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Browsing Theses and Dissertations by Subject "additive manufacturing"

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    Hydroxyapatite structures created by additive manufacturing with extruded photopolymer
    (Colorado State University. Libraries, 2019) López Ambrosio, Katherine Vanesa, author; James, Susan P., advisor; Ma, Kaka, committee member; Prawel, David A., committee member
    Bone tissue has the ability to regenerate and heal itself after fracture trauma. However, this ability can be affected by different risk factors that are related to the patient and the nature of the fracture. Some of the factors are age, gender, diet, health, and habits. Critical-sized defects are particularly difficult, if not impossible, to heal correctly. Particularly in large defects, bone regeneration ability is impeded, disrupting normal healing processes, resulting in defective healing, integration, and non-union. To prevent and treat defective healing or non-union, surgical intervention is needed. Surgeons implant various forms of devices between the ends of the broken bone, usually with external fixation. Implants function by guiding and enabling new bone ingrowth while giving support to the healing tissue. Some of the most common implants are autografts, allografts, and metallic endoprostheses. Unfortunately, these common techniques have drawbacks such as the risk of infection and relatively poor biological or mechanical compatibility with host tissue, in addition to the limited source of donor tissue and high cost, often resulting from secondary surgical interventions. Critical defects are particularly problematic. Hence, there is a necessity for bone implant substitutes that diminish the risk of infection and incompatibility while also providing similar mechanical properties to real bone tissue. Hydroxyapatite (HAp) is a ceramic with a chemical composition similar to bone tissue that has shown biocompatibility and osteoconductive properties with host bone tissue, but it is difficult to manufacture into complex structures with mechanical properties comparable to bone tissue. Therefore, significant efforts are directed to produce materials and methods that could produce HAp synthetic implants to treat bone defects. This research aimed to create and characterize a hydroxyapatite photo-polymeric resin suitable for 3D printing, which could produce dense HAp ceramic parts in complex shapes without requiring support material. We created a HAp-based photopolymer slurry that achieved 41 vol% HAp loading in homogenous slurries. The HAp slurries presented a strong shear thinning behavior and dispersion stability over 20 days under dark storage conditions. The resultant rheological behavior of HAp slurries enabled 3D printing of HAp green bodies in complex shapes using a combination of viscous extrusion and layer-wise photo-curing processes. Complex structures with concave and convex forms and scaffolds with interconnected pores ranging from 130 µm to 600 µm pore sizes and 10% to 40% porosity were successfully built with high resolution and no support material. Moreover, HAp/PEGDMA green bodies presented complete layer cohesion. After 3D printing, sintering was used to densify HAp structures and eliminate the polymer matrix. The resultant HAp structures maintained their complex details, had a relative density of ~78% compared to fully dense HAp and a dimensional shrinkage of ~15% compared to its green body. Sintered HAp structures were found to be non-cytotoxic for ADSCs cells. Flexural properties of HAp green and sintered structures were also determined. It was found that green bodies had a flexural strength of ~30.42MPa comparable to trabecular bone. To summarize, a photopolymerizable resin with 41 vol% of HAp was created to produce ~78% dense HAp complex structures. This was achieved by using additive manufacturing that combined viscous extrusion and layer-wise photo-curing and a sintering process. HAp/PEGDMA showed flexural strength comparable to the trabecular bone, and HAp sintered structures demonstrated non-cytotoxic behavior.
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    ItemOpen Access
    Structural optimization of 3D printed hdyroxyapatite scaffolds
    (Colorado State University. Libraries, 2021) Isaacson, Nelson D., author; Prawel, David, advisor; James, Susan, committee member; Séguin, Bernard, committee member
    Poor healing of critically sized bone defects affects 1.5 million Americans per year and results in more than $1 billion in treatment and therapy cost. Treatment options remain limited and often lead to reoperations, clinical complications, poor functional outcomes, and limb loss, making this one of the biggest challenges in orthopedic medicine, resulting in significant personal and economic cost. Healing strategies using autografts, allografts and xenografts are limited by shortage of available tissue and failure to heal, with complication rates of 50% from delayed or non-union, 30% from allograft fracture, and 15% from infection. Decades of research has been dedicated to solving this problem using a wide variety of bone regeneration techniques. Tissue engineered solutions have emerged that deploy biodegradable, osteoconductive scaffolds to provide structural support and osteoinductive stimulus, with suitable porosity to enable nutrient and waste exchange and angiogenesis. Promising calcium phosphate biomaterials like hydroxyapatite (HAp) and β-tricalcium phosphate are widely studied for bone regeneration scaffolds due to their excellent bioactivity (osteoinductivity, osteoconductivity and osseointegration), mineral composition and tunable degradation rates. Advanced scaffold topologies such as a type of triply periodic minimal surface (TPMS) structure called gyroids are yielding scaffolds that are stiffer and stronger than traditional rectilinear scaffold topologies. Gyroids are ideal candidates for scaffold designs 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. In our study, a method of layer-wise, photopolymerized viscous extrusion, a type of additive manufacturing, was used to fabricate HAp gyroid scaffolds with 60%, 70% and 80% porosities. Our study is the first to use this method to produce and evaluate calcium-phosphate-based scaffolds. Gyroid topology was selected due to its interconnected porosity and superior, isotropic mechanical properties compared to typical rectilinear lattice structures. Our 3D printed scaffolds were mechanically tested in compression and examined to determine the relationship between porosity, ultimate compressive strength, and fracture behavior. Compressive strength increased with decreasing porosity. Ultimate compressive strengths of the 60% and 70% porous gyroids are comparable to that of human cancellous bone, and higher than previously reported for rectilinear scaffolds of the same material. Our gyroid scaffolds exhibited ultimate compressive strength increases between 1.5 and 6.5 times greater than expected, based on volume of material, as porosity decreased. The Weibull moduli, a measure of failure predictability, were predictive of failure mode and found to be in the accepted range for engineering ceramics. The gyroid scaffolds were also found to be self-reinforcing such that initial failures due to minor manufacturing inconsistencies did not appear to be the primary cause of premature failure of the scaffold. The porous gyroids exhibited scaffold failure characteristics that varied with porosity, ranging from monolithic failure to layer-by-layer failure, and demonstrated self-reinforcement in each porosity tested.