Key Words: Thrombosis, Vascular Graft, Microfluidics, Wall Shear Stress / Vascular grafts are essential for the management of cardiovascular disease. However, the lifesaving potential of these devices is undermined by thrombosis arising from material and flow interactions on the blood contacting surface. To combat this issue, the use of antithrombogenic coatings has emerged as a promising strategy for modulating blood and graft interaction in vivo. Although an important determinant of graft performance, hemodynamics are frequently overlooked in the in vitro testing of coatings and their translatability remains poorly understood. We address this limitation with a microscale platform that incorporates vascular prosthesis and coatings with tuneable flow and surface conditions in vitro. As a proof of concept, we use the platform to test the thrombogenic performance of a novel class of lubricant infused (LIS) and antibody lubricant infused (anti-CD34 LIS) coated ePTFE vascular grafts in the presence of arterial wall shear stress, with and without the presence of endothelial cells. Our findings suggest lubricant infused coated ePTFE vascular grafts are thromboresistant under flow and may have potential for in vivo arterial grafting applications. It is moreover apparent that the microscale properties of the device could be advantageous for the testing and translation of novel antithrombogenic coatings or blood contacting prosthesis in general. / Thesis / Master of Applied Science (MASc)

An in vitro arterial flow bioreactor system for the generation of physiological flows in a biological environment was designed, constructed, and characterized. The design was based on models previously used to investigate the response of endothelial cells to shear. The model interfaces a bioreactor with flow elements to compose a flow loop that reproduces arterial flow conditions within the bioreactor. High-resolution (8.6 microns) time-resolved (4 ms) velocity field measurements within the bioreactor were obtained using Particle Image Velocimetry (PIV). Two physiological flows were considered, corresponding to medium human arteries at rest and exercise conditions: first, with an average Reynolds number of 150 and a Womersley parameter of 6.4, and second, with an average Reynolds number of 300 and a Womersley parameter of 9.0. Two cases were considered: first, using a smooth artery section, and second, with a confluent layer of human microvascular endothelial cells grown on the inner surface of the artery section. The instantaneous wall shear stress, time-averaged wall shear stress, and oscillatory shear index were computed from the velocity field measurements and compared for the cases with and without cells. These measurements were used to assess the value of the system for measurement of correlations between fluid dynamics and the response of biological tissue. It was determined that the flow present in such a system is not an accurate reproduction of physiological flow, and that direct measurement of the flow is necessary for accurate quantification of cellular response to fluid parameters. / Master of Science

There is increasing interest in assessing the role of hemodynamics in aneurysm growth and rupture mechanism. Identification of the indicators of rupture risk can prove very valuable in the clinical management of patients. If rupture risk of aneurysms can be predicted, immediate preemptive treatments can be done for the high risk patients whereas others can avoid the risky intervention. Retrospective studies have been performed in the past to filter out indices that differentiate ruptured aneurysms from unruptured aneurysms. However, these differences may not necessarily translate to differences between aneurysms that present unruptured but fork towards growth/rupture and unruptured aneurysms that are invariably stable. The hypothesis of the present study is that hemodynamic indices of unruptured aneurysms when they first presented can be used to predict their longitudinal outcome. A prospective longitudinal cohort study was designed to test this hypothesis. Four clinical centers participated in this study and a total of 198 aneurysms were recruited. These aneurysms were chosen by the physicians to be kept under watchful waiting. Three-dimensional models of aneurysms and their contiguous vasculature generated using the initial scans of patients were used for computational fluid dynamic (CFD) simulations. Both pulsatile and steady flow analyses were performed for each patient. By collating all the prominent hemodynamic indices available in aneurysm literature and developing a few new indices, 25 hemodynamic indices were estimated for each subject. For statistical analysis, it was hypothesized a priori that low wall shear area is different between stable and unstable aneurysms. All other indices were tested in a post-hoc manner. The longitudinal outcome information of these patients was recorded at the clinical centers and the author was blinded until all analyses were complete. Aneurysms that grew during the follow up period were labeled as "grown" and otherwise they were called "stable" by the radiologists. After the hemodynamic analysis was complete, a non-parametric Mann Whitney U test was performed to determine if any index can statistically differentiate the two groups ("grown" versus "stable"). It was found that none of the indices distinguished the two groups with statistical significance. Comparison of the steady and pulsatile flow analysis suggested that the patient population is stratified in the same order by an index, irrespective of whether the index is computed using a steady or pulsatile flow simulation. Pearson correlation coefficient was obtained between basic geometric indices and hemodynamic indices of this population. No strong correlation was found in between morphology and hemodynamics, suggesting uniqueness of the hemodynamic indices. The hypothesis motivating the present study is that aneurysm blood flow based indices can be used as prognostic indicators of growth and/or rupture risk. This study is the first to analyze intracranial aneurysm hemodynamics of a large cohort in a longitudinal prospective manner. Results of the present study indicate that quantitative hemodynamics cannot be used to predict the longitudinal outcome of an aneurysm. Further studies are needed to gain additional clinical insights.

Collecting microlymphatics play a vital role in promoting lymph flow from the initial lymphatics in the interstitial spaces to the large transport lymph ducts. In most tissues, the primary mechanism for producing this flow is the spontaneous contractions of the lymphatic wall. Individual units, known as lymphangion, are separated by valves that help prevent backflow when the vessel contracts, thus promoting flow through the lymphatic network. Lymphatic contractile activity is inhibited by flow in isolated lymphatics, however there are virtually no in situ measurements of lymph flow in these vessels. Initially, a high speed imaging system was set up to image in situ preparations at 500 fps. These images were then manually processed to extract information regarding lymphocyte velocity (-4 to 10 mm/sec), vessel diameter (25 to 165 um), and particle location. Fluid modeling was performed to obtain reasonable estimates of wall shear stress (-8 to 17 dynes/cm2). One of the difficulties encountered was the time consuming methods of manual particle tracking. Using previously captured images, an image correlation method was developed to automate lymphatic flow measurements and to track wall movements as the vessel contracts. Using this method the standard error of prediction for velocity measurements was 0.4 mm/sec and for diameter measurements it was 7.0 µm. It was found that the actual physical quantity being measured through this approach is somewhere between the spatially averaged velocity and the maximum velocity of a Poiseuille flow model.

Collecting microlymphatics play a vital role in promoting lymph flow from the initial lymphatics in the interstitial spaces to the large transport lymph ducts. In most tissues, the primary mechanism for producing this flow is the spontaneous contractions of the lymphatic wall. Individual units, known as lymphangion, are separated by valves that help prevent backflow when the vessel contracts, thus promoting flow through the lymphatic network. Lymphatic contractile activity is inhibited by flow in isolated lymphatics, however there are virtually no in situ measurements of lymph flow in these vessels. Initially, a high speed imaging system was set up to image in situ preparations at 500 fps. These images were then manually processed to extract information regarding lymphocyte velocity (-4 to 10 mm/sec), vessel diameter (25 to 165 um), and particle location. Fluid modeling was performed to obtain reasonable estimates of wall shear stress (-8 to 17 dynes/cm2). One of the difficulties encountered was the time consuming methods of manual particle tracking. Using previously captured images, an image correlation method was developed to automate lymphatic flow measurements and to track wall movements as the vessel contracts. Using this method the standard error of prediction for velocity measurements was 0.4 mm/sec and for diameter measurements it was 7.0 µm. It was found that the actual physical quantity being measured through this approach is somewhere between the spatially averaged velocity and the maximum velocity of a Poiseuille flow model.

Our study was designed to test the hypothesis that flowfield properties such as WSS are closely related to cardiovascular disease. The spatial distribution patterns of several hemodynamic indices (gradient of WSS) were examined and compared with the (known) locations of plaque formation in human aorta. The part of the aorta on which we focused is ascending, aortic arch and descending aorta. Blood flow is influenced by vessel wall motion. Fluid Structure Interaction (FSI) is also investigated and discussed during the description of hemodynamic environment that leads to plaque formation in human aorta. Our Data were DICOM files from Computed Tomography (CT) scans. Using Vascular Modeling Toolkit (VMTK) and these scans as the input, we choose level set segmentation method to extract the geometry of the vessel needed for the simulation. ANSYS CFX Solver was used for the simulation of blood flow. The present numerical study revealed a direct correlation between low WSS values and atherosclerotic plaque localization. The results indicate also that Oscillating Shear Index (OSI) shows clearly points where the possibility of atherogenesis is high enough to be ignored. FSI provides unimportant details when we focused on plaque formation. / Η παρούσα εργασία μελετά την υπόθεση που συνδέει τις ιδιότητες του πεδίου ροής, όπως οι διατμητικές τάσεις (Wall Shear Stresses), με καρδιαγγειακές παθήσεις. Η χωρική κατανομή διάφορων δεικτών αιμοδυναμικής φύσεως (όπως η βάθμωση των διατμητικών τάσεων) μελετήθηκε και τα σημεία που εντοπίστηκαν ως ύποπτα για την ανάπτυξη αθηρωματικών πλακών συγκρίθηκαν με γνωστές από τη βιβλιογραφία περιοχές σχηματισμού τέτοιων φλεγμονών στην ανθρώπινη αορτή. Το τμήμα της αορτής στο οποίο εστιάσαμε είναι η ανιούσα, το αορτικό τόξο και η κατιούσα αορτή. Εξετάστηκε απίσης το ενδεχόμενο να επηρεάζεται η ροή του αίματος από την κίνηση του αρτηριακού τοιχώματος.

This thesis presents a computational analysis of multi-element guard-heated sensors designed to overcome the most severe limitation of conventional thermal sensors for wall shear stress (WSS) measurement in turbulent flows –that of indirect heat conduction through the substrate. The objectives of this thesis are the study of guard-heated sensors {i} to quantify the reduction, over conventional single-element sensors, of substrate heat conduction losses and resultant errors over a range of applied shear and {ii} to examine a range of values of guard heater geometric parameters, in two common fluids, air and water and identify the best designs. Wall-turbulence, the turbulent flow in the vicinity of solid boundaries, has proved difficult to model accurately, due to the lack of accurate WSS measurements. Examples of areas of impact are drag force reduction on transport vehicles in land, sea, air, which today largely translate to reduced fossil fuel use and dependence; aerodynamic noise and control for flight and for wind energy conversion; atmospheric and oceanic transport studies for weather, climate and for pollutant transport; riverbank erosion. Constant-temperature anemometry with MEMS devices, flush-mounted hot-film thermal sensors, is non-intrusive, affords the best temporal resolution and is well-established. However, these hot-film probes suffer from unwanted heat transport to the fluid through the substrate, with errors and nonlinearity large enough to overwhelm quantitative utility of the data. Microfabrication techniques have enabled multi-element guard-heated prototypes to be fabricated. Our results show that errors in sensing-element signals, contributing to spectral distortion, are sensitive to sensor location within the guard heater. These errors can be reduced to below 1% of the signal with proper location of the sensor. Guard heating also reduces the large variation in spatial averaging due to substrate conduction. This makes them suitable for turbulent flows with a large range of fluctuations. / Graduate

京都大学 / 新制・課程博士 / 博士(医学) / 甲第23573号 / 医博第4787号 / 新制||医||1054(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 中本 裕士, 教授 花川 隆, 教授 湊谷 謙司 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

Cardiovascular diseases (CVD) are the number one causes of death in the world. According to the world Health Organization (WHO) 17.5 million people died from cardiovascular disease in 2005, representing 30 % of all global deaths . Of these deaths, 7.6 million were due to heart attacks and 5.7 million due to stroke. If current trends are allowed to continue, by 2015 an estimated 20 million people will die annually from cardiovascular disease. The trends are similar in the United States where on an average 1 person dies every 37 seconds due to CVD. In 2008 an estimated 770,000 Americans will experience a new heart attack (coronary stenosis) and 600,000 will experience a first stroke. Although the exact causes of cardiovascular disease are not well understood, hemodynamics has been long thought to play a primary role in the progression of cardiovascular disease and stroke. There is strong evidence linking the fluid mechanical forces to the transduction mechanisms that trigger biochemical response leading to atherosclerosis or plaque formation. It is hypothesized that the emergence of abnormal fluid mechanical stresses which dictate the cell mechanotransduction mechanisms and lead to disease progression is dependent on the geometry and compliance of arteries, and pulsatility of blood flow. Understanding of such hemodynamic regulation in relation to atherosclerosis is of significant clinical importance in the prediction and progression of heart disease as well as design of prosthetic devices such as stents. The current work will systematically study the effects of compliance and complex geometry and the resulting fluid mechanical forces. The objective of this work is to understand the relationship of fluid mechanics and disease conditions using both experimental and computational methods where (a) Compliance effects are studied in idealized stenosed coronary and peripheral arteries using Digital Particle Image Velocimetry (DPIV), (b) Complex geometric effects of stented arteries with emphasis on its design parameters is investigated using CFD, Also (c) a novel method to improve the accuracy of velocity gradient estimation in the presence of noisy flow fields such as in DPIV where noise is inherently present is introduced with the objective to improve accuracy in the estimation of WSS, which are of paramount hemodynamic importance. The broad impact of the current work extends to the understanding of fundamental physics associated with arterial disease progression which can lead to better design of prosthetic devices, and also to better disease diagnostics. / Ph. D.

<p> This study investigates the flow patterns and wall shear rate distributions downstream from a series of three glass model symmetric bifurcations, typical of the blood vessels in man. The models have a single included angle of 75° and total output to input flow area ratios of 0.75, 1.02 and 1.29, covering the physiological range. The Reynolds numbers studied (based on parent tube) were 400, 800 and 1200 in steady flow.</p> <p> Local fluid velocities were obtained at a number of axial positions along the bifurcation daughter tube via a neutrally buoyant tracer particle technique utilizing cine photography. This provided sufficient information to determine the three velocity components for each particle. The tangential and radial components were in general less than 6% of the mean axial velocity. In the case of the axial components, an analytical representation of the velocity in polar coordinates was obtained. This analytical function permits evaluation of wall shear rate distribution.</p> <p> The velocity pro£iles were found to be symmetric with respect to the plane of the bifurcation. At two diameters downstream from the carina the velocity profiles in the plane of the bifurcation showed a high peak near the inside wall of the branch. With distance downstream the peak was convected tangentially evening out the profile towards an axially symmetric mountain plateau with a dished top.</p> <p> Wall shear rate as a function of θ at constant axial position was represented by displaced cosine function. The highest shear rates always occurred on the inside wall of the daughter tube and the lowest on the outside wall. In general, the largest deviation from developed shear rates occurred close to the carina.</p> <p> The largest positive deviation in wall shear rate from developed values was found in the small area ratio bifurcation and the lowest wall shear rate value was found in the large area ratio bifurcation (a = 1.29) indicating possible flow separation near the carina. The biological implications of the shear rate information generated are discussed.</p> / Thesis / Master of Engineering (MEngr)