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A fibrous herringbone-modified helicoidal architecture is identified within the exocuticle of an impact-resistant crustacean appendage. This previously unreported composite microstructure, which features highly textured apatite mineral templated by an alpha-chitin matrix, provides enhanced stress redistribution and energy absorption over the traditional helicoidal design under compressive loading. Nanoscale toughening mechanisms are also identified using high load nanoindentation and in-situ TEM picoindentation. A Sinusoidally-Architected Helicoidal BiocompositeBy Nicholas A. Yaraghi, Nicolás Guarín-Zapata, Lessa K. Grunenfelder, Eric Hintsala, Sanjit Bhowmick, Jon M. Hiller, Mark Betts, Edward L. Principe, Jae-Young Jung, Leigh Sheppard, Richard Wuhrer, Joanna McKittrick, Pablo D. Zavattieri Keywords: (Composites, Toughness, Impact, Biomineral, Ultrastructure) Submitted to 3 Biologically mineralized composites offer inspiration for the design of next generation structural materials due to their low density, high strength and toughness currently unmatched by engineering technologies. [1][2][3][4][5][6][7][8][9] Such properties are based on the ability for the organism to utilize structural organics and acidic proteins to guide and control the mineralization process to yield hierarchical architectures with well-defined compositional gradients.One notable example is the highly developed raptorial appendage, or dactyl, of the stomatopods, a group of aggressive marine crustaceans that use these structures for feeding upon hard-shelled and soft-bodied prey. [10][11][12][13][14] The dactyls of the "smashers", those that feed primarily on hard-shelled prey, (see Figure 1A) takes the form of a bulbous club ( Figure 1B), which is used to smash through mollusk shells, crab exoskeletons, and other tough mineralized structures with tremendous force and speed. [11][12][13][14][15][16] Achieving accelerations over 10,000g and reaching speeds of 23 m/s from rest, the dactyl strike is recognized as one of the fastest and most powerful impacting events observed in Nature. [11,12] The club is capable of delivering and subsequently enduring repetitive impact forces up to 1500 N and cavitation stresses without catastrophically failing, demonstrating its utility as an exceptionally damage-tolerant natural material.The origins of such a mechanical response lie in the structural design. Previous work identified the club as a multi-regional composite material containing an organic matrix composed of alpha-chitin fibers mineralized by amorphous forms of calcium carbonate and calcium phosphate as well as crystalline apatite. [17,18] These investigations revealed mechanisms responsible for providing damage-tolerance and impact-resistance to the club, which were largely attributed to the interior of the club (periodic region), identified as the primary energy-absorbing layer. [17,18] The combination of soft polymeric nanofibers and stiffer mineral provides a periodic modulus mismatch leading to crack deflection, which in co...

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Nature assembles weak organic and inorganic constituents into sophisticated hierarchical structures, forming structural composites that demonstrate impressive combinations of strength and toughness. Two such composites are the nacre structure forming the inner layer of many mollusk shells, whose brick-and-mortar architecture has been the gold standard for biomimetic composites, and the cuticle forming the arthropod exoskeleton, whose helicoidal fiber-reinforced architecture has only recently attracted interest for structural biomimetics. In this review, we detail recent biomimetic efforts for the fabrication of strong and tough composite materials possessing the brick-and-mortar and helicoidal architectures. Techniques discussed for the fabrication of nacre- and cuticle-mimetic structures include freeze casting, layer-by-layer deposition, spray deposition, magnetically assisted slip casting, fiber-reinforced composite processing, additive manufacturing, and cholesteric self-assembly. Advantages and limitations to these processes are discussed, as well as the future outlook on the biomimetic landscape for structural composite materials.