Shear wave filtering in naturally-occurring Bouligand structures

Preparation and Application of Nature-inspired High-performance Mechanical Materials

Natural materials are valued for their lightweight properties, high strength, impact resistance, and fracture toughness, often outperforming human-made materials. This paper reviews recent research on biomimetic composites, focusing on how composition, microstructure, and interfacial characteristics affect mechanical properties like strength, stiffness, and toughness. It explores biological structures such as mollusk shells, bones, and insect exoskeletons that inspire lightweight designs, including honeycomb structures for weight reduction and impact resistance. The paper also discusses the flexibility and durability of fibrous materials like arachnid proteins and evaluates traditional and modern fabrication techniques, including machine learning. The development of superior, multifunctional, and eco-friendly materials will benefit transportation, mechanical engineering, architecture, and biomedicine, promoting sustainable materials science. STATEMENT OF SIGNIFICANCE: Natural materials excel in strength, lightweight, impact resistance, and fracture toughness. This review focuses on biomimetic composites inspired by nature, examining how composition, microstructure, and interfacial characteristics affect mechanical properties like strength, stiffness, and toughness. It analyzes biological structures such as shells, bones, and exoskeletons, emphasizing honeycomb strength and lightness. The review also explores the flexibility and durability of fibrous materials like arachnid proteins and discusses fabrication techniques for biomaterials. It highlights impact-resistant materials that combine soft and hard components for enhanced strength and toughness, as well as lightweight, wear-resistant biomimetic materials that respond uniquely to cyclic stress. The article aims to advance sustainable materials science by exploring innovations in multifunctional and eco-friendly materials for various applications.

Exploring anisotropic mechanical properties of lobster claw exoskeleton through fractal models

The outstanding mechanical properties of lobster claw exoskeletons are intricately tied to their internal microstructure. Investigating this relationship can offer vital insights for designing high-performance additive manufacturing structures. Fractal theory, with its fractional dimensional perspective, suits the complexity of real-world phenomena. Our study examines fully hydrated lobster claw exoskeletons using a multifaceted approach: four-point bending tests, scanning electron microscopy observations, and fractal models. Test results reveal superior mechanical properties in longitudinal specimens. Scanning electron microscopy shows non-uniform fiber helical structures and porous elements in the exoskeleton. Fracture mechanisms involve both breaking fiber fragments perpendicular to the cross-section and tearing between these fragments. The observed crack propagation paths exhibit statistical self-similarity. Consequently, we develop fractal models for the crack propagation paths in longitudinal and transverse specimens, calculating crack extension forces. Using the box-counting method and its improved variant, we determine the fractal dimensions of specimen sections. The fractal dimension of longitudinal models exceeds that of transverse models, and calculated crack extension forces are higher in longitudinal models. These findings align well with experimental data, demonstrating fractal theory's efficacy in analyzing the lobster claw exoskeleton's anisotropic mechanical properties.

Cuticle ultrastructure of the Early Devonian trigonotarbid arachnid Palaeocharinus

The cuticle is a key evolutionary innovation that played a crucial role in arthropod terrestrialization. Extensive research has elucidated the chemical and structural composition of the cuticle in extant arthropods, while fossil studies have further informed our understanding of cuticle evolution. This study examines the three-dimensionally preserved cuticular structure of the Early Devonian trigonotarbid arachnid genus Palaeocharinus, from the Rhynie chert of Scotland (∼408 Ma). Trigonotarbids, an extinct group of tetrapulmonate arachnids, are among the earliest known unequivocally terrestrial arthropods, and thus may shed light on the evolution of terrestriality. Using high-resolution Confocal Laser Scanning Microscopy (CLSM), we reveal detailed morphological features at the nanometre level. The external cuticle surface of Palaeocharinus is characterized by polygonal scales, sensilla, and small pores identified as the openings of dermal glands and wax canals. Internally, the cuticle exhibits polygonal clusters of pore canals, through which wax was transported from the epidermis to the cuticular surface. The pore canals twist along their vertical axes, reflecting the "twisted plywood" or Bouligand arrangement of chitin-protein microfibril planes characteristic of modern arthropod cuticles. Overall, the cuticle of Palaeocharinus is characteristically thick relative to those of other extinct and extant chelicerates, such thickening being a possible adaptation to terrestrial life.

Bottom-Up Film-to-Bulk Assembly Toward Bioinspired Bulk Structural Nanocomposites

Biological materials, although composed of meager minerals and biopolymers, often exhibit amazing mechanical properties far beyond their components due to hierarchically ordered structures. Understanding their structure-properties relationships and replicating them into artificial materials would boost the development of bulk structural nanocomposites. Layered microstructure widely exists in biological materials, serving as the fundamental structure in nanosheet-based nacres and nanofiber-based Bouligand tissues, and implying superior mechanical properties. High-efficient and scalable fabrication of bioinspired bulk structural nanocomposites with precise layered microstructure is therefore important yet remains difficult. Here, one straightforward bottom-up film-to-bulk assembly strategy is focused for fabricating bioinspired layered bulk structural nanocomposites. The bottom-up assembly strategy inherently offers a methodology for precise construction of bioinspired layered microstructure in bulk form, availability for fabrication of bioinspired bulk structural nanocomposites with large sizes and complex shapes, possibility for design of multiscale interfaces, feasibility for manipulation of diverse heterogeneities. Not limited to discussing what has been achieved by using the current bottom-up film-to-bulk assembly strategy, it is also envisioned how to promote such an assembly strategy to better benefit the development of bioinspired bulk structural nanocomposites. Compared to other assembly strategies, the highlighted strategy provides great opportunities for creating bioinspired bulk structural nanocomposites on demand.

Biomimetic Bouligand chiral fibers array enables strong and superelastic ceramic aerogels

Ceramic aerogels are often used when thermal insulation materials are desired; however, they are still plagued by poor mechanical stability under thermal shock. Here, inspired by the dactyl clubs of mantis shrimp found in nature, which form by directed assembly into hierarchical, chiral and Bouligand (twisted plywood) structure exhibiting superior mechanical properties, we present a compositional and structural engineering strategy to develop strong, superelastic and fatigue resistance ceramic aerogels with chiral fibers array resembling Bouligand architecture. Benefiting from the stress dissipation, crack torsion and mechanical reinforcement of micro-/nano-scale Bouligand array, the tensile strength of these aerogels (170.38 MPa) is between one and two orders of magnitude greater than that of state-of-the-art nanofibrous aerogels. In addition, the developed aerogels feature low density and thermal conductivity, good compressive properties with rapid recovery from 80 % strain, and thermal stability up to 1200 °C, making them ideal for thermal insulation applications.

A study of a bio-inspired impact resistant carbon fiber laminate with a sinusoidal helicoidal structure in the mandibles of trap-jaw ants

The majority of living organisms demonstrate remarkable attributes and have evolved effective mechanisms for synthesizing impact-resistant and damage-tolerant structures. One exemplary instance is the rapid mandible strikes exhibited by trap-jaw ants, which are a highly aggressive species of terrestrial social organisms. An impact-resistant sinusoidal helicoidal architecture is discovered in the mandibles of trap-jaw ants. The bioinspired laminate with a bi-sinusoidal helicoidal structure was manufactured using unidirectional carbon fiber prepreg by mold press forming. This study examines the impact resistance and damage tolerance of a bionic laminate through low velocity impact, computed tomography, and compression after impact tests. The results demonstrate that bionic laminates effectively limit damage propagation within the plane while enhancing energy dissipation capacity. The sinusoidal helicoidal configuration enhances cushioning capability against impact forces, retards penetration under higher loads, hinders crack propagation, and improves residual strength. Bionic laminates provide a valuable solution for damage tolerance through the resistance to through-the-thickness loads. STATEMENT OF SIGNIFICANCE: Helicoidal and sinusoidal helicoidal microstructures have been identified in the cross-section of the jaws of trap-jaw ants. The multiple waviness ratio parameters are designed for fabricating a sinusoidal helicoidal structure laminate using unidirectional carbon fiber prepreg through the mold press forming technique. This results in a damage-tolerant mechanism characterized by reduced delamination damage, which leads to a stiffer mechanical response. Meanwhile, it enhances resistance to crack propagation, leading to the formation of discontinuous delamination areas and the accumulation of sub-critical failures. Additionally, the sinusoidal helicoidal structure laminate combines the cushioning performance of bi-sinusoidal arrangements with the enhanced impact resistance of helical arrangements. This design delays penetration at higher loads, resulting in increased residual strength.

Investigation of bionic composite laminates inspired by the natural impact-resistant helicoidal structure in the mandibles of trap-jaw ants

Many living organisms exhibit exceptional capabilities and have evolved effective strategies to synthesize impact-resistant and damage-tolerant structures. One such example can be observed in the rapid mandible strikes ofOdontomachus monticola, a species of trap-jaw ants from the ponerine subfamily. During trap-jaw strikes, the mandibles can achieve peak speeds of 35.42 m s-1, and the maximum acceleration can reach 71 729 g within an average duration of 0.18 ms. The extreme acceleration results in instantaneous mandible strike forces that can exceed 330 times the ant's body weight, withstanding thousands of impacts. A natural impact-resistant fibrous helicoidal structure is found in the mandibles of trap-jaw ants. This microstructure is characterized by periodic modulus oscillations that increase energy absorption and improve stress redistribution, offering added protection against damage from impact loading. A carbon fiber reinforced helicoidal composite is fabricated based on the microstructure of the trap-jaw ant's mandibles. The results show that the helicoidal composite with a 12° helical-fiber exhibits higher residual strength, making it more capable of withstanding strong collisions. The catastrophic propagation of damage along the thickness direction is prevented by in-plane spreading and redirection of cracks. This research provides useful references for fabricating bionic impact-resistant composites.

Investigation of Dynamic Impact Responses of Layered Polymer-Graphene Nanocomposite Films Using Coarse-Grained Molecular Dynamics Simulations

Polymer nanocomposite films have recently shown superior energy dissipation capability through the micro-projectile impact testing method. However, how stress waves interact with nanointerfaces and the underlying deformation mechanisms have remained largely elusive. This paper investigates the detailed stress wave propagation process and dynamic failure mechanisms of layered poly(methyl methacrylate) (PMMA) - graphene nanocomposite films during piston impact through coarse-grained molecular dynamics simulations. The spatiotemporal contours of stress and local density clearly demonstrate shock front, reflected wave, and release wave. By plotting shock front velocity (U s ) against piston velocity (U p ), we find that the linear Hugoniot U s - U p relationship generally observed for bulk polymer systems also applies to the layered nanocomposite system. When the piston reaches a critical velocity, PMMA crazing can emerge at the location where the major reflected wave and release wave meet. We show that the activation of PMMA crazing significantly enhances the energy dissipation ratio of the nanocomposite films, defined as the ratio between the dissipated energy through irreversible deformation and the input kinetic energy. The ratio maximizes at the critical U p when the PMMA crazing starts to develop and then decreases as U p further increases. We also find that a critical PMMA-graphene interfacial strength is required to activate PMMA crazing instead of interfacial separation. Additionally, layer thickness affects the amount of input kinetic energy and dissipated energy of nanocomposite films under impact. This study provides important insights into the detailed dynamic deformation mechanisms and how nanointerfaces/nanostructures affect the energy dissipation capability of layered nanocomposite films.

Hierarchically structured bioinspired nanocomposites

Next-generation structural materials are expected to be lightweight, high-strength and tough composites with embedded functionalities to sense, adapt, self-repair, morph and restore. This Review highlights recent developments and concepts in bioinspired nanocomposites, emphasizing tailoring of the architecture, interphases and confinement to achieve dynamic and synergetic responses. We highlight cornerstone examples from natural materials with unique mechanical property combinations based on relatively simple building blocks produced in aqueous environments under ambient conditions. A particular focus is on structural hierarchies across multiple length scales to achieve multifunctionality and robustness. We further discuss recent advances, trends and emerging opportunities for combining biological and synthetic components, state-of-the-art characterization and modelling approaches to assess the physical principles underlying nature-inspired design and mechanical responses at multiple length scales. These multidisciplinary approaches promote the synergetic enhancement of individual materials properties and an improved predictive and prescriptive design of the next era of structural materials at multilength scales for a wide range of applications.

Comparison of Lightweight Structures in Bearing Impact Loads during Ice–Hull Interaction

The current study focuses on the impact loading phase characteristic of thin first year ice in inland waterways. We investigate metal grillages, fibre reinforced plastic (FRP) composites and nature-inspired composites using LS Dyna. The impact mode is modelled as (a) simplified impact model with a rigid-body impactor and (b) an experimentally validated ice model represented by cohesive zone elements. The structural concepts are investigated parametrically for strength and stiffness using the simplified model, and an aluminium alloy grillage is analysed with the ice model. The metal–FRP composite was found to be the most favourable concept that offered impact protection as well as being light weight. By weight, FRP composites with a Bouligand ply arrangement were the most favourable but prone to impact damage. Further, aluminium grillage was found to be a significant contender for a range of ice impact velocities. While the ice model is experimentally validated, a drawback of the simplified model is the lack of experimental data. We overcame this by limiting the scope to low velocity impact and investigating only relative structural performance. By doing so, the study identifies significant parameters and parametric trends along with material differences for all structural concepts. The outcomes result in the creation of a viable pool of lightweight variants that fulfil the impact loading phase. Together with outcomes from quasi-static loading phase, it is possible to develop a lightweight ice-going hull concept.

Enhanced Low-Velocity Impact Resistance of Helicoidal Composites by Fused Filament Fabrication (FFF)

Bioinspired composites, capable of tailoring mechanical properties by the strategy of making full use of their advantages and bypassing their drawbacks, are vital for numerous engineering applications such as lightweight ultrahigh-strength, enhanced toughness, improved low-/high- velocity impact resistance, wave filtering, and energy harvesting. Helicoidal composites are examples of them. However, how to optimize the geometric structure to maximize the low-velocity impact resistance of helicoidal composites has been ignored, which is vital to the lightweight and high strength for aerospace, defense, ship, bridge, dam, vessel, and textile industries. Here, we combined experiments and numerical simulations to report the dynamic response of helicoidal composites subjected under low-velocity impact (0-10 m/s). Our helicoidal structures, inspired by the Stomatopod Dactyl club, are fabricated using polylactic acid (PLA) by FFF in a single-phase way. The helicoidal strategy aims to exploit, to a maximum extent, the axial tensile strength of filaments and simultaneously make up the shortage of inter-filament contact strength. We demonstrate experimentally that the low-velocity impact resistance has been enhanced efficiently as the helicoidal angle varies, and that the 15° helicoidal plate is better than others, which has also been confirmed by the numerical simulations. The findings reported here provide a new routine to design composites systems with enhanced impact resistance, offering a method to improve impact performance and expand the application of 3D printing.

Visualization of Polymer Dynamics in Cellulose Nanocrystal Matrices Using Fluorescence Lifetime Measurements

Polymer nanocomposites containing self-assembled cellulose nanocrystals (CNCs) are ideal for advanced applications requiring both strength and toughness as the helicoidal structure of the CNCs deflects crack propagation and the polymer matrix dissipates impact energy. However, any adsorbed water layer surrounding the CNCs may compromise the interfacial adhesion between the polymer matrix and the CNCs, thus impacting stress transfer at that interface. Therefore, it is critical to study the role of water at the interface in connecting the polymer dynamics and the resulting mechanical performance of the nanocomposite. Here, we explore the effect of polymer confinement and water content on polymer dynamics in CNC nanocomposites by covalently attaching a fluorogenic water-sensitive dye to poly(diethylene glycol methyl ether methacrylate) (PMEO₂MA), to provide insights into the observed mechanical performance. Utilizing fluorescence lifetime imaging microscopy (FLIM), the lifetime of dye fluorescence decay was measured to probe the polymer chain dynamics of PMEO₂MA in CNC nanocomposite films. The PMEO₂MA chains experienced distinct regions of differing dynamics within Bouligand structures. A correlation was observed between the average fluorescence lifetime and the mechanical performance of CNC films, indicating that polymer chains with high mobility improved the strain and toughness. These studies demonstrated FLIM as a method to investigate polymer dynamics at the nanosecond timescale that can readily be applied to other composite systems.

Wrinkling pattern formation with periodic nematic orientation: From egg cartons to corrugated surfaces

Egg cartons, known as doubly sinusoidal surfaces, display a rich variety of saddles-cylinder-spherical patches organized with different spatial symmetries and connectivities. Egg carton surfaces, rich in functionalities, are observed in synthetic and biological materials, as well as across atomic and macroscopic scales. In this work we use the liquid crystal shape equation in the absence of elastic effects and normal stress jumps to predict and classify a family of uniaxial, equibiaxial, and biaxial egg cartons, according to the periodicities of the surface director field in nematic (N) and cholesteric (N*) liquid crystals under the presence of anisotropic surface tension (anchoring). Egg carton surface shape periodic solutions to the nonlinear and linearized liquid crystal shape equations predict that the mean curvature is a linear function of the orthogonal (along the surface normal) splay and bend contributions. Mixtures of egg carton surfaces (uniaxial, equibiaxial, and biaxial) emerge according to the symmetries of the nonsingular director field, and the spatial distributions of the director escape into the third dimension; pure uniaxial egg cartons emerge when the director escape has linelike geometries and mixtures of egg cartons arise under source or sink orientation lattices. Orientation symmetry and permutation analysis are incorporated into a full curvature (Casorati, shape parameter, mean curvature, and Gaussian curvature) characterization. Under a fixed anchoring parameter, conditions for maximal nanoscale curvedness and microscale maximal shape gradient diversity are identified. The present results contribute to various pathways to surface pattern formation using intrinsic anisotropic interfacial tension.

Advances in Field-Assisted 3D Printing of Bio-Inspired Composites: From Bioprototyping to Manufacturing

Biocomposite systems evolve to superior structural strategies in adapting to their living environments, using limited materials to form functionality superior to their inherent properties. The synergy of physical-field and Three-dimensional （3D） printing technologies creates unprecedented opportunities that overcome the limitations of traditional manufacturing methods and enable the precise replication of bio-enhanced structures. Here, an overview of typical structural designs in biocomposite systems, their functions and properties, are provided and the recent advances in bio-inspired composites using mechanical, electrical, magnetic, and ultrasound-field-assisted 3D printing techniques are highlighted. Finally, in order to realize the preparation of bionic functional devices and equipment with more superior functions, here an outlook on the development of field-assisted 3D printing technology from three aspects are provided: Materials, technology, and post-processing.

Analysis and simulation of fracture behavior in naturally occurring Bouligand structures

Through natural selection processes, refined biological materials could be created that adapt to various environments and exhibit specific functions. Such materials include typical Bouligand structures that can be widely observed in marine creatures that have hard shells. Consisting of a helicoidal arrangement of aligned fibrils, layered single-twisted Bouligand-type structures (SBS) display exceptional fracture and damage resistance. A much more primitive and rarer type of this formation, the double-twisted Bouligand-type structures (DBS), has been discovered in ancient fish scales, and this architecture could provide added rigidity and significantly contribute to toughness when facing fracture risk. In this work, we describe a computational modeling approach to investigating fracture behaviors and toughening mechanisms in Bouligand structures. To achieve qualitative insights into the fracture behaviors of DBS and SBS, we applied these two configurations, which were identified from corresponding biological materials, to analyze load-displacement responses during single edge notched (SEN) tensile testing; the toughening mechanism is also discussed further. The results clearly show that the arrangement of helix fibrils and interlaminar properties play a major role in the resulting fracture behaviors of Bouligand architectures. This is of interest for the future design of engineering materials and structures that require composites with enhanced toughness, and could deepen our understanding of the structure-property relationship of Bouligand-type structures in bionic design. STATEMENT OF SIGNIFICANCE: In this work, a novel numerical modeling approach based on the extended finite element method (XFEM) has been established to evaluate the fracture behavior of a naturally-occurring Bouligand-type helicoidal structure subjected to the single edge notched (SEN) tensile loading. The roles of the biological features (i.e., layered arrangement of collagen fibrils and interbundle fibrils) on the fracture resistance and toughening mechanism of the Bouligand-type structures have been uncovered and analyzed quantitatively. This is of interest for future design of engineering materials and structures that require composites with enhanced toughness, and can deepen the understanding of the structure-property relationship of the Bouligand-type structure in bionic design.

Structural characterization and regulation of the mechanical properties of the carapace cuticle in tri-spine horseshoe crab (Tachypleus tridentatus)

Horseshoe crab (order Xiphosura) has a large and thick carapace that has evolved as a protective tool to defend against predators and resist impacts from surf-zone turbulence. The naturally occurring spatial variation in the mechanical properties of the carapace cuticle need to be investigated to understand their regulatory mechanism and the underlying design strategies. In this work, we used a combination of high-resolution optical microscopy, scanning electron microscopy, (SEM) and energy-dispersive X-ray spectroscopy (EDS) to evaluate the multiscale microstructure and elemental composition of the cuticle of tri-spine horseshoe crab (Tachypleus tridentatus). The moduli, ultimate strengths, and failure strains of the three individual layers and the entire cuticle were systematically characterized in both the dry and hydrated states. The failure behaviors and energy absorption of the cuticle involved stress stiffening, toughness mechanism and environmental adaptation were analyzed qualitatively and quantitatively and then correlated with the morphological features in different cuticle regions. The mechanical properties are primarily influenced by the endocuticle thickness ratio; a higher thickness ratio corresponds to more stacking of the vertical lamellae, leading to a lower modulus, weaker strength, and greater elongation of the endocuticle. Radial energy is absorbed primarily by the endocuticle, with the energy absorbed in the radial direction being nearly twice that absorbed in the circumferential direction. This is attributed to the larger failure strain and relatively small decrease in the stress plateau in the radial direction. The findings provide a deeper understanding of how nature modulates the cuticle's mechanical properties and inspiration for developing high-performance synthetic composites.

Microstructure and nanomechanical properties of the exoskeleton of an ironclad beetle (Zopherus haldemani)

This study examined natural composite structures within the remarkably strong exoskeleton of the southwestern ironclad beetle (Z. haldemani). Structural and nanomechanical analyses revealed that the exoskeleton's extraordinary resistance to external forces is provided by its exceptional thickness and multi-layered structure, in which each layer performed a distinct function. In detail, the epicuticle, the outmost layer, comprised 3%-5% of the overall thickness with reduced Young's moduli of 2.2-3.2 GPa, in which polygonal-shaped walls (2-3μm in diameter) were observed on the surface. The next layer, the exocuticle, consisted of 17%-20% of the total thickness and exhibited the greatest Young's moduli (∼15 GPa) and hardness (∼800 MPa) values. As such, this layer provided the bulk of the mechanical strength for the exoskeleton. While the endocuticle spanned 70%-75% of the total thickness, it contained lower moduli (∼8-10 GPa) and hardness (∼400 MPa) values than the exocuticle. Instead, this layer may provide flexibility through its specifically organized chitin fiber layers, known as Bouligand structures. Nanoindentation testing further reiterated that the various fibrous layer orientations resulted in different elastic moduli throughout the endocuticle's cross-section. Additionally, this exoskeleton prevented delamination within the composite materials by overlapping approximately 5%-19% of each fibrous stack with neighboring layers. Finally, the innermost layer, the epidermis contributing 5%-7 % of the total thickness, contains attachment sites for muscle and soft tissue that connect the exoskeleton to the beetle. As such, it is the softest region with reduced Young's modulus of ∼2-3 GPa and hardness values of ∼290 MPa. These findings can be applied to the development of innovative, fiber-reinforced composite materials.

Molecular to Macroscale Energy Absorption Mechanisms in Biological Body Armour Illuminated by Scanning X-ray Diffraction with In Situ Compression

Determining multiscale, concurrent strain, and deformation mechanisms in hierarchical biological materials is a crucial engineering goal, to understand structural optimization strategies in Nature. However, experimentally characterizing complex strain and displacement fields within a 3D hierarchical composite, in a multiscale full-field manner, is challenging. Here, we determined the in situ strains at the macro-, meso-, and molecular-levels in stomatopod cuticle simultaneously, by exploiting the anisotropy of the 3D fiber diffraction coupled with sample rotation. The results demonstrate the method, using the mineralized 3D α-chitin fiber networks as strain sensors, can capture submicrometer deformation of a single lamella (mesoscale), can extract strain information on multiple constituents concurrently, and shows that α-chitin fiber networks deform elastically while the surrounding matrix deforms plastically before systematic failure under compression. Further, the results demonstrate a molecular-level prestrain gradient in chitin fibers, resulting from different mineralization degrees in the exo- and endo cuticle.

Nature-Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self-Assembly to DNA Mesophase and Nanocolloids

Liquid crystals (LCs) are omnipresent in living matter, whose chirality is an elegant and distinct feature in certain plant tissues, the cuticles of crabs, beetles, arthropods, and beyond. Taking inspiration from nature, researchers have recently devoted extensive efforts toward developing chiral liquid crystalline materials with self-organized nanostructures and exploring their potential applications in diverse fields ranging from dynamic photonics to energy and safety issues. In this review, an account on the state of the art of emerging chiral liquid crystalline nanostructured materials and their technological applications is provided. First, an overview on the significance of chiral liquid crystalline architectures in various living systems is given. Then, the recent significant progress in different chiral liquid crystalline systems including thermotropic LCs (cholesteric LCs, cubic blue phases, achiral bent-core LCs, etc.) and lyotropic LCs (DNA LCs, nanocellulose LCs, and graphene oxide LCs) is showcased. The review concludes with a perspective on the future scope, opportunities, and challenges in these truly advanced functional soft materials and their promising applications.

Fiber reorientation in hybrid helicoidal composites

Naturally occurring biological materials with stiff fibers embedded in a ductile matrix are commonly known to achieve excellent balance between stiffness, strength and ductility. In particular, biological composite materials with helicoidal architecture have been shown to exhibit enhanced damage tolerance and increased impact energy absorption. However, the role of fiber reorientation inside the flexible matrix of helicoid composites on their mechanical behaviors have not yet been extensively investigated. In the present work, we introduce a Discontinuous Fiber Helicoid (DFH) composite inspired by both the helicoid microstructure in the cuticle of mantis shrimp and the nacreous architecture of the red abalone shell. We employ 3D printed specimens, analytical models and finite element models to analyze and quantify in-plane fiber reorientation in helicoid architectures with different geometrical features. We also introduce additional architectures, i.e., single unidirectional lamina and mono-balanced architectures, for comparison purposes. Compared with associated mono-balanced architectures, helicoid architectures exhibit less fiber reorientation values and lower values of strain stiffening. The explanation for this difference is addressed in terms of the measured in-plane deformation, due to uniaxial tensile of the laminae, correlated to lamina misorientation with respect to the loading direction and lay-up sequence.

Bioinspired Materials: From Living Systems to New Concepts in Materials Chemistry

Nature successfully employs inorganic solid-state materials (i.e., biominerals) and hierarchical composites as sensing elements, weapons, tools, and shelters. Optimized over hundreds of millions of years under evolutionary pressure, these materials are exceptionally well adapted to the specifications of the functions that they perform. As such, they serve today as an extensive library of engineering solutions. Key to their design is the interplay between components across length scales. This hierarchical design-a hallmark of biogenic materials-creates emergent functionality not present in the individual constituents and, moreover, confers a distinctly increased functional density, i.e., less material is needed to provide the same performance. The latter aspect is of special importance today, as climate change drives the need for the sustainable and energy-efficient production of materials. Made from mundane materials, these bioceramics act as blueprints for new concepts in the synthesis and morphosynthesis of multifunctional hierarchical materials under mild conditions. In this review, which also may serve as an introductory guide for those entering this field, we demonstrate how the pursuit of studying biomineralization transforms and enlarges our view on solid-state material design and synthesis, and how bioinspiration may allow us to overcome both conceptual and technical boundaries.

Design of textured multi-layered structures via magnetically assisted slip casting

Multi-layered composites in nature often show functional properties that are determined by the specific orientation of inorganic building blocks within each layer. The shell of bivalve molluscs and the exoskeleton of crustaceans constitute prominent examples. An effective approach to artificially produce textured microstructures inspired by such complex composites is magnetically assisted slip casting (MASC). MASC is a colloidal process in which anisotropic particles are magnetically oriented at arbitrarily defined angles and collected at the surface of a porous mould to grow the material in an additive manner. Whereas a number of proof-of-concept studies have established the potential of the technique, the full design space available for MASC-fabricated structures, and the limits of the approach, have so far not been explored systematically. To fill this gap, we have studied both theoretically and experimentally the various torques that act on the particles at the different stages of the assembly process. We define the boundary conditions of the MASC process for magnetically responsive alumina platelets suspended in a low-viscosity aqueous suspension, considering the composition of the colloidal suspension and the dynamics of the particle alignment process under a rotating magnetic field. These findings lead to design guidelines for the fabrication of bio-inspired composites with customized multi-scale structures for a broad range of applications.

A diecast mineralization process forms the tough mantis shrimp dactyl club

Biomineralization, the process by which mineralized tissues grow and harden via biogenic mineral deposition, is a relatively lengthy process in many mineral-producing organisms, resulting in challenges to study the growth and biomineralization of complex hard mineralized tissues. Arthropods are ideal model organisms to study biomineralization because they regularly molt their exoskeletons and grow new ones in a relatively fast timescale, providing opportunities to track mineralization of entire tissues. Here, we monitored the biomineralization of the mantis shrimp dactyl club-a model bioapatite-based mineralized structure with exceptional mechanical properties-immediately after ecdysis until the formation of the fully functional club and unveil an unusual development mechanism. A flexible membrane initially folded within the club cavity expands to form the new club's envelope. Mineralization proceeds inwards by mineral deposition from this membrane, which contains proteins regulating mineralization. Building a transcriptome of the club tissue and probing it with proteomic data, we identified and sequenced Club Mineralization Protein 1 (CMP-1), an abundant mildly phosphorylated protein from the flexible membrane suggested to be involved in calcium phosphate mineralization of the club, as indicated by in vitro studies using recombinant CMP-1. This work provides a comprehensive picture of the development of a complex hard tissue, from the secretion of its organic macromolecular template to the formation of the fully functional club.

Impact resistance of nanocellulose films with bioinspired Bouligand microstructures

The Bouligand structure features a helicoidal (twisted plywood) layup of fibers that are uniaxially arranged in-plane and is a hallmark of biomaterials that exhibit outstanding impact resistance. Despite its performance advantage, the underlying mechanisms for its outstanding impact resistance remain poorly understood, posing challenges for optimizing the design and development of bio-inspired materials with Bouligand microstructures. Interestingly, many bio-sourced nanomaterials, such as cellulose nanocrystals (CNCs), readily self-assemble into helicoidal thin films with inter-layer (pitch) angles tunable via solvent processing. Taking CNC films as a model Bouligand system, we present atomistically-informed coarse-grained molecular dynamics simulations to measure the ballistic performance of thin films with helicoidally assembled nanocrystals by subjecting them to loading similar to laser-induced projectile impact tests. The effect of pitch angle on the impact performance of CNC films was quantified in the context of their specific ballistic limit velocity and energy absorption. Bouligand structures with low pitch angles (18-42°) were found to display the highest ballistic resistance, significantly outperforming other pitch angle and quasi-isotropic baseline structures. Improved energy dissipation through greater interfacial sliding, larger in-plane crack openings, and through-thickness twisting cracks resulted in improved impact performance of optimal pitch angle Bouligand CNC films. Intriguingly, decreasing interfacial interactions enhanced the impact performance by readily admitting dissipative inter-fibril and inter-layer sliding events without severe fibril fragmentation. This work helps reveal structural and chemical factors that govern the optimal mechanical design of Bouligand microstructures made from high aspect ratio nanocrystals, paving the way for sustainable, impact resistant, and multi-functional films.

Additive Manufacturing as a Method to Design and Optimize Bioinspired Structures

Additive manufacturing (AM) is a current technology undergoing rapid development that is utilized in a wide variety of applications. In the field of biological and bioinspired materials, additive manufacturing is being used to generate intricate prototypes to expand our understanding of the fundamental structure-property relationships that govern nature's spectacular mechanical performance. Herein, recent advances in the use of AM for improving the understanding of the structure-property relationship in biological materials and for the production of bioinspired materials are reviewed. There are four essential components to this work: a) extracting defining characteristics of biological designs, b) designing 3D-printed prototypes, c) performing mechanical testing on 3D-printed prototypes to understand fundamental mechanisms at hand, and d) optimizing design for tailorable performance. It is intended to highlight how the various types of additive manufacturing methods are utilized, to unravel novel discoveries in the field of biological materials. Since AM processing techniques have surpassed antiquated limitations, especially with respect to spatial scales, there has been a surge in their demand as an integral tool for research. In conclusion, current challenges and the technical perspective for further development of bioinspired materials using AM are discussed.

Photonic Crystal Characterization of the Cuticles of Chrysina chrysargyrea and Chrysina optima Jewel Scarab Beetles

A unified description involving structural morphology and composition, dispersion of optical constants, modeled and measured reflection spectra and photonic crystal characterization is devised. Light reflection spectra by the cuticles of scarab beetles (Chrysina chrysargyrea and Chrysina optima), measured in the wavelength range 300-1000 nm, show spectrally structured broad bands. Scanning electron microscopy analysis shows that the pitches of the twisted structures responsible for the left-handed circularly polarized reflected light change monotonically with depth through the cuticles, making it possible to obtain the explicit depth-dependence for each cuticle arrangement considered. This variation is a key aspect, and it will be introduced in the context of Berreman's formalism, which allows us to evaluate reflection spectra whose main features coincide in those displayed in measurements. Through the dispersion relation obtained from the Helmholtz's equation satisfied by the circular components of the propagating fields, the presence of a photonic band gap is established for each case considered. These band gaps depend on depth through the cuticle, and their spectral positions change with depth. This explains the presence of broad bands in the reflection spectra, and their spectral features correlate with details in the variation of the pitch with depth. The twisted structures consist of chitin nanofibrils whose optical anisotropy is not large enough so as to be approached from modeling the measured reflection spectra. The presence of a high birefringence substance embedded in the chitin matrix is required. In this sense, the presence of uric acid crystallites through the cuticle is strongly suggested by frustrated attenuated total reflection and Raman spectroscopy analysis. The complete optical modeling is performed incorporating the wavelength-dependent optical constants of chitin and uric acid.

On the Materials Science of Nature's Arms Race

Biological material systems have evolved unique combinations of mechanical properties to fulfill their specific function through a series of ingenious designs. Seeking lessons from Nature by replicating the underlying principles of such biological materials offers new promise for creating unique combinations of properties in man-made systems. One case in point is Nature's means of attack and defense. During the long-term evolutionary "arms race," naturally evolved weapons have achieved exceptional mechanical efficiency with a synergy of effective offense and persistence-two characteristics that often tend to be mutually exclusive in many synthetic systems-which may present a notable source of new materials science knowledge and inspiration. This review categorizes Nature's weapons into ten distinct groups, and discusses the unique structural and mechanical designs of each group by taking representative systems as examples. The approach described is to extract the common principles underlying such designs that could be translated into man-made materials. Further, recent advances in replicating the design principles of natural weapons at differing lengthscales in artificial materials, devices and tools to tackle practical problems are revisited, and the challenges associated with biological and bioinspired materials research in terms of both processing and properties are discussed.

Smashing mantis shrimp strategically impact shells

Many predators fracture strong mollusk shells, requiring specialized weaponry and behaviors. The current shell fracture paradigm is based on jaw- and claw-based predators that slowly apply forces (high impulse, low peak force). However, predators also strike shells with transient intense impacts (low impulse, high peak force). Toward the goal of incorporating impact fracture strategies into the prevailing paradigm, we measured how mantis shrimp (Neogonodactylus bredini) impact snail shells, tested whether they strike shells in different locations depending on prey shape (Nerita spp., Cenchritis muricatus, Cerithium spp.) and deployed a physical model (Ninjabot) to test the effectiveness of strike locations. We found that, contrary to their formidable reputation, mantis shrimp struck shells tens to hundreds of times while targeting distinct shell locations. They consistently struck the aperture of globular shells and changed from the aperture to the apex of high-spired shells. Ninjabot tests revealed that mantis shrimp avoid strike locations that cause little damage and that reaching the threshold for eating soft tissue is increasingly difficult as fracture progresses. Their ballistic strategy requires feed-forward control, relying on extensive pre-strike set-up, unlike jaw- and claw-based strategies that can use real-time neural feedback when crushing. However, alongside this pre-processing cost to impact fracture comes the ability to circumvent gape limits and thus process larger prey. In sum, mantis shrimp target specific shell regions, alter their strategy depending on shell shape, and present a model system for studying the physics and materials of impact fracture in the context of the rich evolutionary history of predator-prey interactions.

Conduction in the Heart Wall: Helicoidal Fibers Minimize Diffusion Bias

The mammalian heart must function as an efficient pump while simultaneously conducting electrical signals to drive the contraction process. In the ventricles, electrical activation begins at the insertion points of the Purkinje network in the endocardium. How does the diffusion component of the subsequent excitation wave propagate from the endocardium in a healthy heart wall without creating directional biases? We show that this is a consequence of the particular geometric organization of myocytes in the heart wall. Using a generalized helicoid to model fiber orientation, we treat the myocardium as a curved space via Riemannian geometry, and then use stochastic calculus to model local signal diffusion. Our analysis shows that the helicoidal arrangement of myocytes minimizes the directional biases that could lead to aberrant propagation, thereby explaining how electrophysiological principles are consistent with local measurements of cardiac fiber geometry. We discuss our results in the context of the need to balance electrical and mechanical requirements for heart function.

Bioinspired Bouligand cellulose nanocrystal composites: a review of mechanical properties

The twisted plywood, or Bouligand, structure is the most commonly observed microstructural motif in natural materials that possess high mechanical strength and toughness, such as that found in bone and the mantis shrimp dactyl club. These materials are isotropically toughened by a low volume fraction of soft, energy-dissipating polymer and by the Bouligand structure itself, through shear wave filtering and crack twisting, deflection and arrest. Cellulose nanocrystals (CNCs) are excellent candidates for the bottom-up fabrication of these structures, as they naturally self-assemble into 'chiral nematic' films when cast from solutions and possess outstanding mechanical properties. In this article, we present a review of the fabrication techniques and the corresponding mechanical properties of Bouligand biomimetic CNC nanocomposites, while drawing comparison to the performance standards set by tough natural composite materials.This article is part of a discussion meeting issue 'New horizons for cellulose nanotechnology'.

Binary Cellulose Nanocrystal Blends for Bioinspired Damage Tolerant Photonic Films

Most attempts to emulate the mechanical properties of strong and tough natural composites using helicoidal films of wood-derived cellulose nanocrystals (w-CNCs) fall short in mechanical performance due to the limited shear transfer ability between the w-CNCs. This shortcoming is ascribed to the small w-CNC-w-CNC overlap lengths that lower the shear transfer efficiency. Herein, we present a simple strategy to fabricate superior helicoidal CNC films with mechanical properties that rival those of the best natural materials and are some of the best reported for photonic CNC materials thus far. Assembling the short w-CNCs with a minority fraction of high aspect ratio CNCs derived from tunicates (t-CNCs), we report remarkable simultaneous enhancement of all in-plane mechanical properties and out-of-plane flexibility. The important role of t-CNCs is revealed by coarse grained molecular dynamics simulations where the property enhancement are due to increased interaction lengths and the activation of additional toughening mechanisms. At t-CNC contents greater than 5% by mass the mixed films also display UV reflecting behaviour. These damage tolerant optically active materials hold great promise for application as protective coatings. More broadly, we expect the strategy of using length-bidispersity to be adaptable to mechanically enhancing other matrix-free nanoparticle ensembles.

Biomimetic Structural Materials: Inspiration from Design and Assembly

Nature assembles weak organic and inorganic constituents into sophisticated hierarchical structures, forming structural composites that demonstrate impressive combinations of strength and toughness. Two such composites are the nacre structure forming the inner layer of many mollusk shells, whose brick-and-mortar architecture has been the gold standard for biomimetic composites, and the cuticle forming the arthropod exoskeleton, whose helicoidal fiber-reinforced architecture has only recently attracted interest for structural biomimetics. In this review, we detail recent biomimetic efforts for the fabrication of strong and tough composite materials possessing the brick-and-mortar and helicoidal architectures. Techniques discussed for the fabrication of nacre- and cuticle-mimetic structures include freeze casting, layer-by-layer deposition, spray deposition, magnetically assisted slip casting, fiber-reinforced composite processing, additive manufacturing, and cholesteric self-assembly. Advantages and limitations to these processes are discussed, as well as the future outlook on the biomimetic landscape for structural composite materials.

The snapping shrimp dactyl plunger: a thermomechanical damage-tolerant sandwich composite

The dactyl plunger of Alpheus sp. was found to be a layered composite, with mineral-rich outer and inner layers and a chitin-rich middle layer of high porosity. The chitin-rich middle layer is itself composed of several porous chitin laminae. Modelling heat conduction through the plunger cross-section revealed that the chitin-rich layer is able to insulate heat and retard its progress through the material. Heat accumulates in the plunger after a series of successive snaps and as such, its thermally resistant design can be considered most useful under the conditions of successive snapping. The plunger has a concurrent mechanical damage-tolerant design with biogenic mineral layers, viscous (chitin-mineral) interfaces, energy-dissipating porous chitin, and sidewalls composed of ordered, layered aragonite. The snapping shrimp plunger has a design that may protect it and internal soft tissues from thermomechanical damage during plunger-socket compression prior to cavitation bubble release.

Cholesteric liquid crystals in living matter

Liquid crystals play an important role in biology because the combination of order and mobility is a basic requirement for self-organisation and structure formation in living systems. Cholesteric liquid crystals are omnipresent in living matter under both in vivo and in vitro conditions and address the major types of molecules essential to life. In the animal and plant kingdoms, the cholesteric structure is a recurring design, suggesting a convergent evolution to an optimised left-handed helix. Herein, we review the recent advances in the cholesteric organisation of DNA, chromatin, chitin, cellulose, collagen, viruses, silk and cholesterol ester deposition in atherosclerosis. Cholesteric structures can be found in bacteriophages, archaea, eukaryotes, bacterial nucleoids, chromosomes of unicellular algae, sperm nuclei of many vertebrates, cuticles of crustaceans and insects, bone, tendon, cornea, fish scales and scutes, cuttlebone and squid pens, plant cell walls, virus suspensions, silk produced by spiders and silkworms, and arterial wall lesions. This article specifically aims at describing the consequences of the cholesteric geometry in living matter, which are far from being fully defined and understood, and discusses various perspectives. The roles and functions of biological cholesteric liquid crystals include maximisation of packing efficiency, morphogenesis, mechanical stability, optical information, radiation protection and evolution pressure.

Dielectric Characterization of Confined Water in Chiral Cellulose Nanocrystal Films

A known deterrent to the large-scale development and use of cellulose nanocrystals (CNCs) in composite materials is their affinity for moisture, which has a profound effect on dispersion, wetting, interfacial adhesion, matrix crystallization, water uptake, and hydrothermal stability. To quantify and control the hydration and confinement of absorbed water in CNCs, we studied sulfated-CNCs neutralized with sodium cations and CNCs functionalized with less hydrophilic methyl(triphenyl)phosphonium cations. Films were cast from water suspensions at 20 °C under controlled humidity and drying rate, yielding CNC materials with distinguishably different dielectric properties and cholesteric structures. By controlling the evaporation rate, we obtained self-assembled chiral CNC films with extended uniformity, having helical modulation length (nominal pitch) tunable from 1300 to 600 nm. SEM imaging and UV-vis-NIR total reflectance spectra revealed tighter and more uniform CNC packing in films cast at slow evaporation rates or having lower surface energy when modified with phosphonium. The dielectric constant was measured by a noncontact microwave cavity perturbation method and fitted to a classical mixing model employing randomly oriented ellipsoidal water inclusions. The dielectric constant of absorbed water was found to be significantly smaller than that for free liquid indicating a limited mobility due to binding with the CNC "matrix". In the case of hydrophilic Na-modified CNCs, a decreasing pitch led to greater anisotropy in the shape of moisture inclusions (ellipsoidal to platelet-like) and greater confinement. In contrast, the structure of hydrophobic phosphonium-modified CNC films was found to have reduced pitch, yet the shape of confined water remained predominantly spherical. These results provide a useful perspective on the current state of understanding of CNC-water interactions as well as on CNC self-assembly mechanisms. More broadly, we believe that our results are beneficial for the realization of CNC-based functional materials and composites.

Multiwavelength micromirrors in the cuticle of scarab beetle Chrysina gloriosa

Beetles from the genus Chrysina show vivid reflections from bright green to metallic silver-gold as a consequence of the cholesteric liquid crystal organization of chitin molecules. Particularly, the cuticle of Chrysina gloriosa exhibits green and silver stripes. By combining confocal microscopy and spectrophotometry, scanning electron microscopy and numerical simulations, the relationship between the reflectance and the structural parameters for both stripes at the micro- and nanoscales are established. Over the visible and near IR spectra, polygonal cells in tessellated green stripes behave as multiwavelength selective micro-mirrors and the silver stripes as specular broadband mirrors. Thermoregulation, conspecifics or intra-species communication, or camouflage against predators are discussed as possible functions. As a prerequisite to bio-inspired artificial replicas, the physical characteristics of the polygonal texture in Chrysina gloriosa cuticle are compared to their equivalents in synthetic cholesteric oligomers and their fundamental differences are ascertained. It is shown that the cuticle has concave cells whereas the artificial films have convex cells, contrary to expectation and assumption in the literature. The present results may provide inspiration for fabricating multiwavelength selective micromirrors or spatial wavelength-specific light modulators.

STATEMENT OF SIGNIFICANCE

Many insects own a tessellated carapace with bumps, pits or indentations. Little is known on the physical properties of these geometric variations and biological functions are unknown or still debated. We show that the polygonal cells in scarab beetle Chrysina gloriosa behave as multiwavelength selective micromirrors over the visible and infrared spectra, with a variety of spatial patterns. In the context of biomimetic materials, we demonstrate that the carapace has concave cells whereas the artificial films have convex cells, contrary to expectation in the literature. Thermoregulation, communication or camouflage are discussed as advanced functions. Results may provide inspiration for fabricating spatial wavelength-specific light modulators and optical packet switching in routing technologies.

A Sinusoidally Architected Helicoidal Biocomposite

A fibrous herringbone-modified helicoidal architecture is identified within the exocuticle of an impact-resistant crustacean appendage. This previously unreported composite microstructure, which features highly textured apatite mineral templated by an alpha-chitin matrix, provides enhanced stress redistribution and energy absorption over the traditional helicoidal design under compressive loading. Nanoscale toughening mechanisms are also identified using high-load nanoindentation and in situ transmission electron microscopy picoindentation.

Structural Design Elements in Biological Materials: Application to Bioinspiration

Eight structural elements in biological materials are identified as the most common amongst a variety of animal taxa. These are proposed as a new paradigm in the field of biological materials science as they can serve as a toolbox for rationalizing the complex mechanical behavior of structural biological materials and for systematizing the development of bioinspired designs for structural applications. They are employed to improve the mechanical properties, namely strength, wear resistance, stiffness, flexibility, fracture toughness, and energy absorption of different biological materials for a variety of functions (e.g., body support, joint movement, impact protection, weight reduction). The structural elements identified are: fibrous, helical, gradient, layered, tubular, cellular, suture, and overlapping. For each of the structural design elements, critical design parameters are presented along with constitutive equations with a focus on mechanical properties. Additionally, example organisms from varying biological classes are presented for each case to display the wide variety of environments where each of these elements is present. Examples of current bioinspired materials are also introduced for each element.