​How Humans and Nature Make Their Own Plastics
Plastics found in the sea. ​​Image: Ed Bierman / Flickr

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​How Humans and Nature Make Their Own Plastics

Humans and trees don’t have much in common, biologically speaking. But we do share one deep love: long molecules. Really long molecules.

Humans and trees don't have much in common, biologically speaking. But we do share one deep love: long molecules. Really long molecules.

We like long molecules so much, we've given the whole set of them a name: Polymers. All of biology, including humanity, likes polymers because they do some very useful and uncommon things. Like a bowl of spaghetti, these molecules are long enough to get all tangled and knotted up with each other.

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Those long chains of knots and folds provide very useful material properties. High strength, toughness, and elasticity, for example. And these properties are not limited to just solids. Pretty much every weird ​non-Newtonian fluid—Silly Putty, corn starch in water, Jello—is "weird" because these long molecules push and pull on each other within the fluid and give them an almost solid springiness that water and motor oil just don't have.

Nature has its own set of polymers, and it uses them for many of the same purposes we do: protection (think Kevlar vs. chitin in crab shells), style (Rayon vs. butterfly wing patterns), weapons (​3D-printed guns vs. porcupine quills), and building materials (adhesive tape vs. spider silk).

To paraphrase Herman Staudingerr, the Nobel-winning inventor of nylon who brought polymers into the mainstream, there's not much variety you can get out of a few building blocks. But if you have thousands of blocks, the most varied of buildings and halls and structures become possible, whose functions and properties would be otherwise impossible to imagine.

Making Big Molecules From Small Ones

Humans and the rest of biology have independently converged on the fantastic value of polymers, but we've approached them from very different directions.

First, it's useful to have a sense of scale. How long is a "long" molecule? Let's start small, with one of our favorites, H2O. A thimble-full of water has about 10^22 individual molecules of H2O in it. That's 10,000,000,000,000,000,000,000 molecules. If you somehow started plucking individual molecules out of that thimble at the instant of the Big Bang, and counted one molecule every second, you'd only be 0.001 percent through that thimble by now. You would need to keep counting, never stopping, for ten thousand times longer than the age of our known universe in order to count every molecule in the one thimble.

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In comparison, the entire rubber sole of your shoe is a single highly connected molecule of rubber. Just one. And one uncoiled molecule of DNA would stretch a few inches, and the DNA in a single cell would span nearly 10 feet.Considering that the entire cell making that DNA is smaller than the width of a single hair, that's a long molecule.

And long molecules are not the universe's forte. Carl Sagan and Neil deGrasse Tyson have beautifully described how we are all made of "​star stuff..)" Nearly every single atom in the universe, including those in our plastics, was first forged in the blazing heart of a star. But out in space, and even in planetary atmospheres, the biggest molecules are rarely larger than a few atoms. Assembling big molecules from small ones requires clever bending of the universe's rules of thermodynamics and entropy.

And if anything is good at bending rules, it's life.

Humans, for example, far outpace the rest of the biological world in complex tool use. Convection ovens, bicycles, smart watches, atomic bombs, low earth orbit telescopes. We've got a lot of them.

For connecting molecules, we still use the most primitive of human tools: Fire. Good old industrial-scale pressure cookers are still our method of choice to connect monomers together into a polymer, for building plastics. The key lever to make the plastic that comprise our grocery bags is the same one we use to produce the highest-performance polymers for the most demanding technical applications: Nylon, Kevlar, silicone implants.

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Only a select few Marvel heroes might opt for bullet-proof breast implants. To make one or the other, the devil is in the chemical details—the starting materials, catalysts, and reaction rates. But when chemists are ready to start connecting those small molecules together into polymers, nothing beats fire. Heat and pressure can push enough energy into those molecules to get them past the barriers of chemistry and entropy, in order to link them up into one long macromolecule.

The plastic-making life surrounding us is more like the hand-crafting artisans of Portland.

Nature has its polymers, too. Plants and animals want hard ones to protect against their enemies. They want shiny ones to dress in style and attract mates. They want warm coats to keep out the cold. From a molecular point of view, chitosan and cellulose and silk and keratin are like styrofoam and Nalgene and Kevlar. They're long macromolecules synthesized by stringing together smaller building blocks.

Biology doesn't have fire to get past the thermodynamic construction hurdles. The polymer-making life surrounding us is instead more like the hand-crafting artisans of Portland. They collect the building blocks they need from their hyper-local environment, and they slowly but deliberately build signature things.

A tree pulls CO2, water, and minerals from the air and soil as construction material for its cell walls. A spider collects amino acids from its prey to use as eventual sources for silk proteins. A human cell takes up phosphorus, a recurring step along DNA's helical ladder, from that person's diet. Slowly but surely, atom is linked to atom through an elaborate dance of intricate metabolic exchanges built to overcome the same barriers of thermodynamics. Bark grows, silk is spun, genetic information defining your you-ness is twisted into polymeric DNA.

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These very different chemical manufacturing styles have a few major consequences.

For one, natural polymers are information-rich. The long polypeptide sequence of keratin, for example, is a very specific composition and ordering of hundreds of amino acids building blocks. Change their composition and ordering, and you make either claws or hair. Weapon or style. It's all keratin. An even more subtle change will make that hair curly or straight. At the ultimate extreme, the specific connections within a strand of DNA defines the very difference between you and your brother. There's information hidden within those molecules.

Man-made polymers are dumb, in the sense that the entire length of a polymer strand is most often made of identical repeating monomers. Swap the position of a few units in a spaghetti-strand of Kevlar, and it's the same Kevlar. One is no different than the next. There's very little hidden information.

Reversibility is another consequence of different polymer-making approaches. The energetic hammer and chisel of heat and catalyst slams man-made plastics over very high chemical activation barriers, and they will not decompose unless they're pushed back just as hard. That's why our milk jugs can last for centuries when our milk doesn't last the week. This is very useful for making airplane seats that endure over 10,000 butts. It's not useful when non-degradable grocery bags start clogging up the oceans.

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Meanwhile, making biological polymers takes place in incremental, almost infinitesimal, steps in terms of energy use. Those myriad low energy steps mean materials can creep backwards just the same. A chunk of tree bark floating down the river, exposed to the energy of light and microbial enzymes, won't be bark for long. It will soon feed the manufacturing dance of another cell in another organism.

Learning From Nature

With all of life as we know it having converged on the long molecule, humans are slowly learning from nature. We've figured out clever ways to affect the level of organization within molecular linkages enough to make man-made polymers which intentionally organize at the molecular level and respond to environmental triggers like pH and light.

This toolbox already shows some promise in making ​be​tter materials for drug delivery, for inscribing molecule-sized circuits on ​ever-s​hrinking electronics, or for ​creating biodegradable electronics.

But there are still many things we can learn from the way nature's lot uses its long molecules. Lifecycle design, for example, could much more intelligently turn processing waste into useful polymer, which could be further designed to degrade into useful building blocks in their own right. We've started on the first steps down this path, with the ​biodegradable PLA polymers that are finding their way into food packaging and 3D printers. If we can muster the manufacturing patience, growing polymers in complex cell-like steps might even cut down on the massive energy costs of our primitive fire pressure cookers.

The way we can tweak small molecules to make long ones is getting better, in other words—and more complex and more responsible by the year. But we've got a long way to go before we get to spider silk.

Andrew Davis is a science professionalist, hobbyist, and enthusiast, formally trained as a polymer scientist. Follow him on twitter at ​@weemadandrew.