Dissolving electronics – medical sensors that disintegrate

When I last spoke to John Rogers from the University of Illinois, we talked about his new “electronic skin” – a patch that can be applied like a temporary tattoo, that monitor heartbeats and brain activity, and that flexes and bends without breaking. We talked about his curved camera, inspired by the human eye. We spoke about his flexible medical sensors that can mould to the contours of a beating heart or the fissures of a human brain. We chatted about the $500,000 MIT-Lemelson prize that he had won for his inventions.

I tell you all this because I want you to understand that when John Rogers says his team’s new invention is “some of our best stuff ever”, he’s not speaking lightly.

Rogers has now created a line of “transient electronics”, which last for a specified amount of time before completely dissolving away.  Having made his name by taking rigid and brittle electronics and making them flexible and bendy, he has now flipped durability on its head too. Electronics are typically engineered to last as long as possible, but Rogers wants to create machines that will disintegrate after a given time. And his team have already shown how this disappearing tech could be used to make medical implants that are absorbed by the body after their work is done.

Medical implants are an obvious application, and the one that led them down this road in the first place. They have already been working on flexible sensors that can be implanted into the brain and heart, to monitor for signs of epilepsy or heart attacks. “The thing you bump up against is how to get these things to survive in the body for a long time without adverse effects,” says Rogers. “One way to deal with that problem is to move around it. A lot of these implants don’t need to last forever.”

There are other applications too. Scientists could lace the environment with sensors that they wouldn’t have to retrieve – they would just dissolve after their work is done. The military is interested – Rogers’ work is partly supported by a Defense Advanced Research Projects Agency (DARPA) grant. Consumer companies could also benefit by reducing the costs of disposing their unwanted goods, and minimising the toxic chemicals produced in the process. “What if you could make a cell-phone that lasts for two years but then washes away? That would be a useful thing,” asks Rogers. “And because the technology is new, we’re aware that we might not even be thinking of all the possibilities.”

To prove the principle, Suk-Won Hwang in Rogers’ group and former member Dae-Hyeong Kim developed a demo circuit that included a checklist of major components: transistors, diodes, capacitors, resistors, inductors, and connecting wires. All the parts were made of magnesium, magnesium oxide, or plain old silicon, and laid on a sheet of silk.

“We didn’t have to cook up new materials,” says Rogers. “We’re building on materials that are established in electronics and implantable devices.” Silicon is the basis of modern electronics, magnesium has been tested as the basis of intravenous stents, and silk is already used in sutures and other implants that the body can absorb. Rather than dealing with expensive and unworkable new materials, the team is using classics that already have a trillion-dollar consumer industry behind them. The big challenge was to combine them in structures that can dissolve in water.

How is that even possible? Rogers says that silicon will actually dissolve in water, but it does so slowly, and soon saturates the liquid it’s immersed in. “If you took a silicon chip and put it in the body, it would take 1000 years for it to dissolve, and you wouldn’t have enough fluid to make it happen,” he says. But his team has developed ways of carving circuits so thin – 20 nanometres (billionths of a metre) is their current best – that they dissolve away within minutes and hours, even in just a few microlitres (millionths of a litre) of fluid.

The longevity of these circuits can be set by wrapping them in silk. To develop these timed packages, Rogers worked with Fiorenzo Omenetto from Tufts University, who has been studying silk for years. If the fibres cluster in a loose, amorphous way, silk will dissolve almost immediately. In a more tightly arranged crystalline form, they can last for years.

Omenetto has developed methods for processing silk to control its crystallinity, and thus, how quickly it dissolves. And Hu Tao from his team showed that these methods can be used to program the lifespan of the transient electronics. The silk acts like a lit fuse, gradually disappearing until its contents are exposed and disintegrate themselves.

The team have already used their techniques to build a simple camera, which adds solar cells for power and light-detectors for capturing images. The prototype can only take an 8-pixel-by-8-pixel shot, but Rogers says that it can easily be scaled up to much higher resolutions.

They have also created an implantable heater that can be placed under the skin after a surgery, and operated through wireless control. Think of it as an artificial fever. It will burn away any bacteria that might cause infections, and then spontaneously degrade once the risk of such infections has passed. The team inserted these heaters into the skins of mice and rats, and within 2 weeks, they had almost totally disappeared. There were fewer traces of infection at the wound, just faint residues of the heater left, and no signs of any inflammation in the surrounding tissues. It was a timed implant.

The team are already trying to take their transient tech to the next level, especially the medical implants. But Rogers isn’t content with creating ground-breaking inventions. He wants to make ground-breaking inventions that are easy and cheap to manufacture. He is now working with an existing factory to modify its fabrication techniques so that it can produce transient technology. “If that happens, instantly a whole world opens up,” he says. “You could do memory cards or microprocessors. Ultimately, the only way to make a highly functional, low-cost tech is to bootstrap it onto existing infrastructure.”

Reference: Hwang, Tao, Kim, Cheng, Song, Rill, Brenckle, Panilaitis, Won, Kim, Song, Yu, Ameen, Li, Su, Yang, Kaplan, Zakin, Slepian, Huang, Omenetto & Rogers. 2012. A Physically Transient Form of Silicon Electronics. Science. http://dx.doi.org/10.1126/science.1226325

More on John Rogers: Hair-thin ‘electronic skin’ monitors hearts and brains, controls video games

Images and video courtesy of the Beckman Institute, University of Illinois.