Design and Characterisation of Soft Biomaterials and Interfaces for Bio-Adhesives and Cell Culture
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This thesis consists of three chapters which can be split into studies of (1) the impact of length scale, surface adhesion and heterogeneity on the mechanical charac-terisation of soft biomaterials, (2) the effect of non-covalent cross-linking chemistry on the mechanics of CMC hydrogels and (3) analytical characterisation of the interfa-cial mechanics of liquid-liquid interfaces. The mechanical properties of soft materials used in the biomedical field play an important role on their performance. In the field of tissue engineering, it is known that cells sense the mechanical properties of their environment, however some mate-rials, such as Sylard 184 PDMS (poly(dimethylsiloxane)), have failed to elicit such response. It was proposed that differences in the mechanical properties of such soft materials, at different scales, could account for these discrepancies. Indeed, the varia-tion in the elastic moduli obtained for soft materials characterised at different scales can span several orders of magnitude. This called for a side-by-side comparison of the mechanical behaviour of soft materials at different scales. Here we use indentation, rheology and atomic force microscopy nanoidentation (using different tip geometries) to characterise the mechanical properties of PDMS, poly(acrylamide) (PAAm) and carboxymethyl cellulose (CMC) hydrogels at different length scales. Our results high-light the importance of surface adhesion and the resulting changes in contact area, and sample microstructural heterogeneity, in particular for the mechanical characterisation of ultra-soft substrates at the nano- to micro-scale. Next the impact of inorganic divalent cationic and organic cationic polyelec-trolyte cross-linkers on the rheological and adhesive properties of CMC hydrogels was studied. The inorganic cations used in this study were Sr2+ and Ca2+ resulted in no significant change in the bulk rheological properties and apparent chain collapse, but an increase in the adhesive strength. The introduction of organic polyelectrolytes caused complex coacervation with the CMC polymer chains. Some CMC chain bridg-ing occurred, but the interactions between the polyelectrolytes and CMC chains caused complexes to form, resulting in polymer rich complexes to form within a more dilute matrix. The introduction of organic polyelectrolyte cross-links failed to increase the bulk mechanical properties studied by rheology but did increase the adhesive strength. Finally the impact of protein adsorption on interfacial mechanics was studied. The interfacial mechanics was studied by interfacial shear rheology and interfacial nanoindentation by AFM, and PLL, BSA and lysozyme were the proteins chosen for this study. Proteins adsorbed at the interface resulted in a significant increase in the interfacial shear moduli, however this was not seen by nanoindentation. By modelling the interfacial mechanics as the superposition of the film forces and the interfacial surface tension forces we showed that when testing by nanoindentation changes in surface tension and surface potential have a significant impact on the interfacial me-chanics. The addition of protein films was shown to have a surfactant effect on inter-faces essentially softening them, this results in the films essentially having soft sub-strates and is proposed as the main cause of discrepancies between interfacial rheology and nanoindentation by AFM.
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