Bamboo-Inspired Structurally Efficient Materials with a Large Continuous Gradient.

Because of its light weight and high strength, bamboo is used in many applications around the world. Natural bamboo is built from fiber-reinforced material and exhibits a porous graded architecture that provides its remarkable mechanical performance. This porosity gradient is generated through the unique distribution of densified vascular bundles. Scientists and engineers have been trying to mimic this architecture for a very long time with much of the work focusing on the effect of fiber reinforcement. However, there still lacks quantitative studies on the role of pore gradient design on mechanical properties, in part because the fabrication of bamboo-inspired graded materials is challenging. Here, the steep and continuous porosity gradient through an ingenious cellular design in Moso bamboo is revealed. The effect of gradient design on the mechanical performance is systematically studied by using 3D-printed models. The results show that not only the magnitude of gradient but also its continuity have a significant effect. By introducing a continuous and large gradient, the maximum flexural load and energy absorption capability can be increased by 40% and 110% when comparing to the structure without gradient. These bamboo-inspired cellular architectures can offer efficient solutions for the design of damage tolerant engineering structures.

Preferential ice growth on grooved surface for crisscross-aligned graphene aerogel with large negative Poisson's ratio.

Ice formation on solid surfaces is a ubiquitous process in our daily life, and ice orientation plays a critical role in anti-icing/deicing, organ cryo-preservation, and material fabrication. Although previous studies have shown that surface grooves can regulate the orientation of ice crystals, whether the parallel or perpendicular alignment to the grooves is still under debate. Here, we systematically investigate ice formation and its oriented growth on grooved surfaces through both in situ observation and theoretical simulation, and discover a remarkable size effect of the grooves. With the designability of surface groove patterns, the preferential growth of ice crystals is programmed for the fabrication of a crisscross-aligned graphene aerogel with large negative Poisson's ratio. In addition, the size effect provides guidance for the design and fabrication of solid surfaces where the effective control of ice orientation is highly desired, such as efficient deicing, long time organ cryo-preservation, and ice-templated materials.

Anisotropic porous ceramic material with hierarchical architecture for thermal insulation.

Porous ceramic materials are attractive candidates for thermal insulation. However, effective ways to develop porous ceramics with high mechanical and thermal insulation performances are still lacking. Herein, an anisotropic porous silica ceramic with hierarchical architecture, i.e. long-range aligned lamellar layers composed of hollow silica spheres, was fabricated applying a facile bidirectional freezing method. Due to such anisotropic structure, the as-prepared porous silica ceramic displays low thermal conductivity across the layers and high compressive strength along the layers. Additionally, the anisotropic porous silica ceramic is fire-resistant. As a proof of concept, a mini-house was roofed with the anisotropic porous silica ceramic, showing that the indoor temperature could be stabilized against environmental temperature change, making this porous ceramic a promising candidate for energy efficient buildings and other industrial applications. Our study highlights the possibility of combining intrinsically exclusive properties in engineering materials through constructing biomimetic porous structures.

Mechanically Efficient Cellular Materials Inspired by Cuttlebone.

Cellular materials with excellent mechanical efficiency are essential for aerospace structures, lightweight vehicles, and energy absorption. However, current synthetic cellular materials, such as lattice materials with a unit cell arranged in an ordered hierarchy, are still far behind many biological cellular materials in terms of both structural complexity and mechanical performance. Here, the complex porous structure and the mechanics of the cuttlebone are studied, which acts as a rigid buoyancy tank for cuttlefish to resist large hydrostatic pressure in the deep-sea environment. The cuttlebone structure, constructed like lamellar septa, separated by asymmetric, distorted S-shaped walls, exhibits superior strength and energy-absorption capability to the octet-truss lattice and conventional polymer and metal foams. Inspired by these findings, mechanically efficient cellular materials are designed and fabricated by 3D printing, which are greatly demanded for many applications including aerospace structures and tissue-engineering-scaffold. This study represents an effective approach for the design and engineering of high-performance cellular materials through bioinspired 3D printing.

Smart Sponge for Fast Liquid Absorption and Thermal Responsive Self-Squeezing.

Liquid absorption and recycling play a crucial role in many industrial and environmental applications, such as oil spill cleanup and recovery, hemostasis, astronauts' urine recycling, and so on. Although many liquid absorbing materials have been developed, it still remains a grand challenge to achieve both fast absorption and efficient recycling in a cost-effective and energy-saving manner, especially for viscous liquids such as crude oil. A smart polyurethane-based porous sponge with aligned channel structure is prepared by directional freezing. Compared to common sponges with random porous structure, the as-prepared smart sponge has larger liquid absorption speed due to its lower tortuosity and stronger capillary ("tortuosity effect"). More importantly, the absorbed liquid can be remotely squeezed out due to a thermally responsive shape memory effect when the sponge is heated up. Such smart sponges with well-defined porous structure and thermal responsive self-squeezing capability have great potential in efficient liquid absorption and recycling.

Nacre-mimetic composite with intrinsic self-healing and shape-programming capability.

Replicating nacre's multiscale architecture represents a promising approach to design artificial materials with outstanding rigidity and toughness. It is highly desirable yet challenging to incorporate self-healing and shape-programming capabilities into nacre-mimetic composites due to their rigidity and high filler content. Here, we report such a composite obtained by infiltrating a thermally switchable Diels-Alder network polymer into a lamellar scaffold of alumina. The chemical bond switchability and the physical confinement by the filler endows the composite with sufficient molecular mobility without compromising its thermal dimension stability. Consequently, our composite is capable of self-healing internal damages. Additionally, in contrast to the intractable planar shape of other artificial nacres, precise control of the polymer chain dynamics allows the shape of our composite to be programmed permanently via plasticity and temporarily via shape memory effect. Our approach paves a new way for designing durable multifunctional bioinspired structural materials.