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Introduction to Nanocellulose

Cellulose is the most abundant polymer on earth produced by plants, microorganisms, and cell-free systems. Chemically, it is composed of repeated β-d-glucose monomers linked together through β-(1,4) glycoside linkage. The morphology of natural cellulose is usually fibrous with intermittent crystalline and amorphous sections [1]. Separation of fibers results in nanoscale cellulose substances known as nanocellulose, which exists in different morphologies such as cellulose nanocrystals (CNCs) or cellulose nanowhiskers (CNWs) as another name, and cellulose nanofibers (CNFs) (Figure 1.1). Compared to the plant cellulose containing lignin and hemicelluloses, microbial cellulose, termed as bacterial nanocellulose (BNC) and cell-free cellulose, represents the purest form of cellulose. Nanocellulose exhibits distinctive structural and physicochemical, mechanical, and biological characteristics, including reticulate fibrous three-dimensional web-shaped structure, high crystallinity, good mechanical strength, biocompatibility, biodegradability, optical transparency, high specific surface area, polyfunctionality, hydrophilicity, and moldability into 3D structures [2–4]. As a sustainable material, nanocellulose could be extracted from the disintegration of plant and animal cellulose [5], synthesized by different microbial strains [6], and cell-free systems [7]. As a tunable material, nanocellulose both alone and in composite form with other materials is immensely used in various fields such as wound dressing, textiles and clothing, food, cosmetics, regenerative medicines, tissue engineering, energy, optoelectronics, bioprinting, environmental remediation, and so on [2, 8–13]. Nanocellulose exhibits superior structural characteristics than microcrystalline cellulose (MCC), and possesses high mechanical strength and can be facilely surface-modified via different strategies. For example, the longitudinal modulus of CNC extracted from tunicate can reach as high as 151 GPa, while the Young’s modulus of CNF varies between 58 and 180 GPa and its tensile