Eatable electronics

Here’s my latest piece for BBC Future.
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Bacteria resistant to antibiotics – popularly called superbugs – are threatening to make routine, minor surgery the kind of life-threatening intervention it used to be a hundred years ago. Some medical experts are warning that standard operations such as hip replacements and organ transplants could become deadly unless we find ways of warding off these infections.

Bacteria might evolve clever ways of evading chemical agents, but they will always struggle to resist the old-fashioned way of killing them: simply heating them up. It takes only a relatively mild warming to kill the bugs without discomfort or harm to tissues. Now a team of scientists in the United States are developing little electric heaters that could be implanted in a wound and powered wirelessly to fry bacteria while healing takes place – and then dissolves harmlessly into body fluids once its job is done.

This is just one potential application of the bio-resorbable electronic circuits made by John Rogers of the University of Illinois at Urbana-Champaign and his coworkers. The idea itself is not new: Rogers and others have previously reported flexible circuits and electronic devices that can be safely laid directly onto skin, and that even biodegrade over time. But their success in making wireless devices might prove crucial to many of the possible applications, especially in medicine, where it’s desirable to implant them and control or power them without having to puncture the skin to insert leads.

The idea is that radio waves can be used both for remote control of the circuits – to turn them on and off, say – and to provide the power to run them, so that there’s no need for implanted batteries. This kind of radio-frequency (rf) wireless technology is becoming ever more widespread – for example, for tagging consumer goods, food packaging, livestock, even dustbins.

To make rf circuits, you need semiconductors and metals. Those don’t sound like the kinds of materials our bodies will dissolve, but Rogers and colleagues use layers of non-toxic substances so thin that they’ll just disintegrate in water or body fluids. For the metal parts, the researchers use gossamer films (about 0.5 to 50 thousandths of a millimetre thick) of magnesium, which is not only harmless but in fact an essential nutrient: our bodies typically contain about 25 grams of it already. For semiconductors, they use silicon membranes 300 nanometres (millionths of a millimetre) thick, which will also break up in water. Many devices also require an insulating material, for which the researchers used magnesium oxide.

One of the simplest but key components of an rf circuit is an antenna, which picks up radio waves. Rogers and colleagues made these simply from long strips of magnesium foil, which they deposited onto thin films of silk. Silk is the ideal base for such devices, because it is non-toxic, biodegradable, strong and relatively cheap. These antennae, typically about four inches long, will completely dissolve in water in about two hours. Although more deeply buried implants would be hampered by the fact that body tissues absorb radio waves quite strongly, they should still receive enough signal for the kinds of very-low-power applications the researchers are considering.

They have also made a variety of standard circuit components: capacitors, resistors, and crucially, diodes and transistors that incorporate the silicon membranes. Transistors are rather complex structures, requiring delicately patterned films of silicon doped with other elements and sandwiched with metal electrodes and insulating layers. Yet using this suite of materials, Rogers’ team could make versions that would readily dissolve within hours.

One of the first full circuits that the team has made is a radio-frequency “power scavenger”, which can convert up to 15 percent of the radio waves it absorbs (at a particular resonant frequency) into electrical power. Their prototype, measuring about four inches by less than two, can pick up enough power to run a small commercial light-emitting diode. The team can control the rate at which these devices dissolve by fine-tuning the precise molecular structure of the silk sheets on which they are laid down or between which they are sandwiched. This way, they can make devices that last for a week or two if necessary – about the length of time needed to ward off bacteria from a healing wound.

As well as deterring bacteria, Rogers says that implantable, bio-resorbable rf electronics could be used for stimulating nerves for pain relief, and for electrically stimulating the regrowth of bone, a process long proven to work. Conceivably they could also be used to control the release of drugs from implanted reservoirs, in situations where this release must follow a tightly prescribed sequence.

Reference: S.-W. Hwang et al., Advanced Materials advanced online publication, doi: 10.1002/adma201300920.