Browsing by Author "James, Susan, committee member"
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Item Embargo Advanced manufacturing of thermoset polymers and composites(Colorado State University. Libraries, 2023) Ziaee, Morteza, author; Yourdkhani, Mostafa, advisor; Radford, Donald W., committee member; James, Susan, committee member; Bailey, Travis, committee memberThermoset polymers and composites are lightweight materials extensively used in many industries from aerospace to automotive to prosthetics due to their excellent specific mechanical properties and high chemical resistance. However, these products are conventionally manufactured by labor-intensive processes using subtractive manufactured tooling or molds followed by thermal curing inside an oven or autoclave at elevated temperatures for several hours. Hence, conventional manufacturing approaches are energy- and time-consuming and require expensive equipment. Moreover, lack of design flexibility and poor repeatability are additional challenges, which limit the structural and functional capabilities of such products. In this dissertation, I present a novel approach to address the existing limitations in manufacturing thermosets and their composites by developing rapid curing resin systems and integrating them in additive manufacturing (AM) processes. In the first chapter, state-of-the-art manufacturing methods are reviewed and frontal polymerization (FP) as a promising curing strategy for rapid and energy-efficient manufacturing of thermosets and composites is introduced. In the second chapter, the effect of ambient conditioning and resin chemistry on thermal frontal polymerization of a high-performance resin system is explored. In the third chapter, FP is used to demonstrate, for the first time, simultaneous printing and curing of short carbon fiber-reinforced composites for high performance applications. In the following chapter, AM of a soft and stretchable elastomer with tunable thermomechanical properties manufactured via FP is discussed. In the fifth chapter, the printing process is further improved using an external localized heat source, instead of relying on the exothermic heat of polymerization of the resin, to accelerate the curing rate and make the printing process more robust and applicable to the manufacture of large-scale components. Finally in the last chapter, bubble-free frontal polymerization of polyacrylates is introduced for the developed 3D printing process.Item Embargo Advanced solutions for rainfall estimation over complex terrain in the San Francisco Bay area(Colorado State University. Libraries, 2023) Biswas, Sounak Kumar, author; Chandrasekar, V., advisor; Cheney, Margaret, committee member; Gooch, Steven, committee member; James, Susan, committee memberFresh water is an increasingly scarce resource in the western United States and effective management and prediction of flooding and drought have a direct economic impact on almost all aspects of society. Therefore it is critical to monitor and predict water inputs into the hydrological cycle of the Western United States (US). The complex topography of the western US poses a significant challenge in developing physically realistic and spatially accurate estimates of precipitation using remote sensing techniques. The intricate landscape presents a challenging observing environment for weather radar systems. This is further compounded by the complex microphysical processes during the cool season which are influenced by coastal air-sea interactions, as well as orographic effects along the coastal regions of the West. The placement and density of operational National Weather Service (NWS) radars (popularly known as NEXRAD or WSR-88D) pose a challenge in meeting the needs for water resource management in the western US due to the complex terrain of the region. Consequently, areas like the San Francisco Bay Area could use enhanced precipitation monitoring, in terms of amount and type, along watersheds and surrounding rivers and streams. Shorter wavelength radars such as X-Band radar systems are able to augment the WSR-88D network, to observe better the lower atmosphere with higher temporal and spatial resolution. This research investigates and documents the challenges of precipitation monitoring by radars over complex terrain and aims to provide effective and advanced solutions for accurate Quantitative Precipitation Estimation (QPE) using both WSR-88D and the gap-filling X-Band radar systems over the Bay Area on the US West Coast, with a focus on the cool season. Specifically, this study focuses on a precipitation microphysics perspective, aiming to create an algorithm capable of distinguishing orographically enhanced rainfall from cool-season stratiform rainfall using X-Band radar observations. A radar-based rainfall estimator is developed to increase the accuracy of rainfall quantification. Additionally, various other scientific and engineering challenges have been addressed including radar calibration, attenuation correction of the radar beam, radar beam blockage due to terrain, and correction of measurements of the vertical profiles of radar observables. The final QPE product is constructed by merging the X-Band based QPE product with the operational NEXRAD based QPE product, significantly enhancing the overall quality of rainfall mapping within the Bay Area. Case studies reveal that the new product is able to improve QPE accuracy by ~70% in terms of mean absolute error and root mean squared error compared to the operational products. This establishes the overall need for precipitation monitoring by gap-filling X-Band radar systems in the complex terrain of the San Francisco Bay Area.Item Open Access Application of passive flow control to mitigate the thromboembolic potential of bileaflet mechanical heart valves: an in-vitro study(Colorado State University. Libraries, 2014) Forléo, Márcio Henrique, author; Dasi, Lakshmi Prasad, advisor; James, Susan, committee member; Orton, Christopher, committee member; Dinenno, Frank, committee memberImplantation of a bileaflet mechanical heart valve (BMHV) continues to be associated with risk of thromboembolic complications despite anti-coagulation therapy. Mechanical heart valves have been the gold standard in valve heart replacement since the 1950s with BMHVs currently still being the valve of choice for younger patients. Given that a large body of literature points to thromboembolic complications due to poor hemodynamics, improvements to the hemodynamic performance of BMHVs are needed. In this study, we explore the concept of passive flow controls that have been widely used in aerospace industry as a novel approach towards improving BMHV design. Passive flow control elements are small features on solid surfaces, such as vortex generators (VGs), that alter flow to achieve desired performance. The specific aims of this study are (1) develop a methodology to evaluate thromboembolic potential (TEP) of BMHVs using in-vitro particle image velocimetry technique, (2) quantify the efficacy of rectangular VGs distributed on BMHV leaflets to reduce TEP, and (3) quantify the hemodynamic performance impact of rectangular VGs. An in-vitro pulsatile flow loop along with Particle Image Velocimetry (PIV) flow visualization technique was developed, validated, and utilized to acquire time-resolved velocity fields and shear stress loading: Lagrangian particle tracking analysis of the upstream and downstream flow during diastole and systole enabled the calculation of predicted shear stress history and exposure times corresponding to platelets. This information was then used in numerical models of blood damage to predict the TEP of test heart valves using established platelet activation and platelet lysis parameters. BMHV leaflets were constructed using 3D printing technology with VGs based on micro-CT scans of a model BMHV leaflet. Two configurations were constructed: co-rotating VGs and counter-rotating VGs. Co-rotating VGs consist of single features 1mm tall and 2.8mm long spaced equally apart (5mm) at an angle of attack of 23 degrees. Counter-rotating VGs consist of mirrored feature pairs 1mm from each other with the same dimensions as the co-rotating VGs. The leaflets were tested using the methodology described above to elucidate their effect on the TEP of the BMHV compared to the control leaflets. For systolic flow downstream of the valve, we report a decrease in the average platelet activation and average platelet lysis TEP (both normalized by the average exposure time) largely in the central jet, with the vortex generator equipped leaflets compared to the control leaflets at a p-value of 0.05. However, for diastolic flow upstream of the valve, we report an increase in the average platelet lysis TEP and average platelet activation TEP (both normalized by the average exposure time) largely in the regurgitant jet zone with the vortex generator equipped leaflets compared to the control leaflets at a p-value of 0.05. Also, steady and pulsatile flow experiments were conducted to calculate the transvalvular pressure drop across the model BMHV with control leaflets (no VGs) and leaflets containing VGs to calculate effective orifice area (EOA), which is an index of valve performance and is related to the degree to which the valve obstructs blood flow. We report a significant increase in EOA values for valves with leaflets containing passive flow control elements in both steady and pulsatile flow experiments compared to the control leaflets. Under steady flow, the co-rotating VGs configuration had the best EOA value compared to the control leaflet and counter-rotating vortex generator configuration. However, under pulsatile conditions, the counter-rotating VGs configuration had the best EOA value compared to the control leaflet and co-rotating vortex generator configuration. PIV measurements highlight the delay in flow separation caused by the VGs and corroborate the increased pulsatile flow EOA values. This study shows that the TEP of BMHVs can be accurately evaluated using in-vitro PIV techniques and that there is room for improvement in BMHV design using passive flow control elements. With optimization of passive flow control configuration and design, it is possible to further decrease the TEP of BMHVs while increasing their hemodynamic performance; thus creating a safer, more efficient BMHV.Item Open Access Biopolymer nanomaterials for growth factor stabilization and delivery(Colorado State University. Libraries, 2014) Place, Laura Walker, author; Kipper, Matt J., advisor; James, Susan, committee member; Popat, Ketul C., committee member; Miller, Benjamin, committee memberBiopolymers are useful in tissue engineering due to their inherent biochemical signals, including interactions with growth factors. There are six biopolymers used in this work, the glycosaminoglycans (GAGs), heparin (Hep), chondroitin sulfate (CS), and hyaluronan (HA), chitosan (Chi), a GAG-like molecule derived from arthropod exoskeletons, a Chi derivative N,N,N¬-trimethyl chitosan (TMC), and an extracellular matrix (ECM)-derived material, demineralized bone matrix (DBM). The direct delivery of growth factors is complicated by their instability. GAG side chains of proteoglycans stabilize growth factors. GAGs also regulate growth factor-receptor interactions at the cell surface. The majority of proteoglycan function is derived from its GAG side chain composition. Here we report the development of nanoparticles, proteoglycan-mimetic graft copolymers, incorporation of nanoparticles into electrospun nanofibers, and processing methods for electrospinning demineralized bone matrix to fabricate bioactive scaffolds for tissue engineering. The nanoparticles were found to show similar size, composition, and growth factor binding and stabilization as the proteoglycan aggrecan. We use basic fibroblast growth factor (FGF-2) as a model heparin-binding growth factor, demonstrating that nanoparticles can preserve its activity for more than three weeks. Graft copolymers were synthesized with either CS or Hep as the side chains at four different grafting densities. Their chemistry was confirmed via ATR-FTIR and proton NMR. They were shown to increase in effective hydrodynamic diameter with grafting density, resulting in a size range from 90-500 nm. Graft copolymers were tested for their ability to deliver FGF-2 to cells. The CS conditions and the Hep 1:30 performed equally as well as when FGF-2 was delivered in solution. Preliminary dynamic mechanical testing demonstrated that hydrogels containing the copolymers exhibit changes in compressive modulus with cycle frequency. Two electrospinning techniques were developed, using an emulsion and a coaxial needle, for incorporating growth factor into electrospun nanofibers. We bound FGF-2 to aggrecan-mimetic nanoparticles for stabilization throughout electrospinning. The two techniques were characterized for morphology, nanoparticle and FGF-2 incorporation, cytocompatibility, and FGF-2 delivery. We demonstrated that both techniques result in nanofibers within the size range of collagen fiber bundles and dispersion of PCNs throughout the fiber mat, and exhibit cytocompatibility. We determined via ELISA that the coaxial technique is superior to the emulsion for growth factor incorporation. Finally, FGF-2 delivery to MSCs from coaxially electrospun nanofibers was assessed using a cell activity assay. We developed a novel method for tuning the nanostructure of DBM through electrospinning without the use of a carrier polymer. This work surveys solvents and solvent blends for electrospinning DBM. The effects of DBM concentration and dissolution time on solution viscosity are reported and correlated to observed differences in fiber morphology. We also present a survey of techniques to stabilize the resultant fibers with respect to aqueous environments. Glutaraldehyde vapor treatment is successful at maintaining both macroscopic and microscopic structure of the electrospun DBM fibers. Finally, we report results from tensile testing of stabilized DBM nanofiber mats, and preliminary evaluation of their cytocompatibility. The DBM nanofiber mats exhibit good cytocompatibility toward human dermal fibroblasts (HDF) in a 4-day culture.Item Open Access Constitutive modeling of the biaxial mechanics of brain white matter(Colorado State University. Libraries, 2016) Labus, Kevin M., author; Puttlitz, Christian M., advisor; Donahue, Seth, committee member; Heyliger, Paul, committee member; James, Susan, committee memberIt is important to characterize the mechanical behavior of brain tissue to aid in the computational models used for simulated neurosurgery. Due to its anisotropy, it is of particular interest to develop constitutive models of white matter based on experimental data in order to define the material properties in computational models. White matter has been shown to exhibit anisotropic, hyperelastic, and viscoelastic properties. The majority of studies have focused on the shear or compressive properties, while few have tested the tensile properties of the brain. Brain tissue has not previously been tested in a multi-axial loading state, even though in vivo brain tissue is in a constant multi-axial stress state due to fluid pressure, and data from uniaxial experiments do not sufficiently describe multi-axial stresses. The main objective of this project was to characterize the biaxial tensile behavior of brain white matter via experimentation and constitutive modeling. A biaxial experiment was developed specifically for testing brain tissue. Experiments were performed at a quasi-static loading rate, and an Ogden anisotropic hyperelastic model was derived to fit the data. A structural analysis was performed on biaxially tested specimens to relate the structure to the mechanical behavior. The axonal orientation and distribution were measured via histology, and the axon area fraction was measured via transmission electron microscopy. The measured structural parameters were incorporated into the constitutive model. A probabilistic analysis was performed to compare the uncertainty in the stress predictions between models with and without structural parameters. Finally, dynamic biaxial experiments were performed to characterize the anisotropic viscoelastic properties of white matter. Biaxial stress-relaxation experiments were conducted to determine the appropriate form of a viscoelastic model. It was found that the data were accurately modeled by a quasi-linear viscoelastic formulation with an isotropic reduced relaxation tensor and an instantaneous elastic stress defined by an anisotropic Ogden model. Model fits to the stress-relaxation experiments were able to accurately predict the results of dynamic cyclic experiments. The resulting constitutive models from this project build upon previous models of brain white matter mechanics to include biaxial interactions and structural relations, thus improving computational model predictions.Item Open Access Development of a finite element model of supracondylar fractures stabilized with variable stiffness bone plates(Colorado State University. Libraries, 2019) Sutherland, Conor J., author; Puttlitz, Christian M., advisor; McGilvray, Kirk, advisor; Easley, Jeremiah, committee member; James, Susan, committee memberApproximately 10% of orthopaedic fracture fixation cases lead to non-union, requiring surgical intervention. Inadequate fixation device stiffness, which causes unwanted fracture gap motion, is believed to be one of the largest factor in poor healing as it prevents ideal tissue proliferation in the callus. By altering the thickness of orthopaedic bone plates, it was theorized that the fracture gap micro-mechanics could be controlled and driven towards conditions that accommodate good healing. The first goal of the project was to create computational FEA models of an ovine femoral supracondylar fracture stabilized with a plate of varying thickness. The models were used to investigate the mechanical behavior of the plate and the callus under different physiological loading conditions. The second goal of this study was to validate the computational model with bench-top experiments using an ex-vivo ovine femoral fracture model. To achieve these goals, novel plates were designed and manufactured with different stiffnesses (100%, 85%, and 66% relative stiffness) to be used to treat a femoral supracondylar fracture model in ovine test subjects; both in-vivo and ex-vivo. The FE models were shown to accurately predict the stress/strain mechanics on both bone and plate surfaces. Micromechanics (strain and pressure) predictions in the fracture gap were reported and used to make tissue type proliferation predictions based on previously reported mechanics envelopes corresponding to bone remodeling. The results indicated that changing plate thickness successfully altered the construct stiffness and consequently, the predicted healing tissue type at the fracture site. The FE methods described could help improve patient specific fracture care and reduce non-union rates clinically. However, further in vivo testing is required to validate the clinical significance of the methods described in this thesis.Item Open Access Development of an in vitro model of functional mitral valve regurgitation(Colorado State University. Libraries, 2012) Pouching, Kristal, author; Monnet, Eric, advisor; Orton, Christopher, committee member; James, Susan, committee memberFunctional or ischemic mitral regurgitation (FMR) is a common sequelae to various cardiomyopathies which result in altered cardiac geometry secondary to ventricular remodeling. The causative architectural changes are typified by apical tethering of the mitral valve leaflets by ventricular dilation and papillary muscle repositioning, and is usually accompanied by annular dilation and mitral valve leaflet malcoaptation. The resultant mitral regurgitation (MR) and consequent volume overload contributes to further ventricular remodeling and perpetuation of the clinical scenario. The present mainstay of surgical therapy involves annular undersizing with the use of annuloplasty rings. However, surgical interventions thus far have been limited by numerous shortcomings and inconsistent results emphasizing the need for continued research into the mechanics of FMR correction. In this study a novel invitro model of FMR utilizing explanted ovine hearts was introduced as a tool for investigating the mechanism of FMR and determining strategies aimed at correction. In the first phase of model development FMR was induced by either annular dilation or papillary muscle repositioning in a static flow system. Both techniques were individually able to significantly increase the regurgitant volume from baseline (annular dilation: baseline 15.5ml/10s to 78.7 ± 35.3 ml/10s, p =0.02, patch: baseline 7.6ml/10s to 67.4 ± 30.4 ml/10s, p =0.02) with no significant differences between the two groups and a marked increase in regurgitant volume noted when both techniques were applied together (p =0.0001). The devised technique of papillary muscle displacement by patch placement successfully recreated the outward rotation and increased LV sphericity (baseline: 3.25±0.7, patch: 2.34±0.6, p =0.0025) observed clinically. For the second phase of the study the developed model was investigated in a pulsatile flow system with FMR induced by posterior papillary muscle displacement only. A timed, positive pressure valve pump with a set rate of 80 simulated beats/min and approximate flow rate of 6L/min was used and procured results even more pronounced than that recorded for the static flow system (static flow system MR vol: 67.4±30.4ml/10s vs. pulsatile flow system MR vol: 310.5 ± 86.6ml/10s). The final investigation involved subjection of the developed FMR model to geometric modifications aimed at correcting MR in the pulsatile flow system. Attempts were made to correct the modeled papillary muscle displacement until the regurgitant volume was eliminated/minimized and the associated LV dimensions measured. The results showed that correction of the apical tethering of the chordae was sufficient to significantly reduce MR volume (patch: 310±86.6ml/10s vs. displacement correction: 16.1 ± 23.7ml/10s, p =0.0001) despite failure to return to baseline dimensions. In the developed model, which has been demonstrated to be amenable to both pulsatile and static flow systems, annular dilation and posterior papillary repositioning were both able to individually induce FMR and significant increases in regurgitant volume was noted once the two techniques were combined. The role of posterior papillary muscle repositioning in the correction of this disease was emphasized. The developed model provided evidence for the possibility of FMR elimination by geometric alterations beyond restoration of baseline/pre-disease dimensions with direct clinical implications to the surgical treatment of affected patients.Item Open Access Development of N-aryl phenoxazines as strongly reducing organic photoredox catalysts(Colorado State University. Libraries, 2020) McCarthy, Blaine Gould, author; Miyake, Garret, advisor; Bandar, Jeffrey, committee member; Krummel, Amber, committee member; James, Susan, committee memberN-aryl phenoxazines were identified as a new family of organic photoredox catalysts capable of effecting single electron transfer reductions from the photoexcited state. A number of phenoxazines bearing different N-aryl and core substituents were synthesized, characterized, and employed as catalysts. Spectroscopic and electrochemical characterization of these phenoxazines was used to establish structure-property relationships for the design of visible-light absorbing, strongly reducing organic photoredox catalysts. The application of phenoxazines as catalysts for organocatalyzed atom transfer radical polymerization (O-ATRP), a light-driven method for the synthesis of well-defined polymers, revealed the importance of several catalyst properties for achieving control over the polymerization. Investigation of the properties and catalytic performance of N-aryl phenoxazines has provided fundamental insight into the reactivity of organic excited state reductants and photophysical properties of organic molecules. The catalysts developed through this work provide sustainable alternatives to more commonly used precious-metal containing photoredox catalysts.Item Open Access Development of novel mechanical diagnostic techniques for early prediction of bone fracture healing outcome(Colorado State University. Libraries, 2021) Wolynski, Jakob G., author; McGilvray, Kirk, advisor; Puttlitz, Christian, advisor; Heyliger, Paul, committee member; James, Susan, committee member; Wang, Zhijie, committee memberTo view the abstract, please see the full text of the document.Item Open Access Effect of bone geometry on stress distribution patterns in the equine metacarpophalangeal joint(Colorado State University. Libraries, 2012) Easton, Katrina L., author; Kawcak, Chris, advisor; McIlwraith, Wayne, committee member; Puttlitz, Christian, committee member; James, Susan, committee member; Shelburne, Kevin, committee member; Heyliger, Paul, committee memberCatastrophic injury of the equine metacarpophalangeal joint is of major concern for both the equine practitioner and the American public. It is one of the major reasons for retirement and sometimes euthanasia of Thoroughbred racehorses. The most common type of catastrophic injury is fracture of the proximal sesamoid bones and lateral condyle of the third metacarpal bone. Many times these injuries are so disastrous that there is no possibility of fixing them. Even in the injuries that are able to be fixed, complications arising from the fracture such as support limb laminitis may ultimately lead to the demise of the horse. Therefore, prevention of these types of injuries is key. In order to decrease the incidence of injury, it is important to understand the risk factors and pathogenesis of disease that leads to them. This project was established to create a finite element model of the equine metacarpophalangeal joint in order to investigate possible risk factors, namely bone geometry, and its effect on the stress distribution pattern in the joint. The first part of the study involved in vitro experiments in order to provide a comprehensive dataset of ligament, tendon, and bone strain and pressure distribution in the joint with which to validate the finite element model. Eight forelimbs from eight different horses were tested on an MTS machine to a load equivalent to that found at the gallop. Beyond providing data for validation, the study was the first to the author's knowledge to measure surface contact pressure between the distal condyles of the third metacarpal bone and the proximal sesamoid bones. A pressure distribution pattern that could lead to an area of high tension in the area of the parasagittal groove was found. This result could help explain the high incidence of lateral condylar fractures that initiate in this location. The second part of this study focused on the development and validation of a finite element model of the metacarpophalangeal joint. A model was created based on computed tomography (CT) data. It included the third metacarpal bone, the proximal phalanx, the proximal sesamoid bones, the suspensory ligament, medial and lateral collateral ligaments, medial and lateral collateral sesamoidean ligaments, medial and lateral oblique sesamoidean ligaments, and the straight sesamoidean ligament. The mesh resolution was varied to create three models to allow for convergence. The converged model was then validated using data from the previous part of the study as well as data from the literature. The result was a finite element model containing 121,533 nodes, 112,633 hexahedral elements, and 10 non-linear springs. The final section of this study used the converged and validated finite element model to study the effect of varying bone geometry. The model was morphed based on CT data from three horses: control, lateral condylar fracture, and contralateral limb to lateral condylar fracture. There was an area similar between all three groups of increased stress in the palmar aspect of the parasagittal grooves where fractures are thought to initiate. Other results showed distinct differences in the stress distribution pattern between the three groups. Further investigation into these differences may help increase the understanding of a horse's predisposition to injury. In conclusion, this study has shown that joint geometry plays a role in the stress distribution patterns found in the equine metacarpophalangeal joint. The differences in these patterns between the three groups may help explain the increased risk of a catastrophic injury for some horses. Further studies are warranted to better define the parameters leading to these changes.Item Open Access Elastic free-standing RTIL composite membranes for CO2/N2 separation based on sphere-forming triblock/diblock copolymer blends(Colorado State University. Libraries, 2016) Wijayasekara, Dilanji B., author; Bailey, Travis S., advisor; Fisk, John D., committee member; Kipper, Matthew, committee member; James, Susan, committee memberThe main focus of this dissertation was the development of a robust polymeric membrane material for separating CO2 from a gas mixture of CO2 and N2. Flu gas, which is mainly a mixture CO2 and N2, is the single largest form of anthropogenic CO2 emissions to the atmosphere. Capturing CO2 from flu gas is considered as a measure of controlling anthropogenic CO2 emissions. Existing CO2 capturing technologies for flu gas suffer from low efficiency and the low cost effectiveness. Adoption of membrane technology is comparatively the best route towards the economical separations. Challenges faced by existing CO2 separation membrane materials are the lack of high mechanical robustness and the processability required for fabrication of membrane units while maximizing their gas separation properties. We were able to form a novel membrane material that addresses each of these challenges. These novel membranes are based on highly swollen, self-standing films produced using sphere-forming PS-PEO diblock and PS-PEO-PS triblock copolymer blends. The intricate connectivity among spherical domains produced during melt-state assembly (prior to swelling), provides a framework that remains elastically tough even in the presence of large quantities of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTf2N) - a room temperature ionic liquid (RTIL) that has high selectivity for CO2 over N2. Further investigations on improving the robustness of these membranes and the gas separation properties were carried out based on two scenarios. First, potential of improving the thermal stability of these membranes by replacing the thermoplastic polystyrene with a thermoset moiety such as a chemically cross-linked polyisoprene (PI) was researched. Cross-linking chemistry utilized required a post-polymerization modification of PI and it was found that this oxidation modification of olefins on PI caused the decoupling of triblock copolymer in the blend and also substantially hindered melt-state self assembly. The membranes formed with this modification turned out to have inferior mechanical properties compared to the polystyrene based ones, most likely due to the above mentioned complications. Due to the time restrictions, this study was limited to just the identification of the existing challenges in the proposed strategy. Recommendations for addressing the challenges identified are also presented later in the dissertation. The second scenario for improving the performance of these membranes was to increase their productivity by improving both the CO2 permeability and maximizing the trans-membrane pressure differentials possible during operation. To accomplish this we focused on the development of an alternative matrix material (alternative for PEO) enriched with ionic groups. The goal was to increase matrix solubility in the RTIL (improved CO2 permeability) while simultaneously strengthening matrix-RTIL interactions for reduced leaching under higher pressure differentials. Synthetic routes to achieve this task involved a sequential polymerization of isoprene and ethoxy ethyl glycidyl ether (EEGE) monomers. Polymerization of EEGE to yield high molecular weight linear blocks proved to be extremely challenging due to the undesirable chain transfer reaction tendency of EEGE monomer. A great deal of research effort was spent characterizing various anionic reaction conditions and developing measures aimed at suppressing chain transfer. While ultimately unsuccessful, the results of these studies provide significant insight into the challenges of forming high molecular weight linear polyglycidols and will hopefully provide inspiration for the development of future synthetically successful strategies. A series of proof of concept experiments for transforming alcohol functionalities on this polymer system to imidazolium was also completed successfully. The dissertation concludes with a final project completed outside the main objective of the dissertation - a morphological characterization of a series of thermoplastic elastomers with unique molecular architectures. This work is reported separately in the appendix I.Item Unknown Evaluation of osteogenic design factors in electrospun poly(ε-caprolactone) nanofiber scaffolds(Colorado State University. Libraries, 2010) Ruckh, Timothy T., author; Popat, Ketul, advisor; James, Susan, committee member; Kipper, Matt, committee member; Ryan, Stewart, committee memberBiodegradable bone tissue scaffolds have the potential to impact patients with numerous ailments. Starting with fabrication techniques that produce nano-scale features, the ability to manipulate architecture, alter surface chemistry, and deliver biological molecules allows for the design of elegant and highly effective bone scaffolds. This work aimed to develop a porous, nanofiber scaffold with osteogenic design features the capability to deliver an antibiotic molecule from within the nanofibers. Two osteogenic design factors with unique mechanisms of action were selected; hydroxyapatite nanoparticles and oleic acid. Hydroxyapatite (HAp) is the primary inorganic phase of natural bone tissue and has been used to more closely mimic the extracellular environment of synthetic bone tissue scaffolds. Oleic acid (OLA) is an ω-9 fatty acid with suspected osteogenic effects due to activation of peroxisome proliferator-activator receptors (PPARs). In separate in vitro evaluations, OLA significantly increased osteoblast phenotypic behaviors and led to differential expression of the three PPAR isoforms, suggesting that the OLA is activating its anticipated receptor. HAp produced mixed results by inducing a small increase in alkaline phosphatase activity, but decreasing expression levels of bone matrix proteins. An in vivo evaluation of biocompatibility revealed that neither design factor increased the inflammatory response over control nanofiber scaffolds in paravertebral muscle pouches. However, both factors separately increased new osteoid production. Scaffolds with both HAp and OLA elicited the greatest osteogenic response in vivo, suggesting positive synergy between the two design factors. Finally, rifampicin (RIF), an antibiotic molecule was loaded into the nanofibers, and its release into static bacterial culture was effective in inhibiting bacterial population growth for both a Gram-positive and Gram-negative bacterial strain, separately. Overall, these nanofiber scaffolds were demonstrated to be effective carriers of soluble (OLA, RIF) and insoluble signals (HAp) which can modulate cell behaviors. Future work will aim to incorporate additional osteogenic features into the scaffolds and to develop multiple antibiotic release mechanisms from the nanofibers.Item Unknown Fostering employee engagement through supervisory mentoring(Colorado State University. Libraries, 2015) Nowacki, Emily C., author; Byrne, Zinta S., advisor; Kraiger, Kurt, committee member; Vacha-Haase, Tammi, committee member; James, Susan, committee memberEmployee engagement is an increasingly salient topic in organizations given the reported financial, attitudinal, and behavioral gains of having an engaged workforce, and as such, considered a means for achieving effective performance. Supervisors are typically charged with motivating their employees to accomplish work effectively, primarily because of their proximity and often close relationship they have with their subordinates. Consequently, organizations have begun encouraging and expecting supervisors to foster employee engagement. However, little is known about how employees become engaged from observing, working with, and learning from their supervisors. This study contributes to the development of a new theory of how employees, as protégés, become engaged through mentoring received from their supervisors. Using self-report data from 173 employees, I explored the relationships between protégé engagement and perceived mentoring functions (role modeling, career-development, and psychosocial support) in the context of a supervisor-subordinate relationship. Results from this study highlight the theoretical value of mentoring functions, which are understudied aspects in the supervisor-subordinate relationship and are critical for leadership and future leader-development efforts. Thus, this study contributes not only to the theoretical advancement of work engagement, but also to the practical application of efforts to foster employee engagement and to an empirical understanding of how engagement is fostered through satisfaction of intrinsic needs and social learning mechanisms.Item Unknown Mesenchymal stem cell rescue for bone formation following stereotactic radiotherapy of osteosarcoma(Colorado State University. Libraries, 2013) Schwartz, Anthony L., author; Ehrhart, Nicole, advisor; Ryan, Stewart, committee member; James, Susan, committee member; Goodrich, Laurie, committee member; Custis, Jamie, committee memberBackground: Osteosarcoma (OSA) is the most common form of primary bone cancer in dogs and humans. Curative-intent treatment options include amputation, radiation therapy or surgical limb salvage for local tumor control combined with adjuvant chemotherapy for prevention or delay of metastatic disease. Stereotactic radiotherapy (SRT) delivers high dose per fraction radiation to a defined tumor volume with relative sparing of surrounding normal tissues. It has been successfully used as a non-surgical limb salvage procedure to achieve local tumor control of spontaneous OSA in dogs. The most common complication observed with this treatment is pathologic fracture of the irradiated bone. Mesenchymal stem cells (MSCs) are multipotent stem cells that have the capability to differentiate into many cell types including bone. The ability of MSCs to differentiate into bone suggests that they should be investigated as a potential therapy to regenerate bone in SRT treated bone. Methods: In experiments described herein, we developed an orthotopic model of canine osteosarcoma in athymic rats and evaluated the ability of SRT to achieve local tumor control. We then evaluated the ability of MSCs to regenerate bone after SRT treatment of OSA. Results: We demonstrated that the canine OSA cell line reliably engrafted in the rat femur. We characterized progression in order to create a reproducible model in which to replicate a clinical scenario to test MSC behavior following SRT of OSA. Two weeks after OSA cell inoculation was identified as the time period when the same clinical characteristics were observed as in canine OSA cases and was chosen to be an appropriate time for SRT treatment. The optimal SRT protocol to achieve local tumor control while minimizing acute radiation effects was determined to be 3 fractions of 12 Gy delivered on consecutive days. MSCs administered either intravenously or intraosseously 2 weeks after SRT revealed no new bone formation; however, decreased tumor necrosis was observed after MSC treatment. Conclusion: The results herein describe the characterization of an orthotopic rat model of canine OSA. This model was useful for the evaluation of different dose and fractionation SRT protocols along with combination adjuvant therapies that may be clinically relevant for canine or human OSA. The administration of MSCs following SRT did not induce new bone growth. The lack of efficacy is most likely due to the radiation-induced alterations to the bone microenvironment that resulted in conditions poorly suited to MSC survival and/or differentiation.Item Unknown Phase coding and frequency diversity for weather radars(Colorado State University. Libraries, 2020) Kumar, Mohit, author; Chandrasekar, V., advisor; Cheney, Margaret, committee member; James, Susan, committee member; Jayasumana, Anura, committee memberThis thesis has developed three main ideas: 1) Polyphase coding to achieve orthogonality between successive pulses leading to second trip suppression abilities, 2) Frequency diversity on a pulse to pulse basis to achieve second trip suppression and retrieval capability in a weather radar, 3) a multiple input, multiple output (MIMO) configuration using the orthogonality features obtained using ideas in 1 and 2. It is shown in this thesis that this configuration for a radar leads to better spatial resolution by the formation of a bigger virtual array. It is also demonstrated that orthogonality is a big requirement to get this improvement from a MIMO configuration. This thesis addresses this issue with a new polyphase code pair and mismatched filter based framework which gives excellent orthogonal features compared to a matched filter processor. The MIMO platform is a long term goal (technologically) and therefore the polyphase codes were used to demonstrate second trip suppression abilities that uses orthogonal features of these codes to reduce range and velocity ambiguity. These are called as Intra-pulse phase coding techniques. The thesis also demonstrates another technique to achieve orthogonality between pulses by coding them on different frequencies. This is termed as Inter-pulse frequency diversity coding. In the beginning, design and implementation of Intra-pulse polyphase codes and algorithms to generate these codes with good correlation properties are discussed. Next, frequency diversity technique is introduced and compared with other inter-pulse techniques. Other Inter-pulse coding schemes like that based on Chu codes are widely used for second trip suppression or cross-polarization isolation. But here, a novel technique is discussed taking advantage of frequency diverse waveforms. The simulations and tests are accomplished on D3R weather radar system. A new method is described to recover velocity and spectral width due to incoherence in samples from change of frequency pulse to pulse. It is shown that this technique can recover the weather radar moments over a much higher dynamic range of the other trip contamination as compared with the popular systematic phase codes, for second trip suppression and retrieval. For these new features to be incorporated in the D3R radar, it went through upgrade of the IF sections and digital receivers. The NASA dual-frequency, dual-polarization, Doppler radar (D3R) is an important ground validation tool for the global precipitation measurement (GPM) mission's dual-frequency precipitation radar (DPR). It has undergone extensive field trials starting in 2011 and continues to provide observations that enhance our scientific knowledge. This upgrade would enable more research frontiers to be explored with enhanced performance. In the thesis, this upgrade work is also discussed.Item Unknown Spinal cord and meningeal mechanics: viscoelastic characterization and computational modeling(Colorado State University. Libraries, 2018) Ramo, Nicole Lauren, author; Puttlitz, Christian M., advisor; Troyer, Kevin L., advisor; Heyliger, Paul, committee member; James, Susan, committee memberSuffering a spinal cord injury (SCI) can be physically, emotionally, and financially devastating. With the complex loading environment typically seen in SCI events, finite element (FE) computational models provide an important economical and ethical option for investigating the mechanical etiology of SCI, evaluating prevention techniques, and assessing clinical treatments. To this end, numerous research groups have developed FE models of the spinal cord using various degrees of material and structural sophistication. However, the level of model complexity that is necessary to achieve accurate predictions of SCI has not been explicitly investigated as few studies have reported applicable tissue behavior. What are reported in the literature as "spinal cord mechanical properties" are most commonly based on ex-vivo tests of the spinal-cord-pia-arachnoid construct (SCPC). The pia and arachnoid maters are fibrous meningeal tissues that closely envelope the spinal cord, and together are referred to as the pia-arachnoid-complex (PAC). Currently available data demonstrate the PAC's importance in the overall SCPC stiffness and shape restoration following compression. However, only one previous study has reported mechanical properties of isolated spinal PAC, and therefore, conclusions about its contribution to SCPC mechanics are largely unknown. Additionally, it has been shown that SCPC material properties begin to degrade within 90 minutes of death. Considering the experimental difficulties and ethical concerns associated with in-vivo mechanical testing of the SCPC, determining the relationship between in-vivo and ex-vivo viscoelastic properties would allow researchers to more accurately analyze existing ex-vivo data. Therefore, the overarching goal of this work is to address the current gaps in knowledge regarding spinal cord and meningeal tissue mechanics and incorporate the developed material models into a FE model. Comparisons of ex-vivo and in-vivo porcine SCPC non-linear viscoelastic behavior revealed significantly different acute behaviors where the ex-vivo condition exhibited a higher stress response but also relaxed quicker and to a greater extent than the in-vivo condition. Although it only made up less than 6% of the ovine SCPC volume, the PAC was found to significantly affect the non-linear viscoelastic behavior of the SCPC which supports the conclusion that it plays an important protective mechanical role. Examining the fitting and predictive accuracy of linear, quasi-linear, and non-linear viscoelastic formulations to SCPC, cord, and PAC stress-strain data, non-linear formulations are recommended to model the SCPC and cord response to arbitrary loading conditions while the QLV is recommended for the PAC. This work provides researchers with novel insights into the complex mechanical behavior of the spinal cord and PAC. The experimental results represent an important addition to the limited literature on in-vivo versus ex-vivo neural tissue viscoelastic properties; they are also the first to quantify the non-linear elastic behavior of spinal PAC and the non-linear viscoelastic properties of the isolated spinal cord. Finally, the computational portion of this work provides a detailed report of the effects of viscoelastic formulation complexity on FE model prediction accuracy and computational time allowing researchers interested in modeling SCI to make informed decisions about the balance of accuracy and efficiency necessary for their specific modeling efforts.Item Unknown 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 memberPoor 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.Item Open Access Synthesis, postsynthetic modification, and investigation of metal-organic frameworks for environmental and biological applications(Colorado State University. Libraries, 2018) Rubin, Heather N., author; Reynolds, Melissa, advisor; Chen, Eugene, committee member; Finke, Richard, committee member; Van Orden, Alan, committee member; James, Susan, committee memberMetal-organic frameworks (MOFs) are unique porous coordination polymers having record-high surface areas, and tunability at both the organic linkers and metal ions. As such, MOFs are advantageous for various applications including electronics, gas adsorption, and separations amongst others. Despite the advantages associated with MOFs, there are several key challenges that must be addressed in order to broadly expand the practicality of these materials. Such challenges include synthetic pitfalls, structural instability, selectivity, and inefficient heterogeneous catalysis. For instance, most MOFs are not stable in moisture-rich environments, which leads to structural collapse even in the open atmosphere. This instability poses a serious limitation for useful applications. In addition, the synthesis of MOF-related ligands is underdeveloped, which can lead to costly or inaccessible materials. To overcome these challenges, one goal of this research is to develop a solution to enhance the kinetic stability of MOFs to water and another is to execute an efficient and cost-effective synthetic strategy to generate the MOFs used herein. CuBTC (copper benzene-1,3,5-tricarboxylate), a commercially available MOF that has been well-studied and designated as having great potential for many applications, undergoes rapid degradation in humid atmospheres. Therefore, a novel synthetic approach was developed to efficiently access NH2BTC on gram-scale. Postsynthetic modification to the amine of the MOF powder material enhances the kinetic stability of the MOF to water. A distinct linear relationship between the number of carbons in the modification and observed water contact angle is described for the first time. This facilitates the first report of reliable access to mixed-ligand frameworks with predictable, calculated wettability and tunable kinetic stability to water. This work is also the first report of functionalizing copper MOFs as well as MOFs containing a benzene-1,3,5-tricarboxylate ligand to alter hydrophobic characteristics. That initial work inspired further exploration of CuNH2BTC as an antibacterial surface when synthetically grown on the surface of carboxymethylated cotton. The resultant material is capable of tunable Cu2+ ion release (via postsynthetic modification) and exceeds current industry standards for antibacterial agents, exhibiting a log-3 or greater reduction in bacteria both on the surface and in solution. As the scientific community continues to explore and understand MOFs, the implementation of these materials for various applications is dramatically increasing. As such, the second part of this research was devoted to applying and manipulating MOFs to better understand the interactions of MOFs with small molecules and ions. The photophysical properties of CuNH2BTC were investigated and specific interactions between anions and metal ions with MOFs were identified, encouraging the strategic design of MOFs to detect target-analytes via changing fluorescence emission properties in dimethylformamide (quenching or enhancing emission intensity or changing emission wavelength). This work provides a prerequisite study towards the development of improved next-generation MOF chemosensors. In addition, the open coordination site of thermodynamically stable porphyrin-based MOFs was exploited for simultaneous heavy metal detection and metal ion removal from aqueous solutions. Lastly, to better understand heterogenous catalysis with MOFs in biologically relevant media water, a 1HNMR method with solvent suppression was implemented and allows for kinetic and mechanistic studies of biologically relevant MOF-catalyzed decomposition of GSNO with thermodynamically stable MOF CuBTTri in H2O and eventually in blood. As a whole this research provides valuable insights as to how MOFs may be strategically designed, manipulated, and utilized for sensing, catalysis, and antibacterial applications.Item Open Access Viscoelastic characterization and modeling of musculoskeletal soft tissues(Colorado State University. Libraries, 2012) Troyer, Kevin Levi, author; Puttlitz, Christian, advisor; James, Susan, committee member; Heyliger, Paul, committee member; Dasi, Lakshmi, committee memberOver the last decade there has been a dramatic rise in musculoskeletal soft tissue injuries in the general, athletic, and military populations. The etiology of this increase has been largely ascribed to dynamic loading events, including strenuous physical overuse and trauma. Additionally, instability arising from soft tissue pathology or trauma can induce and/or accelerate joint degeneration. Degenerative sequelae, such as post-traumatic osteoarthritis, can cause significant debility and an associated reduction in one's quality of life. Development of successful treatment modalities for joint instability and soft tissue compromise is highly dependent upon a thorough understanding of the affected tissue's mechanical (viscoelastic) behavior. However, current soft tissue viscoelastic characterization paradigms predominantly utilize quasi-linear viscoelastic (QLV) formulae despite substantial empirical evidence which has conclusively demonstrated that these tissues violate its fundamental assumption of elastic and viscous behavior separability. Furthermore, development of more applicable nonlinear viscoelastic formulations has been hindered by the inability of currently-available constitutive models and characterization methodologies to include relaxation manifested during dynamic loading events. As a result, implementation of nonlinear viscoelastic formulae in soft tissue computational models has not been widespread. To surmount these shortcomings, this work develops a novel, nonlinear viscoelastic constitutive formulation and a corresponding experimental characterization technique which can be included in current state-of-the-art computational algorithms. Specifically, the aims of this dissertation were: (1) Develop and validate a nonlinear viscoelastic characterization technique for musculoskeletal soft tissues that incorporates relaxation manifested during loading; (2) Characterize the nonlinear viscoelastic behavior of various types of ligamentous tissues and tendon; (3) Integrate a fully nonlinear viscoelastic constitutive formulation into a finite element algorithm. Aims 1 and 2 were accomplished via development and application of a novel comprehensive viscoelastic characterization (CVC) technique and constitutive formulation to describe the nonlinear viscoelastic behavior of various human cervical spine ligaments (anterior and posterior longitudinal ligament and ligamentum flavum) and ovine Achilles tendon. Additionally, improvements in the predictive accuracy of the CVC fitted coefficients over previously accepted viscoelastic characterization techniques were quantified. Furthermore, a computationally tractable fully nonlinear viscoelastic formulation was developed and validated against an analytical solution (Aim 3). Implementation of the important nonlinear viscoelastic behavior into computational models will greatly accelerate our ability to understand the functional role of soft connective tissues in whole joint mechanics and facilitate future treatment options.