Towards High‐Performance Bioinspired Composites

Biological composites have evolved elaborate hierarchical structures to achieve outstanding mechanical properties using weak but readily available building blocks. Combining the underlying design principles of such biological materials with the rich chemistry accessible in synthetic systems may enable the creation of artificial composites with unprecedented properties and functionalities. This bioinspired approach requires identification, understanding, and quantification of natural design principles and their replication in synthetic materials, taking into account the intrinsic properties of the stronger artificial building blocks and the boundary conditions of engineering applications. In this progress report, the scientific and technological questions that have to be addressed to achieve this goal are highlighted, and examples of recent research efforts to tackle them are presented. These include the local characterization of the heterogeneous architecture of biological materials, the investigation of structure–function relationships to help unveil natural design principles, and the development of synthetic processing routes that can potentially be used to implement some of these principles in synthetic materials. The importance of replicating the design principles of biological materials rather than their structure per se is highlighted, and possible directions for further progress in this fascinating, interdisciplinary field are discussed.     

Bioinspired Photonic Pigments from Colloidal Self‐Assembly

The natural world is a colorful environment. Stunning displays of coloration have evolved throughout nature to optimize camouflage, warning, and communication. The resulting flamboyant visual effects and remarkable dynamic properties, often caused by an intricate structural design at the nano‐ and microscale, continue to inspire scientists to unravel the underlying physics and to recreate the observed effects. Here, the methodologies to create bioinspired photonic pigments using colloidal self‐assembly approaches are considered. The physics governing the interaction of light with structural features and natural examples of structural coloration are briefly introduced. It is then outlined how the self‐assembly of colloidal particles, acting as wavelength‐scale building blocks, can be particularly useful to replicate coloration from nature. Different coloration effects that result from the defined structure of the self‐assembled colloids are introduced and it is highlighted how these optical properties can be translated into photonic pigments by modifications of the assembly processes. The importance of absorbing elements, as well as the role of surface chemistry and wettability to control structural coloration is discussed. Finally, approaches to integrate dynamic control of coloration into such self‐assembled photonic pigments are outlined.     

Carbon Nanotube and Graphene‐based Bioinspired Electrochemical Actuators

Bio‐inspired actuation materials, also called artificial muscles, have attracted great attention in recent decades for their potential application in intelligent robots, biomedical devices, and micro‐electro‐mechanical systems. Among them, ionic polymer metal composite (IPMC) actuator has been intensively studied for their impressive high‐strain under low voltage stimulation and air‐working capability. A typical IPMC actuator is composed of one ion‐conductive electrolyte membrane laminated by two electron‐conductive metal electrode membranes, which can bend back and forth due to the electrode expansion and contraction induced by ion motion under alternating applied voltage. As its actuation performance is mainly dominated by electrochemical and electromechanical process of the electrode layer, the electrode material and structure become to be more crucial to higher performance. The recent discovery of one dimensional carbon nanotube and two dimensional graphene has created a revolution in functional nanomaterials. Their unique structures render them intriguing electrical and mechanical properties, which makes them ideal flexible electrode materials for IPMC actuators in stead of conventional metal electrodes. Currently although the detailed effect caused by those carbon nanomaterial electrodes is not very clear, the presented outstanding actuation performance gives us tremendous motivation to meet the challenge in understanding the mechanism and thus developing more advanced actuator materials. Therefore, in this review IPMC actuators prepared with different kinds of carbon nanomaterials based electrodes or electrolytes are addressed. Key parameters which may generate important influence on actuation process are discussed in order to shed light on possible future research and application of the novel carbon nanomateials based bio‐inspired electrochemical actuators.     

Bioinspired Stimuli‐Responsive and Antifreeze‐Secreting Anti‐Icing Coatings

The detrimental impacts of icing on transportation and power industries are well‐known. Inspired by natural systems that secrete a functional liquid in response to stimuli, this work introduces an anti‐icing coating that responds to surface icing by releasing antifreeze liquid. It consists of an outer porous superhydrophobic epidermis and a wick‐like underlying dermis that is infused with antifreeze liquid. The functionality of the new coating is validated through condensation frosting, simulated freezing fog, and freezing rain experiments. In the tested conditions, the introduced anti‐icing skin delays onset of frost, rime, and glaze accumulation at least ten times longer than anti‐icing superhydrophobic and lubricant impregnated surfaces. Furthermore, the coating delays onset of glaze formation ten times longer than surfaces flooded with a thin film of antifreeze. In each of the icing scenarios, the fundamental mechanisms responsible for antifreeze release and their relation to required antifreeze replenishment rates are described.     

Bioinspired interactive neuromorphic devices

The performance of conventional computer based on von Neumann architecture is limited due to the physical separation of memory and processor. By synergistically integrating various sensors with synaptic devices, recently emerging interactive neuromorphic devices can directly sense/store/process various stimuli information from external environments and implement functions of perception, learning, memory, and computation. In this review, we present the basic model of bioinspired interactive neuromorphic devices and discuss the performance metrics. Next, we summarize the recent progress and development of bioinspired interactive neuromorphic devices, which are classified into neuromorphic tactile systems, visual systems, auditory systems, and multisensory system. They are discussed in detail from the aspects of materials, device architectures, operating mechanisms, synaptic plasticity, and potential applications. Additionally, the bioinspired interactive neuromorphic devices that can fuse multiple/mixed sensing signals are proposed to address more realistic and sophisticated problems. Finally, we discuss the pros and cons regarding to the computing neurons and integrating sensory neurons and deliver the perspectives on interactive neuromorphic devices at the material, device, network, and system levels. It is believed the neuromorphic devices can provide promising solutions to next generation of interactive sensation/memory/computation toward the development of multimodal, low-power, and large-scale intelligent systems endowed with neuromorphic features.     

Liquid-Superrepellent Bioinspired Fibrillar Adhesives

Bioinspired elastomeric fibrillar surfaces have significant potential as reversible dry adhesives, but their adhesion performance is sensitive to the presence of liquids at the contact interface. Like their models in nature, many artificial mimics can effectively repel water, but fail when low-surface-tension liquids are introduced at the contact interface. A bioinspired fibrillar adhesive surface that is liquid-superrepellent even toward ultralow-surface-tension liquids while retaining its adhesive properties is proposed herein. This surface combines the effective adhesion principle of mushroom-shaped fibrillar arrays with liquid repellency based on double re-entrant fibril tip geometry. The adhesion performance of the proposed microfibril structures is retained even when low-surface-tension liquids are added to the contact interface. The extreme liquid repellency enables real-world applications of fibrillar adhesives for surfaces covered with water, oil, and other liquids. Moreover, fully elastomeric liquid-superrepellent surfaces are mechanically not brittle, highly robust against physical contact, and highly deformable and stretchable, which can increase the real-world uses of such antiwetting surfaces.     

Bioinspired supramolecular liquid crystals

A brief account on the historical events leading to the discovery of self-assembling dendrons that generate self-organizable supramolecular dendrimers, or supramolecular polymers, and self-organizable dendronized polymers is provided. These building blocks were accessed by an accelerated design strategy that involves structural and retrostructural analysis of periodic and quasi-periodic assemblies. This design strategy mediated the discovery of porous helical supramolecular structures that self-assembled from dendritic dipeptides. Helical porous columns are the closest mimics of biologically related structures, such as tobacco mosaic virus coat, porous transmembrane proteins, porous pathogens and antibiotics. It is expected that this concept will allow one to investigate the structural origin of functions in synthetic supramolecular materials.     

Unidirectional Wetting Properties on Multi‐Bioinspired Magnetocontrollable Slippery Microcilia

Here, a smart fluid‐controlled surface is designed, via the rational integration of the unique properties of three natural examples, i.e., the unidirectional wetting behaviors of butterfly's wing, liquid‐infused “slippery” surface of the pitcher plant, and the motile microcilia of micro‐organisms. Anisotropic wettability, lubricated surfaces, and magnetoresponsive microstructures are assembled into one unified system. The as‐prepared surface covered by tilted microcilia achieves significant unidirectional droplet adhesion and sliding. Regulating by external magnet field, the directionality of ferromagnetic microcilia can be synergistically switched, which facilitates a continuous and omnidirectional‐controllable water delivery. This work opens an avenue for applications of anisotropic wetting surfaces, such as complex‐flow distribution and liquid delivery, and extend the design approach of multi‐bioinspiration integration.     

Bioinspired Smart Gating of Nanochannels Toward Photoelectric‐Conversion Systems

Learning from nature has inspired the creation of intelligent materials to better understand and imitate biology. Recent studies on bioinspired responsive surfaces that can switch between different states are shown, which open up new avenues for the development of smart materials in two dimensions. Based on this strategy, biomimetic nanochannel systems have been produced by introducing responsive molecules, which closely mimic the gating mechanism of biological nanochannels and show potential applications in many fields such as photoelectric‐conversion systems demonstrated in this paper.     

Bioinspired Catechol‐Terminated Self‐Assembled Monolayers with Enhanced Adhesion Properties

The role of the catechol moiety in the adhesive properties of mussel proteins and related synthetic materials has been extensively studied in the last years but still remains elusive. Here, a simplified model approach is presented based on a self‐assembled monolayer (SAM) of upward‐facing catechols thiol‐bound to epitaxial gold substrates. The orientation of the catechol moieties is confirmed by spectroscopy, which also showed lack of significant amounts of interfering o‐quinones. Local force‐distance curves on the SAM measured by atomic force microscopy (AFM) shows an average adhesion force of 45 nN, stronger than that of a reference polydopamine coating, along with higher reproducibility and less statistical dispersion. This is attributed to the superior chemical and topographical homogeneity of the SAM coating. Catechol‐terminated SAMs are also obtained on high‐roughness gold substrates that show the ability to assemble magnetic nanoparticles, despite their lack of enhanced adhesion at the molecular level. Finally, the influence of the catechol group on the formation and quality of the SAM is explored both theoretically (molecular dynamics simulations) and experimentally using direct‐write AFM lithography.