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ACS nano 11(5), 5148 - 5159 (2017). doi:10.1021/acsnano.7b02305, 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 with the most abundant structuralcomponent on Earth (cellulose nanofibrils (CNFs)) 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 stiff ness of ∼ 55 GPa, strength at break of ∼ 1015 MPa, and toughness of ∼ 55 MJ m$^{−3}$. We also show that addition of small amounts of silk fusion proteins to CNF results in materials with advanced biofunctionalities, 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 strengthand functionalities and bring down our dependence on fossil-based resources., Published by ACS Publ., Washington, DC