## 1 Introduction  
The use of natural fibers (NF) as a renewable material, which can be extracted from renewable resources (e.g., leaves, peels, and trunks of plants), were discovered in the early 1900s [1]. Natural fibers act as biodegradable reinforcing materials and as an alternative for the man-made synthetic fibers (SF) due to their inherent properties. For instance, they are eco-friendly, unlike SF, because they are biodegradable and non-abrasive. On the other hand, NF enhances the mechanical properties of the polymers, i.e., tensile strength, flexural, and impact strengths properties of natural fiber-reinforced polymer composites with different fiber loadings, as presented in the following sections [2, 3]. The most commonly used extract methods to obtain NF have been the mechanical method, chemical method, enzymatic (EM), biological, and combined procedures. The mechanical method has been the most common fiber-matrix procedure because it is fast and can be performed by a machine (e.g., mechanical decorticator), which acts by disintegration [4, 5, 6, 7]. Chemical methods use strong acids (e.g., sulfuric acid or hydrochloric acid) to promote the hydrolysis (in some cases the bleaching [8; 9] and sulfite process [9; 10], which is more suitable for obtaining pulp), which removes the amorphous components of natural fibers and also to separate the cellulose fibrils from the cell wall [11]. After the acid reaction, the parts of the plants need to go through separation steps, namely: centrifugation or filtration and washing (using water or distilled water, acetate buffer, methanol (CH\({}_{3}\)OH), benzoyl chloride (C\({}_{6}\)H\({}_{5}\)COCI), running tap water, etc. [6; 9; 12]). Chemical methods induce a rapid decrease in the degree of polymerization and consequently leads to the production of nano natural fibers through a reduction in the cellulose chain's size. The resulting natural nanofibers are also called whiskers or cellulose nanocrystals. The enzymatic method is currently presented as a more sustainable alternative to chemical methods because it is an environmentally friendly process (e.g., it do not use strong acids or other chemical solvents), which acts by cellulases enzymes (e.g., xylanases), which have very specific catalyzed reactions with high reactivity and selectivity, causing focused results. Usually, enzymatic method can use pectinase, hemicellulose, and poligalacturonase, which modifies the amorphous components of the NF, maintaining the crystalline regions [9; 13; 14; 15]. The enzymatic method can potentially reduce operational costs, energy, and water, and improve the product quality and potential process integration when is compared with the chemical method [6; 16; 17; 18; 19]. However, EM needs detailed knowledge, namely: surface chemistry, available forms of cellulose, specific interactions between the cellulases enzymes, and NF. EM also needs to be fast to avoid complete cellulose degradation [6; 9; 20]. The biological method consists of soaking a vegetable source (e.g., wood, algae, microcrystalline cellulose) for some days in water or distilled water containing several microorganisms. Thus, natural fibers are obtained by the biological action (in anaerobic or aerobic ways), which generates soluble components and gases such as methane [5; 9; 11; 21; 22]. All these extraction methods have been used to originate a complex NF in structure which constitutes composite materials. These materials can be divided into two major groups as far as the surface properties are concerned: a physical group (e.g., morphology, smoothness, roughness, permeability, and specific surface area) and chemical group (e.g., elemental, molecular, composition, and functional). The combination of these two groups determines the thermodynamics of the surface (e.g., free energy, acid-base acceptor, and donor numbers) and gives rise to PC consisting of three basic polymers: cellulose (the main component of plant cell walls), hemicellulose, and lignin [6; 19; 23; 24; 25].