Natural and synthetic polymer-based hybrid materials for tissue regeneration

The world's population is aging and cases of debilitating degenerative diseases are increasing. Bone is the second most transplanted tissue after blood but natural bone grafts are in short supply. Bioglass, which is a particular composition of bioactive glass, stimulates more bone repair than other synthetic bone grafts. However, it is brittle so cannot be used in cyclically loaded sites. A promising solution is the use of hybrid materials that can potentially combine the toughness of polymers with the stiffness and bioactivity of the glass through interpenetrating inorganic-organic networks. Hybrids have the unique feature of tuneable mechanical properties and degradation rates. In this thesis, two very different polymers were investigated as the organic component of hybrids; chitosan and poly(2-hydroxyethyl methacrylate-co-(3-trimethoxysilane)propyl methacrylate). The natural polymer chitosan was incorporated into the silica sol-gel process to produce hybrids and scaffolds were fabricated using freeze drying and foaming techniques. The chemical, morphological, mechanical and degradation properties of the scaffolds were studied. In order to covalently bond the organic and inorganic components, the chitosan was functionalised with an alkoxysilane crosslinker, 3-glycidoxypropyl trimethoxysilane. Using NMR and FTIR, the functionalisation reaction and side-reactions were characterised, discovering that the reaction was only 20% efficient at all pH values. To avoid the inefficient functionalisation reactions and concerns over the reproducibility of natural polymers, the synthetic co-polymer poly(2-hydroxyethyl methacrylate-co-(3-trimethoxysilane)propyl methacrylate) was synthesised by controlled polymerisation techniques (ATRP and ARGET ATRP) and typical free radical polymerisation (FRP). ATRP gave good control over molecular weight distributions, but the copper catalyst had serious implications on the chemical and architectural structure of the polymers. An NMR kinetics study was used to identify alternative polymerisation routes that could avoid the problems associated with the copper catalyst. The polymers were introduced into the sol-gel process to produce entirely synthetic hybrids with non-brittle (tough) behaviour and dissolution rates controlled by the polymer composition. The hybrids also exhibited hydroxyapatite precipitation in simulated body fluid, indicative of potential bioactivity in vivo. Hence, the aim of producing non-brittle, bioactive materials with controllable degradation rates was achieved.