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Chitosan is a polymer derived from naturally-abundant sources of chitin and can be seen referred to in the literature for a wide variety of applications. However, it has its limitations due to lack of strength and stability as a single polymer structure. This problem can be overcome by cross-linking the polymer with other natural or synthetic polymers, in the hope of providing the ‘back-bone’ structure it is missing. This PhD thesis describes the study of a range of organic-inorganic composites which were synthesised by combining chitosan with a tetra ethyl orthosilicate (TEOS) cross-linker to form an interpenetrating polymer network (IPN). In combination with a variety of ordered and disordered polymer and metal nanoparticles, the composites formed were studied with respect to their optical, mechanical and antimicrobial properties. The IPNs made from chitosan and TEOS were prepared in an aqueous-acid environment to form a hydrogel which could then be drop-cast onto a substrate of choice to dry and form a thin film structure. It could also be drop-cast onto pre-existing structures such as photonic crystals, in order to study the resulting optical properties or infused with particles such as gold and silver nanoparticles in order to form composites with enhanced mechanical and antimicrobial properties. The properties of all individual materials have been studied, while the novel composites described above have been studied in-depth the results of which have been published. For this reason, the following thesis has been prepared using a publication format with the main results chapters presented in their published form. Briefly, the main findings of the work are as follows: the pH-sensitivity of chitosan carries forward to the Chi-TEOS IPN and that by combining this feature with the optical properties of a SiO2 photonic crystal provides a pH-sensitive composite which elicits an optical response as an indicator of pH conditions. The mechanical properties of chitosan are found to be enhanced by cross-linking, even though it is occurring by weak bonding interactions. The ideal chitosan: TEOS ratio was determined as 1:1 as this ratio results in a mechanically strong thin film which retains chitosan’s inherent flexibility. This is then further enhanced by the addition of polymer particle structure enhancers in the form of colloidal silica and polystyrene particles. While the addition of Ag and Au NPs did not contribute xii towards enhanced tensile strength these films were also investigated with respect to their antimicrobial abilities. Here it was concluded that the NPs weren’t being released when pH is neutral and so they effectively couldn’t contribute towards antimicrobial activities. In contrast the metal NPs were released under more acidic conditions, potentially providing some possibilities in terms of their use in in-vivo drug release materials. While embedding particles/ drugs for release in neutral conditions isn’t suitable, the Chi-TEOS IPN is an ideal candidate as a stand-alone antimicrobial wound-dressing or for drug release in acidic conditions.