Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item TISSUE ADHESIVE, SPRAYABLE POLYMER BLENDS AS ADJUVANT SURGICAL TOOLS(2022) Erdi, Metecan; Kofinas, Peter; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Commercial materials deployed in surgery for treatment of high-impact clinical pathologies suffer from shortcomings stemming from a combination of poor mechanical properties, difficulty in precise application, and non-specific prevention mechanisms. Work in this dissertation seeks to counteract these concerns through a multitude of blending approaches with biodegradable polymers and therapeutic agents for improved outcomes following traumatic tissue injury. The polymer blends were spray deposited using solution blow spinning, a method of fiber production where material rapidly accumulates onto target tissue substrate and forms a stable interface. The first thrust of this dissertation hones on deposition of a biocompatible, wet tissue adhesive. These tissue adhesives were fabricated through molecular weight ratio blends of poly(lactide-co-caprolactone) (PLCL), a synthetic, biodegradable copolymer with viscoelastic properties fostering pressure-dependent adhesion. High molecular weight PLCL endowed the composite material with rigidity and inherent cohesive strength, while low molecular weight PLCL induced spreadability and adhesive strength. Such optimized material behavior presented an ability to not only adhere to hydrophilic surfaces, but also demonstrated an ability to act as a media for biocompatible and complete wound healing. Efficacy as an adhesive in wound dressings was exhibited through spray deposition of blend adhesives to bandage substrates in a porcine partial thickness burn wound model and comparison with a poly(urethane)-based clinical control material. The second thrust of this dissertation focuses on development of an effectively applied barrier material for prevention of post-operative fibrotic scar tissue termed as adhesions. Rapid generation of tissue-conformal polymer fibers through solution blow spinning yields a material that is inherently flexible, thereby counteracting the brittle architecture of a sheet-like film currently deployed in surgery. Prevention of asymmetric fibrosis was accomplished through tuned surface biodegradation via high and low molecular weight PLCL blends. This strategy seeks to physically prevent prolonged retention of adhesion-generating molecules at the site of injury, as well as biologically counteract underlying inflammatory processes through controlled release of a therapeutic, apolipoprotein mimetic peptide from composite PLCL fiber mat. Adhesion prevention efficacy was qualified in high impact pre-clinical mouse models of cecal ligation and cecal anastomosis, and compared to pre-fabricated, dried hydrogel barrier and aqueous therapeutic suspension controls. Both adhesion severity and resultant wound healing response were significantly improved versus no treatment and clinically adopted controls.Item Adhesion Strength Measurement of Multilayer Structures with Vertical Crack by Four Point Bending Test(2014) Kang, Stephen Junho; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Current microelectronic packages consist of multilayer systems. Adhesion strength is one of the most important factors to the reliability of these systems. Previous studies have used four point bending tests as a method for characterizing the energy release rate to obtain the adhesion strength of bilayer systems. An extension of this work is proposed in this study, where a four point bending test of multilayer structures with a vertical crack is used to measure the adhesion strength, assisted by the presence of a predefined area. The predefined area allows for a weak adhesion horizontal accurate pre-crack which permits crack propagation under loading as well as reducing scatter within the values of critical loads. A numerical analysis is conducted to compute the energy release rate from the critical loads using the concept of the J-integral. Two sets of multilayer specimens were fabricated and tested in the study: one for investigating crack front behavior relative to the compliance change in the load-displacement profile by using transparent substrates, and the other using the previous set as a guideline for testing metal substrates under certain environmental conditions. Experimental results along with visual evidence support the consistent behavior between crack front behavior and compliance change. This correlation can be used as a baseline for testing other electronic packages for interfacial failure.Item Individual and collective dynamics of chemotaxing cells(2011) McCann, Colin Patrick; Losert, Wolfgang; Parent, Carole A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The study of the dynamics of interacting self-propelled entities is a growing area of physics research. This dissertation investigates individual and collective motion of the eukaryote Dictyostelium discoideum, a system amenable to signal manipulation, mathematical modeling, and quantitative analysis. In the wild, Dictyostelium survive adverse conditions through collective behaviors caused by secreting and responding to chemical signals. We explore this collective behavior on size scales ranging from subcellular biochemistry up to dynamics of thousands of communicating cells. To study how individual cells respond to multiple signals, we perform stability analysis on a previously-developed computational model of signal sensing. Polarized cells are linearly stable to perturbations, with a least stable region at about 60 degrees off the polarization axis. This finding is confirmed through simulations of the model response to additional chemical signals. The off-axis sensitivity suggests a mechanism for previously observed zig-zag motion of real cells randomly migrating or chemotaxing in a linear gradient. Moving up in scale, we experimentally investigate the rules of cell motion and interaction in the context of thousands of cells. Migrating Dictyostelium discoideum cells communicate by sensing and secreting directional signals, and we find that this process leads to an initial signal having an increased spatial range of an order of magnitude. While this process steers cells, measurements indicate that intrinsic cell motility remains unaffected. Additionally, migration of individual cells is unaffected by changing cell-surface adhesion energy by nine orders of magnitude, showing that individual motility is a robust process. In contrast, we find that collective dynamics depend on cell-surface adhesion, with greater adhesion causing cells to form smaller collective structures. Overall, this work suggests that the underlying migration ability of individual Dictyostelium cells operates largely independent of environmental conditions. Our gradient-sensing model shows that polarized cells are stable to small perturbations, and our experiments demonstrate that the motility apparatus is robust to considerable changes in cell-surface adhesion or complex signaling fields. However, we find that environmental factors can dramatically affect the collective behavior of cells, emphasizing that the laws governing cell-cell interaction can change migration patterns without altering intrinsic cell motility.