Tribological behaviour, mechanical properties and bio-interface engineering of bio-inspired hydrogels

Abstract

The remarkable load bearing capacity and lubricating properties of natural biphasic materials such as articular cartilage and plant cell walls (PCW) inspire this work to study the mechanics and tribology of biphasic hydrogels. These systems share a similar set of structural and mechanical characteristics, which include a biphasic network structure, presence of friction-modifying surface coatings, as well as incorporation of non-Newtonian and viscoelastic lubricant fluids. In this work, the biomimetic principles are employed in order to create cellulose-based hydrogels with controlled mechanical and tribological performance. The outcomes of this study advance the fundamental knowledge of dynamic mechanical and micro-hydrodynamic properties of biphasic materials, leading to deeper insights into the mechanisms of their friction behaviour. New insights uncovered in this thesis have significant implications for optimising cellulose-based materials for applications across biomaterials and pharmaceuticals as well as in engineering tribology. Previous studies on biomimetic cellulose hydrogels mainly utilized bacterial cellulose as a model system. The most significant limitation of this system is a limited control over pore size distribution and network density. In this work, ionic liquid regeneration process is developed and utilised to fabricate a range of regenerated cellulose-based hydrogels with better-controlled microstructures. Due to cellulose re-crystallisation, regenerated cellulose hydrogels mostly consist of amorphous cellulose as confirmed by 13 C Nuclear Magnetic Resonance spectroscopy and X-Ray Diffraction. The effect of different cellulose sources and ionic liquid types on the regeneration of cellulose is also discussed. Furthermore, PCW-mimetic cellulose-hemicellulose hybrid hydrogels are fabricated using key plant-derived polysaccharides such as tamarind xyloglucan, wheat arabinoxylan and Plantago ovata arabinoxylan. This is achieved by dissolving the controlled amount of hemicellulosic polysaccharides in the ionic liquid solution prior to the regeneration process in water. One of the major discoveries is a three time increase in mechanical modulus of cellulosexyloglucan and cellulose-wheat arabinoxylan hybrid hydrogels, compared to pure cellulose and cellulose-Plantago ovata arabinoxylan hydrogels. Further, analysis of hydrogels' mechanical properties is undertaken using a rheometer-based technique that uniquely incorporates in situ mechanical characterization and enables controlled measurement of friction forces between pairs of hydrogels. The mechanical response is found to be consistent with a generalised biphasic poroviscoelastic deformation model, which accounts for viscoelastic and poroelastic effects in hydrogels undergoing compressive deformation. However, compressibility of cellulose hydrogels makes their mechanical behaviour more complex than described by the model. In particular, the mechanical response of poroviscoelastic cellulose IV Publications during candidature Conference abstracts: