Silk holds the key to devices that dissolve after use

Wrapping something up in silk needn't be just for the sake of achieving a luxurious look. Silk coatings may herald a new breed of electronics that melts away when no longer needed. This could lead to environmentally friendly devices, implants which break down naturally in the body, even self-destructing spy cameras.

A collaboration between electronic engineers, biophysicists and material scientists has led to the creation of nanometre-thick devices with lifespans controlled precisely by an envelope of silk, which is built to degrade at a certain rate.

One test device, a heating circuit powered by beaming radio waves at it, was implanted under the skin of a rat. The idea is to control post-surgical infection by raising the temperature locally if needed, killing bacteria. After the wound has healed, the implant simply melts away.

At the request of US military research agency DARPA ? which funded the work ? the group also built a tiny, dissolvable digital camera. "DARPA have a lot of ideas for applications that are all classified, so I can't say anything about it," says John Rogers of the University of Illinois in Chicago, who led the research.

Flexible lifespan

The lifespan of the silk packaging is easily controlled by varying its crystalline structure and thus the degree to which liquids like water can attack it, explains Fiorenzo Omenetto of Tufts University in Boston, Massachusetts. This means the silk can disintegrate after just a few minutes, or over months or even years.

Team members at Rogers' lab at the University of Illinois built the electronics ? thin, flexible components including antennas and temperature sensors, using magnesium and silicon. The materials are not toxic and the amounts used are so small that they can dissolve quickly in liquid environments like the body. A conventional, millimetre-thick chip would take about 1000 years to dissolve, Rogers says, but the tiny amounts of material used in these new devices can disappear in about two weeks.

The animal implants were carried out at Tufts. One of Tufts' lead authors on the paper, Hu Tao, points out that blood vessels grew into and around the silk package. "This shows that silk is totally biocompatible," he says. Omenetto says no inflammation was seen where the devices were inserted and subsequently dissolved.

The combination of silk and silicon has potential beyond medical implants, and Rogers suggests that the concept of limited-lifespan electronics will foster novel applications. Omenetto's silk material is plasticky and stiff, and looks and feels more like an acetate slide than a silk dress or spider's web, making it an intriguing candidate for biodegradable displays in smartphones, for instance. His group has already made dental implants from the material.

"The achievement here is remarkable," says Chris Bettinger, who works on organic electronics at Carnegie Mellon University in Pittsburgh, Pennsylvania. He says that environmental applications are intriguing, but suggests that the real benefits will be in medicine, cutting out the cost of removing implants which are no longer needed.

Journal reference: Science, DOI: 10.1126/science.1226325

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