Rewriting Life

Plastic from Grass

Engineers seek a cheaper biodegradable polymer.

Jun 5, 2013

Nearly all the plastics sold today come from petroleum and aren’t biodegradable. But researchers at Metabolix in Cambridge, Massachusetts, are genetically engineering switchgrass to produce a biodegradable polymer that can be extracted directly from the plant.

That could transform the economics of making biodegradable polymers. Metabolix already sells such a polymer, but it’s produced by bacteria that feed on plant sugars in expensive fermenters. A plant-based process, which could use crops grown on marginal lands, would require less equipment.

Oliver Peoples cofounded Metabolix with biologist Anthony Sinskey.

Metabolix estimates that it could ultimately sell its plant-based polymers at less than half today’s prices. Whereas today’s end products are niche items like biodegradable plastic shopping bags, more widely used types of products and packaging could then become economical.

The plants-to-plastics vision has gripped Metabolix’s chief scientific officer, Oliver Peoples, a former MIT research scientist, for more than 20 years since he and colleague Anthony Sinskey, an MIT biology professor, discovered metabolic genes that allow bacteria found in soil to naturally produce a polymer known as PHA. But after they founded Metabolix, it took a decade to optimize metabolic systems in the bacteria to produce useful amounts of PHA. Doing so in plants is even more difficult. “It’s much more complex and time-consuming to engineer a complex and slow-growing species like switchgrass versus a very simple bacterium,” Peoples says.

Now, Metabolix plant scientists are working anew on inserting those genes, plus others that regulate growth, into plants including switchgrass, camelina, and sugarcane. In switchgrass, they’re coaxing the plant to produce and store in its tissues a specific type of PHA, known as PHB, that can be used to make injection-molded products such as electronics housings. The company is also working on chemical production steps, including extraction of the PHB using solvents, and a thermal method of converting the PHB into a chemical called crotonic acid, which can be used as a feedstock for polymers. After the PHB is extracted or the crotonic acid produced, remnants of the grass could be burned as a biomass energy source that produces lower net carbon emissions than fossil fuels.

Left: In a petri dish, cell cultures derived from the switchgrass seeds are exposed to a solution containing engineered bacteria that transfer new genes into the cells. Some genes, borrowed from soil bacteria, alter the grass’s metabolic system to produce a polymer known as PHB.
Right: Plastics production begins with switchgrass seeds.

Left With the help of a growth medium, the surviving cells develop into plants whose modified genes enable them to produce plastic.
Right: The cell cultures are placed on a medium that includes an herbicide; cells that failed to incorporate the new genes are killed off. (Modified cells include a gene that allows them to tolerate the herbicide.)

Left: After several weeks, the plants are transferred into peat pellets and later transplanted in a greenhouse.
Right: Months later, the cut and dried grass is ready for industrial processes to extract the PHB.

Peat balls in Metabolix’s greenhouse are kept in plastic bags to maintain humidity. Young plants are tested to ensure that they’re producing PHB. In the background are maturing grasses.

Left: In one promising method to recover and use the PHB, chopped switchgrass is first placed in a stainless-steel chamber.
Right: The chamber (right in photo) is next heated to 300 °C, breaking down the PHB into a chemical called crotonic acid.

The crotonic acid gas is captured and cooled in a condensation process adjacent to the heating chamber.

The resulting crotonic acid, shown here, is a key feedstock for plastics and chemical products.

In one promising method to recover and use the PHB, chopped switchgrass is first placed in a stainless-steel chamber.

Left: A different and more capital-intensive process could directly extract the PHB for use in making plastic products. This method starts by putting the chopped grass in a solvent, which frees the PHB.
Right: The resulting liquid is reserved.

Left: Another solvent is poured in.
Right: This allows the PHB to precipitate out of the solution.

Metabolix calculates that the grass must produce 10 percent of its weight as PHB to be economically competitive with other sources of biodegradable plastics. The company has already nearly doubled the PHB content in switchgrass, from 1.2 percent in 2008 to 2.3 percent last year, including 7 percent in the leaves. The process would still produce some carbon emissions: growing and harvesting plants requires fossil-fuel-based fertilizers and fossil-fuel-powered machines. But Peoples predicts it would be cleaner overall than producing plastic from fossil fuels, though a full analysis has yet to be done. For now, he’s eager to finally realize his plants-to-plastics vision. “This is a testament to sheer bloody single-mindedness,” he says.