A metal/foam compound that can be heated to change its shape, then cooled to regain stiffness, has potential applications in aeronautics and soft robotics.

Developed in the lab of Cornell University Associate Professor of Engineering Rob Shepherd, the compound combines a soft alloy called Field’s metal with a porous silicone foam. In addition to combining the best properties of both—stiffness when it’s called for, and elasticity when a change of shape is required—the material also has the ability to self-heal following damage.

The material combines the best properties of Field’s metal with those of porous silicone foam. Image credit: Rob Shepherd group. “It’s sort of like us—we have a skeleton, plus soft muscles and skin,” Shepherd says. “Unfortunately, that skeleton limits our ability to change shape—unlike an octopus, which does not have a skeleton.”

To develop the material, the researchers dipped the elastomer foam into molten Field's metal, then placed it in a vacuum so that the air in the foam’s pores was removed and replaced by the alloy. The foam has pore sizes of about 2 millimeters, which can be altered to create a stiffer or a more flexible material.

In testing of its strength and elasticity, the material showed the ability to deform when heated above 144 degrees (the melting point of Field's metal), regain rigidity when cooled and return to its original shape and strength when reheated.

Shepherd's research was partially funded by the U.S. Air Force Office of Scientific Research and earmarked for its program on "Co-Continuous Metal-Elastomer Foam Actuators for Morphing Wing MAVs [Micro Air Vehicles]." He says this material would comprise the skin for the morphing wing, giving the MAV the ability to become an underwater vehicle on the fly.

“If you have a wing that’s really broad, you can’t do that because the wing will break off when it hits the water,” he says. “So you need to sweep it back, similar to what a puffin does, and then go under water. And using that new shape, it could be a propeller-driven ship.”

In addition to a morphing-wing application, graduate student and fellow researcher Ilse Van Meerbeek sees this material being used in soft robots that must negotiate tight spaces.

“It could be used in search-and-rescue robots,” she says. “It would be able to go into dangerous and/or unpredictable environments and be able to [move] through narrow cracks, which rigid robots can’t do.”

“Sometimes you want a robot, or any machine, to be stiff,” adds Shepherd. “But when you make them stiff, they can’t morph their shape very well. And to give a soft robot both capabilities—to be able to morph their structure but also to be stiff and bear load—that’s what this material does.”