Tobias Keplinger

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.

A versatile strategy for grafting polymers to wood cell walls.

The hierarchical structure of wood is composed of a cellulose skeleton of high structural order at various length scales. At the nanoscale and microscale the specific structural features of the cells and cell walls result in a lightweight structure with an anisotropic material profile of excellent mechanical performance. By being able to specifically functionalize wood at the level of cell and cell walls one can insert new properties and inevitably upscale them along the intrinsic hierarchical structure, to a level of large-scale engineering materials applications. For this purpose, however, precise control of the spatial distribution of the modifying substances in the complex wood structure is needed. Here we demonstrate a method to insert methacryl groups into wood cell walls using two different chemistry routes. By using these methacryl groups as the anchor points for grafting, various polymers can be inserted into the wood structure. Strikingly, depending on the methacryl precursor, the spatial distribution of the polymer differs strongly. As a proof of concept we grafted polystyrene as a model compound in the second modification step. In the case of methacryloyl chloride the polymer was located mainly at the interface between the cell lumina and the cell wall covering the inner surface of the cells and being traceable up to 2-3 μm in the cell wall, whereas in the case of methacrylic anhydride the polymer was located inside the whole cell wall. Scanning electron microscopy, Fourier transform infrared spectroscopy and especially Raman spectroscopy were used for an in-depth analysis of the modified wood at the cell wall level.

Hybrid wood materials with improved fire retardance by bio-inspired mineralisation on the nano- and submicron level

Inspired by natural matrix-mediated biomineralisation, we present an artificial calcification approach for wood, which predominately targets the hardly accessible nanoporous cell wall structure rather than the micron-sized void system of the cell lumina. CaCO3 can be deposited with this method deep inside the wood structure. Mineralisation of the wood cell wall architecture with CaCO3 offers a green alternative to conventional fire-retardant systems.

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.

Functional lignocellulosic materials prepared by ATRP from a wood scaffold

Wood, a natural and abundant source of organic polymers, has been used as a scaffold to develop novel wood-polymer hybrid materials. Through a two-step surface-initiated Atom Transfer Radical Polymerization (ATRP), the porous wood structure can be effectively modified with polymer chains of various nature. In the present study, polystyrene and poly(N-isopropylacrylamide) were used. As shown with various characterization techniques including confocal Raman microscopy, FTIR, and SEM/EDX, the native wood ultrastructure and features are retained and the polymer chains can be introduced deep within the wood, i.e. inside the wood cell walls. The physical properties of the new materials have been studied, and results indicate that the insertion of polymer chains inside the wood cell wall alters the intrinsic properties of wood to yield a hybrid composite material with new functionalities. This approach to the functionalization of wood could lead to the fabrication of a new class of interesting functional materials and promote innovative utilizations of the renewable resource wood.

Biomaterial Wood: Wood-Based and Bioinspired Materials

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.

Delignified and Densified Cellulose Bulk Materials with Excellent Tensile Properties for Sustainable Engineering

Todays materials research aims at excellent mechanical performance in combination with advanced functionality. In this regard, great progress has been made in tailoring the materials by assembly processes in bottom-up approaches. In the field of wood-derived materials, nanocellulose research has gained increasing attention, and materials with advanced properties were developed. However, there are still unresolved issues concerning upscaling for large-scale applications. Alternatively, the sophisticated hierarchical scaffold of wood can be utilized in a top-down approach to upscale functionalization, and one can profit at the same time from its renewable nature, CO2 storing capacity, light weight, and good mechanical performance. Nevertheless, for bulk wood materials, a wider multipurpose industrial use is so far impeded by concerns regarding durability, natural heterogeneity as well as limitations in terms of functionalization, processing, and shaping. Here, we present a novel cellulose bulk material concept based on delignification and densification of wood resulting in a high-performance material. A delignification process using hydrogen peroxide and acetic acid was optimized to delignify the entire bulk wooden blocks and to retain the highly beneficial structural directionality of wood. In a subsequent step, these cellulosic blocks were densified in a process combining compression and lateral shear to gain a very compact cellulosic material with entangled fibers while retaining unidirectional fiber orientation. The cellulose bulk materials obtained by different densification protocols were structurally, chemically, and mechanically characterized revealing superior tensile properties compared to native wood. Furthermore, after delignification, the cellulose bulk material can be easily formed into different shapes, and the delignification facilitates functionalization of the bioscaffold.

Nano-mechanical characterization of the wood cell wall by AFM studies: comparison between AC- and QI™ mode

BackgroundUnderstanding the arrangement and mechanical properties of wood polymers within the plant cell wall is the basis for unravelling its underlying structure–property relationships. As state of the art Atomic Force Microscopy (AFM) has been used to visualize cell wall layers in contact resonance- and amplitude controlled mode (AC) on embedded samples. Most of the studies have focused on the structural arrangement of the S2 layer and its lamellar structure.ResultsIn this work, a protocol for AFM is proposed to characterize the entire cell wall mechanically by quantitative imaging (QI™) at the nanometer level, without embedding the samples. It is shown that the applied protocol allows for distinguishing between the cell wall layers of the compound middle lamella, S1, and S2 of spruce wood based on their Young’s Moduli. In the transition zone, S12, a stiffness gradient is measured.ConclusionsThe QI™ mode pushes the limit of resolution for mechanical characterization of the plant cell wall to the nanometer range. Comparing QI™- against AC images reveals that the mode of operation strongly influences the visualization of the cell wall.

High-Resolution Adhesion Mapping of the Odd–Even Effect on a Layer-by-Layer Coated Biomaterial by Atomic-Force-Microscopy

The adhesion behavior of polyelectrolyte multilayers consisting of poly(diallyldimethylammonium chloride), PDDA, and poly(styrenesulfonate), PSS, toward a silicon AFM tip was studied during their build-up on wood, a chemically heterogeneous, micrometer rough biomaterial and compared with a nanometer rough substrate, namely quartz. The atomic force microscopy-based force mapping approach generated high-resolution topography-, and adhesion maps within the first bilayers, which point toward a homogeneous layer-by-layer build-up on the biomaterial surface, and therefore indicate an even charge distribution. By analyzing the force-distance curves in every pixel of the mapping, new insights into the specific interactions of the polyelectrolyte multilayers at the surface were achieved. The characteristic odd-even effect of polyelectrolyte multilayers cannot only be determined on quartz, but also on the biomaterial wood, however, only after an offset of two bilayers. This is potentially due to the specific roughness and charge of wood in comparison to commonly used quartz.

Characterization of Wood Derived Hierarchical Cellulose Scaffolds for Multifunctional Applications

Functional materials of high porosity and hierarchical structure, based on renewable building blocks, are highly demanded for material applications. In this regard, substantial progress has been made by functionalizing micro- and nano-sized cellulose followed by its reassembly via bottom-up approaches. However, bottom-up assembly processes are still limited in terms of upscaling and the utilization of these building blocks presupposes the disassembly of the plant feedstock inherit hierarchical cellulose scaffold. To maintain the three-dimensional structure, delignification processes from pulp and paper production were recently adapted for the treatment of bulk wood. Yet, a detailed chemical analysis and the determination of macroscopic swelling/shrinkage parameters for the scaffolds, necessary for a systematic design of cellulose scaffold based materials, are still missing. Here, acidic bleaching and soda pulping were used for producing cellulose scaffolds, for functional materials under retention of their inherent hierarchical structure. Spatially resolved chemical investigations on thin sections by Raman microscopy provided detailed information on the induced alterations at the cell wall level, revealing significant differences in dependence of the chemistry of the pre-treatment. An adaption to bulk wood samples proved the applicability of these treatments at larger scales and volumetric alterations at different atmospheric conditions indicated the effect of the altered porosity of the scaffolds on their hygroscopic behaviour.