Theses and Dissertations
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Item Open Access Investigating group-V doping limits in CdSeTe and potential application of CdSe as tandem top-cells(Colorado State University. Libraries, 2025) Hill, Taylor, author; Sites, James, advisor; Sampath, Walajabad, committee member; Munshi, Amit, committee member; Sambur, Justin, committee member; Rockett, Angus, committee memberCadmium selenium tellurium alloys (CdSeXTe(1-X) known as CST) are a photovoltaic specialist's dream: with ideal single-junction and tandem top-cell bandgaps (based on Se stoichiometry) and large absorption coefficient for all stoichiometries (enabling thin-film applications), CST continues to be a promising material for photovoltaic applications. However, CST is not without its problems. Record efficiency CST devices have demonstrated short-circuit current density (JSC) and fill factor (FF) near their theoretical maximum based on measured bandgaps, but continued improvements to device performance has been limited by the open-circuit voltage (VOC), which has been less than 900mV (< 80% of theoretical maximum) for nearly a decade. Advances in absorber doping for p-type conversion have enabled increased carrier densities, moving from roughly 1014 cm-3 with group-I doping (copper) to 1016 cm-3 using group-V doping (arsenic or phosphorus), but increases in VOC have not been reflected by this fact. This is typically attributed to the so-called "dopant activation" problem, which accounts for the density of acceptor states provided per density of dopant incorporated and tends to be less than 10% in polycrystalline CST. This indicates that roughly 9 out of 10 dopant atoms form defects which may compensate p-type conversion and additionally hinder device performance. Meanwhile, the use of Se alloying to reducing the effective absorber bandgap has afforded increased JSC, but the roles in which Se and group-V dopants play in conjunction with typical device processing is not widely appreciated. In this work, the modern advancements which have allowed for record efficiency CdTe based devices, namely the incorporation of group-V dopants and Se alloying, are examined to address misunderstandings and provide a framework for improving device processing. An investigation into the impact of group-V dopant concentration in CST using optimized device processing conditions reveals that the density of acceptors formed by group-V doping tends to plateau at a point (roughly 1-5 X 1016 cm-3) and further incorporation of dopants tends to reduce device performance through increased radiative recombination. Evaluation of a novel process to increase group-V dopant activation, thereby reducing the concentration of nonactive dopant defects, is presented by use of ion implanted oxygen getters. The presence of oxygen in CST devices is inevitable and an oxidated dopant is effectively an inactive dopant. By implanting elements which have a higher affinity for oxidation relative to dopant atoms, the formation of dopant oxides is reduced, and an increased dopant activation is demonstrated. However, this did not improve device performance in practice, indicating that while the methodology of reducing group-V oxides can increase activation, the process of ion implantation itself may introduce additional lattice defects which negate the increased dopant activation. This leads to an examination of the role Se plays in intrinsic CST absorbers independent of group-V doping, revealing an unexpected n-type intrinsic conductivity, which may be a source of defects which compensate the use of group-V dopants. This indicates that work must be done to carefully balance the distribution of Se throughout the absorber bulk, where a concentration gradient, rather than a uniform ternary stoichiometry, is shown to enable the best performance. Finally, pure CdSe absorbers with a large bandgap of roughly 1.7 eV are examined for potential application in tandem PV devices. CdSe absorbers grown at CSU demonstrate the requisite large bandgap and provide insight into limitations based on absorber thickness. This leads to a discussion on CdSe devices with record VOC. To date, published record efficiency CdSe devices have shown >80% of theoretical short circuit current (JSC/JSCSQ) and >60% of theoretical fill factor (FF/FFSQ). However, such record devices have achieved <50% of the theoretical open circuit voltage (VOC/VOCSQ). The development of CdSe devices using novel transport and contact layer structures involving organic semiconductors and transition metal oxides to achieve >60% of VOCSQ (VOC >900 mV) is presented. The limitations of CdSe absorbers are addressed through temperature and intensity dependent photoluminescence measurements, indicating that low charge mobility due to intrinsic trap states in CdSe bulk are the primary limiting factor to further increasing VOC.Item Embargo Thermoplastic hydrogel elastomer composites through compatibilization with common copolymer end blocks(Colorado State University. Libraries, 2025) Morris, William Boston, author; Bailey, Travis S., advisor; Herrera-Alonso, Margarita, committee member; Prawel, David, committee member; Dandy, David, committee memberThe development of immiscible polymer-polymer composites that unify disparate material properties into a single bulk material has long been hindered by challenges in stabilizing incompatible polymer domains against macrophase separation while achieving sufficient interfacial adhesion for efficient mechanical load transfer. This work introduces a new class of polymer-polymer composites designed to address these challenges based on forming composites between styrenic block copolymer (SBC) thermoplastic elastomers (TPEs) containing immiscible copolymer midblocks with distinct material characteristics. Leveraging the inherent microphase separation behavior of ABA block copolymer systems, which are widely used in TPEs to generate reversible physical crosslinking, this work establishes a simple, scalable, and tunable method for fabricating tough, durable, and intrinsically lubricious hydrogel-elastomer composites; soft, elastic, hydrophilic hydrogel-silicone composites; and porous, hydrophilic elastomers. By combining TPE materials that share common vitreous polystyrene end blocks, each component forms its own phase-separated elastomeric network while simultaneously experiencing enhanced interfacial adhesion. Continuous vitreous domains formed at the component interfaces stabilize the blend morphology and enable mechanical load transfer between phases. Adjusting the compositional ratios and specific TPEs selected allow this composite platform to be tailored to meet a diverse range of performance specifications pertinent in medical devices, soft electronics, and membrane technologies. The thermoplastic hydrogel-elastomer and hydrogel-silicone composites developed in this work specifically address the persistent need for intrinsically lubricious, low-friction polydiene-based elastomers and silicones. Traditional elastomers, while flexible and durable, often suffer from tacky surfaces that require secondary lubricious treatments for medical device applications. Conversely, hydrogels offer excellent lubricity and poroelastic relaxation but typically lack scalable methods for introducing the needed mechanical strength and toughness. By integrating an SBC thermoplastic hydrogel phase into either commercial SBC TPEs or a newly developed PDMS-based SBC TPE, this work demonstrates the creation of versatile hydrogel composites featuring hydrophilic lubricious surfaces and outstanding mechanical durability while remaining thermally processable using standard processing techniques such as injection molding and extrusion, without the use of fluoropolymers. Their commercial application in minimally invasive intravascular catheter componentry was specifically explored through NSF's National I-Corps Program. Finally, by applying the same compatibilization strategy, a novel and straightforward method was developed for generating porosity into TPEs while simultaneously introducing a dense, uniform hydrophilic PEO brush layer conformally coating all internal pore surfaces. The mechanical viability of these porous elastomers towards potential membrane applications is examined.Item Embargo Discovering defect-tolerant hybrid perovskites for semiconductor applications(Colorado State University. Libraries, 2025) Asebiah, Dominic Cudjoe, author; Neilson, James R., advisor; Sambur, Justin B., committee member; Krapf, Diego, committee member; Rappé, Anthony K., committee memberHybrid organic-inorganic semiconductors with a perovskite crystal structure offer a promising pathway to developing defect-tolerant materials, yet their practical application is often hindered by sensitivity to environmental factors such as moisture, oxygen, and radiation, as well as significant structural disorder. This disorder arises from processing- induced and equilibrium defects and the flexibility of the metal-halide framework with mobile organic species. To better understand these effects, vacancy-ordered double perovskites (A2BX6), which feature isolated [BX6] octahedra connected by A-site cations, provide an ideal platform for studying defects and lattice dynamics in perovskite halide semiconductors. Additionally, tin-based perovskite semiconductors like (CH3NH3)SnI3 are prone to unintentional doping, compromising efficiency and performance. To address this issue, we explore how introducing specific tin vacancies can mitigate these challenges and improve stability and electronic properties. While strategies such as chemical substitution have been shown to suppress the decomposition of hybrid perovskites, the exact chemical and physical mechanisms responsible for these stabilizing effects remain unclear. Chapter two examines the solid solution (CH3NH3)1−xCsxSnBr3, focusing on how thermochemistry and structural distortions influence carrier behavior. In line with first principles and Boltzmann scattering predictions, increasing cesium narrows the optical gap but unexpectedly reduces carrier mobility. This is attributed to increased carrier density and scattering. Synchrotron X-ray scattering reveals cubic symmetry at room temperature, but local distortions suggest anharmonic atomic dynamics. Methylammonium-rich compositions retain linear Sn-Br-Sn bonding, while cesium-rich compositions favor bent environments, increasing defect formation and carrier trapping, explaining anomalous microwave transients. Chapter three reports on (NH3(CH2)7NH3)2Sn3I10, a vacancy-ordered perovskite with three-dimensional connectivity. Its structure resembles a Dion-Jacobson perovskite but with [SnI5] square pyramids bridging the layers. Optical studies reveal a sharp absorption onset, photoluminescence emission, and a large Stokes shift. The conductivity measurements show low carrier mobility and density, suggesting polaron-mediated transport. The equilibrium carrier density of (NH3(CH2)7NH3)2Sn3I10 was found to be remarkably low, despite being a Sn(II)-based material. Fast, fluence-dependent, non-radiative recombination indicates localized defect-like states. The photoluminescence behavior aligns with an asymmetric Sn(II) environment, highlighting defect ordering as a strategy to reduce mobile charge carriers. Chapter four explores the structure-electronic properties of (CH3NH3)1−xCsxPbBr3, highlighting a phase transition from cubic to orthorhombic structures with increasing Cs content and a corresponding shift in the optical bandgap. The TRMC measurements reveal that methylammonium-rich samples exhibit higher carrier densities, mobilities, and dielectric constants. Additionally, photoconductivity exhibits wavelength-dependent behavior, with mobility higher under 600 nm excitation than under 520 nm. Chapter five examines the solid solution between CH3NH3SnBr3 and CH3NH3PbBr3, which retains cubic symmetry at room temperature. The optical bandgap follows a nonmonotonic trend due to bandgap bowing. TRMC measurements reveal wavelength- dependent photoconductivity, with Pb-rich increased mobility under 520 nm. Carrier lifetime increases with Pb content. Chapter six investigates the solid solution between CsSnBr3 and CsPbBr3, revealing a phase transition from cubic to orthorhombic perovskite structures as Pb content increases. The optical bandgap shifts systematically, correlating with unit cell expansion. The TRMC measurements show a nonlinear increase in carrier mobility, while carrier density and conductivity decrease with Pb incorporation. Carrier lifetime is longer in CsSnBr3 than in CsPbBr3. Chapter seven examines the solid solutions of (CH3NH3)2Sn1−xTexI6 and Cs2Sn1−xTexI6, both crystallizing into a cubic vacancy-ordered double perovskite structure. The lattice constants shift systematically with increasing Te incorporation, correlating with optical bandgap trends. The pair distribution function analyses reveal significant local structural distortions in Sn-rich samples. The TRMC measurements indicate a nonlinear decrease in carrier lifetime, conductivity, and mobility with Te incorporation. Carrier mobility remains higher in Cs2Sn1−xTexI6, while carrier density follows a nonmonotonic trend in both systems. Chapter eight chapter explores the structure and electronic properties of (NH3(CH2)7NH3)2Bi2I10 and (NH3(CH2)7NH3)2Sn3I10, emphasizing vacancy ordering and charge transport behavior. The TRMC measurements show higher carrier density and longer lifetimes in (NH3(CH2)7NH3)2Bi2I10, while (NH3(CH2)7NH3)2Sn3I10 exhibits higher mobility due to enhanced non-radiative recombination. These insights highlight vacancy ordering as a key factor in optimizing hybrid optoelectronic materials. Chapter nine presents the synthesis and characterization of SnI2 using an-house PXRD and synchrotron X-ray diffraction data, revealing 99.952(18) wt% phase purity. The optical analysis revealed an indirect bandgap of 1.95(4) eV (636 nm). SnI2 exhibited lower carrier mobility than DFT predictions, with higher carrier density but a shorter lifetime. These findings highlight an efficient synthesis route for high-purity SnI2, reinforcing its significance for fundamental studies and applications. The lattice strain in hybrid perovskites is adjusted through complex interactions driven by organic cation dynamics and chemical substitution. These findings indicate that defect ordering reduces mobile charge carriers at equilibrium by moderating carrier trapping and atomic dynamics. The interplay between high-amplitude atomic motions and low defect formation energies in this highly anharmonic system further highlights the structure- dynamics-property relationship.Item Open Access Design & modeling of phase transforming ultra-high temperature metal ceramic multilayer composites(Colorado State University. Libraries, 2025) Stotts, John Carter, author; Weinberger, Christopher R., advisor; Gelfand, Martin, committee member; Ma, Kaka, committee member; Sambur, Justin, committee memberUltra-high temperature ceramics are a class of materials that have found use in high-temperature structural applications due to their high melting temperatures and excellent high-temperature mechanical properties. Although this class of materials is well-suited to these applications at high temperature, they suffer from a low fracture toughness at ambient temperatures where component fabrication and assembly takes place. Thus, during the fabrication and assembly process these materials are highly susceptible to catastrophic brittle failure. In this work, we introduce a novel type of composite that significantly improves the low-temperature fracture toughness without sacrificing the excellent high-temperature properties of the ultra-high temperature ceramics. These novel composites are an innovative approach to metal-ceramic multilayer composites, with the unique ability undergo a phase transformation in the metal layers that results in their disappearance, leaving a single-phase ultra-high temperature ceramic after annealing. In this work, we endeavored to model this phase transformation process and characterize the performance of the composite in order to optimize the material selection and design of the composite. To achieve this goal, we determined the phase transformation time of composites using Finite Element Method simulations and constructed more general, coarse-grained, models of the phase transformation kinetics and toughening. The key to enabling the phase transformation in these composites is to choose group IV transition metal carbides or nitrides and group V transition metal carbides for the ceramic layers of the composite. This group of materials possess a wide range of homogeneity with respect to carbon/nitrogen content in their monocarbide/nitride phases. Additionally, the phase transformation from metal to ceramic in these materials is controlled by the diffusion of the nonmetal atom (carbon/nitrogen) and results in ceramic layer growth with strong adhesion between layers. Thus, a composite can be constructed with a 'frozen-in' non-equilibrium microstructure containing alternating layers of metal and ceramic and be made to transform simply by increasing the temperature, i.e. annealing. Furthermore, this work contains an investigation into the kinetics of carbon diffusion in substoichiometric titanium carbide. This investigation, motivated by an open question in the literature posed by Sarian in 1968, used a computational approach comprised of Monte Carlo, kinetic Monte Carlo, and Density Functional Theory simulations in order to determine the interconnection of diffusion and vacancy-ordered phases. The investigation was multi-pronged, beginning with simulations on a square lattice and then being extended to three-dimension simulations of the titanium carbide carbon-vacancy sublattice.Item Embargo Understanding the electronical and optical properties of 2D TMDs in electrochemical cells for advanced solar energy and photocatalytic applications(Colorado State University. Libraries, 2025) Almaraz, Rafael, author; Sambur, Justin, advisor; Ackerson, Chris, committee member; Bailey, Travis, committee member; Neilson, Jamie, committee memberThis dissertation explores the potential of two-dimensional (2D) transition metal dichalcogenides (TMDs) for advanced solar energy conversion, focusing on the unique behavior of monolayer (ML) molybdenum disulfide (MoS2) in electrochemical environments. The research is motivated by the urgent need for cleaner energy sources, particularly solar energy, which has the potential to significantly reduce global dependence on fossil fuels. In Chapter 1, the introduction highlights the limitations of conventional semiconductors in solar energy conversion, such as thermalization losses, and emphasizes the need for efficient energy storage and conversion systems. The unique properties of 2D semiconductors, such as the potential for hot carrier extraction and tunable band gaps, are introduced as promising solutions to overcome these challenges. In Chapter 2, the theoretical framework for semiconductor photoelectrochemistry is discussed, with an emphasis on the challenges associated with interfacial electron transfer at semiconductor-liquid interfaces. Traditional semiconductor behavior, including band edge stability and electron transfer kinetics, is contrasted with the dynamic behavior observed in 2D materials. This chapter also introduces methods such as Mott-Schottky analysis and in situ spectroelectrochemical techniques to quantify band edge movement and energy levels in 2D semiconductors. Chapter 3 investigates the phenomenon of band gap renormalization (BGR), where the electronic bandgap of 2D materials shifts dramatically due to changes in carrier concentration. Using in situ spectroelectrochemical measurements, the research quantifies the BGR effect in ML-MoS2, showing significant band gap shifts of over 200 meV in response to changes in applied potential and redox conditions. The impact of these shifts on electron transfer kinetics is analyzed, revealing the potential to tune energy levels and enhance solar energy conversion efficiency. In Chapter 4, the research delves deeper into the BGR effect, demonstrating that charge equilibration at the semiconductor-redox electrolyte interface drives substantial band edge movement. By examining ML-MoS2 in various redox environments, the study reveals how redox potentials influence band gap renormalization, providing insights into the energetics of 2D semiconductor-electrolyte interfaces. This chapter highlights the fundamental differences between bulk and 2D semiconductors, opening new avenues for manipulating electron transfer kinetics in energy applications. Chapter 5 discusses ongoing efforts to harness high-energy quantum states in 2D materials for bio-inspired photocatalysis and solar fuel generation. The chapter emphasizes the potential of 2D semiconductors to capture and utilize high-energy carriers from the solar spectrum, offering a pathway to more efficient and sustainable energy conversion processes. Finally, Chapter 6 concludes the thesis with suggestions for subsequent investigations available based on the expertise and resources within the Sambur group at Colorado State University.Item Embargo Degradation and nano-scale structural evolution of geopolymers: effect of temperature, stress and mine process solutions(Colorado State University. Libraries, 2025) Piyathilake, S. A. K. V. M., author; Bareither, Christopher, advisor; Shackelford, Charles D., committee member; Yourdkhani, Mostafa, committee member; Herrera-Alonso, Margarita, committee memberWaste containment barrier systems commonly employ geosynthetics to protect human health and the environment against contaminant release. An example of a robust, composite liner system used to contain mine waste includes a textured polyethylene geomembrane (PE GMX) and a geosynthetic clay liner with two layers of polypropylene geotextiles (PP GCL). A key challenge in barrier system design is forecasting long-term performance of geosynthetic materials in various waste containment applications. A design objective in engineering practice is to predict material stability and lifespan of geomaterials used in waste containment infrastructure. With that objective in mind, this research focused on understanding the degradation mechanisms of polymers used in GMXs and GCLs (i.e., polyethylene and polypropylene), with particular emphasis on micro- and nano-scale changes to chemical and mechanical properties. A preliminary study was conducted to characterize oxidation behavior of geopolymers at room temperature. The analysis confirmed the formation of free radicals and oxidation degradation in the geopolymers. Subsequent studies on the same materials were conducted using in situ small/wide angle X-ray scattering (SAXS/WAXS) experiments with simultaneous tensile tests at elevated temperatures. These novel experiments revealed that chemical and mechanical treatments led to the degradation of the geopolymer, causing it to transition into a more crystalline state and lose its original elasticity. This understanding is vital for the effective use of geopolymers in applications like waste containment. Accelerated exposure experiments were conducted on samples of PE GMXs and PP GCLs in three different solutions (i.e., de-ionized water, bauxite mine process solution, and copper mine process solution) and at three temperatures (i.e., 20 °C, 50 °C, and 80 °C). Differential scanning calorimetry (DSC) and tensile tests were conducted on geopolymers at different durations of exposure up to 300 d. The study found that PE GMX and PP GCL degrade differently under various pH environments and temperature gradients. The degradation is influenced by factors such as the type of polymer, temperature, specific transition metals present, their concentration, and the conditions of the liquid medium. A model to predict the lifetime was developed, and the activation energies for the high and low exposure solutions were extracted.Item Embargo Zwitterionic polymeric nanoparticles for drug delivery(Colorado State University. Libraries, 2024) Lee, Jeonghun, author; Herrera-Alonso, Margarita, advisor; Bailey, Travis S., committee member; Popat, Ketul C., committee member; Peebles, Christie, committee memberBottlebrush block copolymers, characterized by their densely grafted side chains stemming from a highly persistent backbone, offer unique advantages for drug delivery, including enhanced micellar stability, reduced critical micelle concentration, and controlled surface topography, setting them apart from traditional linear polymers. This dissertation focuses on zwitterionic bottlebrush block copolymers (ZBCPs) composed of poly(D, L-lactide) (PLA) and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) side chains, synthesized via a combination of ring-opening and controlled radical polymerization using a grafting-from approach. ZBCPs were self-assembled into uniform spherical micelles and nanoparticles through direct dissolution and rapid mixing methods, and these self-assembled nanostructures were systematically evaluated. Compared to non-ionic PEG micelles (standard), zwitterionic bottlebrush micelles (ZBM) demonstrated superior stability under high salt conditions, elevated temperatures cycles, and in the presence of fetal bovine serum, whereas kinetically assembled nanoparticles (ZBNP) exhibited greater drug loading capacity. Both ZBM and ZBNP also showed excellent hemocompatibility, with ZBM displaying exceptional redispersibility in the absence of cryoprotectants. In parallel, this dissertation investigates boronic acid-functionalized zwitterionic polymers for drug delivery. A linear ABC-type amphiphilic copolymer containing poly(3-aminophenylboronic acid) as the central block was synthesized and compared to its non-functional counterpart. The boronic acid-containing nanoparticles exhibited pH- and oxidation-responsive behavior, enabling controlled drug release. Expanding this concept to a bottlebrush architecture, boronic acid-functionalized bottlebrush triblock copolymers were developed to further enhance nanoparticle performance. The inclusion of a boronic acid interlayer in the bottlebrushes significantly improved redispersibility of drug-loaded nanoparticles while maintaining high drug loading capacity, superior stability, and excellent hemocompatibility. This dissertation provides fundamental insights into solution-based self-assembled nanostructures derived from ZBCPs and boronic acid-functionalized polymers, establishing them as promising advanced drug delivery platforms. These systems offer tunable release kinetics, robust colloidal stability in harsh biological environments, excellent hemocompatibility, and superior redispersibility, thereby enhancing their translational potential in the field of nanomedicine.Item Open Access Development and implementation of novel drug delivery system for transdermal materials(Colorado State University. Libraries, 2024) Sun, Yu, author; Li, Yan Vivian, advisor; Liu, Jiangguo, advisor; Bailey, Travis, committee member; Gentry-Weeks, Claudia, committee memberThere is a constant need for developing transdermal dressing materials with advanced properties such as infection monitoring and wound closure facilitation for effective chronic wound treatments. On the other than, the advancement in drug delivery systems has created innovation of precise targeting of treatment, nontoxicity, cell access, controlled release profiles, treatment variations, and antibiotics activity conservation, which offers a great opportunity in developing novel wound dressing materials. Recently, the application of nanomaterials, especially nanoparticles, in drug delivery systems has shown great potential in effective wound treatment. In this proposal, three projects are focused on developing nanostructured scaffolds loaded with antibiotic agents for novel wound dressing applications. First, the antibiotic encapsulated poly (lactic-co-glycolic acid) (PLGA) nanoparticles will be integrated into monolithic nanofiber scaffolds that can be tested in transdermal materials for antibacterial properties. Second, the antibiotics loaded PLGA nanoparticles incorporated monolithic nanofiber scaffolds will be developed into core-shell fibrous scaffolds to provide a drug delivery system with controlled release of antibiotics. Integration of the nanoparticles into nanofibers includes monolithic and core-shell structures to provide controlled release of antibiotic agents. The mechanisms of controlled release are investigated via experimental and computational methods. Finite difference methods and machine learning are used for developing mathematical models capable of numerically quantifying antibiotic release rates, which provides a theoretical understanding of the release process in the nanostructure scaffolds. The work provides implication of utilizing PLGA nanoparticles in scaffolds to develop effective transdermal materials. Additionally, the computational models would provide tools to understand the mechanism of controlled release process, which may assist in the design of the nanoparticles as well as the nanostructured scaffolds.Item Embargo 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 memberRheumatic 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.Item Open Access 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 memberHealth 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.Item Open Access 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 memberThe 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.Item 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 memberSpark 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.Item 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 memberBlock 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.Item 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 memberThis 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.Item 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 memberFor 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.Item 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 memberIn 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.Item 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 memberWith 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%.Item 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 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 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 memberMayenite 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.Item 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 memberWound 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.