Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item SCAFFOLD DESIGN PARAMETERS TO STIMULATE THE OSTEOGENIC SIGNAL EXPRESSION FOR BONE TISSUE ENGINEERING APPLICATIONS(2010) KIM, KYOBUM; Fisher, John P; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The fundamental components of bone tissue engineering are (a) progenitor cells which subsequently express tissue matrix, (b) scaffolds which can act as temporary frameworks to support bone growth, and (c) growth factors to induce osteoblast regeneration. A variety of growth factors are involved during the differentiation cascade and these chemical and biological signals dynamically interact with cell populations to facilitate the differentiation. Therefore, enhanced expression of endogenous growth factor genes might facilitate abundant existence of growth factors in the surrounding microenvironment, stimulate the osteogenic differentiation of progenitor cell population, and finally induce bone regeneration. This work is focused on the augmentation of osteogenic signal expressions to stimulate the downstream differentiation of transplanted bone marrow stromal cells (BMSCs) population through the optimization of a variety of properties of three dimensional (3D) biodegradable poly(propylene fumarate) (PPF) scaffold. Changes in the microenvironment of cell population would affect the responses of localized cell population and the manipulated scaffold properties might be associated with induction of endogenous osteogenic signal expressions. First, the effect of cell-to-cell paracrine signaling distance, which can by modulated by initial cell seeding density, on the osteogenic signal expressions and osteoblastic differentiation of BMSCs on 2D PPF disks was investigated. Next, in order to investigate the improvement of the 3D macroporous PPF scaffold by the incorporation with nanoparticle filler materials, PPF/hydroxyapatite (HA) nanocomposite scaffolds were fabricated. The effect of HA content and initial cell seeding density on the osteogenic signal expression in 3D porous system was then determined. Finally, the incorporation of diethyl dumarate (DEF) with PPF was tested based on the photocrosslinking characteristics of PPF/DEF composite material with increased mechanical properties. The effect of two scaffold design parameters including the stiffness by modulating the DEF content as well as the pore size of porous scaffold on the signal expression and downstream osteoblastic differentiation was investigated. In addition, the feasibility of PPPF/DEF materials for stereolithographical fabrication was also tested in this work. Controlling these construction parameters to optimize engineered bone substitutes could affect various cellular functions of attachment, proliferation, signal expression, and differentiation. This research provided the insight of stimulation of the expression of target endogenous genes to induce the osteogenic differentiation and bone regeneration as well as the fabrication of improved bone substitute implant materials which is clinically applicable.Item A MULTISCALE MODEL FOR AN ATOMIC LAYER DEPOSITION PROCESS(2010) Dwivedi, Vivek Hari; Adomaitis, Raymond A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atomic layer deposition (ALD) is a deposition technique suitable for the con- trolled growth of thin films. During ALD, precursor gasses are supplied to the reactor in an alternating sequence producing individual atomic layers through self- limiting reactions. Thin films are grown conformally with atomic layer control over surfaces with topographical features. A very promising material system for ALD growth is aluminum oxide. Alu- minum oxide is highly desirable for both its physical and electronic characteristics. Aluminum oxide has a very high band gap (~ 9 ev) and a high dielectric constant (k ~ 9). The choice of precursors for aluminum oxide atomic layer deposition vary from aluminum halide, alkyl, and alkoxides for aluminum-containing molecules; for oxygen-containing molecules choices include oxygen, water, hydrogen peroxide and ozone. For this work a multiscale simulation is presented where aluminum oxide is deposited inside anodic aluminum oxide (AAO) pores for the purposes of tuning the pore diameter. Controlling the pore diameter is an import step in the conversion of AAO into nanostructered catalytic membranes (NCM). Shrinking the pore size to a desired radius allows for the control of the residence time for molecules entering the pore and a method for molecular filtration. Furthermore pore diameter control would allow for the optimization of precursor doses making this a green process. Inherently, the ALD of AAO is characterized by a slow and a faster time scale where film growth is on the order of minutes and hours and surface reactions are near instantaneous. Likewise there are two length scales: film thickness and composition on the order of nanometers and pore length on the order of microns. The surface growth is modeled in terms of a lattice Monte Carlo simulation while the diffusion of the precursor gas along the length of the pore is modeled as a Knudsen diffusion based transport model.Item Computational studies of droplet motion and deformation in a microfluidic channel with a constriction(2010) Lee, Moon Soo; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the present thesis, we investigate the interfacial dynamics of a three-dimensional droplet in a viscous fluid flowing through a square microfluidic channel with a rectangular cross-sectional constriction. The effects of various parameters of the two fluids and the sizes of the constriction geometry are considered. The numerical computation for the current problem requires a highly-accurate and efficient method owing to the very small/large deformation of the droplet shape at low/high flow rates, the small droplet-solid gap and the complicated three-dimensional geometries. An efficient fully-implicit three-dimensional Spectral Boundary Element method developed by Dimitrakopoulos is employed. Our results show that the droplet dynamics is significantly influenced by the non-symmetric shape of the rectangular cross-sectional constriction, i.e. owing to the constriction shape the droplet deforms much less in the flow-direction by forming a flat disk shape. As the capillary number is decreased, the droplet deformation in the flow-direction decreases owing to the larger surface tension. The effects of the viscosity ratio are complicated with viscosity ratio near unity showing the largest deformation.Item COMPUTATIONAL STUDIES ON DROPLET DYNAMICS AT INTERSECTING FLOWS IN MICROFLUIDIC JUNCTIONS(2010) Mamidi, Sai Kishore Reddy; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The current thesis involves a computational study of drop dynamics in microfluidic junctions, at the moderate capillary number of Ca = 0.1. We utilize a three-dimensional Spectral Boundary Element algorithm to determine the drop motion in the presence of intersecting lateral flows in microfluidic, T-junctions and cross-junctions, and analyze the effect on drop deformation and motion with varying shear rates in the channels leading to the junctions, and for viscosity ratios of 0.2 and 20.0 between the drop and the surrounding fluid. We find that the presence of intersecting flows, drastically affects the transient behavior at the junctions, and the drop reaches steady state further away, both up- stream and downstream of these junctions. The time taken to reach steady state in the T-junctions was found to be significantly greater than that in the cross-junction, under identical conditions. Drop velocities were found to be a linear function of the effective shear rate in the channel, and length scale fluctuations as high as 30 percent were observed in the junction region for the cases studied in the thesis. We observed that the excess presure drop with respect to the flow of a single phase fluid was strongly related to the length of the droplet at a given spatial coordinate. The peak surface area of the drop in the junction was found to be a slighly non-linear function of the flow rates in the lateral channels, and almost all the surface area increase was occurring at the head of the drop, in the direction of the flow. Velocity was found to be a weak, inverse function of the viscosity ratio, the increase in drop surface area was found to be greater in drops with lower viscosity. It was found that the junction bend radius/smoothness had a more significant effect on the dynamics of the drop in a T-junction, compared to that in a cross-junction.Item NANOSTRUCTURED THIN FILM POLYMER ELECTROLYTES FOR FLEXIBLE BATTERY APPLICATIONS(2009) Ghosh, Ayan; Kofinas, Peter; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years, the interest in polymeric batteries has increased dramatically. With the advent of lithium ion batteries being used in cell phones and laptop computers, the search for an all solid state battery has continued. Current configurations have a liquid or gel electrolyte along with a separator between the anode and cathode. This leads to problems with electrolyte loss and decreased performance over time. The highly reactive nature of these electrolytes necessitates the use of protective enclosures which add to the size and bulk of the battery. Polymer electrolytes are more compliant than conventional inorganic glass or ceramic electrolytes. The goal of this work was to design and investigate novel nanoscale polymer electrolyte flexible thin films based on the self-assembly of block copolymers. Block copolymers were synthesized, consisting of a larger PEO block and a smaller block consisting of random copolymer of methyl methacrylate (MMA) and the lithium salt of methacrylic acid (MAALi). The diblock copolymer [PEO-b-(PMMA-ran-PMAALi)] with added lithium bis(oxalato)borate, LiBC4O8 (LiBOB) salt (in the molar ratio ethylene oxide:LiBOB = 3:1) was used to form flexible translucent films which exhibited nearly two orders of magnitude greater conductivity than that shown by traditional high molecular weight PEO homopolymer electrolytes, in the absence of ceramic fillers and similar additives. The presence of the smaller second block and the plasticizing effect of the bulky lithium salt were shown to effectively reduce the crystallinity of the solid electrolyte, resulting in improved ion transporting behavior. The tailored solid self-assembled diblock copolymer electrolyte matrix also exhibits an exceptionally high lithium-ion transference number of 0.9, compared to a value between 0.2 and 0.5, shown by typical polymer-lithium salt materials. The electrolyte material also has a wide electrochemical stability window and excellent interfacial behavior with lithium metal electrode. The combination of these properties make electrolyte membranes composed of the diblock copolymer PEO-b-(PMMA-ran-PMAALi) and LiBOB salt, viable electrolyte candidates for flexible lithium ion based energy conversion/storage devices.Item Evaluation of the transcription of small RNA SgrS and glucose transporter mRNA ptsG in E. coli B and E. coli K cultures under high glucose conditions(2009) Ng, Weng Ian; Wang, Nam Sun; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Escherichia coli is commonly used as the production system for recombinant proteins. However, acetate accumulation in fermentation affects cell growth and protein yield. Recent studies have showed that the small RNA SgrS regulates the major glucose transporter mRNA ptsG in a post–transcriptional manner when the metabolic intermediate glucose–6–phosphate is accumulated intracellularly in E. coli K. Here, comparative analysis of the transcription of SgrS and ptsG is performed between E. coli B and E. coli K cultures in both shake flasks and bioreactor. Both strains expressed SgrS when grown on the non–metabolizable glucose analog α–methyl–glucoside. However, under high glucose conditions, only E. coli B showed significant expression of SgrS. This behavior is unaffected by oxygen supply and pH control. E. coli B produced less acetate on glucose than E. coli K in the bioreactor settings. This provides evidence of a possible connection between SgrS and acetate production in aerobic fermentation of E. coli.Item Self-Assembled Photoresponsive and Thermoresponsive Nanostructures(2009) Sun, Kunshan; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Responsive complex fluids based on nanostructures (e.g., micelles, vesicles and nanoparticles) have received considerable attention recently. The ability of these materials to be tuned by light or heat can have many potential applications in the areas of drug delivery, coatings, sensors, or microfluidic valves and dampers. However, most current photoresponsive and thermoresponsive formulations require the synthesis of complex organic molecules, and this prevents them from being used widely for commercial applications. In this dissertation, we seek to develop new classes of photoresponsive (PR) and thermoresponsive (TR) nanostructures based on commercially available, inexpensive precursors. In the first part of this study, we report a new PR fluid based on light-activated nanoparticle assembly. Our system consists of disk-like nanoparticles of laponite along with a surfactant stabilizer (Pluronic F127) and the photoacid generator (PAG), diphenyliodonium-2-carboxylate monohydrate. Initially, the nanoparticles are sterically stabilized by the surfactant and the result is a stable, low-viscosity dispersion. Upon UV irradiation, the PAG gets photolyzed, lowering the pH by about 3 units. In turn, the stabilizing surfactant is displaced from the negatively charged faces of the nanoparticle disks while the edges of the disks become positively charged. The particles are thereby induced to assemble into a 3 dimensional "house-of-cards" network that extends through the sample volume. The net result is a light-induced sol to gel transition, i.e., from a low, water-like viscosity to an infinite viscosity and yield stress. The yield stress of the photogel is sufficiently high to support the weight of small objects. The gel can be converted back to a sol by either increasing the pH or the surfactant content. Evidence for the above mechanism is provided from a variety of techniques, including small-angle neutron scattering (SANS). In the second part of this study, we demonstrate that laponite/PF127 mixtures also show thermogelling, i.e., the fluids transform from low viscosity sols to stiff gels upon heating above a critical temperature. This phenomenon is reversible and it requires the presence of sufficient amounts of both components. At room temperature, PF127 adsorbs onto laponite disks and stabilizes them by steric repulsion. Upon heating, the PF127 layer on the disks becomes thicker, and more importantly, PF127 micelles in the bulk solution grow significantly. Evidence for the growth of micelles is presented from SANS modeling and from transmission electron microscopy (TEM). At a distinct temperature, we believe the micelles induce depletion flocculation of the laponite particles into a gel network. Interestingly, if the PF127 concentration is increased further, the thermogelling is eliminated - this is suggested to be due to the micelles providing depletion stabilization of the particles.Item Polymer capsules as building blocks for soft, connected mesostructures(2009) George, Elijah; Raghavan, Srinivasa; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We show that polymer capsules can serve as soft building blocks for creating a range of mesoscale (0.1 to 10 mm) structures. The central innovation is a new approach for connecting spherical capsules by exploiting electrostatic complexation. Using this approach, connected structures with complex shapes can be easily assembled, and more importantly, a single connected structure can be made to have a diverse array of functions. The modular approach to shape and function is very much like using Lego bricks of different colors. The connected structures can be made responsive (capable of being actuated) by magnetic fields by including magnetic capsules within them. One motivation for creating these structures is to mimic the mechanics and motility of small creatures such as the earthworm or ant - this could eventually enable the design of autonomous biomimetic robots. In addition, soft connected structures could be employed to transport cargo such as drugs or proteins in blood vessels, or to construct valves, rotors, or mixers in microfluidic or lab-on-a-chip devices.Item A Simplified Model of Planetary Chemical Vapor Deposition Reactors(2009) Shahshahan, Negin; Adomaitis, Raymond A.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A simplified model for planetary chemical vapor deposition reactors is proposed and used to compute deposition species mole fraction and deposition rate in the reactor depletion zone. First, the modeling and optimization work performed in the literature is reviewed and their representative deposition rate profiles are extracted. Afterwards, several simplifying assumptions are applied to derive the reactor modeling equation, and the eigenfunction expansion solution is subsequently computed using a previously developed MATLAB object-oriented computational framework. The simulation result for the deposition profile is improved by modifying the inlet boundary condition, and is then compared with the previously published profiles. The MATLAB optimization toolbox is used to find the optimal deposition profile giving the best match with the published, detailed simulator profiles. Finally, an evaluation of the model consistency with the published results is given.Item PHASE BEHAVIOR AND INTERFACIAL PHENOMENA IN TERNARY SYSTEMS(2009) Subramanian, Deepa; Anisimov, Mikhail A; Adomaitis, Raymond A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phase behavior in multi-component systems has a wide variety of applications in the chemical process industry. In this work, the interfaces in two-phase, three-component systems were modeled and studied. Direct calculations of the asymmetric concentration profiles near the critical points of fluid phase separation are very difficult since they are affected by mesoscopic fluctuations. In this study a "complete scaling" approach was used to model interfacial profiles for a highly asymmetric, dilute ternary mixture near the critical point of liquid-liquid separation. The symmetric order parameter profile, the density profile of the lattice gas model, was used to further calculate the asymmetric interfacial concentration profiles at the mesoscale. Fluid asymmetry has been introduced through mixing of the physical field variables into the symmetric scaling theoretical fields. The system-dependent mixing coefficients were calculated from experimental data and a mean-field equation of state, namely, the Margules model. The resultant interfacial profiles for the concentration of water across the methanol-rich and cyclohexane-rich phases show the asymmetry associated with the contribution of the entropy into the symmetric order parameter profile.