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Plastics You Could Eat  

It can be grown in plants. It can be eaten by bacteria. And it can be made into bottles for shampoo. Is PHB the new miracle plastic?

by Kathy Sykes

Do we need biodegradable plastics?

Just in the UK, we produce enough plastic to fill the Royal Albert Hall - three times every single day. That's about six million tonnes a year. And most plastics last indefinitely, taking thousands of years to disintegrate. If the Romans had made plastic, we'd still have their waste with us today. If we don't want our great-grandchildren to have to live on mounds of old plastic, we should try to address the issue.

Recycling plastic is an option, and works well for bottles, but it can cost more in energy terms than using virgin plastic. Burning waste plastic to harness the energy is another option, but the gases released by some plastics are very toxic. The case for biodegradable plastics is strong, especially where packaging gets dirty and can't be re-used.

Several plastics masquerade as biodegradable, but actually break down into tiny invisible bits that could affect the soil and animals such as earthworms. Of the few plastics that are genuinely biodegradable, many contain toxic residues from catalysts. One plastic, that's biodegradable and contains no nasty residues, is called PHB, polyhydroxybutyrate.

Bugs make plastic

'Bugs make plastic' ran the headline when PHB was first commercialized in the early eighties. Most plastics are made from oil but this one is made naturally, all the time, by bacteria. And the supply is limitless. Almost any kind of bacteria store up energy as 'fat' available to be used later by the bacteria. When extracted, this bacteria fat solidifies into a polymer very similar to traditional plastics like polythene. To make PHB on a big scale you just fatten up bacteria with glucose in giant fermenters and effectively 'brew' the plastic.

Not only do they make it - bacteria can eat it. PHB is so rich in energy for bugs, they can't resist it. A discarded piece of PHB will be gobbled up by bacteria in the environment. The time it takes for PHB to degrade just depends on the concentration of bacteria: in the sea it takes PHB a few months to break down; in a sewage works it takes just a few weeks; but on your kitchen or bathroom shelf, it will last decades.

Jamie Hobbs

PHB cystals spiral out from a central point forming beautiful Spherulites

Why isn't PHB filling up our supermarket shelves?

Here is a biodegradable material that could be used for many things: shampoo bottles, plastic cups and packaging materials. But currently it's used for very little. One major factor limiting PHB's use is its brittleness. Drop a brick into a PHB shopping bag and it breaks. The other main factor limiting PHB's use is the cost: we'd have to pay a few pence extra for products packaged in PHB. But there are a few places where PHB is used. The US Navy, having despaired of trying to prevent staff throwing drinking containers overboard, has opted to use PHB cups. Now the sailors are free to throw their empty cups into the waves. In Japan, PHB has found its way into people's bathrooms and appears in the form of disposable razors. Apparently some Japanese women are so uncomfortable about body hair they remove every bit of it. The discretion surrounding the whole activity requires that the razors be thrown down a toilet, and instantly flushed away. PHB's rapid degradation in sewage clearly makes it the right choice.

PHB's success in market penetration could be greatly improved with new governmental legislation. A UN convention in 1973 required that all material thrown directly into the sea must be biodegradable, and it was this gradually coming into force that got the US navy interested in PHB. The Italian government taxes every non-biodegradable plastic bag 250 lira (about 15 US cents), and the Danish government has banned non-recyclable beverage containers. If moves like these continue, there's a better chance that we'll see PHB in our supermarkets.

Even if biodegradable plastics start to be used extensively our waste won't completely disappear. The places where our rubbish is supposed to end up, land-fill sites, are notoriously bad at enabling anything to degrade - even paper lasts decades. There aren't enough bacteria because there isn't enough oxygen around to keep them alive. Large-scale composting facilities will be needed and these cost money.

Using PHB in the Body

Since the bacteria in our own stomachs make PHB to store energy our bodies don't reject it. PHB can be used in medicine, in sutures for example, where the body gradually replaces PHB over time. The plastic could also be used as a material that slowly releases drugs into the body. After surgical implantation, its gradual degradation releases locked-in drugs, right at the point where they are needed most. The fact that the human body accepts PHB, means that you could eat the wrapper along with the chocolate bar.

Jamie Hobbs

The exact form of the PHB spherulites depends on the temperature at which they crystallize: These grew at 80°, 55° and 40° respectively.

Beautiful crystals

Plastic bags may not look or feel very crystalline, but if you zoom into a plastic bag by a factor of a million, you can actually see lots of tiny crystals. These little crystals form regular structures themselves: they spiral out from a central point, forming little spheres called 'spherulites'. The crystals spiral completely regularly, and the resulting spherulite structures can be incredibly beautiful.

For years, polymer physicists used polythene, or more correctly polyethylene, as a model to represent the behaviour of other polymers. But over the last 20 years some scientists have opted to use PHB instead. It crystallises more slowly than polyethylene, and so is easier to study. It's been a remarkably useful material, and many new discoveries have been made thanks to its big, slowly-forming crystals.

An unanswered question

PHB is everywhere. Trace amounts in short chains of only about 150 units have been found in the cells of yeast, carrots, spinach, sheep, pigs, cattle and even in humans. It exists in the cells of a staggering variety of different organisms. In fact, it seems that you can find PHB in any cell that you care to choose, if you look hard enough. And nobody knows what it's there for. Surely, for something to be so ubiquitous, it must have some function. It's inconceivable that it's just an accident that PHB is present in so many places. Some scientists have even claimed that PHB could be as important as proteins and that HB units (hydroxy butyrate) might have been present in the primordial soup on earth, before amino acids and proteins. These claims may be extravagant, but whatever the real story is, watch this space; PHB must do something significant in cells.

It grows on trees...almost

A recent development in the PHB story has perhaps changed its destiny forever. A team at the DoE Plant research lab at Michigan State University considered that there might be a faster way of making PHB. In 1992, they took two genes from PHB-making bacteria and inserted them directly into two cress plants and then crossed them. Some of the offspring plants incorporated both the new genes and produced PHB in their leaves. They had managed to create a plant that could grow plastic.

Chris Somerville

White PHB blobs inside a cressleaf

There were problems -the new cress plants were pathetic: their growth was severely stunted. The leading geneticist Chris Sommerville (by now at the Carnegie Institute, Washington), wanted to try out an unusual idea. He decided to add more DNA to the cress plants, but this time from a pea plant. It worked. Now instead of PHB being manufactured randomly over the plant's leaves, it was only made in one place, the chloroplasts, and the plant's growth was completely normal. The PHB formed 14% of the dry weight of the leaves.

Now some scientists are trying to do the same thing in potatoes so it's easier to harvest the PHB. It's quite a nice thought, plastic coming from fields of potatoes instead of from oil refineries. But scientists really are not yet sure about how easy it is to confine genetically modified plants in one area, or about the impact so much PHB would have on the environment. It's too early yet to risk growing PHB on a large scale outdoors.


Factories of the Future?

The future for PHB

There's a real need for biodegradable plastics. PHB has a myriad of potential uses and scientific interest has been steadily growing since it became easily available in the early eighties. Despite this, it seems its future is very uncertain.

In 1996 Monsanto bought all patents for making PHB from ICI/Zeneca. But Monsanto's fermenters producing PHB from bacteria were closed down at the start of last year. That doesn't mean that Monsanto had given up on PHB, quite the opposite -instead they had high hopes for producing PHB from plants instead of bacteria. But now with so much media attention on GM crops, there has been little news of Monsanto's plans for PHB.

All the scientists who've been working on PHB are hoping that Monsanto will keep producing the PHB that they need for their research. Whatever the future for PHB, I'm pretty sure this intriguing plastic will hit the news again in the not too distant future.


Copyright © FirstScience.com

Dr Kathy Sykes researched PHB's crystallization behaviour for five years, before becoming Head of Science for Explore @t Bristol, a new hands-on science centre in Bristol. She has been responsible for an area called 'You Are Amazing', about our bodies, brains and senses. Kathy has previously worked at Tomorrow's World (BBC Television), taught physics in Zimbabwe and also had a brief spate at being a magician's assistant in Florence.>

To find out more about: Explore @t Bristol
With thanks to Jamie Hobbs for the pictures.

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