Materials with enhanced structure derived from crustaceans and seaweed could be part of a next-generation answer to the challenge of replacing petroleum-based plastic films, according to new research from North Carolina State University. The research has been supported by the National Science Foundation.
Combining chitosan, a biopolymer that makes crab shells hard, with agarose, a biopolymer extracted from seaweed that is used to make gels, creates unique biopolymer composite films with increased strength. The films are also biodegradable, anti-bacterial, water repellent and transparent. The findings could eventually lead to sustainable packaging films for food and consumer goods.
Orlin Velev, S. Frank and Doris Culberson Distinguished Professor of Chemical and Biomolecular Engineering at NC State and corresponding author of a paper describing the research asked:
How do we find sustainable replacements for synthetic polymers? Synthetic polymers make very good films, but we want to replace them with natural biopolymers. The question is, how do we tailor the common structure of these natural polymers - in our case, agarose and chitosan - so that we have all the desirable properties of synthetic polymers in a sustainable, biodegradable film?
Simply mixing chitosan and agarose together may not be enough. Velev says that previous attempts to make such blends reported improvements in properties, but when dried, they produced gritty films that may not have the right strength.
Instead, Velev and his collaborators took a different approach, reinforcing the agarose films with fibrillated colloidal scale material - called soft dendritic colloids - made from chitosan. The strong micro- and nanoscale chitosan fibrils are hierarchically branched to give strength and stability to the agarose film in which they are embedded.
Yosra Kotb, an NC State doctoral student and first author of the paper said:
It is challenging to chemically modify natural polymers, but we can change their morphology and use them as composites. We use dendritic chitosan particles to reinforce the agarose matrix because the compatibility of the two materials results in good mechanical properties; chitosan particles also have an opposite charge to agarose. When mixed, these charges are neutralised, making the resulting materials more resistant to water.
The biopolymer composites are about four times stronger than agarose films alone, the research shows, and are also resistant to E. coli, a commonly studied bacterium. The paper also showed that a film made from biopolymer composites degraded significantly after a month underground, while a common plastic sandwich bag remained completely intact after the same period underground.
"Interestingly, our composite is initially strongly antibacterial," Velev said, "but because it is made from natural materials, bacteria will still colonise it after some time, so after a month underground, it will easily biodegrade".
Velev added that his lab will continue to work on improving the structure of the biopolymer composite films, with the aim of eventually matching the properties of synthetic polymer films.
“If you package food, you want the package to be impermeable to oxygen and water,” he said. “But natural materials are permeable, so we will continue to work to make our films more impermeable to water and oxygen”.
