Tunable Wood by Reversible Interlocking and Bioinspired Mechanical Gradients

Superstrong, sustainable, origami wood paper enabled by dual-phase nanostructure regulation in cell walls

Constructing a crystalline-amorphous hybrid structure is an effective strategy to overcome the conflict between the strength and toughness of materials. However, achieving such a material structure often involves complex, energy-intensive processing. Here, we leverage the natural wood featuring coexisting crystalline and amorphous regions to achieve superstrong and ultratough wood paper (W-paper) via a dual-phase nanostructure regulation strategy. After partially removing amorphous hemicellulose and eliminating most lignin, the treated wood can self-densify through an energy-efficient air drying, resulting in a W-paper with high tensile strength, toughness, and folding endurance. Coarse-grained molecular dynamics simulations reveal the underlying deformation mechanism of the crystalline and amorphous regions inside cell walls and the failure mechanism of the W-paper under tension. Life cycle assessment reveals that W-paper shows a lower environmental impact than commercial paper and common plastics. This dual-phase nanostructure regulation based on natural wood may provide valuable insights for developing high-performance and sustainable film materials.

Renewable biomass-based aerogels: from structural design to functional regulation

Global population growth and industrialization have exacerbated the nonrenewable energy crises and environmental issues, thereby stimulating an enormous demand for producing environmentally friendly materials. Typically, biomass-based aerogels (BAs), which are mainly composed of biomass materials, show great application prospects in various fields because of their exceptional properties such as biocompatibility, degradability, and renewability. To improve the performance of BAs to meet the usage requirements of different scenarios, a large number of innovative works in the past few decades have emphasized the importance of micro-structural design in regulating macroscopic functions. Inspired by the ubiquitous random or regularly arranged structures of materials in nature ranging from micro to meso and macro scales, constructing different microstructures often corresponds to completely different functions even with similar biomolecular compositions. This review focuses on the preparation process, design concepts, regulation methods, and the synergistic combination of chemical compositions and microstructures of BAs with different porous structures from the perspective of gel skeleton and pore structure. It not only comprehensively introduces the effect of various microstructures on the physical properties of BAs, but also analyzes their potential applications in the corresponding fields of thermal management, water treatment, atmospheric water harvesting, CO2 absorption, energy storage and conversion, electromagnetic interference (EMI) shielding, biological applications, etc. Finally, we provide our perspectives regarding the challenges and future opportunities of BAs. Overall, our goal is to provide researchers with a thorough understanding of the relationship between the microstructures and properties of BAs, supported by a comprehensive analysis of the available data.

Lightweight, Strong, and Transparent Wood Films Produced by Capillary Driven Self-Densification

Wood delignification and densification enable the production of high strength and/or transparent wood materials with exceptional properties. However, processing needs to be more sustainable and besides the chemical delignification treatments, energy intense hot-pressing calls for alternative approaches. Here, this study shows that additional softening of delignified wood via a mild swelling process using an ionic liquid-water mixture enables the densification of tube-line wood cells into layer-by-layer sheet structures without hot-pressing. The natural capillary force induces self-densification in a simple drying process resulting in a transparent wood film. The as-prepared films with ≈150 µm thickness possess an optical transmittance ≈70%, while maintaining optical haze >95%. Due to the densely packed sheet structure with a large interfacial area, the reassembled wood film is fivefold stronger and stiffer than the delignified wood in fiber direction. Owing to a low density, the specific tensile strength and elastic modulus are as high as 282 MPa cm3 g-1 and 31 GPa cm3 g-1. A facile and highly energy efficient wood nanotechnology approach are demonstrated toward more sustainable materials and processes by directly converting delignified wood into transparent wood omitting polymeric matrix infiltration or mechanical pressing.

Wet-Stable Lamellar Wood Sponge with High Elasticity and Fatigue Resistance Enabled by Chemical Cross-Linking

The excessive consumption of fossil-based plastics and the associated environmental concerns motivate the increasing exploitation of sustainable biomass-based materials for advanced applications. Natural wood-derived lamellar wood sponges via a top-down approach have recently attracted significant attention; however, the insufficient compressive fatigue resistance and lack of structural stability in water limit their wide applications. Here, we report a facile chemical cross-linking strategy to tackle these challenges, by which the cellulose fibrils in the lamellas are covalently bridged to enhance their connectivity. The cross-linked wood sponges demonstrate high compressibility up to 70% strain and exceptional compressive fatigue resistance (∼5% plastic deformation after 10,000 cycles at 50% strain). The interfibrillar cross-linking inhibits the swelling of cellulose fibrils and preserves the arch-shaped lamellas of the sponge in water, endowing the wood sponge with excellent wet stability. Such highly elastic and wet-stable lamellar wood sponges offer a sustainable alternative to synthetic polymer-based sponges used in diverse applications.

Anisotropic wood-hydrogel composites: Extending mechanical properties of wood towards soft materials' applications

Delignified wood (DW) offers a versatile platform for the manufacturing of composites, with material properties ranging from stiff to soft and flexible by preserving the preferential fiber directionality of natural wood through a structure-retaining production process. This study presents a facile method for fabricating anisotropic and mechanically tunable DW-hydrogel composites. These composites were produced by infiltrating delignified spruce wood with an aqueous gelatin solution followed by chemical crosslinking. The mechanical properties could be modulated across a broad strength and stiffness range (1.2-18.3 MPa and 170-1455 MPa, respectively) by varying the crosslinking time. The diffusion-led crosslinking further allowed to manufacture mechanically graded structures. The resulting uniaxial, tubular structure of the anisotropic DW-hydrogel composite enabled the alignment of murine fibroblasts in vitro, which could be utilized in future studies on potential applications in tissue engineering.

Physicochemical Characterization of Chitosan/Poly-γ-Glutamic Acid Glass-like Materials

This paper sets up a new route for producing non-covalently crosslinked bio-composites by blending poly-γ-glutamic acid (γ-PGA) of microbial origin and chitosan (CH) through poly-electrolyte complexation under specific experimental conditions. CH and two different molecular weight γ-PGA fractions have been blended at different mass ratios (1/9, 2/8 and 3/7) under acidic pH. The developed materials seemed to behave like moldable hydrogels with a soft rubbery consistency. However, after dehydration, they became exceedingly hard, glass-like materials completely insoluble in water and organic solvents. The native biopolymers and their blends underwent comprehensive structural, physicochemical, and thermal analyses. The study confirmed strong physical interactions between polysaccharide and polyamide chains, facilitated by electrostatic attraction and hydrogen bonding. The materials exhibited both crystalline and amorphous structures and demonstrated good thermal stability and degradability. Described as thermoplastic and saloplastic, these bio-composites offer vast opportunities in the realm of polyelectrolyte complexes (PECs). This unique combination of properties allowed the bio-composites to function as glass-like materials, making them highly versatile for potential applications in various fields. They hold potential for use in regenerative medicine, biomedical devices, food packaging, and 3D printing. Their environmentally friendly properties make them attractive candidates for sustainable material development in various industries.

In-situ magnetite deposited wood composites with extensive electromagnetic interference shielding performance

Wood is an insulator material, using its porous structure to endow it with efficient microwave absorption and broaden its application range is still a major challenge. Here, wood-based Fe₃O₄ composites with excellent microwave absorption properties and high mechanical strength were prepared by alkaline sulfite method, in-situ co-precipitation method and compression densification method. The results showed that the magnetic Fe₃O₄ was densely deposited in the wood cells, and the prepared wood-based microwave absorption composites had both high electrical conductivity, magnetic loss, excellent impedance matching performance and attenuation performance, as well as effective microwave absorption properties. In the frequency range of 2-18 GHz, the minimum reflection loss value was -25.32 dB. At the same time, it had high mechanical properties. Compared with the untreated wood, its modulus of elasticity (MOE) in bending increased by 98.77%, and modulus of rapture (MOR) in bending improved by 67.9%. The developed wood-based microwave absorption composite is expected to be used in electromagnetic shielding fields such as anti-radiation and anti-interference.

Deformable hard tissue with high fatigue resistance in the hinge of bivalve Cristaria plicata

The hinge of bivalve shells can sustain hundreds of thousands of repeating opening-and-closing valve motions throughout their lifetime. We studied the hierarchical design of the mineralized tissue in the hinge of the bivalve Cristaria plicata, which endows the tissue with deformability and fatigue resistance and consequently underlies the repeating motion capability. This folding fan-shaped tissue consists of radially aligned, brittle aragonite nanowires embedded in a resilient matrix and can translate external radial loads to circumferential deformation. The hard-soft complex microstructure can suppress stress concentration within the tissue. Coherent nanotwin boundaries along the longitudinal direction of the nanowires increase their resistance to bending fracture. The unusual biomineral, which exploits the inherent properties of each component through multiscale structural design, provides insights into the evolution of antifatigue structural materials.

An Oxidative Enzyme Boosting Mechanical and Optical Performance of Densified Wood Films

Nature has evolved elegant ways to alter the wood cell wall structure through carbohydrate-active enzymes, offering environmentally friendly solutions to tailor the microstructure of wood for high-performance materials. In this work, the cell wall structure of delignified wood is modified under mild reaction conditions using an oxidative enzyme, lytic polysaccharide monooxygenase (LPMO). LPMO oxidation results in nanofibrillation of cellulose microfibril bundles inside the wood cell wall, allowing densification of delignified wood under ambient conditions and low pressure into transparent anisotropic films. The enzymatic nanofibrillation facilitates microfibril fusion and enhances the adhesion between the adjacent wood fiber cells during densification process, thereby significantly improving the mechanical performance of the films in both longitudinal and transverse directions. These results improve the understanding of LPMO-induced microstructural changes in wood and offer an environmentally friendly alternative for harsh chemical treatments and energy-intensive densification processes thus representing a significant advance in sustainable production of high-performance wood-derived materials.

Sustainability in Wood Products: A New Perspective for Handling Natural Diversity

Wood is a renewable resource with excellent qualities and the potential to become a key element of a future bioeconomy. The increasing environmental awareness and drive to achieve sustainability is leading to a resurgence of research on wood materials. Nevertheless, the global climate changes and associated consequences will soon challenge the wood-value chains in several regions (e.g., central Europe). To cope with these challenges, it is necessary to rethink the current practice of wood sourcing and transformation. The goal of this review is to address the intrinsic natural diversity of wood, from its origin to its technological consequences for the present and future manufacturing of wood products. So far, industrial processes have been optimized to repress the variability of wood properties, enabling more efficient processing and production of reliable products. However, the need to preserve biodiversity and the impact of climate change on forests call for new wood processing techniques and green chemistry protocols for wood modification as enabling factors necessary for managing a more diverse wood provision in the future. This article discusses the past developments that have resulted in the current wood value chains and provides a perspective about how natural variability could be turned into an asset for making truly sustainable wood products. After briefly introducing the chemical and structural complexity of wood, the methods conventionally adopted for industrial homogenization and modification of wood are discussed in relation to their evolution toward increased sustainability. Finally, a perspective is given on technological potentials of machine learning techniques and of novel functional wood materials. Here the main message is that through a combination of sustainable forestry, adherence to green chemistry principles and adapted processes based on machine learning, the wood industry could not only overcome current challenges but also thrive in the near future despite the awaiting challenges.

Autonomous self-burying seed carriers for aerial seeding

Aerial seeding can quickly cover large and physically inaccessible areas¹ to improve soil quality and scavenge residual nitrogen in agriculture², and for postfire reforestation^3-5 and wildland restoration^6,7. However, it suffers from low germination rates, due to the direct exposure of unburied seeds to harsh sunlight, wind and granivorous birds, as well as undesirable air humidity and temperature^1,8,9. Here, inspired by Erodium seeds^10-14, we design and fabricate self-drilling seed carriers, turning wood veneer into highly stiff (about 4.9 GPa when dry, and about 1.3 GPa when wet) and hygromorphic bending or coiling actuators with an extremely large bending curvature (1,854 m^-1), 45 times larger than the values in the literature^15-18. Our three-tailed carrier has an 80% drilling success rate on flat land after two triggering cycles, due to the beneficial resting angle (25°-30°) of its tail anchoring, whereas the natural Erodium seed's success rate is 0%. Our carriers can carry payloads of various sizes and contents including biofertilizers and plant seeds as large as those of whitebark pine, which are about 11 mm in length and about 72 mg. We compare data from experiments and numerical simulation to elucidate the curvature transformation and actuation mechanisms to guide the design and optimization of the seed carriers. Our system will improve the effectiveness of aerial seeding to relieve agricultural and environmental stresses, and has potential applications in energy harvesting, soft robotics and sustainable buildings.

Allomorphic regulation of bamboo cellulose by mild alkaline peroxide for holocellulose nanofibrils production

The exploration of sustainable lignocellulosic nanomaterials with unique properties and applicable functions is receiving growing interest. In this work, holocellulose nanofibrils (HCNFs) were prepared from moso bamboo using mild alkaline peroxide bleaching method (MAPB) followed by mechanical nanofibrillation. MAPB was proved to effectively remove lignin and retain hemicellulose. Meanwhile, partial allomorphic changes from cellulose I to cellulose II were revealed together with varying degrees of crystallinity. Thermogravimetric analysis (TGA) experiment showed an increasing thermal stability trend due to more allomorphic changes into anti-parallel cellulose II. Well-dispersed HCNFs suspensions were successfully prepared by homogenization and HCNFs films with high transparency and flexibility were fabricated. The films reached the maximum tensile strength of 55.8 MPa and tensile strain of 1.55 % along with a calculated toughness of 25 MJ/m3. Moreover, the prepared materials are biocompatible and completely non-toxic, which will theoretically support the application of HCNFs materials in fields of biology, medicine and food industry.

Modification of Poplar Wood via Polyethylene Glycol Impregnation Coupled with Compression

Wood permeability and compressibility are affected by cell wall structure and chemical composition. These properties can be improved by appropriate wood pretreatments. Low-density poplar wood was converted to a more dense structure by the following steps: First, lignin and hemicellulose were removed using a mixture of NaOH and Na2SO3. Second they were impregnated with polyethylene glycol (PEG, mean molecular weight of 1200), nano-SiO2, and a silane coupling agent at atmospheric temperature and pressure. Finally, impregnated wood was compressed at 150 °C. Results showed that the tracheid lumens on the transverse section of the compressed wood almost vanished. Specifically, the lumens in the wood cells, especially those that were compressed, were almost completely filled with PEG. In FTIR, the asymmetric absorption peaks of Si–O–Si at 1078–1076 cm−1 were clearly observed, which confirms the existence of bonding between nano-SiO2 and wood. The highest melting enthalpy and crystallization enthalpy showed a heat storage capacity of modified wood, which were 20.7 and 9.8 J/g, respectively. Such phase change capabilities may have potential applications in regulating the rate of change of room temperature. In summary, the modified wood could be utilized as material for construction to conserve energy.

Intercellular Matrix Infiltration Improves the Wet Strength of Delignified Wood Composites

Delignified wood (DW) represents a promising bio-based fibrous material as a reinforcing component in high-performance composites. These cellulose composites possess excellent strength and stiffness in the dry state, which are significantly higher than for natural wood. However, in the wet state, a penetrating water layer enters the intercellular regions and disrupts the stress transfer mechanisms between cell fibers in fully DW. This water layer initially facilitates complex shaping of the material but imparts DW composites with very low wet stiffness and strength. Therefore, a sufficient stress transfer in the wet state necessitates a resin impregnation of these intercellular regions, establishing bonding mechanisms between adjacent fibers. Here, we utilize a water-based dimethyloldihydroxyethylene urea thermosetting matrix (DMDHEU) and compare it with a non-water-based epoxy matrix. We infiltrate these resins into DW and investigate their spatial distribution by scanning electron microscopy, atomic force microscopy, and confocal Raman spectroscopy. The water-based resin impregnates the intercellular areas and generates an artificial compound middle lamella, while the epoxy infiltrates only the cell lumina of the dry DW. Tensile tests in the dry and wet states show that the DMDHEU matrix infiltration of the intercellular areas and the cell wall results in a higher tensile strength and stiffness compared to the epoxy resin. Here, the artificial compound middle lamella made of DMDHEU bonds adjacent fibers together and substantially increases the composites' wet strength. This study elucidates the importance of the interaction and spatial distribution of the resin system within the DW structure to improve mechanical properties, particularly in the wet state.

Wood Derived Cellulose Scaffolds-Processing and Mechanics

Wood-derived cellulose materials obtained by structure-retaining delignification are attracting increasing attention due to their excellent mechanical properties and great potential to serve as renewable and CO₂ storing cellulose scaffolds for advanced hybrid materials with embedded functionality. Various delignification protocols and a multitude of further processing steps including polymer impregnation and densification are applied resulting in a large range of properties. However, treatment optimization requires a more comprehensive characterization of the developed materials in terms of structure, chemical composition, and mechanical properties for faster progress in the field. Herein, the current protocols for structure-retaining delignification are reviewed and the emphasis is placed on the mechanical characterization at different hierarchical levels of the cellulose scaffolds by experiments and modeling to reveal the underlying structure-property relationships.

Plant Nanomaterials and Inspiration from Nature: Water Interactions and Hierarchically Structured Hydrogels

Recent developments in the area of plant-based hydrogels are introduced, especially those derived from wood as a widely available, multiscale, and hierarchical source of nanomaterials, as well as other cell wall elements. With water being fundamental in a hydrogel, water interactions, hydration, and swelling, all critically important in designing, processing, and achieving the desired properties of sustainable and functional hydrogels, are highlighted. A plant, by itself, is a form of a hydrogel, at least at given states of development, and for this reason phenomena such as fluid transport, diffusion, capillarity, and ionic effects are examined. These aspects are highly relevant not only to plants, especially lignified tissues, but also to the porous structures produced after removal of water (foams, sponges, cryogels, xerogels, and aerogels). Thus, a useful source of critical and comprehensive information is provided regarding the synthesis of hydrogels from plant materials (and especially wood nanostructures), and about the role of water, not only for processing but for developing hydrogel properties and uses.

Mechanics Design in Cellulose-Enabled High-Performance Functional Materials

The abundance of cellulose found in natural resources such as wood, and the wide spectrum of structural diversity of cellulose nanomaterials in the form of micro-nano-sized particles and fibers, have sparked a tremendous interest to utilize cellulose's intriguing mechanical properties in designing high-performance functional materials, where cellulose's structure-mechanics relationships are pivotal. In this progress report, multiscale mechanics understanding of cellulose, including the key role of hydrogen bonding, the dependence of structural interfaces on the spatial hydrogen bond density, the effect of nanofiber size and orientation on the fracture toughness, are discussed along with recent development on enabling experimental design techniques such as structural alteration, manipulation of anisotropy, interface and topology engineering. Progress in these fronts renders cellulose a prospect of being effectuated in an array of emerging sustainable applications and being fabricated into high-performance structural materials that are both strong and tough.

Wood and the Activity of Dead Tissue

Wood is a prototypical biological material, which adapts to mechanical requirements. The microarchitecture of cellulose fibrils determines the mechanical properties of woody materials, as well as their actuation properties, based on absorption and desorption of water. Herein it is argued that cellulose fiber orientation corresponds to an analog code that determines the response of wood to humidity as an active material. Examples for the harvesting of wood activity, as well as bioinspiration, are given.

Functional Materials from Nanocellulose: Utilizing Structure-Property Relationships in Bottom-Up Fabrication

It is inherently challenging to recapitulate the precise hierarchical architectures found throughout nature (such as in wood, antler, bone, and silk) using synthetic bottom-up fabrication strategies. However, as a renewable and naturally sourced nanoscale building block, nanocellulose-both cellulose nanocrystals and cellulose nanofibrils-has gained significant research interest within this area. Altogether, the intrinsic shape anisotropy, surface charge/chemistry, and mechanical/rheological properties are some of the critical material properties leading to advanced structure-based functionality within nanocellulose-based bottom-up fabricated materials. Herein, the organization of nanocellulose into biomimetic-aligned, porous, and fibrous materials through a variety of fabrication techniques is presented. Moreover, sophisticated material structuring arising from both the alignment of nanocellulose and via specific process-induced methods is covered. In particular, design rules based on the underlying fundamental properties of nanocellulose are established and discussed as related to their influence on material assembly and resulting structure/function. Finally, key advancements and critical challenges within the field are highlighted, paving the way for the fabrication of truly advanced materials from nanocellulose.

Effects of Fiber Angle on the Tensile Properties of Partially Delignified and Densified Wood

Partial delignification and densification provide a pathway to significant improvement in the mechanical performance of wood. In order to elucidate potential effects of this treatment on the mechanical anisotropy of wood, partially delignified and densified spruce wood veneers were characterized at varying degrees of off-axis alignment. While the tensile strength and the modulus of elasticity (MOE) were clearly improved in parallel to the axis of wood fibers, this improvement quickly leveled off at misalignment angles ≥30°. For transverse tensile strength, the performance of alkaline-treated and densified wood was even inferior to that of untreated wood. Microscopic examination revealed the presence of microscopic cracks in treated wood, which are assumed to be responsible for this observation. It is concluded that impaired transverse tensile properties are a weakness of partially delignified and densified wood and should be considered when a potential usage in load-bearing applications is intended.

Luminescent and Hydrophobic Wood Films as Optical Lighting Materials

Most materials used for optical lighting applications need to produce a uniform illumination and require high mechanical and hydrophobic properties. However, they are rarely eco-friendly. Herein, a bio-based, polymer matrix-free, luminescent, and hydrophobic film with excellent mechanical properties for optical lighting purposes is demonstrated. A template is prepared by turning a wood veneer into porous scaffold from which most of the lignin and half of the hemicelluloses are removed. The infiltration of quantum dots (CdSe/ZnS) into the porous template prior to densification resulted in almost uniform luminescence (isotropic light scattering) and could be extended to various quantum dot particles, generating different light colors. In a subsequent step, the luminescent wood film is coated with hexadecyltrimethoxysilane (HDTMS) via chemical vapor deposition. The presence of the quantum dots coupled with the HDTMS coating renders the film hydrophobic (water contact angle ≈ 140°). This top-down process strongly eliminates lumen cavities and preserves the orientation of the original cellulose fibrils to create luminescent and polymer matrix-free films with high modulus and strength in the direction of fibers. The proposed optical lighting material could be attractive for interior designs (e.g., lamps and laminated cover panels), photonics, and laser devices.

Delignification and Ionic Liquid Treatment of Wood toward Multifunctional High-Performance Structural Materials

Wood-based multifunctional materials with excellent mechanical performance are increasingly considered for sustainable advanced applications due to their unique hierarchical structure and inherent reinforcing cellulose phase orientation. Nonetheless, a wider multipurpose utilization of wood materials is so far hampered because of constraints arising from scalable functionalization, efficient processing, facile shaping as well asnatural heterogeneity and durability. This study introduces a multifunctional all-wood material fabrication method relying on delignification, ionic liquid (IL) treatment, and pressure-assisted consolidation of wood. Structure-retaining controlled delignification of wood was performed to enable direct access to the hierarchical cellulose assembly, while preserving the highly aligned and thus beneficial wood structural directionality. As a following step, the obtained biobased scaffold with an increased porosity was infiltrated with an IL and heat-activated to partially dissolve and soften the cellulose fiber surface. Samples washed with water to remove IL exhibited pronounced isotropic flexibility, which upon combined compression and lateral shear allowed the fabrication of various 3D shapes with adjustable fiber architecture. The obtained very compact and totally additive-free all-wood materials were extensively characterized, revealing superior mechanical performance, and gained multifunctionality compared to native wood.

Atomic force microscopy imaging of delignified secondary cell walls in liquid conditions facilitates interpretation of wood ultrastructure

Deep understanding of the physicochemical and structural characteristics of wood at the nanoscale is essential for improving wood usage in biorefining and advancing new high performance materials design. Herein, we use in situ atomic force microscopy and a simple delignification treatment to elucidate the nanoscale architecture of individual secondary cell wall layers. Advantages of this approach are: (i) minimal sample preparation that reduces the introduction of potential artifacts; (ii) prevention of structural rearrangements due to dehydration; (iii) increased accessibility to structural details masked by the lignin matrix; and (iv) possibility to complement results with other analytical techniques without sample manipulation. The methodology permits the visualization of parallel and helicoidally arranged microfibril aggregates in the S1 layer and the determination of lignin contribution to microfibril aggregates forming S2 layers. Cellulose and hemicelluloses constitute the core of the aggregates with a mean diameter of approximately 19 nm, and lignin encloses the core forming single structural entities of about 30 nm diameter. Furthermore, we highlight the implications of sample preparation and imaging parameters on the characterization of microfibril aggregates by AFM.

Wood Deformation Leads to Rearrangement of Molecules at the Nanoscale

Wood, as the most abundant carbon dioxide storing bioresource, is currently driven beyond its traditional use through creative innovations and nanotechnology. For many properties the micro- and nanostructure plays a crucial role and one key challenge is control and detection of chemical and physical processes in the confined microstructure and nanopores of the wooden cell wall. In this study, correlative Raman and atomic force microscopy show high potential for tracking in situ molecular rearrangement of wood polymers during compression. More water molecules (interpreted as wider cellulose microfibril distances) and disentangling of hemicellulose chains are detected in the opened cell wall regions, whereas an increase of lignin is revealed in the compressed areas. These results support a new more "loose" cell wall model based on flexible lignin nanodomains and advance our knowledge of the molecular reorganization during deformation of wood for optimized processing and utilization.

Green Synthesis of Hierarchical Metal-Organic Framework/Wood Functional Composites with Superior Mechanical Properties

The applicability of advanced composite materials with hierarchical structure that conjugate metal-organic frameworks (MOFs) with macroporous materials is commonly limited by their inferior mechanical properties. Here, a universal green synthesis method for the in situ growth of MOF nanocrystals within wood substrates is introduced. Nucleation sites for different types of MOFs are readily created by a sodium hydroxide treatment, which is demonstrated to be broadly applicable to different wood species. The resulting MOF/wood composite exhibits hierarchical porosity with 130 times larger specific surface area compared to native wood. Assessment of the CO2 adsorption capacity demonstrates the efficient utilization of the MOF loading along with similar adsorption ability to that of pure MOF. Compression and tensile tests reveal superior mechanical properties, which surpass those obtained for polymer substrates. The functionalization strategy offers a stable, sustainable, and scalable platform for the fabrication of multifunctional MOF/wood-derived composites with potential applications in environmental- and energy-related fields.

Designed for the enhancement of structure mechanostability and strength: Suture-serrate margins of bivalve shells

Through biological evolution, bivalve mollusks developed shells to improve the utilization of metabolic energy and provide protection against external threats. In addition to the mechanical optimization of the microstructure, the design of the macroscopic shape of a bivalve shell naturally becomes a potential approach to achieving the aforementioned purposes. While the functions of some features of mollusk shells have been studied, the role of the suture-serrate margins, a common morphology of bivalve shell edges, in the global mechanical behaviors of bivalve shells requires further exploration. Here, we present how the serrate margins contribute to the global mechanical properties of bivalve shells. The results of the compression tests employed on a typical bivalve, M. mercenaria, showed that the complete bivalve shells with suture-serrate margins perform better in terms of strength and work to fracture than those without the margins under the same conditions (dry and wet). The primary failure types observed during compression reveal that the failure mechanisms of valve shells are dependent on the suture-serrate margin morphology and water content. Using numerical simulations, the mechanical functions of the suture-serrate margins were demonstrated. Specifically, serrate margins provide mutual resistance by "locking" complementary valves to redistribute and eliminate stress concentrations around pre-existing defects, thereby enhancing the mechanostability and strength of the entire structure.

Delignified Wood-Polymer Interpenetrating Composites Exceeding the Rule of Mixtures

Wood is increasingly considered in sustainable structural materials development due to its hierarchical structure, including an oriented reinforcing cellulose phase combined with carbon capturing and renewability. Top-down manufacturing techniques can provide direct access to this hierarchical cellulose scaffold for use in new functional materials. For high-performance load-bearing wood-based materials, the volume content of the reinforcing phase needs to be increased to much higher fiber volume contents (FVCs). This has been achieved by structure-retaining delignification followed by densification. The obtained matrix-free materials possess high tensile stiffness due to preservation of hierarchical fiber alignment; however, they demonstrate low mechanical properties in bending and cannot be used in moist conditions due to their propensity for water absorption. In order to address these two challenges, an interpenetrating wood polymer phase composite is developed using a delignified wood scaffold as a continuous reinforcing phase and epoxy resin as the interconnected matrix phase. We utilize the continuous flow channels in delignified wood for vacuum-assisted matrix infiltration in a condition of open continuous porosity in the wood scaffold. Prior to matrix curing, the material is densified in order to increase the FVC, decrease porosity, and reduce density variations in the wood scaffold. Due to the compressibility of delignified cellulose fibers, interpenetrating phase composites (IPCs) with very high FVCs of up to 80% could be produced, leading to exceptionally high tensile stiffness and strength of up to 70 GPa and 600 MPa. The obtained stiffness values far exceed the upper limit of the rule of mixtures due to an enhanced stress transfer through mechanically interlocked fiber-fiber interfaces combined with the stiffness providing matrix phase that further aids stress transfer between neighboring wood cells via their pits. This new approach paves the way for an efficient production of high-performance sustainable materials that can be used as alternative for glass fiber reinforced composites or natural fiber composites.

The Puzzle of the Walnut Shell: A Novel Cell Type with Interlocked Packing

The outer protective shells of nuts can have remarkable toughness and strength, which are typically achieved by a layered arrangement of sclerenchyma cells and fibers with a polygonal form. Here, the tissue structure of walnut shells is analyzed in depth, revealing that the shells consist of a single, never reported cell type: the polylobate sclereid cells. These irregularly lobed cells with concave and convex parts are on average interlocked with 14 neighboring cells. The result is an intricate arrangement that cannot be disassembled when conceived as a 3D puzzle. Mechanical testing reveals a significantly higher ultimate tensile strength of the interlocked walnut cell tissue compared to the sclerenchyma tissue of a pine seed coat lacking the lobed cell structure. The higher strength value of the walnut shell is explained by the observation that the crack cannot simply detach intact cells but has to cut through the lobes due to the interlocking. Understanding the identified nutshell structure and its development will inspire biomimetic material design and packaging concepts. Furthermore, these unique unit cells might be of special interest for utilizing nutshells in terms of food waste valorization, considering that walnuts are the most widespread tree nuts in the world.

Mesoporosity of Delignified Wood Investigated by Water Vapor Sorption

Wood represents a highly suitable biobased scaffold for the development of mechanically robust and functional materials. Its functionalizability can be enhanced by means of delignification, resulting in an increase in porosity due to partial or complete removal of lignin and hemicellulose constituents. In this work, the impact of partial and complete delignification on the mesoporous structure is investigated via water vapor sorption isotherms and deuterium exchange. Pore size distributions of wood samples with five different delignification levels were compared to native wood. The derived pore size distributions at the water swollen state reveal an increase in porosity with decreasing lignin content. However, after complete lignin removal, drying causes a nonreversible collapse of the cell wall, which results in reduced porosity.