Nacre-Inspired Structural Composites: Performance-Enhancement Strategy and Perspective

Inorganic-organic hybrid metamaterials with switchable high stiffness and elasticity

In the pursuit of replicating the remarkable mechanical properties of natural biological composites like bone and seashell, developing artificial bulk materials that seamlessly integrate rigid inorganic components with ductile organic constituents has been a longstanding challenge. A key hurdle has been the establishment of robust and reliable linkages between these disparate building blocks. Mechanical metamaterials achieved by well-designed chemical structures, however, offer a promising solution to address this challenge. In this study, we demonstrate that the calcium phosphate-based inorganic-organic hybrid metamaterials trapping inorganic nanoparticles within long-chain polymeric networks and anchoring inorganic blocks to these networks via short-chain organic crosslinkers exhibit switchable and tunable high stiffness and elasticity. Additionally, these metamaterials not only exhibit peculiar mechanical characteristics, but also present excellent biocompatibility, as demonstrated by the in vivo tests using male rats and the in vitro tests. These results suggest a wide range of potential clinical applications.

Nacre-inspired graphene oxide/chitosan supported Pd species composite paper-like membrane with superior catalytic performance

Recent studies have shown that graphene oxide (GO) nanosheets can form a nacre-like bioinspired layered structure with polysaccharide of chitosan (CS), leading to composites with excellent mechanical properties. In this study, we go further steps by immobilization of Pd species (both Pd2+ and Pd0) within nacre-like bioinspired layered GO-CS composite paper-like membranes by vacuum-assisted self-assembly process to fabricate novel GO-CS-Pd composite membrane catalysts for the first time. Synergistic interactions from hydrogen bonding (between the GO nanosheets and CS chains) and ionic bonding (between the GO nanosheets and Pd2+ ions) have been efficiently achieved, resulting in significantly improvement of the mechanical properties. Meanwhile, the in-situ grown Pd0 nanoparticles were homogeneously incorporated in the interstices of the nacre-like GO-CS composite membranes. The mechanical properties, specific area performances, and Pd0 nanoparticles size of the resultant GO-CS-Pd composite membrane are mainly tuned by the loading amount of CS. The membranes are high active for Suzuki reactions of aromatic halides and phenylboronic acid with catalyst loading as low as 0.05 mol%, and can be recycled for 8 runs without significant loss of activities. Positron annihilation lifetime spectroscopy and other structural characterization methods are implemented to characterize the unique compartmentalization structure in the nacre-like composite membranes.

Multiscale integral synchronous assembly of cuttlebone-inspired structural materials by predesigned hydrogels

The overall structural integrity plays a vital role in the unique performance of living organisms, but the integral synchronous preparation of different multiscale architectures remains challenging. Inspired by the cuttlebone's rigid cavity-wall structure with excellent energy absorption, we develop a robust hierarchical predesigned hydrogel assembly strategy to integrally synchronously assemble multiple organic and inorganic micro-nano building blocks to different structures. The two types of predesigned hydrogels, combined with hydrogen, covalent bonding, and electrostatic interactions, are layer-by-layer assembled into brick-and-mortar structures and close-packed rigid micro hollow structures in a cuttlebone-inspired structural material, respectively. The cuttlebone-inspired structural materials gain crack growth resistance, high strength, and energy absorption characteristics beyond typical energy-absorbing materials with similar densities. This hierarchical hydrogel integral synchronous assembly strategy is promising for the integrated fabrication guidance of bioinspired structural materials with multiple different micro-nano architectures.

Modulating Electrochemical Energy Storage and Multi-Spectra Defense of MXenes by Interfacial Dual-Filler Engineering

MXenes have attracted growing interest in electrochemical energy storage owing to their high electronic conductivity and editable surface chemistry. Besides, rendering MXenes with spectrum defense properties further broadens their versatile applications. However, the development of MXenes suffers from weak van der Waal interaction-driven self-restacking that leads to random alignment and inferior interface microenvironments. Herein, a nacre-inspired MXene film is tailored by dual-filling of 2-ureido-4[1H]-pyrimidinone (UPy)-modified polyvinyl alcohol (PVA-UPy) and carbon nanotubes (CNTs). The dual-nanofillers engineering endows the nanocomposite film with a highly ordered structure (a Herman's order value of 0.838), a high mechanical strength (139.5 MPa), and continuous conductive pathways of both the ab plane and c-axis. As a proof-of-concept, the tailored nanocomposite film achieves a considerable capacitance of 508.2 F cm-3 and long-term cycling stability without performance degradation for 10 000 cycles. It is efficient for spectra defense in radar and infrared bands, displaying a high electromagnetic shielding capacity (19186 dB cm2 g-1) and a super-low infrared (IR) emissivity (0.16), with negligible performance decay after saving in the air for 1 year, responsible for the applications in specific and complex conditions. This interfacial dual-filler engineering concept showcases effective nanotechnology toward sustainable energy applications with a long lifetime and safety.

Nacre-Inspired Hybrid Multilayer Insulation Composites

Superinsulation aerogels are characterized by low tensile strength and brittleness due to their high porosity. To address these limitations, multiscale architectural design inspired by nacre can be employed. This materials design approach offers a promising strategy for enhancing the mechanical strength of aerogel thermal insulation. In this study, we present nacre-inspired multilayer cellulose-silica aerogel configurations. The cellulose "brick" network imparts structural strength to effectively redistribute energy, while the nanoporous "mortar" silica blocks heat transfer, maintaining insulation and fire retardance. The multilayer composites, with a layering configuration of five cellulose layers with four silica layers (5 + 4) and a cellulose layer thickness of 1.42 mm, exhibit a thermal conductivity of 31.3 mW/(m·K), a flexural modulus of 505 MPa, and an impact strength of 7.33 kJ/m2. The hydrophobic composite shows a water contact angle of 127°, enhanced soundproofing with a 27% noise reduction, and a carbon footprint of 0.49 kgCO2eq/kg. The multilayer cellulose-silica aerogel design provides a robust, eco-friendly thermal insulation solution for green building applications.

Formation mechanism of a novel core-shell with tetradecyl dimethyl benzyl ammonium-modified montmorillonite interlayer nanofibrous membrane and its antimicrobial properties

A novel core-shell with a tetradecyl dimethyl benzyl ammonium chloride-modified montmorillonite (TDMBA/MMT) interlayer silk fibroin (SF)/poly(lactic acid) (PLLA) nanofibrous membrane was fabricated using a simple conventional electrospinning method. Scanning electron microscopy and pore size analyses revealed that this core-shell with TDMBA/MMT interlayer maintained its nanofibrous morphology and larger pore structure more successfully than SF/PLLA nanofibrous membranes after treatment with 75% ethanol vapor. Transmission electron microscopy and energy-dispersive X-ray spectroscopy analyses testified that the SF/PLLA-TDMBA/MMT nanofibers exhibited a core-shell with an interlayer structure, with SF/PLLA in the core-shell layer and TDMBA/MMT in the interlayer. The formation of a core-shell with interlayer nanofibers was primarily attributed to the uniform dispersion of TDMBA/MMT nanosheets in a solution owing to its exfoliation using hexafluoroisopropanol and then preparing a stable spinning solution similar to an emulsion. Compared to SF/PLLA nanofibrous membranes, the core-shell structure with TDMBA/MMT interlayers of SF/PLLA nanofibrous membranes exhibited enhanced hydrophilicity, thermal stability, mechanical properties as well as improved and long-lasting antimicrobial performance against Escherichia coli and Staphylococcus aureus without cytotoxicity.

Nacre-like block lattice metamaterials with targeted phononic band gap and mechanical properties

The extraordinary quasi-static mechanical properties of nacre-like composite metamaterials, such as high specific strength, stiffness, and toughness, are due to the periodic arrangement of two distinct phases in a "brick and mortar" structure. It is also theorized that the hierarchical periodic structure of nacre structures can provide wider band gaps at different frequency scales. However, the function of hierarchy in the dynamic behavior of metamaterials is largely unknown, and most current investigations are focused on a single objective and specialized applications. Nature, on the other hand, appears to develop systems that represent a trade-off between multiple objectives, such as stiffness, fatigue resistance, and wave attenuation. Given the wide range of design options available to these systems, a multidisciplinary strategy combining diverse objectives may be a useful opportunity provided by bioinspired artificial systems. This paper describes a class of hierarchically-architected block lattice metamaterials with simultaneous wave filtering and enhanced mechanical properties, using deep learning based on artificial neural networks (ANN), to overcome the shortcomings of traditional design methods for forward prediction, parameter design, and topology design of block lattice metamaterial. Our approach uses ANN to efficiently describe the complicated interactions between nacre geometry and its attributes, and then use the Bayesian optimization technique to determine the optimal geometry constants that match the given fitness requirements. We numerically demonstrate that complete band gaps, that is attributed to the coupling effects of local resonances and Bragg scattering, exist. The coupling effects are naturally influenced by the topological arrangements of the continuous structures and the mechanical characteristics of the component phases. We also demonstrate how we can tune the frequency of the complete band gap by modifying the geometrical configurations and volume fraction distribution of the metamaterials. This research contributes to the development of mechanically robust block lattice metamaterials and lenses capable of controlling acoustic and elastic waves in hostile settings.

Guidelines derived from biomineralized tissues for design and construction of high-performance biomimetic materials: from weak to strong

Living organisms in nature have undergone continuous evolution over billions of years, resulting in the formation of high-performance fracture-resistant biomineralized tissues such as bones and teeth to fulfill mechanical and biological functions, despite the fact that most inorganic biominerals that constitute biomineralized tissues are weak and brittle. During the long-period evolution process, nature has evolved a number of highly effective and smart strategies to design chemical compositions and structures of biomineralized tissues to enable superior properties and to adapt to surrounding environments. Most biomineralized tissues have hierarchically ordered structures consisting of very small building blocks on the nanometer scale (nanoparticles, nanofibers or nanoflakes) to reduce the inherent weaknesses and brittleness of corresponding inorganic biominerals, to prevent crack initiation and propagation, and to allow high defect tolerance. The bioinspired principles derived from biomineralized tissues are indispensable for designing and constructing high-performance biomimetic materials. In recent years, a large number of high-performance biomimetic materials have been prepared based on these bioinspired principles with a large volume of literature covering this topic. Therefore, a timely and comprehensive review on this hot topic is highly important and contributes to the future development of this rapidly evolving research field. This review article aims to be comprehensive, authoritative, and critical with wide general interest to the science community, summarizing recent advances in revealing the formation processes, composition, and structures of biomineralized tissues, providing in-depth insights into guidelines derived from biomineralized tissues for the design and construction of high-performance biomimetic materials, and discussing recent progress, current research trends, key problems, future main research directions and challenges, and future perspectives in this exciting and rapidly evolving research field.

3D printable strong and tough composite organo-hydrogels inspired by natural hierarchical composite design principles

Fabrication of composite hydrogels can effectively enhance the mechanical and functional properties of conventional hydrogels. While ceramic reinforcement is common in many hard biological tissues, ceramic-reinforced hydrogels lack a similar natural prototype for bioinspiration. This raises a key question: How can we still attain bioinspired mechanical mechanisms in composite hydrogels without mimicking a specific composition and structure? Abstracting the hierarchical composite design principles of natural materials, this study proposes a hierarchical fabrication strategy for ceramic-reinforced organo-hydrogels, featuring (1) aligned ceramic platelets through direct-ink-write printing, (2) poly(vinyl alcohol) organo-hydrogel matrix reinforced by solution substitution, and (3) silane-treated platelet-matrix interfaces. Unit filaments are further printed into a selection of bioinspired macro-architectures, leading to high stiffness, strength, and toughness (fracture energy up to 31.1 kJ/m2), achieved through synergistic multi-scale energy dissipation. The materials also exhibit wide operation tolerance and electrical conductivity for flexible electronics in mechanically demanding conditions. Hence, this study demonstrates a model strategy that extends the fundamental design principles of natural materials to fabricate composite hydrogels with synergistic mechanical and functional enhancement.

Design and synthesis of bioinspired nanomaterials for biomedical application

Natural materials and bioprocesses provide abundant inspirations for the design and synthesis of high-performance nanomaterials. In the past several decades, bioinspired nanomaterials have shown great potential in the application of biomedical fields, such as tissue engineering, drug delivery, and cancer therapy, and so on. In this review, three types of bioinspired strategies for biomedical nanomaterials, that is, inspired by the natural structures, biomolecules, and bioprocesses, are mainly introduced. We summarize and discuss the design concepts and synthesis approaches of various bioinspired nanomaterials along with their specific roles in biomedical applications. Additionally, we discuss the challenges for the development of bioinspired biomedical nanomaterials, such as mechanical failure in wet environment, limitation in scale-up fabrication, and lack of deep understanding of biological properties. It is expected that the development and clinical translation of bioinspired biomedical nanomaterials will be further promoted under the cooperation of interdisciplinary subjects in future. This article is categorized under: Implantable Materials and Surgical Technologies > Nanomaterials and Implants Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Multifunctional nacre-like materials

Nacre, the iridescent inner layer of seashells, displays an exceptional combination of strength and toughness due to its 'brick-wall' architecture. Significant research has been devoted to replicating nacre's architecture and its associated deformation and failure mechanisms. Using the resulting materials in applications necessitates adding functionalities such as self-healing, force sensing, bioactivity, heat conductivity and resistance, transparency, and electromagnetic interference shielding. Herein, progress in the fabrication, mechanics, and multi-functionality of nacre-like materials, particularly over the past three years is systematically and critically reviewed. The fabrication techniques reviewed include 3D printing, freeze-casting, mixing/coating-assembling, and laser engraving. The mechanical properties of the resulting materials are discussed in comparison with their constituents and previously developed nacre mimics. Subsequently, the progress in incorporating multifunctionalities and the resulting physical, chemical, and biological properties are evaluated. We finally provide suggestions based on 3D/4D printing, advanced modelling techniques, and machine elements to make reprogrammable nacre-like components with complex shapes and small building blocks, tackling some of the main challenges in the science and translation of these materials.

Ultrastrong Ionotronic Films Showing Electrochemical Osmotic Actuation

A multifunctional soft material with high ionic and electrical conductivity, combined with high mechanical properties and the ability to change shape can enable bioinspired responsive devices and systems. The incorporation of all these characteristics in a single material is very challenging, as the improvement of one property tends to reduce other properties. Here, a nanocomposite film based on charged, high-aspect-ratio 1D flexible nanocellulose fibrils, and 2D Ti3 C2 Tx MXene is presented. The self-assembly process results in a stratified structure with the nanoparticles aligned in-plane, providing high ionotronic conductivity and mechanical strength, as well as large water uptake. In hydrogel form with 20 wt% liquid, the electrical conductivity is over 200 S cm-1 and the in-plane tensile strength is close to 100 MPa. This multifunctional performance results from the uniquely layered composite structure at nano- and mesoscales. A new type of electrical soft actuator is assembled where voltage as low as ±1 V resulted in osmotic effects and giant reversible out-of-plane swelling, reaching 85% strain.

A Strong, Tough and Fire-Retardant Biomimetic Multifunctional Wooden Laminate

Mildly delignified wood showed a well-preserved wood cell wall framework, and its derived compressed materials demonstrate excellent mechanical properties and advanced functional material potential. Here, we proposed a simple yet effective approach for making strong, tough, and fire-retardant wooden laminate by a three-step process of mild delignification, infiltrating potassium nonafluoro-1-butanesulfonate (PFBS), and hot-pressing to densify the material. PFBS can be infiltrated into the micro/nano-structures of the mildly delignified wood to achieve a good flame-resistant protective barrier. Flame retardant tests showed that this strong, tough, and fire-retardant wooden laminate has a superior flame-retardant performance to natural wood. Additionally, the wooden laminate also exhibits a simultaneously enhanced tensile strength (175.6 MPa vs. 89.9 MPa for natural wood) and toughness (22.9 MJ m-3vs. 10.9 MJ m-3 for natural wood). Given these attributes, the resulting wooden laminates are identified as promising candidates for high-performance structural applications, fulfilling stringent requirements for both mechanical resilience and flame-retardant efficacy.

Multi-objective Bayesian optimization for the design of nacre-inspired composites: optimizing and understanding biomimetics through AI

The hierarchical structures found in biological materials lead to an outstanding balance of multiple material properties, and numerous research studies have been initiated to emulate the key concepts for the designing of engineering materials, the so-called bioinspired composites. However, the optimization of bioinspired composites has long been difficult as it usually falls into the category of 'black-box problem', the objective functions not being available in a functional form. Also, bioinspired composites possess multiple material properties that are in a trade-off relationship, making it impossible to reach a unique optimal design solution. As a breakthrough, we propose a data-driven material design framework which can generate bioinspired composite designs with an optimal balance of material properties. In this study, a nacre-inspired composite is chosen as the subject of study and the optimization framework is applied to determine the designs that have an optimal balance of strength, toughness, and specific volume. Gaussian process regression was adopted for the modeling of a complex input-output relationship, and the model was trained with the data generated from the crack phase-field simulation. Then, multi-objective Bayesian optimization was carried out to determine pareto-optimal composite designs. As a result, the proposed data-driven algorithm could generate a 3D pareto surface of optimal composite design solutions, from which a user can choose a design that suits his/her requirement. To validate the result, several pareto-optimal designs are built using a PolyJet 3D printer, and their tensile test results show that each of the characteristic designs is well optimized for its specific target objective.

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.

Role of Inorganic Amorphous Constituents in Highly Mineralized Biomaterials and Their Imitations

A highly mineralized biomaterial is one kind of biomaterial that usually possesses a high content of crystal minerals and hierarchical microstructure, exhibiting excellent mechanical properties to support the living body. Recent studies have revealed the presence of inorganic amorphous constituents (IAC) either during the biomineralization process or in some mature bodies, which heavily affects the formation and performance of highly mineralized biomaterials. These results are surprising given the preceding intensive research into the microstructure design of these materials. Herein, we highlight the role of IAC in highly mineralized biomaterials. We focused on summarizing works demonstrating the presence or phase transformation of IAC and discussed in detail how IAC affects the formation and performance of highly mineralized biomaterials. Furthermore, we described some imitations of highly mineralized biomaterials that use IAC as the synthetic precursor or final strengthening phase. Finally, we briefly summarized the role of IAC in biomaterials and provided an outlook on the challenges and opportunities for future IAC and IAC-containing bioinspired materials researches.

Bioinspired Laminated Bioceramics with High Toughness for Bone Tissue Engineering

For the research of biomaterials in bone tissue engineering, it is still a challenge to fabricate bioceramics that overcome brittleness while maintaining the great biological performance. Here, inspired by the toughness of natural materials with hierarchical laminated structure, we presented a directional assembly-sintering approach to fabricate laminated MXene/calcium silicate-based (L-M/CS) bioceramics. Benefiting from the orderly laminated structure, the L-M/CS bioceramics exhibited significantly enhanced toughness (2.23 MPa·m^1/2) and high flexural strength (145 MPa), which were close to the mechanical properties of cortical bone. Furthermore, the L-M/CS bioceramics possessed more suitable degradability than traditional CaSiO₃ bioceramics due to the newly formed CaTiSiO₅ after sintering. Moreover, the L-M/CS bioceramics showed good biocompatibility and could stimulate the expression of osteogenesis-related genes. The mechanism of promoting osteogenic differentiation had been shown to be related to the Wnt signaling pathway. This work not only fabricated calcium silicate-based bioceramics with excellent mechanical and biological properties for bone tissue engineering but also provided a strategy for the combination of bionics and bioceramics.

Myriophyllum spicatum Leaves: Aerophily for Gas Collection and Transportation in Water

The unique living environment of aquatic plants makes them produce many fantastic properties different from land ones. For instance, the leaves of Myriophyllum spicatum show excellent hydrophobicity and aerophily characteristics. In this paper, the abundant morphological structure, composition, and aerophily properties of Myriophyllum spicatum leaves are revealed. The contact angle of the leaf surface can reach 122° in air, exhibiting wonderful gas collection ability under water. The results showed that the aerophily of the leaves is attributed to the multistage micro-nanostructure and waxy layer on the surface. The gas transportation toward the tips of leaves is based on the void gradient formed by the nanoscale morphology at different growth stages and the buoyancy as well. These features provide bionic experience for gas collection, bubble transportation, and liquid resistance reduction in water environments.

Bioinspired High-Strength Montmorillonite-Alginate Hybrid Film: The Effect of Different Divalent Metal Cation Crosslinking

The natural nacre has a regular ordered layered structure of calcium carbonate tablets and ion crosslinking proteins stacked alternately, showing outstanding mechanical properties. Inspired by nacre, we fabricated different divalent metal cation-crosslinked montmorillonite-alginate hybrid films (MMT-ALG-X2+; X2+ = Cu2+, Cd2+, Ba2+, Ca2+, Ni2+, Co2+ or Mn2+). The effect of ionic crosslinking strength and hydrogen bond interaction on the mechanical properties of the nacre-mimetics was studied. With the cations affinities with ALG being increased (Mn2+ < Co2+ = Ni2+ < Ca2+ < Ba2+ < Cd2+ < Cu2+), the tensile strength of nacre-mimetics showed two opposite influence trends: Weak ionic crosslinking (Mn2+, Co2+, Ni2+ and Ca2+) can synergize with hydrogen bonds to greatly increase the tensile properties of the sample; Strong ionic crosslinking (Ba2+, Cd2+, Cu2+) and hydrogen bonding form a competitive relationship, resulting in a rapid decrease in mechanical properties. Mn2+ crosslinking generates optimal strength of 288.0 ± 15.2 MPa with an ultimate strain of 5.35 ± 0.6%, obviously superior to natural nacre (135 MPa and 2%). These excellent mechanical properties arise from the optimum synergy of ion crosslinking and interfacial hydrogen bonds between crosslinked ALG and MMT nanosheets. In addition, these metal ion-crosslinked composite films show different colors, high visible transparency, and excellent UV shielding properties.

Pearl-inspired graphene oxide-collagen microgel with multi-layer mineralization through microarray chips for bone defect repair

Biomineralization of natural polymers in simulated body fluid (SBF) can significantly improve its biocompatibility, osteoconductivity, and osteoinductivity because of the hydroxyapatite (HAp) deposition. Nevertheless, the superficial HAp crystal deposition hamper the deep inorganic ions exchange in porous microgels, thus gradually leading to a nonuniform regeneration effect. Inspired by the pearl forming process, this article uses the microarray chips to fabricate the multi-layer mineralized graphene oxide (GO)-collagen (Col)-hydroxyapatite (HAp) microgel, denoted as MMGCH. These fabricated MMGCH microgels exhibit porous structure and uniform HAp distribution. Furthermore, the suitable microenvironment offered by microgel promotes the time-dependent proliferation and osteogenic differentiation of stem cells, which resulted in upregulated osteogenesis-related genes and proteins, such as alkaline phosphatase, osteocalcin, and collagen-1. Finally, the MMGCH microgels possess favorable bone regeneration capacities both in cranial bone defects and mandibular bone defects via providing a suitable microenvironment for host-derived cells to form new bone tissues. This work presents a biomimetic means aiming to achieve full-thickness and uniform HAp deposition in hydrogel for bone defect repair.

A Bioinspired Ultratough Composite Produced by Integration of Inorganic Ionic Oligomers within Polymer Networks

The nacre-inspired laminates are promising materials for their excellent mechanics. However, the interfacial defects between organic-inorganic phases commonly lead to the crack propagation and fracture failure of these materials under stress. A natural biomineral, bone, has much higher bending toughness than the nacre. The small size of inorganic building units in bone improves the organic-inorganic interaction, which optimizes the material toughness. Inspired by these biological structures, here, an ultratough nanocomposite laminate is prepared by the integration of ultrasmall calcium phosphate oligomers (CPO, 1 nm in diameter) within poly(vinyl alcohol) (PVA) and sodium alginate (Alg) networks through a simple three-step strategy. Owing to the small size of inorganic building units, strong multiple molecular interactions within integrated organic-inorganic hierarchical structure are built. The resulting laminates exhibit ultrahigh bending strain (>50% without fracture) and toughness (21.5-31.0 MJ m^-3), which surpass natural nacre and almost all of the synthetic laminate materials that have been reported so far. Moreover, the mechanics of this laminate is tunable by changing the water content within the bulk structure. This work provides a way for the development of organic-inorganic nanocomposites with ultrahigh bending toughness by using inorganic ionic oligomers, which can be useful in the fields of tough protective materials and energy absorbing materials.

Bioinspired Techniques in Freeze Casting: A Survey of Processes, Current Advances, and Future Directions

Freeze casting, popularly known as ice templating or freeze gelation, is a mechanical method to fabricate scaffolds of desirable properties and materials. Aerospace engineering, the healthcare sector, manufacturing department, and automotive industries are the different fields where freeze casting has been used. Bioinspiration refers to the translation of biological systems into new and innovative creations. Bioinspired materials are extensively used in freeze casting methods such as ceramide, spines of porcupine fish, and collagen. Due to the tunable properties and production of complex structures with ease, biomaterials have found numerous applications in the ice templating method. This review rigorously explains the freeze casting process and the effect of thermal conductivity, stress, and electrostatic repulsion on the porous materials. Also, we have discussed the different biomaterial polymers used in freeze casting along with different methods involved.

Beyond homogeneous dispersion: oriented conductive fillers for high κ nanocomposites

Rational design of structures for regulating the thermal conductivities (κ) of materials is critical to many components and products employed in electrical, electronic, energy, construction, aerospace, and medical applications. As such, considerable efforts have been devoted to developing polymer composites with tailored conducting filler architectures and thermal conduits for highly improved κ. This paper is dedicated to overviewing recent advances in this area to offer perspectives for the next level of future development. The limitations of conventional particulate-filled composites and the issue of percolation are discussed. In view of different directions of heat dissipation in polymer composites for different end applications, various approaches for designing the micro- and macroscopic structures of thermally conductive networks in the polymer matrix are highlighted. Methodological approaches devised to significantly ameliorate thermal conduction are categorized with respect to the pathways of heat dissipation. Future prospects for the development of thermally conductive polymer composites with modulated thermal conduction pathways are highlighted.

Nacre-Inspired Strong and Multifunctional Soy Protein-Based Nanocomposite Materials for Easy Heat-Dissipative Mobile Phone Shell

Inspired by the hierarchically ordered "brick and mortar" (BM) architecture of natural nacre, in this study a rational assembly of boron nitride (BN) nanosheets was introduced into a mixture of trimethylolpropane triglycidyl ether (TTE) and soy protein isolate (SPI), and a strong and multifunctional SPI-based nanocomposite film with multinetwork structure was synthesized. At a low BN loading (<0.5%), the resulting multifunctional film was flexible, antiultraviolet, and nearly transparent and also displayed good thermal diffusion ability and exhibited an excellent combination of high tensile strength (36.4 MPa) and thermal conductivity (TC, 2.40 W·m^-1·K^-1), surpassing the performances of various types of petroleum-based plastics (displayed a tensile strength ranging from 1.9 to 21 MPa and TC ranging from 0.55-2.13 W·m^-1·K^-1), including nine different types of materials currently utilized for mobile phone shells, suggesting its vast potential in practical applications.

Nacreous aramid-mica bulk materials with excellent mechanical properties and environmental stability

Low density, high strength and toughness, together with good environmental stability are always desirable but hardly to achieve simultaneously for man-made structural materials. Replicating the design motifs of natural nacre clearly provides one promising route to obtain such kind of materials, but fundamental challenges remain. Herein, by choosing aramid nanofibers and mica microplatelets as building blocks, we produce a nacreous aramid-mica bulk material with a favorable combination of low density (∼1.7 g cm-3), high strength (∼387 MPa) and toughness (∼14.3 MPa m1/2), and impressive mechanical stability in some harsh environments, including acid/alkali solutions, strong ultraviolet radiation, boiling water, and liquid nitrogen, standing out from previously reported biomimetic bulk composites. Moreover, the obtained material outperforms other bulk nacre-mimetics and most engineering structural materials in terms of its specific strength (227 MPa/[Mg m-3]) and specific toughness (8.4 MPa m1/2/[Mg m-3]), making it a new promising engineering structural material for different technical fields.

Mechanically Robust and Broadband Blackbody Composite Films Based on Self-Assembled Layered Structures

Inspired by nacre that is mechanically strong and versatile in light manipulation, large-scale black films with a nacre-like microstructure and carbon nanotube inclusion were prepared using a facile self-assembly technique. A layered structure promoting blackness and toughness simultaneously was realized, affording robust films with a solar-absorptivity as high as 96.9%. Our design strategy and fabrication process will be beneficial for the facile access to various advanced blackbody coatings.

Mechanical Properties of Nacre-Like Composites: A Bottom-Up Approach

Nacre is a highly organized hierarchical structure of the mineral and organic components at all scales down to the molecular-scale guided by organic molecules. The mechanical properties of the mineral component of nacre have been studied and well established for decades. In the present work, the shear modulus of the organic matrix of nacre was obtained using two of its important proteineous components, Perlucin and Lustrin A. The shear modulus value of the organic matrix was computed to be in the range of 1.25–1.45 GPa using atomistic molecular dynamics (MD) simulations. Moreover, finite element (FE) simulations were conducted on the three-dimensional (3D) models of the nacre-like composite while varying the relative composition of mineral and organic constituents. The nacre-like composite models with 10–20% by volume of organic part estimated high toughness. The exact optimum value will depend on the mechanical properties of the organic matrix used in the synthesis of nacre-like material. The study is an advancement in the modeling of nacre, sheds light on macroscale properties of nacre-like composites, and opens up new avenues for continuum studies of nacre mechanics, including its mysterious toughening mechanism.

Constructing Bone-Mimicking High-Performance Structured Poly(lactic acid) by an Elongational Flow Field and Facile Annealing Process

Poly(lactic acid) (PLA) as one of the most promising biodegradable polymers is being tremendously restricted in large-scale applications by the notorious toughness, ductility, and heat distortion resistance. Manufacturing PLA with excellent toughness, considerable ductility, balanced strength, and great heat distortion resistance simultaneously is still a great challenge. Natural structural materials usually possess excellent strength and toughness by elaborately constructed sophisticated hierarchical architectures, however, completely reproducing natural structural materials' architecture have evidenced to be difficult. Inspired by the hierarchical construction of the compact bone, an innovational method with an intensive and continuous elongational flow field and facile annealing process was developed to create bone-mimicking structured PLA at an industrial scale. The bone-mimicking structured PLA with unique and novel hierarchical architectures of interlocked 3D network lamellae and large extended-chain lamellae connecting the regular lamellae was constructed by in situ formed oriented thermoplastic poly(ether)urethane nanofibers (TNFs) acting as "collagen fibers", orderly staggered PLA lamellae behaving as "hydroxyapatite (HA) nanocrystals", and the tenacious interface functioning as a "soft protein" adhesive layer. Attributed to the unique structure, it possesses super toughness (90.3 KJ/m²), high stiffness (2.15 GPa), balanced strength (52.6 MPa), and notable heat distortion resistance (holding at 163 °C for 1 h) simultaneously. These excellent performances of the structured PLA provide it with immense potential applications in both structural and bio-engineering materials fields such as artificial bones and tissue scaffolds.

Nature-Inspired Nacre-Like Composites Combining Human Tooth-Matching Elasticity and Hardness with Exceptional Damage Tolerance

Making replacements for the human body similar to natural tissue offers significant advantages but remains a key challenge. This is pertinent for synthetic dental materials, which rarely reproduce the actual properties of human teeth and generally demonstrate relatively poor damage tolerance. Here new bioinspired ceramic-polymer composites with nacre-mimetic lamellar and brick-and-mortar architectures are reported, which resemble, respectively, human dentin and enamel in hardness, stiffness, and strength and exhibit exceptional fracture toughness. These composites are additionally distinguished by outstanding machinability, energy-dissipating capability under cyclic loading, and diminished abrasion to antagonist teeth. The underlying design principles and toughening mechanisms of these materials are elucidated in terms of their distinct architectures. It is demonstrated that these composites are promising candidates for dental applications, such as new-generation tooth replacements. Finally, it is believed that this notion of bioinspired design of new materials with unprecedented biologically comparable properties can be extended to a wide range of material systems for improved mechanical performance.

Water-Evaporation-Powered Fast Actuators with Multimodal Motion Based on Robust Nacre-Mimetic Composite Film

Water evaporation as a source of energy to trigger moisture-responsive soft materials is an emerging field in a variety of energy-harvesting devices, which has attracted widespread attention. Here, we design and fabricate bioinspired nacrelike composite film actuators consisting of graphene oxide and sodium alginate, which demonstrate an obvious shrinkage in volume when their state transfers from wet to dry and the contractile stress is up to 42.3 MPa. Based on these features, the film actuators can show rapid and continuous movements under the water gradient. The flipping frequency of the actuators can reach up to 76 rounds min^-1, which is much faster than those in previous reports. The film can flap back and forth quickly on water vapor even after loading a cargo that is 9 times its own weight. Moreover, high mobility with multimodal motion including blooming, stretching, folding, and twisting can also be achieved by modulating the shapes of films. Thus, film actuators may hold great potential in many fields, such as microrobots, artificial muscles, and sensors on grounds of their rapid response speed and adjustable motion models.

Bioinspired Design and Fabrication of Polymer Composite Films Consisting of a Strong and Stiff Organic Matrix and Microsized Inorganic Platelets

Intensive studies on nacre-inspired composites with exceptional mechanical properties based on an organic/inorganic hierarchical layered structure have been conducted; however, integrating high strength, stiffness, and toughness for engineering materials still remains a challenge. We herein report the design and fabrication of polymer composites through a hydrogel-film casting method that allow for building uniformly layered organic/inorganic microstructure. Alginate (Alg) was used for an organic matrix, whose mechanical properties were controlled by Ca²⁺ cross-linking toward the simultaneously strong, stiff, and tough resultant composite. Alumina (Alu) microplatelets were used for horizontally aligned inorganic phase, and their alignment and interactions with the organic matrix were improved by polyvinylpyrrolidone (PVP) coating on the platelet. The composite film exhibits well-balanced elastic and plastic deformation under tensile stress, leading to high stiffness and toughness, which have not been generally achieved in microplatelet-based composite films developed in previous studies. The synergistic effect of Ca²⁺ cross-linking and PVP-coated Alu platelets on the mechanical properties improved polymer-platelet interfacial interactions, and platelet alignment is clearly demonstrated through mechanical tests and Fourier transform infrared and X-ray diffraction analyses. We further demonstrate that the reinforcing effect of the Alu platelet and PVP-coated platelet on the mechanical properties is dependent on humidity. Such effects are maximized at highly dry conditions, which is consistent with the model estimation. Furthermore, a thick bulk composite was produced by laminating thin films and showed high mechanical properties under flexural stress. Our design and fabrication strategies combined with the understanding of their mechanism yield an alternative approach to produce engineered composite materials.

Bioinspired nacre-like alumina with a bulk-metallic glass-forming alloy as a compliant phase

Bioinspired ceramics with micron-scale ceramic "bricks" bonded by a metallic "mortar" are projected to result in higher strength and toughness ceramics, but their processing is challenging as metals do not typically wet ceramics. To resolve this issue, we made alumina structures using rapid pressureless infiltration of a zirconium-based bulk-metallic glass mortar that reactively wets the surface of freeze-cast alumina preforms. The mechanical properties of the resulting Al2O3 with a glass-forming compliant-phase change with infiltration temperature and ceramic content, leading to a trade-off between flexural strength (varying from 89 to 800 MPa) and fracture toughness (varying from 4 to more than 9 MPa·m½). The high toughness levels are attributed to brick pull-out and crack deflection along the ceramic/metal interfaces. Since these mechanisms are enabled by interfacial failure rather than failure within the metallic mortar, the potential for optimizing these bioinspired materials for damage tolerance has still not been fully realized.

Achieving Color and Function with Structure: Optical and Catalytic Support Properties of ZrO2 Inverse Opal Thin Films

Taking inspiration from natural photonic crystal architectures, we report herein the successful fabrication of zirconia inverse opal (ZrO₂ IO) thin-film photonic crystals possessing striking iridescence at visible wavelengths. Poly(methyl methacrylate) (PMMA) colloidal crystal thin films (synthetic opals) were first deposited on glass microscope slides, after which the interstitial voids in the films were filled with a Zr(IV) sol. Controlled calcination of the resulting composite films yielded iridescent ZrO₂ IO thin films with pseudo photonic band gaps (PBGs) along the surface normal at visible wavelengths. The PBG position was dependent on the macropore diameter (D) in the inverse opals (and thus proportional to the diameter of the PMMA colloids in the sacrificial templates), the incident angle of light with respect to the surface normal (θ), and also the refractive index of the medium filling the macropores, all of which were accurately described by a modified Bragg's law expression. Au/ZrO₂ IO catalysts prepared using the ZrO₂ IO films demonstrated outstanding performance for the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH₄, which can be attributed to the interconnected macroporosity in the films, which afforded a high Au nanoparticle dispersion and also facile diffusion of 4-nitrophenol to the catalytically active Au sites.

Facile and On-Demand Cross-Linking of Nacre-Mimetic Nanocomposites Using Tailor-Made Polymers with Latent Reactivity

The development of on-demand cross-linking strategies is a key aspect in promoting mechanical properties of high-performance bioinspired nanocomposites. Here, we embed styrene sulfonyl azide groups with latent chemical reactivity into water-soluble copolymers and assemble those with high-aspect-ratio synthetic nanoclays to generate well-defined layered polymer/nanoclay nacre-mimetics. A considerable stiffening and strengthening occurs upon activation of the covalent cross-linking using simple heating. Varying the amount of cross-linkable units allows molecular control of mechanical properties from ductile to stiff and strong. Moreover, the covalent cross-linking enhances the moisture stability of water-borne nacre-mimetics. The strategy is facile and versatile allowing for a transfer into applications.

A Bioinspired Interface Design for Improving the Strength and Electrical Conductivity of Graphene-Based Fibers

Graphene-based fibers (GBFs) are attractive for next-generation wearable electronics due to their potentially high mechanical strength, superior flexibility, and excellent electrical and thermal conductivity. Many efforts have been devoted to improving these properties of GBFs in the past few years. However, fabricating GBFs with high strength and electrical conductivity simultaneously remains as a great challenge. Herein, inspired by nacre-like multilevel structural design, an interface-reinforced method is developed to improve both the mechanical property and electrical conductivity of the GBFs by introducing polydopamine-derived N-doped carbon species as resistance enhancers, binding agents, and conductive connection "bridges." Remarkably, both the tensile strength and electrical conductivity of the obtained GBFs are significantly improved to ≈724 MPa and ≈6.6 × 10⁴ S m^-1 , respectively, demonstrating great superiority compared to previously reported similar GBFs. These outstanding integrated performances of the GBFs provide it with great application potential in the fields of flexible and wearable microdevices such as sensors, actuators, supercapacitors, and batteries.