Materials programmed to shape shift

By Paul Rincon
Science editor, BBC News website

media captionScientists have pre-programmed materials to transform their shape.

Scientists have pre-programmed materials to change their shape over time.

Previous shape-shifting materials have needed some external trigger to tell them to transform, like light or heat.

Now, a US-based team has encoded a sequence of shape transformations into the very substance of a polymer, with each change occurring at a pre-determined time.

The principles could be applied in implants that deliver medicine from within the human body and the technology could also see use in heavy industry.

Professor Sergei Sheiko from the University of North Carolina at Chapel Hill, and colleagues introduced two types of chemical bond to their polymer: permanent bonds and dynamic (or reversible) bonds.

The permanent bonds store the material's final shape, while the dynamic bonds control how quickly it can reach this shape.

Prof Sheiko said there were several parameters in the material which, when adjusted, allowed the scientists to control the changes.

"One is the strength of the individual bond - or the energy of dissociation of the individual bond. The other is the concentration of these bonds," he explained.

"There is a third parameter: several individual hydrogen bonds (dynamic bonds) can form a cluster. This cluster of hydrogen bonds can then form stronger cross-links."

In bloom

As a proof of concept, the team designed a synthetic flower which "bloomed" in a pre-programmed fashion.

"We wanted to make the concept more explicit. So there are plenty of examples in nature, like flowers, which change their shape with time," Prof Sheiko told BBC News.

"One of the advantages of our technology is that you can assemble a complex shape of individual pieces like these petals. Usually shape memory materials are just made up of one chunk that changes shape."

He said each of the pieces could be programmed individually, with different timings.

Asked how precisely the timing of the changes could be controlled, Prof Sheiko explained: "We cannot control accurately between 20 and 21 seconds. But we can control between 20 and 60 seconds, two minutes and five minutes.

"We can control [the shape changes] pretty accurately on a scale of minutes and hours."

Potential applications include drug delivery systems, which allow medicines to be released within the body according to a specific timescale.

"People want a material that changes shape without a stimulus. The reason is very practical: there is often no way to apply one.

"In the body, for example, it is pitch black inside and the temperature is super-stable. Our bodies work very hard to maintain a constant temperature. It's a similar situation in space, or down an oil borehole."

Prof Sheiko said his team had been asked about the possibility of producing a smart cement for the oil and gas industry. This could be poured down a borehole but would remain liquid for a while before setting at a specified time.

He explained: "It's a very interesting challenge indeed, to design materials that would change their properties - which could be colour, shape, density, or mechanical properties - simply as a function of time."

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