Sustainable liquid metal-induced conductive nacre.

Nacre has inspired research to fabricate tough bulk composites for practical applications using inorganic nanomaterials as building blocks. However, with the considerable pressure to reduce global carbon emissions, preparing nacre-inspired composites remains a significant challenge using more economical and environmentally friendly building blocks. Here we demonstrate tough and conductive nacre by assembling aragonite platelets exfoliated from natural nacre, with liquid metal and sodium alginate used as the "mortar". The formation of GaOC coordination bonding between the gallium ions and sodium alginate molecules reduces the voids and improves compactness. The resultant conductive nacre exhibits much higher mechanical properties than natural nacre. It also shows excellent impact resistance attributed to the synergistic strengthening and toughening fracture mechanisms induced by liquid metal and sodium alginate. Furthermore, our conductive nacre exhibits exceptional self-monitoring sensitivity for maintaining structural integrity. The proposed strategy provides a novel avenue for turning natural nacre into a valuable green composite.

Tough and Conductive Nacre-inspired MXene/Epoxy Layered Bulk Nanocomposites.

A long-standing quest in materials science has been the development of tough epoxy resin nanocomposites for use in numerous applications. Inspired by nacre, here we report tough and conductive MXene/epoxy layered bulk nanocomposites. The orientation of MXene lamellar scaffolds is enhanced by annealing treatment. The improved interfacial interactions between MXene lamellar scaffold and epoxy through surface chemical modification resulted in a synergistic effect. Tailoring the interlayer spacing of MXene nanosheets to a critical distance resulted in a fracture toughness about eight times higher than that of pure epoxy, surpassing other epoxy nanocomposites. Our nacre-inspired MXene/epoxy layered bulk nanocomposites also show high electrical conductivity that provides self-monitoring capability for structural integrity and exhibits an excellent electromagnetic interference shielding efficiency. Our proposed strategy provides an avenue for fabricating high-performance epoxy nanocomposites.

Ginkgo seed shell provides a unique model for bioinspired design.

Natural structural materials typically feature complex hierarchical anisotropic architectures, resulting in excellent damage tolerance. Such highly anisotropic structures, however, also provide an easy path for crack propagation, often leading to catastrophic fracture as evidenced, for example, by wood splitting. Here, we describe the weakly anisotropic structure of Ginkgo biloba (ginkgo) seed shell, which has excellent crack resistance in different directions. Ginkgo seed shell is composed of tightly packed polygonal sclereids with cell walls in which the cellulose microfibrils are oriented in a helicoidal pattern. We found that the sclereids contain distinct pits, special fine tubes like a "screw fastener," that interlock the helicoidal cell walls together. As a result, ginkgo seed shell demonstrates crack resistance in all directions, exhibiting specific fracture toughness that can rival other highly anisotropic natural materials, such as wood, bone, insect cuticle, and nacre. In situ characterization reveals ginkgo's unique toughening mechanism: pit-guided crack propagation. This mechanism forces the crack to depart from the weak compound middle lamella and enter into the sclereid, where the helicoidal cell wall significantly inhibits crack growth by the cleavage and breakage of the fibril-based cell walls. Ginkgo's toughening mechanism could provide guidelines for a new bioinspired strategy for the design of high-performance bulk materials.

Stiff and tough PDMS-MMT layered nanocomposites visualized by AIE luminogens.

Polydimethylsiloxane (PDMS) is a widely used soft material that exhibits excellent stability and transparency. But the difficulty of fine-tuning its Young's modulus and its low toughness significantly hinder its application in fields such as tissue engineering and flexible devices. Inspired by nacre, here we report on the development of PDMS-montmorillonite layered (PDMS-MMT-L) nanocomposites via the ice-templating technique, resulting in 23 and 12 times improvement in Young's modulus and toughness as compared with pure PDMS. Confocal fluorescence microscopy assisted by aggregation-induced emission (AIE) luminogens reveals three-dimensional reconstruction and in situ crack tracing of the nacre-inspired PDMS-MMT-L nanocomposite. The PDMS-MMT-L nanocomposite is toughened with mechanisms such as crack deflection and bridging. The AIE-assisted visualization of the crack propagation for nacre-inspired layered nanocomposites provides an advanced and universal characterization technique for organic-inorganic nanocomposites.

Bioinspired Color Switchable Photonic Crystal Silicone Elastomer Kirigami.

Bioinspired dynamic structural color has great potential for use in dynamic displays, sensors, cryptography, and camouflage. However, it is quite rare for artificial structural color devices to withstand thousands of cycles. Male hummingbird's crowns and gorgets are brightly colored, demonstrating frequent color switching that is induced by regulating the orientation of the feathers through movement of skin or joints. Inspired by this unique structural color modulation, we demonstrate a flexible, mechanically triggered color switchable sheet based on a photonic crystal (PhC)-coated polydimethylsiloxane (PDMS) kirigami (PhC-PDMS kirigami) made by laser cutting. Finite element modeling (FEM) simulation reveals that the thickness of PDMS kirigami and the chamfer at the incision induced by laser cutting both dominate the out-of-plane deformation through in-plane stretching. The bioinspired PhC-PDMS kirigami shows precisely programmable structural color and keeps the color very well after recycling over 10 000 times. This bioinspired PhC-PDMS kirigami also shows excellent viewability even in bright sunlight, high readability, robust functionality, technical flexibility, and mechanical durability, which are readily exploitable for applications, such as chromic mechanical monitors for the sports industry or for medical applications, wearable camouflage, and security systems.

A Butterfly-Inspired Hierarchical Light-Trapping Structure towards a High-Performance Polarization-Sensitive Perovskite Photodetector.

Extensive applications for photodetectors have led to demand for high-responsivity polarization-sensitive light detection. Inspired by the elaborate architecture of butterfly Papilio paris, a 1D nanograting bonded porous 2D photonic crystal perovskite photodetector (G-PC-PD) using a commercial DVD master and 2D crystalline colloidal arrays template was fabricated. The coupling effect from grating diffraction and reflection of the PC stopband renders the enhanced light harvesting of G-PC-PD. The porous scaffold and nanoimprinting process afford a highly crystalline perovskite film. White light responsivity and detectivity of G-PC-PD are up to 12.67 A W-1 and 3.22×1013  Jones (6∼7 times that of a pristine perovskite photodetector). The highly ordered nanograting arrays of G-PC-PD enable polarization-sensitive light detection with a rate of -0.72 nA deg-1 . This hierarchical perovskite integrated nanograting and 2D PC architecture opens a new avenue to high-performance optoelectronic devices.

Ultra-Tough Inverse Artificial Nacre Based on Epoxy-Graphene by Freeze-Casting.

Epoxy nanocomposites combining high toughness with advantageous functional properties are needed in many fields. However, fabricating high-performance homogeneous epoxy nanocomposites with traditional methods remains a great challenge. Nacre with outstanding fracture toughness presents an ideal blueprint for the development of future epoxy nanocomposites. Now, high-performance epoxy-graphene layered nanocomposites were demonstrated with ultrahigh toughness and temperature-sensing properties. These nanocomposites are composed of ca. 99 wt % organic epoxy, which is in contrast to the composition of natural nacre (ca. 96 wt % inorganic aragonite). These nanocomposites are named an inverse artificial nacre. The fracture toughness reaches about 4.2 times higher than that of pure epoxy. The electrical resistance is temperature-sensitive and stable under various humidity conditions. This strategy opens an avenue for fabricating high-performance epoxy nanocomposites with functional properties.

Ultratough Bioinspired Graphene Fiber via Sequential Toughening of Hydrogen and Ionic Bonding.

Graphene-based fibers synthesized under ambient temperature have not achieved excellent mechanical properties of high toughness or tensile strength compared with those synthesized by hydrothermal strategy or graphitization and annealing treatment. Inspired by the relationship between organic/inorganic hierarchical structure, interfacial interactions, and moderate growth temperature of natural nacre, we fabricate an ultratough graphene fiber via sequential toughening of hydrogen and ionic bonding through a wet-spinning method under ambient temperature. A slight amount of chitosan is introduced to form hydrogen bonding with graphene oxide nanosheets, and the ionic bonding is formed between graphene oxide nanosheets and divalent calcium ions. The optimized sequential toughening of hydrogen and ionic bonding results in an ultratough graphene fiber with toughness of 26.3 MJ/m3 and ultimate tensile strength of 743.6 MPa. Meanwhile, the electrical conductivity of the resultant graphene fiber is as high as 179.0 S/cm. This kind of multifunctional graphene fiber shows promising applications in photovoltaic wires, flexible supercapacitor electrodes, wearable electronic textiles, fiber motors, etc. Furthermore, the strategy of sequential toughening of hydrogen and ionic bonding interactions also offers an avenue for constructing high-performance graphene-based fibers in the near future.

Bioinspired Supertough Graphene Fiber through Sequential Interfacial Interactions.

Natural nacre exhibits extraordinary functional and structural diversity, combining high strength and toughness. The mechanical properties of nacre are attributed to (i) a highly arranged hierarchical layered structure of inorganic minerals (95 vol %) containing a small amount only of organic materials (5 vol %), (ii) abundant synergistic interfacial interactions, and (iii) formation under ambient temperature. Herein, inspired by these three design principles originating from natural nacre, the supertough bioinspired graphene-based nanocomposite fibers (BGNFs) are prepared under room temperature via sequential interfacial interactions of ionic bonding and π-π interactions. The resultant synergistic effect leads to a super toughness of 18.7 MJ m-3 as well as a high tensile strength of 740.1 MPa. In addition, the electrical conductivity of these supertough BGNFs is as high as 384.3 S cm-1. They can retain almost 80% of this conductivity even after 1000 cycles of loading-unloading testing, which makes these BGNFs promising candidates for application in flexible and stable electrical devices, such as strain sensors and actuators.

Smart Nacre-inspired Nanocomposites.

Nacre-inspired nanocomposites with excellent mechanical properties have achieved remarkable attention in the past decades. The high performance of nacre-inspired nanocomposites is a good basis for the further application of smart devices. Recently, some smart nanocomposites inspired by nacre have demonstrated good mechanical properties as well as effective and stable stimuli-responsive functions. In this Concept, we summarize the recent development of smart nacre-inspired nanocomposites, including 1D fibers, 2D films and 3D bulk nanocomposites, in response to temperature, moisture, light, strain, and so on. We show that diverse smart nanocomposites could be designed by combining various conventional fabrication methods of nacre-inspired nanocomposites with responsive building blocks and interface interactions. The nacre-inspired strategy is versatile for different kinds of smart nanocomposites in extensive applications, such as strain sensors, displays, artificial muscles, robotics, and so on, and may act as an effective roadmap for designing smart nanocomposites in the future.

High-Performance Nanocomposites Inspired by Nature.

Natural materials, including nacre, bone, and the lobster cuticle, exhibit excellent mechanical properties, combining high strength and toughness. Such materials have the added benefit of being light in weight. These advantageous features are due to such natural materials' orderly hierarchical architectures and abundant interface interactions. How to utilize these design principles created by nature to fabricate high-performance bioinspired nanocomposites remains a great research challenge. A logical roadmap for developing these nanocomposites can be described as "discovery, invention, and creation." Here, the discovery of the relationship between natural materials' design principles and such materials' extraordinary mechanical properties is discussed. Then, the invention of bioinspired strategies for mimicking natural materials is considered and representative strategies addressed. Next, the creation of multifunctional nanocomposites is discussed and bioinspired nanocomposites, including fiber nanocomposites, 2D film nanocomposites, and 3D bulk nanocomposites reviewed. Finally, a perspective and outlook for future directions in making bioinspired nanocomposites is provided to offer inspiration to the community and a clear vision for future research.

Thermochromic Artificial Nacre Based on Montmorillonite.

Nacre-inspired nanocomposites have attracted a great deal of attention in recent years because of their special mechanical properties and universality of the underlying principles of materials engineering. The ability to respond to external stimuli will augment the high toughness and high strength of artificial nacre-like composites and open new technological horizons for these materials. Herein, we fabricated robust artificial nacre based on montmorillonite (MMT) that combines robustness with reversible thermochromism. Our artificial nacre shows great potential in various fields such as aerospace and sensors and opens an avenue to fabricate artificial nacre responsive to other external stimuli in the future.

Bioinspired Graphene-Based Nanocomposites and Their Application in Flexible Energy Devices.

Graphene is the strongest and stiffest material ever identified and the best electrical conductor known to date, making it an ideal candidate for constructing nanocomposites used in flexible energy devices. However, it remains a great challenge to assemble graphene nanosheets into macro-sized high-performance nanocomposites in practical applications of flexible energy devices using traditional approaches. Nacre, the gold standard for biomimicry, provides an excellent example and guideline for assembling two-dimensional nanosheets into high-performance nanocomposites. This review summarizes recent research on the bioinspired graphene-based nanocomposites (BGBNs), and discusses different bioinspired assembly strategies for constructing integrated high-strength and -toughness graphene-based nanocomposites through various synergistic effects. Fundamental properties of graphene-based nanocomposites, such as strength, toughness, and electrical conductivities, are highlighted. Applications of the BGBNs in flexible energy devices, as well as potential challenges, are addressed. Inspired from the past work done by the community a roadmap for the future of the BGBNs in flexible energy device applications is depicted.

Graphene-based artificial nacre nanocomposites.

With its extraordinary properties as the strongest and stiffest material ever measured and the best-known electrical conductor, graphene could have promising applications in many fields, especially in the area of nanocomposites. However, processing graphene-based nanocomposites is very difficult. So far, graphene-based nanocomposites exhibit rather poor properties. Nacre, the gold standard for biomimicry, provides an excellent example and guidelines for assembling two-dimensional nanosheets into high performance nanocomposites. The inspiration from nacre overcomes the bottleneck of traditional approaches for constructing nanocomposites, such as poor dispersion, low loading, and weak interface interactions. This tutorial review summarizes recent research on graphene-based artificial nacre nanocomposites and focuses on the design of interface interactions and synergistic effects for constructing high performance nanocomposites. This tutorial review also focuses on a perspective of the dynamic area of graphene-based nanocomposites, commenting on whether the concept is viable and practical, on what has been achieved to date, and most importantly, what is likely to be achieved in the future.

Nacre-inspired integrated strong and tough reduced graphene oxide-poly(acrylic acid) nanocomposites.

Inspired by the relationship between interface interactions and the high performance mechanical properties of nacre, a strong and tough nacre-inspired nanocomposite was demonstrated based on graphene oxide (GO) and polyacrylic acid (PAA) prepared via a vacuum-assisted filtration self-assembly process. The abundant hydrogen bonding between GO and PAA results in both high strength and toughness of the bioinspired nanocomposites, which are 2 and 3.3 times higher than that of pure reduced GO film, respectively. In addition, the effect of environmental relative humidity on the mechanical properties of bioinspired nanocomposites is also investigated, and is consistent with previous theoretical predictions. Moreover, this nacre-inspired nanocomposite also displays high electrical conductivity of 108.9 S cm(-1). These excellent physical properties allow this type of nacre-inspired nanocomposite to be used in many applications, such as flexible electrodes, aerospace applications, and artificial muscles etc. This nacre-inspired strategy also opens an avenue for constructing integrated high performance graphene-based nanocomposites in the near future.

Use of Synergistic Interactions to Fabricate Strong, Tough, and Conductive Artificial Nacre Based on Graphene Oxide and Chitosan.

Graphene is the strongest and stiffest material, leading to the development of promising applications in many fields. However, the assembly of graphene nanosheets into macrosized nanocomposites for practical applications remains a challenge. Nacre in its natural form sets the "gold standard" for toughness and strength, which serves as a guide to the assembly of graphene nanosheets into high-performance nanocomposites. Here we show the strong, tough, conductive artificial nacre based on graphene oxide through synergistic interactions of hydrogen and covalent bonding. Tensile strength and toughness was 4 and 10 times higher, respectively, than that of natural nacre. The exceptional integrated strong and tough artificial nacre has promising applications in aerospace, artificial muscle, and tissue engineering, especially for flexible supercapacitor electrodes due to its high electrical conductivity. The use of synergistic interactions is a strategy for the development of high-performance nanocomposites.

Synergistic toughening of graphene oxide-molybdenum disulfide-thermoplastic polyurethane ternary artificial nacre.

Inspired by the ternary structure of natural nacre, robust ternary artificial nacre is constructed through synergistic toughening of graphene oxide (GO) and molybdenum disulfide (MoS2) nanosheets via a vacuum-assisted filtration self-assembly process. The synergistic toughening effect from high mechanical properties of GO and lubrication of MoS2 nanosheets is successfully demonstrated. Meanwhile, the artificial nacre shows high electrical conductivity. This approach for constructing robust artificial nacre by synergistic effect from GO and MoS2 provides a creative opportunity for designing and fabricating integrated artificial nacre in the near future, and this kind of ternary artificial nacre has great potential applications in aerospace, flexible supercapacitor electrodes, artificial muscle, and tissue engineering.