Mechanical Engineering

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Now showing 1 - 7 of 7
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    Experimental Characterization of Vascular Tissue Viscoelasticity with Emphasis on Elastin's Role
    (2010) Shahmirzadi, Danial; Hsieh, Adam; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Elucidating how cardiovascular biomechanics is regulated during health and disease is critical for developing diagnostic and therapeutic methods. The extracellular matrix of cardiovascular tissue is composed of multiple fibrillar networks embedded in an amorphous ground substance and has been found to reveal time-dependent mechanical behavior. Given the multiscale nature of tissue biomechanics, an accurate description of cardiovascular biomechanics can be obtained only when microstructural morphology is characterized and put together in correlation with tissue-scale mechanics. This study constitutes the initial steps toward a full description of cardiovascular tissue biomechanics by examining two fundamental questions: How does the elastin microstructure change with tissue-level deformations? And how does the extracellular matrix composition affect tissue biomechanics? The outcome of this dissertation is believed to contribute to the field of cardiovascular tissue biomechanics by addressing some of the fundamental existing questions therein. Assessing alterations in microstructural morphology requires quantified measures which can be challenging given the complex, local and interconnected conformations of tissue structural components embedded in the extracellular matrix. In this study, new image-based methods for quantification of tissue microstructure were developed and examined on aortic tissue under different deformation states. Although in their infancy stages of development, the methods yielded encouraging results consistent with existing perceptions of tissue deformation. Changes in microstructure were investigated by examining histological images of deformed and undeformed tissues. The observations shed light on roles of elastin network in regulating tissue deformation. The viscoelastic behavior of specimens was studied using native, collagen-denatured, and elastin-isolated aortic tissues. The stress-relaxation responses of specimens provide insight into the significance of extracellular matrix composition on tissue biomechanics and how the tissue hydration affects the relaxation behavior. The responses were approximated by traditional spring-dashpot models and the results were interpreted in regards to microstructural composition.
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    DIRECT NUMERICAL SIMULATIONS OF TRANSITIONAL PULSATILE FLOWS
    (2008-07-11) Beratlis, Nikolaos George; Balaras, Elias; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In the present work a numerical study of transitional pulsatile flow through planar and cylindrical constrictions is presented. First, a simulation carefully coordinated with an experiment is carried out for validation purposes and results are in good agreement with the experiment. The parametric space that we adopted is similar to the one reported in a variety of past experiments relevant to the flow through stenosed arteries. In general, the flow just downstream of the constriction is dominated by the dynamics of the accelerating/decelerating jet that forms during each pulsatile cycle. It is found that the disturbance environment upstream of the stenosis has an effect on the spatial and temporal localization of the transition process in the post-stenotic area. The flow in the reattached area further downstream, is also affected by the jet dynamics. A 'synthetic', turbulent-like, wall-layer develops, and is constantly supported by streamwise vortices that originate from the spanwise instabilities of the large, coherent structures generated by the jet. The relation of these structures to the phase-averaged turbulent statistics and the turbulent kinetic energy budgets is discussed. The flow physics in the cylindrical configuration are qualitatively similar to those in the planar cases. The effect of blood rheology on the flow characteristics is also assessed by employing a biviscosity model in the simulations and it is found not to have a big effect on the turbulent intensities.
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    The Equilibrium Geometry Theory for Bone Fracture Healing
    (2008-04-29) Yew, Alvin Garwai; Hsieh, Adam H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Models describing the impact of mechanical stimuli on bone fracture healing can be used to design improved fixation devices and optimize clinical treatment. Existing models however, are limited because they fail to consider the changing fracture callus morphology and probabilistic behavior of biological systems. To resolve these issues, the Equilibrium Geometry Theory (EGT) was conceptualized and when coupled with a mechanoregulation algorithm for differentiation, it provides a way to simulate cell processes at the fracture site. A three-dimensional, anisotropic random walk model with an adaptive finite element domain was developed for studying the entire course of fracture healing based on EGT fundamentals. Although a coarse cell dispersal lattice and finite element mesh were used for analyses, the computational platform provides exceptional latitude for visualizing the growth and remodeling of tissue. Preliminary parameter and sensitivity studies show that simulations can be fine-tuned for a wide variety of clinical and research applications.
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    Design of a dielectrophoretic cell loading device
    (2007-08-09) Urdaneta, Mario Gustavo; Smela, Elisabeth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In recent years there has been an increasing interest in studying individual cells, and structures that physically entrap one or few cells have been developed for this purpose, but the approaches to load cells into these structures leave a lot to be desired. This dissertation discusses the design of a device that loads cells suspended in a solution into microvials using a combination of dielectrophoresis and fluid flow, which offers significant advantages over previous loading approaches. The basic concept is to use fluid flow and dielectrophoretic forces to position a given cell above a given vial, within an array of similar vials, and then bringing the cell into the vial. The loading of several cells flowing in a channel into a vial in a matter of seconds is demonstrated. The design of the loading device spurred the development of novel topics in the area of dielectrophoresis. The structures into which cells are loaded produce "parasitic cages". The effect of multiple electric fields and at multiple frequencies had to be explored to eliminate the parasitic cages, and new theory was developed to describe the phenomenon in a straight forward and convenient way. The design process of dielectrophoretic structures known as flow through sorters was simplified significantly using a method that relies on non dimensional analysis and a figure of merit. These topics investigated have broader applications than just loading cells into vials. The dissertation demonstrates technologies and design and fabrication methods key to the cell loading design. The dissertation ends by describing the design of a device that can be implemented to load cells into vials on integrated circuit chips and outlining this device's expected characteristics and performance based on the theory and methods presented through the dissertation.
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    Biomechanics of the Intervertebral Disc: The Effects of Load History on Mechanical Behavior
    (2007-06-20) Gabai, Adam Shabtai; Hsieh, Adam; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Degenerative disc disease is associated with back pain, and can be a debilitating disorder. In addition to the biological contributions of genetics and aging, mechanical factors have been implicated in accelerating the progression of disc degeneration. Two studies were performed in order to explore the effects of various loading conditions on disc biomechanics. The first study explores the effects of compressive historical loads and disc hydration on subsequent creep loading and recovery. The second study investigates the restorative powers of creep distraction between compressive loading periods. In both cases three commonly applied mathematical models were employed to characterize disc behavior and the effectiveness of each model was validated. The studies confirm that hydration level has a significant impact on disc stiffness and time dependent behavior. Distraction and conditioning phases are shown to have a significant impact on hydration level and thus subsequent mechanical behavior.
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    THERMAL CYCLING DESIGN ALTERNATIVES FOR THE POLYMERASE CHAIN REACTION
    (2005-09-27) Lewis, Monte Allen; Herold, Keith E; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    PCR (polymerase chain reaction) is a process by which a small amount of DNA is amplified many times to yield an easily detectable amount. This process is widely used for the detection of bacterial pathogens for biodefense, in basic research, criminal identification, and disease detection in humans. The reaction mixture must be cycled repeatedly between three different temperature levels in PCR. The reaction mixture is first heated at 94 °C, cooled to 54 °C, and then heated to 72 °C. This cycle is repeated 20-40 times. The main objective of the work reported here is to evaluate alternative heating/cooling schemes for PCR with the ultimate goal of speeding up a PCR reaction. A secondary goal is to arrive at a design that is consistent with battery operation to allow for a portable PCR device. Insight is gained about interactions between the PCR reaction and the engineering system.
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    A NOVEL AIRFLOW CONTROL VALVE FOR USE IN MEDICAL APPLICATIONS
    (2004-05-26) Hitchcock, Kathryn Elizabeth; Hristu-Varsakelis, Dimitrios; Mechanical Engineering
    Rapid improvements in digital technology over the last two decades have led to artificial ventilators that drastically improve physicians' ability to measure and control aspects of their patients' breathing. However, the mechanical systems paired with the new digital controllers have not advanced in parallel with them. As a result, mechanical ventilators do not respond sufficiently fast to changes in operating conditions and can injure patients by allowing the air volume or pressure in their lungs to become too high. This thesis describes a new air flow control valve that can be incorporated in existing ventilators to correct this condition. The valve's low mass and short stroke result in rapid full-range motion with low actuator force and travel. These qualities also make the valve well-suited for use as a flow-change mechanism in instruments that measure airway resistance, including the Airflow Perturbation Device (APD). We describe a series of experiments that verify the valve's performance in both ventilator and APD applications.