Thesis (M.S.)--University of Alabama at Birmingham, 2009. / Title from PDF title page (viewed Jan. 29, 2010). Includes bibliographical references (p. 66-70).

Thesis (Ph. D.)--University of Hong Kong, 2010. / Includes bibliographical references (p. 253-274). Also available in print.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Acetylcholine is a neurotransmitter that regulates neurite branching, induces neurite outgrowth, and synapse formation. Because of its various roles in neuronal activities, acetylcholine-based materials may also be useful in nerve repair. We present a series of biodegradable polymers with varying concentrations of acetylcholine-like motifs. We hypothesize that neurite sprouting and extension can be enhanced by using materials to present biochemical and physical cues. Acetylcholine-like motifs were incorporated by the polycondensation of diglycidyl sebacate, aminoethyl acetate, and leucine ethyl ester, which permitted control over acetylcholine motif concentration. Interactions between the polymers and neurons were characterized using rat dorsal root ganglia explants (DRG). We screened the potential application of these materials in nerve tissue engineering using the following criteria: 1) neurite sprouting, 2) neurite length, and 3) distribution of the neurite lengths. The ability of DRG to sprout neurites was influenced by the concentration of acetylcholine motifs of the polymer. Addition of acetylcholine receptor antagonists to DRG cultured on the polymers significantly decreased neurite sprouting, suggesting acetylcholine receptors mediate sprouting on the polymers. Future studies may examine how neurons on acetylcholine-based polymers exhibit changes in downstream signaling events and cell excitability that are associated with receptor activation. In preparation for testing the acetylcholine-based polymers in vivo, porous scaffolds with longitudinally oriented channels were fabricated using fiber templating and salt leaching. Micro computed tomography, scanning electron microscopy, and cryo-sectioning revealed the presence of longitudinally oriented channels. Channel volume and average pore size of the scaffolds were controlled by the number of fibers and salt fusion time. Future studies may involve testing the effect of acetylcholine-motifs by coating polymers onto such scaffolds or assessing the effect of the scaffold's dimensional properties on nerve regeneration.

The biocompatibility and biodegradability of poly(lactic acid) (PLA) facilitate its use in a variety of biomedical applications, ranging from sutures to drug delivery. However, uncontrolled interactions with cells and insufficient mechanical properties have prevented PLA from reaching its full potential as a scaffold for use in tissue engineering. Methods to improve the mechanical, chemical and biological properties of PLA are limited by the lack of functional groups along the backbone of the polymer. One possible approach towards overcoming these limitations involves the incorporation of functional groups into the backbone of the polymer through the copolymerization of monomers bearing protected functional groups. Deprotection and modification of these functional groups could provide the opportunity to direct the attachment of cells, and enhance to the physical properties of the polymer. We have developed a general methodology for the synthesis of lactide monomers substituted with protected functional groups (alcohols protected as benzyl ethers, amines protected as benzyl carbamates and carboxylic acids protected as benzyl esters). The monomers were homopolymerized, and copolymerized with lactide, and deprotected to give functional PLA copolymers with pendant hydroxyl, amine, and carboxyl groups. A thorough investigation of the chemical modification of PLA copolymers bearing functional groups along the polymer backbone was performed on a copolymer prepared by copolymerizarion of a dibenzyloxy-substituted lactide monomer with lactide followed by reductive debenzylation. Reaction of the resulting hydroxyl-substited PLA with succinic anhydride resulted in an acid-substituted PLA that is amenable to standard EDC/NHS coupling. The utility of this copolymer was illustrated by coupling with an amine derivative of biotin, and an RGD-containing peptide sequence. The preparation of the biodegradable polyester substituted with RGD, a ubiquitous adhesion peptide, provided us with control over cellular attachment to the hybrid material. We also explored approaches to make use of the pendant functional groups on PLA to enhance the physical properties of polymer foams. Copolymers with pendant photocrosslinkable cinnamate groups were prepared by reaction of the hydroxyl-substited PLA copolymers with cinnamoyl chloride. The copolymer was foamed using thermally-induced phase separation (TIPS), and photocrosslinked upon irradiation at 300 nm. Irradiation resulted in an increase in the compressive modulus of the foams. Crosslinking also led to a decrease in the rate of hydrolytic degradation of the foams, thereby demonstrating the potential for use of these strategies in the development of porous scaffolds for bioengineering. Another potential approach towards the preparation of robust polymer foams is the incorporation of a rigid polymer block which can phase separate during foam formation to provide additional structural integrity. Several poly(norbornene)-PLA diblock copolymer compositions were prepared by the ring-opening of lactide by a hydroxyl-terminated poly(norbornene) macroinitiator. The ability of the diblock copolymer to phase separate at elevated temperature was verified using small-angle x-ray scattering and wide-angle x-ray scattering.

[Truncated abstract] The synthetic hydrogel poly(2-hydroxyethyl methacrylate) (PHEMA) has been used as a biocompatible biomaterial in ocular devices, such as soft contact lenses, intraocular lenses and an artificial cornea. Due to its favourable properties as an already established (but non-biodegradable) biomaterial, PHEMA is an interesting candidate for use as a material for scaffolds in tissue engineering. A tenant of tissue engineering scaffolds is obtaining the appropriate porous morphology to allow for successful cellular attachment and support. PHEMA hydrogels exhibit varied morphological features, which range from non-porous (homogeneous) to macroporous (heterogeneous) and can be readily obtained by fine-tuning the polymerisation conditions. A desirable feature for matrices that are to be used as tissue supports is the ability to biodegrade in a biological environment. This thesis describes the preparation and enzymatic biodegradation behaviour of novel porous PHEMA hydrogels that have been crosslinked with biodegradable peptide-based crosslinking agents. Peptide-based crosslinking agents were designed to contain two terminal polymerisable groups flanking an internal biodegradable backbone. This backbone was specifically designed to be targeted by the proteolytic enzyme papain. The general design template allowed for the development of a synthetic methodology that was readily implemented for the production of a range of olefin-peptide conjugates. A suite of olefin-peptide conjugates of general structure I were synthesised, characterised and further tested with papain to determine their biodegradation properties. ... The second strategy for producing bioresorbable degradation fragments involved the incorporation of the highly hydrophilic comonomer, poly(ethylene glycol) PEG into the PHEMA backbone. The addition of PEG to PHEMA resulted in the formation of homogeneous hydrogels that had an improved hydrophilicity compared to their heterogeneous PHEMA counterparts. The synthetic conditions for the preparation of PHEMA and PHEMA-co-PEG hydrogels by photoinitiated polymerisation were thoroughly investigated. It was found that the pore morphology and general properties (non-porous to macroporous) of these hydrogels could be controlled by the appropriate choice of polymerisation conditions. The hydrogels were characterised by scanning electron microscopy, thermal gravimetric analysis and differential scanning calorimetry. The peptide-based crosslinking agents were successfully co-polymerised with the HEMA and PEGMA via photoinitiated polymerisation to provide a range of PHEMA and PHEMA-co-PEG hydrogels that displayed both homogeneous and heterogeneous hydrogel properties. The final crosslinked hydrogels were characterised by scanning electron microscopy and were subjected to enzymatic hydrolysis. The PHEMA-peptide conjugate hydrogels proved to be biodegradable, with degradation behaviour dependent on the hydrogel formulation and the length of the peptide-based crosslinking agent.

Introduction: Embolic agents are used to block blood flow of hypervascular tumours, ultimately resulting in target tissue necrosis. However, this therapy is limited by the formation of new blood vessels within the tumour, a process known as angiogenesis. Targeting angiogenesis led to the discovery of anti-angiogenic factors, large molecular weight proteins that can block the angiogenic process. The aim of this research is development of poly (vinyl alcohol) (PVA) aqueous solutions that cross-link in situ to form a hydrogel that functions as an embolic agent for delivery of macromolecular drugs. Methods: PVA (14 kDa, 83% hydrolysed), functionalised by 7 acrylamide groups per chain, was used to prepare 10, 15, and 20wt% non-degradable hydrogels, cured by UV or redox initiation. Structural properties were characterised and the release of FITCDextran (20kDa) was quantified. Degradable networks were then prepared by attaching to PVA (83% and 98 % hydrolysed) ester linkages with an acrylate end group. The effect on degradation profiles was assessed by varying parameters such as macromer concentration, cross-linking density, polymer backbone and curing method. To further enhance the technology, radiopaque degradable PVA was synthesised, and degradation profiles were determined. Cell growth inhibition of modified PVA and degradable products were also investigated. Results: Redox initiation resulted in non-degradable PVA networks of well-controlled structural properties. Increasing the solid content from 10 to 20wt% prolonged the release time from few hours to ~ 2 days but had no effect on the percent release, with only a maximum release of 65% achieved. Ester attachment to the PVA allowed flexibility in designing networks of variable swelling behaviors and degradation times allowing ease of tailoring for specific clinical requirements. Synthesis of radiopaque degradable PVA hydrogels was successful without affecting the polymer solubility in water or its ability to polymerize by redox. This suggested that this novel hydrogel is a potential liquid embolic with enhanced X-ray visibility. Degradable products had negligible cytotoxicity. Conclusion: Novel non-degradable and radiopaque degradable PVA hydrogels cured by redox initiation were developed in this research. The developed PVA hydrogels showed characteristics in vitro that are desirable for the in vivo application as release systems for anti-angiogenic factors.

Introduction: Embolic agents are used to block blood flow of hypervascular tumours, ultimately resulting in target tissue necrosis. However, this therapy is limited by the formation of new blood vessels within the tumour, a process known as angiogenesis. Targeting angiogenesis led to the discovery of anti-angiogenic factors, large molecular weight proteins that can block the angiogenic process. The aim of this research is development of poly (vinyl alcohol) (PVA) aqueous solutions that cross-link in situ to form a hydrogel that functions as an embolic agent for delivery of macromolecular drugs. Methods: PVA (14 kDa, 83% hydrolysed), functionalised by 7 acrylamide groups per chain, was used to prepare 10, 15, and 20wt% non-degradable hydrogels, cured by UV or redox initiation. Structural properties were characterised and the release of FITCDextran (20kDa) was quantified. Degradable networks were then prepared by attaching to PVA (83% and 98 % hydrolysed) ester linkages with an acrylate end group. The effect on degradation profiles was assessed by varying parameters such as macromer concentration, cross-linking density, polymer backbone and curing method. To further enhance the technology, radiopaque degradable PVA was synthesised, and degradation profiles were determined. Cell growth inhibition of modified PVA and degradable products were also investigated. Results: Redox initiation resulted in non-degradable PVA networks of well-controlled structural properties. Increasing the solid content from 10 to 20wt% prolonged the release time from few hours to ~ 2 days but had no effect on the percent release, with only a maximum release of 65% achieved. Ester attachment to the PVA allowed flexibility in designing networks of variable swelling behaviors and degradation times allowing ease of tailoring for specific clinical requirements. Synthesis of radiopaque degradable PVA hydrogels was successful without affecting the polymer solubility in water or its ability to polymerize by redox. This suggested that this novel hydrogel is a potential liquid embolic with enhanced X-ray visibility. Degradable products had negligible cytotoxicity. Conclusion: Novel non-degradable and radiopaque degradable PVA hydrogels cured by redox initiation were developed in this research. The developed PVA hydrogels showed characteristics in vitro that are desirable for the in vivo application as release systems for anti-angiogenic factors.

Thesis (Ph.D.)--Flinders University, School of Chemistry, Physics and Earth Sciences. / Typescript bound. Includes bibliographical references and list of publications. Also available online.

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 113-116).