Biologically Inspired High Performance Material - Coacervation of genetically engineered silk-like fusion proteins as an intermediate step toward fabrication of next generation fiber, adhesive and composite

Nature can serve as a source of inspiration for the design of the next generation high performance materials. Proteins can play a major role in structuring the novel sustainable and advanced functional materials. Given the precise design of proteins at molecular level together with expanding knowledge of new protein sequences, the ease of gene synthesis, cloning strategies and optimized biological production, various potential designs and applications can be anticipated. However, one of the main challenge toward this goal is the lack of understanding of the processes in which such materials could be assembled and form their functional molecular interactions. Inspired by the natural structural material, this thesis highlights solutions to some of the fundamental challenges related to the design strategies and processing routes with the extends the scopes toward potential applications. In publication 1, the general problem of how to directly assemble genetically engineered and recombinantly produced fusion proteins toward functional states was touched from a biological structural materials perspective. This was approached by exploring how the overall protein architecture and modularity affect liquid-liquid demixing and coacervate formation as the functioning intermediate entities toward assemblies of protein based fiber and also adhesive fiber. In publication 2, the nature of phase separated liquid-like coacervate assemblies was characterized in detail using various state-of-the-art techniques. Overall, the assemblies showed a range of properties including low surface tension, low viscosity, fast molecular diffusion, coalescence, cohesiveness and difformability under shear flow. It was further demonstrated how these could be used as an intermediate state for strong water based assemblies between various cellulosic surfaces. In publication 3, cellulose nanofibril (CNF) was used to fabricate high performance fibers with exceptional mechanical properties. This was carried out by exploring how CNF could be aligned under shear forces while being extruded through rela-tively long and thin capillaries.In publication 4, the central challenge of biocomposite mimicry as to how to minimize stress singularity at interfaces of dissimilar components for achieving high toughness was approached. This was carried out by effective infiltration and crosslinking of oriented CNF network (publication 3) through the use of low surface energy, adhesive and energy dissipating phase separated coacervates (publication 1 and 2). The resulting material showed exceptional strength, stiffness and overall toughness