09 September 2013

Scientists seek to grow biodegradable, carbon-neutral plastic in plants

It’s inescapable — plastics are everywhere. From the alarm clock you slap a hand down on the moment you wake up, to the smartphone you cradle in your hands before bed, plastics make modern technology possible. But what happens to all that plastic when it’s discarded? The sad fact is most of the old alarm clocks and broken cell phones you throw away will outlive you. A group of researchers at a company called Metabolix in Cambridge, Massachusetts hope to change that by producing inexpensive and versatile biodegradable plastics. This isn’t just another petrochemical trick — these folks are growing plastic in plants.

The plants are the end stage, though. To understand what Metabolix is doing, we need to look at where it started. Oliver Peoples is the chief science officer at Metabolix and was a researcher at MIT for over 20 years. During that time, he discovered a series of genes in soil bacteria that naturally produce a carbon polymer called PHA. This piqued his interest.

PHA — or polyhydroxyalkanoates if you want to be formal — is a linear carbon polymer produced in the bacterium Alcaligenes eutrophus by the fermentation of sugars. This organism uses PHA as an energy storage medium in much the same way animals (including humans) use glycogen. What Peoples realized is that PHA could be used to assemble plastics that break down when disposed of.

It took a decade of painstaking work to coax the bacteria into producing usable amounts of PHA for testing.
Even at optimal production levels, the bacteria will never make enough PHA to be a viable production method. The breakthrough may come in the form of trans-species genetic engineering.(We’ve received word from Metabolix. A representative noted that “Metabolix has, and will, make PHA biopolymer economically in microbes.”)

Metabolix has spent the last few years attempting to insert the bacterial genes responsible for the production of PHA into plants — switchgrass to be precise. You may be familiar with switchgrass from its role in the production of cellulosic ethanol. Metabolix is using it for the same reason ethanol producers do — it’s fast-growing, hearty, and produces a lot of carbon biomass.

These modified plants store some of the carbon from their CO2 intake in a form of PHA called PHB that can be used to make plastic housings and packaging. Metabolix estimates that for the process to be viable, the switchgrass needs to be roughly 10% PHB compounds by weight, and it’s getting close. Early experiments in 2008 reached 1.2% PHB, which rose to 2.3% by last year. The tantalizing hint that researchers are on the right track — some parts of the plants have gotten as high as 7% PHB.

In advance of reaching the magic number, Metabolix has been developing the necessary technology to extract the polymers. The first method involves baking chopped switchgrass at 300° Celsius. This breaks down the PHB into crotonic acid, which is then captured and used to make plastic. Another more expensive, but promising method is to use solvents to directly precipitate the PHB out of chopped switchgrass.

This process has huge potential in waste management, but also in environmental protection. Because the carbon polymers are produced from COtaken in by plants, it is essentially carbon-neutral — no fossil fuels required. Research is continuing for now, but there may come a day in the not too distant future that the busted old cell phone you thoughtlessly toss in the trash will break down into harmless microscopic bits long before you do.

Now read: How to grow plants pre-processed for biofuel production


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