Theses and Dissertations

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Open Access
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.
Open Access
Preparation and characterization of poly lactic-co-glycolic nanoparticles encapsulated with gentamicin for drug delivery applications
(Colorado State University. Libraries, 2019) Sun, Yu, author; Li, Yan Vivian, advisor; Bailey, Travis, committee member; Wang, Zhijie, committee member
Wound treatment has always been a popular topic around the world. Since the emergence of nanotechnology, the development and design of novel wound dressing materials have been dramatically improved. The ues of nanoparticles encapsulated with antibiotics to deliver drugs has been shown to be a potentially effective approach to control bacterial infections at a wound position. Recently, biodegradable and biocompatible polymers have drawn lots of attention for the manufacture of drug-loaded nanoparticles in the pharmaceutical industry. In this work, poly-lactic-co-glycolic acid (PLGA) was used in nanoparticle synthesis due to its biodegradability, biocompatibility, and nontoxicity. For this work, gentamicin was loaded into the PLGA nanoparticles as an antibiotic because it is a broad-spectrum antibiotic effective in wound treatments. PLGA nanoparticles were prepared while gentamicin was loaded in the nanoparticles via a double emulsion evaporation method. Poly vinyl alcohol (PVA) was a surfactant that was an important factor in determining the most probable nanoparticle size and morphology. When the PVA concentrations were 9% and 12%, the nanoparticles demonstrated a spherical structure with a porous surface. The porous surface of a nanoparticle was promising for the purpose of releasing encapsulated antibiotics. Another important factor in determining the formation of nanoparticles was the PLGA concentration. Poly lactic-co-glycolic acid (PLGA) was the main material affecting PLGA nanoparticles' properties. PLGA nanoparticles would have various release profiles, morphology, and size distribution with different PLGA concentrations. The results suggested that different PLGA concentrations can endow PLGA nanoparticles with various properties which can lead to different applications of PLGA nanoparticles.
Open Access
Processing of mayenite electride and its composites in spark plasma sintering
(Colorado State University. Libraries, 2019) Kuehster, Adam Edward, author; Ma, Kaka, advisor; Weinberger, Chris, committee member; Williams, John, committee member
Mayenite electride, as the first inorganic room temperature stable electride, has attracted intensive research interests since the early 2000s due to its great potential in various applications such as catalysts, conductive oxides and thermionic emission materials. Mayenite electride is developed from mayenite, a stoichiometric compound of CaO and Al2O3 (12CaOˑ7Al2O3, referred to as C12A7 hereafter) that has a cubic unit cell with a positively charged lattice framework [Ca24Al28O64]4+ of twelve crystallographic subnano-cages per unit and O2- anions clathrated in the cages to maintain charge neutrality. When mayenite is heat treated in a reducing environment, electrons replace O2- ions clathrated in the cages. The electrons can migrate through the inter-cage framework, leading to the formation of electride (C12A7:e-), an electrically conductive form of C12A7. A variety of methods to make C12A7:e- powder and bulk materials have been investigated in the literature, all of which involve multiple steps and long-time (days to weeks) of heat treatment at high temperatures (>1100 ˚C). Although fundamental knowledge of the structure and functionality of C12A7:e- is advancing in the field, the formation of other calcium aluminate phases during the synthesis of mayenite or its electride has been overlooked. Most of the previous studies also lack detailed microstructure characterization. In addition, monolithic C12A7:e- does not provide continuous ohmic contact due to the destruction of the surface cages during processing, which limits its direct use in thermionic emission devices. To address the aforementioned practical issues and to fill in the fundamental knowledge gap, we investigated the effect of adding different reinforcing particles, including carbon black (CB), Ti, and TiB2, on the formation of C12A7:e- via spark plasma sintering (SPS), with attention particularly paid to address phase formation during the processing. Specifically, preformed C12A7 powder was synthesized via a solid-state reaction and used as the precursor base in SPS to study the effect of additives. In addition, a novel approach using in-situ reaction in SPS was proposed in the present work to significantly reduce the processing time. My research revealed that both Ti and TiB2 effectively reduced C12A7 to its electride phase, C12A7:e-. However, addition of Ti and TiB2 also led to partial decomposition of C12A7 into secondary calcium aluminate phases, primarily Al2O3-rich calcium monoaluminate (CA) and CaO-rich tricalcium aluminate (C3A). Although CB did not effectively reduce C12A7 to C12A7:e-. it did not result in the formation of any secondary calcium aluminate phases. Using Ti foils on the top and bottom of the preformed C12A7 powder in SPS created C12A7:e- with a near-theoretical maximum electron concentration ~ 10^21/cm^3. For the in-situ reaction approach, the chemical homogeneity and size distribution of precursor powders are critical to forming C12A7:e- in the typical processing time frame of SPS (5-15 minutes). The fast heating rate and C-rich environment in SPS increased the CaCO3 decomposition temperature to above 930°C, which is consequential to the calcium aluminate formation reaction. Adding Ti powder lowered the CaCO3 decomposition temperature in SPS and allowed for the formation of C12A7:e- via in-situ reaction sintering. The work function of a 50-50wt% C12A7:e- -Ti composite in this study is ~ 2.6 eV.
Open Access
Cationic-doping of mayenite electride: synthesis, processing, and effect on thermal stability
(Colorado State University. Libraries, 2021) DeBoer, Brodderic, author; Ma, Kaka, advisor; Weinberger, Chris, committee member; Bailey, Travis, committee member; Bandhauer, Todd, committee member
Mayenite electride is an electrically conductive ceramic developed from its parent phase, oxy-mayenite (12CaO•7Al2O3, commonly referred to as C12A7). C12A7 has a unique unit cell that consists of a positively charged [Ca24Al28O64]4+ framework containing twelve cages and two extra-framework O2- ions located inside two cages. The extra-framework O2- ions can be replaced with electrons when C12A7 is heated in a reducing environment, and those extra-framework electrons act like anions, forming the mayenite electride phase, denoted as C12A7:e- hereafter. The anionic electrons enable peculiar properties of C12A7:e- such as high electrical conductivity and low work function, making it a promising material for field emission devices, thermionic-cooling, and as a hallow cathode for electrical propulsion. Compared to other electride materials such as Ca2N, which barely sustain their electride properties even at ambient conditions, C12A7:e- has been reported to be stable up to 400 °C. This temperature is yet not high enough to enable its applications in the technologies mentioned above. Doped derivatives of C12A7:e- emerged in recent years to improve its electronic properties, mainly electron density and electrical conductivity. However, the effects of doping on the oxidation resistance and thermal stability of C12A7:e- remained unclear. Experimental effort on cationic doping of C12A7:e- was particularly lacking in the literature. Therefore, the goal of this study is two-fold: (1) to develop processing routes for successful cationic doping of C12A7:e-, and (2) to test if cationic doping can improve the thermal stability of C12A7:e-. Copper (Cu) and niobium (Nb) were selected as cationic dopants in this study to elucidate how cationic doping affects the thermal stability of the mayenite electride. First, effort was focused on developing synthesis and processing methods to effectively dope Cu and Nb into C12A7:e-. Three different methods were investigated, including diffusion doping; in conventional furnace or via spark plasma sintering (SPS), single-step in-situ formation via SPS, and a solid-state reaction (SSR) synthesis followed by reduction. The phase constitutions, lattice parameters, and microstructure of the various C12A7:e- samples fabricated via the aforementioned methods were characterized to verify if cationic doping was successfully achieved. Electrical conductivity was measured to verify the electride phase is sustained after the doping. Thermal analysis was performed to determine the thermal stability of the cation-doped C12A7:e- compared to undoped counterparts, including onset temperature and peak temperature of oxidation, oxidation rate, mass gain percentage resulted from oxidation, and any decomposition reaction. The key findings of this study include: (1) both Cu-doping and Nb-doping improved the thermal stability of the C12A7:e- by increasing the onset temperatures of oxidation; (2) Cu-doping was effectively and efficiently achieved via the novel SPS diffusion doping method. SPS diffusion doping of Cu at 800 °C gave rise to a minimum lattice parameter (a = 11.942 Å) of C12A7:e-, the lowest oxidation rate, and the smallest mass gain percent at 1050 °C; (3) Using oxy-mayenite and Nb2O5 as precursor for reaction sintering and in-situ reduction in SPS led to successful Nb-doping into the C12A7:e-. Despite the increased onset oxidation temperature resulted from Nb addition, pest oxidation occurred in Nb-doped C12A7:e- samples, leading to high oxidation rate, high total mass gain percentage, and fracture of the solid samples at temperature above 700 °C. In conclusion, Cu-doping was experimentally proved to be an effective approach to improve the thermal stability of C12A7:e- and meanwhile increase the electrical conductivity.
Open 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.
Open Access
Sustainable recycling of metal machining swarf via spark plasma sintering
(Colorado State University. Libraries, 2021) Sutherland, Alexandra E., author; Ma, Kaka, advisor; Sambur, Justin, committee member; Simske, Steve, committee member
In general, extracting virgin metals from natural resources exerts a significant environmental and economic impact on our earth and society. Production of virgin stainless steels and titanium (Ti) alloys have particularly caused concerns because of the high demands of these two classes of metals across many industries, with low fractions of scraps (less than one-third for steels and one-fourth for Ti alloys) that are currently recirculated back into supply. In addition, the conventional recycling methods for metals require multiple steps and significant energy consumption. With the overarching goal of reducing energy consumption and streamlining recycling practices, the present research investigated the effectiveness of direct reuse of stainless steel swarf and Ti6Al-4V alloy swarf as feedstock for spark plasma sintering (SPS) to make solid bulk samples. The parts made from machining swarf were characterized to tackle material challenges associated with the metal swarf such as irregular shapes and a higher amount of oxygen content. The hypothesis was that while solid bulk parts made from metal swarf would contain undesired pores that degrade mechanical performance, some mechanical properties (e.g., hardness) can be comparable or even outperform the industrial standard counterparts made from virgin materials, because of cold working and grain refinement that occurred to the swarf during machining and the capability of SPS to retain ultrafine microstructures. 304L stainless steel and Ti-6Al-4V (Ti64) alloy swarf were collected directly from machining processes, cleaned, and then consolidated to bulk samples by SPS with or without addition of gas atomized powder. Nanoindentation and Vickers indentation were utilized to evaluate the hardness at two length scales. Ball milling was performed on Ti64 to assess the energy consumption required to effectively convert swarf to varied morphologies. In addition, to provide insight into the macroscale mechanical behavior of the materials made by SPS of recycled swarf, finite element modeling (FEM) was used to predict tensile stress-strain curves and the corresponding stress distributions in the samples. The key findings from my research proved that reuse of austenitic stainless steel chips and Ti64 alloy swarf as feedstock for SPS is an effective and energy efficient approach to recycle metal scraps, compared to the production and use of virgin gas atomized powders, or conventional metal recycling routes. The mechanical performance of the samples made from metal swarf outperformed the relevant industrial standard materials in terms of hardness while the ductility remains a concern due to the presence of pores. Therefore, future work is proposed to continue to address the challenges associated with mechanical performance, including but not limited to, tuning the SPS processing parameters, quantifying an appropriate amount of addition of powder as a sintering aid, and refining the morphology of the swarf by ball milling. It is critical for the health of our planet to always consider the tradeoff between energy consumption and materials performance.
Open Access
Development of surface modifications on titanium for biomedical applications
(Colorado State University. Libraries, 2021) Maia Sabino, Roberta, author; Popat, Ketul C., advisor; Martins, Alessandro F, advisor; Herrera-Alonso, Margarita, committee member; Li, Yan Vivian, committee member; Wang, Zhijie, committee member
For decades, titanium-based implants have been largely employed for different medical applications due to their excellent mechanical properties, corrosion resistance, and remarkable biocompatibility with many body tissues. However, even titanium-based materials can cause adverse effects which ultimately lead to implant failure and a need for revision surgeries. The major causes for implant failure are thrombus formation, bacterial infection, and poor osseointegration. Therefore, it is essential to develop multifunctional surfaces that can prevent clot formation and microbial infections, as well as better integrate into the body tissue. To address these challenges, two different surface modifications on titanium were investigated in this dissertation. The first one was the fabrication of superhemophobic titania nanotube (NT) surfaces. The second approach was the development of tanfloc-based polyelectrolyte multilayers (PEMs) on NT. The hemocompatibility and the ability of these surfaces to promote cell growth and to prevent bacterial infection were investigated. The results indicate that both surface modifications on titanium enhance blood compatibility, and that tanfloc-based PEMs on NT improve cell proliferation and differentiation, and antibacterial properties, thus being a promising approach for designing biomedical devices.
Open Access
Tuning interfacial biomolecule interactions with massively parallel nanopore arrays
(Colorado State University. Libraries, 2021) Wang, Dafu, author; Kipper, Matt J., advisor; Snow, Chris D., advisor; Bailey, Travis S., committee member; Stasevich, Tim J., committee member
This project studied interfacial interactions of macromolecules with nanoporous materials, with an ultimate goal of exploiting these interactions in functional biomaterials. We quantified interaction forces and energies for guest molecules threaded into the pores of protein crystals via nano-mechanical atomic force microscopy (AFM) pulling experiments. We demonstrated that both double-stranded DNA and poly(ethylene glycol) are rapidly absorbed within porous protein crystals, where they presumably bind to the inner "wall" surfaces of the protein crystal nanopores. These "guest" molecules can be retrieved from the "host" crystal by chemically modified AFM tips, enabling precise measurements of the adhesion forces and interaction energies. Based on these experiments, machine learning approaches were developed to classify hundreds of thousands of individual force-distance curves obtained in the AFM experiments. Furthermore, we showed that the interactions between protein crystal "hosts" and "guest" macromolecules can be used to modulate cell behavior, by presenting cell adhesion ligands tethered to different lengths of macromolecules that thereby modulate the maximum traction force cells can apply before rupturing bonds tethering the adhesion ligand to the porous protein crystal interior. This method affords the opportunity to create biomaterials that store an internal reservoir of cell-specific signals that can be presented to independently modulate the behavior of different cell populations in a single material. In the first chapter, some recent advancements, and methodologies of measuring interfacial biomolecule interactions are reviewed and compared. The reviewed technics include atomic force microscopy, fluorescence recovery after photobleaching, the total internal reflection fluorescence, confocal microscopy, and optical tweezers. Furthermore, this chapter interduces the application of machine learning to assist the interfacial biomolecule interaction studies, especially the AFM measurements. This chapter further prospects of the future of interfacial biomolecule interactions studies. In the second chapter, the methodologies of probing and observing the surface of highly porous Camphylobacter Jejuni formed protein crystals (CJ protein crystals) by high-resolution AFM are introduced. Throughout this chapter, the morphologies of CJ protein crystals are comprehensively investigated by AFM and have been discussed in this chapter. In the third chapter, for the first time, the interactions of DNA with porous protein crystals are quantitatively measured by high-resolution AFM and chemical force microscopy. The surface structure of protein crystals with unusually large pores was observed in liquid via high-resolution AFM. Force-distance (F-D) curves were also obtained using AFM tips modified to present or capture DNA. The interactions of DNA molecules with protein crystals to be quantitatively studied while revealing the morphology of the protein crystal surface in detail, in buffer, reveals how a new protein-based biomaterial can be used to bind DNA guest molecules. In the fourth chapter, strategies of machine learning are introduced which pioneered the use of machine learning to classify and cluster the interaction patterns between DNA and protein crystals, enabling us to process thousands of F-D curves collected by AFM. Finally, in the fifth chapter, we quantitatively measure and take advantage of the interaction between poly(ethylene glycol) (PEG)-arginine-glycine-aspartic acid (RGD) complex and nanoporous protein crystals to understand how non-covalent surface presentation of peptide adhesion ligands can influence cell behavior. Through AFM, F-D curves of interactions between PEG-RGD and host protein crystals were obtained for the first time. Furthermore, a strategy is developed that enables us to design surfaces that non-covalently present multiple different ligands to cells with tunable adhesive strength for each ligand, and with an internal reservoir to replenish the precisely defined crystalline surface.
Open Access
Stability of thin-film CdTe solar cells with various back contacts
(Colorado State University. Libraries, 2021) Hill, Taylor D., author; Sites, James, advisor; Sampath, Walajabad, advisor; Bradley, Mark, committee member
With an increasing reliance on photovoltaic energy comes an ever-increasing demand to understand the mechanisms of failure which lead one to having an under-performing solar module. Recent technological advances have proven CdTe solar cells to be competitive with traditional Si, taking up 5% of the world solar market and reaching efficiency upwards of 22.1% for small area scale and 18.6% for module scale. This thesis explores various back-contact configurations to reduce the contact barrier height as well as how they hold up under accelerated lifetime testing. Various degradation mechanisms, such as diffusion of species, drift within the built-in fields, and formations of various impurities/complexes on the surface and within the bulk were explored. The results of accelerated-lifetime experiments revealed the instability of devices with large amounts of Cu and those containing the colloidal Ni based paint solution as a metallic back contact. Sputtered films of nickel doped with vanadium (Ni:V) and chromium (Cr) demonstrated the capability to produce cells with efficiencies between 12-13% with fill factors up to 75%. Metallic bilayers containing a metallic cap of aluminum (Al) were then evaluated, demonstrating an increase in efficiency up to 15.1%. Buffer layers of NiO revealed the presence of a large back-contact barrier via the rollover effect in forward bias, leading to devices with efficiency of only 3%, but subsequent work revealed that by applying the NiO buffer prior to CdCl2 passivation reduces the back barrier and produces cells with peak efficiency of 14.8%.
Open Access
Understanding and utilization of thermal gradients in spark plasma sintering for graded microstructure and mechanical properties
(Colorado State University. Libraries, 2022) Preston, Alexander David, author; Ma, Kaka, advisor; Weinberger, Chris, committee member; Neilson, Jamie, committee member; Heyliger, Paul, committee member
Spark plasma sintering (SPS), also commonly known as electric field assisted sintering, utilizes high density electric currents and pressure to achieve rapid heating and significantly shorter sintering times for consolidating metal and ceramic powders, which could otherwise be difficult, time consuming, and energy intensive. SPS has attracted extensive research interests since the early 1990's, with the promise of efficient manufacturing of refractory materials, ultrahigh temperature ceramics, nanostructured materials, functionally graded materials, and non-equilibrium materials. Thermal gradients occur in SPS tooling and the samples during sintering, which can be a drawback if homogeneous properties are desirable, as the temperature inhomogeneity can lead to large gradients in microstructure such as porosity, grain size, and phase distribution. Many researchers have looked to mitigate or control these gradients by design and use of specialized tooling. However, the effect of the starting powder is relatively less investigated or overlooked. Feedstock powders can come in various shapes, particle size distributions, and surface chemistry. Effects of these powder characteristics on the SPS process and the consequent microstructure of the sintered parts remain as a gap in the fundamental knowledge of SPS. To fill in this gap, my research investigated the role of thermal gradients during SPS, and how the thermal gradients subsequently affect the location-specific pore distribution, and the consequent mechanical properties of the materials. From a practical point of view, design and fabrication of a bulk sample with a fully dense surface and an engineered pore architecture in the sample interior via one-step SPS will enable mechanical properties unattainable via conventional processing of fully dense bulk materials, such as alike combination of lightweight, high surface hardness, and wear resistance, and high toughness. Therefore, the overarching goal of my research was to provide fundamental insights into the material processing - microstructure - properties correlation so that the field assisted sintering technology can be advanced to control location-specific microstructure. To fulfill this goal, two metallic materials were selected in my study, austenitic stainless steel and commercially pure titanium, representing inherently heavy but widely used alloys, and a pure metal that is inherently lightweight, these materials were used to investigate the effects of powder morphology on the sintering behavior. The pure Ti was selected specifically to gain fundamental insight into the effect of powder shape on sintering, while mitigating the concern of alloying/precipitation events and integrating FEM with my experimental work. This work identified a relationship between decreasing pore size and increasing yield strength in stainless steel, which was attributed to fine precipitate formation surrounding submicron pores inducing local stiffening. Whereas larger pores where precipitates were not found are concluded to not have the necessary driving force for the precipitation event to occur. Ball milled stainless steel powders with higher aspect ratios were also shown to have smaller porosity gradients in comparison to their spherical gas atomized counterparts. A thermal electric finite element model is also proposed which incorporates the master sintering curve to simulate densification as an alternative to the more computationally costly and difficult to parametrize fully coupled thermal-electric-mechanical finite element model. Results from the combined model indicate strong agreement with experimental results within 2% accuracy of measured densification. Additionally, the model predicts higher porosity gradients for gas atomized powders in comparison to ball milled powders which is experimentally verified.
Open Access
Stable and unstable tiling patterns of ABC miktoarm triblock terpolymers studied via GPU-accelerated self-consistent field calculations
(Colorado State University. Libraries, 2022) Hawthorne, Cody, author; Wang, David, advisor; Bailey, Travis, committee member; Miyake, Garrett M., committee member; Szamel, Grzegorz, committee member
Block copolymers are macromolecules formed from linking together two or more chemically distinct types of polymers. Provided the different monomers that make up each polymer are immiscible enough, melts of these molecules will self-assemble into highly ordered, periodic structures at length scales typically on the order of nanometers. The exemplary and simplest material in this respect is the AB diblock copolymer, a linear macromolecule formed by bonding together two immiscible polymers (or 'blocks') A and B. This material is capable of assembling into lamellar, cylindrical, spherical, and networked morphologies depending on the length of the A block and degree of immiscibility between A and B. The ability to control bulk properties of block copolymers via tuning these molecular properties, as well as the length scales that these ordered structures form at, makes them intriguing candidates for next generation technological applications in lithography, photonics, and transport. In order to realize these applications it is imperative to have an intimate understanding of the phase behavior of the materials such that the morphology that will form at a given combination of parameters can be predicted reliably. Self-consistent field theory, or SCFT, has emerged as a useful theory for investigating block copolymer phase behavior. This statistical-mechanical theory has been successfully used to construct phase diagrams of the self-assembled morphologies of various block copolymer systems. These phase diagrams provide the connection between molecular properties (such as block lengths, block incompatibility, and chain architecture) and bulk properties necessary in order to control the behavior of the material. The theory must, in general, be solved numerically – an open-source software termed 'PSCFPP' has recently been made available for this purpose, capable of implementing high-performance SCFT calculations for arbitrarily complex acyclic block copolymers by taking advantage of the massive parallelization of GPUs. In this work, PSCFPP is used to apply SCFT to a neat melt of complex ABC miktoarm triblock terpolymers, which are an interesting class of block copolymer formed by linking three distinct polymers A, B, and C at a single junction point. The resulting star-shaped macromolecule is referred to as a 'miktoarm' and exhibits unique morphologies such as the Archimedean tiling patterns that cannot be found in other block copolymer materials. To focus on the effect of composition, which has not yet been fully elucidated, we restrict the interaction parameters between monomers ABC to the symmetric case where all are equivalent. The central region of the phase diagram, where the effect of the miktoarm architecture is most significant, is mapped out in detail and a 3D morphology previously thought to be metastable is shown to be a stable phase. Further, discrepancies in the literature concerning the stability of multiple 2D tiling patterns are resolved such that the phase diagram presented is the most accurate for the system to date. Finally, a 2D morphology of some interest owing to the possibility of exhibiting photonic band gaps is definitively shown to be stable in this system and its thermodynamic properties analyzed to ascertain what drives its formation. These results provide a solid foundation for further refinement of our understanding of ABC miktoarm phase behavior and demonstrate the utility of a software such as PSCFPP for obtaining high-accuracy SCF results.
Embargo
Development of bioinspired hyaluronan enhanced synthetic polymers for medical applications
(Colorado State University. Libraries, 2023) Gangwish, Justin Phillip, author; James, Susan P., advisor; Bailey, Travis S., advisor; Dasi, Lakshmi Prasad, committee member; Popat, Ketul, committee member; Monnet, Eric, committee member
The first transcatheter aortic valve replacement (TAVR) was performed in 2002, and over the past two decades has become as prevalent as surgical aortic valve replacements in America. TAVR's have significant potential to provide relief for patients with aortic valve disease, but due to their high cost remain accessible primarily in wealthy nations. Furthermore, TAVR's have limited lifespans and are radiotransparent so they can only be evaluated for function through echocardiography. Thus, an expensive medical implant with limited durability is only replaced after it has begun to fail and cause the patient further stress on their heart. To address these issues this dissertation reviews the research performed in generating a novel heart valve leaflet material that is radiopaque, made primarily out of linear low-density polyethylene (LLDPE), and incorporates the biological polymer hyaluronan (HA). These leaflets could significantly reduce the cost of TAVR's potentially allowing for global adoption of the technology. They are radiopaque and easily imaged using x-ray fluoroscopy allowing changes in leaflet shape or movement to be identified prior to when the echocardiography would have shown a deleterious effect to function. They incorporate HA which is found in the interior lumen of blood vessels and has been shown to decrease calcification and thrombosis, both of which have caused polymeric leaflets to fail in previous research. Finally, they can be easily shaped and reinforced allowing for a potentially far greater device lifespan. The leaflets are made by melt pressing in radiopaque powders of either tungsten or bismuth trioxide into sheets of LLDPE followed by treatment with HA to form an interpenetrating polymer network. The material properties of the leaflets were evaluated for their tensile mechanical properties, their hydrophilicity, their radio transparency (or lack thereof), and their hemodynamics. The biocompatibility of the leaflets was evaluated through a cytotoxicity assay, whole blood clotting on the surface of the material, and the ability for platelets to adhere and activate on the material surface. The results demonstrate the material has significant potential to function as a heart valve leaflet in a TAVR. Beyond evaluating this novel material, the process by which HA is incorporated into LLDPE was examined and optimized for commercial scale up. First, the need for solvent distillation and nitrogen blankets during treatment were determined to be unnecessary to produce an HA IPN with LLDPE. Then the rate at which the LLDPE is drawn from the HA solution as well as the vacuum pressure and temperature during this process was found to affect the amount of active HA at the surface of the material. Finally initial evidence was found that shows that HA IPN does not form through the bulk of the material, but rather in the first few microns of the LLDPE. Unrelated to TAVR's a study was performed to enhance the non-woven polypropylene used in surgical masks and N-95 masks against COVID-19 using HA and polyethylene glycol (PEG). HA was used to form a microcomposite on the surface of the non-woven polypropylene while PEG was grafted to the surface with oxygen plasma. The resulting materials were evaluated for their tensile mechanical properties, breathability, chemical composition, hydrophilicity, cytotoxicity, and ability to adsorb COVID-19 spike protein. The results indicate the material has notable potential to make masks more effective at preventing the transfer of COVID-19, however further studies using live SARS-CoV-2 virus beyond the capabilities of this laboratory are necessary before that potential can be fully confirmed.
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Advanced nanostructured materials for enhancing bioactivity
(Colorado State University. Libraries, 2024) Bhattacharjee, Abhishek, author; Popat, Ketul C., advisor; Sampath, Walajabad, committee member; Herrera-Alonso, Margarita, committee member; Wang, Zhijie, committee member
Health hazards such as pathogenic infection, communicable diseases, and bone damage and injuries cause enormous human suffering and pain worldwide. Biomaterials such as orthopedic implants and biosensors are crucial tools to remedy these complications. Development of novel biomaterials and modifying existing materials can help enhance medical device efficacy. One of the key aspects of improving biomaterials is the utilization of nanotechnology. Nanoscale surface features can improve the interaction between materials and biological agents, thus improving their bioactivity. In this dissertation research, two different biomaterials were used for two distinct applications. Firstly, titanium, a common material for orthopedic implants, was used. Ti is a popular implant material because of its superior corrosion resistance, lightweight, and excellent biocompatibility. However, 10% of Ti implants fail each year due to pathogenic bacterial infection and poor osseointegration resulting in revision surgeries and immense suffering of the patients. Nanostructured surface modification approaches can potentially reduce the failure rate of Ti implants. In this study, TiO2 nanotube arrays (NT) were fabricated followed by zinc (Zn) and strontium (Sr) doping. These elements provide important signals to mesenchymal stem cells to differentiate into osteoblasts which helps in bone healing. Zn also reduces bacterial adhesion to the implant surface. Results showed that the modified surfaces could significantly reduce bacterial adhesion and improved osseointegration properties of the mesenchymal stem cells. Secondly, a polydiacetylene (PDA)-based electrospun nanofiber biosensor was prepared that is flexible in nature for monitoring bacterial or viral infection. The nanofiber biosensor could selectively detect Gram-negative bacteria via a vivid blue-to-red color transition. Since the color transition is visible to the naked eye, the biosensor offers immense potential to be used as a screening device for Gram-negative bacterial infection in various industries such as food packaging, medical, intelligence, and national security. During the COVID-19 pandemic, the PDA biosensing platform was utilized to detect the spike (S) protein of the SARS-CoV-2. For this, the surface chemistry of the PDA fibers was modified, and a receptor protein was conjugated at the end of the PDA polymer chain. When the modified PDA fibers were incubated with the S protein, the blue-to-red color transition happened, thus sensing the presence of S protein in the environment. This result indicated that PDA nanofiber biosensor is a flexible sensing platform for effectively detecting both bacteria and viruses. The two biomaterials investigated in this research indicated that the use of nanotechnology can help in enhancing their bioactivity.
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Failure analysis and durability enhancement of polymeric heart valve leaflets
(Colorado State University. Libraries, 2024) Khair, Nipa, author; James, Susan P., advisor; Bailey, Travis S., advisor; Li, Vivian, committee member; McGilvray, Kirk, committee member
Rheumatic and calcified aortic heart valve disease is prevalent globally among all aged people, and the number is rapidly increasing. Clinically accepted, minimally invasive xenograft-based transcatheter aortic heart valve replacement (TAVR) shows limited durability (<10 years). Hyaluronic acid (HA) enhanced polyethylene polymeric TAVR shows excellent in vitro and in vivo anti-calcific, anti-thrombotic, and hydrodynamic performance, making it a suitable candidate for heart valve leaflets. The main problem, however, is during durability testing, cyclic impact loading causes premature failure in a consistent fashion related to TAVR assembly. This dissertation investigates leaflet premature failure mechanisms and provides two plausible solutions to upgrade heart valve durability without sacrificing performance. With regard to the failure mechanism, representative areas of retrieved failed leaflets are examined under electron microscopy and small angle x-ray scattering. The investigation finds abrasive wear, wear polishing, fine scratching, and imprints of the metal stent of the leaflet surface, indicating surface wearing from soft plastic rubbing against hard metal. A strong permanganate oxidizer etches away low-energy amorphous domain to unveil stable spherulitic structures of approximately 3 µm, bridging and tie molecular domains of pristine LLDPE. The oxidizer partially etches away polymeric buildups of failed leaflets only to reveal thinned-out and fractured spherulites beneath them, identifying the buildups as stress precursors. SAXS study reports local lamellar disruption further confirming the SEM results. Most. Notably, this is the first study that, to our knowledge, to directly image stable craze cross-tie microstructure that formed due to chain disentanglement from high amplitude cyclic stress. The SEM images validate previous theoretical and computational molecular dynamics models of cross-tie structure architecture. Therefore, leaflet premature failures are the compound effect of cyclic fatigue-initiated crazing and surface wear. Heart valve leaflet durability can be upgraded by controlling crazing and surface wearing. Both the crazing and surface wearing can be controlled by crosslinking of randomly folded amorphous chains. Because they are direct impacts of chain disentanglement under high amplitude cyclic stress. Crosslinked covalent bonds of polymer limit chain movements. LLDPE thin sheets are crosslinked at 50, 70, 100, and 150 kGy doses using 200 KeV (low energy) and 4 MeV (low energy) electron beams at room temperature in the air. Their effects are characterized by measuring gel content percentage, tensile testing, Differential Scanning Calorimetry (DSC), nanoindentation, and nano scratch test. Crosslinked LLDPE heart valve leaflet tested in in vitro flow loop and wear tester to determine valve performance and durability, respectively. Low energy electron beam (LEEB) forms 28% xylene insoluble gel whereas high energy electron beam (HEEB) forms 58 % gel at 100 kGy doses. LEEB does not affect mechanical properties, but HEEB significantly increases stiffness and yield strength. A slight reduction of melting temperature is found for LLDPE crosslinked by both of the energy sources. Nanomechanical tests show crosslinking improves hardness and coefficient of friction, an indication of improving surface wear resistance, which can explain durability improvement. Heart valve durability can also be improved by strengthening the leaflet with fiber reinforcement. A thin plastic sheet is assembled into a cylindrical form by welding two ends, which never fails. The weld at the commissure post is found to be mechanically stronger than the rest of the leaflet, which protected this region. Braided fibers are embedded on the leaflet regions of the commissure post perpendicular to the valve circumference, mimicking the weld but at a much higher strength. Leaflet durability skyrockets from a few million ISO 5840-2005 cycles to 73 million. The entire cardiac cycle of the heart valve with embedded fibers of varying angles, lengths, and numbers is simulated in Finite Element Analysis (FEA) to study their effects on leaflet maximum principal stress and leaflet opening dynamics. Horizontal fibers wrap the leaflet 360° to relax the leaflet completely during peak diastolic. However, the leaflet has a higher coaptation gap and delayed opening. The heart valve with embedded horizontal fibers is physically manufactured and tested in an in vitro flow loop and wear tester, which showed improved durability, but compromised hemodynamics. Finally, strategically crosslinked leaflet was simulated in FEA where leaflet regions of the commissure post and stent line are assigned with stiff crosslinked LLDPE material property, but the rest of the cusps undergo maximum bending are assigned with uncrosslinked LLDPE material property. Results show that strategically crosslinked leaflets open more easily than fully crosslinked leaflets. The final chapter discusses 3D shaped LLDPE leaflet bio enhancement process. Leaflets are 3D shaped in a vacuum thermoformer followed by the HA enhancement. Whole blood clotting resistance, platelet adhesion, activation, and cytotoxicity studies are conducted to determine at 10-4 µmol/mm2 ranged HA population density is required to achieve the best biocompatibility. Generally, water contact angle, Toluidine Blue O (TBO) elution assays, ATR-FTIR are used to determine overall HA presence on the leaflet. This study reports TBO staining and elution is the most effective and accurate measurement tool for determining HA population density. Fiber-reinforced LLDPE, and crosslinked LLDPE are HA-treated, and TBO staining predicts heavily populated HA surface density.

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