John Rogers is a magician of materials science, particularly when it comes to combining electronics with biology. His latest trick is to conjure up silicon circuits that disappear when their job is done.
Rogers, a professor at the University of Illinois, described his new “transient electronics” last week to the American Association for the Advancement of Science in Chicago. It is a follow-up to his previous achievement of giving flexibility to rigid semiconductors in the form of “smart skin” that bends as the body moves.
The key to transient electronics is to make the silicon circuitry so thin (100 nanometres or less) that they not only bend but also dissolve in water – or body fluids. We think of silicon as a fairly inert material but, in fact, it reacts with water, very slowly, producing silicic acid. This reaction is irrelevant for normal silicon chips but Rogers’ are so thin – less than a thousandth the thickness of a human hair – that the reaction eats them away in a matter of days or weeks. Their survival time can be adjusted by making the circuitry a bit thicker or thinner, according to the application.
“There are many medical uses of electronics in the body where you don’t want the device to remain for ever and you don’t want to have to go in and remove it with another operation,” Rogers says.
His first transient device, currently being tested in animals, is applied to a surgical site to prevent infection. It produces slight inductive heating of a degree or two for a couple of weeks – enough to deter opportunistic pathogens but not to affect healing.
Not only the circuitry but also the battery that powers the patch is soluble. “The resorbable battery is a simple design, with magnesium foil as one electrode, molybdenum as the other and salt water [electrolyte] in between,” Rogers says.
Transient “electroceuticals” – combinations of electronics and pharmaceuticals – could also measure metabolic functions, for instance, or even deliver drugs for a set period inside the body.
Rogers says he adapts silicon, rather than looking for new biologically compatible materials, because it is well understood and affordable. “The technology has to move into the commercial world,” he says. “We are using silicon because the fabrication procedures are not too different from mainstream electronics.”
Transient electronics has other potential applications beyond medicine. The first may be for the radio-frequency identification (RFID) tags used to track objects in many industries. “We have ways to print RFID antennas out of soluble metals and the silicon guts can be made transient too,” Rogers says.
The two main advantages of transience in consumer electronics could be security – your data would dissolve away if your device was lost or stolen – and a reduction of the electronic waste going to landfill sites. “The vision would be to make a cellphone that’s completely water soluble or, even if you couldn’t do that, make certain components water soluble,” Rogers says.