The complex structure of wood, one of the most abundant biomaterials on Earth, has been optimized over 270 million years of tree evolution. This optimization has led to the highly efficient water and nutrient transport, mechanical stability and durability of wood. The unique material structure and pronounced anisotropy of wood endows it with an array of remarkable properties, yielding opportunities for the design of functional materials. In this Review, we provide a materials and structural perspective on how wood can be redesigned via structural engineering, chemical and/or thermal modification to alter its mechanical, fluidic, ionic, optical and thermal properties. These modifications enable a diverse range of applications, including the development of high-performance structural materials, energy storage and conversion, environmental remediation, nanoionics, nanofluidics, and light and thermal management. We also highlight advanced characterization and computational-simulation approaches for understanding the structure–property–function relationships of natural and modified wood, as well as informing bio-inspired synthetic designs. In addition, we provide our perspective on the future directions of wood research and the challenges and opportunities for industrialization. The porous hierarchical structure and anisotropy of wood make it a strong candidate for the design of materials with various functions, including load bearing, multiscale mass transport, and optical and thermal management. In this Review, the composition, structure, characterization methods, modification strategies, properties and applications of natural and modified wood are discussed.

Structure-property-function relationships of natural and engineered wood

There has been a rapid growth in research and innovation of bio-based adhesives in the engineered wood product industry. This article reviews the recent research published over the last few decades on the synthesis of bio-adhesives derived from such renewable resources as lignin, starch, and plant proteins. The chemical structure of these biopolymers is described and discussed to highlight the active functional groups that are used in the synthesis of bio-adhesives. The potentials and drawbacks of each biomass are then discussed in detail; some methods have been suggested to modify their chemical structures and to improve their properties including water resistance and bonding strength for their ultimate application as wood adhesives. Moreover, this article includes discussion of techniques commonly used for evaluating the petroleum-based wood adhesives in terms of mechanical properties and penetration behavior, which are expected to be more widely applied to bio-based wood adhesives to better evaluate their prospect for wood composites application.

https://www.mdpi.com/2073-4360/9/2/70/pdf

Bio-Based Adhesives and Evaluation for Wood Composites Application.

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.

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

Recent developments in the conservation of materials properties of historical wood

Non-destructive testing of wood and wood-based materials

To understand better the structure-property relationships of wood in situ, nondestructive synchrotron-based tomographic microscopy (SbTM) with subcellular resolution is useful. In this context, an in situ testing device was developed to determine the cellular response of wood to mechanical loading. Different rotationally symmetric specimens were tested to synchronize the failure areas to the given scanning areas. Norway spruce samples were uniaxially compressed in the longitudinal direction and scanned in situ at several increasing relative forces ending up in the plastic deformation regime. A sufficiently high quality in situ tomography was demonstrated. The reconstructed data allowed the observation of the load-dependent development of failure regions: cracks and buckling on the microstructure were clearly visible. Future investigations with SbTM on different wood species, loading directions, and different moisture contents are promising in terms of the micromechanical behavior of wood.

/pdf/synchrotron-based-tomographic-microscopy-sbtm-of-wood-201cyuoyk6.pdf

Synchrotron-based tomographic microscopy (SbTM) of wood: development of a testing device and observation of plastic deformation of uniaxially compressed Norway spruce samples

Tensile tests of miniature spruce wood specimens have been performed to investigate the damage evolution in wood at the microscopic scale. For this purpose, the samples were stepwise tensile loaded in the longitudinal (L) and radial (R) directions and the damage evolution was monitored in real-time by acoustic emission (AE) and synchrotron radiation micro-computed tomography (SRμCT). This combination is of outstanding benefit as SRμCT monitoring provides an insight on the crack evolution and the final fracture at microscopic scale, whereas AE permits the detection of the associated accumulation and interaction of single damage events on all length scales with high time resolution. A significant drawback of the AE testing of wood has been overcome by means of calibrating the AE amplitudes with the underlying crack length development. Thus, a setup-dependent and wood species-dependent calibration value was estimated, which associates 1 μm 2 crack area generating of 0.0038 mV in the detected AE amplitude. Furthermore, for both L and R specimens, AE signals were classified into two clusters by using a frequency-based approach of unsupervised pattern recognition. The shares of AE signals of both clusters correlate with the ratio of the relative crack area of the interwall and transwall cracks gained from the fractographic analysis of SRμCT scans.

/pdf/damage-evolution-in-wood-synchrotron-radiation-micro-1way24vz68.pdf

Damage evolution in wood: synchrotron radiation micro-computed tomography (SRμCT) as a complementary tool for interpreting acoustic emission (AE) behavior

Micro X-ray computed tomography (XCT) is an emerging technology that has found many applications in biology and the study of materials. Synchrotron-based micro computed tomography has been adopted for the study of adhesive bonding in wood. This paper reviews recent developments of an integrated project that uses micro XCT to assist with modeling of adhesive bonds and to assess the role of cell wall penetration on moisture resistance.  The research includes study of: anatomical features of several commercially important wood species, penetration of three adhesive types into wood, moisture effects on bonding, and mechanical performance of bonds during XCT scanning.

/pdf/micro-x-ray-computed-tomography-of-adhesive-bonds-in-wood-3f7ua391vp.pdf

Micro x-ray computed tomography of adhesive bonds in wood

The lasting plastic deformation of the cellular elements of beech, fir, and spruce wood under uniaxial compression has been investigated by the combination of in situ loading and synchrotron micro-computed tomography. The deformation of singular elements embedded in the tissue and their influence on the deformation lines and surrounding tissue was examined by tomographic reconstructions. An automatic observation of the failure area was applied in the case of the softwoods, which permitted the determination of the densification degree. The development of failure lines differed from the expected pattern by often showing branching. However, the 3D observations confirmed the effects known from 2D examinations in many cases.

/pdf/failure-and-failure-mechanisms-of-wood-during-longitudinal-22mve09irs.pdf

Failure and failure mechanisms of wood during longitudinal compression monitored by synchrotron micro-computed tomography

Wood-based composites hold the promise of sustainable construction. Understanding the influence on wood cellular microstructure in the macroscopic mechanical behavior is key for engineering high-performance composites. In this work, we report a novel Individual Cell Tracking (ICT) approach for in-situ quantification of nanometer-scale deformations of individual wood cells during mechanical loading of macroscopic millimeter-scale wood samples. Softwood samples containing > 104 cells were subjected to controlled radial tensile and longitudinal compressive load in a synchrotron radiation micro-computed tomography (SRµCT) setup. Tracheid and wood ray cells were automatically segmented, and their geometric variations were tracked during load. Finally, interactions between microstructure deformations (lumen geometry, cell wall thickness), cellular arrangement (annual growth rings, anisotropy, wood ray presence) with the macroscopic deformation response were investigated. The results provide cellular insight into macroscopic relations, such as anisotropic Poisson effects, and allow direct observation of previously suspected wood ray reinforcing effects. The method is also appropriate for investigation of non-linear deformation effects, such as buckling and deformation recovery after failure, and gives insight into less studied aspects, such as changes in lumen diameter and cell wall thickness during uniaxial load. ICT provides an experimental tool for direct validation of hierarchical mechanical models on real biological composites.

/pdf/in-situ-quantification-of-microscopic-contributions-of-3e17ax4d5m.pdf

Michaela Zauner

Papers

Synchrotron-based tomographic microscopy (SbTM) of wood: development of a testing device and observation of plastic deformation of uniaxially compressed Norway spruce samples

Damage evolution in wood: synchrotron radiation micro-computed tomography (SRμCT) as a complementary tool for interpreting acoustic emission (AE) behavior

Micro x-ray computed tomography of adhesive bonds in wood

Failure and failure mechanisms of wood during longitudinal compression monitored by synchrotron micro-computed tomography

In-situ quantification of microscopic contributions of individual cells to macroscopic wood deformation with synchrotron computed tomography.