## 2 Cellulose Nanomaterials  
Cellulose is the most abundant renewable source on Earth; it can be found in plants, algae, microorganisms, such as some bacteria, and other natural sources. In fact, its annual production is approximately 1.5 \(\times\)\(10^{12}\) tons [20; 26]. Chemically, it is composed of a linear polysaccharide produced from ringed glucose monomers linked together through \(\beta\)-(1,4) glycosidic bonding, and its microscopic morphology corresponds to alternating regions of crystalline and amorphous zones, which, together, form the characteristic macroscopic fibrous structure of cellulose (Figure 1) [5; 27]. The ordered crystalline domains are more resistant to chemical, mechanical, and enzymatic treatments and thus have a higher resistance to degradation compared to the amorphous ones; these crystalline regions are held together by hydrogen bonds that make cellulose stable, but decrease its solubility in water and other solvents [5; 7]. The hierarchical structure of cellulose is described in Figure 2.  
This polymer, seen as an inexhaustible source of raw materials and, consequently, all of the nanocellulose materials obtained from it, has gained increasing interest thanks to its attractive characteristics, such as its ease of availability, high surface area, good mechanical properties, renewability, and biocompatibility [6; 28]. To define cellulose nanomaterials, the nomenclature established by the Technical Association of the Pulp and Paper Industry (TAPPI) of the Nanotechnology Division  
Figure 1: Amorphous and crystalline regions of cellulose micro fibrils. Figure 2: The hierarchical structure of cellulose. Reproduced under terms of the CC-BY license [6]. Copyright © 2020, The Authors, published by Front. Chem. must be considered, and, from the standardization TAPPI WI 3021, it is possible to define nanocellulose as the crystallite or cellulose fiber possessing at least one dimension in the nanoscale range (1-100 nm) [6]. The class of nanocellulose materials includes all those derived from cellulose with several shapes, dimensions, chemical surface, and properties. Therefore, from the separation of cellulose fibers, different types of nanoscale cellulose can be obtained; in particular, through the function of their sources, the degree of crystallinity, and the extraction and production method, if a bottom-up or top-down technique is used, it is possible to distinguish three categories of nanocellulose: cellulose nanocrystals (CNCs), nano-fibrilated cellulose (CNF), and bacterial nanocellulose (BNC). CNCs consist of cylindrical, elongated, inflexible, and rod-like nanoparticles with dimensions of 4-70 nm in width and 100-6000 nm in length and a crystallinity index of around 54-88%; they are usually obtained by hydrolysis. Nano-fibrilated cellulose (CNF), commonly obtained by mechanical treatment, with a braided network structure, is composed of longer and flexible fibers with dimensions of 20-100 nm in width and >10,000 nm in length and a low crystallinity index (<50%). Finally, bacterial nanocellulose (BNC), also known as microbial nanocellulose, is the most promising and cost-effective biomaterial, especially for the biomedical industry, with its highest crystallinity index > 88% [20]. The BNC consists of ultrafine nanofibers that are 20-100 nm in diameter and micrometers lengths that form an identifying 3D network [6; 20; 29]. All of these categories of nanocellulose are related to each other because they can all be obtained from the same natural sources, and each can form stable structures in a liquid medium or matrix via hydrogen bonds. In fact, nanocellulose contains a great number of hydroxyl groups (-OH) that can form hydrogen bonds either between different chains (intermolecular bonds) or in the same chain (intramolecular bonds), making this material highly hydrophilic and allowing us to modify it via different chemical and physical strategies [4]. Its hydrophilicity can sometimes be a drawback, especially in the case of applications where humidity can be a problem [10].