Engineering chitosan-silk fibroin laminates for use as strong, tough, and biodegradable alternatives to plastic packaging

Abstract

Plastics are strong, resilient, and lightweight materials that have become essential to modern life. However, the mass production of plastic has also generated 6.3 billion metric tons of plastic waste, with devastating effects on wildlife, natural ecosystems, and even human health. Thus a need exists for biodegradable plastic substitutes that perform like conventional plastics. This study fabricates, characterizes, and optimizes chitosan-silk fibroin laminate films (termed “Shrilk”) for use as biodegradable alternatives to plastic packaging, which makes up 40% of all plastic waste. Shrilk—a portmanteau of the two words, “shrimp” (because chitosan is often isolated from shrimp shells) and “silk” (from silk fibroin)—demonstrated optical clarity and tensile properties comparable to common plastic packaging materials. The laminate structure resulted in higher tensile strength than both pure and blended films. Raman spectroscopy indicated a region of chitosan-silk fibroin overlap at the interface, suggesting that the laminar arrangement enables chitosan and silk fibroin to interact in an organized form of mechanical entanglement. Shrilk films were confirmed to be biodegradable, losing 84% of their total mass after four weeks in simulated landfill conditions. Casein, collagen, and keratin were investigated as potential substitutes for silk fibroin, but were unable to reproduce the adhesion, flexibility, and strength found in Shrilk films. The molecular weight of chitosan, thickness of films, number of laminate layers, and amount of glycerol were varied in order to alter Shrilk’s mechanical properties, but generally had little effect. However, covering Shrilk films with a hydrophobic coating effectively prevented Shrilk from completely losing its mechanical properties when wet, and even suggested that a moderate amount of water could act as a plasticizer to increase the films’ toughness and elongation at break. Overall, this work presents a strong foundation for future research into chitosan-protein laminates as a potential solution to plastic pollution.