Imagine a small autonomous vehicle that can drive on land, stop, and suddenly squash into a quadcopter drone. After the rotor began to rotate, the drone flew away. Looking at it more closely, what do you think you'll see? What mechanism transformed it from a land vehicle into a flying quadcopter drone? You might imagine a part like a gear, maybe a series of tiny servo motors pulling all of its parts into place. The mechanism was designed by a team at Virginia Tech led by Michael Bartlet, an associate professor in the Department of Mechanical Engineering, who proposed a new way to change shape at the material level.
The researchers used rubber, metal and temperature to deform the material and hold them in place without motors or pulleys. The team's work has been published in Scientific Robotics. Co-authors of the paper include graduate students Dohgyu Hwang and Edward J. Barron III and postdoctoral researcher A.B. M. Tahidul Haque。
There are many creatures in nature that change shape to perform different functions. Octopuses dramatically change shape to move, eat, and interact with their environment; humans bend muscles to support loads and maintain shape; and plants move to capture sunlight throughout the day. But how can we create a material that can achieve these functions in order to enable a new type of multifunctional, deformed robot?
"When we started this project, we wanted a material that could do three things: change shape, keep that shape, and then go back to the original configuration and do that in many cycles," Bartlet said. One of the challenges was to create a material that was soft enough to change shape dramatically, but hard enough to create adaptive machines that could perform different functions. ”
To create a structure that could be deformed, the team turned to kirigami, a Japanese art of making shapes by cutting paper. (This method is different from origami, which uses folding.) By looking at the strength of those kirigami patterns in rubber and composites, the team was able to create a material architecture that repeats geometric patterns.
Next, they needed a material that could both maintain shape and eliminate shape as needed. Here, they introduced an inner skeleton made of a low melting point alloy (LMPA), embedded inside the rubber skin. Typically, when a metal is pulled too long, the metal is permanently bent, cracked, or pulled into a fixed, unusable shape. With this special metal embedded in rubber, however, the researchers turned this typical failure mechanism into a force. When stretched, this composite material now quickly retains the desired shape, perfect for soft deformed materials that can immediately become load-bearing.
Finally, the material must restore the structure to its original shape. Here, the team added a soft, tendrils-like heater next to the LMPA net. These heaters convert the metal into a liquid at 60 degrees Celsius (140 degrees Fahrenheit), or 10% of the melting temperature of aluminum. The elastomer skin keeps the molten metal in place and then pulls the material back into its original shape, twisting and stretching, giving the composite what researchers call "reversible plasticity." After the metal has cooled, it again helps to maintain the shape of the structure.
"These composites have a metal inner skeleton embedded in the rubber with a soft heater, where a kirigami-inspired cut defines an array of metal beams." "These cuts, combined with the unique properties of the material, are really important for deforming, quickly fixing into shape, and then returning to their original shape," Hwang said. ”
The researchers found that this kirigami-inspired composite design could create complex shapes, from cylinders to balls to bumps at the base of peppers. Shape changes can also be achieved quickly. After impacting with the ball, the shape changes and is fixed in place in less than 0.1 seconds. In addition, if the material breaks, it can be healed multiple times by melting and modifying the metal endoskeleton.
The application of this technology has only just begun. By combining this material with onboard power, control and motors, the team created a functional drone that can autonomously fly from the ground to the air. The team also created a small, deployable submarine that used the deformation and return of material to fish objects out of the aquarium by scraping the submarine's abdomen along the bottom.
"We are excited about the opportunities this material presents to multifunctional robots." "These composites are strong enough to withstand the forces coming from the motor or propulsion system, but can easily deform shapes, which allows machines to adapt to their environment," Barron said. ”
Looking ahead, the researchers envision deformed composites playing a role in the emerging field of soft robotics, creating machines capable of performing different functions, repairing themselves after damage to improve resilience, and stimulating different ideas for human-machine interfaces and wearables.