Environmental and Occupational Health Sciences Institute
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Cardio-metabolic effects of one hundred hours of exoskeletal-assisted walking in individuals with spinal cord injury
No full textThe prevalence of cardiovascular disease (CVD) is significantly higher in individuals with spinal cord injury (SCI) compared to able-bodied counterparts. Current upper body aerobic exercise modalities, such as arm crank ergometry and rowing, offer limited benefits to significantly improve CVD risk factors in SCI. While upright exoskeletal-assisted walking (EAW), improves cardiorespiratory fitness, its effects on CVD risk in SCI remain uncertain. This study aims to assess the impact of an EAW program on body composition and lipid profile in individuals with chronic incomplete SCI. Dual-energy x-ray absorptiometry obtained fat mass and lean mass measurements. Overnight fasted blood draws were analyzed for lipid profile parameters (High Density Lipoprotein Cholesterol, Low Density Lipoprotein Cholesterol; LDL-c, triglycerides, & total cholesterol). All outcome measures were collected at three points, baseline, 50-hours, and 100-hours. A linear mixed model and clinically significant changes (5% reduction for total body fat mass and 10% reduction for total LDL-c), were utilized to observe group and within-subject effects. Findings from 19 participants (13 paraplegia, 6 tetraplegia, & average body mass index 43.4 ± 6.0 kg/m²) yielded significant reductions in total body fat mass (0.70 ± 2.66 kgs, p = 0.047) and arm fat mass (0.11 ± 0.36 kgs, p = 0.042) post 100-hour EAW intervention. Clinically significant changes in total body fat mass were present in one third of participants (7/19). Of these participants, the majority had a paraplegia SCI (6/7) and a lower extremity motor score of 11 or higher (5/7). For lipid changes, 10 participants showed clinically significant improvements, with most of them also having paraplegia SCI (7/10) and a lower extremity motor score of 11 or higher (6/10). Statistical differences in pattern of change were found by level of injury (total body fat mass, total cholesterol, & LDL-c), lower extremity motor score (Visceral adipose tissue, leg lean mass, & total cholesterol), and time since SCI (High Density Lipoprotein Cholesterol). Findings of the present investigation demonstrate that EAW can contribute to the improvement of risks associated with CVD. The significant differences in pattern of change between subgroups support further investigating the implications of EAW across SCI subgroups for promoting optimal cardiovascular health across individuals with SCI.Ph.D.Includes bibliographical referencesIncludes vit
Machine learning-based approach for thermospheric density prediction
No full textFor satellites at low altitudes, atmospheric drag is the dominant non-conservative perturbation force and is the most difficult cause for predicting the orbits of low Earth orbit (LEO) satellites. Among all the factors affecting the drag force, thermospheric density is presently the predominantly uncertain term. This dissertation presents a significant advancement in thermospheric density prediction through the development of a data-driven framework based on machine learning methods. The proposed framework integrates data from the empirical models including the Jacchia-Bowman (JB-2008) and Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Extended (NRLMSISE-00), geomagnetic and solar indices, the corresponding time indices and the geographic positions, and accelerometer-derived density data along satellites to predict thermospheric density.Key contributions of this dissertation include an in-depth exploration of the critical inputs Dst and SymH indices in the framework, which help to improve the accuracy of the model in predicting thermospheric density. The framework is initially validated using the Gaussian processes (GPs) and neural network model with dropout to predict the thermospheric density along the Challenging Minisatellite Payload (CHAMP) satellite during both quiet and storm periods. The predicted results based on the proposed framework show high accuracy and quality uncertainty estimations. Afterward, we introduced and applied the deep evidential methods to predict density along CHAMP based on the proposed framework. Based on the deep evidential model, we analyze the performance of the estimated uncertainty on both storm and quiet periods.
A global thermospheric density prediction framework based on the deep evidential method is then developed and validated on various satellites. The results show higher predicted accuracy than the empirical models with reliable uncertainty estimations, even for testing on future periods or on new satellites that are not included in the training data. The model's ability to predict global thermospheric density across a wide range of geophysical settings is also demonstrated and compared with the empirical models JB-2008 and NRLMSISE-00.
To demonstrate the practical application of our model, we applied it to orbit propagation and compared the results with those densities derived from JB-2008 and NRLMSISE-00. The simulation lasts for 10 hours based on a storm period in 2003 and a quiet period in 2007. The results show that the orbit propagation based on the proposed model is significantly closer to the actual observed orbits than the empirical models, highlighting its superior accuracy and robustness.Ph.D.Includes bibliographical reference
Tabletop object rearrangement: structure, complexity, and efficient combinatorial search-based solutions
No full textThis thesis aims to provide a complete structural analysis and efficient algorithmic solutions to tabletop object rearrangement with overhand grasps (TORO ). This problem captures a common task that we solve on a daily basis and is essential in enabling truly intelligent robotic manipulation. When rearranging many objects in a confined workspace,on the one hand, action sequencing with the least pick-n-places in TORO is NP-hard[1]; on the other hand, temporarily relocating objects to some free space (“buffer poses”) may be necessary but highly challenging in a cluttered environment. Focusing on these two challenges, the thesis covers TORO in four different setups, including varied workspace assumptions (with/without external buffers) and manipulator settings (single/dual-arms or a mobile manipulator). The thesis first explores TORO with external buffers (TORE), addressing the size of needed space for temporary object relocation (“running buffers”). This study shows that finding the maximum running buffers (MRB) is NP-hard and that MRB can grow unbounded with an increasing number of objects, even with uniform shapes. Exact algorithmsdeveloped for both labeled and unlabeled settings can scale to over 100 objects. The thesis further extends the TORE algorithms to tabletop rearrangement with internal buffers (TORI), where all temporary object placements need to be inside the workspace. A two-step baseline planner is developed, generating a primitive plan based on object-object collisions and selecting buffer locations. Using a “lazy” planner within a bi-directionaltree search framework, the method efficiently produces robust, high-quality solutions in simulations, outperforming existing approaches for large-scale problems. The thesis later explores lazy buffer verification for dual-arm task planning in non-monotone rearrangement tasks. Handling complex dependencies often requires moving objects multiple times and coordinating hand-offs between two arms in a shared workspace. Our task planning algorithms effectively sequence and distribute pick-and-place tasks,yielding significant time savings over greedy or single-robot approaches. For motion planning, we introduce a tightly integrated pipeline that combines novel sampling methods with advanced trajectory optimization. The proposed planner achieves superior execution times and trajectory compliance with acceleration and jerk constraints, advancing coordination
in dual-arm systems. Finally, the thesis incorporates lazy buffer verification into an A* framework (ORLA*) for time-optimal multi-object rearrangement in mobile robot tabletop setups. The proposed method applies delayed evaluation to optimize object pick-and-place sequences, factoring in both end-effector and robot base movements. With learning-based stability predictions,ORLA* can handle complex, multi-layered rearrangement tasks. Extensive simulations and ablation studies validate ORLA* ’s effectiveness, providing high-quality solutions for challenging rearrangement scenarios.Ph.D.Includes bibliographical reference
Controlling metal-polymer interfaces for optoelectronic devices
No full textOrganic semiconducting conjugated polymers are commonly used in optoelectronic devices, replacing inorganic materials due to their many advantages, such as, ease of processability, thin form factors, compatibility with solution deposition techniques, and mechanical flexibility. They are made of large number of aromatic monomer repeat units, creating macromolecular chains, whose electronic properties can be tuned by, for example, chemical doping, introducing chemical moieties or varying the side chain length. Organic semiconducting polymers are particularly advantageous because, they can be formed into thin films using atmospheric pressure deposition techniques, like spin coating or solution printingprocesses, compared to expensive vacuum deposition processes that are used for inorganic and organic small molecules. However, organic semiconducting polymers also suffer from several disadvantages, including low stability when exposed to air and moisture; and low device efficiencies, due to low charge carrier mobilities and the presence of triplet excited states. In optoelectronic devices, particularly in organic light-emitting diodes (OLEDs), only ~25 % of the light emitted is useful due to poor light extraction efficiency, and the device stability is low, especially for blue-emitting active layers, due to long lived triplet states. Previously, several methods have been used to overcome these issues by adding different additive and buffer layers or developing new device architectures. Commercially available OLEDs are fabricated with indium-tin oxide (ITO) transparent electrodes, which are extremely robust, although they involve high cost and high embodied energy. Alternatives forITO have been actively investigated, with research into new transparent conducting electrode materials (TCE) for bottom-emitting OLED devices. Another approach to this problem is to build the device on a metallic anode instead of ITO that can be patterned to give a plasmonic electrode, with the light coming out through a transparent top cathode. Plasmonic structures can enhance the light extraction efficiency due to resonant scattering or surface plasmon polariton extraction. Further, the overall device stability and performance can be improved due to the near-field optical effects of plasmonic structures, such as the Purcell effect, by strongly interacting with the organic semiconductor emitters.
In the present thesis, we have investigated interfacial layers and plasmonic structures that can be effective for improving the performance of bottom-emitting and top-emitting polymer OLEDs. The first part of the thesis (Chapter 2) deals with one of the major problems in building a solution processed optoelectronic device on a metallic electrode, which is the creation of a Schottky barrier at the metal-polymer interface. This means that the injection of charges at the interface between the metal electrode and the polymer active layer depends on the difference between the work function of the metal electrode and frontier orbital energy levels of the polymer. Several methods may be used to overcome this barrier by using interfacial layers, oxides or buffer layers. One of the common methods used in organic field-effect transistors (OFETs) to overcome this barrier is to use a self-assembled monolayer (SAM) of thiol-containing compounds on metallic electrodes like Au or Ag. The other main challenge in using a metal like Ag is that it undergoes oxidation readily during processing of the device.
In that regard, SAMs making S-Ag bonds can also help with passivation of the Ag surface. We employ (±) α-Lipoic acid derivatives to form SAMs on Ag for the study of electrical injection and electronic properties by building inverted-organic hole only conjugated polymer devices. (±) α-Lipoic acid with its two S atoms can form stronger bonds with Ag, and, by varying the functional groups at the end, we can vary the overall molecular dipole moment and interfacial interactions at the metal-polymer interface. We find significant improvements in the performance (about 1.5 times) and passivation with (±) α-Lipoic acid. Additionally, we studied the change in work-function, charge injection barrier and polymer-morphology in the presence of novel SAMs. In the second part of the thesis (Chapter 3), we develop a transparent conducting cathode material that can be used as a top contact in OLEDs built on Ag. For n-type electrode materials, a combination of ZnO and Ag layers are very popular, especially in solar cells. We have developed a multilayer of ZnO/Ag/ZnO suitable for deposition directly on the conjugatedpolymer active layer by optimizing and varying the ZnO and Ag thickness using mild sputtering conditions. We demonstrate a TCE that has a transparency of ~85% and a sheet resistance of 1.47 Ω/sq, which is lower than that of the industry standard TCE, ITO. We also demonstrate the usefulness of this TCE in a transparent OLED built on ITO. The third part of the thesis (Chapter 4) deals with fabricating solution processed OLEDs that incorporate plasmonic nanoparticles. The biggest roadblock in OLED display technology is the stability of blue OLED sub-pixels. Compared to the red and green sub-pixels, which have achieved an external quantum efficiency (EQE) of ~20%, the EQE of blue sub-pixels still remains at 5%. We first optimize a deep-blue phosphorescent OLED, and incorporate Ag nanoparticles onto the surface of the ITO electrode to study plasmonic and Purcell effects. We study transient photoluminescence lifetime, quantum yield and device stability to understand the effects of the plasmonic nanoparticles on the emitter, as well as on the device performance. We find that, even though there is a plasmonic and stability enhancement, especially in the case of colloidal Ag particles on ITO, the overall device performance is dominated by other optical and electronic effects. Further studies on the band alignment and charge transfer between the ITO, Ag nanoparticles and the hole transport layer (HTL), and electronic interactions between the HTL and emissive layer are necessary to isolate and improve the plasmonic and Purcell effects. In summary, in this thesis, we demonstrate different device architectures, electrodes and interfaces that can lead to efficiency and stability improvements in organic optoelectronic devices specifically, polymer-based OLEDs. We study the modification of metal-polymer interfaces for top-emitting OLEDs and develop a new transparent electrode material as thetop-contact electrode. We also study at the effects of plasmonic nanoparticles on the stability and efficiency in deep-blue phosphorescent OLEDs. These studies have the potential to impact OLED technology, specifically for display and solid-state lighting applications.Ph.D.Includes bibliographical reference
Development of screening platforms for chemoenzymatic oligosaccharide synthesis
No full textGlycans, among the most structurally diverse and functionally essential biopolymers, are key mediators in metabolic pathways, cellular recognition, and host-pathogen interactions. Yet their biosynthesis is not directly encoded in the genome, complicating both the study of their biological roles and the development of synthetic strategies for biomedical and biotechnological applications. Natural glycan assembly typically depends on membrane-bound glycosyltransferases, which can be challenging to express, purify, and use at scale, with few robust screening systems available. As an alternative, researchers have turned to glycosyl hydrolases (GHs), seeking to harness their capacity for transglycosylation. However, GHs generally favor hydrolysis over transglycosylation, resulting in low product yields and limiting their utility. This has led to growing interest in glycosynthases - engineered GH derivatives whose improved mechanistic understanding and performance are pivotal to expanding our toolkit for glycan synthesis.In this work, we developed and validated a medium-throughput screening platform employing an azide-based biosensor to identify improved glycosynthase (GS) variants in vitro. Unlike many conventional assays, this microplate-compatible system provides both speed and quantitative precision, facilitating the rapid discovery of promising glycosynthase mutants. Focusing on Thermotoga maritima α-L-fucosidase (TmAfc), we used error-prone PCR to target three regions within its active site and screened the resulting variants with our azide biosensor platform. The starting template (TmAfc_D224G) was produced through site-directed mutagenesis (SDM) of the enzyme’s catalytic nucleophile, and subsequent activity tests confirmed glycosynthase function. This approach successfully identified variants with varied glycosynthase activity using glycosyl azides as donor substrates and can use potentially diverse acceptors, including human milk oligosaccharides.
Further investigations applied a site-saturation variants (SSV) strategy within a 10 Å radius of the active site. By correlating a chemical rescue assay with glycosynthase activity and verifying this relationship with Liquid Chromatography - Mass Spectrometry (LC–MS) product characterization, we established a useful proxy for pinpointing improved GS mutants. Beyond experimental methods, we developed a bioinformatics toolkit to streamline retrieval of Carbohydrate Active Enzymes (CAZymes) and assist with multiple sequence alignments, thereby guiding future protein engineering efforts. In parallel, we introduced an aptazyme-based GDP biosensor to potentially monitor nucleotide sugar formation in glycosyltransferase reactions, further supporting ongoing CAZyme engineering and cellular glycoengineering initiatives.
Collectively, these advances provide valuable screening methods and insights for protein engineering and ultimately contributing to understanding of glycan biosynthesis and its applications.Ph.D.Includes bibliographical reference
Strengthening and plastic flow of dislocation networks in face-centered cubic metals
No full textPrecipitate-dislocation interactions and forest interactions are the fundamental interaction mechanism in aluminum alloys. However there are still blanks left in the investigation of the two interaction mechanisms above. In chapter 2, we investigated the critical looping stresses of the plate-like precipitates with and without misfit fields, and our dipole model isable to predict the critical looping stresses of plate-like precipitates without misfit fields. In chapter 3, we employed forest model to investigate the forest interactions including junction residence time, link length distribution, flow rule, and other mechanical behaviors of dislocation links.
The crystal plasticity (CP) model, containing the flow rule and evolution mechanisms, is able to predict the mechanical response of a single crystal or polycrystalline materials. In chapter 4, first we investigated the link elongation rate of dislocation links to figure out the evolution mechanisms of dislocation links. Next we examined the correctness of link length model including the flow rule and evolution mechanisms. Finally we constructed crystal plasticity model based on link length model to predict the mechanical response of an aluminum single crystal.Ph.D.Includes bibliographical reference
Investigation of lithium metal stabilization through electrolyte and alloy optimization
No full textLi-ion batteries are a widely utilized energy storage tool found within many consumer and commercial devices. To meet the energy and power density demands of these sectors and allow for the continued practical use of Li-ion battery technology, there must be improvements upon currently utilized Li-ion battery materials. At present, graphite negative electrodes and lithium intercalation positive electrodes fail to meet accelerating energy density demands. The incorporation of Li-metal as an electrode material offers high theoretical capacity (~3860mAh/g), low redox potentials, and low density, allowing for significant improvements in gravimetric and volumetric energy densities when paired with high energy density cathode materials. Practical utilization of Li-metal as an electrode material, however, is hindered by heterogenous Li plating behavior, manifesting in dendrite formation, and resulting in poor performance and safety concerns. There are several strategies investigated to address this shortcoming, where emphasis is placed on altering electrode materials and electrolyte compositions in order to induce favorable Li-metal nucleation, plating and morphological evolution with cycling. In this work, a novel dual-salt fluoroganosiyl (FOS) based electrolyte was developed, based on the investigation of the initial formation reactions of the SEI, and found to enable high efficiency in-situ formed Li-metal batteries (<3% initial loss at 4.0mAh/cm2). Ultra-low areal plating capacities (0.08mAh/cm2) were utilized within Li-metal half cells, to amplify these initial SEI formation reactions, and correlate SEI formation efficiency with a given electrolyte modification. This amplification of SEI formation reactions was found to be predictive of capacity fade and dendrite formation under higher areal plating capacities (2.5, 4.0, 6.5 mAh/cm2), and thus can be utilized as a tool for relating initial SEI formation efficiency and longer-term performance. Further evaluation of this novel electrolyte using XPS characterization in the pre-SEI and early Li-plating environments revealed that contributions to the SEI from this novel electrolyte may include Li-C-N, Li3N and B-N components. Further work looking to stabilize the Li-plating interface involved the investigation of low voltage and high-capacity Li-alloying materials, such as Ag (<2150mAh/g) and Mg (<670mAh/g). The incorporation of Mg and Ag as negative electrode materials may allow for more favorable Li-plating behavior, where a continuum between Li metal plating and Li alloying reactions may be established. Here, a novel composition 7:1:0.125 Li:Mg:Ag has been found to enable low irreversible loss and stable cyclability vs Li-containing LCO and non-Li containing Nb0.99Ta0.1PO5 cathodes. Here, Li2AgMg and Li0.9Mg0.1 phases formed within this composition were shown to enable this observed high performance, where a mechanistic relationship between these phases is established.Ph.D.Includes bibliographical reference
Localization spectral sequences for the knot floer homology of strongly invertible knots
No full textWe establish two spectral sequences in knot Floer homology associated to a directedstrongly invertible knot K: one from the knot Floer homology of K to a two dimen-
sional vector space, and one from the singular knot Floer homology of a singular knot
associated to K to the knot Floer homology quotient knot of K. The first of these
spectral sequences is used to define a numerical invariant of strongly invertible knots.
The second spectral sequence splits along the Alexander grading in a way that we
pin down preciselyPh.D.Includes bibliographical reference
SPE-21, a palmitoyltransferase that is necessary for spermiogenesis and fertility in Caenorhabditis Elegans
No full textIn most animals, meiosis results in the formation of spermatids that must further differentiate into fertilization-competent spermatozoa. This process is known as spermiogenesis or spermatid activation in Caenorhabditis elegans, and it results in the transformation of round, non-motile spermatids into amoeboid, motile spermatozoa. C. elegans spermatid activation is associated with the fusion of Golgi-derived membranous organelles (MOs) with the sperm plasma membrane. This fusion process places MO-derived proteins onto the cell surface and prepares the plasma membrane of spermatozoa for fertilization. In our current study, we have identified, cloned, and characterized the role of spe-21, also designated as dhhc-5, during hermaphrodite and male sperm activation. spe-21 null mutant worms are severely sub-fertile at all studied growth temperatures due to sperm-associated defects. spe-21 mutant spermatids fail to activate both in vivo and in vitro after treatment with known chemical activators. We have identified the function of spe-21 in MO fusion during spermiogenesis. The gene spe-21 encodes a predicted four-pass transmembrane protein with a conserved Asp-His-His-Cys tetrapeptide, cysteine-rich zinc finger motif (DHHC-CRD type zinc finger motif). Generally, proteins with DHHC-CRD motifs are residents of Golgi or Golgi-derived vesicle membranes and catalyze post-translational addition of palmitate to their protein substrates. Therefore, they are called palmitoyltransferases or palmitoylacyltransferases (PATs). We also found that SPE-21 localizes to the MOs of spermatids. Our findings suggest that SPE-21, a MO-localized palmitoyltransferase, is required for proper spermatid activation and the creation of fertilization-competent sperm. Our findings also highlight the importance of lipid-related functions in gametes and fertilization.Ph.D.Includes bibliographical reference