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.

Even Visually Intact Cell Walls in Waterlogged Archaeological Wood Are Chemically Deteriorated and Mechanically Fragile: A Case of a 170 Year-Old Shipwreck.

Structural and chemical deterioration and its impact on cell wall mechanics were investigated for visually intact cell walls (VICWs) in waterlogged archaeological wood (WAW). Cell wall mechanical properties were examined by nanoindentation without prior embedding. WAW showed more than 25% decrease of both hardness and elastic modulus. Changes of cell wall composition, cellulose crystallite structure and porosity were investigated by ATR-FTIR imaging, Raman imaging, wet chemistry, 13C-solid state NMR, pyrolysis-GC/MS, wide angle X-ray scattering, and N2 nitrogen adsorption. VICWs in WAW possessed a cleavage of carboxyl in side chains of xylan, a serious loss of polysaccharides, and a partial breakage of ß-O-4 interlinks in lignin. This was accompanied by a higher amount of mesopores in cell walls. Even VICWs in WAW were severely deteriorated at the nanoscale with impact on mechanics, which has strong implications for the conservation of archaeological shipwrecks.

Solvent-Controlled Spatial Distribution of SI-AGET-ATRP Grafted Polymers in Lignocellulosic Materials.

In the current quest for the design of advanced complex materials, the functionalization of biological materials having hierarchical structures has been of high interest. In the case of lignocellulosic materials, various modification techniques have allowed one to obtain materials with outstanding properties. However, the control over the spatial distribution of the modification inside the wood scaffold, which is an important parameter to obtain the desired properties, has yet to be understood. In this study, the use of solvents with different wood-swelling capabilities is proposed to control the spatial polymer-modification distribution inside the hierarchical wood structure. Wood cubes were functionalized via SI-AGET-ATRP using solvents with different wood-swelling capabilities. Spectroscopic (Raman and FTIR) and electron microscopy techniques showed that a good wood-swelling solvent as reaction media can transport the polymerization initiator molecule into the cell wall, allowing it to react with all the available -OH groups in the wood structure. Conversely, the use of a bad wood-swelling solvent limits the reaction to the available -OH groups at the lumen/cell wall interface. The subsequently added polymers grow from the available initiator sites and therefore show similar spatial distribution. This diffusion limitation is visible not only at the microscopic level (cellular structure) but also at the macroscopic level (over the length of the sample).

Monolayer Graphene Coupled to a Flexible Plasmonic Nanograting for Ultrasensitive Strain Monitoring.

Plasmonically coupled graphene structures have shown great promise for sensing applications. Their complex and cumbersome fabrication, however, has prohibited their widespread application and limited their use to rigid, planar surfaces. Here, a plasmonic sensor based on gold nanowire arrays on an elastomer with an added graphene monolayer is introduced. The stretchable plasmonic nanostructures not only significantly enhance the Raman signal from graphene, but can also be used by themselves as a sensor platform for 2D strain sensing. These nanowire arrays on an elastomer are fabricated by template-stripping based nanotransfer printing, which enables a simple and fast production of stable nanogratings. The ultrasmooth surfaces of such transferred structures facilitate reliable large-area transfers of graphene monolayers. The resulting coupled graphene-nanograting construct exhibits ultrahigh sensitivity to applied strain, which can be detected by shifts in the plasmonic-enhanced Raman spectrum. Furthermore, this sensor enables the detection of adsorbed molecules on nonplanar surfaces through graphene-assisted surface enhanced Raman spectroscopy (SERS). The simple fabrication of the plasmonic nanowire array platform and the graphene-coupled devices have the potential to trigger widespread SERS applications and open up new opportunities for high-sensitivity strain sensing applications.

A close-up view of the wood cell wall ultrastructure and its mechanics at different cutting angles by atomic force microscopy.

MAIN CONCLUSION: AFM measurements on spruce sample cross-sections reveal that the structural appearance of the S2 layer changes from a network structure to a concentric lamellar texture depending on the cutting angle. The structural assembly of wood constituents within the secondary cell wall has been subject of numerous studies over the last decades, which has resulted in contradicting models on the spatial arrangement and orientation of the wood macromolecules. Here, we use multichannel atomic force microscopy by means of quantitative imaging, to gain new insights into the macromolecular assembly. Cross-sections of spruce wood, which had been cut at different angles ranging from 0° to 30° were investigated. Strikingly, depending on the cutting angle, the structural appearance of the S2 layer changed from a network-like structure to a distinct concentric lamellar texture. This makes us conclude that the often visualized lamellar organization of the secondary cell wall is not the consequence of a continuous inherent ring pattern, but rather a result of the specific surface cross-section appearance of cellulose aggregates at larger cutting angles. By analyzing the recorded force distance curves in every pixel, a nano-mechanical characterization of the secondary cell wall was conducted. Substantially lower indentation modulus values were obtained compared to nanoindentation values reported in the literature. This is potentially due to a smaller interaction volume of the probe with a by far less deep indentation.

Plant material features responsible for bamboo's excellent mechanical performance: a comparison of tensile properties of bamboo and spruce at the tissue, fibre and cell wall levels.

BACKGROUND AND AIMS: Bamboo is well known for its fast growth and excellent mechanical performance, but the underlying relationships between its structure and properties are only partially known. Since it lacks secondary thickening, bamboo cannot use adaptive growth in the same way as a tree would in order to modify the geometry of the stem and increase its moment of inertia to cope with bending stresses caused by wind loads. Consequently, mechanical adaptation can only be achieved at the tissue level, and this study aims to examine how this is achieved by comparison with a softwood tree species at the tissue, fibre and cell wall levels. METHODS: The mechanical properties of single fibres and tissue slices of stems of mature moso bamboo (Phyllostachys pubescens) and spruce (Picea abies) latewood were investigated in microtensile tests. Cell parameters, cellulose microfibril angles and chemical composition were determined using light and electron microscopy, wide-angle X-ray scattering and confocal Raman microscopy. KEY RESULTS: Pronounced differences in tensile stiffness and strength were found at the tissue and fibre levels, but not at the cell wall level. Thus, under tensile loads, the differing wall structures of bamboo (multilayered) and spruce (sandwich-like) appear to be of minor relevance. CONCLUSIONS: The superior tensile properties of bamboo fibres and fibre bundles are mainly a result of amplified cell wall formation, leading to a densely packed tissue, rather than being based on specific cell wall properties. The material optimization towards extremely compact fibres with a multi-lamellar cell wall in bamboo might be a result of a plant growth strategy that compensates for the lack of secondary thickening growth at the tissue level, which is not only favourable for the biomechanics of the plant but is also increasingly utilized in terms of engineering products made from bamboo culms.

Biodegradable and Flexible Wood-Gelatin Composites for Soft Actuating Systems.

Compliant materials are indispensable for many emerging soft robotics applications. Hence, concerns regarding sustainability and end-of-life options for these materials are growing, given that they are predominantly petroleum-based and non-recyclable. Despite efforts to explore alternative bio-derived soft materials like gelatin, they frequently fall short in delivering the mechanical performance required for soft actuating systems. To address this issue, we reinforced a compliant and transparent gelatin-glycerol matrix with structure-retained delignified wood, resulting in a flexible and entirely biobased composite (DW-flex). This DW-flex composite exhibits highly anisotropic mechanical behavior, possessing higher strength and stiffness in the fiber direction and high deformability perpendicular to it. Implementing a distinct anisotropy in otherwise isotropic soft materials unlocks new possibilities for more complex movement patterns. To demonstrate the capability and potential of DW-flex, we built and modeled a fin ray-inspired gripper finger, which deforms based on a twist-bending-coupled motion that is tailorable by adjusting the fiber direction. Moreover, we designed a demonstrator for a proof-of-concept suitable for gripping a soft object with a complex shape, i.e., a strawberry. We show that this composite is entirely biodegradable in soil, enabling more sustainable approaches for soft actuators in robotics applications.

Plant micro- and nanomechanics: experimental techniques for plant cell-wall analysis.

In the last few decades, micro- and nanomechanical methods have become increasingly important analytical techniques to gain deeper insight into the nanostructure and mechanical design of plant cell walls. The objective of this article is to review the most common micro- and nanomechanical approaches that are utilized to study primary and secondary cell walls from a biomechanics perspective. In light of their quite disparate functions, the common and opposing structural features of primary and secondary cell walls are reviewed briefly. A significant part of the article is devoted to an overview of the methodological aspects of the mechanical characterization techniques with a particular focus on new developments and advancements in the field of nanomechanics. This is followed and complemented by a review of numerous studies on the mechanical role of cellulose fibrils and the various matrix components as well as the polymer interactions in the context of primary and secondary cell-wall function.

Passive climate regulation with transpiring wood for buildings with increased energy efficiency.

Buildings are significant end-users of global energy. About 20% of the energy consumption worldwide is used for maintaining a comfortable indoor climate. Therefore, passive systems for indoor temperature and humidity regulation that can respond to environmental changes are very promising to reduce buildings' energy consumption. We developed a process to improve the responsiveness of wood to humidity changes by laser-drilling microscopic holes and incorporating a hygroscopic salt (calcium chloride). The resulting "transpiring wood" displays superior water adsorption capacity and high moisture exchange rate, allowing regulation of humidity and temperature by the exchange of moisture with the surrounding air. We proved that the hygrothermal performance of transpiring wood can be used to regulate indoor climate, with associated energy savings, for various climate types, thus favoring its application in the building sector. The reduction of temperature fluctuations, thanks to the buffering of temperature peaks, can lead to an indirect energy saving of about 10% for cooling and between 4-27% for heating depending on the climate. Furthermore, our transpiring wood meets different sustainability criteria, from raw materials to the fabrication process, resulting in a product with a low overall environmental impact and that is easy to recycle.

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.

Review on design strategies and applications of metal-organic framework-cellulose composites.

Metal-organic frameworks (MOFs) are among the most attractive functional porous materials. However, their processability and handling remains a substantial challenge because MOFs generally occur in powder form due to their crystalline nature. Combining MOFs and cellulose substrates to fabricate engineered materials offers an ideal solution to broaden their utilization as functional materials. MOF/cellulose composites further provide remarkable mechanical properties, tunable porosity, and accessible active sites of MOFs. In this review, we summarize current state-of-the-art fabrication routes for MOF/cellulose composites, with a specific focus on the unique potential of utilizing three-dimensional bio-based cellulosic scaffolds. We highlight their utilization as adsorbents in the gas and liquid phase, for antibacterial and protein immobilization, chemical sensors, electrical energy storage, and other emerging applications. In addition, we discuss current limitations and potential future research directions in the field of MOF/cellulose composites for advanced functional materials.

Thermoresponsive Smart Gating Wood Membranes.

Smart membranes that can open and/or close their pores in a controlled manner by external stimuli possess potential in various applications, such as water flow manipulation, indoor climate regulation, and sensing. The design of smart gating membranes with high flux, immediate response, and mechanical robustness is still an open challenge, limiting their versatility and practical applicability. Inspired by the controlled opening and closure of plant stomata, we have developed a smart gating wood membrane, taking advantage of the unique wood scaffold with its hierarchical porous structure to carry thermoresponsive hydrogel gates. Laser drilling was applied to cut channels in the wood scaffold with well-aligned pores to incorporate the smart gating membranes. In situ polymerization of poly(N-isopropylacrylamide) above its lower critical solution temperature inside the channels resulted in a hydrogel with a heterogeneous microstructure acting as a thermoresponsive gate. The wood-based smart gating membranes exhibited reversible and stable pore opening/closing under heating/cooling stimuli. The achieved rapid response and feasibility of scale-up open the venue for various practical applications. In this work, we demonstrated their potential for indoor light regulation and as a water flow manipulator.

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.

Sustainable wood electronics by iron-catalyzed laser-induced graphitization for large-scale applications.

Ecologically friendly wood electronics will help alleviating the shortcomings of state-of-art cellulose-based "green electronics". Here we introduce iron-catalyzed laser-induced graphitization (IC-LIG) as an innovative approach for engraving large-scale electrically conductive structures on wood with very high quality and efficiency, overcoming the limitations of conventional LIG including high ablation, thermal damages, need for multiple lasing steps, use of fire retardants and inert atmospheres. An aqueous bio-based coating, inspired by historical iron-gall ink, protects wood from laser ablation and thermal damage while promoting efficient graphitization and smoothening substrate irregularities. Large-scale (100 cm2), highly conductive (≥2500 S m-1) and homogeneous surface areas are engraved single-step in ambient atmosphere with a conventional CO2 laser, even on very thin (∼450 µm) wood veneers. We demonstrate the validity of our approach by turning wood into highly durable strain sensors, flexible electrodes, capacitive touch panels and an electroluminescent LIG-based device.

Cellulose lattice strains and stress transfer in native and delignified wood.

Small specimens of spruce wood with different degrees of delignification were studied using in-situ tensile tests and simultaneous synchrotron X-ray diffraction to reveal the effect of delignification and densification on their tensile properties at relative humidities of 70-80 %. In addition to mechanical properties, these analyses yield the ratio of strains in the cellulose crystals and in the bulk, which reflects the stress-transfer to crystalline cellulose. While the specific modulus of elasticity slightly increases from native wood by partial or complete delignification, the lattice strain ratio does not show a significant change. This could indicate a compensatory effect from the decomposition of the amorphous matrix by delignification and from a tighter packing of cellulose crystals that would increase the stress transfer. The reduced strain to failure and maximum lattice strain of delignified specimens suggests that the removal of lignin affects the stress-strain behavior with fracture at lower strain levels.

Natural Wood-Based Catalytic Membrane Microreactors for Continuous Hydrogen Generation.

The development of controlled processes for continuous hydrogen generation from solid-state storage chemicals such as ammonia borane is central to integrating renewable hydrogen into a clean energy mix. However, to date, most reported platforms operate in batch mode, posing a challenge for controllable hydrogen release, catalyst reusability, and large-scale operation. To address these issues, we developed flow-through wood-based catalytic microreactors, characterized by inherent natural oriented microchannels. The prepared structured catalysts utilize silver-promoted palladium nanoparticles supported on metal-organic framework (MOF)-coated wood microreactors as the active phase. Catalytic tests demonstrate their highly controllable hydrogen production in continuous mode, and by adjusting the ammonia borane flow and wood species, we reach stable productivities of up to 10.4 cmH23 min-1 cmcat-3. The modular design of the structured catalysts proves readily scalable. Our versatile approach is applicable for other metals and MOF combinations, thus comprising a sustainable and scalable platform for catalytic dehydrogenations and applications in the energy-water nexus.

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.

Analyzing Plant Cell Wall Ultrastructure by Scanning Near-Field Optical Microscopy (SNOM).

The importance of lignocellulosic materials in various research areas including for example biorefinery processes and new biomaterials is constantly rising. Therefore, a detailed knowledge on the macromolecular assembly of plant cell walls is needed. However, despite the tremendous progress in the structural and chemical analysis of plant cell walls there are still uncertainties concerning the ultrastructure. This is either due to Rayleigh criterion based limitations of the analytical methods or the relatively low chemical information gained with high spatial resolution techniques. In this chapter scanning near field optical microscopy (SNOM) is presented as a powerful tool for the subdiffraction limited chemical and structural characterization of plant cell walls.

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.

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.