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.

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

This investigation concerns the design, build, and testing of a new class of skin friction sensor capable of performing favorably in high-speed, high-enthalpy flow conditions, such as that found in atmospheric re-entry vehicles, scramjets, jet engines, material testing, and industrial processes. Fully understanding and optimizing these complex flows requires an understanding of aerodynamic properties at high enthalpies, which, in turn, requires numerical and analytical modeling as well as reliable diagnostic instrumentation. Skin friction is a key quantity in assessing the overall flight and engine performance, and also plays an important role in identifying and correcting problem areas. The sensor design is founded on a direct-measuring, cantilever arrangement. The design incorporates two fundamental types of materials in regards to thermal conductivity and voltage resistivity properties. The non-conducting material distinction greatly deters the effect of heat soak and prevents EMI transmission throughout the sensor. Four custom fabricated metal-foil strain gauges are arranged in a Wheatstone bridge configuration to increase sensitivity and to provide further compensation for sensitivity effects. The sensor is actively cooled via a copper water channel to minimize the temperature gradient across the electronic systems. The design offers a unique immunity to many of the interfering influences found in complex, high-speed, high-enthalpy flows that would otherwise overshadow the desired wall shear measurement. The need to develop an encompassing design methodology was recognized and became a principal focus of this research effort. The sensor design was developed through a refined, multi-disciplinary approach. Concepts were matured through an extensive and iterative program of evolving key performance parameters. Extensive use of finite element analysis (FEA) was critical to the design and analysis of the sensor. A software package was developed to utilize the powerful advantage of FEA methods and optimization techniques over the traditional trial and error methods. Each sensor endured a thorough series of calibrations designed to systematically evaluate individual aspects of its functionality in static, dynamic, pressure, and thermal responses. Bench-test facilities at Virginia Tech (VT) and Air Force Research Laboratory (AFRL) further characterized the design vibrational effects and electromagnetic interference countermeasure effectiveness. Through iterations of past designs, sources of error have been identified, controlled, and minimized. The total uncertainty of the skin friction sensor measurement capability was determined to be ±8.7% at 95% confidence and remained fairly independent of each test facility. A rigorous, multi-step approach was developed to systematically test the skin friction sensor in various facilities, where flow enthalpy and run duration were progressively increased. Initial validation testing was conducted at the VT Hypersonic Tunnel. Testing at AFRL was first performed in the RC-19 facility under high-temperature, mixing flow conditions. Final testing was conducted under simulated scramjet flight conditions in the AFRL RC-18 facility. Performance of the skin friction sensors was thoroughly analyzed across all three facilities. The flow stagnation enthalpies upward of 1053 kJ/kg (453 Btu/lbm) were tested. A nominal Mach 2.0 to 3.0 flow speed range was studied and stagnation pressure ranged from 172 to 995 kPa (25 to 144 psia). Wall shear was measured between 94 and 750 Pa (1.96 and 15.7 psf). Multiple entries were conducted at each condition with good repeatability at ±5% variation. The sensor was also able to clearly indicate the transient flow conditions of a full scramjet combustion operability cycle to include shock train movement and backflow along the isolator wall. The measured experimental wall shear data demonstrated good agreement with simple, flat-plate analytical estimations and historic data (where available). Numerical CFD predictions of the scramjet flow path gave favorable results for steady cold and hot flow conditions, but had to be refined to handle the various fueling injection schemes with burning in the downstream combustor and surface roughness models. In comparing CFD wall shear predictions to the experimental measurements, in a few cases, the sensor measurement was adversely affected by shock and complex flow interaction. This made comparisons difficult for these cases. The sensor maintained full functionality under sustained high-enthalpy conditions. No degradation in performance was noted over the course of the tests. This dissertation research and development program has proven successful in advancing the development of a skin friction sensor for applications in high-speed, high-enthalpy flows. The sensor was systematically tested in relevant, high-fidelity laboratory environments to demonstrate its technology readiness and to successfully achieve a technology readiness level (TRL) 6 milestone. The instrumentation technology is currently being transitioned from laboratory development to the end users in the hypersonic test community. / Ph. D.

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

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Atherosclerosis and related cardiovascular diseases (CVDs) are amongst the largest causes of morbidity and mortality in the developed world, causing considerable monetary pressure on public health systems worldwide. Atherosclerosis is characterised by the build up of vascular plaque in medium and large arteries and is a direct precursor to acute vascular syndromes such a myocardial infarction, stroke or peripheral arterial diseases. The causative factors leading to CVD still remain relatively poorly understood, but are becoming increasingly identifiable as a dysfunction of the endothelial cells that line the arterial wall. It is well known that the endothelium responds to the prevailing fluid mechanic (i.e. haemodynamic) environment, which plays a crucial role in the localised occurrence of atherosclerosis near vessel bends and bifurcations. In these areas, disturbed haemodynamics lead to flow separation and very low wall shear stress (WSS), which directly affects the functionality of the endothelium and impedes the transport of important blood borne agonists and antagonists. Detailed full field measurements assessing complex haemodynamics are sparse and consequently this thesis aims to address some of the important questions related to arterial haemodynamics and CVD by performing in-vitro flow measurements in physiologically relevant conditions. In particular, this research develops and uses state-of-the-art Particle Image Velocimetry (PIV) techniques to measure three-dimensional velocity and WSS fields in scaled models of the human carotid artery. For this purpose, the necessary theoretical and experimental concepts are developed and in-depth analyses of the underlying factors affecting the local haemodynamics and their relation to CVD are carried out. In the first part, a methodology for the construct of transparent hydraulic flow phantoms from medical imaging data is developed. The arterial geometries are reproduced in optically clear silicone and the flowing blood is modelled with a refractive index matched blood analogue. Subsequently, planar and Stereo-PIV techniques are developed and verified. A novel interfacial PIV (iPIV) technique is introduced to directly measure WSS by inferring the velocity gradient from the recorded particle images. The new technique offers a maximal achievable resolution of 1 pixel and therefore removes the resolution limit near the wall usually associated with PIV. Furthermore, the iPIV performance is assessed on a number of numerical and experimental test cases and iPIV offers a significantly improved measurement accuracy compared to more traditional techniques. Subsequently, the developed methodologies are applied in three studies to characterise the velocity and WSS fields in the human carotid artery under a number of physiological and experimental conditions. The first study focuses on idealised vessel geometries with and without disease and establishes a general understanding of the haemodynamic environment. Secondly, a physiological accurate vessel geometry under pulsatile flow conditions is investigated to provide a more realistic representation of the true in-vivo flow conditions. The prevailing flow structure in both cases is characterised by flow separation, strong secondary flows and large spatial and temporal variations in WSS. Large spatial and temporal differences exist between the different geometries and flow conditions; spatial variations appear to be more significant than transient events. Thirdly, the three-dimensional flow structure in the physiological carotid artery model is investigated by means of stereoscopic and tomographic PIV, permitting for the first time the measurement of the full 3D-3C velocity field and shear stress tensor in such geometries. The flow field within the model is complex and three-dimensional and inherently determined by the vessel geometry and the build up of an adverse pressure gradient. The main features include strong heliocoidal flow motions and large spatial variations in WSS. Lastly, the physiological implications of the current results are discussed in detail and reference to previous work is given. In summary, the present research develops a novel and versatile PIV methodology for haemodynamic in vitro studies and the functionality and accuracy is demonstrated through a number of physiological relevant flow measurements.