Cellulose nanocrystals

This chapter focuses on the use of nanotechnology in the development of cellulose and chitin nanoctystals and their novel biomedical applications. It consists of four main sections. The first section is a brief introduction. The second section focuses on cellulose nanocrystals (CNCs) and their preparation procedure, physical properties, and surface modifications. Cationic modification of CNCs is also presented to produce positively charged CNCs. Various bioapplications of CNCs in bionanocomposites, drug delivery, and biosensors are discussed as well. The third section focuses on chitin nanoctystals (CHNCs). Except for a short introduction on chitin and its structure, the methods of isolation and characterization of chitin are discussed and the surface modifications and properties of CHNCs are summarized. The applications of CHNCs as reinforcing fillers in nanocomposites and several biomedical applications are discussed. The fourth section is a summary and perspective highlighting the future directions on the application of these natural nanoctystals in various key industries related to biomedicine. 

Cellulose nanociystals (CNCs) are rigid rod-like ciystals with diameters in the range of 3-50 nm and lengths of 100 nm to several pm, depending on 

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The reaction appears to be well suited for selective conversion of biomass carbohydrates into their corresponding oxidized derivatives. This system serves to oxidize several polymeric carbohydrates including starches and pullulan.445 49 More recent work has described the TEMPO-catalyzed introduction of carboxyl groups in native cellulose and its different morphological forms,45(M52 cellulose derivatives,453 and the surface of cellulose nanocrystals.454 The related biopolymer, chitin, also is oxidized under these conditions.455... 

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Ultrasonic treatments [70] have the potential to change the morphology of the formed whiskers. When hydrolysis is carried out under ultrasonic treatment, a combination of both sulfuric and hydrochloric acids during the hydrolysis appears to generate spherical cellulose nanocrystals instead of rod like nanocrystal which are the typical morphology of whiskers. These spherical cellulose nanocrystals show better thermal stability mainly because they possess fewer sulfate groups on their surfaces [71]. 

Table 2.1 Geometrical characteristics, length and width, of cellulose nanocrystals (adapted from ref [38])...

The cellulose based materials that are used as nano-reinforcements are cellulose nanocrystals (i.e. whiskers and nanospheres), nanofibrillated cellulose, regenerated cellulose nanoparticles and electrospun nanofibers. A wide range of polymer matrices have been used to form cellulose nanocomposites. Synthetic polymers such as polypropylene, poly(vinyl chloride) (PVC) [102], waterborne epoxy [103], waterborne polyurethane [104], polyurethane [105], poly-(styrene-co-butyl acrylate) [106], poly(oxyethylene) [107], polysiloxanes [108], polysulfonates [109], cellulose acetate butyrate [110,111], poly(caprolactone) [112], poly(viny 1 alcohol) [113] and poly (vinyl acetate) [114]. Different biopolymers such as starch-based... 

Electrospinning processes are used to prepare nanocomposite fiber mats. In order to prepare nanocomposite fiber mats, mixtures containing the polymer solution and cellulose whiskers solution are placed in the appropriated electrospinning setup. Peresin et al. [128] have produced nanocomposite mats of poly (vinyl alcohol) (PVA) reinforced with cellulose nanocrystals using this electrospinning technique. Smooth nonwoven mats with homogeneous nanofibers were obtained. Park et al. [129] have also incorporated cellulose whiskers into nanofibers of polyethylene oxide (PEO) by the electrospiiming process. 

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