The aim of this thesis was to develop a hydrogel system with enhanced mechanical performance. This hydrogel system has to be preferentially electrically conductive to facilitate possible controlled drug release. To fabricate a tough hydrogel system, a double network (DN) approach was employed by forming two polymer networks interpenetrated in each other with considerably different crosslinking ratios.
The new developments in tough hydrogel materials are highlighted in Chapter 1, and their enhanced mechanical performance and corresponding toughening mechanisms are discussed. These tough hydrogels have been mainly developed over the past ten years with many now showing mechanical properties comparable with those of natural tissues. The possibility of employing a conductive hydrogel system for controlled drug release purposes was investigated by studying chitosan hydrogel films containing carbon nanotubes in Chapter 2. A modulated release behaviour was demonstrated by tuning the strength and polarity of the applied voltage, ranging from -0.8 to +0.15 V. Attempts to make stronger hydrogels based on chitosan and other synthetic hydrogel networks resulted in fabricating chitosan-poly(acrylamide) fibres in Chapter 3, with up to, respectively, 11 and 8 times enhancement in modulus and tensile strength compared to PAAm hydrogel. Furthermore, to combine the strengthening mechanisms of hydrogen-bonding and double network hydrogels in forming a toughened hydrogel system, a double network system based on poly(acrylic acid) and a bottlebrush network made of poly(ethylene glycol) methyl ether methacrylates oligomers was made and characterized in Chapter 4. Mechanical properties (tensile, compression) and swelling behaviour of this system at various pHs were studied systematically, along with other physical properties such as transparency and surface contact angle. The results indicated that this system is strongly pH sensitive, with all of the mechanical and physical properties affected by the pH.
Finally, a conducting polymer (PEDOT) and carbon nanotubes were employed to introduce conductivity to the aforementioned hydrogel network, and the results are presented in, respectively, Chapter 5 and Chapter 6. Conductivity of hydrogels at various pHs was studied in Chapter 5, showing the DN-PEDOT hydrogels have remained pH sensitive with a conductivity up to 4.3 S/cm at acidic pH. In Chapter 6 the formation of a carbon nanotube-rich sheath around a tough double network hydrogel core via a phase segregation process is described. This phenomenon was observed in various double network hydrogel structures, regardless of the nature and composition of the networks. The obtained hydrogels are potentially applicable in the field of controlled drug release. The conclusion chapter (Chapter 7) summarises the thesis, with a few suggestions for future studies in this field.
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