WHY THIS MATTERS IN BRIEF

Who needs to buy real flowers any longer when plastic ones are good enough?

Tick tock. Shape-shifting objects can now have their own version of a biological clock, thanks to a new material that transforms at a given time.

Morphing materials are interesting because they allow objects to change shape, and thus function. But they typically need a trigger to start morphing, like a change in light levels, temperature or pH. Now Sergei Sheiko from the University of North Carolina at Chapel Hill and his colleagues have created a type of putty with an internal clock that allows it to transform over time.

“In certain situations, like inside your body or in space, external triggers are not permissible or are ineffective,” says Sheiko.

“You simply want an object to change shape at a given moment.”

The team started with a conventional soft polymer then tweaked its molecular structure. A small proportion of links between molecules in a polymer are permanent, allowing the material to act like a spring, snapping back to its original form when stretched and released, like a piece of rubber. But most of the bonds are shape-shifting, breaking and rearranging themselves over time. The rate of shape shifting can be modified, allowing the researchers to control how the material changes over the course of several hours.

“Most bonds snap in a split second, so our goal was to extend their lifetime,” says Sheiko.

To demonstrate the concept, the team used the material to create a delivery box which opened automatically on one side when it reached its destination. But the morphing polymer is likely to be more useful for designing medical implants that are folded up for insertion then change shape inside the body.

Designing complex shapes proved difficult, but the team came up with a workaround. They found that intricate designs could be assembled from building blocks that could each be programmed to change at different times.

“Nobody has ever done this before,” says Sheiko. Their most sophisticated design is an artificial flower with individually programmed petals that blooms autonomously.

Programming the material to change at a constant rate was easy. But Sheiko and his team struggled to introduce a dormant period or to accelerate change at certain times. Although they didn’t find a way to modify the timing mechanism for this purpose, giving an object an extra ‘skin’ solved the problem. By adding a water-soluble shell, and tweaking its thickness based on the desired time delay, an additional clock could be added to the system when dropped in water.

“We plan to explore this further,” says Sheiko.