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The ideal scaffold for tissue engineering provides an architecture similar to natural tissue. For cardiac tissue repair, this three dimensional environment can be modelled on the myocardial extracellular matrix (ECM). To approximate the material composition of the ECM, collagen type I can be used, as it constitutes the majority of the cardiac ECM in vivo. Furthermore, it can be easily extracted and processed, and offers a range of potential cell binding motifs. However, although its biophysical properties can be controlled via cross-linking, recent reports have suggested that this process has detrimental effects on bioactivity. The aim of this work is to optimise the properties of collagen scaffolds, modelled on the cardiac ECM, by varying cross-linking and freeze-drying conditions and thereby create an environment in which cardiomyocytes exhibit physiological behaviour. As its starting point, this thesis investigates porcine myocardial ECM. Tissue was decellularised using a combination of chemical and physical agents and the architecture and biophysical properties of the resulting cell-free samples were characterised. It was found that these samples comprised interconnected, isotropic pore structures, showed the capacity to swell i.e. increase their weight by) 2,800 % in 14 days, exhibited no mass loss over 14 days and had a Young's modulus (E) of 8.0 +/- 0.4 kPa. To mimic the observed properties, bovine collagen type I (Collagen Solutions, plc) was freeze-dried to produce isotropic pores and stabilised using a N-hydroxy-succinimide and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimidehydrochloride, (NHS/EDC) cross-linking protocol. It was hypothesised that by tailoring the degree of cross-linking, the biophysical properties of the scaffolds could be modified to match the properties observed in the cardiac ECM. A mass of 0.69 g of NHS and 1.15 g of EDC, when combined in 100 ml of 96 % ethanol was defined as a cross-linking solution of 100 % concentration (100 % XL). Based on this definition, scaffolds were treated using solutions diluted to 10 %, 3 % and 1 %. Samples treated in 96 % ethanol alone, were defined as 0 % XL. It was found that while 100 % XL scaffolds possessed properties most similar to the ECM (swelling capacity of 4,200 % , mass loss of 10.8 % over 14 days, E = 6.9 +/- 0.5 kPa), cells showed limited attachment and survival at this cross-linking degree. By comparison, on 3 % XL samples, cell attachment and survival were significantly improved. In addition, the 3 % XL samples possessed sufficient stability to be used in cell culture (swelling capacity of 4,500 %, mass loss of 32.8 % over 14 days, E = 3.6 +/- 0.2 kPa). In vivo, cardiomyocytes have a spindle-shaped morphology and populate elongated, anisotropic pores. It was hypothesised that this porous structure could be imitated by imposing increased temperature gradients on the collagen suspension during freezing. Finite element modelling was used to simulate the freeze-drying process and indicated that the temperature gradient during freezing could be increased from 4.4 °C to 23 °C, over the 10 mm sample height, using polycarbonate moulds with conductive bases. Experimentally this led to successful creation of scaffolds with anisotropic pores and demonstrated that temperature gradient is ultimately responsible for anisotropic pore architecture. In a final set of experiments, isotropic and anisotropic 3 % XL scaffolds were seeded, in two different experiments, with either myoblasts or hESC-derived cardiomyocytes. The myoblast cell line was used to investigate cell access to the scaffolds and it was found that cells were able to invade isotropic scaffolds. However, in anisotropic scaffolds with horizontally aligned pores, invasion was limited to the top 60 µm of the samples, leading to an increased cell density in this area. This was subsequently confirmed with the hESC-derived cardiomyocytes. Seven days after cell seeding, isotropic scaffolds showed non-coordinated contractions, while anisotropic scaffolds with horizontally aligned pores, exhibited large-scale, coordinated contractions in the high cell density areas, deforming scaffolds up to 1170 nm over a length of 10 mm along the direction of the pores. It has been demonstrated that, based on the characteristics of the cardiac ECM, three-dimensional scaffolds can be created which allow cardiomyocytes to connect and exhibit physiological behaviour. These scaffolds show considerable potential in the field of regenerative medicine and the insights gained during their development will contribute significantly to the further advancement of the field.

Poly(L-lactide) (PLLA) is a well-recognised bioresorbable polymer known to degrade after 1.5 to 5 years by hydrolysis. For certain medical device or drug delivery applications, it would be desirable to reduce this degradation time as strategies for tailoring degradation and drug release rates remain limited. This work aimed to examine a consistent series of polymers based on a large block of PLLA and small quantities of hydrophilic poly(ethylene glycol) (PEG) initiator. The polymers had PLLA number average molecular weight (Mn) values ranging between about 60 kDa and 200 kDa and PEG Mns ranging between 550 Da and 5000 Da giving very low PEG wt% values ranging between 0.1 and 1.5 wt%. There are currently no studies which consider high molecular weight PLLA polymers with small quantities of PEG for potential use in structural implants. Furthermore, reports in the literature do not consider the individual effects of PEG addition and PEG and PLLA lengths. The focus of this project was on the impact of processing, hydrolytic degradation and drug release on the morphological aspects of the materials. The materials were thoroughly characterised in their as-synthesised and processed forms. The assynthesised polymers were semi-crystalline and retained the unit cell of PLLA. The glass transition temperature (Tg) was significantly reduced by PEG functionalisation. After injection moulding, nuclear magnetic resonance (NMR) indicated that the PEG component was still present. The Mn of the PEG functionalised samples decreased by approximately two-fold compared with the as-synthesised materials while the PLLA control polymers, processed beyond 200 °C, were more affected as the processing temperature was increased. The degradation properties of the materials were considered. The processed materials were submerged in phosphate buffered saline (PBS) (pH = 7.4) at 37 °C over an 8-month degradation study. During hydrolytic degradation, PEG functionalisation resulted in an increased water uptake. Mass loss began in all polymers when the Mn fell below a threshold of about 20 kDa. In the PEG functionalised samples, the degree of crystallinity increased with time, facilitated by plasticisation from PEG and the increased water content. The molecular degradation rate, k for the PEG-functionalised polymers was dependent on the presence of PEG functionalisation but was little affected by PEG length or PLLA length in the ranges studied. The time taken to reach the critical Mn, and hence the time for mass loss to begin, therefore depended on both the initial Mn and the presence or absence of PEG functionalisation. In the presence of PEG, k, was dramatically enhanced: k for PEG-functionalised polymers fell in the range of 6 x $10^{-4}$ $h^{-1}$ to 1 x $10^{-3}$ $h^{-1}$, as compared with that of the PLLA control of 2.9 x $10^{-5}$ $h^{-1}$. The mechanism of drug release from an analogous series of polymers was investigated. Propranolol. HCl was selected as a model drug for the drug release studies due to its thermal stability and solubility in PBS. Drug loading of propranolol.HCl was achieved by mixing the polymer and drug then injection moulding. A second method of drug incorporation using supercritical CO2 to load propranolol into as-synthesised polymer granules before injection moulding was examined for comparison. The materials processed through injection moulding showed that while drug crystals were present at the surface and in the polymer matrix, a level of drug solubility was also achieved in the PEG-functionalised polymers whereas the PLLA control showed no signs of polymer-drug interaction and only a distribution of drug crystals confined to the surface. The presence of drug crystals on the surface of the PLLA control resulted in the instant dissolution of propranolol.HCl and gave a burst release compared with an initial burst release in the PEG-functionalised polymers followed by a gradual release of the drug. This initial burst release was eliminated from the profile of the samples processed via supercritical CO2. The amorphous dispersion of the drug in the matrix gave a slow, sustained release throughout the duration of the drug release study. The results in this thesis have elucidated the intricate mechanisms of degradation and drug release from PEG-functionalised PLLA polymers. The overall outcome shows new ways of controlling the degradation and drug release rates of already medically established poly(-hydroxy acid) polymers extending their potential for use within temporary structural implants.

Understanding, and ultimately, predicting the degradation of bioresorbable composites made of biodegradable polyesters and calcium-based ceramics is paramount in order to fully unlock the potential of these materials, which are heavily used in orthopaedic applications and also being considered for stents. A modelling framework which characterises the degradation of bioresorbable composites was generated by generalising a computational model previously reported in literature. The framework uses mathematical expressions to represent the interwoven phenomena present during degradation. Three ceramic-specific models were then created by particularising the framework for three common calcium-based fillers, namely tricalcium phosphate (TCP), hydroxyapatite (HA) and calcium carbonate (CC). In these models, the degradation of a bioresorbable composite is described with four parameters: the non-catalytic and auto-catalytic polymer degradation rates, $k_1$ and $k_2'$ respectively and the ceramic dissolution rate and exponent, $A_\text{d}$ and $\theta$ respectively. A comprehensive data mining exercise was carried out by surveying the existing literature in order to obtain quantitative degradation data for bioresorbable composites containing TCP, HA and CC. This resulted in a database with a variety of case studies. Subsequently, each case study was analysed using the corresponding ceramic-specific model returning a set of values for the four degradation constants. Both cases with agreement and disagreement between model prediction and experimental data were studied. 76% of the 107 analysed case studies displayed the expected behaviour. In general terms, the analysis of the harvested data with the models showed that a wide range of degradation behaviours can be attained using different polymeric matrix - ceramic filler combinations. Furthermore, the existence of discrepancies in degradation behaviour between a priori similar bioresorbable composites became apparent, highlighting the high number of hidden factors affecting composite degradation such as polymer tacticity or ceramic impurities. The analysis of the case studies also highlighted that the ceramic dissolution rate needed to depict the portrayed degradation behaviours is significantly higher than that reported for ceramics alone in dissolution studies under physiological conditions, indicating that studies of the filler elements alone do not provide a complete picture. Lastly, the computational analysis provided insight into the complex influence of factors such as sample porosity and degradation protocol in the degradation behaviour. In addition to the computational analysis of literature data, an experimental degradation study was carried out with nanocomposites made of calcium carbonate and poly(D,L-lactide-co-glycolide). This study showed the existence of a clear buering effect with the addition of the ceramic filler and confirmed the assumptions employed in the modelling framework in this particular bioresorbable composite. The detailed nature and modest size of these data enabled a more precise and thorough analysis using the CC composites degradation model. In summary, the modelling framework is able to capture the main degradation behaviour of bioresorbable composites and also point to factors responsible for dissimilar behaviours. The degradation maps generated with the values of $k_1$, $k_2'$, $A_\text{d}$ and $\theta$ output by the models appear to be a good tool to summarise, classify and facilitate the analysis and search of specific bioresorbable composites.

Macromolecular biomaterials often require covalent crosslinking to achieve adequate stability and mechanical strength for their given application. However, the use of auxiliary chemicals may be associated with long-term toxicity in the body. Oppositely-charged polyelectrolytes (PEs) have the advantage that they can self-crosslink electrostatically and those derived from marine organisms are an inexpensive alternative to glycosaminoglycans present in the extracellular matrix of human tissues. A range of different combinations of PEs and preparation conditions have been reported in the literature. However, although there has been some work on complex formation between chitosan (CS) and carrageenan (CRG), much of the work undertaken has ignored the effect of pH on the consequent physicochemical properties of self-crosslinked polyelectrolyte complex (PEC) gels, films and scaffolds. Chitosan is a positively-charged polysaccharide with NH3+ side groups derived from shrimp shells and, carrageenan is a negatively-charged polysaccharide with OSO3- side groups derived from red seaweed. These abundant polysaccharides possess advantageous properties such as biodegradability and low toxicity. However, at present, there is no clear consensus on the cell binding properties of CS and CRG or CS-CRG PEC materials. The aim of this study was to explore the properties of crosslinker-free PEC gels, solvent-cast PEC films and freeze-dried PEC scaffolds based on CS and CRG precursors for medical applications. The objective was to characterise the effect of pH of the production conditions on the physicochemical and biological properties of CS-CRG PECs. Experimental work focused on the interaction between PEs, the composition of PECs, the rheological properties of PEC gels and the mechanical properties of PEC films and scaffolds. In addition, cell and protein attachment to the PEC films was assessed to determine their interactions in a biological environment. For biomedical applications, these materials should ideally be stable when produced such that they can be processed to form either a film or a scaffold and have mechanical properties comparable to those of collagenous soft tissues. FTIR was used to confirm PEC formation. Zeta potential measurements indicated that the PECs produced at pH 2-6 had a high strength of electrostatic interaction with the highest occurring at pH 4-5. This resulted in stronger intra-crosslinking in the PEC gels which led to the formation of higher yield, solid content, viscosity and fibre content in PEC gels. The weaker interaction at pH 7-12 resulted in higher levels of CS incorporated into the complex and the formation of inter-crosslinking through entanglements between PEC units. This resulted in the production of strong and stiff PEC films and scaffolds appropriate for soft tissue implants. The PECs prepared at pH 7.4 and 9 also exhibited low swelling and mass loss, which was thought to be due to the high CS content and entanglements. From the range of samples tested, the PECs produced at pH 7.4 appeared to show the optimum combination of yield, stability and homogeneity for soft tissue implants. Biological studies were performed on CS, CRG and PECs prepared at pH 3, 5, 7.4 and 9. All of the PE and PEC films were found to be non-cytotoxic. When the response of three different cell types and a high binding affinity protein (tropoelastin) was evaluated; it was found that the CS-CRG PEC films displayed anti-adhesive properties. Based on these experimental observations and previous studies, a mechanistic model of the anti-adhesive behaviour of PEC surfaces was proposed. It was therefore concluded that the CS-CRG PECs produced might be suitable for non-biofouling applications.