If you could safely give your old mobile for lunch to your dog, you would not only save space in those drawers full of old electronic junk at home. E-waste is a much wider problem: almost half of the 500,000 tonnes of household hazardous waste produced in the UK in 2004 were electronic products and plastics. And the numbers have likely increased exponentially since then.
In the last few years, the idea of electronics that can be literally eaten – or at least can safely dissolve in the body or in the environment – has unexpectedly evaded the science fiction realm, mostly thanks to advances in nanotechnology.
In 2010, a group led by Siegfried Bauer – a physicist at the Johannes Kepler University in Linz, Germany – built a prototype of a transistor fully made of edible materials. "Polymers were made from corn, dielectric materials from sugar-like substances, semiconductors from carrot components and electrodes from thin layers of gold – that have an E number in Europe, that is, they can be taken in as food," says Bauer.
Beta-carotene, indigo, caffeine, glucose, colouring materials and DNA could be key elements of future sensors that, for example, could be injected in fruits to detect whether they are ripe or not. These sensors could then be thrown away by the consumer or eaten safely. Other applications for these sensors could include injecting them in the body for medical purposes.
Bauer has produced a range of electronic devices made mostly of organic materials. Rather than pursuing fully edible materials, his research is now oriented to "imperceptible" electronics: ultrathin solar cells, stretchable LEDs and other devices that could be easily coupled to the body's surfaces. A key feature of these devices is that they must be manufactured in layers that are not thicker than a few hundred nanometres.
John Rogers, a physical chemist at the University of Illinois at Urbana-Champaign, has taken a different approach to the same issue. Rather than using organic materials, he employs standard silicon electronics substances: once these are miniaturised to a nanoscopic size, they become safely dissolvable in the body and in the environment.
For example, Rogers has successfully injected in rodents a circuit that can be activated remotely to generate heat, and shortly after dissolves in the body. It could be used, for example, to kill the pathogens in a wound. Now Rogers is working at a prototype circuit that is also loaded with antibiotics, releases them at a wireless trigger signal, and then vanishes.
Rogers is also contemplating dissolvable circuits that can be injected in the brain to mitigate chronic pain, or in the bones to stimulate their growth. "In all these cases, you want the circuit to work for a limited period of time, and then disappear, to avoid an unnecessary load of devices in the body," says Rogers. A layer of silicon loses 1 to 3 nanometres per day in normal conditions. If a device is manufactured to be just a few hundred nanometres thick, then it will vanish in just a few weeks. "What remains is silicic acid, a naturally occurring substance in the body", Rogers points out.
Rogers refers to these devices as "transient" electronics. If they are safe for the body, then they can be also spread in the environment. This is why they could be spread in thousands to clean a chemical spill, for example, and dissolve after they have done their remediation job. With this idea in mind, Rogers has developed prototypes of sensors for temperature, pH scale and chemicals.
"But the area from which we currently get most of our funding is the military," says Rogers. Transient electronics could contribute to war and spying with electronic systems that disappear naturally or at a specific trigger signal.
Although most of this research has not gone out of the laboratory, there is a field in which it may impact in the short term. Radio-frequency identification (RFID) tags – small passive circuits attached to consumer goods to track them – are produced in billions per year and there is an increasing concern for their environmental impact. Rogers is working at changes that may make them transient using dissolvable antennae and vanishing integrated circuits.
"The advantage of using standard electronics materials is that the industry to work with them is already set up," says Rogers. The challenge is to adapt the production, to introduce steps (like slicing devices to nano-sized layers) that make the outcome transient.
Another important limitation is that while silicon dissolves easily, another key electronic material, copper, does not. "It tends to create a protective oxide coating that blocks dissolution," Rogers explains. So, one must use less conductive metals like tungsten, magnesium or zinc. "However, the penalty one must pay in terms of resistance in circuits is not large: in biomedical applications, you don't need an electronics as fast as state-of-the-art consumer electronics," says Rogers.
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