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Capsules for Self-Healing Circuits

Nanotube-filled capsules could restore conductivity to damaged electronics.
September 11, 2009

Dropping a cell phone or laptop can, of course, cause irreparable damage. Now researchers are developing a material that could let a circuit self-repair small but critical damage caused by such an impact.

Electrical band-aid: Polymer capsules filled with carbon nanotubes can restore conductivity to electrical circuits when ripped open. The nanotube suspension inside the capsules is visible in the light microscope image above; the image below, from a scanning-electron microscope, shows the surface of the polymer capsules.

Capsules, filled with conductive nanotubes, that rip open under mechanical stress could be placed on circuit boards in failure-prone areas. When stress causes a crack in the circuit, some of the capsules would also rupture and release nanotubes to bridge the break. The researchers, from the University of Illinois at Urbana-Champaign, are also working on capsule additives designed to heal failures in lithium-ion battery electrodes, to prevent the short-circuiting that can sometimes cause a fire.

Previous research into self-healing materials has mostly focused on restoring mechanical properties after a damaging event. The University of Illinois researchers have, for example, already made self-healing coatings that can repair scratches and prevent corrosion on boats or car chassis. Now the group has brought the same techniques to the problem of restoring electronic properties.

“We want to address common failures in cell phones and other portable electronics,” says Paul Braun, a professor of materials science and engineering at the University of Illinois who leads the research project with Jeffrey Moore, a professor of chemistry, materials science, and engineering. These failures may become an even bigger problem as flexible electronics, which are subject to much more mechanical stress, become widespread, says Braun.

To make their self-healing material, Braun and Moore encapsulated carbon nanotubes inside polymer spheres about 200 micrometers in diameter each. They selected carbon nanotubes because of their high electrical conductivity and because their elongated shape does a good job of lining up to bridge gaps.

In proof-of-concept studies, the researchers ripped the capsules apart and placed the resulting mixture between the tips of two electrical probes. The released nanotubes formed a bridge that completed the circuit between the two probes. Though the polymer itself isn’t conductive, this didn’t impede the flow of current – there was a net positive increase in conductivity after the rupture. Details of the experiments were published last week in Journal of Materials Chemistry.

“The restoration of electronic properties is fantastic,” says Christopher Bielawski, associate professor of chemistry at the University of Texas in Austin, who is also developing self-healing electronic materials.

“Many times when a device fails, it’s because a circuit or capacitor burns out,” says Bielawski. “This is critical in situations where you can’t repair it – in satellites or submarines.” To address the problem, engineers currently build redundancy into a system. Self-healing circuits could make devices for remote applications more lightweight, more efficient, and cheaper, says Bielawski.

Mark Hersam, professor of materials science and engineering at Northwestern University in Evanston, IL, also sees potential for the materials to be used in batteries. “Lithium-ion battery failures can be catastrophic,” leading to explosive fires that happen when corrosion causes electrical short circuits, says Hersam. Last month, Hersam began a Department of Energy-sponsored collaboration with the University of Illinois researchers to develop self-healing materials for batteries. It should also be possible to select capsule polymers that respond to chemical changes such as corrosion, he says.

“Of course, you don’t want to weigh down a battery with extra stuff,” says Braun. The same holds for circuit boards. But Braun says it isn’t necessary to use large quantities of the capsules: “You could add the capsules in small quantities because these failures tend to occur at the same point in the structure every time.”

The researchers are currently developing ways to precisely position the spheres. Braun says the group has had a paper accepted describing the use of a technique called electrospraying to place the nanotube bubbles. The group is also working on more realistic tests for the capsules, including fracture studies in conductive materials.

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