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Purpose The purpose of this study is to quantify the vertical shrinkage rates and the mechanical strength of three-dimensional (3D) printed parts for a variety of wood-based materials for liquid deposition modeling. Design/methodology/approach The overall hypothesis was that a well-chosen combination of binders, fibers and fillers could reduce shrinkage in the Z dimension and increase compressive and flexural strength (DIN 52185, 52186). To test this assumption, eight sub-hypotheses were formulated. Mixtures of the ingredients were chosen in different ratios to measure the performance of prints. For time efficiency, an iterative heuristic approach was used – not testing all variations of all variables in even increments, but cutting off lines of testing when mixtures were clearly performing poorly. Findings The results showed that some mixtures had high dimensional accuracy and strength, while others had neither, and others had one but not the other. Shrinkage of 3D printed objects was mainly caused by water release during drying. An increase of the wood as well as the cement, sand, salt and gypsum content led to reduced vertical shrinkage, which varied between 0 and 23%. Compressive and flexural strength showed mixed trends. An increase in wood and salt content worsened both strength properties. The addition of fibers improved flexural, and the addition of cement improved compression strength. The highest strength values of 14 MPa for compressive and 8 MPa for flexural strength were obtained in the test series with gypsum. Originality/value This paper is an important milestone in the development of environmentally friendly materials for additive manufacturing. The potential of many ingredients to improve physical properties could be demonstrated.

The material properties of wood are intimately tied to the amount of moisture contained in the wood cell walls. The moisture content depends on the environmental conditions, i.e. temperature and relative humidity, but also on material characteristics of the wood itself. The exact mechanisms governing moisture equilibrium between wood cell walls and environmental conditions remain obscure, likely because multiple material characteristics have been proposed to be involved. In this study, we used a data exploration approach to illuminate the important wood characteristics determining the cell wall moisture content in the full moisture range. Specimens of nine different wood species (two softwoods and seven hardwoods) were examined in terms of their material characteristics at multiple scales and their cell wall moisture content was measured in equilibrium with both hygroscopic conditions and at water-saturation. By statistical analysis, the chemical composition was found to be the most important predictor of the cell wall moisture content in the full moisture range. For the other wood characteristics the importance differed between the low moisture range and the humid and saturated conditions. In the low moisture range, the cellulose crystallinity and hydroxyl accessibility were found to be important predictors, while at high moisture contents the microfibril orientation in the S1 and S3 layers of the cell walls was important. Overall, the results highlighted that no single wood characteristic were decisive for the cell wall moisture content, and each of the predictors identified by the analysis had only a small effect in themselves on the cell wall moisture content. Wood characteristics with a major effect on the cell wall moisture content were, therefore, not identified., Cellulose, 30 (3), ISSN:1572-882X, ISSN:0969-0239

The increased risk of fracture in the elderly associated with metabolic conditions like osteoporosis poses a significant strain on health care systems worldwide. Due to bone's hierarchical nature, it is necessary to study its mechanical properties and failure mechanisms at several length scales. We conducted micropillar compression experiments on ovine cortical bone to assess the anisotropic mechanical response at the lamellar scale over a wide range of strain rates (10−4 to 8·102 s−1). At the microscale, lamellar bone exhibits a strain rate sensitivity similar to what is reported at the macroscale suggesting that it is an intrinsic property of the extracellular matrix. Significant shear band thickening was observed at high strain rates by HRSEM and STEM imaging. This is likely caused by the material's inability to accommodate the imposed deformation by propagation of thin kink bands and shear cracks at high strain rates, leading to shear band thickening and nucleation. The post-yield behavior is strain rate and direction dependent: hardening was observed for transverse oriented micropillars and hardening modulus increases with strain rate by a factor of almost 2, while axially oriented micropillars showed strain softening and an increase of the softening peak width and work to ultimate stress as a function of strain rate. This suggests that for compression at the micrometer scale, energy absorption in bone increases with strain rate. This study highlights the importance of investigating bone strength and post-yield behavior at lower length scales, under hydrated conditions and at clinically relevant strain rates. Statement of significance We performed micropillar compression experiments of ovine cortical bone at two different orientations and over seven orders of magnitude of strain rate. Experiments were performed under humid condition to mimic the natural conditions of bone in a human body using a newly developed micro-indenter setup. The strain rate sensitivity was found to be of a similar magnitude to what has been reported for higher length scales, suggesting that the strain rate sensitivity is an intrinsic property of the bone extracellular matrix. In addition, localized shear deformation in thick bands was observed for the first time at high strain rates, highlighting the importance of investigating bone under conditions representative of an accident or fall at several length scales., Acta Biomaterialia, 131, ISSN:1742-7061, ISSN:1878-7568

The application of adhesives in modern timber engineering often introduces moisture into the wood, leading to permanent residual stresses after hardening. This paper proposes a novel approach to assess these residual stresses by using wooden bilayers as a reporter system. For thin bilayers, moisture-induced stresses lead to pronounced visible flexion that can be used to identify the stress-driving parameters of the adhesive’s gelation process. These parameters depend solely on the wood/adhesive combination and are inversely determined by fitting a finite element method model on the experimentally obtained flexion state. In a subsequent step, the determined parameters are used to calculate the residual stresses in the adhesive bondline of cross-laminated timber plates, emphasizing this approach’s scale independence and general applicability to larger scale structures. All combinations of European beech and Norway spruce with the adhesives Melamine–Urea–Formaldehyde (MUF), Phenol–Resorcinol–Formaldehyde (PRF), and Polyurethane (PUR) were investigated., Wood Science and Technology, 56 (5), ISSN:0043-7719, ISSN:1432-5225

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., Carbohydrate Polymers, 296, ISSN:0144-8617, ISSN:1879-1344

Background and Aims Fruit pedicels have to deal with increasing loads after pollination due to continuous growth of the fruits. Thus, they represent interesting tissues from a mechanical as well as a developmental point of view. However, only a few studies exist on fruit pedicels. In this study, we unravel the anatomy and structural–mechanical relationships of the pedicel of Adansonia digitata, reaching up to 90 cm in length. Methods Morphological and anatomical analyses included examination of stained cross-sections from various positions along the stalk as well as X-ray microtomography and scanning electron microscopy. For mechanical testing, fibre bundles derived from the mature pedicels were examined via tension tests. For establishing the structural–mechanical relationships, the density of the fibre bundles as well as their cellulose microfibril distribution and chemical composition were analysed. Key Results While in the peduncle the vascular tissue and the fibres are arranged in a concentric ring-like way, this organization shifts to the polystelic structure of separate fibre bundles in the pedicel. The polystelic pedicel possesses five vascular strands that consist of strong bast fibre bundles. The fibre bundles have a Young’s modulus of up to 5 GPa, a tensile strength of up to 400 MPa, a high density (>1 g cm−3) and a high microfibril angle of around 20°. Conclusions The structural arrangement as well as the combination of high density and high microfibril angle of the bast fibre bundles are probably optimized for bearing considerable strain in torsion and bending while at the same time allowing for carrying high-tension loads.

Polarized THz time domain spectroscopy was used to study the anisotropic properties of wood-based materials for potential optical elements in the THz range, such as half-wave and quarter-wave plates. Wood samples of different species and sample thickness were studied experimentally showing high birefringence but rather high absorption. We elaborate on two approaches to optimize the optical properties for use as wave plates and assess them based on a figure of merit describing their efficiency as a function of birefringence and absorption. The first approach is to dry the wood samples, which significantly improves the efficiency of wave plates. The second approach is the use of artificially produced cellulose samples using 3D printing and freeze drying techniques, which also show birefringence caused by their similar macroscopic cellulose fibre structure. These materials have the potential as cost effective THz elements that are easy to tailor and produce for use at specific frequencies.

The sensory hairs of the Venus flytrap (Dionaea muscipula Ellis) detect mechanical stimuli imparted by their prey and fire bursts of electrical signals called action potentials (APs). APs are elicited when the hairs are sufficiently stimulated and two consecutive APs can trigger closure of the trap. Earlier experiments have identified thresholds for the relevant stimulus parameters, namely the angular displacement $$\theta $$ θ and angular velocity $$\omega $$ ω . However, these experiments could not trace the deformation of the trigger hair’s sensory cells, which are known to transduce the mechanical stimulus. To understand the kinematics at the cellular level, we investigate the role of two relevant mechanical phenomena: viscoelasticity and intercellular fluid transport using a multi-scale numerical model of the sensory hair. We hypothesize that the combined influence of these two phenomena and $$\omega $$ ω contribute to the flytrap’s rate-dependent response to stimuli. In this study, we firstly perform sustained deflection tests on the hair to estimate the viscoelastic material properties of the tissue. Thereafter, through simulations of hair deflection tests at different loading rates, we were able to establish a multi-scale kinematic link between $$\omega $$ ω and the cell wall stretch $$\delta $$ δ . Furthermore, we find that the rate at which $$\delta $$ δ evolves during a stimulus is also proportional to $$\omega $$ ω . This suggests that mechanosensitive ion channels, expected to be stretch-activated and localized in the plasma membrane of the sensory cells, could be additionally sensitive to the rate at which stretch is applied.

Rubber wood utilization in Nigeria has become a viable alternative because of over-exploitation of more durable species. The use of rubber wood in its native form has some disadvantages including dimensional instability, low durability and susceptibility to weathering in outdoor uses. However, it may be improved through chemical modification such as acetylation. In this study, we revealed the effectiveness of acetylation on the protection of rubber wood against weathering in terms of weight loss (WL), colour change, lignin degradation, and mechanical properties. Acetylation was carried out using two different reaction times to achieve weight percent gains (WPG) of around 7% and 10%. Understanding of the effect of acetylation and subsequent weathering on mechanical properties was enabled by analysing wood chemistry with Fourier transform infrared (FTIR) analysis and Raman spectroscopy. The mechanical tests of weathered unmodified and acetylated rubber wood revealed a decrease of tensile stiffness of the unmodified samples as a function of weathering time, while stiffness was retained for the acetylated samples. Weathered unmodified samples showed a bi-phasic stress-strain pattern with a high strain at breakage indicating a slippage of fibres under stress due to degradation of the middle lamella. This was hardly visible for acetylated samples. Thus, acetylation was shown to be effective for protecting rubber wood used in outdoor conditions.

Chemical modification of wood mainly aims at improving dimensional stability, resistance to biodeterioration and surface degradation. In some cases, it is specifically targeted at adapting or improving mechanical performance. However, a general understanding of the effects of chemical modification on the mechanical properties of wood, which would facilitate more efficient modification strategies, is missing. Here, a combined experimental and simulation study is provided to gain a more general understanding of the mechanical behaviour of chemically modified wood. In the first part of this study, the mechanical properties of chemically modified Norway spruce are studied experimentally. In Mora Mendez et al. (Wood Sci Technol 2019), simulations of different types of chemical modifications will be presented using a multi-scale model and the outcome will be compared with the obtained experimental data. Chemical modification was based on a two-step modification process. The first step involved methacrylation of the OH-groups in the cell wall. In the second step, in situ polymerization of styrene was induced in the methacrylated samples, which resulted in a partial cell wall and lumen filling. Tensile stiffness and rolling shear stiffness were analysed for methacrylated and polymerized samples. Whereas only small changes in mechanical properties were found for methacrylated samples, the polymerization process led to pronounced increases in elastic modulus and shear stiffness because of weight percent gains of 60–95%. Yet, the specific stiffness was lowered, as the density increase was disproportionate to the stiffness increase. Moreover, a pronounced improvement in rolling shear modulus (GRT) by a factor of 4.5 was obtained for the in situ polymerized specimen.

For robotic fabrication of wooden structures, the simple, quick and tight joining of elements can be solved using swelling hardwood dowels. This topic has been the focus of the present study, and the set-recovery capacity of densified wood (dW) as dowel material was investigated. European beech was compressed in the radial direction at 103°C and 10% moisture content (MC) to a compression ratio of 40%. Multiple swelling and shrinkage cycles were applied to measure swelling behavior, swelling pressure development and combined swelling and creep under compressive loading. It has been demonstrated that dW shows increased swelling and more persisting swelling pressures than native wood (nW). The set-recovery prevents significant contact-stress relaxation over multiple cycles of MC change. Application as a structural joining element for robotic fabrication was studied by shear lap joint tests on round double-dovetail swelling dowels.

Rubberwood is widely planted for latex production. At the cessation of latex yield it becomes a viable timber source in the wood industry. While good bonding performance of rubberwood has been reported, quantitative information to support this statement is missing. In this study, the tensile shear strength (TSS) and wood failure percentage (WFP) of unmodified and acetylated rubberwood in both wet and dry conditions were examined. Three frequently used adhesives were selected: one-component polyurethane (1C PUR), melamine-urea-formaldehyde (MUF), and phenol-resorcinol-formaldehyde (PRF). Furthermore, fatigue behaviour was analysed for PRF-bonded samples by cyclic loading. Results showed that in dry state, the TSS and WFP of acetylated rubberwood is comparable to the unmodified samples for all adhesives. In wet condition, the performance of bonded rubberwood was improved by acetylation. Cyclic loading revealed comparable fatigue behaviour of bonded unmodified and acetylated rubberwood. This investigation provides first quantitative information on the performance of bonded rubberwood, which can be a valuable input for the production of laminated timber products for structural purposes., European Journal of Wood and Wood Products, 79 (1), ISSN:0018-3768, ISSN:1436-736X

Violin varnishes influence the vibrational properties of tonewood. However, the frequency dependence of the varnish influence and mechanical properties of typical varnishes has received little attention. The viscoelastic properties of various violin varnish materials over the audible frequency range were characterized by dynamic mechanical analysis. The properties of the studied varnishes showed comparable frequency dependencies. For all varnishes, E increased and tan(δ) decreased with increasing frequency. The results were in good agreement with an analytical mechanical model, which was used for additional numerical FEM calculations. The approach of numerically determining varnish-induced changes in the vibrational properties on basis of the individual wood and varnish properties was confirmed through comparison with experimental results obtained in an earlier study. The latter procedure was subsequently used to analyse varnish-induced changes in the eigenfrequencies of a violin soundboard. The results revealed that the frequency dependence of the varnish properties determined the specific influence of varnishes on the vibrational properties of tonewood, which should be taken into account when assessing the impact of varnishes., Results in Materials, 9

The bending of wooden bilayers in response to the change of relative humidity can be utilized to build autonomous actuation devices. For bending, the properties of the adhesive may play a crucial role in terms of humidity transfer and mechanical performance. Hence, the type of the adhesive may have an impact on the bilayer responsiveness upon humidity changes. In this study, bilayers with various sizes were fabricated and glued with five different adhesives. The same samples were exposed at first to a single desorption phase for 48 h and in a second experiment to ten de-/adsorption cycles with a cycle length of 24 h. The initial moisture contents (MC) of the bilayers differed depending on the type of adhesive. Due to the additional water uptake of the wood layers bonded with water introducing adhesives (PVAC, MUF and PRF), these bilayers had a higher initial MC than those bonded with non-water introducing adhesives (PUR and Epoxy). During the de-/adsorption cycles, the overall amplitude of bending stayed relatively constant. However, the start- and the endpoints of the movement were shifted in the first three cycles, since here, the cycle length was not sufficiently long to reach an equilibrated MC state. However, after this initial adjustment phase, the bending of the wooden bilayers was reversible and repeatable. No significant differences in the kinetics of bilayers manufactured with the different adhesive systems could be observed. Thus, the specific adhesive properties do not seem to influence the responsiveness and performance of the wooden bilayers.

The carnivorous Venus flytrap catches prey by an ingenious snapping mechanism. Based on work over nearly 200 years, it has become generally accepted that two touches of the trap’s sensory hairs within 30 s, each one generating an action potential, are required to trigger closure of the trap. We developed an electromechanical model, which, however, suggests that under certain circumstances one touch is sufficient to generate two action potentials. Using a force-sensing microrobotic system, we precisely quantified the sensory-hair deflection parameters necessary to trigger trap closure and correlated them with the elicited action potentials in vivo. Our results confirm the model’s predictions, suggesting that the Venus flytrap may be adapted to a wider range of prey movements than previously assumed., It is generally accepted that two touches of the Venus flytrap’s sensory hairs within 30 seconds are required to trigger closure of the trap. Here, however, quantification of the plant’s sensory hair deflection parameters reveals that one stimulus is sufficient.

Light affects many aspects of cell development. Tomato seedlings growing at different light qualities (white, blue, green, red, far-red) and in the dark displayed alterations in cell wall structure and composition. A strong and negative correlation was found between cell wall thickness and hypocotyl growth. Cell walls was thicker under blue and white lights and thinner under far-red light and in the dark, while intermediate values was observed for red or green lights. Additionally, the inside layer surface of cell wall presented random deposited microfibrillae angles under far-red light and in the dark. However, longitudinal transmission electron microscopy indicates a high frequency of microfibrils close to parallels related to the elongation axis in the outer layer. This was confirmed by ultra-high resolution small angle X-ray scattering. These data suggest that cellulose microfibrils would be passively reoriented in the longitudinal direction. As the cell expands, the most recently deposited layers (inside) behave differentially oriented compared to older (outer) layers in the dark or under FR lights, agreeing with the multinet growth hypothesis. High Ca and pectin levels were found in the cell wall of seedlings growing under blue and white light, also contributing to the low extensibility of the cell wall. Low Ca and pectin contents were found in the dark and under far-red light. Auxins marginally stimulated growth in thin cell wall circumstances. Hypocotyl growth was stimulated by gibberellins under blue light.

For the performance of wood as a building material, its dimensional changes in response to alterations of relative humidity are commonly perceived as an adverse effect. Recently, this material inherent property has been proposed to be utilized in a smart way. Employing the bilayer principle, controlled and reversible shape changes in response to changes of relative humidity were demonstrated. Wood naturally inherits a unique combination of material properties specifically suitable for large-scale shape-changing parts. While being environmentally responsive, it offers high mechanical stiffness throughout shape-change, ease of machining and working, and sustainable availability in large sizes and quantities. In this study, we demonstrate design principles for achieving a range of shape changing patterns such as uni- and bi-directional surface curvature of wood and wood-hybrid bilayers with both negative (hyperboloid curvature) and positive Gaussian curvature (spherical curvature). In parallel, we have developed suitable joints to join multiple elements to facilitate upscaling in length and width while maintaining shape-change. The ability to design and control the type and magnitude of curvature for specific sizes, shapes, and aspect ratios open the opportunity for a new class of large-scale weather responsive elements and self-forming building components.

Climate Adaptive Building Shells can help to reach the aimed and required global reduction of energy consumption in the building sector. By being implemented as, e.g., facade shading systems, their adaptability to environmental changes can improve energy efficiency and indoor comfort of buildings. Autonomous, humidity-driven wood bilayers are proposed as an alternative to motor-driven facade shading elements. Due the hygro-responsiveness of the wood material, the changes of relative humidity during day and night as well as the drying effect of direct solar radiation can be utilized for inducing cyclic programmed shape changes of wood bilayers for aperture opening and closing of such adaptive facade shading systems. The kinetics of such autonomous shape changes of wood bilayers have been analyzed at small scale, but the application-relevant upscaling remains a challenge. So far, the proposed solutions do not allow maintaining a sufficiently high rate of shape change and the required mechanical stability of the wood bilayers at the same time. Here, we present the coupling of two wood bilayers as one possible solution for the implementation as shading elements. By coupling, the rate of aperture opening and closing can be amplified without increasing the (limited) rate of shape change of the single wood bilayers. The coupling and the resulting combination of shape change and rotation are characterized for five different typologies of lever arm configuration. Based on first studies with bilayer strips, an upscaling in width is conducted and its implementation for a humidity-driven shading system is presented by means of a demonstrator.

Transmission terahertz (THz) spectroscopy on heterogeneous materials often shows structured spectra due to interference of waves passing through the object along different optical paths. Of special interest is wood with its strong birefringence and pronounced orientation-dependent structure. In this paper, we show that at specific polarization angles, the THz transmission spectra show gaps at distinct frequencies. The frequency changes of these gaps can be used for a fast and accurate determination of the dependence of birefringence on humidity. We present results for spruce and beech with different sample thicknesses and orientations, and study and model their optical properties as a function of water content.

Wood is a fiber-reinforced, orthotropic composite material, which shows complex anisotropic optical characteristics induced by the preferential orientation of the cellulose microfibrils in the THz domain. We hypothesize that in addition to their birefringent properties the helical arrangement of microfibrils could also exhibit optical activity. This would allow determining microfibril angles in wood, a key feature for its mechanical properties. We propose to detect optical activity and birefringence independently using THz time-domain spectroscopy (THz-TDS) despite interference with annual ring structure. Our procedure is based on the Jones calculus formalism. In this report, we demonstrate the ability to detect left-and right-handed chirality of prototype helical structures made of copper wires and birefringence of spruce wood.

The carnivorous Venus flytrap catches prey by an ingenious snapping mechanism. Based on work over the past 190 years, it has become generally accepted that two touches of the trap’s sensory hairs within 30 seconds, each one generating an action potential, are required to trigger closure of the trap. We developed an electromechanical model which, however, suggests that under certain circumstances one touch is sufficient to generate two action potentials. Using a force-sensing microrobotics system, we precisely quantified the sensory hair deflection parameters necessary to trigger trap closure, and correlated them with the elicited action potentials in vivo. Our results confirm the model’s predictions, suggesting that the Venus flytrap may be adapted to a wider range of prey movement than previously assumed.

Modifications alter hygro-mechanical properties of wood in non-trivial ways that depend on modification treatment and wood microstructure. Generic micromechanical models with modifications on the cellular scale of spruce are proposed and studied, such as partial and entire lumen filling with isotropic materials, as well as modification of S2-layer properties. Based on a hierarchical micromechanical model, hygro-mechanical response surfaces of the modified, orthotropic material are predicted. Simulation results are compared to experimental data. The findings can be used for optimizing modification treatments, as well as for calculating the behavior in graded situations, common to treatments with limited modification depth.

Based on a combination of micropillar compression experiments and modelling of the secondary cell wall (cw) using continuum micromechanics, the shear yield stress of the polymer matrix is identified for both normal and compression wood of Norway spruce. It is shown that the model is able to capture the differences in mechanical properties between the two tissues based on the knowledge of composition of the samples, microfibril angle, as well as phase properties on the nanometer scale. By testing an isolated piece of the cell wall with a homogeneous and uniaxial stress field on the micrometer scale and using the micromechanical model to determine average stress fields on the nanometer scale, it is possible to identify the shear yield stress of the polymer matrix in wood, which was found to be in the range of 14.9–17.5 MPa for normal and compression wood. It was shown that this corresponds to a stress in the lignin phase of approx. 17 MPa. This combined study thus demonstrates a new approach for validating ...

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.

International audience; Main conclusion CAD-deficient poplars enabled studying the influence of altered lignin composition on mechanical properties. Severe alterations in lignin composition did not influence the mechanical properties. Wood represents a hierarchical fiber-composite material with excellent mechanical properties. Despite its wide use and versatility, its mechanical behavior has not been entirely understood. It has especially been challenging to unravel the mechanical function of the cell wall matrix. Lignin engineering has been a useful tool to increase the knowledge on the mechanical function of lignin as it allows for modifications of lignin content and composition and the subsequent studying of the mechanical properties of these transgenics. Hereby, in most cases, both lignin composition and content are altered and the specific influence of lignin composition has hardly been revealed. Here, we have performed a comprehensive micromechanical, structural, and spectroscopic analysis on xylem strips of transgenic poplar plants, which are downregulated for cinnamyl alcohol dehydrogenase (CAD) by a hairpin-RNA-mediated silencing approach. All parameters were evaluated on the same samples. Raman microscopy revealed that the lignin of the hpCAD poplars was significantly enriched in aldehydes and reduced in the (relative) amount of G-units. FTIR spectra indicated pronounced changes in lignin composition, whereas lignin content was not significantly changed between WT and the hpCAD poplars. Microfibril angles were in the range of 18A degrees-24A degrees and were not significantly different between WT and transgenics. No significant changes were observed in mechanical properties, such as tensile stiffness, ultimate stress, and yield stress. The specific findings on hpCAD poplar allowed studying the specific influence of lignin composition on mechanics. It can be concluded that the changes in lignin composition in hpCAD poplars did not affect the micromechanical tensile properties.

The mechanical properties of walnut (Juglans regia L.) and cherry (Prunus avium L.) woods, as frequent raw materials in cultural heritage objects, have been investigated as a function of the anatomical directions and the moisture content (MC). The strength data are decreasing with increasing MC, whereas the tensile strength in the longitudinal direction is higher by factors of 1.5–2 compared to the compression strength. Moreover, the inequality of tensile and compressive stiffness is discussed, which is a matter of debate since a long time. This so-called bimodular behavior is difficult to describe in a generalized mode due to the high data variability if tension and compression properties are analyzed on different samples. If tensile and compressive stiffness tests are performed on the same samples of walnut and cherry wood, the ratio between these properties is significantly higher than 1., Holzforschung, 72 (1), ISSN:0018-3830, ISSN:1437-434X

Xylan is one of the main compounds determining wood properties in hardwood species. The xylan backbone is thought to be synthesized by a synthase complex comprising two members of the GT43 family. We downregulated all GT43 genes in hybrid aspen (Populus tremula × tremuloides) to understand their involvement in xylan biosynthesis. All three clades of the GT43 family were targeted for downregulation using RNA interference individually or in different combinations, either constitutively or specifically in developing wood. Simultaneous downregulation in developing wood of the B (IRX9) and C (IRX14) clades resulted in reduced xylan Xyl content relative to reducing end sequence, supporting their role in xylan backbone biosynthesis. This was accompanied by a higher lignocellulose saccharification efficiency. Unexpectedly, GT43 suppression in developing wood led to an overall growth stimulation, xylem cell wall thinning and a shift in cellulose orientation. Transcriptome profiling of these transgenic lines indicated that cell cycling was stimulated and secondary wall biosynthesis was repressed. We suggest that the reduced xylan elongation is sensed by the cell wall integrity surveying mechanism in developing wood. Our results show that wood-specific suppression of xylan-biosynthetic GT43 genes activates signaling responses, leading to increased growth and improved lignocellulose saccharification.

Wood exhibits anisotropic swelling and shrinking upon changes of wood moisture content (MC). By manufacturing bi-layered structures with adapted grain orientation in the two bonded layers, humidity-driven actuators are generated, which have the potential to be used for autonomous climate adaptive building with tile. The present study deals with design principles for upscaling the size of the bilayers and for increasing the rate of MC change and, thus, rate of shape change. Wood bilayers with widths of up to half a meter were subjected to changes of relative humidity (RH). Moisture and curvature changes were recorded. Bilayers with different widths showed curvature exclusively along their length. Next to this, the performance was compared between bilayers with and without milled-in grooves. These grooves lead to shorter diffusion paths along fibre direction for increasing the rate of MC change. The highest rates of MC change were visible for the samples with the smallest width within the first hours after change of RH. Later on, all samples showed similar rates. The milling of grooves increased the moisture change rate substantially compared to the non-milled samples resulting in a higher rate of curvature change. The increase is especially pronounced for cyclic changes of RH. This study shows that, by applying material specific design principles, the shape change of wood bilayers can be adapted and the rate of the MC change can be increased by keeping diffusion paths short along fibre direction. These principles may facilitate the use of large-scale wood bilayers as lamellae in shading systems.

In this study, several theoretical models to numerically estimate shear properties of orthotropic materials are introduced. These approaches are based on the combination of Hankinson’s empirically derived formula with other empirical and analytical calculations. Next to shear moduli, which are estimated from the elastic moduli and Poisson’s ratios, shear strengths are also estimated from the in-axis strengths. The models are validated by mechanical tests on walnut wood (Juglans regia L.), for which a sufficient data set can be found in literature. The Arcan test is used to estimate the shear moduli, while the shear block test is used to estimate the shear strengths. The results show that the model, which is based on a combined use of Hankinson’s formula and tensor rotation, gives the best estimation of shear moduli as evaluated by the minimum differences to the experimentally obtained results. For the shear strengths, a combination of Hankinson’s formula and Norris’ failure criterion shows the best agreement in comparison to the experimental data. The theoretical calculations may be used for a time efficient estimation of shear modulus and strength in comparison to the very time-consuming experimental estimation.

SummaryResearch conductedThe phytohormone ethylene impacts secondary stem growth in plants by stimulating cambial activity, xylem development and fiber over vessel formation. Here we report the effect of ethylene on secondary cell wall formation and the molecular connection between ethylene signaling and wood formation.MethodsWe applied exogenous ethylene or its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) to wild type and ethylene insensitive hybrid aspen trees(Populus tremula x tremuloides)and studied secondary cell wall anatomy, chemistry and ultrastructure. We furthermore analyzed the transcriptome (RNA Seq) after ACC application to wild type and ethylene insensitive trees.Key resultsWe demonstrate that ACC and ethylene induce gelatinous-layers (G-layers) and alter the fiber cell wall cellulose microfibril angle. G-layers are tertiary wall layers rich in cellulose, typically found in tension wood of aspen trees. A vast majority of transcripts affected by ACC are downstream of ethylene perception and include a large number of transcription factors (TFs). Motif-analyses reveal potential connections between ethylene TFs (ERFs, EIN3/EIL1) and wood formation.ConclusionG-layer formation upon ethylene application suggests that the increase in ethylene biosynthesis observed during tension wood formation is important for its formation. Ethylene-regulated TFs of the ERF and EIN3/EIL1 type could transmit the ethylene signal.

Summary Lignin engineering is a promising tool to reduce the energy input and the need of chemical pre-treatments for the efficient conversion of plant biomass into fermentable sugars for downstream applications. At the same time, lignin engineering can offer new insight into structure-function relationships of plant cell walls by combined mechanical, structural and chemical analyses. Here, such a comprehensive approach was applied on poplar trees (Populus tremula x Populus alba) downregulated for CINNAMYL ALCOHOL DEHYDROGENASE (CAD) for gaining insight into the impact of lignin reduction on mechanical properties. Downregulation of CAD resulted in a significant decrease in both elastic modulus and yield stress. Since wood density and cellulose microfibril angle did not show any significant differences between the wild type and the transgenic lines, these structural features could be excluded as influencing factors. Fourier transform infrared spectroscopy (FTIR) and Raman imaging were performed to elucidate changes in the chemical composition directly on the mechanically tested tissue sections. Lignin content was identified as a mechanical relevant factor since a correlation with a coefficient of determination (r²) of 0.65 between lignin absorbance (as an indicator of lignin content) and tensile stiffness was found. A comparison of the present results with those of previous investigations shows that the mechanical impact of lignin alteration under tension stress depends on certain structural conditions such as a high cellulose microfibril angle, which stresses the complex relationship between chemistry and mechanical properties in plant cell walls. This article is protected by copyright. All rights reserved.

Background Lignocellulose from fast growing hardwood species is a preferred source of polysaccharides for advanced biofuels and “green” chemicals. However, the extensive acetylation of hardwood xylan hinders lignocellulose saccharification by obstructing enzymatic xylan hydrolysis and causing inhibitory acetic acid concentrations during microbial sugar fermentation. To optimize lignocellulose for cost-effective saccharification and biofuel production, an acetyl xylan esterase AnAXE1 from Aspergillus niger was introduced into aspen and targeted to cell walls. Results AnAXE1-expressing plants exhibited reduced xylan acetylation and grew normally. Without pretreatment, their lignocellulose yielded over 25% more glucose per unit mass of wood (dry weight) than wild-type plants. Glucose yields were less improved (+7%) after acid pretreatment, which hydrolyses xylan. The results indicate that AnAXE1 expression also reduced the molecular weight of xylan, and xylan–lignin complexes and/or lignin co-extracted with xylan, increased cellulose crystallinity, altered the lignin composition, reducing its syringyl to guaiacyl ratio, and increased lignin solubility in dioxane and hot water. Lignin-associated carbohydrates became enriched in xylose residues, indicating a higher content of xylo-oligosaccharides. Conclusions This work revealed several changes in plant cell walls caused by deacetylation of xylan. We propose that deacetylated xylan is partially hydrolyzed in the cell walls, liberating xylo-oligosaccharides and their associated lignin oligomers from the cell wall network. Deacetylating xylan thus not only increases its susceptibility to hydrolytic enzymes during saccharification but also changes the cell wall architecture, increasing the extractability of lignin and xylan and facilitating saccharification. Electronic supplementary material The online version of this article (doi:10.1186/s13068-017-0782-4) contains supplementary material, which is available to authorized users.

Structural hierarchy, heterogeneity, and an adapted interfacial design are hallmarks of biological materials and have been exploited in engineering to optimize the mechanical response of composite materials used in load-bearing applications. In this chapter, we first revisit the exquisite hierarchical structure of plants, in particular the palm tree, to show how multiscale gradients provide an elegant means to optimize the stress distribution and architecture of load-bearing, resilient biological materials. We then turn our attention to synthetic heterogeneous composites that harness the concept of graded designs to enable mechanical coupling between hard and soft components of interest in the emerging field of stretchable electronics. By discussing similarities and disparities between these selected natural and artificial systems, a critical analysis on the benefits and challenges in the field of biomimetic materials research is presented. While synthetic systems remain far from reaching the level of architectural control found in natural materials, recently developed fabrication technologies provide encouraging demonstrations that suggest a promising future for biologically inspired advanced materials.

The orientation distribution of cellulose microfibrils in the plant cell wall is a key parameter for understanding anisotropic plant growth and mechanical behavior. However, precisely visualizing cellulose orientation in the plant cell wall has ever been a challenge due to the small size of the cellulose microfibrils and the complex network of polymers in the plant cell wall. X-ray diffraction is one of the most frequently used methods for analyzing cellulose orientation in single cells and plant tissues, but the interpretation of the diffraction images is complex. Traditionally, circular or square cells and Gaussian orientation of the cellulose microfibrils have been assumed to elucidate cellulose orientation from the diffraction images. However, the complex tissue structures of common model plant systems such as Arabidopsis or aspen (Populus) require a more sophisticated approach. We present an evaluation procedure which takes into account the precise cell geometry and is able to deal with complex microfibril orientation distributions. The evaluation procedure reveals the entire orientation distribution of the cellulose microfibrils, reflecting different orientations within the multi-layered cell wall. By analyzing aspen wood and Arabidopsis stems we demonstrate the versatility of this method and show that simplifying assumptions on geometry and orientation distributions can lead to errors in the calculated microfibril orientation pattern. The simulation routine is intended to be used as a valuable tool for nanostructural analysis of plant cell walls and is freely available from the authors on request.

In view of the aimed transition toward sustainable societies, wood has a great future potential in being a renewable and carbon-storing resource, as well as due to its peculiar structural multiscale features and excellent properties. A useful approach for developing wood or wood-based materials with improved properties or new functionalities is to combine bioinspired materials science and wood materials research. For this, recent developments in polymer chemistry and nanotechnology can be used to transfer and apply the basic principles and mechanisms from nature for the fabrication of hybrid wood materials. In this chapter, we review the recent developments in the field and provide examples of our own work on wood materials synthesis and characterization, which illustrate the general concept and its future potential.

Palms can grow at sites exposed to high winds experiencing large dynamic wind and gust loads. Their stems represent a system of stiff fibrous elements embedded in the soft parenchymatous tissue. The proper design of the interface of the stiffening elements and the parenchyma is crucial for the functioning of the stem. The strategy of the palm to compromise between stiff fibre caps and the soft parenchymatous tissue may serve as a model system for avoiding stress discontinuities in inhomogeneous and anisotropic fibre-reinforced composite materials. We investigated the mechanical, structural and biochemical properties of the fibre caps of the palm Washingtonia robusta at different levels of hierarchy with high spatial resolution. A gradual decrease in stiffness across the fibre cap towards the surrounding parenchymatous tissue was observed. Structural adaptations at the tissue level were found in terms of changes in cell cross sections and cell wall thickness. At the cell wall level, gradients across the fibre cap were found in the degree of orientation of the microfibrils and in the lignin level and composition. The impact of these structural variations in the local material stiffness distribution is discussed.

Biological composites are typically based on an adhesive matrix that interlocks rigid reinforcing elements in fiber composite or brick-and-mortar assemblies. In nature, the adhesive matrix is often made up of proteins, which are also interesting model systems, as they are unique among polymers in that we know how to engineer their structures with atomic detail and to select protein elements for specific interactions with other components. Here we studied how fusion proteins that consist of cellulose binding proteins linked to proteins that show a natural tendency to form multimer complexes act as an adhesive matrix in combination with nanofibrillated cellulose. We found that the fusion proteins are retained with the cellulose and that the proteins mainly affect the plastic yield behavior of the cellulose material as a function of water content. Interestingly, the proteins increased the moisture absorption of the composite, but the well-known plastifying effect of water was clearly decreased. The work helps to understand the functional basis of nanocellulose composites as materials and aims toward building model systems for molecular biomimetic materials.

Implementing programmable actuation into materials and structures is a major topic in the field of smart materials. In particular the bilayer principle has been employed to develop actuators that respond to various kinds of stimuli. A multitude of small scale applications down to micrometer size have been developed, but up-scaling remains challenging due to either limitations in mechanical stiffness of the material or in the manufacturing processes. Here, we demonstrate the actuation of wooden bilayers in response to changes in relative humidity, making use of the high material stiffness and a good machinability to reach large scale actuation and application. Amplitude and response time of the actuation were measured and can be predicted and controlled by adapting the geometry and the constitution of the bilayers. Field tests in full weathering conditions revealed long-term stability of the actuation. The potential of the concept is shown by a first demonstrator. With the sensor and actuator intrinsically incorporated in the wooden bilayers, the daily change in relative humidity is exploited for an autonomous and solar powered movement of a tracker for solar modules., PLoS ONE, 10 (4), ISSN:1932-6203

Background X-ray scattering is a well-established method for measuring cellulose microfibril angles in secondary cell walls. However, little data is available on the much thinner primary cell walls. Here, we show that microfibril orientation distributions can be determined by small angle X-ray scattering (SAXS) even in primary cell walls. The technique offers a number of advantages: samples can be analyzed in the native hydrated state without any preparation which minimizes the risk of artifacts and allows for fast data acquisition. The method provides data averaged over a specimen region, determined by the size of the used X-ray beam and, thus, yields the microfibril orientation distribution within this region. Results Cellulose microfibril orientation distributions were obtained for single cells of the alga Chara corallina, as well as for the multicellular hypocotyl of Arabidopsis thaliana. In both, Chara and Arabidopsis, distributions with a broad scattering around mean microfibril angles of approximately 0° and 90° towards the longitudinal axis of the cells were found. Conclusions With SAXS, the structure of primary cell walls can be analysed in their native state and new insights into the cellulose microfibril orientation of primary cell walls can be gained. The data shows that SAXS can serve as a valuable tool for the analysis of cellulose microfibril orientation in primary cell walls and, in consequence, add to the understanding of its mechanical behaviour and the intriguing mechanisms behind cell growth., Plant Methods, 10, ISSN:1746-4811