Wood Nanomaterials and Nanotechnologies

Wood is an abundant, intrinsically renewable natural resource, with over three trillion mature trees on Earth that serve as an important carbon sink during growth. These features when combined with new technologies and processing techniques have endowed wood with rich, emerging functionality that make it a natural material choice toward sustainability. Around 300 million years of tree evolution has yielded over 60 000 woody species, each of which is an engineering masterpiece of nature. Evolution has led to wood's unique hierarchically porous structure, with cell walls primarily composed of cellulose, hemicellulose, and lignin, which can support trees 300 feet tall while also providing efficient water and nutrient transport. This lignocellulosic structure offers an intriguing material platform for diverse functionalization and applications. Recent research breakthroughs around the world epitomize the tremendous yet largely unexplored potential of such advanced wood-based functional materials—a paradigm shift from the conventional use of wood in furniture and building construction. Many efforts have been dedicated to tailoring wood's composition and structure at multiple length scales to impart new functionality and synthesize novel bio-based functional composites, which are explored in this special issue of Advanced Materials. Two major approaches, bottom-up and top-down, have been developed in fabricating wood-based functional materials. Most are made through bottom-up approaches that involve breaking down the cellulose fibers of wood into cellulose fibrils via a chemical and/or physical process and then assembling those building blocks into various macroscopic structures. Recently, the top-down approach, involving the structural and/or compositional engineering of wood without breaking the material down into its constituent fibrils, has gained increasing interest due to this strategy's capability to maintain wood's original anisotropic structure. Such top-down approaches have led to the invention of new wood-based functional materials with improved or even new properties and functions, largely expanding the application fields of wood. In addition to fabrication techniques, advanced wood materials have benefited from progress in wood characterization technologies and computational simulation methodologies. Characterization methods such as X-ray diffraction, X-ray tomography, Raman spectroscopy, electron microscopy, atomic force microscopy, nuclear magnetic resonance spectroscopy, and neutron diffraction enable the structures and behaviors of wood-based functional materials to be identified down to the micro- and nanoscale. Computational studies also play a key role in assisting and accelerating the discovery of mechanical, fluidic, ionic, electronic, optical, thermal, and acoustic properties in wood-based functional materials as well as determining important structure–property–function relationships that are essential for informing applied research. This special issue of Advanced Materials presents recent developments in wood nanomaterials and nanotechnologies toward various sustainable applications. In terms of wood nanomaterials and nanotechnologies, especially fabrication technologies, the past decade has witnessed great progress. Among the various fabrication technologies, bottom-up approaches have gained the most attention. Fuxiang Chu and co-workers (article number 2001135) present a comprehensive review on wood-derived functional polymeric materials fabricated by means of macromolecular engineering with a focus on the utilization of controlled/living polymerization, click chemistry, and dynamic bond chemistry for wood-based modification from the perspective of molecular design. Through a dissolving and regeneration process, regenerated cellulose can be made from wood, which can be further assembled into various macroscopic structures, such as macrofibers, films, hydrogels, and aerogels, as summarized by Lina Zhang and co-workers (article number 2000682). More comprehensively, sustainable and functional hydrogels derived from plant materials are reviewed by Orlando J. Rojas and co-workers (article number 2001085) with a focus on the water interactions, hydration, and swelling that are important in designing, processing hydrogels and achieving desired properties. Holocellulose fibers with well-preserved cellulose nanofibril structure in the mesoporous fiber cell wall can be fabricated via mild delignification treatment of wood, which can be further assembled into fibers, paper, biocomposites, and compression molded fiber materials with good mechanical properties (reviewed by Lars Berglund and Xuan Yang in article number 2001118). Among the various wood-derived nanomaterials, nanocellulose has gained increasing interest and been widely investigated and used toward sustainability. Gustav Nyström and co-workers (article number 2000657) review the organization of nanocellulose into biomimetic aligned, porous, and fibrous materials through a variety of bottom-up fabrication techniques. In particular, fabrication techniques such as spinning (reviewed by L. Daniel Söderberg and co-workers in article number 2001238), layer-by-layer assembly (reviewed by Lars Wågberg and Johan Erlandsson in article number 2001474), and the dispersion, suspension, and emulsion polymerization of nanocellulose in both emulsions and heterogeneous water-based systems (reviewed by Emily D. Cranston and co-workers in article number 2002404) show great potential in the production of cellulose fibers, films, aerogels, and laminated bulk structures with excellent mechanical properties and desired functions. Through the hybridization of nanocellulose with other non-wood components, cellulose nanocomposites can be fabricated via solvent blending, infiltration, direct solid blending, solution processing, melt-extrusion, milling/pulverizing, and in situ polymerization approaches (reviewed by Robert J. Moon, Jeffrey P. Youngblood, and co-workers in article number 2000718). The fabricated nanocellulose materials can be modified toward improved properties by surface and interface engineering (reviewed by Subir Kumar Biswas, Jingquan Han, Hiroyuki Yano, and co-workers in article number 2002264), and by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation and phosphorylation and counterion exchange of charged groups (reviewed by Akira Isogai in article number 2000630). In contrast to the bottom-up strategies, cellulose scaffolds that retain the natural wood structure by various delignification techniques can be fabricated by top-down approaches (reviewed by Ingo Burgert and co-workers in article number 2001375). This anisotropic cellulose scaffold can be further processed by polymerization and densification toward a large range of properties. Alternatively, by mimicking the hierarchically cellular structure of wood, artificial wood materials with a microstructure similar to wood but less hierarchical and complex can be fabricated (reviewed by Shuhong Yu and co-workers in article number 2001086). In addition to the advanced fabrication methods, characterization technologies and computational methodologies have also rapidly developed in the past decades. Stephen J. Eichhorn and co-workers (article number 2001613) provide a comprehensive review on recent developments in the use of advanced imaging techniques for studying the structural properties of wood in relation to its mechanical properties. Meanwhile, Markus J. Buehler and co-workers (article number 2003206) summarize recent advances in computational simulation approaches, such as density functional theory (DFT) and molecular dynamics (MD), that have been used to understand the structure, properties, and reactions of cellulose, lignin, and the wood cell walls. This progress in wood fabrication, characterization, and computational simulation technologies has led to the accelerated development of advanced wood-based materials with a large range of properties and functions. Liangbing Hu and Chaoji Chen (article number 2002890) present a comprehensive and critical review on recent advances, challenges, and future opportunities in the nanoscale regulation of ions in top-down and bottom-up wood-based structures, as well as their applications in various devices that utilize this ion regulation capability for energy storage, environmental remediation, sensing, and signaling. Further discussion of the electrochemical energy storage and solar evaporation applications of wood is given by Sang-Young Lee, Leif Nyholm, and co-workers (article number 2000892) and Young-Shin Jun, Srikanth Singamaneni, and co-workers (article number 2000922), respectively, with a particular emphasis on cellulose nanomaterials. The recent development of top-down nanowood materials for applications relating to the water–energy nexus, with the aim of addressing the global energy and water crises, is reviewed by Jason Ren and co-workers (article number 2001240). In addition to the water and energy fields, wood-based materials also demonstrate great promise in light management (reviewed by Han Yang, Silvia Vignolini, and co-workers in article number 2001215 and Jian Li, Shouxin Liu, and co-workers in article number 2000596) and thermal management (reviewed by Ivan I. Smalyukh in article number 2001228 and Lennart Bergström and co-workers in article number 2001839), as well as intelligent electronics (reviewed by Haipeng Yu, Yiqiang Wu, and co-workers in article number 2000619), bioapplications (reviewed by Kai Zhang and co-workers in article number 2000717), and lightweight structural materials (Teng Li, Shuze Zhu, and co-workers in article number 2002504). In another comprehensive review by Hongli Zhu and co-workers (article number 2001654), functional materials derived from different parts of wood for use in different fields such as energy, electronics, biomedical, and water treatment are discussed. Wood and lignocellulosic materials also serve as a source of bioinspiration for actuators, soft robotic systems, and architectures and may even be considered as materially programmed information (reviewed by Peter Fratzl and co-workers in article number 2001412). The growing use of wood nanomaterials offers a sustainable solution toward addressing global challenges in energy, water, and the environment. We believe the numerous scientific and technological breakthroughs of wood nanomaterials and nanotechnologies over the past ten years covered by this special issue will eventually lead to revolutionary advances in material research and development, create a paradigm shift from nonrenewable materials to renewable and more sustainable biosourced composites, expand the influence of wood nanomaterials and wood nanotechnologies to a broader community, and benefit societal goals of sustainability. Finally, we would like to take this opportunity to express our appreciation to Dr. Peter Gregory, Dr. Jos Lenders, Dr. Valentina Lombardo, and the editorial team of Advanced Materials for their enthusiastic dedication and professional editing. The great efforts of all the coauthors who have made important contributions to this special issue are highly appreciated. Chaoji Chen received his B.S. (2010) and Ph.D. (2015) degrees in materials science and engineering from Huazhong University of Science and Technology (HUST), P. R. China. He is currently a postdoctoral associate in Prof. Liangbing Hu's group at the University of Maryland, College Park. His research focuses on biomass (e.g., wood, cellulose, chitin, etc.) processing, engineering, and functionalization toward sustainable applications, including energy storage and conversion, environmental remediation, lightweight structural materials, thermal management, electronics, and advanced nanomanufacturing (e.g., 3D printing, microwave heating, thermal shock heating) of functional materials. Lars Berglund studied materials science and engineering at Luleå University of Technology (LTU), Luleå, Sweden, and polymer science and engineering at University of Massachusetts, Amherst, USA. He obtained a Ph.D. from Linköping University in 1987. From 1991 to 2002, he was a professor at LTU, and from 2002 a professor of wood and wood composites at the Kungliga Tekniska Högskolan (KTH) Royal Institute of Technology, Stockholm, Sweden. He was the director of the Wallenberg Wood Science Center at KTH from 2009 to 2020. Ingo Burgert received his diploma in wood science and technology at the University of Hamburg, Germany, in 1995, followed by a Ph.D. in 2000. From 2000 to 2003 he was a postdoctoral associate at the University of Natural Resources and Life Sciences (BOKU), Vienna, Austria. From 2003 to 2011, he was a research group leader at the Max Planck Institute of Colloids and Interfaces, Potsdam, Germany. Since 2011, he has been a professor of wood materials science at Eidgenössische Technische Hochschule (ETH) Zurich and group leader at Eidgenössische Materialprüfungs-und Forschungsanstalt (EMPA), Dübendorf. Liangbing Hu received his B.S. degree in applied physics from the University of Science and Technology of China (USTC) in 2002. He did his Ph.D. at University of California, Los Angeles. In 2006, he joined Unidym, Inc. as a co-founding scientist. He did his postdoctoral studies at Stanford University from 2009 to 2011. Currently, he is Herbert Rabin Distinguished Professor at the University of Maryland, College Park and the Director of the Center for Materials Innovation (CMI). His research interests include emerging energy storage technologies, sustainable nanomaterials for energy and environmental applications, and ultrahigh temperature manufacturing.