Biological composites-complex structures for functional diversity

Ultrastable and High-Performance Silk Energy Harvesting Textiles

Energy harvesting textiles (EHTs) have attracted much attention in wearable electronics and the internet-of-things for real-time mechanical energy harvesting associated with human activities. However, to satisfy practical application requirements, especially the demand for long-term use, it is challenging to construct an energy harvesting textile with elegant trade-off between mechanical and triboelectric performance. In this study, an energy harvesting textile was constructed using natural silk inspired hierarchical structural designs combined with rational material screening; this design strategy provides multiscale opportunities to optimize the mechanical and triboelectric performance of the final textile system. The resulting EHTs with traditional advantages of textiles showed good mechanical properties (tensile strength of 237 ± 13 MPa and toughness of 4.5 ± 0.4 MJ m-3 for single yarns), high power output (3.5 mW m-2), and excellent structural stability (99% conductivity maintained after 2.3 million multi-type cyclic deformations without severe change in appearance), exhibiting broad application prospects in integrated intelligent clothing, energy harvesting, and human-interactive interfaces.

Understanding the multiscale self-assembly of metal-organic polyhedra towards functionally graded porous gels

Spatial heterogeneity and gradients within porous materials are key for controlling their mechanical properties and mass/energy transport, both in biological and synthetic materials. However, it is still challenging to induce such complexity in well-defined microporous materials such as crystalline metal-organic frameworks (MOFs). Here we show a method to generate a continuous gradient of porosity over multiple length scales by taking advantage of the amorphous nature of supramolecular polymers based on metal-organic polyhedra (MOPs). First, we use time-resolved dynamic light scattering (TRDLS) to elucidate the mechanism of hierarchical self-assembly of MOPs into colloidal gels and to understand the relationship between the MOP concentrations and the architecture of the resulting colloidal networks. These features directly impact the viscoelastic response of the gels and their mechanical strength. We then show that gradients of stiffness and porosity can be created within the gel by applying centrifugal force at the point of colloidal aggregation. These results with the creation of asymmetric and graded pore configuration in soft materials could lead to the emergence of advanced properties that are coupled to asymmetric molecule/ion transport as seen in biological systems.

Bio-Inspired Synthetic Ivory as a Sustainable Material for Piano Keys

Natural ivory is no longer readily or legally available, as it is obtained primarily from elephant tusks, which now enjoy international protection. Ivory, however, is the best material known for piano keys. We present a hydroxylapatite–gelatin biocomposite that is chemically identical to natural ivory but with functional properties optimized to replace it. As this biocomposite is fabricated from abundant materials in an environmentally friendly process and is furthermore biodegradable, it is a sustainable solution for piano keys with the ideal functional properties of natural ivory.

Crack engineering for the construction of arbitrary hierarchical architectures

Despite emerging breakthroughs in the achievement of numerous elegant biomimetic structures that impart fascinating functionalities, bioinspired materials still suffer from poor structural durability, chemical reliability, flexibility, and optical transparency, as well as unaffordable cost and low throughput, thus preventing their broad real-life applications. In striking contrast to conventional wisdom, we demonstrate that the usually avoided and detrimental elastic crack phenomenon can be translated into powerful configurable-crack engineering to achieve structures and functions that are impossible to realize even using state-of-the-art techniques. Our approach dramatically enriches the freedom and flexibility in the design of materials to mimic various natural living organisms and paves the road for translating nature’s inspirations into real-world applications.

Three-dimensional hierarchical morphologies widely exist in natural and biomimetic materials, which impart preferential functions including liquid and mass transport, energy conversion, and signal transmission for various applications. While notable progress has been made in the design and manufacturing of various hierarchical materials, the state-of-the-art approaches suffer from limited materials selection, high costs, as well as low processing throughput. Herein, by harnessing the configurable elastic crack engineering—controlled formation and configuration of cracks in elastic materials—an effect normally avoided in various industrial processes, we report the development of a facile and powerful technique that enables the faithful transfer of arbitrary hierarchical structures with broad material compatibility and structural and functional integrity. Our work paves the way for the cost-effective, large-scale production of a variety of flexible, inexpensive, and transparent 3D hierarchical and biomimetic materials.

Water: How Does It Influence the CaCO3 Formation?

Nature produces biomineral-based materials with a fascinating set of properties using only a limited number of elements. This set of properties is obtained by closely controlling the structure and local composition of the biominerals. We are far from achieving the same degree of control over the properties of synthetic biomineral-based composites. One reason for this inferior control is our incomplete understanding of the influence of the synthesis conditions and additives on the structure and composition of the forming biominerals. In this Review, we provide an overview of the current understanding of the influence of synthesis conditions and additives during different formation stages of CaCO₃ , one of the most abundant biominerals, on the structure, composition, and properties of the resulting CaCO₃ crystals. In addition, we summarize currently known means to tune these parameters. Throughout the Review, we put special emphasis on the role of water in mediating the formation of CaCO₃ and thereby influencing its structure and properties, an often overlooked aspect that is of high relevance.

Fiber with Butterfly Wings: Creating Colored Carbon Fibers with Increased Strength, Adhesion, and Reversible Malleability

Colored and color-changing materials are central to perception and interaction in nature and have been exploited in an array of modern technologies such as sensors, visual displays, and smart materials. Attempts to introduce color into carbon fiber materials have been limited by deleterious impacts on fiber properties, and the extension of colored fibers toward "smart composites" remains in its infancy. We present carbon fibers incorporating structural color, similar to that observed on the surface of soap bubbles and various insects and birds, by modifying the fiber surface through in situ polymerization grafting. When dry, the treated fibers exhibit a striking blue color, but when exposed to a volatile solvent, a cascade of colors across the visible light region is observed as the film first swells and then shrinks as the solvent evaporates. The treated fibers not only possess a unique color and color-changing ability but also can be reversibly formed into complex shapes and bear significant loads even without being encased in a supporting polymer. The tensile strength of treated fibers shows a statistically significant increase (+12%), and evaluation of the fiber-to-matrix adhesion of these polymers to an epoxy resin shows more than 300% improvement over control fibers. This approach creates a new platform for the multifaceted advance of smart composites.

Durability of the Exterior Transparent Coatings on Nano-Photostabilized English Oak Wood and Possibility of Its Prediction before Artificial Accelerated Weathering

Changes in surface material characteristics can significantly affect the adhesion and overall life of coatings on wood. In order to increase the durability of transparent exterior coatings, it is possible to use the surface modification of wood with UV-stabilizing substances. In this work, selected types of surface modifications using benzotriazoles, HALS, ZnO and TiO₂ nanoparticles, and their combinations were applied to oak wood (Quercus robur, L.). On such modified surfaces, the surface free energy, roughness, and contact wetting angle with three selected types of exterior transparent coatings were subsequently determined. An oil-based coating, waterborne acrylic thick layer coating, and thin-layer synthetic coating were tested and interaction with the aforementioned surface modifications was investigated after 6 weeks of accelerated artificial weathering. The results of changes in the initially measured surface characteristics of the modified oak wood were compared to the real results of degradation of coatings after artificial accelerated weathering. The positive effect of surface modification, in particular the mixture of benzotriazoles, HALS, and ZnO nanoparticles on all kinds of coatings was proven, and the best results were observed in thick-film waterborne acrylic coating. The changes in the measured surface characteristics corresponded to the observed durability of the coatings only when measured by wetting using drops of the tested coatings.

Evaluating the hierarchical, hygroscopic deformation of the Daucus carota umbel through structural characterization and mechanical analysis

Many physically immobile plants develop passive yet ingenious strategies for active seed dispersal through self-deformation in response to external stimuli, such as humidity. These hygroscopic deformations are usually driven by the internal heterogeneous architecture, which provides valuable, inspiring information for the development of novel actuating systems. The Daucus carota compound umbel is an interesting structure showing a distinct hygroscopic deformation that operates at hierarchical levels among these plants. Here, we investigate the structure of the primary and secondary rays of the umbel associated with their deformation through mechanical analyses. We reveal that through controlling both the cellulose microfibril angle (MFA) and lignification, the multi-level bending behavior of the umbel is achieved, which contributes to efficient seed protection and dispersal. The primary rays generally show more significant bending curvature changes than the secondary rays, and within each level, the outer rays exhibit a larger motion amplitude than the middle and inner rays. Mechanical testing and theoretical analysis support that adjusting the lignin content within the ray structure compensates for the effect of the small differences in cellulose MFA on its bending behavior, which contributes to the overall hygroscopic deformation. Findings also show that the primary outer ray can generate reaction forces that are more than 700 times its weight, which is higher than that for the pine cone scales. The new insights from this work are instructive for bioinspired designs of complex, self-deforming structures and devices. STATEMENT OF SIGNIFICANCE: The carrot (Daucus carota) compound umbels exhibit a unique hierarchical, hygroscopic deformation for seed dispersal among immobile plants. In this work, we elucidate that the multi-level bending behavior of the umbel is achieved through manipulating the cellulose microfibril angle (MFA) and lignification of the primary and secondary rays for the first time. We also discover that adjusting the degree of lignification compensates for the effect of small cellulose MFA differences on the bending behavior theoretically and experimentally. The primary outer rays deform in a highly efficient manner, in which reactions forces about more than 700 times its weight are generated. The findings presented are instructive for bioinspired designs of complex, self-deforming structures and devices.

Plant-Derived Nanocellulose as Structural and Mechanical Reinforcement of Freeze-Cast Chitosan Scaffolds for Biomedical Applications

Despite considerable recent interest in micro- and nanofibrillated cellulose as constituents of lightweight structures and scaffolds for applications that range from thermal insulation to filtration, few systematic studies have been reported to date on structure-property-processing correlations in freeze-cast chitosan-nanocellulose composite scaffolds, in general, and their application in tissue regeneration, in particular. Reported in this study are the effects of the addition of plant-derived nanocellulose fibrils (CNF), crystals (CNCs), or a blend of the two (CNB) to the biopolymer chitosan on the structure and properties of the resulting composites. Chitosan-nanocellulose composite scaffolds were freeze-cast at 10 and 1 °C/min, and their microstructures were quantified in both the dry and fully hydrated states using scanning electron and confocal microscopy, respectively. The modulus, yield strength, and toughness (work to 60% strain) were determined in compression parallel and the modulus also perpendicular to the freezing direction to quantify anisotropy. Observed were the preferential alignments of CNCs and/or fibrils parallel to the freezing direction. Additionally, observed was the self-assembly of the nanocellulose into microstruts and microbridges between adjacent cell walls (lamellae), features that affected the mechanical properties of the scaffolds. When freeze-cast at 1 °C/min, chitosan-CNF scaffolds had the highest modulus, yield strength, toughness, and smallest anisotropy ratio, followed by chitosan and the composites made with the nanocellulose blend, and that with crystalline cellulose. These results illustrate that the nanocellulose additions homogenize the mechanical properties of the scaffold through cell-wall material self-assembly, on the one hand, and add architectural features such as bridges and pillars, on the other. The latter transfer loads and enable the scaffolds to resist deformation also perpendicular to the freezing direction. The observed property profile and the materials' proven biocompatibility highlight the promise of chitosan-nanocellulose composites for a large range of applications, including those for biomedical implants and devices.

Analysis of hygroscopic self-shaping wood at large scale for curved mass timber structures

The growing timber manufacturing industry faces challenges due to increasing geometric complexity of architectural designs. Complex and structurally efficient curved geometries are nowadays easily designed but still involve intensive manufacturing and excessive machining. We propose an efficient form-giving mechanism for large-scale curved mass timber by using bilayered wood structures capable of self-shaping by moisture content changes. The challenge lies in the requirement of profound material knowledge for analysis and prediction of the deformation in function of setup and boundary conditions. Using time- and moisture-dependent mechanical simulations, we demonstrate the contributions of different wood-specific deformation mechanisms on the self-shaping of large-scale elements. Our results outline how to address problems such as shape prediction, sharp moisture gradients, and natural variability in material parameters in light of an efficient industrial manufacturing.

Maximally stiffening composites require maximally coupled rather than maximally entangled polymer species

Polymer composites are ideal candidates for next generation biomimetic soft materials because of their exquisite bottom-up designability. However, the richness of behaviours comes at a price: the need for precise and extensive characterisation of material properties over a highly-dimensional parameter space, as well as a quantitative understanding of the physical principles underlying desirable features. Here we couple large-scale Molecular Dynamics simulations with optical tweezers microrheology to characterise the viscoelastic response of DNA-actin composites. We discover that the previously observed non-monotonic stress-stiffening of these composites is robust, yet tunable, in a broad range of the parameter space that spans two orders of magnitude in DNA length. Importantly, we discover that the most pronounced stiffening is achieved when the species are maximally coupled, i.e., have similar number of entanglements, and not when the number of entanglements per DNA chain is largest. We further report novel dynamical oscillations of the microstructure of the composites, alternating between mixed and bundled phases, opening the door to future investigations. The generic nature of our system renders our results applicable to the behaviour of a broad class of polymer composites.

Aerosol-based synthesis of pure and stable amorphous calcium carbonate

A facile aerosol-based method for the synthesis of pure and stable amorphous calcium carbonate (ACC) is presented. The method relies on the instantaneous carbonation of calcium hydroxide aerosols with carbon dioxide followed by rapid drying of the freshly formed ACC. The ACC display extended stability against humidity induced crystallization.

Repair of tooth enamel by a biomimetic mineralization frontier ensuring epitaxial growth

The regeneration of tooth enamel, the hardest biological tissue, remains a considerable challenge because its complicated and well-aligned apatite structure has not been duplicated artificially. We herein reveal that a rationally designed material composed of calcium phosphate ion clusters can be used to produce a precursor layer to induce the epitaxial crystal growth of enamel apatite, which mimics the biomineralization crystalline-amorphous frontier of hard tissue development in nature. After repair, the damaged enamel can be recovered completely because its hierarchical structure and mechanical properties are identical to those of natural enamel. The suggested phase transformation-based epitaxial growth follows a promising strategy for enamel regeneration and, more generally, for biomimetic reproduction of materials with complicated structure.

Design of Hierarchical Beads for Efficient Label-Free Cell Capture

Defined hierarchical materials promise cell analysis and call for application-driven design in practical use. The further issue is to develop advanced materials and devices for efficient label-free cell capture with minimum instrumentation. Herein, the design of hierarchical beads is reported for efficient label-free cell capture. Silica nanoparticles (size of ≈15 nm) are coated onto silica spheres (size of ≈200 nm) to achieve nanoscale surface roughness, and then the rough silica spheres are combined with microbeads (≈150-1000 µm in diameter) to assemble hierarchical structures. These hierarchical beads are built via electrostatic interaction, covalent bonding, and nanoparticle adherence. Further, after functionalization by hyaluronic acid (HA), the hierarchical beads display desirable surface hydrophilicity, biocompatibility, and chemical/structural stability. Due to the controlled surface topology and chemistry, HA-functionalized hierarchical beads afford high cell capture efficiency up to 98.7% in a facile label-free manner. This work guides the development of label-free cell capture techniques and contributes to the construction of smart interfaces in bio-systems.

Adaptation and Evolution of Biological Materials

Research into biological materials often centers on the impressive material properties produced in Nature. In the process, however, this research often neglects the ecologies of the materials, the organismal contexts relating to how a biological material is actually used. In biology, materials are vital to organismal interactions with their environment and their physiology, and also provide records of their phylogenetic relationships and the selective pressures that drive biological novelties. With the papers in this symposium, we provide a view on cutting-edge work in biological materials science. The collected research delivers new perspectives on fundamental materials concepts, offering surprising insights into biological innovations and challenging the boundaries of materials’ characterization techniques. The topics, systems, and disciplines covered offer a glimpse into the wide range of contemporary biological materials work. They also demonstrate the need for progressive “whole organism thinking” when characterizing biological materials, and the importance of framing biological materials research in relevant, biological contexts.

Using Wool Keratin as a Basic Resist Material to Fabricate Precise Protein Patterns

The ability to pattern natural polymers at different scales is extremely important for many research areas, such as cell culture, regenerative medicine, bioelectronics, tissue engineering, degradable implants, and photonics. For the first time, the use of wool keratin (WK) as a structural biomaterial for fabricating precise protein microarchitectures is presented. Through straightforward biochemical processes, modified WK proteins become intrinsically photoreactive without significant changes in protein structure or function. Under light irradiation, intermolecular chemical crosslinking between WK molecules can be successfully initiated by using commercially available photoinitiators. As a result, high-performance WK patterning on the micrometer scale (µm) can be achieved through a combination of water-based photolithography techniques. By simply mixing with nanoparticles, enzymes, and other dopants, various "functional WK resists" can be generated. In addition, without the addition of any cell-adhesive ligands, these patterned protein microstructures are demonstrated as bio-friendly cellular substrates for the spatial guidance of cells on their surface. Furthermore, periodic microfabricated WK structures in complex patterns that display typical iridescent behavior can be designed and formed over macroscale areas (cm).

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.

Bridging functional nanocomposites to robust macroscale devices

At the intersection of the outwardly disparate fields of nanoparticle science and three-dimensional printing lies the promise of revolutionary new “nanocomposite” materials. Emergent phenomena deriving from the nanoscale constituents pave the way for a new class of transformative materials with encoded functionality amplified by new couplings between electrical, optical, transport, and mechanical properties. We provide an overview of key scientific advances that empower the development of such materials: nanoparticle synthesis and assembly, multiscale assembly and patterning, and mechanical characterization to assess stability. The focus is on recent illustrations of approaches that bridge these fields, facilitate the design of ordered nanocomposites, and offer clear pathways to device integration. We conclude by highlighting the remaining scientific challenges, including the critical need for assembly-compatible particle–fluid systems that ultimately yield mechanically robust materials. The role of domain boundaries and/or defects emerges as an important open question to address, with recent advances in fabrication setting the stage for future work in this area.

Spatial and temporal tunability of magnetically-actuated gradient nanocomposites

Natural biological materials usually adopt functional gradient designs within interfacial regions to fulfil unusual mechanically-challenging demands. Manufacturing analogous gradients to alleviate premature failures for synthetic interfaces has remained challenging until recently, where magnetically-actuated gradient nanocomposites (MA-G-NCs) have emerged as a promising processing technique. The essence of this technique lies in controlling the spatial distribution of nanoreinforcements (usually particles) inside a polymer matrix through a magnetophoresis process. Herein, we present a theory-experiment-combined study on the evolution kinetics and equilibrium distribution of the nanoparticles during the magnetophoresis process and consequently to explore the spatial and temporal tunability of the MA-G-NCs. Using a simplified drift-diffusion theory as the guide, we determine two critical processing parameters for the MA-G-NCs: the applied magnetic field and the actuation duration. By systematically varying these two parameters independently, we experimentally demonstrate that the profile of the nanoparticle distribution inside the MA-G-NCs can be finely tuned both spatially and temporally. In order to quantify the volume fraction of the nanoparticles along the cross section of the MA-G-NCs, we propose a mechanics-based method by site-specifically measuring the local elastic modulus and converting back to the volume fractions based on an established modulus-fraction correlation. The nanoparticle concentration profiles obtained thereby are validated by morphological characterizations and also agree well with theoretical predictions based on the drift-diffusion theory. Our combined results indicate that the magnetophoresis-induced evolution of the nanoparticles follows approximately the drift-diffusion transport process and the gradient profile of the MA-G-NCs is highly controllable and programmable. The presented study not only advances the fundamental understanding of the evolution kinetics of the nanoparticles under the effect of magnetophoresis, but also establishes the critical processing-structure-property relationships for the MA-G-NCs that should guide future development of customized interfaces with desired mechanical and physical property gradients.

Characterization and Scaled-Up Production of Azido-Functionalized Silk Fiber Produced by Transgenic Silkworms with an Expanded Genetic Code

The creation of functional materials from renewable resources has attracted much interest. We previously reported on the genetic code expansion of the domesticated silkworm Bombyx mori to functionalize silk fiber with synthetic amino acids such as 4-azido-L-phenylalanine (AzPhe). The azido groups act as selective handles for biorthogonal chemical reactions. Here we report the characterization and scaled-up production of azido-functionalized silk fiber for textile, healthcare, and medical applications. To increase the productivity of azido-functionalized silk fiber, the original transgenic line was hybridized with a high silk-producing strain. The F₁ hybrid produced circa 1.5 times more silk fibroin than the original transgenic line. The incorporation efficiency of AzPhe into silk fibroin was retained after hybridization. The tensile properties of the azido-functionalized silk fiber were equal to those of normal silk fiber. Scaled-up production of the azido-functionalized silk fiber was demonstrated by rearing circa 1000 transgenic silkworms. Differently-colored fluorescent silk fibers were successfully prepared by click chemistry reactions, demonstrating the utility of the azido-functionalized silk fiber for developing silk-based materials with desired functions.