Chemical and Biomolecular Engineering - Theses
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ItemTunable polymer capsules for therapeutic delivery applications( 2013)The area of therapeutic drug and gene delivery has made rapid progress over the last two decades. This interest in the development of drug delivery systems is aimed at protecting the body from the non-specific side effects of drugs and biomolecules; protecting therapeutic compounds from premature degradation and directing them specifically to target sites; and having tunable carrier degradation and cargo release to optimise the therapeutic efficacy. Of the drug delivery systems studied, the Layer-by-Layer (LbL) approach for the assembly of polymer multilayer films and capsules is of particular interest because it is a facile and highly versatile assembly technique that allows for specific properties to be tailored into the assembled materials to fulfill various criteria needed for the successful bioapplication of these systems. This thesis focuses on the development of a biologically compatible polymer capsule system based on the LbL technique and the incorporation of chemical and physical approaches to achieve stable cross-linked capsules, cargo loading and controlled release. Specifically, the work aims to (i) study the film build up and characteristics of cross-linked polymer films and capsules; (ii) develop stable polymer capsules capable of loading a cargo; (iii) demonstrate a modular approach in achieving tunable capsule degradation and cargo release; and (iv) provide some insight into the behaviour of these capsules in in vitro cell studies. This is demonstrated through the fundamental studies of the assembly of a low-fouling poly(N-vinyl pyrrolidone) (PVPON) capsule which uses a cross-linker in film stabilisation. The cross-linker has a reducible disulphide bond that endows the capsules with stimuli-responsive degradable properties. The optimised capsule system is loaded with a model plasmid DNA and used to demonstrate tunable carrier degradation and cargo release behaviour, through the use of the cross-linkers, in simulated cellular reducing environment. The study is extended to the assembly of hybrid systems, which is based on incorporating a charge-shifting polymer poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) and a block copolymer poly(2-(methacryloyloxy)ethyl-phosphorylcholine)-b-poly(2-(diisopropyl-amino)ethyl methacrylate) (PMPC-PDPA), into films and capsules. These hybrid systems are investigated for their properties. Finally, these different capsule systems are investigated for their interaction and internalisation behaviour in in vitro cell studies. The preliminary results offer some insight into the potential use of these highly engineered drug delivery carriers in biomedical applications.
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ItemPoly(methacrylic acid) hydrogel capsules as a platform for biomedical applications( 2012)The design and assembly of biocompatible nanoengineered carriers is of interest due to their potential applications in biotechnology as tools for catalysis and sensing, in biomedicine as systems for drug delivery, in diagnostics, and in vivo imaging. The layer-by-layer (LbL) technology is a prominent technique to design carrier systems for biomedical applications. In recent years, poly(methacrylic acid) hydrogel capsules (PMA HCs), based on disulfide-stabilized poly(methacrylic acid), have been fabricated from the LbL technique, and thoroughly studied to gain control over their stability, degradability and cargo release. These capsules are obtained by the sequential deposition of thiolated poly(methacrylic acid) (PMASH) and poly(N-vinylpyrrolidone) (PVPON) onto silica particles via hydrogen bonding. Upon controlled crosslinking and removal of the silica template, PVPON is released at physiological conditions due to the disrupted hydrogen bonding between PMASH and PVPON, which results in single-component PMA hydrogel capsules. This work provides an insight into designing a novel architecture and biofunctionalization of PMA HCs for enhanced and targeted drug delivery. Specifically, this research describes novel hydrogel capsule architectures, namely subcompartmentalized hydrogel capsules (SHCs), which are designed for potential applications in drug delivery and microencapsulated biocatalysis. Examples of SHCs with tens of subcompartments are demonstrated with their successful drug/cargo loading, as well as selective degradation of the SHC carrier and/or sub-units in response to multiple chemical stimuli. To develop a facile surface functionalization approach of the PMA hydrogel capsules, retention of PVPON was employed through modification of the polymer to obtain a bifunctional polymeric linker. The antibody-functionalized PMA/PVPON HCs demonstrate significantly enhanced cellular binding and internalization to specific cells, suggesting these capsules can specifically interact with cells through antibody/antigen recognition. To understand the impact of aspect ratio on cellular function, PMA HCs were prepared with various aspect ratios. Careful control over aspect ratio of the silica rods provided the ability to control the aspect ratio of the PMA HCs. Upon incubation of these capsules with living cells, varied behavior was observed, suggesting different mechanisms for their interactions with cells.
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ItemCapsosomes: en route toward synthetic cellular systems( 2011)Engineering artificial cells is currently an emerging area of research that involves constructing mimics of biological cells. These biomimetic cellular structures hold tremendous promise for the creation of next-generation therapeutic tools due to their ability to restore lost cellular functions. Amongst their potential applications, replenishing absent or malfunctioning enzymatic activities to degrade waste products or to support the synthesis of medically relevant biomolecules is a chief goal, which can provide long-term therapeutic solutions for chronic diseases. Artificial cells do not require the complex multifunctionality of their biological counterparts and can be more simply designed to perform a specific activity. A key approach in designing a cell-like system is a subcompartmentalized assembly, which is one of the features of biological cells that enable the performance of multiple complex biochemical reactions within confined environments. This thesis focuses on developing a bottom-up approach to assemble micron-sized vessels with a controlled number of enzyme-loaded subcompartments toward cell mimicry. Capsosomes, polymer hydrogel capsules containing controlled amounts of intact cargo-loaded liposomal subcompartments, were developed in this thesis and they represent a novel class of carrier system toward the design of bioinspired vehicles. Polymer capsules, assembled via the sequential deposition of interacting polymers onto particle templates (layer-by-layer technique, LbL) followed by core removal, serve as structurally stable scaffolds with tunable permeability that allow exchange of reagents and nutrients between the internal and external milieu – resembling cell membranes. On the other hand, liposomes divide the interior of the capsules into subcompartments and can stably encapsulate fragile hydrophobic and hydrophilic cargo, e.g., enzymes in order to conduct encapsulated catalysis – resembling cell organelles. The creation of (bio)degradable capsosomes is based on the sequential assembly of cargo-loaded liposomes and polymers onto sacrificial particle templates, followed by the LbL deposition of poly(N-vinylpyrrolidone) (PVP) and thiol-functionalized poly(methacrylic acid) (PMASH) via hydrogen bonding. Upon crosslinking the thiols of the PMASH and dissolution of the particle templates, colloidally stable capsosomes are obtained. The coassembly of polymers and liposomes was optimized via novel noncovalent linkage concept using tailor-made cholesterol-modified polymers and this unique approach facilitates stable incorporation of intact liposomes into polymer films. Spatial position of the subcompartments can be controlled, which yields capsosomes containing membrane-associated or “free-floating” subunits. Capsosomes exhibit size-dependent retention of the encapsulated cargo within the liposomal subunits. To prolong the stability of the liposomes in the compartmentalized assembly against degradative enzymes, the outer membrane of the capsosomes was surface functionalized with poly (ethylene glycol) (PEG). The functionality of capsosomes was demonstrated by triggered encapsulated (two-step) enzymatic catalysis. Capsosomes encapsulating glutathione reductase were able to generate glutathione, a potent antioxidant, while simultaneously releasing small therapeutic molecules, which highlights the ability of this subcompartmentalized assembly in addressing the complexity in therapeutic cell mimicry. The phase transition temperature of the liposomes was used as a trigger to initiate the enzymatic reactions, allowing capsosomes to be repeatedly used for multiple subsequent catalysis. Capsosomes with tailored properties present new opportunities en route to the development of functional cell mimics and the presented studies highlight crucial aspects for the successful applications of capsosomes as therapeutic artificial cells.
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ItemBiologically compatible thin films by click chemistry( 2010)The assembly of materials by the Layer-by-Layer (LbL) method provides a versatile and flexible method of assembling nanoscale structures with tailored composition, thickness, size, shape and functionality. Many conventional methods of assembling materials such as thin films and hollow polymer capsules fall short in achieving versatile, covalently bound multilayers. Combining click chemistry, a highly efficient and specific set of reactions, with the established LbL technique provides a number of advantages for the preparation of thin films and hollow polymer capsules. The click-LbL approach permits the production of stable, tailored and functionalisable materials through the modular assembly of a variety of building blocks. This thesis focuses on developing biologically compatible thin films and capsules by the click- LbL technique. Specifically, this work aims to: (i) investigate the fundamental factors controlling the growth and characteristics of click-LbL films; (ii) develop stable, biocompatible films; (iii) demonstrate that click chemistry is a facile and flexible means of functionalising multilayer films; and (iv) demonstrate that click-LbL films are capable of performing specific roles within biological systems. This will be demonstrated through a fundamental study on the assembly of thin films and capsules through assembly of alkyne- and azide-functional poly(acrylic acid) building blocks by the click-LbL technique. The technique is then extended to the assembly of covalently bonded films comprised solely of poly(ethylene glycol) (PEG) acrylates that limit the adsorption of serum proteins. By functionalising click-LbL films of PEG with a cell adhesion promoting peptide, these films then demonstrate the specific adhesion and growth of cells onto a surface. Multilayer films are also assembled using temperature-responsive PEG methacrylate copolymers and their ability to resist the adsorption of proteins from human serum is tested. Hollow capsules comprised of poly(N-vinyl pyrrolidone), with demonstrated low-cytotoxicity, are then produced by multilayer assembly onto colloidal silica. These capsules are covalently stabilised through a click-functionalised cross-linking agent, which can be cleaved under reducing conditions, to deconstruct capsules. The demonstrated properties of click-LbL films and capsules presented herein, offer potential for chemical engineering, pharmaceutical and in particular, biological applications.