# Cellulose Nanofibers and Other Biopolymers for Biomedical Applications. A Review
###### Abstract  
Biopolymers are materials synthesised or derived from natural sources, such as plants, animals, microorganisms or any other living organism. The use of these polymers has grown significantly in recent years as industry shifts away from unsustainable fossil fuel resources and looks towards a softer and more sustainable environmental approach. This review article covers the main classes of biopolymers: Polysaccharides, proteins, microbial-derived and lignin. In addition, an overview of the leading biomedical applications of biopolymers is also provided, which includes tissue engineering, medical implants, wound dressings, and the delivery of bioactive molecules. The future clinical applications of biopolymers are vast, due to their inherent biocompatibility, biodegradability and low immunogenicity. All properties which their synthetic counterparts do not share. biopolymers; cellulose nanofibers; polysaccharides; proteins; tissue engineering; medical implants; wound healing; biocompatibility +
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to dissolve the amorphous regions of the cellulose chains and release the crystalline domains [8]. To prepare NCC, the cellulose fibers must undergo a purification process. In this process, the extraction with organic solvents is first carried out in order to eliminate the extractables from the biomass followed by an alkaline treatment with sodium hydroxide (NaOH) or potassium hydroxide (KOH) to purify the cellulose, and finally a bleaching process with sodium chlorite (NaClO\({}_{2}\)). This process is key to ensure the effectiveness of acid hydrolysis and facilitate the attack on amorphous regions. The alkaline treatment aims to solubilise most of the pectin and hemicellulose, and the bleaching process, decompose the phenolic compounds or molecules with chromophore groups in the lignin, and eliminate the by-products of such decomposition [9]. Once the cellulose has been purified, acid hydrolysis is carried out using different acids. The most commonly used acid is sulphuric acid (H\({}_{2}\)SO\({}_{4}\)), due to the esterification process of the anionic sulphate ester groups (-OSO\({}_{3}\)\({}^{-}\)) with the hydroxyl groups on the surface of the cellulose, which induces the formation of a negative electrostatic layer on the surface of the nanocrystals, and facilitates their dispersion in water. During acid hydrolysis, the hydronium ions (H\({}_{3}\)O\({}^{-}\)) penetrate the cellulose chain by hydrolysing the glycosidic bonds of the amorphous regions releasing the individual crystalline regions [10]. The resulting product is dialysed against distilled water and then dispersed with a sonication stage. These nanocrystals have a short rod shape between 2-20 nm in diameter and 100-500 nm in length, being composed of 100% cellulose with a high crystallinity (54-88%) [6; 11]. Cellulose nanofibers (CNF), also known as nanofibrilated cellulose, are a type of flexible, elongated, cross-linked nanocellulose that can be extracted from cellulose fibers by mechanical treatment [12]. In order to obtain cellulose nanofibers, it is necessary to subject the raw material to a purification process similar to that which is carried out to obtain CNC. NFC is generally produced by mechanical delamination of cellulose fibers after a previous treatment (pre-treatment) that facilitates disintegration. These pre-treatments avoid the occlusion of the equipment where the disintegration process takes place, and reduces the number of passes or treatment time, with the reduction of energy that this entails. CNF was first isolated in 1983 by [PERSON] et al. using bleached softwood fiber and subjected to a high-pressure homogenisation process [13]. However, the high energy consumption necessary for its production limited the scientific and industrial interest.