Hierarchically designed structures with architectural features that span across multiple length scales are found in numerous hard biomaterials, like bone, wood, and glass sponge skeletons, as well as manmade structures, like the Eiffel Tower. It has been hypothesized that their mechanical robustness and damage tolerance stem from sophisticated ordering within the constituents, but the specific role of hierarchy remains to be fully described and understood. We apply the principles of hierarchical design to create structural metamaterials from three material systems: (i) polymer, (ii) hollow ceramic, and (iii) ceramic-polymer composites that are patterned into self-similar unit cells in a fractal-like geometry. In situ nanomechanical experiments revealed (i) a nearly theoretical scaling of structural strength and stiffness with relative density, which outperforms existing nonhierarchical nanolattices; (ii) recoverability, with hollow alumina samples recovering up to 98% of their original height after compression to ≥50% strain; (iii) suppression of brittle failure and structural instabilities in hollow ceramic hierarchical nanolattices; and (iv) a range of deformation mechanisms that can be tuned by changing the slenderness ratios of the beams. Additional levels of hierarchy beyond a second order did not increase the strength or stiffness, which suggests the existence of an optimal degree of hierarchy to amplify resilience. We developed a computational model that captures local stress distributions within the nanolattices under compression and explains some of the underlying deformation mechanisms as well as validates the measured effective stiffness to be interpreted as a metamaterial property.H ierarchy is ubiquitous in the natural world; characterizing it, understanding its origins, and discovering its role in enhancing material properties are essential to designing new advanced materials (1-4). Natural structural materials, like Euplectella sponges, radiolarians, and bone, are exceptionally resilient against extreme mechanical environments and seem to draw their robustness from intricate mechanical networks that contain multiple levels of hierarchy (3-7). Hierarchical engineered structures are used in modern architecture, with notable examples being the Eiffel tower and the Garabit viaduct (8); today, hierarchy is seen commonly in construction cranes and building scaffolding. Both natural and engineered structures use the concept of hierarchical design to minimize material use while optimizing structural integrity.The hierarchical scale of a material is defined by its order, which represents the number of distinct structural length scales (2). Design principles and theories describing hierarchical structural materials exist (2, 9), and macroscopic second-and thirdorder 2D cellular solids, like honeycombs (10, 11) and corrugated core sandwich panels (12)(13)(14), have been designed and tested experimentally. Theories that describe the design and optimization of 3D hierarchical trusses have been proposed (15-18)...

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A continuous immersion system has been developed to deposit multifunctional, multilayer thin films using layerby-layer assembly. An initial demonstration of this device is presented here by coating cotton fabric, a complex three-dimensional substrate, with a flame-retardant recipe composed of chitosan (CH) and poly(sodium phosphate) (PSP) and a conductive recipe composed of poly(diallyldimethylammonium chloride) and multiwalled carbon nanotubes stabilized with sodium deoxycholate. Electron microscopy images reveal an unaltered weave structure for the conductive coating and the formation of a CH/PSP protective thermal barrier on fabric coated by the machine. Flame retardant and conductivity measurements of continuously coated fabric achieve properties comparable to handmade samples, and the large-scale coater showed a high level of consistency. This unique device provides an important proof-of-concept and lays the groundwork for scale-up to an industrial-scale system that could be used to produce these nanocoatings commercially.

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Hierarchy in natural and synthetic materials has been shown to grant these architected materials properties unattainable independently by their constituent materials. While exceptional mechanical properties such as extreme resilience and high deformability have been realized in many human-made three-dimensional (3D) architected materials using beam-and-junction-based architectures, stress concentrations and constraints induced by the junctions limit their mechanical performance. A new hierarchical architecture in which fibers are interwoven to construct effective beams is presented. In situ tension and compression experiments of additively manufactured woven and monolithic lattices with 30 µm unit cells demonstrate the superior ability of woven architectures to achieve high tensile and compressive strains (>50%)-without failure events-via smooth reconfiguration of woven microfibers in the effective beams and junctions. Cyclic compression experiments reveal that woven lattices accrue less damage compared to lattices with monolithic beams. Numerical studies of woven beams with varying geometric parameters present new design spaces to develop architected materials with tailored compliance that is unachievable by similarly configured monolithic-beam architectures. Woven hierarchical design offers a pathway to make traditionally stiff and brittle materials more deformable and introduces a new building block for 3D architected materials with complex nonlinear mechanics.