Phase-changing robots on horizon

A new material built from a combination of wax and foam is capable of switching between hard and soft states, according to research published today by the Massachusetts Institute of Technology (MIT).

The material, which has been developed by Anette Hosoi, a professor of mechanical engineering and applied mathematics at MIT, and a number of researcher partners, could be used to build deformable surgical robots, research suggests.

“If you’re trying to squeeze under a door, for example, you should opt for a soft state

MIT professor Anette Hosoi

If built, a robot made of this material could feasibly move through the body to reach a particular point without damaging any of the organs or vessels along the way.

Working with robotics company Boston Dynamics, the researchers began developing the material as part of the Chemical Robots program of the Defense Advanced Research Projects Agency (DARPA).

However, for a robot to perform meaningful tasks, it needs to be able to exert a reasonable amount of force on its surroundings, Hosoi said.

“You can’t just create a bowl of Jell-O, because if the Jell-O has to manipulate an object, it would simply deform without applying significant pressure to the thing it was trying to move.”

Hosoi explained that controlling a very soft structure is extremely difficult. It is much harder to predict how the material will move, and what shapes it will form, than it is with a rigid robot, she said.

Therefore, the research team concluded that the only way to build a deformable robot would be to develop a material that can switch between a soft and hard state.

“If you’re trying to squeeze under a door, for example, you should opt for a soft state, but if you want to pick up a hammer or open a window, you need at least part of the machine to be rigid,” Hosoi said.

To build a material capable of shifting between squishy and rigid states, the researchers coated a foam structure in wax. They chose foam because it can be squeezed into a small fraction of its normal size, but once released will revert back to its original shape.

The wax coating is capable of changing from a hard outer shell to a soft, pliable surface with moderate heating. This could be done by running a wire along each of the coated foam struts and then applying a current to heat up and melt the surrounding wax, the research suggests.

Turning off the current would allow the material to cool down and return to its rigid state.

“This material is self-healing,” Hosoi said. “So if you push it too far and fracture the coating, you can heat it and then cool it, and the structure returns to its original configuration.”

To build the material, the researchers placed the polyurethane foam in a bath of melted wax. They then squeezed the foam to encourage it to soak up the wax, explained graduate student and research partner Nadia Cheng.

“A lot of materials innovation can be very expensive, but in this case you could just buy really low-cost polyurethane foam and some wax from a craft store,” Cheng said.

Hosoi is now investigating the use of other unconventional materials for robotics, such as magnetorheological and electrorheological fluids.

These materials consist of a liquid with particles suspended inside, and can be made to switch from a soft to a rigid state with the application of a magnetic or electric field.