Technological watch

PRINCETON, N.J.—Researchers at Princeton University have developed "bubble casting," as a new way to make soft robots using balloons that change shape in predictable ways when inflated.

Bubble casting, according to a Nov. 11 Princeton news release, uses "basic rules of fluid mechanics" to offer "a simple, flexible way to create actuators for soft robots."

The method involves injecting a liquid elastomer into a mold which could be as simple as a drinking straw or a more complex shape, like a spiral or flipper.

The researchers then injected air into the liquid elastomer to create a long bubble throughout the length of the mold.

Helped by gravity, the bubble slowly rises to the top as the elastomer drains to the bottom.

Once the elastomer hardens, it can be removed from the mold and inflated with air, which causes the thin side with the bubble to stretch and curl in on the thicker base.

"If it's allowed more time to drain before curing, the film at the top will be thinner," said first author Trevor Jones, a graduate student in chemical and biological engineering.

And the thinner the film, the more it will stretch when inflated, enabling greater overall bending, according to the researchers.

By controlling factors such as the thickness of the elastomer coating the mold, how quickly the elastomer settles and cure-time, the researchers claimed they can dictate how the resulting actuator will behave.

So far, the researchers have cast star-shaped "hands" that gently grip a blackberry, a coil that contracts like a muscle and a set of "fingers" that curl up one by one as the entire system is inflated.

The actuators deform when inflated with air, compared to other soft robotics systems that employ magnetic fields, electric fields or changes in temperature or humidity.