Low melting metal enables morphing robots

Imagine a small autonomous vehicle that could drive over land, stop, and flatten itself into a quadcopter; well, you don't have to imagine very hard as engineers from Virginia Tech have combined the use of rubber, low melting point metal, and temperature to create a robot that can morph to perform different functions.

Nature alone is rife with organisms that can change their shape to achieve different tasks, like an octopus that can dramatically shift and mold itself to move, eat, and interact with its environment, and that is what researchers at Virginia Tech, led by assistant professor in mechanical engineering Michael Bartlett, would publish in "Science Robotics."

"When we started the project, we wanted a material that could do three things: change shape, hold that shape, and then return to the original configuration, and to do this over many cycles," said Bartlett. "One of the challenges was to create a material that was soft enough to dramatically change shape, yet rigid enough to create adaptable machines that can perform different functions."

How do you create a material that achieves these functions to enable new types of multifunctional, morphing robots?

To create a structure that could be morphed, the team turned to kirigami, the Japanese art of making shapes out of paper by cutting. Similar to the art of folding paper – origami – kirigami allows for a far more diverse architecture, creating a strong geometric pattern.

After determining the method, they needed to ensure that the material would hold shape but allow that shape to be erased on demand. Here, they introduced an endoskeleton made of a low melting point alloy (LMPA) embedded inside a rubber skin.

Ordinarily, when a metal is stretched too far, it becomes permanently bent, cracked, or set into a fixed, unusable shape. However, the researchers turned this typical failure mechanism into a strength with an alloy most likely comprised of bismuth, cadmium, lead, tin, and indium embedded in the rubber.

When stretched, this composite would now hold the desired shape rapidly, ideal for soft morphing material that needs to become instantly load bearing.

Finally, the material had to return the structure to its original shape. Here, the team incorporated soft, tendril-like heaters next to the LMPA mesh. The heaters cause the metal to be converted to a liquid at 60 degrees Celsius (140 degrees Fahrenheit), or 10% of the melting temperature of aluminum.

The elastic skin keeps the melted metal contained and in place and then pulls the material back into its previous, original form, reversing the stretching and giving what the team calls "reversible plasticity." After the metal cools, it again contributes to holding the shape of the structure.

"These composites have a metal endoskeleton embedded into a rubber with soft heaters, where the kirigami-inspired cuts define an array of metal beams," said co-author of the paper Dohgyu Hwang. "These cuts combined with unique properties of the materials were really important to morph, fix into shape rapidly, then return to the original shape."

After building its robot, the researchers at Virginia Tech found that this kirigami-inspired design could create complex shapes, from cylinders to balls to the bumpy shape of the bottom of a bell pepper.

Shape change could also be achieved quickly: after impact with a ball, the shape changed and fixed into place in less than one-tenth of a second. Furthermore, if the material broke, it could be healed multiple times by melting and reforming the metal endoskeleton.

While the concept of transforming material-mechanical robots is nothing new, and even many have appeared in recent years, the applications for such a technology are still being discovered.

By combining Virginia Tech's recent addition with onboard power, control, and motors, the team was able to create a functional drone that autonomously morphs from a ground to air vehicle. They even made a small, deployable submarine, using the morphing and returning aspects of its latest hybrid material robot to retrieve objects from an aquarium by scraping the belly of the sub along the bottom.

"We're excited about the opportunities this material presents for multifunctional robots," said another co-author on the paper, Edward Barron III. "These composites are strong enough to withstand the forces from motors or propulsion systems, yet can readily shape morph, which allows machines to adapt to their environment."

Looking forward, the researchers envision morphing composites to play a role in the emerging field of soft robotics to create machines that can perform diverse functions, self-heal after being damaged to increase resilience, and spur different ideas in human-machine interfaces and wearable devices.