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Bio-integrated circuitry melds man and machine

Flexible circuitry mimics skin, brain, human tissue

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Flexibility is fine, but what about stretchability?

All well and good – but not good enough for bio-integrated electronics. "Think about your skin or your heart or your brain," Rogers said. "These are systems that not only flex, but they also stretch, in the sense that they undergo large strain deformations – and there's no thickness at which you can make silicon in this way that will make it stretchy.

Problem – but one solved by Rogers and his team by taking a page from that much-maligned musical instrument known more for "Lady of Spain" than for microelectronic elegance: the accordion.

The solution was to pre-stretch a silicone rubber substrate, bond the silicon to it, then relax the rubber. What you get is an accordion pleat–type of material – or, as Rogers put it, "it leads to a non-linear buckling instability and the creation of a wavy form of silicon."

The final trick that allowed the creation of circuitry on this stretchy, deformable, durable, bio-friendly substrate was to connect the device's small, discrete functional elements with traces constructed in snaky patterns that could bend and stretch without breaking.

Bio-integated electronics

A stretchable silicon-on-silicone substrate requires circuitry to also be able to stretch – hence the snaky traces

"It's all about mechanical engineering," Rogers said. "Instead of using a uniform sheet of silicon or integrated circuit, cut the system into an open spider-web mesh consisting of these very thin, narrow, filamentary serpentine structures, bond the whole thing onto a very low-modulus thin sheet of silicone rubber – maybe 50 kilopascals. And if you do all of that with care to the mechanics, guided by full 3D finite-element modeling simulations, then you can generate integrated circuits in this kind of heterogeneous integrated format that have stress-strain properties almost perfectly matched to the epidermis."

Got that? If not, here's the take-away: using his techniques, a device designer can create pliable, stretchable integrated circuits that can bond to human skin or tissue with no discomfort to the user.

Placing such as device on the skin is simple. The silicon-silicone circuit system is stuck onto a water-soluble plastic film as a mounting surface and placed onto the skin. The mounting surface is then washed away, and the device adheres to the surface merely by van de Waals forces. "No penetrating pins, no glue, no straps, no adhesive tapes," Rogers explained.

Of course, simply applying one of these devices to the skin won't make it permanent. Skin, after all, exfoliates on its own, with new cells constantly regenerating beneath the surface layer. Still, Rogers says that such a device should last about two weeks.

But since it's a fully functional circuit, and not merely, say, an electrode, it can do quite a bit of work during its lifetime, seeing that circuits could contain such devices as transistors, resistors, strain gauges and other sensors, photovoltaics for generating power, RF antennas, and other elements.

As one example of such a device's usage, Rogers suggested placing one upon a person's neck where it could perform electromycography (EMG), measuring the electrical signals of muscle-controlling motor neurons. You could then use the data from the EMG spectrogram, using pattern-matching algorithms to compare it with predetermined speech-pattern spectrograms, and create a user interface, provide control over a prosthesis, or build a speaking device for people with diseases of the trachea.

The user-interface capabilities of the neck-mounted EMG sensor were used by some of Rogers' students to control video games through speech, and an on-hand sensor was built to pilot a quadricopter with hand and fist movements.

Transient electronics

In addition to bio-integrated electronics, Rogers has developed 'transient electronics' – circuits that dissolve inside you

A more-serious example that Rogers provided was neonatal care. Currently, if a newborn is in a neonatal ICU, he or she is plugged into a welter of devices through a maze of electrodes. Using a bio-integrated device that was equipped with both sensors and RF capabilities, the level of invasiveness could be reduced significantly. ®

Bootnote

In addition to talking about bio-integrated devices, Rogers also discussed his team's work on what he calls transient electronics – devices that can be implanted, swallowed, or otherwise placed in a body, and which will then dissolve in a predetermined amount of time and be safely absorbed with no ill affects to the subject.

To demonstrate the safety of the silicon and magnesium devices that his group is working on, Rogers produced a edible Colpitts RF oscillator (FM transmitter) that he popped into his mouth and swallowed. "Tastes just like chicken," he told his IEDM audience.

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