Chemical and Biomolecular Engineering - Theses

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    Formation and Destruction of Environmental Contaminants in High Temperature Processes
    Narimani, Milad ( 2022)
    This thesis deals with the high temperature processes with the potential to generate environmental contaminants which adversely affect wildlife and human health. Identification of decomposition pathways and kinetics of reactions at moderate to elevated temperatures are required to avoid or limit the formation of contaminants. The text emphasizes both degradation chemistry and process design aspects as individual steps to form an efficient process. Safe and efficient implementation of a thermal process is highly dependent on understanding the chemistry of the process. Chapter 1 provides a brief overview of different high temperature processes with the potential for the formation and destruction of environmental contaminants ranging from industrial scale to personal devices. Waste-to-energy plants, water sludge and contaminated soil treatment, and chemical stockpile destruction facilities are industrial-scale processes that can be designed to use thermal methods for man-made chemical destruction. On a smaller scale, vaping devices are another group of thermal processes which can form a low concentration of contaminants but these chemicals directly inhaled by humans can be a serious threat to human health. Chapter 2 covers detailed chemical kinetic modeling of the two most regulated compounds of per- and poly-fluoroalkyl substances (PFAS) family namely perfluorooctane sulfonic acid (PFOS), and perfluorooctanoic acid (PFOA). A high amount of these forever chemicals can be found in solid waste material feeding to waste-to-energy plants. Also, these compounds can be presented in a lower concentration in wastewater sludge and contaminated soil due to their environmental persistence and wide array of applications. Kinetic modeling of perfluorinated sulfonamides, an emerging compound of PFAS family, is detailed in Chapter 3. To examine the effectiveness of thermal processes for the disposal of phosphorus chemical stockpiles a preliminary study using theoretical methods is crucial to be applied due to hazardous safety issues linked to their laboratory use. Thermochemical properties of less toxic simulant compounds, such as triethyl phosphate (TEP) and diethyl-methyl phosphonate (DEMP), and their decomposition intermediates along with their kinetic rate coefficients were calculated to modify the developed kinetic models for combustion and pyrolysis processes (Chapter 4). Glyphosate as the world’s widespread weed killer is another phosphorous compound classified as “probably carcinogenic to humans” and has the potential to be fully banned in the future. The thermal decomposition mechanism of glyphosate and its metabolite aminomethylphosphonic acid (AMPA) were presented in Chapter 5. In chapter 6, the biomolecular degradation of AMPA through a reaction with OH radicals in the atmosphere was studied. This reaction is of importance in high-temperature processes due to the presence of OH radicals. Chapter 7 focused on identifying the pyrolysis chemistry of triclosan and its chlorinated derivatives and then developing a chemical kinetic model to predict their pyrolysis products. This compound belongs to the family of pharmaceuticals and personal care products (PPCPs) and ended in biosolids from wastewater treatment as biosolids are used to fuel waste-to-energy plants. High temperature chemistry can be important even in a condition with a trace amount of target compound in the reaction chamber. In the vaping process, the decomposition products are directly inhaled into the human lung, and the presence of a low concentration (ppm) of an acutely toxic compound can be lethal. Vaping of vitamin E acetate (VEA) under dry hit vaping conditions was examined to predict toxic ketene formation potential (Chapter 8). This chapter presents the concentration of VEA decomposition reaction products versus vaping temperature. Chapter 9 focused on identifying the high-temperature chemistry of ethyl ester flavor additives and simulated vaping process in a plug flow reactor to find the concentration of vaping products in the inhaled stream.
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    Recycling of Lithium-ion Batteries by Solvent Extraction
    LU, Junnan ( 2022)
    The increasing demand for lithium-ion batteries (LiBs) is putting a strain on the critical metal supply chain. Moreover, a significant proportion of spent LiBs are disposed of in landfills. The linear LiB value chain has a massive environmental footprint, making it imperative to create a circular economy for LiBs by recycling spent LiBs to manufacture new batteries. Among the recycling processes, extractive hydrometallurgy is a promising technology for recovering valuable metals from black mass leachate. The purpose of this study is to establish a theoretical foundation for developing a cost-effective and robust solvent extraction process that can prepare mixed valuable metals LiB cathode resynthesis. A thermodynamic model, which assumes solvent extraction as a heterogeneous polynuclear complexation, was developed first. The model, namely equilibrium status iterations (ESI), required experimental data from mixed metal chloride solution to regress parameters related to the stoichiometric ratio and equilibrium constants. The ESI was used to describe the performance of solvent extraction of lithium, nickel, cobalt, and manganese under varying operating conditions. The experimental equilibria suggested that the solvent containing Cyanex 272 as the extractant, TBP (tributyl phosphate) as the modifier, and kerosene as the diluent was suitable for extracting lithium, cobalt, manganese, and nickel, however with low selectivity of cobalt and manganese. To reduce the separation cost, it was proposed not to separate the valuable metals during the recycling process as these would be mixed in the downstream cathode synthesis. To obtain the essential mass balance data for process design, synthetic leachate was prepared to capture all potential metals in practice, including lithium, cobalt, manganese, nickel, aluminium, copper, and iron. The experimental data confirmed ESI could predict the equilibria of solvent extraction of synthetic solution with seven metal ions in a sodium chloride solution. ESI was further applied to scale up the process, using a three-stage counter current mixer-settler to increase the lithium extraction yield, and a three-step stripping process to produce streams with different compositions. As a key novelty of this study, metals were not separated and purified individually. Instead, the solvent extraction process provided streams with mixed compositions, stoichiometrically aligned to manufacture cathode active material precursors. This novel approach saved significant costs compared to conventional solvent extraction processes, making it a recommended process in this study.
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    The performance of perfluoro(butenyl vinyl ether) based polymeric membranes in gas and vapor separation
    El Okazy, Moataz Ali Mostufa ( 2023)
    Amorphous perfluoropolymers have gained attention recently as a membrane material for gas separation because of their wide range of interesting features. They are chemically inert, thermally stable, have high gas permeability and display superior separation performance for several gas pairs including He/H2, He/CH4, N2/CH4 and CO2/CH4. They also have high resilience against plasticisation by condensable compounds such as water vapor and BTEX components (Benzene, toluene, ethylbenzene, and xylenes) due to the unfavorable interactions between their polymer matrix and these hydrogen containing molecules. Amorphous perfluoropolymers are thus promising membrane materials for CO2 capture from natural gas. CyclAFlorTM copolymers are amorphous perfluoropolymers in commercial use for optical fibers, but they have never been tested for gas separation. The gas separation performance of these perfluoropolymers was thus assessed in this thesis. The permeability of a series of pure gases was measured in CyclAFlorTM with two different monomer ratios at 35 C and higher temperatures below their glass transition temperatures. The permeability of these gases was also measured in Cytop for comparison. Permeability coefficients were found to be independent of pressure, while they followed Arrhenius behavior with temperature with a positive activation energy of permeation for all studied gases. The CyclAFlorTM copolymer with low FFV (Poly(88%PBVE-co-12%PDD)) showed separation performance close to that of Cytop for all gas pairs, while the one with high FFV (Poly(50%PBVE-co-50%PDD)) showed an interesting combination of high permeability and selectivity for several gas pairs compared to Teflon AF 1600. CyclAFlorTM copolymers operate in the vicinity of 2008 Upper Bound (UB) for several gas pairs including He/CH4, N2/CH4, N2/CH4 and He/N2. Cytop tested in the present study operates closer to the UB for several gas pairs than the performance reported in literature due to the greater removal of residual solvent. CyclAFlorTM and Cytop exhibited CO2/CH4 separation performance under mixed conditions comparable to the performance recorded with pure gases. The interesting separation performance of CyclAFlorTM and Cytop then led to a need to understand more about their superior performance compared to Teflon AFs and HyflonADs. The sorption of CO2 and CH4 were thus measured gravimetrically in Cytop and poly(50%PBVE-co-50%PDD) at different temperatures and pressures. The Non-Equilibrium Lattice Fluid (NELF) model and dual mode of sorption (DMS) model were then used to model the sorption isotherms in Cytop and poly(50%PBVE-co-50%PDD), respectively. The sorption data was consistent with the polymer fractional free volume (FFV), and the unfavorable interactions between methane and perfluoropolymers were confirmed. Both Cytop and poly(50%PBVE-co-50%PDD) showed CO2/CH4 solubility selectivity higher than other commercial perfluoropolymers. Gas diffusivity was then calculated using two different approaches for comparison: the transport model developed by Minelli and Sarti; and molecular dynamics (MD) simulations. The transport model confirmed the higher CO2/CH4 diffusivity selectivity of both polymers than Teflon AF 1600. The diffusion coefficients predicted by MD were overestimated relative to the transport model data, particularly for large penetrants (i.e. O2, N2 and CH4). The presence of condensable impurities such as water vapor, BTEX compounds and liquid glycol carryover can impact the performance of polymeric membranes. Therefore, the effect of water, toluene and xylene vapors on CO2/CH4 separation performance of Cytop and poly(50%PBVE-co-50%PDD) was studied over a wide range of vapor activities at 35 C with 10% CO2 in CH4 mixture at 9 bar. The sorption of these penetrants was also studied. Water vapor had no impact on the separation performance of Cytop and poly(50%PBVE-co-50%PDD) because of the low concentration of water vapor and the high FFV of these perfluoropolymers. Water permeability showed a decreasing trend with activity due to the formed water clusters, particularly at high activities. Toluene and xylene had no significant impact on the separation performance of Cytop, while both condensable penetrants caused a slight and gradual drop in CO2 and CH4 permeability with activity because of competitive sorption or/and pore blocking. Interestingly, convex sorption isotherms were noted with toluene and xylene in Cytop, while concave ones were obtained in poly(50%PBVE-co-50%PDD). This was attributed to the limited accessibility of toluene and xylene molecules to the Langmuir cavities in Cytop, unlike poly(50%PBVE-co-50%PDD). Liquid glycols carryover also had no impact on Cytop, while a slight plasticisation of poly(50%PBVE-co-50%PDD) was noted. High CO2 partial pressure (~ 7 bar) seemed to have no impact on the performance of Poly(50%PBVE-co-50%PDD), while a slight plasticisation was experienced with Cytop (3% increase in CO2 permeability). Overall, the thesis demonstrates that PBVE based perfluoropolymers are outstanding membrane materials for He/CH4, N2/CH4 and CO2/CH4 separation, with promising performance in the presence of a wide range of process stream impurities.
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    A multi-scale computational fluid dynamic study of buoyancy driven droplet coalescence
    Vadakkal Rasheed, Abdul Raize ( 2022)
    Liquid-liquid extraction is one of the critical mass transfer processing operations used in the mining and petroleum industries. High performance extraction columns achieve high mass transfer rates while processing large material throughputs. Droplet coalescence plays a vital role in determining the mass transfer rate in an extraction column due primarily to its role in determining droplet size distributions. However, models for determining droplet coalescence under realistic operating conditions are presently not available or poorly validated. The present study validates a numerical tool able to predict droplet coalescence in industrially relevant scenarios. Within the present study, we use simulations to predict the impact and possible coalescence between two hydrocarbon droplets within a water continuous phase. Two different collision scenarios are investigated: In the first case, the upper droplet is held stationary on a needle while the lower droplet rises via buoyancy and impacts the top drop - referred to as a ‘fixed’ collision. In the second case, both drops are free and moving towards each other - referred to as a ‘free’ collision. We use a previously developed MSIC (Multi-Scale Interface Capturing) Multiphysics algorithm to predict the collision outcome. The technique employs a novel sub-grid scale method to calculate the film pressure between the colliding droplets and the volume of fluid method (VOF) for capturing droplet interface deformation. The model is validated against experimental data for both bounce and coalescence outcomes produced by our collaborators within the Department of Chemical Engineering at the University of Melbourne. We obtained a phase map showing collision outcome as a function of diameter ratio and Weber number (We, interpreted as a non-dimensional impact velocity). The diameter ratio is defined as the ratio of the diameters of the bottom drop to the top drop. To obtain the collision phase map, a systematic series of droplet collision simulations were carried out. The results of the fixed collision simulations show a transition of bouncing to coalescence as We is increased, under conditions where the diameter ratio is less than or equal to one; however, the critical We numbers that define the bouncing to coalescence transition are different from those previously reported in the literature, which are for a gas-liquid system. For fixed collisions between droplets having different diameters the results are more complicated, with larger diameter ratios tending to favour bounce outcomes. In contrast, for free collisions, regardless of the diameter ratio and We considered in the present analysis the outcome was bounce. In addition, we obtained the contact/drainage time during the droplet interactions and also the rebound velocity in the case of bounce events. As well as predicting the bounce and coalescence outcomes, the simulations can be used to examine the physics of the film drainage process in more detail. We compared the pressure distribution and the flow dynamics inside the thin film for both mobile and immobile droplet interfaces. Notably, for the mobile interface case, the effect of interfacial velocities resulted in a non-monotonic pressure distribution within the film, resulting in surface fluctuations. In addition, we noted that if the change in pressure gradient exceeds a critical limit, surface tension can no longer keep the interface flat/smooth, leading to the growth of interface instabilities that could lead to coalescence. Whereas for an immobile interface, the pressure distribution was observed to generally decrease monotonically towards the rim, thus creating a stable smooth interface during the interaction. As a result, the outcome obtained with the immobile interface was bounce even for the maximum impact We studied in the present case. The results generated within this work show that 2D axisymmetric droplet collisions can be rigorously determined using this novel computational fluid dynamics tool. Future work will extend the model to include non-axisymmetric collisions, and more detailed interfacial chemistry, focusing particularly on industrially relevant applications.
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    Exploring additive manufacturing and non-solvent induced phase separation for fluoropolymer membrane production
    Imtiaz, Beenish ( 2022)
    The alarming rise in scarcity of fresh and safe drinking water around the world has prompted an increase in search for new materials to fabricate more efficient water filtration membranes. Poly(vinylidene difluoride) (PVDF) is a popular membrane material however its use as long lasting membranes is hindered by fouling, compaction, and caustic damage. The overarching aim of this thesis was to integrate engineering manufacturing with membrane production in order to explore innovative methods for fabrication of flat sheet and patterned fluoropolymer membranes. The first part of this thesis reports on the bulk synthesis of dehydrofluorinated PVDF (dPVDF), catalysed by a reaction of PVDF with ethylenediamine (EDA), which results in alkene moieties along the backbone of the polymer chain, as confirmed by attenuated total reflectance – Fourier transform infrared (ATR-FTIR) and Raman spectroscopies. The second study of this thesis investigated the suitability of dPVDF solutions (prepared in N,N-dimethylacetamide (DMAc), with or without poly(vinyl pyrrolidone) (PVP)) for direct ink writing (DIW). Steady-state rheology confirmed the non-Newtonian and shear-thinning character of these solutions, whereas oscillatory rheology demonstrated superior gelling behaviour of dPVDF solutions with PVP. Additionally, the presence and changes in microstructure of the solutions were studied via modified Cole-Cole plots and Gurp-Palmen plots. Moreover, a stepwise oscillatory shear rate cycling was carried out to simulate the behaviour of the printing inks during DIW, that indicated complete recovery of microstructure in dPVDF ink post-deposition. The third study presents a hybrid process, implying DIW + ex situ non-solvent induced phase separation (NIPS), for the fabrication of flat sheet dPVDF microfiltration (MF) membranes. To produce inks for DIW, the dPVDF was dissolved in DMAc along with a pore-forming agent, PVP (at 5-30 wt%, relative to dPVDF concentration). Membranes were produced by DIW of the inks into continuous wet films - followed by NIPS in deionised (DI) water. The fabricated dPVDF membranes were more hydrophobic (water contact angle, WCA = 115 degrees) than the similarly fabricated PVDF membranes (WCA = 99 degrees) yet had greater equilibrium water content (EWC) and porosity, which correlated to the morphology of the fabricated membranes. The dPVDF membranes with 30 wt% PVP not only demonstrated stability in a caustic environment (1 M NaOH for 90 min), but also had a pure water flux of approximately 4300 LMH, within the range of commercially available PVDF membranes (approximately 6300 – 8100 LMH). Among the many physical methods of improving fouling resistance is the fabrication of patterned membranes, where the patterns provide the functionality of an integrated spacer, thus providing turbulence to the incoming feed. Despite fluoropolymers being common polymers for membrane fabrication, their commercial production into membranes remains dominated by simple casting and solvent phase separation. The final study of this thesis demonstrated a rapid and simple approach to produce patterned fluoropolymer membranes, with profiled surfaces, via immersion precipitation printing (ipP). This study utilised DIW + in situ NIPS in isopropyl alcohol (IPA), followed by ex situ NIPS in DI water. The direct phase inversion of the patterns during membrane production induces a porous morphology. Further, pure water permeability studies were performed on unpatterned membranes and membranes with patterns fabricated with a combination of different fluoropolymer materials (PVDF and synthesised dPVDF). This simple ipP approach showed potential as a viable alternative for the production of fluoropolymer membranes where the complete control over pattern height, fidelity, and shape is required. In summary, this thesis has developed a hybrid approach comprising of DIW + NIPS for membrane production, in which PVDF has been dehydrofluorinated to produce to dPVDF using EDA, and then fabricated into flat sheet membranes. These membranes have shown to be used as microfiltration (MF) membrane with improved caustic resistance relative to commercial membranes. Moreover, ipP has been utilised for the first time to fabricate patterned membranes with varying pattern shapes and heights from PVDF and/or dPVDF. This potentially points towards fabrication of membranes from different materials with variable geometries and tuneable porosities based on solvents, non-solvents, and pore-formers. The implications of this research are far-reaching as it points towards the integration of engineering manufacturing and membrane preservation. These discoveries will form the basis of future work expanding into smart membranes, bespoke membrane form factors, and new filtration applications.
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    Multiscale characterisation and sub-nanometre engineering of reduced graphene oxide membranes
    Cao, Yang ( 2022)
    Multilayered 2D nanomaterial-based membranes with sub-nanometre pores or channels hold great promise for widespread applications such as compact energy storage, ion sieving, membrane separation and bio-electronics. Understanding their assembly structure across multiple length scales, particularly at the sub-nanometre scale, is crucial to realise their precise structural engineering for desired applications but has largely been overlooked in the literature. With the chemical conversion of multilayered graphene oxide membranes to their electroconductive form as a model system, this thesis is devoted to investigating the stacking behaviour of 2D nanoscale building blocks in laminar membranes across multiple length scales during their chemical conversion. With a combined approach of various light scattering techniques and dynamic electrosorption analysis, this study establishes a hierarchical structural model of ultra-dense graphene membranes to describe how the graphitised clusters, sub-nanometre channels and large slit-like voids co-exist in the membranes in an interconnected manner. These structural features and their hierarchical organisation are dependent on the synthetic conditions, including the reduction agent, reduction degree, reduction kinetics, concentration of electrolyte and nature of solvent in the reduction environment. This study also reveals that the (sub-) nanotexture of the graphene membranes, including the size and distribution of the graphitised clusters, the connectivity and structural ordering of the nanochannels, and the extent of graphitisation/crystallinity, can have a significant effect on the transport of ions through the membranes under electrification. The results enable the development of several new methods for engineering their sub-nanometre structure for compact capacitive energy storage and future nanoionics applications.
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    A Detailed Chemical Kinetic Modeling of Nitrogen Containing Fuel Oxidation
    Alam, Mohammad Ashraful ( 2022)
    Nitrogen-containing organic fuels i.e., Methylamine (CH3NH2), Dimethylamine (CH3NHCH3), Ethylamine (CH3CH2NH2), and/or solvent i.e., 2-aminoethanol (NH2CH2CH2OH) are the most common aliphatic amines extensively used in the chemical, petrochemical, energy industries, or carbon capture technology in natural gas or coal power plant. In the oxidation or combustion process, these organic fuels or solvents are released into the atmosphere as a form of hazardous compounds e.g., NH3, CO2, NOx, or volatile organic compounds (VOCs) which leads to the formation of nitramine and nitrosamines in the atmospheric oxidation process. In recent decades, the number of kinetic models of nitrogen base amine fuel combustion has been elucidated in a wide range of gas phases and supercritical water conditions. Although extensive experiment or computational research evaluated the number of intermediates and products including identified the relation to environmental degradation products, a detailed understanding of major nitrogen containing intermediates contributing to NH3, CO2, NOx, and N2 formation are still lacking, as well as has not been well demonstrated the efficiency of hydroperoxy radical reaction chemistry. A detailed analysis and understanding in-sight into the combustion or oxidation process of these nitrogen containing organic fuels or solvent are important to elucidate the mechanism by which these hazardous or polluant compounds are formed and may be achieved through a refined or developed reaction kinetic model. In this dissertation, peroxy radical chemistry and its role in nitrogen-based organic fuel combustion or solvent degradation in supercritical water or gas phase conditions have been thoroughly investigated. This work also investigated the formation of an imine and decomposition reaction followed by hydrolysis reaction chemistry. This dissertation proposed new reaction sets which have been computationally tested and validated against several experiments under a variety of conditions, for instance, fuel equivalence ratios, temperature, and concentrations of diluent gas, which have simultaneously described the importance of new reactions set in the combustion model of nitrogen containing fuels oxidation and degradation in the gas phase and supercritical water condition.
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    Structural Control of Synthetic Polypeptides in Biomaterials at Multiple Dimensional Scales
    Chan, Nicholas Jun-An ( 2022)
    Proteins and peptides hold particular interest amongst biopolymers as major structural and functional natural components of many materials. Polypeptides themselves are their molecular structure which are sequences of amino acids with proteins being approximately 100 amino acids or greater in length while peptides are below this length. Once covalently linked, side chains interact with the molecular backbone resulting in spontaneous protein folding and thus resulting in well-defined, sequence-specific secondary structures including alpha-helices, beta-sheets and random coils. However, such secondary structures can be manipulated through processing to yield materials with unique structures. As secondary structures and their eventual overall conformation results in different physical and chemical properties, this study aims to address the combination of these issues for the development of different materials. This study investigates a range of novel fabrication methods for manipulating and utilizing the secondary structures of non-sequence specific polypeptides in materials at different dimensional scales. Specifically, strategies to fabricate hydrogels, films and fibers have all been developed to produce materials which either utilize secondary structure-based nanostructures which have been traditionally hard to utilize such as beta-sheet nanocrystals or produces materials with secondary structures contradictory to their usual structure. Subsequent properties resulting from this secondary structure control were investigated to discern their impact and subsequent potential in various applications. For the development of hydrogels, synthetic polypeptides are generally used to form secondary structure-based physical crosslinks. beta-sheet crosslinks are especially favourable due to their role in tough natural silks and thus been used to develop tough hydrogels. In this study, chemically crosslinked hydrogels were initially synthesized with pendant amine groups to initiate the polymerization of poly(L-valine) (PVal) and poly(L-valine-r-glycine) (PLVG) as beta-sheet forming polypeptide. By grafting from the hydrogel, issues of steric hindrance are mitigated and thus hydrogels with high beta-sheet content can be produced. The resulting hydrogels displayed greatly improved mechanical strength over their counterparts, lacking beta-sheets with improved compressive strength and stiffness of up to 30 MPa (300 times greater than the initial network) and 6 MPa (100 times greater than the initial network), respectively. Furthermore, this technique was found to be applicable to a range of different polymer hydrogels, with demonstrated applicability in 3D printed structures. For the development of modified surfaces, synthetic polypeptides have previously been applied to deposition- and polymerization-based techniques. Our group has previously utilized a unique surface modification method whereby macrocrosslinkers with multiple polymerizable sidechains are utilized in a process called the continuous assembly of polymers (CAP). To observe the effect of using this specific conformation on polypeptides, poly(L-lysine) (PLLys) based polymers were utilized as a model for CAP utilizing reversible addition-fragmentation chain-transfer (RAFT) polymerization as the polymerization medium. Through manipulation of different RAFT agents and conditions, surface films of up to 36.1 +/- 1.1 nm thick which could be increased to 94.9 +/- 8.2 nm upon multiple applications of this process. Versatility was found in using poly(L-glutamic acid) (PLGlu) as another model polypeptide. Furthermore, films had high random coil conformation which is especially unusual PLLys films which would favour alpha-helical conformations. Enzymatic degradation was found using different digestive and wound healing proteases with different rates of degradation depending on the use of PLLys or PLGlu- based macrocrosslinkers. Finally, fibers utilising beta-sheets have been developed using sequence-specific methods to control aggregation. Other methods utilizing beta-sheets require multi-step or complicated processes to yield the precursor polymer required for polymer formation. Thus, this study develops a method of fabricating fibers using a broad range of preexisting polymers by introducing the valine-based monomer into the precursor solution (i.e. the fiber spinning dope) and triggering polymerization during fiber spinning resulting in PVal. Beta-sheets could be successfully integrated into all polymeric fibers, but with greatly differing impacts on substructures based on intermolecular bonding. Polymers (polycaprolactone (PCL) and cellulose acetate) with low hydrogen bonding potential were found to show increased crystallization. Such polymers displayed increase tensile strength upon beta-sheet introduction (2.2 and 4.3 times increase respectively for PCL and cellulose acetate fibers). In contrast, poly(amide) based polymers with high hydrogen-bonding potential (Nylon 6 and poly(benzyl-L-glutamate) (PBLG)) did not show any increased crystallization, though an increase in extensibility at break (2.9 and 1.8 times increase respectively for nylon 6 and PBLG) was observed. Finally, in other polymers without either of these features, intercalation of beta-sheets into the usual polymeric assembly of the polymer results in either minimal or reduced mechanical properties. In summary, fabrication processes for a broad range of different polypeptide based materials have been investigated to control both secondary structure and polymeric substructures. From this, properties such as mechanical and degradation potential have been studied in relation to these properties and thus revealed a range of novel approaches to polypeptide-based material fabrication.
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    Encapsulation of Lactic Acid Bacteria and γ-Amino Butyric Acid using Exopolysaccharides for Food Applications
    Pandey, Pooja ( 2022)
    The demand for functional foods containing probiotics and bioactives is increasing. In this thesis, lactic acid bacteria as probiotics and GABA as a bioactive were coencapsulated using different techniques viz spray drying, freeze drying and double emulsion using ultrasonication in a biocompatible matrix. Bulk encapsulation was achieved using spray and freeze drying by optimizing the wall material composition. The optimum shell composition obtained by D optimal mixture design was 0.4% inulin, 4.6% dextran, and 8.4% of maltodextrin respectively. Various homogenization techniques have been used to encapsulate bacterial cells via emulsification using homogenizers. However, the encapsulation of bacteria using ultrasound is relatively unexplored, and the survival of bacteria during ultrasonication is yet to be investigated. The possibilities of encapsulating L. plantarum and GABA in an inner aqueous phase of (W1/O) single emulsion using ultrasound was explored. The bacteria cells were treated using a 20 kHz, 3 mm microtip ultrasonic horn for varying sonication times at 10 W calorimetric power and the cell viability was assessed. No significant differences were observed in the bacterial counts of control and the sonicated samples. Single emulsions (W1/O) were prepared using W1 (2% w/v bacterial cells and 5% w/v GABA) and soyabean oil (O) with 1% w/w PGPR, with a volume ratio of 40:60 and ultrasonication (3 W/mL for 30 s) to produce a stable emulsion. Double emulsion microcapsules (W1/O/W2) were prepared using the single emulsion (W1/O) mixed with W2 (5% w/v dextran or 5% w/v whey protein solution, or 5% dextran and 5% GABA solution) by ultrasonication (3W/mL for 10 s) at volume ratio of 20:80. During sequential in vitro digestion, dextran capsules were stable in simulated stomach and small intestine juices. Dextran capsules were freeze dried to form shelf-stable true capsules. Spray dried (SD), freeze dried (FD), double emulsion (DE) and double emulsion freeze dried capsules (DFD) were compared and found that SD powders have good flowability and better shelf life compared to DFD powders. However, the DFD powders were able to provide reasonable protection to the LAB during both storage and digestion, whereas the bulk powders could confer good protection during storage but limited protection during in vitro digestion. A grain-based functional beverage premix and cookies were developed by incorporating SD and DFD microcapsules. SD powder was added at 10 or 20% w/w to beverage premix samples and stored for 75 days under refrigerated (5 degree C, 90% RH) or ambient (25 degree C, 40 % RH) conditions. Physicochemical properties such as nutritional profile, water absorption index (WAI), water solubility index (WSI), color, the viability of probiotics, GABA content, and sensory attributes were analyzed. The incorporation of 10% w/w microencapsulated probiotic powder was adequate to retain desirable functional properties in the resulting beverages with an overall acceptability score of 7 on the hedonic scale. The results showed that beverage premix-maintained L. plantarum viability (>10^7 CFU/g) and GABA content (> 30 mg/g) during storage of up to 75 days. Free bacteria died during baking while some of the microencapsulated bacteria survived in the cookies. Microencapsulation improved the viability of L. plantarum during baking up to 15 min in oven at 160 degree C. GABA was stable during the baking process for both the control and microencapsulated cookies. Thus, the microencapsuled L. plantarum and GABA can be used in developing functional food products.
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    Spider silk-inspired functional materials with tailored surface properties for biomedical applications
    Lentz, Sarah ( 2022)
    Biomaterials science is an increasingly important and constantly evolving field of science. Only intensive cooperation between different disciplines and a deep understanding of the physical and chemical interactions within developed materials and the biological system as a whole lead to the successful development of new biomaterials. Biocompatibility plays a central role here. It must be possible to assess whether the material is compatible with the respective application, e.g., implantation in hard or soft tissue. Here, a further distinction can be made between structural and surface compatibility. Structural compatibility covers the structure, shape, and mechanical property interactions in a biological environment. Surface compatibility summarizes the adaptation of chemical, physical, biological, and morphological surface properties to the biological environment. Consequently, the surface properties of a biomaterial are crucial for its biocompatibility and interactions with the host system. Materials used as biomaterials must fulfill a wide range of requirements. They should have excellent mechanical stability, be biocompatible and, depending on the requirements, bioinert or bioactive. For example, bioactive biomaterials are used to increase or control interaction with cells. Synthetic polymers usually have excellent mechanical properties but then lack biocompatibility, whereas natural polymers often have excellent biocompatibility but then are mechanically very weak and therefore not suitable for applications with high mechanical stress. A promising material that exhibits the advantages of both classes of polymers is spider silk. Spider silk has been used since ancient times as wound dressings and suture material as it is mechanically resilient and elastic and elicits little to no immune response. Natural spider silk cannot be used as a biomaterial on a large scale due to the cannibalistic behavior of most spider species and changing quality of silk. Therefore, this work presents two approaches utilizing materials inspired from natural spider silk to create functional, modifiable, mechanically resilient, and biocompatible coatings. The first approach is bioengineered recombinant spider silk proteins. Before biotechnological production, these proteins produced can be genetically modified in E.Coli bacteria. In this work, twelve different spider silk protein variants are used and investigated concerning their biocompatibility, biodegradability, and interaction with proteins, cells, and human blood. These spider silk protein variants are non-toxic and can be resorbed by the body as they consist solely of amino acids. The second approach is based on synthetic polypeptides prepared by the continuous assembly polymerization (CAP) method, published for the first time, using reversible-addition-fragmentation chain-transfer (RAFT) polymerization, or CAP-RAFT. Polypeptides were selected based on amino acids found in natural spider silk (L-lysine and L-glutamic acid). These coatings based on synthetic polypeptides were investigated concerning secondary structure and biodegradability. CAP-RAFT was established as a viable strategy to prepare surface-limited cross-linked polypeptide films with precise film thickness control and novel properties such as specific secondary structure formation and biodegradation. This variability of secondary structure combined with enzymatic degradation shows high potential for numerous biological applications. In the present work, secondary structure formation and assembly of the spider silk-inspired materials on coatings were investigated in detail. Firstly, the effect of coating thickness on the structural properties (beta-sheet fraction) was investigated from the nanoscale to the microscale. A coating thickness-dependent assembly and phase separation model is presented. In addition, the orientation of beta-sheets in recombinant spider silk coatings was investigated. Another important aspect of surface biocompatibility is the structure-property relationship of these spider silk-based materials. Concerning applications in the biomedical field, the interaction between material and biological environment is essential. Several aspects are studied in detail: specifically surface charge, surface chemistry, surface topography, and surface hydrophilicity. These aspects were analyzed to understand the interaction with proteins, cells, and blood as well as their biodegradability. Based on the results of the respective studies, it was possible to categorize the different spider silk variants into bioinert and bioactive variants and assign their subsequent potential biomedical applications. Positively charged spider silk protein variants are bioactive and have the most significant interaction with cells and blood. Modification with the cell-binding peptide improved cell adhesion of all variants used. Amino acid sequences based on the natural Araneus diadematus fibroin (ADF) 3 protein showed significantly faster enzymatic degradation than the protein variants based on the amino acid sequence of ADF4. The introduction of three-dimensional patterns on the coating surface can significantly increase the adhesion of cells to material (negatively charged variant), which shows little adhesion of cells as a smooth coating. In this dissertation, the structure formation, assembly, and structure-property relationships of spider silk-inspired materials were systematically investigated. These spider silk-inspired materials possessed a high potential for application in various biomedicine fields due to the diverse modification possibilities in terms of morphology, amino acid sequence, and charge.