"Bubble Casting" and "Fancy Balloons" Could Be Key to Flexible, Low-Cost Soft Robotic Actuators

A team of engineers at Princeton University has developed a manufacturing technique dubbed "bubble casting" as a means of making "fancy balloons" that, the team claims, will push the field of soft robotics forward.

"[Traditional robots] will not be able to hold your hands and allow you to move somewhere without breaking your wrist," explains Pierre-Thomas Brun, assistant professor of chemical and biological engineering and the lead researcher on the study, of the need for soft robotics. "They're not naturally geared to interact with the soft stuff, like humans or tomatoes."

Key to the field of soft robotics are actuators, and it's here where the team has claimed a breakthrough: A means of manufacturing soft robotic actuators which doesn't require 3D printers, laser cutters, or other expensive equipment — and which can scale to actuators over 10 feet in length and with features as small as 100 microns.

"What's really smart is this idea to shape the structure just by natural fluid motion," said François Gallaire, a professor of fluid dynamics at the EPFL who was not involved in the research, opined on the team's work. "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."

The manufacturing technique, which the team calls "bubble casting," sees elastomer injected into a mold then injected with air — creating the bubble. The bubble slowly rises to the top as the elastomer drains from the bottom, and the result is a "fancy balloon" which has a thin side — causing it to stretch and curl in a predictable way once inflated with air.

The technique has already been used to produce star-shaped soft robotic "hands," which can softly grip a blackberry, a coil designed to act as an artificial muscle, and a set of fingers which curl one-by-one. The next step, beyond improving the reliability in the face of over-inflation: More complex actuators, including a rippling system which mimics a millipede's locomotion and chambers which mimic the beating of the human heart.

The team's work has been published in the journal Nature under closed-access terms.