2,264 research outputs found

    Effects of complex vessel geometries on neutrophil margination and adhesion in post-capillary venules

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    The inflammatory process is a regular occurrence within a healthy body. As part of the inflammatory process, leukocytes flow through blood vessels and are recruited to the region of the injury. Neutrophils play a significant role in this process; however the margination of neutrophils to particular locations in micro vessels is not fully understood. Post capillary venules, in particular, have complex geometries which may contribute to non-uniform adhesion of neutrophils. Margination is a phenomenon that occurs during the relatively early phases of inflammation; as a result of dilation of capillaries and slowing of the bloodstream, leukocytes tend to occupy the periphery of the cross-sectional lumen. Other investigations have looked at the adhesion of neutrophils in vivo or flow patterns in converging tubes, but the correlation between flow patterns in complex geometries and neutrophil margination is not well understood. This study seeks to investigate correlations between margination and bulk flow patterns as well as parameters that affect bulk flow properties. The primary aim of this investigation is to create specific computational and in vitro models based on in vivo data that isolate the hydrodynamic mechanisms associated with complex geometries. Main geometric factors that were investigated were surface roughness, branch geometries, number of convergences and squared vs. rounded t-junctions. To determine the effect of surface roughness a large scale parallel plate flow chamber model as well as a microfabrication technique to simulate roughness at the blood vessel scale were created that simulate surface roughness due to endothelial cell nuclei. CFD modeling was also used to determine effects of other geometric factors including branch geometries, number of convergences and squared vs. rounded t-junctions. Overall, results from this study suggest that complex geometries can have a significant role on neutrophil margination and adhesion in blood vessels. A preliminary relationship between wall shear stress and margination was established

    Numerical and experimental haemodynamic studies of stenotic coronary arteries

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    Dissertação de mestrado integrado em Engenharia Biomédica (área de especialização em Biomateriais, Reabilitação e Biomecânica)Cardiovascular diseases remain the most frequent cause of mortality worldwide and constitute a major healthcare challenge. Among them, coronary artery disease causes nearly half of the deaths and, thus it is of great interest to better understand its development and effects. This disease is characterized by the narrowing (stenosis) of coronary arteries due to plaque deposition at the arterial wall, a pathological process known as atherosclerosis. This dissertation aimed to study the hemodynamics in stenotic coronary arteries, in order to get a deeper understanding of the effects of this pathology on the blood flow behavior. For this purpose, both numerical and experimental studies were conducted using idealized models. The numerical research was carried out using Ansys® software by means of computational fluid dynamics which applies the finite volume method. The experimental approach was performed using a high-speed video microscopy system, to visualize and investigate the blood flow in the in vitro stenotic biomodels. Initially, the influence of roughness in flow visualizations was studied, and the best biomodel was the one printed with the lowest resolution having been, therefore, the selected to perform the hemodynamic studies. To compare those results with numerical data, the flow was set to be laminar and stationary and the fluid was considered Newtonian. In general, the numerical and experimental results were in good agreement, not only in the prediction of the flow behavior with the appearance of recirculation zones in the post-stenotic section, but also in the velocity profiles. In a posterior phase, a pulsatile inlet condition was applied to compare the use of laminar and turbulent assumptions, using the SST k- model. The results obtained allowed to conclude that the second one is more appropriate to simulate the blood flow. Subsequently, the main differences in hemodynamics were examined considering blood as a Newtonian and non-Newtonian fluid (Carreau model). For these models, the differences were very slight in terms of velocity fields, but more significant for the wall shear stress measurements, with the Newtonian model predicting lower values. The remaining simulations were performed using the Carreau model and a transient inlet flow, having observed an increase in the velocities and wall shear stress values with the degree of stenosis, which is associated with a greater risk of thrombosis.As doenças cardiovasculares continuam a ser a causa mais frequente de mortalidade em todo o mundo e constituem um grande desafio para a saúde. Entre elas, a doença arterial coronariana causa quase metade das mortes e, portanto, é de enorme interesse entender melhor o seu desenvolvimento e efeitos. Esta doença é caracterizada pelo estreitamento (estenose) das artérias coronárias devido à deposição de placas na parede arterial, um processo patológico conhecido como aterosclerose. Esta dissertação teve como objetivo estudar a hemodinâmica nas artérias coronárias estenóticas, a fim de obter uma compreensão mais profunda dos efeitos desta patologia no comportamento do fluxo sanguíneo. Para tal, foram realizados estudos numéricos e experimentais, utilizando modelos idealizados. A investigação numérica foi realizada no software Ansys®, através da dinâmica computacional dos fluidos, que aplica o método dos volumes finitos. A abordagem experimental foi realizada utilizando um sistema de microscopia de vídeo de alta velocidade, para visualizar e investigar o fluxo sanguíneo nos biomodelos estenóticos in vitro. Inicialmente, estudou-se a influência da rugosidade nas visualizações do escoamento, e o melhor biomodelo foi o impresso com menor resolução tendo sido, portanto, o selecionado para a realização dos estudos hemodinâmicos. Para comparar esses resultados com dados numéricos, o escoamento foi definido como laminar e estacionário e o fluído foi considerado Newtoniano. Em geral, os resultados numéricos e experimentais foram concordantes, não só na previsão do comportamento do fluxo com aparecimento de zonas de recirculação na zona pós-estenótica, mas também nos perfis de velocidade. Numa fase posterior, foi aplicada uma condição de entrada pulsátil para comparar o uso de simulações de natureza laminar e turbulenta, usando o modelo SST k-. Os resultados obtidos permitiram concluir que a segunda é mais apropriado para simular o fluxo sanguíneo. Posteriormente, foram examinadas as principais diferenças hemodinâmicas, considerando o sangue como fluído Newtoniano e não-Newtoniano (modelo de Carreau). Para estes modelos, as diferenças foram muito pequenas nos perfis de velocidade, mas mais significativas nas tensões de corte na parede medidas, com o modelo Newtoniano a prever valores mais baixos. As restantes simulações foram realizadas usando o modelo de Carreau e um escoamento de entrada transiente, tendo-se observado um aumento dos valores das velocidades e da tensão de corte na parede com o grau de estenose, o que está associado a um maior risco de trombose

    Assessment of surface roughness and blood rheology on local coronary hemodynamics: a multi-scale computational fluid dynamics study

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    The surface roughness of the coronary artery is associated with the onset of atherosclerosis. The study applies, for the first time, the micro-scale variation of the artery surface to a 3D coronary model, investigating the impact on haemodynamic parameters which are indicators for atherosclerosis. The surface roughness of porcine coronary arteries have been detailed based on optical microscopy and implemented into a cylindrical section of coronary artery. Several approaches to rheology are compared to determine the benefits/limitations of both single and multiphase models for multi-scale geometry. Haemodynamic parameters averaged over the rough/smooth sections are similar; however, the rough surface experiences a much wider range, with maximum wall shear stress greater than 6 Pa compared to the approximately 3 Pa on the smooth segment. This suggests the smooth-walled assumption may neglect important near-wall haemodynamics. While rheological models lack sufficient definition to truly encompass the micro-scale effects occurring over the rough surface, single-phase models (Newtonian and non-Newtonian) provide numerically stable and comparable results to other coronary simulations. Multiphase models allow for phase interactions between plasma and red blood cells which is more suited to such multi-scale models. These models require additional physical laws to govern advection/aggregation of particulates in the near-wall region

    3D printed biomodels for flow visualization in stenotic vessels: an experimental and numerical study

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    Atherosclerosis is one of the most serious and common forms of cardiovascular disease and a major cause of death and disability worldwide. It is a multifactorial and complex disease that promoted several hemodynamic studies. Although in vivo studies more accurately represent the physiological conditions, in vitro experiments more reliably control several physiological variables and most adequately validate numerical flow studies. Here, a hemodynamic study in idealized stenotic and healthy coronary arteries is presented by applying both numerical and in vitro approaches through computational fluid dynamics simulations and a high-speed video microscopy technique, respectively. By means of stereolithography 3D printing technology, biomodels with three different resolutions were used to perform experimental flow studies. The results showed that the biomodel printed with a resolution of 50 μm was able to most accurately visualize flow due to its lowest roughness values (Ra = 1.8 μm). The flow experimental results showed a qualitatively good agreement with the blood flow numerical data, providing a clear observation of recirculation regions when the diameter reduction reached 60%.This work was supported by FCT-Fundacao para a Ciencia e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020, UIDB/04077/2020, and NORTE-01-0145-FEDER-030171, funded by COMPETE2020, NORTE 2020, PORTUGAL 2020, and FEDER. This project received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 798014. This project received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 828835

    Pressure drop and recovery in cases of cardiovascular disease: a computational study

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    The presence of disease in the cardiovascular system results in changes in flow and pressure patterns. Increased resistance to the flow observed in cases of aortic valve and coronary artery disease can have as a consequence abnormally high pressure gradients, which may lead to overexertion of the heart muscle, limited tissue perfusion and tissue damage. In the past, computational fluid dynamics (CFD) methods have been used coupled with medical imaging data to study haemodynamics, and it has been shown that CFD has great potential as a way to study patient-specific cases of cardiovascular disease in vivo, non-invasively, in great detail and at low cost. CFD can be particularly useful in evaluating the effectiveness of new diagnostic and treatment techniques, especially at early ‘concept’ stages. The main aim of this thesis is to use CFD to investigate the relationship between pressure and flow in cases of disease in the coronary arteries and the aortic valve, with the purpose of helping improve diagnosis and treatment, respectively. A transitional flow CFD model is used to investigate the phenomenon of pressure recovery in idealised models of aortic valve stenosis. Energy lost as turbulence in the wake of a diseased valve hinders pressure recovery, which occurs naturally when no energy losses are observed. A “concept” study testing the potential of a device that could maximise pressure recovery to reduce the pressure load on the heart muscle was conducted. The results indicate that, under certain conditions, such a device could prove useful. Fully patient-specific CFD studies of the coronary arteries are fewer than studies in larger vessels, mostly due to past limitations in the imaging and velocity data quality. A new method to reconstruct coronary anatomy from optical coherence tomography (OCT) data is presented in the thesis. The resulting models were combined with invasively acquired pressure and flow velocity data in transient CFD simulations, in order to test the ability of CFD to match the invasively measured pressure drop. A positive correlation and no bias were found between the calculated and measured results. The use of lower resolution reconstruction methods resulted in no correlation between the calculated and measured results, highlighting the importance of anatomical accuracy in the effectiveness of the CFD model. However, it was considered imperative that the limitations of CFD in predicting pressure gradients be further explored. It was found that the CFD-derived pressure drop is sensitive to changes in the volumetric flow rate, while bench-top experiments showed that the estimation of volumetric flow rate from invasively measured velocity data is subject to errors and uncertainties that may have a random effect on the CFD pressure result. This study demonstrated that the relationship between geometry, pressure and flow can be used to evaluate new diagnostic and treatment methods. In the case of aortic stenosis, further experimental work is required to turn the concept of a pressure recovery device into a potential clinical tool. In the coronary study it was shown that, though CFD has great power as a study tool, its limitations, especially those pertaining to the volumetric flow rate boundary condition, must be further studied and become fully understood before CFD can be reliably used to aid diagnosis in clinical practice.Open Acces

    In vitro biomodels in stenotic arteries to perform blood analogues flow visualizations and measurements: a review

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    Cardiovascular diseases are one of the leading causes of death globally and the most common pathological process is atherosclerosis. Over the years, these cardiovascular complications have been extensively studied by applying in vivo, in vitro and numerical methods (in silico). In vivo studies represent more accurately the physiological conditions and provide the most realistic data. Nevertheless, these approaches are expensive, and it is complex to control several physiological variables. Hence, the continuous effort to find reliable alternative methods has been growing. In the last decades, numerical simulations have been widely used to assess the blood flow behavior in stenotic arteries and, consequently, providing insights into the cardiovascular disease condition, its progression and therapeutic optimization. However, it is necessary to ensure its accuracy and reliability by comparing the numerical simulations with clinical and experimental data. For this reason, with the progress of the in vitro flow measurement techniques and rapid prototyping, experimental investigation of hemodynamics has gained widespread attention. The present work reviews state-of-the-art in vitro macro-scale arterial stenotic biomodels for flow measurements, summarizing the different fabrication methods, blood analogues and highlighting advantages and limitations of the most used techniques.This work has been supported by FCT – Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020, UIDB/04077/2020, UIDB/00690/2020, UIDB/04436/2020 and NORTE-01-0145-FEDER-030171, NORTE-01-0145-FEDER-029394 funded by COMPETE2020, NORTE 2020, PORTUGAL 2020, Lisb@2020 and FEDER.info:eu-repo/semantics/publishedVersio

    Reproducibility of the computational fluid dynamic analysis of a cerebral aneurysm monitored over a decade

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    Computational fluid dynamics (CFD) simulations are increasingly utilised to evaluate intracranial aneurysm (IA) haemodynamics to aid in the prediction of morphological changes and rupture risk. However, these models vary and differences in published results warrant the investigation of IA-CFD reproducibility. This study aims to explore sources of intra-team variability and determine its impact on the aneurysm morphology and CFD parameters. A team of four operators were given six sets of magnetic resonance angiography data spanning a decade from one patient with a middle cerebral aneurysm. All operators were given the same protocol and software for model reconstruction and numerical analysis. The morphology and haemodynamics of the operator models were then compared. The segmentation, smoothing factor, inlet and outflow branch lengths were found to cause intra-team variability. There was 80% reproducibility in the time-averaged wall shear stress distribution among operators with the major difference attributed to the level of smoothing. Based on these findings, it was concluded that the clinical applicability of CFD simulations may be feasible if a standardised segmentation protocol is developed. Moreover, when analysing the aneurysm shape change over a decade, it was noted that the co-existence of positive and negative values of the wall shear stress divergence (WSSD) contributed to the growth of a daughter sac

    Fluid Dynamic Modeling of Biological Fluids: From the Cerebrospinal Fluid to Blood Thrombosis

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    1noL'abstract è presente nell'allegato / the abstract is in the attachmentopen718. INGEGNERIA CIVILE E AMBIENTALEnoopenCardillo, Giuli

    Estudio de la influencia de las tensiones tangenciales en el endotelio mediante un sistema microfluídico de placa de ateroma

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    Las enfermedades cardiovasculares son la principal causa de muerte en todo el mundo. Entre ellas, la aterosclerosis es la etiología más común. La aterosclerosis consiste en el estrechamiento progresivo de un vaso sanguíneo debido a la deposición de una placa de ateroma. A pesar de la elevada incidencia de esta enfermedad, su mecanismo aún no se conoce completamente.La aterosclerosis es una patología localizada; por lo tanto, los factores sistémicos en sangre no son los únicos iniciadores de esta enfermedad. Las zonas más susceptibles de desarrollar aterosclerosis comparten patrones de flujo turbulento, debido principalmente a la presencia de bifurcaciones. Por todo esto, la biomecánica y la hemodinámica de esta patología están ganando cada vez mayor atención.A nivel celular, el flujo sanguíneo influye en el comportamiento de las células endoteliales de la pared del vaso. Estas células pueden modificar su forma y orientación, provocando una alteración en la permeabilidad de la pared. Un aumento de la permeabilidad del endotelio es el primer paso para desarrollar aterosclerosis. Por lo tanto, el estudio del efecto del flujo en la forma y orientación celular es esencial.En este trabajo, se ha desarrollado un ensayo experimental para estudiar las células endoteliales coronarias humanas (HCAECs) y su respuesta al flujo sanguíneo. En particular, el flujo se ha caracterizado mediante la tensión tangencial en la pared (WSS), tensión que experimentan las células debido al paso de la sangre. Para ello, se ha diseñado y fabricado un dispositivo microfluídico, se ha establecido el protocolo experimental y se ha desarrollado un programa de análisis de imágenes para posprocesar la información obtenida en el microscopio.Hasta ahora se han llevado a cabo dos lotes de experimentos. Se ha encontrado respuesta celular en términos de forma y orientación a los diferentes valores de WSS en todos los casos estudiados.<br /
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