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Theses and Dissertations

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Browsing Theses and Dissertations by Subject "3D printing"

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    Design, fabrication, and characterization of 3D printed ceramic scaffolds for bone regeneration
    (Colorado State University. Libraries, 2024) Baumer, Vail Olin, author; Prawel, David, advisor; McGilvray, Kirk, committee member; Heyliger, Paul, committee member
    Synthetic bone tissue scaffolds are a promising alternative to current clinical techniques for treating critically large bone defects. Scaffolds provide a three-dimensional (3D) environment that mimics the properties of bone to accelerate bone regeneration. Optimal scaffolds should match the mechanical properties of the implantation site, feature a highly porous network of interconnected channels to facilitate mass transport, and exhibit surface properties for the attachment, proliferation, and differentiation of bone cell lineages. 3D printing has enabled the manufacture of complex scaffold topologies that meet these requirements in a variety of biomaterials which has led to rapidly expanding research. Structural innovations such as triply periodic minimal surfaces (TPMS) are enabling the production of scaffolds that are stiffer and stronger than traditional rectilinear topologies. TPMS are proving to be ideal candidates for bone tissue engineering (BTE) due to their relatively high mechanical energy absorption and robustness, interconnected internal porous structure, scalable unit cell topology, and smooth internal surfaces with relatively high surface area per volume. Among the material options, calcium phosphate-based ceramics, such as hydroxyapatite and tricalcium phosphate, are popular for BTE due to their high levels of bioactivity (osteoconductivity, osteoinductivity and osteointegration), compositional similarities to human bone mineral, non-immunogenicity, tunable degradation rates, and promising drug delivery capabilities. Despite the potential for TPMS ceramic scaffolds in BTE, few studies have explored beyond the popular Gyroid topology. Of the many TPMS options, the Fischer Koch S (FKS) has been simulated to be stronger, be more isotropic, have higher surface area, and absorb more energy than Gyroid at high porosities. In this report, we present a method for photocasting any TPMS in hydroxyapatite which is used to 3D print the first FKS ceramic scaffold. Results indicated that the resolution and accuracy of the process is suitable for BTE, and the custom software for producing the scaffolds was made available to the open-source community. Then, FKS and Gyroid scaffolds were designed to match the properties of trabecular bone using this method for use in critical bone defect repair. The scaffolds were printed and characterized using compressive and flow-based testing to reveal that, while both designs could mimic the low end of natural bone performance, the FKS were 32% stronger and only 11% less permeable than Gyroid. These findings emphasized the need for further characterization of these scaffolds beyond mechanical analysis and into studies of cell growth. To accomplish this, a custom multi-channel perfusion bioreactor was designed to culture cells on these scaffolds to investigate differences in cell behavior with higher efficiency than current designs. The design, capable of culturing many samples simultaneously, was validated using computational fluid dynamics and cell growth assays to demonstrate osteogenic effects and repeatability. In this work, novel TPMS scaffolds were fabricated from hydroxyapatite with sufficient accuracy and quality for large defects, testing of these scaffolds matched trabecular bone performance and suggested that FKS may be superior to Gyroid, and lastly, a four-channel bioreactor system was designed and validated to enable researchers to further characterize scaffolds for BTE.
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    ItemOpen Access
    Low work function, long lifetime filament for electron beam-based, wire-fed metal additive manufacturing
    (Colorado State University. Libraries, 2018) Nguyen, Bao Gia, author; Bradley, Thomas, advisor; Williams, John, advisor; de la Venta Granda, Jose, committee member
    Tantalum filaments are used in electron beam additive manufacturing to thermionically emit electrons that are used to build near-net shape, metal parts. High operating temperatures are required to emit electrons which consequently limits the lifetime of these filaments. This thesis presents the thermionic emission characteristics of drop-in filament replacements that incorporate barium calcium aluminate cermets. Barium calcium aluminate is a low work function material used with hollow cathodes in electric propulsion devices to provide very long service lifetimes by acting as a moderate temperature, electron source. A marriage of these two technologies may limit downtime and increase the productivity and output of electron beam additive manufacturing. Results of extended runtime tests are presented from configurations that immerse the modified filament in plasma and operate it as a vacuum emitter. The effect of contamination by air and fabrication methods are examined and evaluated based on effective work function and current density measurements. The latter includes formation methods for barium diffusion orifices as well as surface preparation methods for cermets. The experimental data collected were used to validate a predictive model that evaluates emission current densities, in both temperature and space-charge limited conditions, and effective work functions based on the fractional surface coverage of barium over a tantalum substrate.
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    ItemOpen Access
    Toolless out of build plane manufacturing of intricate continuous fiber reinforced thermoplastic composites with a 3D printing system
    (Colorado State University. Libraries, 2019) Bourgeois, Mark Elliott, author; Radford, Donald, advisor; Ma, Kaka, committee member; Maciejewski, Anthony, committee member
    Continuous fiber reinforced composite materials are manufactured using a variety of techniques ranging from manual layup to highly automated tape and fiber placement, yet all of the processes require significant tooling to act as a form which gives the composite the desired shape until processing is complete. Once processed and rigid, the composite is removed from the tooling and the tooling is, usually, then prepared and another composite component shaped on the tool. Manufacturing on such tooling has the advantage of offering a repeatable shape in a large batch production of fiber reinforced composite parts; however, the tooling itself can be a significant time to manufacture and cost challenge. It may take a large volume of composite parts to effectively amortize the cost of the tooling, which has a finite service life. Further, once the tooling is produced, making geometry changes during a production cycle is almost impossible. Geometry changes need either remanufacture of the tooling or the development of completely new tooling sets. Thus, technologies which could reduce the required tooling for composites production are highly desirable. With the advent of additive manufacture, it has become commonplace to expect the development of components of very complex geometry built from a simple surface. However, unlike continuous fiber reinforced composites, these complex geometry 3D printed components have material properties which are, for the most part, non-directional. While fiber reinforced composites are produced in a layer-by-layer additive fashion, the key to the performance of this material family is the positioning and orientation of the continuous fiber over complex contours, which has resulted in a need for substantial tooling. Thus, if concepts related to 3D printing could be mapped into the continuous fiber reinforced composite manufacturing space, the potential may exist for a radical reduction in the amount of tooling required and a corresponding increase in the flexibility of manufacture. The current research effort implements concepts common to 3D printing to investigate an approach to producing continuous fiber reinforced structures which require no tooling. Sandwich panels are commonly used as structure based on fiber reinforced composites, with the goal of high flexural stiffness and low mass. It is most common to separate two high performance composite laminates (facesheets) with a low-density core material, generally in the form of a foam of honeycomb. A recent concept has been to replace these traditional core materials with fiber reinforced truss-like structures, with the goal of further reducing mass; however, a manufacturable solution for these truss core sandwich panels has not been developed and those processes that do exist are tooling intensive. In this work, a system was developed and demonstrated that can radically reduce the amount of tooling required for truss core sandwich panels. Pyramidal truss core sandwich panels were manufactured to test the positional fidelity of out of build plane, unsupported space manufacturing. Laminates with different lamina counts were manufactured on a substrate and in unsupported space and tested for consolidation quality. Lap shear specimens were manufactured on a substrate and in unsupported space and tested for interlaminar bond quality. Individual continuous fiber reinforced composite strand specimens were manufactured in unsupported space at varying temperatures and tested for stiffness. These truss core panels, manufactured without tooling, were compared to similar truss core panels produced by more traditional techniques. The outcome of the research performed indicates that structures could be manufactured, unsupported, in free space with good precision. The void content of laminates manufactured in unsupported space decreased by 15% as the laminate was built up while the laminate manufactured on the substrate had no significant change in void content. Unidirectional laminates placed in space showed no statistical difference in strength when compared to laminates placed on a substrate. Crossply laminates had a 33% reduction in strength compared to similar laminates placed on a substrate. Composite truss core sandwich panels manufactured with the system developed in this work were more precise than composite truss core sandwich panels manufactured with compression molding and heat fusion bonding. Increasing the placement temperature of continuous fiber reinforced thermoplastic strands increases the quality of the strand by up to 44%. Improvements to the MAGIC system have increased the composite quality by 25%. Thus, manufacturing techniques were implemented to place fiber not only within the build plane, X-Y, but also to place continuous fiber out of the build plane, X Y Z. Intricate continuous fiber reinforced thermoplastic composites were manufactured without the use of tooling. While the composites produced with the new system were less stiff than composites made with compression molding further improvements to the manufacturing system have closed the stiffness gap between the two manufacturing methods.