Strings, rings and automobiles: Kozo Ito’s revolutionary polymers

It is a breakthrough that could lead to robots with artificial muscles and lightweight automobiles that can travel twice as far on a single battery charge, but for University of Tokyo professor Kozo Ito it all began on a flight to China in 1995.

He was on his way to a scientific conference when he found himself seated next to a fellow scientist called Akira Harada. Harada, a chemist, began to talk about a molecule he had recently created, the properties of which seemed hard to believe.

The molecule was called a polyrotaxane and the best way to visualise it is as a necklace of beads on a string.

In his synthesis, Harada explained, the beads threaded themselves on to the molecular string, where they could slide back and forth, with a stopper at each end to prevent them coming off. “Poly” means many; “rota” means wheel; and “axis” means axle. They are a molecule made of many little rings on an axle.

This kind of substance is strikingly different from familiar molecules encountered in high-school chemistry, such as water or carbon dioxide, where atoms bond by sharing electrons with one another.

The components of a polyrotaxane interlock physically, like keys on a key ring, without the atoms having to interact with one another at all.

Polyrotaxanes are part of a wider class of “mechanically linked molecules” used to develop molecular machines — an achievement for which Jean-Pierre Sauvage, Fraser Stoddart and Ben Feringa were awarded the Nobel Prize in chemistry in 2016.

“The broadest view of this field is: how can we thread molecules through one another and what are their properties once that’s done?” says Steve Goldup, a professor of chemistry at the University of Southampton in the UK.

To Ito — who was not a chemist but a polymer physicist — such molecules sounded fantastical. Polymers are long molecular chains and include everything from plastics such as polystyrene to natural molecules such as DNA. Ito studied their physical properties.

“At that stage, the polymer physics field knew nothing about it. I was fascinated by what I was hearing,” he says. “I was so surprised to learn about how polyrotaxanes self-assemble I couldn’t believe it at first. But sitting on that plane, Prof Harada told me about various X-ray and other experiments.

“I started off thinking it was a fib,” Ito says, “but by the time I got off that plane I was totally convinced. When I got back to my lab, I decided to go for it and changed my research topic.”

A young associate professor at the time, Ito was casting around for a fresh idea, and this odd class of molecule sounded like an exciting prospect. He gathered a few graduate students and set to work. But since he was coming from polymer physics, his interests were rather different from those already researching in the field.

“At the time I started, there were lots of chemists involved, and chemists love to make beautiful, tightly packed structures. But I was a physicist and I wasn’t so bothered about that. I wanted to know how on earth those rings got on there in the first place. How did those rings get on the thread? It was so mysterious. And once they were on, they could move so quickly. I wanted to measure how fast,” he says. “I was good at measurement.”

Measuring how fast one part of a molecule moved relative to the rest of it was no easy task. Ito and his team had to make the molecule’s string longer and longer so that the rings had freedom to slide.

“There were all these chemists around the world making densely packed structures and just me making them freely moving so I could measure the speed. Then one of my students asked, ‘What happens if we connect them up?’ ”

To cross-link the polymer strings would be nothing new: that is what it means to vulcanise rubber. But polyrotaxanes were peculiar strings with rings sliding along them. What Ito and his student Yasushi Okumura did in 1999 was join the rings to one another, making cross-links like a figure of eight.

If joining polymer strings gives you something like a fishing net, then joining the rings gives a net with a little pulley at every point where two threads meet, resulting in a material with some spectacular properties. Apply stress and, instead of tearing, the strings whizz along through the rings like a rope going through a pulley. Remove the stress and they relax back to how they were.

At the level of the naked eye, you can dish out a remarkable level of abuse — stretching a gel to more than 20 times its starting size or compressing it to a 20th — and the slide-ring material returns undamaged to its starting point.

The potential value of such a material was immediately obvious, and the University of Tokyo team took out a basic patent.

“The properties of these materials are really cool, particularly given that the polymer strings themselves are very simple,” says Goldup.

What makes them so interesting, he adds, is that the slide-ring observation is general. It does not apply to just a single chemical but can in theory be applied wherever there is a need to make a polymer stronger. As with all polymers, the basic building blocks are familiar atoms such as carbon, hydrogen and oxygen, but many variations are possible.

Ito is a cheerful communicator, knows his way around a government committee and has an entrepreneurial bent unusual in Japanese academia. Straight away, he pushed to commercialise his invention, spinning a venture company called Advanced Softmaterials out of Tokyo University.

As with many scientific discoveries, however, it has taken decades to figure out where slide-ring materials are useful and shift them from research into development.

The team’s first thought was contact lenses, but advances in that field meant there was little room for a new material. Their next attempt was a coating that took advantage of the incredible ability of slide-ring materials to absorb stress and snap back to their original form, properties that could heal scratches on the surface of a screen.

“NTT Docomo [Japan’s biggest mobile company] made about 300,000 phones. Self-healing materials weren’t yet in vogue but that was the start,” says Ito.

Once again, however, the innovation was not quite in tune with the market, which was moving heavily towards the iPhone, with its touchscreen made out of scratch-resistant glass.

All the applications to that point had involved soft materials. But in 2014, Ito became the manager of a joint academic and commercial programme called ImPACT, backed by a huge Japanese government grant, which aimed at making breakthroughs in polymers.

Japan is a world leader in polymer science and home to many top companies in the field. That is partly, says Ito, because polymers were still a young and active field as Japanese basic research began to find its feet in the 1950s and 1960s.

As one part of ImPACT, he worked with Toray — one of the world’s biggest producers of carbon fibre — to combine a slide-ring polyrotaxane with its carbon-fibre reinforced plastics.

The result is a plastic that is strong enough to serve as the chassis for an automobile. To prove the concept, the project built a car called ItoP made entirely from plastic.

ItoP is meant to stand for “Iron to Polymer”, but, as several participants note with a smile, it could also be read as “Ito Plastics”. The vehicle weighs about 850kg, compared with 1,300kg or 1,400kg for a normal car, and has the potential to solve some of the range issues associated with battery vehicles. The less weight there is to move, the further the car can go on a given charge.

“We’ve established the concept,” Sadayuki Kobayashi, chief research associate in the plastics research laboratory at Toray, told the Financial Times last year. But he warned it would take a long time to get slide-ring materials into regular use.

“The key material, the polyrotaxane, has no commercial base at all. So we have to go through a big process of scaling up the manufacturing,” he said.

In the meantime, there are slide-ring golf balls and golf tees, and Ito is excited by the “e-Rubber” sensors and actuators (or “movers”) under development at Toyoda Gosei, an offshoot of the Toyota automobile group. Toyoda Gosei has used slide-ring materials to create a substance that can contract and vibrate when an electric current passes through it.

The initial application may be in a touchscreen or some kind of tactile feedback system, but the real prize — which remains a long way off for now — is different. This is a substance that moves without a motor.

“The most amazing thing would be artificial muscle,” says Ito. “A robot moves using motors but they all have a magnet inside so they’re very heavy. If you used this material, you could make an incredibly light robot with something like muscle.”

One of the main reasons why roboticists do not expect their creations to take over the world any time soon is that they are weaker than babies: it takes 10kg of robot to move 1kg of weight. But actuators made using the Toyoda Gosei material are 10 times more powerful.

“It really would be like an android — something close to a human — if you could use artificial muscles,” says Ito.

In terms of basic research, polyrotaxanes and slide-ring materials are closely linked to the world of molecular machines, which were the focus of the 2016 Nobel Prize. Slide-ring materials rely on the rings moving under an external force but their properties also make it possible to create machines such as switches, motors and transistors at the molecular level.

A big question, says Ito, is whether these tiny movements can be used to create effects in the macroscopic world we live in. “A molecule may move on the nanoscale, but can that somehow be used to create a bigger mechanical movement like a robot? That’s a line of research I want to pursue.”

The citation for the 2016 Nobel Prize said: “In terms of development, the molecular motor is at the same stage as the electric motor was in the 1830s, when scientists displayed various spinning cranks and wheels, unaware that they would lead to washing machines, fans and food processors.”

Ito is one of those working to translate these tiny molecular motions into materials that change the everyday world. If one day you find yourself behind the wheel of a plastic car, you will know who to thank.

Robin Harding is the FT’s Tokyo bureau chief

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