1,720,987 research outputs found
Viscoelastic response of hydrogel materials at finite strains
No full textHydrogel materials are very soft materials consisting of polymer networks and solvent molecules. The materials may exhibit large volume changes depending on its external chemical and mechanical environment and have viscoelastic properties which is common for many polymeric materials. In order to model the material response with the finite element method, a hydrogel constitutive model have been combined with finite viscoelastic theory and the resulting viscoelastic hydrogel constitutive model have been coded in a UMAT-subroutine for analysis with the ABAQUS/Standard finite element modeling software. Material parameters have been extracted from a hydrogel relaxation experiment, and while the experimental data is variable, the constitutive model have successfully been able to mimic the viscoelastic material response shown in the experimental data.
The Neo-Hookean and Yeoh hyperelastic models have also been combined with finite viscoelastic theory in order to model compression experiments performed on acrylic Ugelstad particles. The material models where not able to model the complex force-deflection curve shown in the experimental data and some different hyperelastic models should be considered in order to properly model the material.
The constitutive models have been numerically tested, with finite element creep and relaxation tests, which show that the models are numerically stable at large deformations
Viscoelastic response of hydrogel materials at finite strains
Get PDFHydrogel materials are very soft materials consisting of polymer networks and solvent molecules. The materials may exhibit large volume changes depending on its external chemical and mechanical environment and have viscoelastic properties which is common for many polymeric materials. In order to model the material response with the finite element method, a hydrogel constitutive model have been combined with finite viscoelastic theory and the resulting viscoelastic hydrogel constitutive model have been coded in a UMAT-subroutine for analysis with the ABAQUS/Standard finite element modeling software. Material parameters have been extracted from a hydrogel relaxation experiment, and while the experimental data is variable, the constitutive model have successfully been able to mimic the viscoelastic material response shown in the experimental data.
The Neo-Hookean and Yeoh hyperelastic models have also been combined with finite viscoelastic theory in order to model compression experiments performed on acrylic Ugelstad particles. The material models where not able to model the complex force-deflection curve shown in the experimental data and some different hyperelastic models should be considered in order to properly model the material.
The constitutive models have been numerically tested, with finite element creep and relaxation tests, which show that the models are numerically stable at large deformations
Influence of the Dynamic Motion of the Atrial Wall on the Hemodynamics in the Human Left Atrium: A Computational Fluid Dynamics Study
No full textHjerte- og karsykdommer er den ledende dødsårsaken i Europa, der atrieflimmer (AF) er en av de viktigste årsakene, og utbredelsen forventes å øke de kommende årene. Etterspørselen etter bedre diagnostikk og behandling av AF vil derfor øke. Numerisk fluiddynamikk (CFD) blir sett på som en viktig bidragsyter til å forstå hemodynamikken i det kardiovaskulære systemet og hvordan forskjellige hjerte- og karsykdommer påvirker blodstrømmen. CFD-analyse av venstre atrium (LA), den mest kritiske av atriene, har enda ikke nådd klinisk praksis på grunn av flere usikkerheter i modellen. En av disse usikkerhetene er nødvendigheten av å ha en dynamisk modell av LA eller hvis en statisk modell er tilstrekkelig, som denne oppgaven tar i sikte for å svare på.
Fra magnetisk resonansavbildning (MRI) har en pasientspesifikk LA-geometri blitt segmentert i 25 tidstrinn av hjertesyklusen. Det dynamiske meshet i CFD-studien er opprettet ved å bruke bilderegistrering mellom de segmenterte volumene, og et forskyvningsfelt blir generert og påført overflatemeshet. Resultater fra simuleringene innebærer veldig forskjellige strømningsegenskaper i den intraatriale strømningen mellom de statiske og dynamiske tilfellene. Det dynamiske tilfellet viser mer enighet med målt intraatriale strømningsfelt i andre studier. Det statiske tilfellet viser også høyere tendenser til blodstase der risikoen for trombedannelse kan oppstå. To forskjellige geometriske statiske tilfeller ble også sammenlignet og viste liten forskjell i deres egenskaper. Konklusjonen er derfor at en dynamisk LA-modell anbefales for LA CVD-studier. Hvis en statisk modell brukes, er det ingen vesentlig forskjell i hvilket tidstrinn geometrien segmenteres fra.
Resultatene viser imidlertid vanskeligheter med deres konvergens og krever videre studier for å lage en modell nøyaktig nok til å bli brukt i klinisk praksis. Den presenterte dynamiske modellen har også forbedringsområder når det gjelder meshkvalitet
Influence of the Dynamic Motion of the Atrial Wall on the Hemodynamics in the Human Left Atrium: A Computational Fluid Dynamics Study
Get PDFHjerte- og karsykdommer er den ledende dødsårsaken i Europa, der atrieflimmer (AF) er en av de viktigste årsakene, og utbredelsen forventes å øke de kommende årene. Etterspørselen etter bedre diagnostikk og behandling av AF vil derfor øke. Numerisk fluiddynamikk (CFD) blir sett på som en viktig bidragsyter til å forstå hemodynamikken i det kardiovaskulære systemet og hvordan forskjellige hjerte- og karsykdommer påvirker blodstrømmen. CFD-analyse av venstre atrium (LA), den mest kritiske av atriene, har enda ikke nådd klinisk praksis på grunn av flere usikkerheter i modellen. En av disse usikkerhetene er nødvendigheten av å ha en dynamisk modell av LA eller hvis en statisk modell er tilstrekkelig, som denne oppgaven tar i sikte for å svare på.
Fra magnetisk resonansavbildning (MRI) har en pasientspesifikk LA-geometri blitt segmentert i 25 tidstrinn av hjertesyklusen. Det dynamiske meshet i CFD-studien er opprettet ved å bruke bilderegistrering mellom de segmenterte volumene, og et forskyvningsfelt blir generert og påført overflatemeshet. Resultater fra simuleringene innebærer veldig forskjellige strømningsegenskaper i den intraatriale strømningen mellom de statiske og dynamiske tilfellene. Det dynamiske tilfellet viser mer enighet med målt intraatriale strømningsfelt i andre studier. Det statiske tilfellet viser også høyere tendenser til blodstase der risikoen for trombedannelse kan oppstå. To forskjellige geometriske statiske tilfeller ble også sammenlignet og viste liten forskjell i deres egenskaper. Konklusjonen er derfor at en dynamisk LA-modell anbefales for LA CVD-studier. Hvis en statisk modell brukes, er det ingen vesentlig forskjell i hvilket tidstrinn geometrien segmenteres fra.
Resultatene viser imidlertid vanskeligheter med deres konvergens og krever videre studier for å lage en modell nøyaktig nok til å bli brukt i klinisk praksis. Den presenterte dynamiske modellen har også forbedringsområder når det gjelder meshkvalitet.Cardiovascular Diseases (CVDs) are the leading cause of death in Europe, where Atrial Fibrillation (AF) is one of the major causes, and the prevalence is expected to increase for the coming years. Demand for better diagnostic and treatment of AF is therefore increasing. Computational Fluid Dynamics (CFD) is viewed as an essential contributor to understanding the hemodynamics in the cardiovascular system and how different CVDs impact the blood flow. CFD analysis of the Left Atrium (LA), the most critical of the atria, have not yet reached clinical practice due to several model uncertainties. One of these uncertainties is the necessity of having a dynamic model of the LA or if a static model is sufficient, which this thesis aims to answer.
From Magnetic Resonance Imaging (MRI), a patient-specific LA geometry has been segmented for 25 time-steps in the cardiac cycle. The dynamic mesh in the CFD study is created by using image registration between the segmented volumes, and a displacement field is extracted and applied to the surface mesh of the LA. Results from the simulations imply very different flow characteristics in the intra-atrial flow between the static and dynamic cases. Compared to measured intra-atrial flow field in other studies, the dynamic case shows more agreement. The static case also shows higher tendencies of blood stasis where the risk of thrombus formation may occur. Two different geometrical static cases were also compared and showed little difference in their characteristics. The conclusion is therefore that a dynamic LA model is recommended for LA CVD studies. If a static model is used, there is no substantial difference in which time-step the geometry is extracted from.
The results do, however, exhibit difficulties in their convergence and require further study in order to create a model accurate enough to be used in clinical practice. The dynamic model presented also have areas of improvement concerning mesh quality
Modelling and numerical analysis of the porcine and human mitral apparatus
Get PDFPhD i konstruksjonsteknikkPhD in Structural Engineerin
Spatiotemporal mapping of cascading processes in oligonucleotide-functionalized responsive hydrogels
No full textHydrogels – crosslinked polymer networks holding a large amount of water – have many properties that make them useful for medical applications, such as their resemblance to biological tissues, biocompatibility and biodegradability. So-called responsive hydrogels are also able to change their properties in response to a change in their surroundings. Some of their potential applications include manufacturing of sensors, delivering drugs to a specific location in the body, or serving as scaffolds for tissue engineering. The aim of the thesis was to investigate the processes that lead to swelling of oligonucleotide-functionalized hydrogels which respond to the presence of a target DNA or other oligonucleotide. An understanding of these processes and how changing one affects the rest is important in order to be able to adjust the hydrogels to their intended applications. The swelling of the hydrogels and the transport of the DNA within them were observed using interferometry. Then the binding strength of the target to the network was varied to investigate how the transport and the overall swelling are affected
Spatiotemporal mapping of cascading processes in oligonucleotide-functionalized responsive hydrogels
Get PDFHydrogels – crosslinked polymer networks holding a large amount of water – have many properties that make them useful for medical applications, such as their resemblance to biological tissues, biocompatibility and biodegradability. So-called responsive hydrogels are also able to change their properties in response to a change in their surroundings. Some of their potential applications include manufacturing of sensors, delivering drugs to a specific location in the body, or serving as scaffolds for tissue engineering. The aim of the thesis was to investigate the processes that lead to swelling of oligonucleotide-functionalized hydrogels which respond to the presence of a target DNA or other oligonucleotide. An understanding of these processes and how changing one affects the rest is important in order to be able to adjust the hydrogels to their intended applications. The swelling of the hydrogels and the transport of the DNA within them were observed using interferometry. Then the binding strength of the target to the network was varied to investigate how the transport and the overall swelling are affected
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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