## 2 Cellulose Nanomaterials
### Cellulose Nanofibrils (CNFs)  
Cellulose nanofibrils, also called nano-fibrillated cellulose (CNF), are primarily obtained from wood with a mechanical treatment of wood pulp, but they can also be extracted from sugar beets, potato tubers, hemp, and flax [13].  
Their characteristic structure is formed by an entangled network of long and flexible fibers, with dimensions of 20-100 nm in width and >10,000 nm in length [6].  
This type of nanocellulose presents the same amount of amorphous and crystalline regions; the mechanical treatments are the most common methods used to produce it, such as mechanical defibrillation or disintegration caused by mechanical forces. The main disadvantage of these methods is the significant energy consumption. For this reason, and mainly to prevent the aggregation of the micro and nanofibrils in the final product, the mechanical treatments are usually combined with enzymatic and/or chemical treatment, such as the catalytic 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation method, which allows us to obtain nanofibrils by mechanical disintegration, generating CNFs with a uniform size. This approach, moreover, does not change the crystallographic forms of the cellulosic fibers while increasing the surface charge due to the carboxyl groups introduced by TEMPO, which is an interesting advantage for the production of nanocellulose composite materials [34; 35; 36]. Nevertheless, the mechanical production methods of this type of nanocellulose remain the most advantageous; in fact, they do not require additives, do not modify the surface charge compared to the starting material, and give excellent yields [37].  
CNFs have many interesting properties, such as their mechanical strength and rigidity, that make them an alternative to artificial fillers used for the reinforcement of plastic composite materials; moreover, the low thermal expansion and limited oxygen transmission rate permit their application in electronic devices, food packaging, and printing. Furthermore, the easy functionalization of CNFs also allows for their use in hydrophobic matrices with which they would have little affinity [10].  
\begin{table}
\begin{tabular}{c c c} \hline
**Method** & **Source** & **Remarks** \\ \hline H\({}_{2}\)SO\({}_{4}\) hydrolysis & Bacteria & L (nm) = 100–1000; W (nm) = 10–50; Aspect ratio (L/D) = 2–100. \\ H\({}_{2}\)SO\({}_{4}\) hydrolysis & Valonia & L (nm) = 100–2000; W (nm) = 10–20; Aspect ratio (L/D) = 50–200. \\ H\({}_{2}\)SO\({}_{4}\) hydrolysis & Ramie & L (nm) = 70–200; W (nm) = 5–15; Aspect ratio (L/D) = \(\sim\)12. \\ H\({}_{2}\)SO\({}_{4}\) hydrolysis & Wood & L (nm) = 100–300; W (nm) = 3–5; Aspect ratio (L/D) = 20–100. \\ H\({}_{2}\)SO\({}_{4}\) hydrolysis & Sisal & L (nm) = 100–300; W (nm) = 3–5; Aspect ratio (L/D) = \(\sim\)60. \\ H\({}_{2}\)SO\({}_{4}\) hydrolysis & Tunicates & L (nm) = 1000; W (nm) = 10–20; Aspect ratio (L/D) = \(\sim\)100. \\ HCl hydrolysis & Bacteria & L (nm) = 160–420; W (nm) = 15–25; Aspect ratio (L/D) = 7–23. \\ HCl hydrolysis & Cotton & L (nm) = 100–300; W (nm) = 3–5; Aspect ratio (L/D) = 20–100. \\ \hline \end{tabular}
\end{table}
Table 1: Dimension of nanocellulose with respect to the various parameters. Reproduced with permission [7]. Copyright © 2018, Elsevier.