The secret of self-healing robot muscles

Wax-coated polyurethane changes stiffness with a change in temperature.

Anette Hosoi and Nadia Cheng, MIT

Think of the shape-shifting T-1000 cyborg in "Terminator 2" that oozed from liquid to a robot with super-human strength. Or an example from nature: a mouse squeezing through an impossibly small hole, coming out the other end to drag a large piece of cheese home.

Both mouse and movie muscles have the ability to change from squishy to stiff and back again. Now MIT scientists have found a way to make materials with that property that could someday be used as artificial muscles for robots. The ingredients are so simple, you could make the materials at home.

And even more importantly, if the robot is injured, it can be healed with a short zap of heat. This YouTube video shows how it works:

Anette Hosoi, professor of mechanical engineering and applied mathematics at MIT, and her former graduate student Nadia Cheng, said their material could be used to build deformable robots that could navigate through complex spaces, like worming through rubble during a search-and-rescue operation. A micro version could someday be used to weave through the body during surgery.

But Hosoi told MIT News, creating a robot with squishy muscles solves only half of the problem. "If a robot is going to perform meaningful tasks, it needs to be able to exert a reasonable amount of force on its surroundings. 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."

So the researchers decided to develop a material that could switch between a soft and hard state, like the tentacle of an octopus. Hosoi says. "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."

The researchers' material is made by dipping polyurethane foam into a bath of melted wax. Heat it and it is squishy. Cool it and it is stiff. Cheng says: "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."

They are now looking for other materials to make artificial muscles controlled by electricity or magnets.