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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Subject: search.filters.subject.morphology

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    Direct visualization of nanoparticle morphology in thermally sintered nanoparticle ink traces and the relationship among nanoparticle morphology, incomplete polymer removal, and trace conductivity
    (Institute of Physics, 2023-06-19) Chandel, Ghansham Rajendrasingh; Sun, Jiayue; Etha, Sai Ankit; Zhao, Beihan; Sivasankar, Vishal Sankar; Nikfarjam, Shakiba; Wang, Mei; Hines, Daniel R.; Dasgupta, Abhijit; Woehl, Taylor; Das, Siddartha
    A key challenge encountered by printed electronics is that the conductivity of sintered metal nanoparticle (NP) traces is always several times smaller than the bulk metal conductivity. Identifying the relative roles of the voids and the residual polymers on NP surfaces in sintered NP traces, in determining such reduced conductivity, is essential. In this paper, we employ a combination of electron microscopy imaging and detailed simulations to quantify the relative roles of such voids and residual polymers in the conductivity of sintered traces of a commercial (Novacentrix) silver nanoparticle-based ink. High resolution transmission electron microscopy imaging revealed details of the morphology of the inks before and after being sintered at 150 °C. Prior to sintering, NPs were randomly close packed into aggregates with nanometer thick polymer layers in the interstices. The 2D porosity in the aggregates prior to sintering was near 20%. After heating at 150 °C, NPs sintered together into dense aggregates (nanoaggregates or NAgs) with sizes ranging from 100 to 500 nm and the 2D porosity decreased to near 10%. Within the NAgs, the NPs were mostly connected via sintered metal bridges, while the outer surfaces of the NAgs were coated with a nanometer thick layer of polymer. Motivated by these experimental results, we developed a computational model for calculating the effective conductivity of the ink deposit represented by a prototypical NAg consisting of NPs connected by metallic bonds and having a polymer layer on its outer surface placed in a surrounding medium. The calculations reveal that a NAg that is 35%–40% covered by a nanometer thick polymeric layer has a similar conductivity compared to prior experimental measurements. The findings also demonstrate that the conductivity is less influenced by the polymer layer thickness or the absolute value of the NAg dimensions. Most importantly, we are able to infer that the reduced value of the conductivity of the sintered traces is less dependent on the void fraction and is primarily attributed to the incomplete removal of the polymeric material even after sintering.
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    Effects of physico-chemical cues from the blood-brain barrier microenvironment on tumor cell migration and morphology
    (2019) Pranda, Marina Alexandrovna; Stroka, Kimberly M; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Cancer metastasis is particularly deadly, leading to 90% of cancer deaths. During metastasis, tumor cells break off from a primary tumor and travel to distant sites. Metastasis to the brain results in a poor patient prognosis. However, several common cancers, such as breast cancer and melanoma, metastasize to the brain. In order to metastasize to the brain, tumor cells have been shown to cross the highly selective blood-brain barrier (BBB), which separates the brain parenchyma from the circulatory system. The BBB is highly impermeable, even for many chemotherapeutics, however, tumor cells are able to cross it by a poorly understood mechanism. The BBB consists of endothelial cells connected by tight junctions, and is supported by cells of the neurovascular unit, such as astrocytes. Furthermore, the composition of the extracellular matrix beyond the BBB is unique and contains hyaluronic acid (HA). In disease, HA organization or biophysical properties may become altered. The goal of this study was to investigate how specific physico-chemical interactions of tumor cells and the BBB microenvironment may impact tumor cell behavior at the BBB, as well as explore cold atmospheric plasma (CAP) as potential cancer treatment. This understanding could lead to better future therapeutics and a better prognosis for patients. We hypothesized that biophysical and biochemical cues from the BBB microenvironment, as well as the tumor cell phenotype, can influence tumor cells’ migration and morphology. In this dissertation, we investigated the interaction of tumor cells with astrocyte-secreted biochemical cues and the biophysical cues from a HA/gelatin extracellular matrix on tumor cell morphology, migration, and incorporation into an endothelium. Our results showed that tumor cell migration and morphology are significantly altered by astrocyte-secreted factors and the HA/gelatin extracellular matrix; however, the extracellular matrix is less significant during incorporation. We also showed that brain- and bone-seeking tumor cells display varied morphologies on matrices with niche-relevant mechanical properties. Finally, we demonstrated that CAP selectivity for reducing migration of tumor vs. normal cells is highly sensitive to cell culture media formulation. Together, these results provide new insights into tumor cell behavior at the BBB and inform future studies and therapeutic development.
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    SYNTHESIS AND CONTROL OF MORPHOLOGY OF POLY METHYL METHACRYLATE AND POLY ACRYLIC ACID MICRO-PARTICLES BY THE MODIFIED SUSPENSION POLYMERIZATION TECHNIQUES
    (2015) Kim, Yunju Jung; Choi, Kyu Yong; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis studied the synthesis and control of morphology of two kinds of polymers by modified suspension polymerization techniques. The first polymer, poly (methyl methacrylate), is a transparent thermoplastic polymer, which is typically used in diffusing film in the backlight unit of an LCD. Also, the synthesis of micron-sized polymer particles with complex internal morphologies such as hollows, multihollows, and multiporous structures is of growing interest in many technological applications such as microelectronic displays and microencapsulation. The direct synthesis of such materials is carried out in heterogeneous processes with controlled phase separation mechanisms. In such systems, detailed knowledge of heterogeneous polymerization kinetics and phase separation phenomena is essential for investigating the process characteristics. An in situ polymerization and phase separation technique has been used to construct a ternary phase diagram for the free radical precipitation polymerization of methyl methacrylate (MMA), n-hexane, and poly(methyl methacrylate) (PMMA) system. The onset of the phase separation point during polymerization is directly monitored in real time by laser light scattering (LLS) technique for a broad range of polymer concentrations. The presented method overcomes the difficulty of determining the cloud points by titrating unreactive blends of polymer and solvent at high initial monomer concentrations that lead to high polymer concentration and high viscosity of the mixture fluid at the system phase separation point. We present the micro dispersive suspension polymerization (MDSP) process to produce complex particle morphologies in a single-stage process. MDSP is a hybrid of suspension and dispersion polymerizations. The micron-sized polymer particles are polymerized by suspension polymerization, and the internal morphology of particle is polymerized by dispersion polymerization inside the polymer particles. Varying the initial conditions for the phase separation in precipitation and dispersion polymerizations, final particles’ morphology may change from solid polymer particles to complex porous polymeric structures. In this heterogeneous process, the system evolution depends on the composition and molecular characteristics of the coexisting phases and on the characteristics of the interface. Using MDSP, we were able to develop a phase diagram to show the regions of multi-hollow/porous and core-shell/pomegranate-like poly (methyl methacrylate) (PMMA) particles. We also show that controlling morphology of polymer particles by thermodynamic and kinetic variables is technically feasible. The second polymer, poly (acrylic acid), is an absorbing polymer. Superabsorbent polymers (SAP) can absorb and retain extremely large amounts of water or aqueous solutions relative to their own mass. Partially neutralized sodium polyacrylate is industrially a very important polymer for many applications. However, in industry sodium polyacrylate is mostly manufactured by bulk polymerization, and the resultant bulk polymer is pulverized using a kneader to obtain small discrete polymer particles. It is environment-unfriendly process and the produced granules from bulk have irregular shapes, rather than a spherical shape. This study is aimed at investigating the inverse suspension polymerization of acrylic acid to make spherical polymer particles. In particular, the study is focused on how the resulting polymer morphology and characteristics are affected by the polymerization conditions. A feasible and simple technique to obtain Na-polyacrylate microparticles with sizes below 10 µm was investigated using a high shear mixing device. To maintain the stability of submicron size of aqueous droplets in the oil medium, a co-surfactant system containing Span 80 and Tween 80 was used. Neutralization of acrylic acid was proved from EDX analysis. Na-polyacrylate submicron particles were characterized by size, surface morphology, swelling capacity, and conversion. When the speed of homogenization was lowered from 3000 rpm to 1000 rpm, particles over 10 µm were obtained, but more nano-sized particles were present outside. We also developed the technique above to increase polymer particle size to tens of microns. In this process, a wrinkled and cracked surface of Na-polyacrylate particles was observed in the special environment of post treatment. Surface area, swelling capacity, and swelling speed of different morphologies and sizes were characterized and analyzed. In order to synthesize spherical Na-polyacrylate particles with smooth surface regardless of post treatment, polymerization time was progressed longer than 20 hr. Na-polyacrylate particles had a solid structure at high conversion over 0.996 after longer than 15 hrs of polymerization, which made particles maintain their shapes regardless of post treatment. When a high monomer concentration was used in this polymerization, perfectly smooth and spherical polymer particles were obtained after 9 hr, which was faster than when a lower monomer concentration was used.