, mostrando unas prestaciones comparables con las de las utilizadas por los grupos de investigación más potentes en el campo de la electrofisiología.[CA] Les taquiarítmies auriculars tenen una alta prevalença en el món desenvolupat, a més diversos estudis poblacionals indiquen que en les pròximes dècades aquesta es veurà incrementada. Els mecanismes de micro o macro-reentrada dels fronts d'ona elèctrics que regeixen el comportament mecànic del cor, es presenten com una de les principals causes del manteniment d'aquestes arítmies. El flutter auricular és mantingut per una macro-reentrada al voltant d'un obstacle anatòmic o funcional en les aurícules, mentre que en el cas de la fibril·lació auricular la hipòtesi que defineix als rotors d'alta freqüència com a elements dominants i responsables del manteniment de l'arítmia s'ha anat imposant a la resta en els últims anys. No obstant això, les teràpies que tenen com a objectiu finalitzar o aïllar aquestes reentrades tenen encara una eficàcia limitada. L'ablació per radiofreqüència permet eliminar zones del teixit cardíac resultant en la interrupció del circuit de reentrada en el cas de macro-reentrades o l'aïllament de comportaments micro-reentrants. La localització no invasiva dels circuits reentrants incrementaria l'eficàcia d'aquestes teràpies i reduiria la durada de les intervencions quirúrgiques. D'altra banda, les teràpies farmacològiques alteren les expressions iòniques associades a la excitabilitat i la refractaritat del teixit amb la finalitat de dificultar el manteniment de comportaments reentrants. Aquest tipus de teràpies exigeixen incrementar el coneixement dels mecanismes subjacents que expliquen el procés de reentrada i les seues propietats, la recerca d'aquests mecanismes permet definir les dianes terapèutiques que milloren l'eficàcia en l'extinció d'aquests comportaments. En aquesta tesi el modelatge matemàtic s'utilitza per a fer un pas important en la minimització de les limitacions associades a aquests tractaments. La cartografia elèctrica de superfície ha sigut testada, clínicament i amb simulacions matemàtiques com a tècnica de diagnòstic i localització de circuits macro-reentrants. L'anàlisi de mapes de fase obtinguts a partir dels registres multicanal de derivacions electrocardiogràfiques distribuïdes en la superfície del tors permet diferenciar diferents circuits de reentrada. És per açò que aquesta tècnica de registre i anàlisi es presenta com una eina per a la localització no invasiva de circuits macro i micro-reentrants. Una població de models matemàtics, dissenyada en aquesta tesi a partir dels registres dels potencials d'acció de 149 pacients, ha permès avaluar els mecanismes iònics que defineixen les propietats associades als processos de reentrada. Açò ha permès apuntar al bloqueig del corrent ICaL com a diana terapèutica. Aquesta s'associa a l'increment del moviment del nucli que facilita l'impacte del rotor amb altres fronts d'ona o obstacles extingint-se així el comportament reentrant. La variabilitat entre pacients reflectida en la població de models ha permès a més mostrar els mecanismes pels quals un mateix tractament pot mostrar efectes divergents, així l'ús de poblacions de models matemàtics permetrà prevenir efectes secundaris associats a la variabilitat entre pacients i aprofundir en el desenvolupament de teràpies individualitzades. Aquests treballs es fonamenten sobre una plataforma de simulació de electrofisiologia cardíaca basat en Unitats de Processament Gràfic (GPUs) i desenvolupada en aquesta tesi. La plataforma permet la simulació de models cel·lulars cardíacs així com de teixits o òrgans amb geometria realista, mostrant unes prestacions comparables amb les de les utilitzades per els grups de recerca més importants en aquesta área.Liberos Mascarell, A. (2016). Mathematical modeling approaches for the diagnosis and treatment of reentrant atrial tachyarrhythmias [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/62166TESI

Extended Duration Simulation and Testing of Cellular and Decellularised Heart Valve Roots

Heart valve disease can affect people of all ages, and can be treated by either valve repair or valve replacement surgery. Currently available replacement heart valves, including mechanical prostheses, bioprostheses, autografts and allografts improve patient survival and quality of life, but have limitations. Key limitations include the risk of immunological reaction and the lack of growth potential and regeneration, which is of particular importance in young patients. To address these limitations, low concentration sodium dodecyl sulphate (SDS) decellularised human aortic, human pulmonary, porcine aortic and porcine pulmonary heart valve roots have been developed. Decellularisation of allografts would potentially reduce the risk of immunological reaction, and the development of a decellularised porcine pulmonary heart valve root would potentially provide an option for right ventricular outflow reconstruction in younger patients who have undergone the Ross Procedure. Before moving to clinical trials, the functional performance of decellularised heart valve roots needs to be pre-clinically assessed appropriately to determine mechanical safety. Whilst there are recommended test methods in place for the in vitro functional performance assessment of newly manufactured and modified surgical replacement heart valves, they need to be optimised or replaced with novel methods suitable for decellularised heart valve roots, due to their time dependent viscoelastic properties. The main aim of this research was to optimise in vitro hydrodynamic and biomechanical performance test methods and develop a novel real time fatigue test method for biological heart valve roots. The secondary aim was to apply the developed in vitro test methods to cellular and decellularised (human and porcine) heart valve roots to evaluate the effect of decellularisation, prior to the decellularised heart valve roots being implanted in patients for clinical trials. In collaboration with NHS Blood and Transplant, Tissue and Eye Services, in vitro biomechanical and hydrodynamic performance of decellularised human aortic and pulmonary heart valve roots was evaluated for the first time in this research. This research determined that the hydrodynamic and functional biomechanical performance of human aortic and pulmonary heart valve roots was not affected by decellularisation treatment. Decellularisation, however, significantly altered some of the directional material properties of pulmonary and aortic heart valve root leaflets. To support clinical translation of decellularised porcine pulmonary heart valve roots, material properties of pulmonary heart valve roots was evaluated following 12 months implantation in sheep. In addition, the effect of the processing steps of cryopreservation and decellularisation on the material properties of porcine pulmonary heart valve roots was investigated. Cryopreservation was shown not to alter the material properties of cellular porcine pulmonary heart valve roots, however, decellularisation did have an effect on the material properties of the porcine pulmonary heart valve root wall. Following 12 months implantation in sheep, the decellularised porcine pulmonary heart valve root wall and leaflets showed a trend for decreasing stiffness and strength; becoming more like the cellular ovine, potentially indicating constructive remodelling. A novel method was developed to investigate the real time fatigue of biological heart valve roots, which was then applied to porcine cellular aortic heart valve roots and porcine decellularised aortic heart valve roots at 120 bpm under physiological cyclic pressures for a maximum of 1.2 million cycles. The results showed no fatigue difference between the cellular and decellularised heart valve roots. Overall, a portfolio of in vitro pre-clinical test methods were developed, optimised and applied to assess the hydrodynamic, biomechanical and fatigue performance of biological heart valve roots including decellularised human and porcine heart valve roots. The in vitro pre-clinical test methods developed in this study will lead to the refinement of in vivo large animal studies and revision of international standards; and the data will help in the development of the next generation of replacement biological heart valve roots, such as decellularised heart valve roots

Technical paper contest for women 1992. Space challenges: Earth and beyond

Two of the major concerns of the NASA Ames Research Center (NASA ARC) Advisory Committee for Women (ACW) are that recruitment of women scientists, engineers, and technicians needs to increase and that barriers to advancement need to be removed for improved representation of women in middle and upper management and scientific positions. One strategy that addressed this concern was the ACW sponsorship of a Technical Paper Contest for Women at Ames Research Center. Other sponsors of the Contest were the Ames Equal Opportunity Council and the Ames Contractor Council. The Technical Paper Contest for Women greatly increased the visibility of both the civil service women and the women who work for contractors at Ames. The women had the opportunity to hone their written and oral presentation skills. Networking among Ames women increased