The fluid mechanics of convection and diffusion in medical devices for visualization in arteries and clearing of urea in hemodialysis. Analysis, design, and optimization via CFD and experimental analysis

Abstract

This thesis explores the understanding of convection and diffusion transport mechanisms in medical devices. The specific goals for this thesis is understanding the fluid mechanics of two different biomedical systems – visualization in the cardiovascular system and urea removal in hemodialysis. This study investigates the fluid dynamics of saline flushing in endovascular catheters used in the treatment and diagnosis of atherosclerotic plaque in arteries. This section of the thesis focuses on the convection-dominated physics, which are observed during saline flushing. The process involves saline injection, mixing with blood, and advection of the mixture away from the region of interest to provide a clear optical path for imaging. The objective of the study is to achieve clearance in the artery for optical imaging without the use of a balloon occlusion to obstruct antegrade flow into the imaging region of the artery. Computational fluid simulations (CFD) simulations are used as a rapid turn-around tool for the understanding of the influence of different parameters on the flow process that control the transport and mixing of saline and blood, leading to an evolutionary design process of an endovascular catheter that combines imaging forward-viewing element with saline flushing lumens. A novel design and control technique is developed that provides the method to regulate the pressure in a blocked artery during saline flushing, so only small deviations from physiological pressure are exerted on the damaged artery wall, minimizing risk of rupture. In vitro experiments carried out saline flushing to achieve optical clearing in phantoms simulating chronic total occlusions in coronary arteries, as well as in partially occluded coronary arteries with an opaque blood surrogate. Different plaque-catheter configurations, including plaque size, distance from the catheter, injection flow rates and orientation of the catheter with respect to the plaque and occlusions near bifurcations, are studied via CFD. An additional optical ray analysis provides confirmation of the clearing of the artery occluded by plaque. A new catheter design is demonstrated to produce optical clearing via saline flushing, leading safely and effectively to a clear optical imaging field, and an improved technique is outlined that overcomes some practical challenges and limitations commonly encountered in angioscopy.The second part of this thesis investigates the fluid physics of urea transport in photo-catalytic urea oxidation channels, for portable hemodialysis in kidney disease patients. This section of the thesis focuses on inducing mixing at low Reynolds numbers to overcome the diffusion-limited urea transport from the bulk dialysate flow in the channels to the catalyst surface. The study is inspired by recent advancements in photo-electrochemical oxidation of urea by TiO2. A numerical CFD model is used to analyze the impact of the design and operational parameters on the performance of the dialysis system. This involved a grid independence study to resolve the high Peclet number transport of urea and different models to represent the oxidation of urea and its effect of urea transport to the catalyst surface. Innovative channel designs (thin channel, wavy channel and chevron channel) were studied for performance, and their metrics compared to improve dialysis efficiency, contributing to the development of novel wearable dialysis devices.