Advanced materials with low density and high strength impose transformative impacts in the construction, aerospace, and automobile industries. These materials can be realized by assembling well-designed modular building units (BUs) into interconnected structures. This study uses a hierarchical design strategy to demonstrate a new class of carbon-based, ultralight, strong, and even superelastic closed-cellular network structures. Here, the BUs are prepared by a multiscale design approach starting from the controlled synthesis of functionalized graphene oxide nanosheets at the molecular- and nanoscale, leading to the microfluidic fabrication of spherical solid-shelled bubbles at the microscale. Then, bubbles are strategically assembled into centimeter-scale 3D structures. Subsequently, these structures are transformed into self-interconnected and structurally reinforced closed-cellular network structures with plesiohedral cellular units through post-treatment, resulting in the generation of 3D graphene lattices with rhombic dodecahedral honeycomb structure at the centimeter-scale. The 3D graphene suprastructure concurrently exhibits the Young's modulus above 300 kPa while retaining a light density of 7.7 mg cm and sustaining the elasticity against up to 87% of the compressive strain benefiting from efficient stress dissipation through the complete space-filling closed-cellular network. The method of fabricating the 3D graphene closed-cellular structure opens a new pathway for designing lightweight, strong, and superelastic materials.

In this study, we fabricated a three-dimensionally assembled architecture made of reduced graphene oxide (rGO) and utilized it as an ultralightweight strain gauge. Building units for the assembly were prepared over the multiscale starting from functionalized GO nanosheets at the nanoscale to microfluidically processed solid-shelled bubbles at the microscale. These GO solid bubbles were elaborately assembled into close-packed 3D structures over the centimeter scale and then reduced by thermal treatment. Thermally reduced rGO assembly of which the internal structure was spontaneously transformed into a closed-cellular structure such as the 3D rhombic dodecahedral honeycomb lattice during thermal reduction could manifest superior elasticity against a strain of 30% by virtue of the hierarchically interconnected network while securing a low density of about 10 mg/cm3 and mechanical robustness, which was then applied as a strain gauge. The strain gauge with a thermally reduced 3D rGO structure exhibited a gauge factor of around 4 and excellent mechanical durability over 250 cycles, suggesting a new pathway for implementing ultralightweight strain-sensitive materials.

Carbon nanotubes (CNTs) have attracted considerable attention because of their high electrical conductivity and outstanding mechanical properties. As such, there have been numerous attempts to form CNTs into diverse structures for use in a wide range of applications. However, the intrinsic high aspect ratios of CNTs and resulting deformability have prevented the fabrication of sophisticated CNT-based structures, especially for three-dimensional (3D) cellular architectures. To challenge this limitation, we present a novel method to fabricate a 3D CNT cellular network from the assembly of microfluidically synthesized CNT-shelled microbubbles. We successfully generated stable spherical CNT-shelled bubbles with excellent size and shape uniformity by precisely controlling bubble dimensions by varying microfluidic variables. We also developed a fundamental understanding of the bubble stability, which allowed us to suppress shrinkage-induced deformation. The synthesized CNT-shelled bubbles were assembled into a 3D close-packed structure, followed by treatment with thermal reduction to induce interfacial bonding and transformation into a closed cellular network structure. Overall, this work provides a new strategy of assembling 1D nanomaterials as the building blocks for well-regulated 3D closed cellular architectures with improved structural or physical properties.

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