Design and fabrication of chitin/chitosan aerogels

Abstract

Chitin is the second-most abundant, non-toxic natural biopolymer containing N-acetylglucosamines (GlcNAc) linked by β-1,4 glycosidic bonds. The N-deacetylation of chitin yields chitosan which improves the water solubility under acidic environment. Both chitin and chitosan are widely used in many applications including biomedicine, catalysis, agriculture and purification and separation techniques due to their biocompatibility and biodegradability [1-4]. For the target specific biological applications, hydrogel matrices of chitin/chitosan and their derivatives have been widely employed [5, 6]. Because of their available -OH, -NH2 and -NH-C(O)-CH3 functional groups, they can be physically or chemically modified by desired molecules. Engineering randomly connected fiber matrices of chitin/chitosan into various dimensions can be achieved in laboratories by inducing either the self-assembly of the molecular chains or percolated network formation of nanowhiskers/nanofibrils. There is a series of actions happening in the wet state-forming higher-order structures of chitin/chitosan matrices: (a) association of molecular chains in a fashion producing the smallest building blocks called nanofibers; (b) formation of a randomly interconnected three-dimensional network in a small domain; (c) finally the complete formation of pure matrices of chitin/chitosan filled with liquid. The hydrogels can be designed into nanoparticles, beads, fibers, sheets and monoliths of different dimensions. The hydrogel matrices can also be fabricated as specific three-dimensional design by using 3D-bioplotter [7]. In a general work-flow of aerogel preparation, after hydrogel formation, the chitin/chitosan matrices are treated in several steps: neutralizing/washing, cleaning, solvent exchange and supercritical/freeze drying. We have developed in the last years a novel green method to the syntheses of the aerogels of chitin, chitosan and chitosan derivatives [8, 9]. In most of our designs, as a target, the nucleophile -NH2 functional groups was chemically modified. In order to form hydrogels, all variable parameters such as pH values, acids, bases, temperature and solvent medium have been controlled. As a result, we have developed a variety of open porous chitin/chitosan materials. For instance, change in acid strength in the chitosan hydrogel formation resulted in huge difference in stability and porous network formation. In general, the microstructures of randomly interconnected nanofibrillar networks have been obtained. The physical properties have shown low density, high porosity (>93%) and high surface area (150-350 m2/g).