Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
9 results
Search Results
Item DEFORMATION MECHANICS OF SOFT MATTER UNDER EXTERNAL STIMULI(2019) Cheng, Jian; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Artificial soft matters are a class of materials which can be easily deformed by external stress, typical examples include foams, colloids, elastomers, and hydrogels. Due to their unprecedented and unique properties, such as large deformability, high resemblance to biological systems, versatile response to multi-physical stimuli, and biological compatibility, soft matters have found applications in fields like soft actuators and robots, soft sensors, bio-mimicking material systems, micro-fluidic system control, biomedical engineering, etc. In these applications, the large deformability of soft matters has taken an enabling role. The deformation theory of polymeric soft matters can date back to 1940s in the early infancy of the statistical mechanics sketch of rubbery materials, with a fast growth in the most recent decade concurring the latest progress in soft matters. However, the mechanical modeling of soft matter leaves many open questions. This doctorate research is devoted to advance the understanding of the deformation mechanics of soft matter, specifically, from the following aspects: (1) how the chemo-mechanical interaction between the solvent molecules and the polymeric network invokes anomalous behaviors of a thin-walled hydrogel structure under internal pressure, in contrast to its polymer counterpart; (2) the application of the dielectric elastomer as sensing medium in soft sensor technology; (3) the development of a novel light-responsive hydrogel material system with the application in bio-mimicking shape transform; (4) and enriching the existing theory to facilitate the mechanistic understanding of the deformational behaviors of a type of fiber-reinforce anisotropic hydrogels. For that, this dissertation (1) reveals the delayed burst of hydrogel thin-shell structures as a new failure mechanism, which is dissimilar from the instantaneous burst of a rubber shell: at a subcritical applied pressure the burst occurs with a delay in time; (2) presents a facile design of capacitive tactile force sensor using a dielectric elastomer subjected to a modest voltage and a pre-stretch; (3) develops a theoretical framework to simulate the light-responsive deformation of the proposed hybrid hydrogel system; and (4) from the perspective of micromechanics, constructs a constitutive model suitable for the microfiber-reinforced anisotropic hydrogel, with large deformation, mass transportation, and the origin of anisotropy are intrinsically captured.Item Dynamic Control of Fiber Orientation for Additive Manufacturing via a Soft-Actuating Nozzle(2019) Armstrong, Connor; Bigio, David I; Sochol, Ryan D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recently, additive manufacturing of fiber-reinforced composite hydrogels has been used to create self-assembling and self-folding structures through hydration-triggered shape change. Additive manufacturing of shape-changing structures has applications in spatially-limited environments such as in-vivo biological implants and components for space travel. Fiber orientation in composite hydrogels dictates the degree of anisotropic swelling deformation of hydrated structures. This thesis explores the impact of extrusion channel geometry on fiber orientation as well as the relationship between fiber orientation and swelling deformation of composite hydrogels. To study the impact of fiber orientation on swelling deformation, fiber orientation in composite hydrogels was varied using diverging extrusion dies of increasing divergence angles. It was found that increasing channel divergence angle reduced the number of fibers oriented in the direction of flow, which led to increasingly isotropic swelling deformations. To create a gradient of fiber orientations in extruded structures, an extrusion nozzle utilizing soft actuators to alter its divergence angle in real-time was developed. Hydrogels extruded through the soft-actuated dynamic nozzle exhibited similar fiber orientation and swelling behavior to those extruded through the fixed divergence angles. Spatially-varied swelling deformation characteristics promise to improve additive manufacturing of self-assembling and self-folding structures by increasing the complexity of controllable shape change geometries achievable in extruded composite polymer structures.Item Designing Hydrogels that Transform their Shape in Response to Molecular Cues(2016) Athas, Jasmin; Raghavan, Srinivasa R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There has been considerable interest in developing shape-changing soft materials for potential applications in drug delivery, microfluidics and biosensing. These shape- changing materials are inspired by the morphological changes exhibited by plants in nature, such as the Venus flytrap. One specific class of shape-change is that from a flat sheet to a folded structure (e.g., a tube). Such “self-folding” materials are usually composed of polymer hydrogels, and these typically fold in response to external stimuli such as pH and temperature. In order to develop these hydrogels for the previously described applications, it is necessary to expand the range of triggers. The focus of this dissertation is the advancement of shape-changing polymer hydrogels that are sensitive to uncommon cues such as specific biomolecules (enzymes), the substrates for such enzymes, or specific multivalent cations. First, we describe a hybrid gel that responds to the presence of low concentrations of a class of enzymes known as matrix metalloproteinases (MMPs). The hybrid gel was created by utilizing photolithographic techniques to combine two or more gels with distinct chemical composition into the same material. Certain portions of the hybrid gel are composed of a biopolymer derivative with crosslinkable groups. The hybrid gel is flat in water; however, in the presence of MMPs, the regions containing the biopolymer are degraded and the flat sheet folds to form a 3D structure. We demonstrate that hydrogels with different patterns can transform into different 3D structures such as tubes, helices and pancakes. Furthermore, this shape change can be made to occur at physiological concentrations of enzymes. Next, we report a gel with two layers that undergoes a shape change in the presence of glucose. The enzyme glucose oxidase (GOx) is immobilized in one of the layers. GOx catalyzes the conversion of glucose to gluconic acid. The production of gluconic acid decreases the local pH. The decrease in local pH causes one of the layers to swell. As a result, the flat sheet folds to form a tube. The tube unfolds to form a flat sheet when it is transferred to a solution with no glucose present. Therefore, this biomolecule- triggered shape transformation is reversible, meaning the glucose sensing gel is reusable. Furthermore, this shape change only occurs in the presence of glucose and it does not occur in the presence of other small sugars such as fructose. In our final study, we report the shape change of a gel with two layers in the presence of multivalent ions such as Ca2+ and Sr2+. The gel consists of a passive layer and an active layer. The passive layer is composed of dimethylyacrylamide (DMAA), which does not interact with multivalent ions. The active layer consists of DMAA and the biopolymer alginate. In the presence of Ca2+ ions, the alginate chains crosslink and the active layer shrinks. As a result, the gel converts from a flat sheet to a folded tube. What is particularly unusual is the direction of folding. In most cases, when flat rectangular gels fold, they do so about their short-side. However, our gels typically fold about their long-side. We hypothesize that non-homogeneous swelling determines the folding axis.Item A NEW CLASS OF HYBRID HYDROGELS(2013) Fernandes, Neville Justine; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hybrid hydrogels are a novel way of combining materials with different properties and retaining their individual functionalities within the same composite gel. Here we attempt to demonstrate how this approach can be used to create hydrogels whose morphologies can be altered depending on external stimuli. First we report the creation of hollow hybrid gels which are similar to the previously created solid hybrid gels but have the advantage of a faster and enhanced response to external stimuli. Two stimuli that we have specifically investigated are temperature and solvent composition. We show how to modify the type and extent of response of the gels, i.e. make them shrink or swell, by changing the composition of the polymer as well as the crosslinker within the gel. Thereafter, we also demonstrate how the responses can be manipulated to change the morphology of the hybrid gel itself.Item Enzymatic Activity Preservation through Entrapment within Degradable Hydrogel Networks(2012) Mariani, Angela Marie; Kofinas, Peter; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation aimed to design and develop a "biogel;" a reproducible, abiotic, and biocompatible polymer hydrogel matrix, that prolongs enzymatic stability allowing for rapid production of biomolecules. The researched entrapment method preserves enzyme activity within an amicable environment while resisting activity reduction in the presence of increased pH environmental challenges. These biogels can be used in a number of applications including repeated production of small molecules and in biosensors. Five main objectives were accomplished: 1) Biogels capable of maintaining enzymatic functionality post-entrapment procedures were fabricated; 2) Biogel activity dependence on crosslinker type and crosslink density was determined; 3) Biogel composition effects on sustained activity after storage were compared; 4) Biogel activity dependence on charged monomer moieties was evaluated, and 5) Combined optimization knowledge gained from the first four objectives was utilized to determine the protection of enzymes within hydrogels when challenged with an increased pH above 8. Biogels were fabricated by entrapping beta-galactosidase (lactase) enzyme within acrylamide (ACR) gels crosslinked with poly(ethylene glycol) diacrylate (PEGDA, degradable through hydrolysis) or N,N'-methylenebisacrylamide (BIS, non-degradable). Initial hydrogel entrapment reduced activity to 40% in ACR/PEGDA gels, compared to a 75% reduction in initial activity of ACR/BIS biogels. Once entrapped, these enzymes resist activity reduction in the presence of environmental challenges, such as altering the pH from 7 to above 8. When biogels were challenged at a pH of 8, activity retention positively correlated to PEGDA crosslinker density; increasing from 48% to 91% retention in 30 to 40 mole % PEGDA biogels as compared to solution based control which retained only 23%. Retention of activity when perturbed from pH 7 is advantageous for biogel applications including the repeated production of desired small molecules and biosensors. Biogels with positive or negative monomer moiety functionalities were also investigated to increase enzyme-matrix interactions and enzyme stability. The researched entrapment method illustrates the potential to sterically hinder and diffusively impede enzymes from performing their function, potentially enabling the reactivation of the enzyme at a site and time dictated by the user by degrading the crosslinks of the network.Item Optimization of xanthan chitosan polyelectrolytic hydrogels for microencapsulation of probiotic bacteria(2011) Soma, Pavan Kumar; Lo, Martin; Food Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effectiveness of microencapsulation system for targeted delivery of probiotics depends on its ability to protect cells from harsh gastrointestinal conditions of stomach followed by effectively releasing the cells in intestinal conditions. Oppositely charged xanthan and chitosan form stable polyelectrolytic hydrogels capable of encapsulating enzymes and cells. The present study aims at developing an effective microencapsulation system for probiotics by screening and optimizing the factors critical to xanthan-chitosan hydrogel (XCH) capsule formation. The changes in the core pH of the hydrogel capsule in response to simulated gastric juice (SGJ) were characterized. Increase in xanthan concentration and chitosan molecular weight improved the barrier properties, however, increasing complexation time beyond 40 min had the opposite effect. Increase in molecular weight of chitosan resulted in improved viability of probiotic bacteria, Lactobacillus acidophilus, after SGJ treatment, which could be attributed to the differences in hydrogel membrane thickness at the surface of capsule, as evidenced by scanning electron micrographs (SEM). Introducing XCH capsules made with high molecular weight (HMW) chitosan into xanthan solution resulted in the formation of xanthan-chitosan-xanthan hydrogel (XCXH) capsules. Unlike HMW and medium molecular weight (MMW) chitosan, low molecular weight (LMW) chitosan did not form the outer layer beyond XCH, suggesting the significance of chitosan molecular weight in the formation of XCXH. The increased hydrogel thickness of XCXH capsules formed with HMW chitosan compared to XCH capsules rendered better retention of cells in SGJ treatment for a longer period of time, further suggesting the importance of membrane thickness on the hydrogel stability and its barrier properties. Furthermore, complete release of cells from XCXH in simulated intestinal fluid (SIF) was extended by approximately an hour compared to XCH capsules. Smaller, nozzle-sprayed XCXH capsules using HMW chitosan protected probiotic bacteria in SGJ albeit one-log reduction in its protective efficacy compared to syringe extruded capsules. When incorporated into stirred yogurt, XCXH microcapsules improved the viability of L. acidophilus by ~1 log CFU/ml between 15 and 30 days of storage. The stability of bacteria against bile salts was significantly improved, enabling the delivery of prescribed number of cells to attain the claimed health benefits.Item Desing of Click Hydrogels for Cell Encapsulation(2011) Breger, Joyce; Wang, Nam Sun; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The long-term stability of ionically crosslinked alginate hinders the development of a bioartificial pancreas for the treatment of Type I Diabetes. Ionically crosslinked alginate with divalent cations is traditionally utilized to encapsulate islets of Langerhans serving as a protective barrier between the host's immune system and the donor islets of Langerhans. However, due to ion exchange with monovalent ions from the surrounding serum, alginate degrades exposing donor tissue to the host's immune system. The overall goal of this dissertation was to explore the possibility of utilizing `click' chemistry to introduce covalent crosslinking in alginate for therapeutic cell encapsulation. `Click' chemistry is customarily defined as the Cu (I) catalyzed reaction between an azide and alkyne to form a 1,2,3 triazole ring. To achieve the goal of covalently crosslinked polysaccharides, the following aims were determined: (1) synthesis and characterization of functionalized polysaccharides (alginate and/or hyaluronic acid) with alkyne or azide end groups; (2) measurement and comparison of the stability and transport properties of covalently crosslinked alginate hydrogels to that of ionically crosslinked alginate hydrogels; (3) determination of the inflammatory potential and cytotoxicity of these functionalized polysaccharides and `click' reagents by employing RAW264.7, a murine macrophage cell line under various simulated inflammatory states (with or without endotoxin, with or with out the inflammatory cytokine gamma-interferon); (4) optimization of the `click' reaction for therapeutic cell encapsulation utilizing RIN-5F, a rat insulinoma cell line, while minimizing cytotoxicity and maintaining insulin production; (5) encapsulation of primary porcine islets of Langerhans in either ionically and/or covalently crosslinked alginate capsulation and comparing insulin response to a glucose challenge. The results of these experiments demonstrate the utility of employing `click' chemistry to increase the overall stability of alginate hydrogels while maintaining therapeutic cell function.Item Blood Coagulation Inducing Synthetic Polymer Hydrogel(2010) Casey, Brendan John; Kofinas, Peter; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Uncontrolled hemorrhaging, or blood loss, accounts for upwards of 3 million deaths each year and is the leading cause of preventable deaths after hospital admission around the world. Biological-based hemostatics are quite effective at controlling blood loss, but prohibitively expensive for people in developing countries where over 90 % of these deaths are occurring. Synthetic-based hemostatics are less expensive, yet not nearly as effective as their biological counterparts. A better understanding of how synthetic materials interact with and affect the body's natural clotting response is vital to the development of future hemostatic material technology which will help millions around the world. Initial in vitro experimentation focused on investigating the key chemical and structural material properties which affect Factor VII (FVII) activation in citrated human plasma. Enzyme-linked assays were utilized to confirm the ability of specifically formulated charged hydrogels to induce FVII activation and provided insight into the critical material parameters involved in this activation. Dynamic mechanical analysis was used to establish a correlation between polymeric microstructure and FVII activation. Experiments utilizing coagulation factor depleted and inhibited plasmas indicated that FVII, FX, FII, and FI are all vital to the process outlining the general mechanism of fibrin formation from the onset of FVII activation. The ability of the polymer to induce fibrin formation in "artificial plasma" explicitly lacking calcium, TF, and platelets suggested that a specifically designed material surface has the capability to substitute for these vital cofactors. Clinical diagnostic experimentation using sheep blood indicated that hydrogels containing higher amounts of electrostatic positive charge and lower cross-link density were able to induce faster, more robust clot formation in the presence of a coagulation cascade activator. Subsequent in vivo animal experimentation clearly demonstrated the ability of such hydrogels to aggregate platelets and erythrocytes promoting the formation of an effective hemostatic seal at the wound site. Moreover, in vivo testing confirmed the viability of such a charged polymer hydrogel to effectively control blood loss in a clinically relevant model.Item Molecularly Imprinted Polymers for the Selective Recognition of Proteins(2009) Janiak, Daniel S.; Kofinas, Peter; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Molecular imprinting is a technique used to synthesize polymers that display selective recognition for a given template molecule of interest. In this study, the role of hydrogel electrostatic charge density on the recognition properties of protein-imprinted hydrogels was explored. Using 3-methacrylamidopropyl trimethylammonium chloride (MAPTAC) as a positively charged monomer and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) as a negatively charged monomer, a number of acrylamide-based polyelectrolyte hydrogels with varying positive and negative charge densities were prepared. The imprinted hydrogels were synthesized in the presence of the target molecule bovine hemoglobin (Bhb). The ability of the hydrogels to selectively recognize Bhb was examined using a competitive template molecule, cytochrome c. The Bhb imprinted gels exhibited template recognition properties that were dependent on both the monomer charge density and on whether the chosen monomer carried a positive or negative charge. In addition to polyelectrolye hydrogels, polyampholyte hydrogels containing both positively and negatively charged monomers were also synthesized. The simultaneous presence of two oppositely charged monomers in the pre-polymerization mixture resulted in imprinted hydrogels with cavities that contain highly specific functional group orientation. The polyampholyte hydrogels exhibited decreased swelling when compared to their polyelectrolyte counterparts, due to the shielding of repulsive interactions between oppositely charge monomers. This decreased swelling resulted in greater template recognition, but lower selectivity, when compared to their polyelectrolyte counterparts. In addition, we found that common agents used in template extraction may be responsible for the specific and selective binding properties exhibited by molecularly imprinted polymers in many published studies, and the effect of variations of the template extraction protocol on the MIP recognition properties were also studied in depth.