Nanostructured Biopolymeric Materials : Hydrodynamic Assembly, Mechanical Properties and Bio-Functionalities

Abstract: The need for high-end multifunctional materials from renewable resources has evolved given a rapidly increasing population and accompanying environmental concerns. Scalable assembly methods are and will be imperative in designing high-performance environmentally friendly materials, requiring new processes allowing control on all hierarchical levels. In this thesis, engineering concepts for manipulation of nanoscale components from biopolymeric resources have been applied to achieve extraordinary macroscale performance. The route chosen has been fluid-phase assembly as it is one of the most promising methods for producing large, ordered structures from nanoscale objects. The thesis has three main parts; assembly of cellulose nanofibrils (CNFs) and fundamentals associated with the processing technique, the combination of CNFs with silk fusion proteins and finally the assembly of amyloid-like protein nanofibrils (PNFs). In the CNFs assembly part, we have pursued the challenge of transferring the full potential of CNFs to macroscale materials. CNFs are the most abundant structural elements in biological systems and have impressively high strength and stiffness, yet natural or man-made cellulose composites are much weaker than the CNFs. We fabricated nanocellulose fibers in pursuit of maximal mechanical performance by hydrodynamically controlling the structural ordering of nanofibrils, resulting in continuous fibers with mechanical properties higher than any natural or man-made macroscale biopolymeric material (Young’s modulus 86 GPa and a tensile strength 1.57 GPa). As the hydrodynamic assembly process is largely dependent on fundamental phenomenon controlling rotational and translational diffusion, we have applied a novel methodology based on birefringence allowing time-resolved in-situ investigations of diffusion and network dynamics of nanofibrils including effects of anisotropic orientation distributions. Genetic engineering enables the synthesis of bioengineered silk fusion proteins that can serve as a foundation of new biomaterials. However, silk proteins are difficult to process and cannot be obtained in large quantities from spiders. By combining CNFs with recombinant spider silk proteins (RSPs) we have fabricated strong, tough and bioactive nanocomposites.   We demonstrate how small amounts of silk fusion proteins added to CNFs give advanced bio-functionalities unattainable to wood-based CNFs alone. Finally, flow-assisted assembly is applied to fabricate a material from 100% non-crystalline protein building blocks with whey protein, a mixture with β-lactoglobulin as the main component, which self-assemble into amyloid-like PNFs stabilized by hydrogen bonds. We show how conditions during the fibrillation process affect properties and morphology of the PNFs. Furthermore, we compare the assembly of whey PNFs of distinct morphologies and show that PNFs can be assembled into strong microfibers without the addition of plasticizers or crosslinkers.