Structure, Biochemistry and Mechanical Properties of Bioinspired Thin Films and Composites

After billions of years of evolution, it is no wonder that biological systems are treated as an invaluable source of inspiration in the development of new materials. Despite the limited number of constituents in their physical environment, natural materials display a wide range of mechanical properties and functions unmatched by their engineering counterparts. Capitalizing on the infinite reservoir of inspiration that nature offers, this work focuses on the fundamental understanding of the structure-chemical composition-property relationships of constituents present in several marine species and provides a platform for designing new materials inspired by these species./nMarine mussels secrete several adhesive proteins that function in aqueous environments against turbulent conditions without mechanical failure and are a prominent example of permanent adhesion in nature, in which human-made adhesives cannot compete. Mussel-inspired coatings, derived from the spontaneous polymerization of dopamine, so-called ‘polydopamine’ coatings (pDA), have been widely reported in the literature for their ability to be deposited on a variety of substrates. However, the composition of pDA remains unknown, mainly due to its insoluble and heterogeneous nature. In addition, pDA coatings exhibit poor mechanical resistance to delamination and abrasion. The first part of this thesis is focused on understanding the molecular mechanisms that give rise to the adhesion of pDA using single-molecule force spectroscopy (SMFS). The results favor a ‘polymer’ model of pDA structure, casting doubt on models suggesting polydopamine is a supramolecular aggregate of small molecules and oligomers. Next, considering that pDA is polymeric in nature, thermal and laser annealing methods that enhance the scratch and shear resistance of pDA are reported./nThe second part of this thesis is devoted to elucidating the underlying physicochemical principles employed by non-mineralized tissues to achieve hard, stiff, and tough properties. Specifically, the jaws of Glycera worm -a venom injecting bloodworm- comprised of protein, melanin, and copper ions combine a unique range of mechanical properties usually encountered in highly mineralized tissues. The molecular and nanoscale mechanics, the morphology and structure of protein and melanin, and the interactions among individual constituents in Glycera jaws are investigated. Firstly, the Glycine- and Histidine-rich Glycera multi-tasking protein (GMTP) using a recombinant jaw protein is characterized, revealing six functions critical for jaw formation and performance. Subsequently, the tolerance of the jaws to induced contact damage is studied to probe the reinforcement strategies employed. Results showed that multiscale interfacial energy dissipation mechanisms could achieve an optimized balance among mutually exclusive properties such as strength and toughness. These studies can offer new opportunities for harnessing and exploiting bioinspired strategies in designing lightweight yet strong polymer composites, blends, networks and/or thin films.

Advisor: Messersmith, Phillip B.
Ritchie, Robert O.