3D printing takes its lead from the pop-up book

We have pop-up books and pop-up tents, now a team of US scientists has made pop-up electronics. The technique, a world-first, was reported in Science. It involves printing circuits on to pre-stretched silicone rubber – when the tension is released they pop up into tiny gadgets. “In just one shot you get your structure,” says Yonggang Huang, an engineer at Northwestern University, one of the authors of the study.

Miniaturisation is the name of the game in electronics. That quest has taken us from flat circuits to 3D. But manufacturing microscopic 3D structures is a convoluted process that has resisted attempts over the past decade to speed it up. And it’s becoming more difficult. Intel describes its latest chips as “the most complex structures ever manufactured”. They contain billions of transistors connected by a 3D maze of copper all on a wafer of silicon the size of a fingernail. Making them takes more than three weeks as the device is shuttled back and forth between vast processing machines. They are patterned, etched, coated with metal and etched again, before they are ready to go to work.

In a bid to speed up the process, some engineers have tried to copy nature. Tadpoles, for instance, are able to transform their organs, turning gills into lungs, in a single day. That’s a piece of engineering more complex than manufacturing a microchip. Examples such as this troubled David Gracias, a materials engineer at Johns Hopkins University in Baltimore. “What nags me is that all of nature is self-assembled,” he said.

In 2001 Gracias constructed a flat pattern of tiny panels that, when heated, folded themselves into a box that could fit on the head of a pin. More recent work inspired by the Japanese art of origami has led to complex objects such as self-folding flowers. But while these constructions are ingenious, they are far from being functional devices.

“None offer the ability to build microstructures that embed high performance semiconductors, such as silicon,” says Huang.

Then, late one night, Huang’s Northwestern University colleague Yihui Zhang made an unexpected breakthrough while working on stretchable electronics – implantable medical devices that stretch or bend to conform to the body. One of his sample patterns was a snaking wire that had been printed on to pre-stretched silicone rubber. When he let go, he was surprised and delighted to see the wire spring up to form a 3D helix.  He immediately realised he was on to something.

To turn this phenomenon into a new technology, Zhang enlisted Huang, an expert in nanoscale engineering, as well as John Rogers, who heads an advanced fabrication group at the University of Illinois. One of the first problems they faced was how to turn the idea into a manufacturing method. They knew they could make a helix but what about other 3D shapes?

The key was to control the bonding between the pattern and the silicone rubber it is printed on. It needed to be strong at hinge points but weak elsewhere. That way, when the tension was released, the strongly bonded points anchored the structure down while the weaker regions broke away from the surface, allowing the whole structure to pop up like the pictures in a pop-up book.

“This is what we call an inverse problem,” says Zhang. “You know what you want in the end but you don’t know what structure to begin with.”

Huang developed a computer model to solve the problem. Then they put it through its paces figuring out the necessary 2D shape to print, along with sticking points, for more than 40 different 3D structures. Their paper demonstrates a wonderful array of structures including flowers, baskets, tents and mini-cages small enough to trap a dust mite.

The approach worked equally well for a wide range of advanced materials, including electronic device grade silicon and metals. The team also made structures that combined two different materials – such as a flower with two gold petals and four polymer petals.  

“This opens the door to the creation of high quality electronic devices,” says Gracias. But questions remain about the strength of the sticking points anchoring the structures down. “Would this adhesion be sufficient to be used in a realistic setting?” asks Garcia. “This is a key issue that needs more vetting.”

So far Zhang and his team have created and tested a simple 3D induction coil, which performed exactly as predicted.

Pop-up Intel chips are still a way off, but the researchers are already working on real-world devices, including a new kind of bioreactor that uses pop-up 3D electrodes to guide and monitor the growth of cells in the lab. They are also developing implantable electronic devices, such as biomedical sensors, that use bendy, stretchy, pop-up helical wires that can conform to the body’s movement.

“The pop-up technique will expand what is possible,” says Huang.