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Micro/nanostructure of fish scale and biomimetic fabrication and characterization. (a) arapaima giga; (b-d) micro/nanostructure of fish scale, three colored dotted lines represent three periodically arranged fiber layers; (e-f) biomimetic bottom-up assembly strategy; (g) biomimetic twisted plywood structural material and microstructure. Credit: Science China Press

Over hundreds of millions of years of evolution, the magical nature has given birth to a myriad of biological materials which serve either as the skeletons of the organisms or as defensive or offensive weapons. Although these natural structural materials are derived from relatively sterile natural components, such as fragile minerals and ductile biopolymers, they often exhibit extraordinary mechanical properties due to their highly ordered hierarchical structures and sophisticated interfacial design. Therefore, they are always the main object for researchers to investigate and imitate aiming to create advanced artificial structural materials.

Through microstructural observation, we can find that many biological materials, including fish scale, crab claw and bone, all have typical twisted plywood structure that consists of highly ordered arrangement of micro/nanoscale fiber lamellas. They are structurally sophisticated natural fiber reinforced composites and often exhibit excellent damage tolerance that is urgently needed but difficult to be obtained for engineering structural materials. Therefore, fully mimicking this kind of natural hierarchical structure and interfacial design, by using artificial synthetic and abundant one-dimensional micro/nanoscale fibers as building blocks, is expected to produce high-performance new-style artificial structural materials that are expected to exceed existing engineering structural materials. However, due to the lack of micro/nanoscale assembly technology, especially the lack of means to efficiently integrate one-dimensional micro/nanoscale structural units into macroscopic bulk form, mimicking natural fiber reinforced composites has always been a major challenge, and there has rarely been reported so far.

In response to this challenge, recently, inspired by the micro/nanoscale twisted plywood structure of the natural Arapaima giga scale armour (a-d), for the first time, the biomimetic research team led by Professor Shu-Hong Yu from the University of Science and Technology of China (USTC) proposed a high-efficient bottom-up 'brushing-and-laminating' assembly strategy (e-f) with the biocompatible micro/nanofibers as structural units, successfully fabricated three-dimensional bulk biomimetic twisted plywood structural materials (g). Through hierarchically controlling the fiber alignment in the biopolymer matrix, the mechanical properties of resultant materials can be precisely modulated. It was found that the obtained artificial materials highly replicate the multiscale structure and toughening mechanisms of their natural counterparts, realizing excellent mechanical properties far beyond fundamental structural components and can be comparable with those of natural bone and many other natural and artificial materials. More importantly, the proposed assembly strategy is eco-friendly, programmed and scalable, and can be easily extended to other materials systems. Therefore, it provides a new technological space for designing more advanced biomimetic fiber reinforced structural materials (especially armour protection materials).

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