The studies of genipin-crosslinked chitosan-poly(vinyl pyrrolidone) hydrogels as smart pH responsive materials

Abstract

Smart hydrogels are macromolecular networks that can exhibit significant and reversible changes in volume upon application of an external stimulus such as pH or temperature. These polymers are well suited for use in delivery systems, tissue engineering and actuation, although currently volume changes must be externally initiated and release is often uncontrolled. Gels able to self-oscillate and distribute absorbed moieties in a controlled regime can be realised by coupling such smart materials to oscillatory chemical reactions that oscillate in the relevant stimuli to induce a volume change within a polymer gel. This work comprises of the synthesis and characterisation of a pH-sensitive hydrogel that has shown to be a good candidate for combination with appropriate oscillatory reactions. Genipin-crosslinked chitosan-poly(vinyl pyrrolidone) hydrogels were synthesised and investigated for their potential to be coupled to chemical oscillators. This semi-interpenetrating network combines biocompatibility, low toxicity, biodegradability and haemocompatibility attributes using genipin, a natural crosslinking agent with low cytotoxicity that autofluoresces upon crosslink formation. Effects of gel composition, gelation time and post synthetic manipulation on the structure, swelling, responsiveness and the mechanical properties are evaluated. Particular emphasis is placed upon improving network stability via freezing and thawing hydrogels for various timescales and cycles. Mechanical properties of the hydrogels were determined via uniaxial compression tests in multiple environments where they may find application. These results are related to the morphology and swelling potential of the networks. Further to determining volume variations gravimetrically, the hydrogels are also characterised optically, examining the change in surface area and height of the specimens. The force response upon swelling and collapse is investigated under an oscillatory regime and subsequently applied with the bromate-sulfite-ferrocyanide oscillatory reaction. In view of prospective applications, the potential of the hydrogel to release absorbed constituents from within the matrix is also determined. This thesis aids the development towards applications in controlled, pulsatile and sustained actuation and release.