Nitesh Mittal, Ronnie Jansson, Mona Widhe, Tobias Benselfelt, Karl M. O. Håkansson, Fredrik Lundell, My Hedhammar, L. Daniel Söderberg

Nature's design of functional materials relies on smart combinations of simple components to achieve desired properties. Silk and cellulose are two clever examples from nature - spider silk being tough due to high extensibility, whereas cellulose possesses unparalleled strength and stiffness among natural materials. Unfortunately, silk proteins cannot be obtained in large quantities from spiders and recombinant production processes are so far rather expensive. We have therefore combined small amounts of functionalized recombinant spider silk proteins (RSP) with the most abundant structural component on earth (cellulose nanofibrils (CNF)) to fabricate isotropic as well as anisotropic hierarchical structures. Our approach for the fabrication of bio-based anisotropic fibers results in previously unreached but highly desirable mechanical performance with a stiffness ∼55 GPa, strength at break ∼1015 MPa, and toughness ∼55 MJ m(-3). We also show that addition of small amounts of silk fusion proteins to CNF result in materials with advanced bio-functionalities, which cannot be anticipated for the wood-based CNF alone. These findings suggest that bio-based materials provide abundant opportunities to design composites with high strength and functionalities and bring down our dependence on fossil-based resources.