4D printing technique makes objects that transform with heat
Scientists at Singapore's University of Technology and Design have developed a 4D printing technique that can create complex structures, including cardiac stents and drug capsules, in just a few minutes. The process creates 3D printed structures which react to stimuli such as heat, moisture or light. The resulting objects are made from a shape-memory polymer and a rubbery elastomer. Whilst the structures are single use and cannot be changed following the heat exposure, they offer potential for the quick manufacturing of medical implants. 
A flower… now in 4D Ding et al. Sci. Adv. 2017;3:e1602890

A new 4D-printing technique that creates complex structures in minutes could be used to make temperature-activated cardiac stents, drug capsules and flat-pack furniture.

4D printing creates 3D objects that change their shape over time in response to stimuli such as heat, moisture or light. It is useful for making structures that can adapt to their environment, but is often a laborious process. The most common materials used in 4D printing, shape-memory polymers, normally require at least five steps to make them into adaptable objects. Hydrogels are simpler to use, but too soft to fashion into rigid structures.

Zhen Ding at the Singapore University of Technology and Design and his colleagues have now developed a way to rapidly print rigid 4D objects with a commercial 3D printer and a heat source.

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They created a variety of objects, including a delicate flower that closes its petals, a flat star shape that morphs into a dome, and lattices that contract and elongate. The structures were made from flat 3D-printed strips that were then heated to make them curve.

The strips, which can be printed in less than a minute, are made from layers of a stiff shape-memory polymer paired with a rubbery elastomer – a polymer with elastic properties.

When heated to 45°C, the shape-memory polymer component relaxes and allows the elastomer to bend. As the strip cools, the shape-memory polymer stiffens again and locks the object into its new, curved configuration.

Fixed in place

One limitation to the technique is that it permanently fixes the structure in place after one heating cycle, says Geoff Spinks at the University of Wollongong in Australia. “This rules out applications that require reversible shape changes, like artificial muscles for robots and prosthetics,” he says.

But the method could be used to make complex structures that don’t require such shape-shifting, says Spinks.

For example, compact cardiac stents – tubes for placing in blood vessels to keep them open – could open up in an artery in response to body temperature. By fine-tuning the temperature transition point, medicine capsules could also be designed to bend and break open when body temperature rises with infection. And flat-pack furniture could assemble itself when heated.

Moreover, the technology could easily be used by people with other ideas, says Spinks. “No new chemistries, materials or equipment are involved, so it’s ready for anyone to start using immediately.”

Journal reference: Science Advances, DOI: 10.1126/sciadv.1602890