Researchers at the Wyss Institute for Biologically Inspired Engineering have found a way to replace plastic with a naturally abundant and fully degradable bioplastic. For the past two years, the Harvard institute researchers have been working to create a material they call Shrilk, which is a combination of silk protein and a substance called chitosan that is found in crustacean shells and insect parts. Most bioplastics currently on the market are mainly made from wood cellulose, which can only be made into food and drink containers and does not fully degrade in landfills. In contrast, not only is Shrilk fully degradable, but it also releases plant nutrients into the soil. And unlike bioplastic from wood cellulose, Shrilk can be used to make items such as grocery and trash bags, packaging materials, and diapers. It also poses no threat to trees. A lead researcher said Wyss has been approached by a variety of entrepreneurs and companies interested in Shrilk since March, when researchers made an announcement about their ability to make products from chitosan that could only be made in the past with petroleum-based plastics.
Shrilk could be an environmental game-changer. Only seven percent of the more than 34 million tons of petroleum-based plastic waste that is produced every year gets recycled. The rest finds its way to landfills, where it takes 1,000 years to degrade, or to the ocean. Within one year, some types of plastic begin breaking down in the ocean, leaching toxic chemicals as part of the disintegration process. While the chemicals in this plastic soup can potentially cause cancer in humans, they are thought to be even more toxic to the ocean’s simpler life-forms. There is an estimated 100 million tons of plastic that is currently swirling around in the ocean. Some of that plastic is not of the type that rapidly breaks down, but it still causes problems. According to a 2008 study published in Environmental Research, plastic garbage affects 267 marine species, and it is eaten by approximately 44 percent of all seabirds.
Chitosan is a resilient form of chitin, which is “the second-most abundant organic material on Earth,” according to the Wyss Institute. Most chitin currently comes from discarded shrimp shells. It is used in cosmetics, dietary supplements, and fertilizers. Shrimpers around the world, and particularly those in Honduras, Vietnam, and India, are always looking out for economically viable ways to make use of their shrimp shells. Chitosan-based products could provide an additional source of revenue for these shrimpers. Another possible source for chitosan is the copepod, a species of plankton-sized crustaceans. Wyss Institute lead researcher Javier Fernandez says that the copepod is thought to produce billions of tons of chitin per year. This means, according to Fernandez, that the amount of chitin the crustacean has produced in the last year equals the amount of plastic the world has produced in the last five years.
Shrilk is a complex material that needs to be fabricated in specialized environments, and chitosan is not waterproof like petroleum-based plastics, so a coating of beeswax is required to create a water barrier. Also, some modifications will be required to use the current technologies of injection molding and casting to mass-manufacture Shrilk-based products. Fernandez says that the ways manufacturers have worked with synthetic polymers and ideas that have developed around them will also need to be retooled. But once those minor hurdles are cleared, Shrilk products will be on their way to replacing petroleum-based plastics.
By Donna Westlund