Melt-in-your-mouth silicon

A biodegradable computer chip would be a breakthrough with many practical uses, but the real potential is in health care. Elizabeth Finkel reports.  

Scientists have wrapped super-thin soluble silicon in silk to create a biodegradable chip with multiple applications.
The silicon chip has legendary durability, but what if it could dissolve after its use-by date? Imagine biodegradable iPhones, nanobots reporting on oil spills then dissolving in the muck, or patients with spinal injuries fitted with chips that guide the rewiring of their nerves.

Now, an international team of scientists has provided a proof of concept, reported in a September 2012 issue of the journal Science.

They implanted a 2.5cm-long biodegradable chip into a mouse, and it started to melt after 15 days. But not before the device raised the skin temperature over that period by 5°C. Like a localised fever, it was enough to kill bacteria and viruses. In humans, implants such as this might one day be used as a drug-free antibiotic in the critical 15 days after injury.

But that's hardly the most exciting application. Bionic eye engineer Rylie Green, at Bionic Vision Australia at the University of New South Wales in Sydney, imagines chips that could guide the repair of damaged spinal cords. Electrical pulses delivered through 'sieve electrodes' are already used experimentally to stimulate and guide growing nerves. To be useful clinically, these sieve electrodes would need to melt away.

"Nerve guides are the obvious place for this degradable technology," suggested Green. Arthur Lowery, an electrical engineer and director of Monash Vision Group at Monash University in Melbourne, envisions a boon to spying and defence technologies. "Imagine a bug or camera that can be swallowed on capture", or a drone that dissolves if it falls into enemy hands.

A soluble silicon chip sounds like a contradiction in terms. A standard chip will last 1,000 years before wearing away. But it dissolves in water at the very slow rate of a few nanometres a day. The trick, then, is to make silicon chips less than 100 nanometres thick.

"There are ways to slice silicon in incredibly thin geometries so that you reduce - by many orders of magnitude - the total amount of material that has to dissolve, but at the same time it's thick enough that you can build reliable, highperformance transistors," explained co-author John Rogers, a bioengineer at the University of Illinois, USA. To manufacture circuitry like the antenna that allows for wireless control of the implant, the researchers used soluble magnesium rather than insoluble metals such as zinc or copper.

The functioning chip is protected by wrapping it in another legendary material - strong, flexible silk. By varying the constitution and thickness of the silk, researchers control the rate at which it dissolves and the working life of the device.

Ultimately, the silk wrapper and the silicon circuit dissolve without a trace - in the reported case, after about a month.

"In the future, we envision 'electronically enhanced organs'. For instance, imagine an ultrathin, stretchy 'electronic pericardium' that wraps the heart, and is able to measure arrhythmias in their very earliest stages, and then eliminate them before they evolve into a serious health problem," said Rogers.

Achieving 'enhanced organs' may take a while. But perhaps by the time iPhone 10 comes out, those waiting in the queues can just dump their old phones in the compost bin - guilt free.

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