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Theses and Dissertations

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Browsing Theses and Dissertations by Author "Bailey, Travis S., committee member"

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    Control of polymer network structure and degradation
    (Colorado State University. Libraries, 2017) Yaghoubi Rad, Ima, author; Stansbury, Jeffrey W., advisor; Kipper, Matthew J., advisor; James, Susan P., committee member; Bailey, Travis S., committee member
    This thesis presents work performed evaluating controlled degradation in polymeric networks via incorporation of nanogels either as precursor or as a component of pseudo-interpenetrating polymeric networks. These polymeric crosslinked nanoparticles have applications in drug/gene delivery, cell imaging, and inert or functional prepolymer nano-fillers, therefore controlling their molecular weight (size) and structural properties are mandatory requirements. In addition to the primary effects of reactant selection on the nanogel formation, the solvent used and the agitation rate can provide additional parameters to control nanogel size. This work also develops a practical understanding of polymer characteristics and degradation kinetics of networks constructed from reactive nanogels with regiospecifically degradable linkages. Analogous non-degradable control structures are also prepared for each experimental condition. Clear, monolithic photopolymers are prepared from 50 wt% solvent-based dispersions of the reactive nanogels. The results of equilibrium swelling, mass loss, and compressive modulus (dry/swollen) demonstrate interplay between hydrophilic/hydrophobic effects, labile linkage location, and the crosslinking density that appears to dominate many of predicted property trends. The introduction of hydrolytically degradable linkages (PLA) into the internal crosslink structure of the nanogel promotes greater hydrophobic character compared to PLA placement in external reactive side chains. Consequently, nanogel-based networks with shorter hydrophilic crosslinker and lower crosslinker concentration show lower mass swelling rate, higher Tg, and lower compressive modulus reduction. Nanogels unlock an immense potential in designing superior alternatives for accepted materials with significantly reduced network heterogeneity compared to conventional hydrogels, which ultimately appoint them as novel candidates for controlled drug delivery and tissue engineering applications. Another part of this work demonstrates the effects of nano-scale pre-crosslinked hydrophobic particles as additive to model labile monomer on hydrolytic degradation. The modification of hydrolytically vulnerable polymers through the intimate integration of secondary networks based on styrenic nanogel structures is intended to reduce or even eliminate hydrolytic degradation potential. Nanogel addition at any level produces reduction in network swelling and mass loss proportional to nanogel content. The flexural modulus and ultimate transverse strength of nanogel-loaded resin monomer (TEGDMA) does not change compared to neat resin homopolymer as one control material in addition to the homopolymer of PEG2000PLADMA, which includes polylactic acid segments in the crosslinks. The use of a monomer-swollen highly crosslinked hydrophobic nanogel offers a versatile platform from which hydrolytic and potentially enzymatic degradation can be suppressed in a variety of applications such as polymer-based dental restoratives while retaining resin formulation, handling and mechanical properties. Some of the most important challenges in processing high performance materials are their high viscosity and limited solubility as a result of high molecular weight, intermolecular interactions, and rigid monomeric structure. Alternatively, high strength thermoset materials formed by ambient photopolymerization are limited in their performance by incomplete, vitrification-limited conversion, and relatively low glass transition temperature. In non-biological applications, significant effort has been focused on improving processing techniques and advanced machinery, and notably trivial attention has been paid to upgrade molecular structure of the resins. On the other hand, in biological applications photocuring is the perfect choice since applying high temperatures is practically impossible. As a result, another objective of this work was to develop an alternative photocurable material with enhanced processability, yet retaining thermal and mechanical properties of conventional resins. The average diameter of these polymeric particles is less than 20 nm with glass transition temperatures greater than 200 °C. These paradoxical properties trace back to molecular-level rearrangements of the same monomeric building blocks used in current thermoplastic/thermoset resins.
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    Disentangle model differences and fluctuation effects in DPD simulations of diblock copolymers
    (Colorado State University. Libraries, 2013) Sandhu, Paramvir, author; Wang, Qiang (David), advisor; Bailey, Travis S., committee member; Szamel, Grzegorz, committee member
    In the widely used dissipative particle dynamics (DPD) simulations 1, polymers are commonly modeled as discrete Gaussian chains interacting with soft, finite-range repulsions. In the original DPD simulations of microphase separation of diblock copolymer melts by Groot and Madden 2 , the simulation results were compared and found to be consistent with the phase diagram for the "standard model" of continuous Gaussian chains with Dirac δ-function interactions obtained from self-consistent field (SCF) calculations. Since SCF theory is a mean-field theory neglecting system fluctuations/correlations while DPD simulations fully incorporate such effects, the model differences are mixed with the fluctuation/correlation effects in their comparison. Here we report the SCF phase diagram for exactly the same model system as used in DPD simulations. Comparing our phase diagram with that for the standard model highlights the effects of chain discretization and finite-range interactions, while comparing our phase diagram with DPD simulation results unambiguously (without any parameter-fitting) reveal the effects of system fluctuations/correlations neglected in the SCF theory.
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    Fast off-lattice Monte Carlo simulations of phase transitions in block copolymers and liquid crystals
    (Colorado State University. Libraries, 2015) Zong, Jing, author; Wang, Qiang, advisor; Bailey, Travis S., committee member; Szamel, Grzegorz, committee member; Watson, A. Ted, committee member
    The basic idea of the so-called fast off-lattice Monte Carlo (FOMC) simulations is to perform particle-based Monte Carlo (MC) simulations in continuum with the excluded-volume interactions modeled by soft repulsive potentials that allow particle complete overlapping, where using soft potentials naturally arises from the application of coarse-grained models. This method is particularly suitable for the study of equilibrium properties of soft matter. One apparent advantage of FOMC is that using soft potentials can greatly improve the sampling efficiency in the simulations. Another advantage is that FOMC simulations can be performed in any statistical ensemble, and all the advanced off-lattice MC techniques proposed to date can be readily applied to further improve the sampling efficiency. Moreover, it provides a powerful methodology to directly compare theoretical results with simulation results without any parameter fitting. Last but not least, using FOMC is the only way to study experimentally accessible fluctuation/correlation effects in many-chain systems. This work makes use of FOMC simulations to study phase transitions in block copolymers and liquid crystals. To compare with the simulations results, various theoretical methods are also applied in the research. Chapter 2 is devoted to study the classic yet unsolved problem of fluctuation/correlation effects on the order-disorder transition (ODT) of symmetric diblock copolymer (DBC). In Chapter 3, we highlight the importance of quantitative and parameter-fitting-free comparisons among different models/methods. In Chapter 4, we investigate the effect of system compressibility on the ODT of DBC melts. In Chapter 5, we extend FOMC simulations to study the isotropic-nematic transition of liquid crystals. Finally, in Chapter 6, we briefly summarize all the studies in this dissertation and give some directions to future work.
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    Membrane adsorbers and novel affinity peptides for recombinant protein purification
    (Colorado State University. Libraries, 2015) Weaver, Justin, author; Wickramasinghne, S. Ranil, advisor; Qian, Xianghong, committee member; Carlson, Jon O., committee member; Bailey, Travis S., committee member
    The purification of recombinant proteins for use as pharmaceutically active ingredients represents one of the largest costs of drug development and production. Of the different classes of recombinant protein therapeutics monoclonal antibodies represent the largest percentage of protein therapeutics currently on the market with even more in clinical development. The work presented in this thesis describes the evaluation of both commercial and newly designed anion exchange and hydrophobic interaction (HIC) membrane adsorbers as well as identification of novel affinity peptides for the purification of recombinant proteins, specifically monoclonal antibodies. Commercially available anion-exchange membrane adsorbers were evaluated for their potential to remove impurities commonly present at low concentration in recombinant protein solutions expressed in mammalian cell culture. These so-called trace impurities include virus, host cell proteins, and DNA; these impurities are of particular concern because they are highly immunogenic at very low concentrations. Ionic strength and pH were shown to be the dominant factors affecting impurity binding on quaternary amine (Q) membranes indicating these ligands interact with the impurities primarily through electrostatic interactions. It is likely impurity interactions with primary amine ligands involved not only electrostatic but hydrogen bonding interactions which stabilized impurity-ligand interactions enabling greater removal at a broader range of solution pH and ionic strength conditions. Binding of host cell proteins with a broad range of isoelectric points was also demonstrated using the primary amine ligand as compared to the Q ligands. The effect of solution pH, ionic strength, flow rate, and the presence of competing anionic species was investigated. In addition to commercially available anion-exchange membrane adsorbers novel anion-exchange membranes, developed by Dr. Bharat Bhut and Prof. Scott Husson at Clemson University, were evaluated for binding capacity and virus removal. Regenerated cellulose microfiltration membranes were modified with a negatively-charged quaternary amine polymer, systematically varying the polymer chain density and length. IgG and DNA binding capacity, as well as minute virus of mice removal, was evaluated as a function of polymer chain density and length. It was shown that IgG binding capacity increased with polymer chain density indicating IgG access to binding sites was not a limiting factor. Similarly, high polymer chain density and longer polymerization time (translating to longer polymer chain length) resulted in higher DNA binding and virus removal again indicating ligand accessibility was not an issue even with large solutes such as virus. Environmentally-responsive hydrophobic interaction membranes were also developed in the Wickramasinghe lab and evaluated for protein binding capacity and recovery. Three-dimensional polymer brushes were grafted from 0.45 µm pore size regenerated cellulose membrane surfaces. The dynamic binding capacity of human IgG was greater than current commercially available hydrophobic interaction membranes with comparable recoveries. Affinity purification using novel small peptides was also explored as an antibody purification tool. Several heptapeptide affinity ligands were identified that bound specifically to the Fc region of IgG. These peptides have similar function to Staphylococcus Aureus Protein A, which is used extensively as an affinity purification ligand for monoclonal antibodies in the pharmaceutical industry. A large library of seven amino acid-long peptides was screened via M13 Phage Display for specific binding to the Fc, or constant region, of human IgG antibody. After initial identification, specificity of binding only to IgG was demonstrated through subsequent competitive ELISA assays. Though the affinity peptides were initially screened against human IgG₄ Fc, binding to a larger subset of human and non-human antibodies was shown indicating the peptides were binding to highly conserved regions on the antibodies. Because Protein A has some limitations in industrial process applications, these novel heptapeptides may provide an alternative solution for affinity purification of monoclonal antibodies.
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    Polysaccharide-based nanostructures for growth factor delivery and mesenchymal stem cell activation
    (Colorado State University. Libraries, 2011) Almodovar Montanez, Jorge Luis, author; Kipper, Matt J., advisor; Bailey, Travis S., committee member; Kisiday, John D., committee member; Prasad, Ashok, committee member
    Mesenchymal stem cells (MSCs) are very promising in tissue engineering and regenerative medicine because of their ability to differentiate into different type of cells including bone and cartilage. MSCs differentiation can be modulated using both chemical (i.e. proteins) and physical cues (ie. topography). This thesis presents work performed evaluating polysaccharide-based nanostructures for growth factor delivery and MSCs activation. Different polysaccharide-based nanostructures were developed and characterized including polyelectrolyte multilayers (PEMs) and electrospun nanofibers. On flat gold-coated glass surfaces, PEMs were constructed using the polycations chitosan and N,N,N -trimethyl chitosan, and the polyanions hyaluronan, chondroitin sulfate, and heparin. An exhaustive spectroscopic study was performed on all of the PEMs pairs to investigate the effects of polyelectrolyte charge density on thickness, swelling, composition, and ion-pairing. The results demonstrated that hydrophilicity and swelling are reduced when one polyelectrolyte is strong and the other is weak, while ion pairing is increased. The stability of adsorbed proteins to PEMs was also investigated using IR spectroscopy. Construction of PEMs and adsorption of basic fibroblast growth factor (FGF-2) was evaluated on heparin chitosan PEMs constructed on gold-coated glass, tissue culture polystyrene (TCPS), and titanium. In vitro testing of the FGF-2-loaded PEM constructed on TCPS and titanium was performed using ovine bone marrow-derived MSCs. It was noted that FGF-2 activity is enhanced, with regards to MSCs proliferation, when delivered from PEMs compared to delivery in solution. Chitosan nanofibers were successfully electrospun from a trifluoroacetic acid and dichloromethane solution. A new technique was developed to modify electrospun chitosan nanofibers with polyelectrolyte multilayers using N,N,N -trimethyl chitosan and heparin. Controlled release of bioactive FGF-2, complexed with heparin-chitosan polyelectrolyte complex nanoparticles, from electrospun chitosan nanofiber mats was achieved with zero order kinetics over a period of 27 days. When the nanofibers are further modified with a single PEM bilayer (PEM, composed of N,N,N -trimethyl chitosan and heparin), the release is completely prevented. The mitogenic activity of the released FGF-2 was also evaluated, with respect to the proliferation of ovine bone marrow-derived MSCs. The effect on osteogenic differentiation of bone marrow-derived ovine and equine MSCs seeded on electrospun chitosan nanofibers versus flat TCPS was investigated. The effect of dexamethasone on osteogenic differentiation was also investigated. We found that we can successfully grow and maintain both equine and ovine MSCs on electrospun chitosan nanofibers. Also, both MSCs exhibit higher differentiation markers (alkaline phosphatase activity) when cultured on chitosan nanofibers compared to flat TCPS surfaces. This work demonstrates new systems for stabilizing and controlling the delivery of heparin-binding growth factors for the activation of bone marrow-derived MSCs, using polysaccharide-based nanomaterials. These novel materials have potential applications in musculoskeletal tissue regeneration.