Technique combines silicon, magnesium and silk for medical implants, transistors and digital cameras that can melt away

By Rachel Ehrenberg

Web edition: September 27, 2012
Print edition: November 3, 2012; Vol.182 #9 (p. 15)

Imagine your old phone dissolving away after you’ve traded up, or a pacemaker that’s absorbed by the body when it’s no longer needed. Such gadgetry may not be far off: Scientists have developed a technique for making electronic devices that disappear without a trace. Constructed of silicon, magnesium and silk, the transient electronics can be tuned to last for days, weeks or even a year — and then disappear.

Scientists used the approach to make a bacteria-fighting medical implant that melts away after a few weeks, and a simple 64-pixel sensor array like those found in digital cameras, which was designed to last for about a day. The researchers also made temperature and strain sensors, solar cells, transistors, radio antennas and wireless power coils — all of which degrade into nothing. The team describes the work in the Sept. 28 Science.

“This is a huge step. It is a pinnacle,” says materials engineer Mihai Irimia-Vladu of Johannes Kepler University in Austria. “It’s a very elegant demonstration of making functional devices that are biodegradable.”

Superthin slices of semiconducting silicon and components made of magnesium perform the hardware and semiconducting tasks. The silk serves as scaffolding and packaging, which largely determines the lifetime of the device. So a unit might have a magnesium resistor, a silicon diode and a capacitor made out of magnesium and magnesium oxide. These delicate structures are stamped onto a sheet of silkworm silk and then packaged in more silk.

By liquefying the silk beforehand and then manipulating the concentration of various silk proteins, the researchers can package the device so it lasts for just a few days or for up to a year or longer, says study coauthor John Rogers of the University of Illinois at Urbana–Champaign. Extensive calculations that incorporate chemical reaction rates, such as rates of solubility and diffusion, allow the researchers to predict and program the lifetime of a particular device.

In one demonstration, the scientists made a wireless-controlled implant that emits heat, killing bacteria. Three weeks after implanting it on a rat’s surgical wound, the device had nearly disappeared.

While there is still testing to be done before such implants are used in people, the ingredients have a good track record: Silk has been used as sutures for wounds for decades and is known to safely disintegrate. The quantity of magnesium used in the devices is far less than that in a daily vitamin. Silicon has also been under investigation for some time as a means of delivering drugs to a specific site in the body.

In the new study, the rat’s implant was designed to break down after absorbing a certain amount of body fluid. But it’s possible that pH, temperature or other environmental cues might kick off the disappearing act.

In addition to implants that release drugs or heat at a wound site, the approach could be used for crafting other temporary implants, such as pacemakers needed for a short time following heart surgery. The environmental realm is another promising area: Say there’s an oil spill you want to monitor for a year. Instead of dropping sensors you have to collect later or that would drop to the bottom of the sea, they could just decompose. Eventually, such transient electronics might even make their way into consumer devices.

“Many electronics are built to last forever, and that’s fine, but think about smart phones today — nobody wants to keep them after a couple of years,” says Rogers. “In the longer term this is also about not contributing to an unmanageable waste stream.”