3D-printed soft robot hops over engineering obstacles

A group of Harvard engineers have combined the autonomy and speed of rigid robots with the adaptability of a soft 3D-printed robot.

Traditionally, industrial robots are stiff and made mostly of metal, but accordingly fast, precise and powerful. Soft robots, on the other hand, are adaptable and resilient, but slow, difficult to manufacture and challenging to make autonomous because most motors, pumps, batteries, sensors, and microcontrollers are rigid.

Harvard engineers have combined the two, developing one of the first 3D-printed, soft robots that moves – or rather hops – autonomously. The development means the team has found a solution to this particular engineering challenge that has plagued soft robotics, namely the integration of rigid and soft materials.

The robot’s body transitions from soft to hard, reducing the stress where the rigid electronic components join the body and increasing the robot’s resiliency. Its monolithic design – created in one continuous print job, using several different materials – increases its strength and robustness. It has no sliding parts or traditional joints, so the robot isn't victim to dirt or debris like its more intricate cousins, making it a good candidate for use in harsh terrains.

Senior author Robert J. Wood said: “The vision for the field of soft robotics is to create robots that are entirely soft, but for practical reasons our soft robots typically have some rigid components – things like batteries and control electronics. This robot is a demonstration of a method to integrate the rigid components with the body of the soft robot through a gradient of material properties, eliminating an abrupt hard-to-soft transition that is often a failure point.”

The combustion-powered robot, which shares the look of a suction-powered rubber popper, is comprised of two main parts: a soft plunger-like body with three pneumatic legs and the rigid core module, containing power and control components and protected by a semi-soft 3D printed shield.

To initiate movement, the robot inflates its pneumatic legs to tilt its body in the direction it wants to go. Then butane and oxygen are mixed and ignited, catapulting the robot into the air. It’s a powerful jumper, reaching up to six times its body height in vertical leaps and half its body width in lateral jumps. In the field, the hopping motion could be an effective way to move quickly and easily around obstacles.

Nicholas Bartlett, first author of the paper, said: “The wonderful thing about soft robots is that they lend themselves nicely to abuse.”

“The robot’s stiffness gradient allows it to withstand the impact of dozens of landings and to survive the combustion event required for jumping. Consequently, the robot not only shows improved overall robustness, but can move much more quickly than traditional soft robots,” he said.

The robot’s jumping ability and soft body would come in handy in harsh and unpredictable environments or disaster situations, allowing it to survive large falls and other unexpected situations.

This new design demonstrates the possibilities of 3D printing in soft robotics. Traditional methods of fabrication – custom moulds and multi-step assembly – are costly and slow. The ever-increasing variety of materials compatible with 3D printers is allowing engineers to prototype new designs faster and any increased complexity does not necessarily lead to increased cost.

“Soft robotics is a relativity nascent subfield and 3D printing is adding to the repertoire of things we can do in a really practical way,” said Wood.

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