Squishy Robots


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Two 3D-printed soft, flexible scaffolds. Image: Anette Hosoi

An international collaboration has developed a material made from wax and foam that is capable of switching between hard and soft states. Its developers say it could result in low-cost robots that can squeeze through small spaces and then regain their shapes.

The material could be used to build deformable surgical robots that could move through the body to reach a particular point without damaging organs or blood vessels along the way. It was developed by Massachusetts Institute of Technology mechanical engineering professor Anette Hosoi and her former graduate student, Nadia Cheng, alongside researchers at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, and Stony Brook University in New York.

Phase-changing material could allow even low-cost robots to switch between hard and soft states.
Image: MIT.edu

Partnering with Industry

Working with robotics company Boston Dynamics of Waltham, MA, the researchers began developing the material as part of the Chemical Robots Program of the Defense Advanced Research Projects Agency. The agency wanted robots capable of squeezing through tight spaces and then expanding again to move around a given area, much as octopuses do, Hosoi said.

“But 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 says.

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 adds.

To build such a shape-shifting material, the researchers coated a foam structure in wax. Foam can be squeezed into a small fraction of its normal size and will bounce back to its original shape once released.

The wax coating, meanwhile, can change from a hard outer shell to a soft, pliable surface with moderate heating. Running a wire along each of the coated foam struts and then applying a current can heat and soften the surrounding wax, Hosoi says.

“This material is self-healing,” she says. “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 a polyurethane foam lattice in a bath of melted wax. They squeezed the foam to encourage it to soak up the wax, Cheng says.

They used a 3D printer to build a second version of a foam lattice, to allow them to carefully control the position of each of the struts and pores. The printed lattice was more controllable than the original polyurethane foam, but would cost more. The first version works, but the printed lattice can be refined by analysis and changed according to need.

The wax coating could also be replaced by a stronger material, such as solder, she adds.

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This material is self-healing. 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.

Prof. Annette Hosoi, Massachusetts Institute of Technology

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September 2014

by Jean Thilmany, Associate Editor, Mechanical Engineering Magazine