Bubble casting: it’s not hard to make soft robots

Soft robots – made from malleable material – can get into and around thousands of places their hard counterparts cannot. But they’re trickier to make.

A new technique called bubble casting could change things, according to a new paper, published in Nature. This method uses “fancy balloons” that predictably change shape when inflated.

Traditional robots have plenty of uses, but they aren’t known for being gentle.

“They will not be able to hold your hands and allow you to move somewhere without breaking your wrist,” says Professor Pierre-Thomas Brun, lead researcher on the study. “They’re not naturally geared to interact with the soft stuff, like humans or tomatoes.”

Soft robots could be used for harvesting produce, grabbing delicate items on a conveyor belt, or providing personal care to humans.

Developed by researchers at Princeton University, US, the new system involves injecting bubbles into a liquid polymer. Once the polymer has set, it can bend and move to grip things.

The team use bubble casting to develop and create hands that grip, flapping fish tails, and slinky-like coils that can retrieve balls.

The researchers hope this simple design could accelerate development of other soft robots.

Bubble casting uses the physics of fluids. The liquid polymer is an elastomer, which sets into a rubbery material similar to the outside of a balloon. Using straws or spirals as a mould, air is pumped into the liquid polymer to create a bubble that floats to the top as the polymer sets.

That film around the bubble top can then be inflated, wrapping around the hard bottom as it increases in size.

“If it’s allowed more time to drain before curing, the film at the top will be thinner,” explains first author Trevor Jones, a graduate student in chemical and biological engineering. “And the thinner the film, the more it will stretch when you inflate it and cause greater overall bending.”

The researchers successfully cast star-shaped ‘hands’ that gently picked up a blueberry and even ‘fingers’ that curl up one by one, almost as through they were playing a piano.

“What’s really smart is this idea to shape the structure just by natural fluid motion,” say Professor François Gallaire, a professor of fluid dynamics at École polytechnique fédérale de Lausanne, Switzerland, who was not involved in the research.

“These processes are going to work at many different scales, including for very tiny things. That’s exciting because casting these tubes with typical fabrication methods could be really difficult, so there’s the potential to make very small tubes.”