When one tiny circuit on a computer chip cracks or fails, the chip and the device it powers can crash. But what if the circuit could repair itself? Researchers have created a self-healing method that fixes chip mishaps faster than a user can recognize a problem with their device. The technique, developed by aerospace engineering professor Scott R. White, materials science and engineering professor Nancy R. Sottos, and colleagues at the University of Illinois at Urbana-Champaign, could extend the longevity of electronic devices and batteries—and reduce waste.
Conductivity patch-ups could make building-in redundancies or a sensory diagnostics system into chips unnecessary, so the technique could lead to less expensive devices. It could also reduce or delay gadget obsolescence.
The breakthrough comes as electronic devices are evolving to perform more sophisticated tasks, so manufacturers are packing as much density on a chip as possible. But as devices fatigue, the chip density compounds reliability problems. Individual transistors, on-chip wiring, and chip-to-board connections all cause chips to fail. The problem is both long-standing and widespread, hindering advanced electronic packaging.
The University of Illinois at Urbana-Champaign researchers induced self-repair in gold circuits with microcapsules of gallium-indium, one of a handful of liquid metal alloys that both flow and are highly conductive. The researchers enveloped minute amounts of alloy in shells of polymeric urea formaldehyde, and embedded them in the dielectric layer above the conductive pathway.
Using mechanical stress, the team then damaged circuits, mimicking the constant flexing of substrates when a cell phone keypad is pressed. The fracture also strained the capsules, which ruptured. The liquid metal seeped into the crack, filling it and restoring electron transport, almost like micro-solder.
White and his team found that large capsules did not work as well as small ones. Those with diameters of 200 micrometers returned 90% of broken circuits to 99% of their original conductivity, whereas all capsules with 10-micrometer diameters healed with 98% efficiency. The results were published in the December 20 issue of the journal Advanced Materials.
The speed of repair surprised the researchers. "We had to use special equipment to see the event," says White. "When using traditional data acquisition, the rate was too slow to see damage and healing take place. I was scratching my head and saying, 'It’s not breaking.' " But once they collected data at a high enough rate, they saw conductivity briefly plummet to zero and then resume after an average healing time of 160 microseconds. That’s seven orders of magnitude faster than the group’s results a decade ago with microencapsulated monomers—a matter of minutes versus microseconds—which were published in Nature.
This is the first time a microcapsule-based healing approach has been used to restore a circuit’s conductivity rather than to repair the structure, according to the researchers. Because the emphasis was on the flow of electric current, the researchers did not study weight-carrying ability, but White does not think load-testing results would have been very good because liquid metals do not carry loads.
How durable are the repaired circuits? To find out, White’s group monitored sample circuits for four months and saw no change in conductivity. "They’re pretty stable," he says. Re-repair is also possible, but with qualifications. "If a crack opened up one capsule at one spot and repair happened, and the next day it broke again at that exact same spot," White says, "you’ve already released the healing agent in that capsule, so it can’t heal again." But the stress of a nearby break can shake up a neighboring capsule. "As soon as the crack propagates a short distance away, it comes into contact with a capsule that wasn’t ruptured the first time," explains White, "which will release and backflow into that area. So repair will happen again."
It is not clear exactly how much the method will extend the longevity of circuits and chips, but White says showing that they can self-heal almost 100% of the time "has to translate to significant advancement in a chip's lifetime."
Manufacturers are unlikely to spend more than necessary to produce chips, however. But self-healing will eventually be inexpensive, says White. During the next five years, the technology might be used in the circuits of devices for which there is no alternative, like orbiting satellites that can’t be repaired, decommissioned, and replaced. "Whatever the added expense is, it will be well worth it," he says.
Self-healing consumer electronics are a decade away, White estimates. "I do fully expect gadgets that last longer using these kinds of technologies," he says. "They’re very new, but it's the wave of future."
Karen A. Frenkel is a science and technology writer and author who writes for Bloomberg.com, Bloomberg BusinessWeek, FastCompany.com, Science, and Science NOW.
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