Tomer, Ron; (1994) Some thermo-, photo- and electro-responsive hydrogels. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

This work has focused on externally stimulated hydrogels which are thermo- responsive, photo-responsive and electro-responsive as matrices for drug delivery. Thermo-responsive hydrogels were fabricated from N-isopropylacrylamide (NiPAAm) and exhibited behaviour analogous to a lower critical solution temperature(LCST) phenomenon- being highly swollen at room temperature but deswelling when the temperature was raised and collapsing fully above the LCST. Copolymerization of different monomers and changing the crosslinking concentration controlled the swelling profiles of the gels, and the release of water soluble model drugs and solutes was investigated. On-off release of flurbiprofen was achieved by temperature modulation. Photo-responsive hydrogels were prepared by incorporating an azobenzene chromophore into the NiPAAm network. Azobenzene naturally occurring as the trans isomer undergoes reversible isomerisation to cis under irradiation. Two different azobenzene derivatives were used- methacryloylaminoazobenzene and di(methacryloylamino)azobenzene, the first used as a pendant group inside the network, and the second as crosslinking unit. When using azobenzene as a pendant group, solute release from the network increases upon UV irradiation, while as a crosslinking unit, isomerization decreases the release of the model solute, although the maximum difference was not greater than 10%. Hyaluronic acid (HA) was used to synthesize an electro-responsive release hydrogel. HA is a naturally occurring polysaccharide and due to its carboxylic groups enables the formation of highly swollen polyelectrolyte gels. Unloaded HA gels deswell dramatically under an electric field due to electro-osmosis that causes partial protonation of the network. The responsive swelling behaviour of loaded HA gels was dependent upon the initial swelling state of those gels. Release of two negatively charged macromolecules was investigated and on-off switchable release was demonstrated. Possible future responsive release systems based on the knowledge acquired in this work are envisioned in the end of this thesis.

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