## 1 Introduction  
Cellulose is one of the most abundant biopolymers on Earth [1; 2; 3]. This natural polymer is a long-chain polysaccharide, made up of 7000-20,000 \(\beta\)-(1\(\rightarrow\)4)-linked anhydro-D-glucose units [4; 5; 6] that can be found in wood (hardwood and softwood), annual plants (hemp, flax, jute ramie), seeds (e.g., cotton coir, etc. ), grasses (bamboo, bagasse, etc. ), marine animals (tunicates), algae, fungi, invertebrates, and bacteria [7]. However, wood constitutes the main supply source for the production of cellulose-based materials owing to its wide availability worldwide [8; 9]. In the following, we will mainly focus on the properties and uses of wood-based nanocellulose (Figure 1)  
In the hierarchical structure of plant cell walls, cellulose chains are aggregated in microfibrils that are few-nanometer-thick slender fibres (Figure 1a-c) [1; 2; 3; 4; 6; 10; 11]. The structure of microfibrils is also semi-crystalline. However, the exact organisation of crystallites in the microfibrils is still not completely understood [5]. Recent studies [12; 13] have shown that amorphous regions in microfibrils are very small and/or are presumably more correctly only \"slightly disordered\" and not fully amorphous. On a larger scale, the microfibrils themselves are aligned and bound in the form of bundles by hemicelluloses and either pectin or lignin (Figure 1c). Nanocelluloses (cellulose nanocrystals, CNCs, or cellulose nanofibrils, CNFs) are portions of plant cell walls with at least one dimension in the nanometer range [5; 19; 20]. CNFs (Figure 1d) are mechanically produced by delaminating plant-based cellulose, yielding slender nanoscale  
Figure 1: Hierarchical structure of wood fibres (**a**), showing the various layers (S1, S2, S3) in wood cell wall (**b**) [14] (Adapted from [PERSON] [14] with permission from © Springer-Verlag 2007). (**c**) Portion of a wood cell wall showing the cellulose microfibrils aligned and bound in the form of bundles by hemicelluloses and lignin. (Adapted from [PERSON] [15] with permission from licensee Springer. 2011). Atomic Force Microscope (AFM) micrographs of CNFs (**d**) [16] and CNCs (**e**) (Adapted from [PERSON] et al. [5] with permission from Elsevier). In Figure (**d**), the photograph inset shows a TEMPO-oxidised CNF water suspension with solids content of 1 wt%. Note that the TEMPO-oxidation is a chemical pre-treatment used to improve the delamination of cellulose fibres. This pre-treatment leads to a regioselective oxidation of cellulose primary alcohol groups (-OH) at the surface of the fibrils into carboxylate ones (-COO\({}^{-}\)). As such, negatively charged CNFs are more easily isolated from the fibre walls during the disintegration process [17; 18]. fibrous particles (length \(l>1\) um and width \(d\approx 5\)-100 nm) containing both disordered and crystalline regions [1; 5; 20]. The delamination process of cellulose fibres is often combined with pre-treatments such as enzymatic hydrolysis [21; 22], carboxylation via TEMPO-mediated oxidation or periodate chlorite oxidation [17; 18], carboxymethyl [23], designed to reduce energy input needed for fibrillation. As such, CNFs present a wide diversity of microstructures and surface chemistries. In contrast, CNCs (Figure 1e) are mainly extracted from cellulose fibres using an acid hydrolysis process [20; 24]. The isolation of CNCs by acid hydrolysis selectively degrades disordered cellulose chains, resulting in highly crystalline rod-shaped nanoparticles [5]. CNCs are considerably less slender than CNFs (length \(l\approx 100\)-200 nm and width \(d\approx 5\)-20 nm). Many review articles [6; 8; 25; 26; 27] and books [20; 28] dealing with the production processes of nanocelluloses are available for interested readers. Nanocelluloses in the form of CNCs and CNFs have received considerable attention over the last two decades.