Keratin forms feathers, horns, and wool. As abundant food industry waste, it is a promising sustainable raw material for bioplastics. But the heavily cross-linked protein is difficult to process and typically results in brittle films. People use it as powder or add “a billion plasticizers so you lose all the mechanical properties of keratin,” says Giovanni Perotto, a materials scientist at the Italian Institute of Technology.
Perotto and colleagues took advantage of the thiol groups on the cysteine amino acids in keratin. They thought they could link the thiols to carbon-carbon double bonds using a simple Michael-type addition.
The researchers first extracted keratin by dissolving wool in an aqueous solution of urea and sodium metabisulfite, which creates a thick soup of long, unfolded protein strands. Then they added a small amount of polyethylene glycol (PEG), epoxidized soybean oil acrylate, or other building blocks containing C-C double bonds. These get grafted on the thiol groups. “The strategy should work for so many molecules; we’ve barely scratched the surface,” Perotto says.
After removing the urea, sulfites, and ungrafted molecules, the researchers cast or molded the engineered keratin into a solid bioplastic. The building blocks bestow keratin with new properties. The PEG-keratin material could be heated and shaped like a thermoplastic, while the soy oil–keratin material could be made into flexible films.
Both the keratin extraction method and Michael addition are known, but combining them to make bioplastic is new, says Naba Dutta, a polymer scientist and engineer at RMIT University. He says the complex purification steps may affect the cost-effectiveness of the material. Nevertheless, he finds the fully biobased soy oil–keratin plastic especially promising for sustainable bioplastics.
Olarewaju Oluba, a medical biochemist at David Umahi Federal University of Health Sciences, says the material’s industrial feasibility and biodegradability warrant further investigation because it could be part of “a sustainable strategy for the conversion of low-value waste into high-value biodegradable plastics with applications in packaging, construction, and medicine.”