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Review
. 2021 Jul;33(28):e2001613.
doi: 10.1002/adma.202001613. Epub 2020 Aug 23.

Beyond What Meets the Eye: Imaging and Imagining Wood Mechanical-Structural Properties

Eleni Toumpanaki  1 , Darshil U Shah  2 , Stephen J Eichhorn  1
Affiliations
Review

Beyond What Meets the Eye: Imaging and Imagining Wood Mechanical-Structural Properties

Eleni Toumpanaki et al. Adv Mater. 2021 Jul.

Abstract

Wood presents a hierarchical structure, containing features at all length scales: from the tracheids or vessels that make up its cellular structure, through to the microfibrils within the cell walls, down to the molecular architecture of the cellulose, lignin, and hemicelluloses that comprise its chemical makeup. This structure renders it with high mechanical (e.g., modulus and strength) and interesting physical (e.g., optical) properties. A better understanding of this structure, and how it plays a role in governing mechanical and other physical parameters, will help to better exploit this sustainable resource. Here, recent developments on the use of advanced imaging techniques for studying the structural properties of wood in relation to its mechanical properties are explored. The focus is on synchrotron nuclear magnetic resonance spectroscopy, X-ray diffraction, X-ray tomographical imaging, Raman and infrared spectroscopies, confocal microscopy, electron microscopy, and atomic force microscopy. Critical discussion on the role of imaging techniques and how fields are developing rapidly to incorporate both spatial and temporal ranges of analysis is presented.

Keywords: diffraction; imaging; spectroscopy; structure-property relationships; wood.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
a) Schematic of the length scales of wood—from the size of trees themselves (many meters high) down to the molecular scale of the components of the wood cell wall, e.g., cellulose, hemicellulose (xylan for dicots) and lignin. Image of wood tissue, a wood cell, and a cellulose microfibril with a short xylan chain, and the generic structural arrangement of secondary cell wall polymers (labeled as cellulose, xylan, and lignin). b) Asbhy plot showing the relationship between Young's modulus of different materials and their densities—comparing native woods and densified woods,[ 19 , 20 ] with polymers, composites, and metals and alloys. Lines show the relationships between Young's modulus (E) and density (ρ) with E/ρ2 (solid line) and E/ρ (dotted line). Image of wood tissue: Adapted with permission.[ 16 ] Copyright 2018 The Authors, published by the Royal Society. Image of wood cell: Adapted with permission.[ 17 ] Copyright 2018, Elsevier. Images of cellulose microfibril and the generic structural arrangement of secondary cell wall polymers: Adapted under the terms of the CC‐BY Creative Commons Attribution 3.0 Unported license (https://creativecommons.org/licenses/by/3.0/).[ 18 ] Copyright 2014 The Authors, published by Society for Experimental Biology and John Wiley & Sons Ltd.
Figure 2
Figure 2
Proposed model of softwood molecular architecture: a) microfibril cellulose structures and hemicellulose–cellulose interactions and b) macrofibril structure. a,b) Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses(by/4.0).[ 24 ] Copyright 2019, The Authors, published by Springer Nature.
Figure 3
Figure 3
Main lignin–carbohydrate linkages: a) phenyl glycoside, b) γ‐ester, and c) benzyl ether. Adapted with permission.[ 44 ] Copyright 2007, De Gruyter.
Figure 4
Figure 4
a) Schematic of a wood cell tracheid showing the microfibril angle (M) in the S2 layer top). The relationship between the longitudinal Young's modulus of Pinus radiata and M bottom). Data are reproduced from Cave[ 54 ] with units converted to SI; typical diffraction patterns from wood cell walls. b) Wide‐angle X‐ray diffraction pattern and c) small‐angle X‐ray diffraction pattern showing how to determine M. b,c) Reproduced with permission.[ 58 ] Copyright 2001, Springer Nature.
Figure 5
Figure 5
a) Schematic of the cell wall structure for fibers found in cherry bark (C. sargentii) showing (top) the flattened geometry of the fibers indicating dimensions and longitudinal (L), transverse (T), and radial (R) directions and (bottom) the compositions of the outer layer of the fibers of aliphatic/aromatic lipids and the inner layer of helically wound cellulose microfibrils. b) A typical stress–strain curve of cherry bark top) and the change in microfibril helical angle and the intensity of the scattering peaks used to derive this quantity bottom). The inset shows a close‐up view of the change in helical angle in the elastic regime. a,b) Reproduced with permission.[ 57 ] Copyright 2018, Wiley‐VCH.
Figure 6
Figure 6
a) Schematic of a typical X‐ray diffraction pattern from the end section of a wood cell wall indicating the approach to determine the microfibril angle; b) mesh scan over a complete wood cell in cross section with parts of neighboring cells, pixel size: 2 × 2 mm. Dark regions correspond to lumina, bright regions showing a scattering signal correspond to cell walls. Each pixel corresponds to an individual diffraction pattern from which the microfibril angle can be derived. a,b) Adapted with permission.[ 59 ] Copyright 1999, International Union of Crystallography (Reproduced with permission of the International Union of Crystallography; https://doi.org/10.1107/S0021889899010961).
Figure 7
Figure 7
Confocal microscopy has found a variety of uses in wood research. a) 2D optical section of ring‐porous hardwood tissue (top), enabling 3D rendering of 61 tangential optical sections at 1 µm intervals. Vessels are labelled as A and B. 100 µm scale bars. b) Intensity of cell wall polymer distribution (lignin in red, and polysaccharides in green) measured across regions of interest (ROI) following short periods (2 or 12 min) of mechanical milling. Scale bar: 10 µm. c) 3D reconstruction of a bordered pit, with the greater and lesser rings (Gr, Lr) visible and pectin (red) and crystalline cellulose (green) are labeled. 36 µm × 36 µm square. d) Adhesive penetration. Scale bar: 25 µm. a) Adaptated with permission.[ 106 ] Copyright 2004, Botanical Society of America, published by John Wiley and Sons. b) Reproduced with permission.[ 122 ] Copyright 2017, Elsevier. c) Reproduced with permission.[ 115 ] Copyright 2013, Botanical Society of America, published by Wiley. d) Adapted with permission.[ 118 ] Copyright 2004, Springer Nature.
Figure 8
Figure 8
AFM has enabled imaging cell wall ultrastructure, as well as directly relating structure to bio‐chemical, mechanical and thermal properties. a) Imaging individual cellulose aggregates in the S2 layer. b) Spatial distribution of indentation modulus of cell walls at the adhesive bond line to study resin penetration into cell walls. c) Topological image of a cell wall junction which was scanned in AFM mode to measure differences in Young's modulus of cell wall layers. d) Imaging differences in thermal conductivity of cell wall layers of Norway spruce, relating to the different microfibril angle and cellulose to noncellulosic polymer ratio of each layer; CML is the central middle lamella. e) Imaging differences in polarity of the inner S3 layer and transverse sections S2 layer, as a function of time in days. a) Adapted with permission.[ 135 ] Copyright 2003, Springer Nature. b) Adapted with permission.[ 145 ] Copyright 2004, Elsevier. c) Adapted under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/).[ 140 ] Copyright 2017, The Authors, published by BioMed Central, part of Springer Nature. e) Adapted with permission.[ 148 ] Copyright 2014, Elsevier.
Figure 9
Figure 9
The range of spatial and temporal scales enveloped by the various spectro‐microscopy techniques in imaging woods (and similar plant‐based materials). The relevant length scales of wood's hierarchical structure are presented for reference. Figure drawn using data from refs. [ 158 , 159 ].
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