Cellulose, chitin and silk: the cornerstones of green composites

This overview article is concerned with fabrication and applications of the composites of three major biopolymers, cellulose (Cel), chitin (Chn)/chitosan (Chs), and silk fibroin (SF). A brief discussion of their molecular structures shows that they carry functional groups (-OH, -NH-COCH3, -NH2, -CONH-) whose hydrogen-bonding, and der Waals interactions lead to semi-crystalline structures in the solid phase. There are several classes of solvents that disrupt these interactions, hence dissolve the above-mentioned biopolymers. These include solutions of inorganic and organic electrolytes in dipolar aprotic solvents (DASs), ionic liquids (ILs), and their solutions in DASs. Mixing of biopolymer solutions leads to efficient mutual interactions, hence formation of relatively homogeneous composites. These are then regenerated in non-solvents (water, ethanol, acetone) in different physical forms, e.g., fibers, nanoparticles and films. We discuss the fabrication of these products that have enormous potential use in the textile industry, in medicine, in the food industry, and decontamination of fluids. These applications will most certainly expand due to the attractive characteristics of these composites (renewability, sustainability, biodegradation) and the increased public concern about the adverse environmental impact of petroleum-based polymers, as recently shown by the presence of microplastics in air, water, land, and food (Akdogan \& Guven in Environ Pollut. 254:113011 (2019)). (AU)