"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.