# Cellulose Nanofibers and Other Biopolymers for Biomedical Applications. A Review
###### Abstract
In order to reduce energy consumption and make disintegration more efficient, different pretreatments, such as enzymatic hydrolysis [14], partial carboxymethyl [15], catalytic oxidation using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) [16], and mechanical refining [17] were employed. After pre-treatment, the fiber is subjected to a mechanical disintegration process, including high pressure, microfluidisation, friction grinding, extrusion, cryopreservation and high intensity ultrasonication [18]. Bacterial nanocellulose (BNC) is another type of nanocellulose that has special features compared to the other types. While CNC and CNF are isolated from lignocellulosic biomass using top-down deconstruction methods of cellulose fiber, BNC is produced by a down-top process, which consists of the assembly of low molecular weight sugars by bacteria [19]. The main microorganism producing BNC is _Gluconacetobacter xylinus_; however, several species are able to synthesise this biopolymer. The production of nanocellulose by bacteria consists of three main stages: (i) Polymerisation of glucose residues in \(\beta\)-1-4 glucan, (ii) extracellular secretion of linear chains, and (iii) organisation and crystallisation of glucan chains through hydrogen bonds and van der Walls forces, arranged in a hierarchy in fibers [20]. Unlike the other types of nanocellulose mentioned above, BNC is produced in a pure way, without lignin, hemicellulose, pectin or any other component of the lignocellulosic matrix of biomass. Its high purity, mechanical resistance, insolubility, biodegradability and non-toxic properties make it an ideal starting material for the production of high value-added products in the biomedical field [21]. Therefore, cellulose materials possess properties of biodegradability and renewability. These materials also have low environmental, animal/human health, and safety risks [22]. Indeed, they have already been incorporated in the daily life for thousands of years, for instance, in the textile, food and pharmaceutical industries, among others [22; 23]. In addition, nanomaterials derived from cellulose possess high specific strength and modulus, high specific surface area, and the extensive ability for chemical modification [22]. Due to all the above, nanocellulose is an excellent candidate to be used in biomedical applications. Biopolymers are polymers produced from natural sources either chemically synthesised from biological material or entirely biosynthesised by living organisms. These include: Polysaccharides, such as cellulose, hemicellulose, starch, or chitosan; protein animal-based polymers, such as silk and collagen; and lignin, a high-molecular weight biopolymer present in the cell walls of vascular plants, which has complex and variable structures. Lignin is formed by randomly cross-linked networks of methoxylated and hydroxylated phenylpropane [24; 25; 26]. 'Biopolymer' can be considered an umbrella term as it covers polymers which can be characterised as one or more of the following [27]:  
* Biobased: Polymers composed entirely or in some part from biomass (natural sources). * Biodegradable: Polymers capable of being composed by bacteria or other organisms. * Biocompatible: Polymers which are compatible with human or animal tissue, i.e., does not harm the body whilst fulfilling its function. This review will focus on common biopolymers and their biomedical applications. Naturally derived polymers possess huge potential for medical use due to their inherent biocompatibility, biodegradability, non-toxicity and ability to deliver bioactive molecules [28].