Chitin nanocrystals

Over the past decades, CHNCs have been widely studied and used in various chemical and biomedical applications, such as polymer/CHNC nanocomposites, tissue engineering scaffolds, hydrogels, and medical wound dressings. In the following sections, the structure, preparation, characterization, and applications of CHNCs in the biomedical area will be briefly reviewed. 

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Many significant scientific and technological advances have recently been made in the area of the isolation of nanofibrils (otherwise called chitin nanocrystals, or whiskers). In particular, articles have been pubhshed dealing with the following advances (1) nanochitin isolated after hydrolysis with HCl (2) nanochitin isolated mechanically in the presence of minor amounts of acetic acid (3) nanochitosan obtained from partially deacetylated chitin and (4) defibrillation of chitosan. These advances are of such importance that they overshadow the technology developed during the previous years, for reasons that will be apparent below [33]. 

Very recently Mathew et al. [79] reported on the crystal studies of chitosan/ chitin crystal nanocomposites. The chitosan (a) exhibits a highly amorphous nature with broad and ill-dehned signals at 29 = 9-10° and 18-20° (Fig. 3.13). The chitin nanocrystals (b) show a strong peak at 29 = 8.8 and 19° and shoulders at 29 = 20 and 22°, conhrming its crystalline structure as a-chitin. 

Goodrich, J.D., Winter, W.T. a-Chitin Nanocrystals Prepared from shrimp shells and their specific surface area measurement. Biomactomolecules. 8, 252-257 (2007)... 

Fan, Y., Saito, T., Isogai, A. Chitin nanocrystals prepared by TEMPO-mediated oxidation of alpha-chitin. Biomacromolecules. 9, 192-198 (2008b)... 

Cationic Cellulose and Chitin Nanocrystals for Novel Therapeutic Applications... 

Later, Rinki et al. proposed a green approach to prepare nanoscaffolds from CHNCs using supercritical carhon dioxide (scCOa). The scCOa method was found to be more time and energy efficient, with improved scaffold properties compared to the lyophilization method. An increase in surface area, pore volume, and pore size confirmed formation of the network structure. This type of highly porous biocompatible scaffold is an attractive material for tissue engineering applications. Lertwattanaseri et al. also reported a microwave technique for the preparation of a chitosan scaffold from chitin nanocrystals. 

Gandini A, Belgacem MN (2008) Furan derivatives and furan chemistry at the service of macromolecular materials. In Belgacem MN, Gandini A (eds) Monomers, polymers and composites from renewable resources. Elsevier, Amsterdam Goodrich JD, Winter WT (2007) a-Chitin nanocrystals prepared from shrimp shells and their... 

The 1 wt% of chitin nanociystals were added to poly(lactic acid) plasticized with glycerol triacetate plasticizer in blown film application. The addition of chitin nanocrystals increased the tear strength by 175% and the puncture strength by 300%. Also, the glass transition temperature increased by 4°C and the films degree of ciystallinity was... 

Saralegi A, Fernandes SCM, Alonso-Varona A, Palomares T, Foster EJ, Weder C, et al. Shape-memory bionanocomposites based on chitin nanocrystals and thermoplastic polyurethane with a highly crystalline soft segment. Biomacromolecules December 2013 14(12) 4475-82. 

Chitin has been known to form microfibrillar arrangements embedded in a protein matrix, and these microfibrils have diameters ranging from 2.5 to 2.8 nm [81]. Crustacean cuticles possess chitin microfibrils with diameters as large as 25 nm [82, 83]. Although it has never been specifically measured, the stiffness of chitin nanocrystals thought to be at least 150 GPa, based on the observation that cellulose is about 130 GPa and the extra bonding in chitin crystallites is going to stiffen it further [84]. 

Mathew et al. [196] developed cross-Hnked bionanocomposites using chitosan reinforced with chitin nanocrystals and gluteraldehyde as the cross-linker. These composites were characterized by FTIR, XRD, and atomic force microscopy (AFM). The authors found that cross-hnking and chitin whiskers content were both found to impact the water uptake mechanism. Cross-hnking provided dimensional stabihty in addic medium and significantly decreased the equihbrium water uptake. Moreover, incorporation of chitin nanocrystals provided increased permeation selectivity to chitosan in neutral and addic medium. 

On the other hand, chitin and chitosan are employed as reinforcement for polymers and in particular for PUs. Saad et al. synthesized PUs with block copolymers of PHB and they reinforced them with chitin nanocrystals. They reported an improvement on the thermal stability due to the addition of nanofillers [128]. Barikani et al. developed hybrids of PU and chitosan and found that the chitosan increases the thermal stability of the PU matrix [129]. Similarly, Zia et al. added chitin to the polyurethane backbone, reporting an enhancement on their thermal properties [130]. 

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