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Biological nanocomposites such as nacre, bone and wood synergistically combine strength, stiffness and toughness with lightweight structure, whereas most manmade engineering materials with higher densities follow the rule-of-mixtures, according to which strength and toughness are mutually exclusive properties. Biomimetic approaches study and mimic nature's concepts and material structures with the aim of developing high-performance bioinspired materials. Recent studies have shown that many of the properties of natural nanocomposites arise from their hierarchical structures from multiple length scales. Molecular level control and design are known to be crucial for the performance of the natural materials especially at the interfaces of the softer matrix and the harder reinforcing elements. In this work, examples of biopolymer matrices were studied from the mechanical perspective in order to understand how biological components, such as genetically engineered proteins and graphene flakes, could be used to design an organic matrix at the molecular level and to control its macroscopic material properties. The results indicated that the biopolymer networks can be functionalized noncovalently in aqueous and mild conditions directly via self-assembly in order to influence the mechanical properties. In publications I and II, genetically engineered fusion proteins, incorporating hydrophobin - double cellulose binding domain or plain double cellulose binding domain, were used to tune the nanofibrillar cellulose network under conditions of controlled humidity. In publication III, another genetically engineered fusion protein, chitin binding domain - aspein, was used to modify nanofibrillated chitin matrix through ionic interactions and biomimetic mineralization of calcium carbonate. In publication IV, multilayered graphene flakes were exfoliated directly into native nanofibrillated cellulose networks in order to create nanocomposites with improved mechanical properties. Non-covalent modification of the colloidal biopolymer matrices is an efficient route to construct and study multifunctional nanocomposite materials by engineering the interfaces between the soft and hard phases. Importantly, genetically engineered proteins could pave the way towards new functional components for biomimetic structural nanocomposite materials while Nature's materials continue to provide the constructing principles and inspiration for the development of biomimetic materials. |
