THE idea of creating biodegradable electronics for implantation into the human body has been around for a couple of decades, but no one has yet managed to do it. However, Zhenan Bao and Christopher Bettinger, who are chemical engineers at Stanford University, have just made a start. They have created transistors from a sulphur-containing hydrocarbon called thiophene, a polyester called polylactide co-glycolide (PLGA, for short) and polyvinyl alcohol (PVA). All of these chemicals are approved by America's Food and Drug Administration for human use.
To make their transistors, Dr Bao and Dr Bettinger chopped sheets of PLGA into 1cm squares and deposited thin silver contacts on to them. They then coated the PLGA with thiophene, which acts as a semiconductor, interspersed with PVA, which acts as an insulator. The result is a flexible transistor a few millimetres thick.
Such organic transistors are not as efficient and powerful as silicon transistors, nor are they anything like as small. But they can be manufactured cheaply. More to the point, they break down slowly in warm, salty conditions of the sort found in the body. In laboratory tests intended to mimic the body's interior, they disappeared after 70 days, leaving behind only the wires—which, being made of silver, are not toxic.
To create working devices it will be necessary to double-check that the other ingredients are not toxic, too, despite their FDA clearance, and also to make the other components of electrical circuits, such as capacitors and resistors, in a similar way. Dr Bao is also investigating other compounds with potential uses in this area, such as beta-carotene (the molecule that gives carrots their colour), which she thinks might, like thiophene, act as a semiconductor. If she succeeds, it should be possible to devise electrical control circuits for instruments such as drug-delivery devices that could be implanted and then left alone to do their thing for a month or two before vanishing.
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