A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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    Directed Kinetic Self-Assembly of Mounds on Patterned GaAs (001): Tunable Arrangement, Pattern Amplification and Self-Limiting Growth
    (MDPI, 2014-05-12) Lin, Chuan-Fu; Kan, Hung-Chih; Kanakaraju, Subramaniam; Richardson, Christopher; Phaneuf, Raymond
    We present results demonstrating directed self-assembly of nanometer-scale mounds during molecular beam epitaxial growth on patterned GaAs (001) surfaces. The mound arrangement is tunable via the growth temperature, with an inverse spacing or spatial frequency which can exceed that of the features of the template. We find that the range of film thickness over which particular mound arrangements persist is finite, due to an evolution of the shape of the mounds which causes their growth to self-limit. A difference in the film thickness at which mounds at different sites self-limit provides a means by which different arrangements can be produced.
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    LEVERAGING SELF-ASSEMBLY AND BIOPHYSICAL DESIGN TO BUILD NEXT-GENERATION IMMUNOTHERAPIES
    (2022) Froimchuk, Yevgeniy; Jewell, Christopher M; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The immune system has evolved mechanisms to respond not only to specific molecular signals, but also to biophysical cues. Interestingly, research at the interface of biomaterials and immunology has also revealed that the biophysical properties and form of vaccines and immunotherapies impact immunological outcomes. For example, the intermolecular distance between antigen molecules on the surface of nanoparticles can impact formation of T cell receptor clusters that are critical during T cell activation. Despite the importance of biophysical cues in tuning the immune response, the connections between these parameters and immunological outcomes are poorly understood in the context of immunotherapy. Immunotherapies harness an individual’s immune system to battle diseases such as autoimmunity. During autoimmune disease, the immune system malfunctions and mistakenly attacks self-tissue. Immunotherapies can help tailor and guide more effective responses in these settings, as evidenced by recent advances with monoclonal antibodies and adoptive cell therapies. However, despite the transformative gains of immunotherapies for patients, many therapies are not curative, work only for a small subset of patients, and lack specificity in distinguishing between healthy and diseased cells, which can cause severe side effects. To overcome these challenges, experimental strategies are attempting to co-deliver self-antigens and modulatory cues to reprogram dysfunctional responses against self-antigens without hindering normal immune function. These strategies have shown exciting potential in pre-clinical models of autoimmune disease but are unproven in clinical research. Understanding how biophysical features are linked to immunological mechanisms in these settings would add a critical dimension to designing translatable, antigen-specific immunotherapies. Self-assembling materials are a class of biomaterials that spontaneously assemble in aqueous solution. Self-assembling modalities are useful technologies to study the links between biophysical parameters and immune outcomes because they offer precise control and uniformity of the biophysical properties of assembled moieties. Our lab leveraged the benefits of self-assembly to pioneer development of “carrier-free” immunotherapies composed entirely of immune signals. The therapies are composed of self-antigens modified with cationic amino acid residues and anionic, nucleic acid based modulatory cues. These signals are self-assembled into nanostructured complexes via electrostatic interactions. The research in this dissertation utilizes this platform as a tool to understand how tuning the biophysical properties of self-antigens impacts molecular interactions during self-assembly and in turn, how changes in biophysical features are linked to immunological outcomes. Surface plasmon resonance studies revealed that the binding affinity between signals can be tuned by altering overall cationic charge and charge density of self-antigen, and by anchoring the self-antigen with arginine or lysine residues. For example, the binding affinity between signals can be increased by increasing the total cationic charge on the self-antigen, and by anchoring the self-antigen with arginine residues rather than lysine residues. Computational modeling approaches generated insights into how molecular interactions between signals, such as hydrogen bonding, salt-bridges, and hydrophobic interactions, change with different design parameters. In vitro assays revealed that a lower binding affinity between self-assembled signals was associated with greater reduction of inflammatory gene expression in dendritic cells and more differentiation of self-reactive T cells towards regulatory phenotypes that are protective during autoimmunity. Taken all together, these insights help intuit how to use biophysical design to improve modularity of the self-assembly platform to incorporate a range of antigens for distinct disease targets. This granular understanding of nanomaterial-immune interactions contributes to more rational immunotherapy design.
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    Gel Formation by the Self-Assembly of Small Molecules: Insights from Solubility Parameters
    (2014) Diehn, Kevin; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Many small molecules can self-assemble into long fibers and thereby gel organic liquids. However, no capability exists to predict whether a molecule in a given solvent will form a gel, a thin solution (sol), or an insoluble precipitate. In this thesis, we build a framework for gelation via a common gelator based on Hansen solubility parameters (HSPs). Using HSPs, we construct 3-D plots showing regions of solubility (S), slow gelation (SG), instant gelation (IG), and insolubility (I) for DBS in different solvents. Our central finding is that these regions radiate out as concentric shells. The distance (R0) from the central sphere quantifies the incompatibility between gelator and solvent. The elastic moduli of the gels increase with R0, while the time to gelation decreases with R0. Our approach can be used to design organogels of desired strength and gelation time by judicious choice of a solvent or a blend of solvents.
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    Self-assembly in aqueous solutions of a non-ionic hydrotrope
    (2012) Subramanian, Deepa; Anisimov, Mikhail A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Hydrotropes are amphiphilic molecules, too small to cause spontaneous self-assembly towards equilibrium mesoscale structures in aqueous solutions, but they form dynamic, noncovalent assemblies, which may create microscopic regions of lowered polarity. This enhances the solubilization of hydrophobic compounds, also known as solubilizates, in aqueous solutions and may cause further aggregation to larger structures. In this work, unusual mesoscopic properties of aqueous solutions of a non-ionic hydrotrope, namely tertiary butyl alcohol (TBA) have been investigated by light scattering, microscopy, and chromatography. Aqueous TBA solutions show anomalous thermodynamic and structural properties in the range of concentrations 3-8 mol % TBA and temperatures 0 - 25 °C. These anomalies appear to be associated with short-lived, short-ranged micelle-like structural fluctuations, distinctly different from usual concentration fluctuations in non-ideal solutions. Molecular dynamics simulations and neutron-scattering experiments show clustering of TBA molecules on a nanometer scale, interacting through hydrogen bonds with a shell of water molecules. In this concentration range, TBA aqueous solutions, although macroscopically homogeneous, occasionally show the presence of "mysterious" inhomogeneities on a 100 nm scale. We have found that the emergence of such inhomogeneities strongly correlates with impurities present in commercial TBA samples. Experiments with controlled addition of a third component, such as propylene oxide, isobutyl alcohol, or cyclohexane, reveal the mechanism of formation of these inhomogeneities through stabilization of micelle-like fluctuations by a solubilizate. These structures are long-lived, i.e., stable from a few days up to many months. We have confirmed that mesoscale structures in aqueous solutions can be generated from self-assembly of small molecules, without involvement of surfactants or polymers. This kind of self-assembly may potentially result in the development of novel nanomaterials.
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    ACCELERATED SELF-ASSEMBLY OF PEPTIDE-BASED NANOFIBERS USING NANOMECHANICAL STIMULUS
    (2010) Chang, Jonathan Paul; Seog, Joonil; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    One-dimensional nanostructures are ideal building blocks for functional nanoscale assembly. Peptide-based nanofibers have great potential for building smart hierarchical structures due to their tunable structures at a single residue level and their ability to reconfigure themselves in response to environmental stimuli. In this study, it was observed that a pre-adsorbed silk-elastin-based protein polymer self-assembled into nanofibers through a conformational change on the mica substrate. Furthermore, using atomic force microscopy, it was shown that the rate of the self-assembling process was significantly enhanced by applying a nanomechanical stimulus. The orientation of the newly grown nanofiber was mostly perpendicular to the scanning direction, implying that the new nanofiber assembly was locally activated with a directional control. The method developed as a part of this study provides a novel way to prepare a nanofiber patterned substrate using a bottom-up approach.
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    Controlled liposome formation and solute encapsulation with continuous-flow microfluidic hydrodynamic focusing
    (2008-12-11) Jahn, Andreas; DeVoe, Don L; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Liposomes enable the compartmentalization of compounds making them interesting as drug delivery systems. A drug delivery system (DDS) is a transport vehicle for a drug for in vivo drug administration. Drugs can be encapsulated, bound, or otherwise tethered to the carrier which can vary in size from tens of nanometers to a few micrometers. Liposomal DDSs have shown their capability to deliver drugs in a new fashion, allowing exclusive sales of encapsulated drugs to be extended beyond the initial compound's patent expiration date. However, existing methods to form liposomes and encapsulate drugs are based on bulk mixing techniques with limited process control and the produced liposomes frequently require post-processing steps. In this dissertation, a new method is demonstrated to control liposome formation and compound encapsulation that pushes beyond existing benchmarks in liposome size homogeneity and adjustable encapsulation. The technology utilizes microfluidics for future pharmacy-on-a-chip applications. The microfluidic system allows for precise control of mixing via molecular diffusion with reproducible and controlled physicochemical conditions compared to traditional bulk-phase preparation techniques (i.e. test tubes and beakers). The laminar flow and facile fluidic control in microchannels enables reproducible self-assembly of lipids into liposomes in a sheathed flow-field. Confining a water-soluble compound to be encapsulated to the immediate vicinity where liposome formation is expected to occur reduces sample consumption without affecting liposome loading. The ability to alter the concentration and control the amount of encapsulated compounds within liposomes in a continuous-flow mode is another interesting feature towards tailored liposomal drug delivery. The liposome formation strategy demonstrated in this dissertation offers potential for point-of-care drug encapsulation, eliminating shelf-life limitations inherent to current liposome preparation techniques.
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    Development of self-assembled ZnO nanostructures in diblock copolymers on large area Si wafers and gas sensor applications
    (2008-05-27) Ali, Hasina Afroz; Iliadis, Agis A.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    ZnO nanoparticles with improved optical properties and increased surface areas have the potential for advanced optoelectronic, gas sensor and biosensor applications. In order to exploit these unique properties of ZnO nanoparticles for the realization of nanoscale devices, we developed novel techniques for the self-assembly and functionalization of ZnO nanoparticles through diblock copolymers on large area (100)Si surfaces. These novel techniques allowed us to subsequently develop the first ZnO nanoparticle based device. Thus, a novel ZnO-nanocomposite/Si n-p heterojunction diode and a high performance hydrogen gas nanosensor have been developed, for the first time. The thesis presents the novel technique developed for the self-assembly of ZnO nanostructures with spherical morphology through diblock copolymers on large area Si substrates. Correlation between the physical parameters of the nanoparticles and the copolymers was evaluated from AFM studies. Control of the nanoparticle size and density was achieved by varying copolymer block lengths. The largest nanoparticles had average sizes of 250 nm and densities of 1x107cm-2 while the smallest nanoparticles had average sizes of 20nm and densities of 1x101010cm-2. XRD studies showed that the wurtzite crystal structured nanoparticles assumed the same orientation (100) as the Si substrate, indicating a pseudo-epitaxial nanostructure. Room temperature photoluminescence studies showed quantum confinement effects with a blue shift from 372 nm (large particles) to 363 nm (small particles). A broad defect related green-yellow luminescence centered at 555 nm indicative of n-type conductivity of the nanoparticles was also observed. The n-type nanoparticles on p-type Si resulted in the development of a ZnO-nanocomposite/pSi n-p heterojunction diode for the first time. The nanodiode showed good rectification and low leakage currents. LogI-V characteristics gave built-in voltages of 0.69 and 0.7 eV, saturation currents of 2 and 2.34 x10-8A, and ideality factors of 5.9 and 5.7 for the small and large particles, respectively. The transport mechanisms of the nano-diodes were studied. C-V characteristics showed abrupt p-n junctions, suggesting an intimate junction interface consistent with the pseudo-epitaxial nature of the structure. A novel hydrogen gas nanosensor based on the ZnO-nanocomposite/Si heterojunction diode was developed for high sensitivity, rapid, room temperature sensing. Response and recovery times were reduced by a factor of 100 and smaller and denser nanoparticles were found to be faster and more sensitive.
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    Growth and Characterization of Multiferroic BaTiO3-CoFe2O4 Thin Film Nanostructures
    (2004-12-08) Zheng, Haimei; Salamanca-Riba, Lourdes; Ramesh, Ramamoorthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Multiferroic materials which display simultaneous ferroelectricity and magnetism have been stimulating significant interest both from the basic science and application point of view. It was proposed that composites with one piezoelectric phase and one magnetostrictive phase can be magnetoelectrically coupled via a stress mediation. The coexistence of magnetic and electric subsystems as well as the magnetoelectric effect of the material allows an additional degree of freedom in the design of actuators, transducers, and storage devices. Previous work on such materials has been focused on bulk ceramics. In the present work, we created vertically aligned multiferroic BaTiO3-CoFe2O4 thin film nanostructures using pulsed laser deposition. Spinel CoFe2O4 and perovskite BaTiO3 spontaneously separated during the film growth. CoFe2O4 forms nano-pillar arrays embedded in a BaTiO3 matrix, which show three-dimensional heteroepitaxy. CoFe2O4 pillars have uniform size and spacing. As the growth temperature increases the lateral size of the pillars also increases. The size of the CoFe2O4 pillars as a function of growth temperature at a constant growth rate follows an Arrhenius behaviour. The formation of the BaTiO3-CoFe2O4 nanostructures is a process directed by both thermodynamic equilibrium and kinetic diffusion. Lattice mismatch strain, interface energy, elastic moduli and molar ratio of the two phases, etc., are considered to play important roles in the growth dynamics leading to the nanoscale pattern formation of BaTiO3-CoFe2O4 nanostructures. Magnetic measurements exhibit that all the films have a large uniaxial magnetic anisotropy with an easy axis normal to the film plane. It was calculated that stress anisotropy is the main contribution to the anisotropy field. We measured the ferroelectric and piezoelectric properties of the films, which correspond to the present of BaTiO3 phase. The system shows a strong coupling of the two order parameters of polarization and magnetization through the coupled lattices. This approach to the formation of self-assembled ferroelectric/ferro(ferri-)magnetic nanostructures is generic. We have created similar nanostructures from other spinel-perovskite systems such as BiFeO3-CoFe2O4, BaTiO3-NiFe2O4, etc., thus making it of great interest and value to a broad materials community.