Sorry, Bender. With each new breakthrough in soft robotics, it's looking more and more like the robots of the future won't be housed in hard, rigid bodies a la Futurama. That's because plushy, pliable robots are more adaptable and resilient to damage than their stiff counterparts, and they're also worlds safer when it comes to interacting with humans. But here's the catch: Many of today's soft robots are slow, expensive, difficult to connect to rigid parts like batteries and motors, and a total pain to fabricate.

Today, a team of engineers led by Nicholas Bartlett at Harvard University has unveiled a soft, jumping robot that overcomes many of these challenges. Bartlett's durable, baseball-cap sized robot moves with explosive speed, and was rapidly 3D-printed rather than painstakingly molded and cast. Even cooler, the robot's body consists of nine blended layers, including material as soft as rubber up to as rigid as metal. That makes it not only one of the first 3D-printed soft robots but also the first robot to be 3D-printed with a gradient of stiffness. The research was published today in the journal Science.

Plushy, pliable robots are more adaptable and resilient to damage than their stiff counterparts

"I think the takeaway is that 3D printing, right now, opens up a whole new fabrication strategy for soft robotics," Bartlett says. "It was also really surprising and encouraging to see just how durable this 3D-printed technique of blending [gradients of material stiffness] made our robot."

Explosive jumps

Wyss Institute for Biologically Inspired Engineering and the Harvard John A. Paulson School of Engineering and Applied Sciences

Each of the robot's leaps is powered by a single explosion of butane and oxygen—a blast that rapidly inflates the bottom of the bot's semi-spherical body, thrusting it off the ground and into the air. Before each jump, the robot's three rocket engine-shaped legs can be inflated with different amounts of air, expanding them and angling the leaping robot in whichever direction it needs to jump. On top of the robot's body there's a stiff roll cage covering its guts—a battery, computer, pump, and stores of butane and oxygen.

"3D printing, right now, opens up a whole new fabrication strategy for soft robotics."

So why such a strange design? Everything beneath the robot's stiff components (such as the battery) is 3D-printed as a singular appendage. Each appendage interacts with the rest of the bot via only four tubes—three to carry air to inflate the legs, and one to deliver the fuel—plus a pair of wires which spark the fuel's ignition. That simple setup makes the jumper a breeze to fabricate. Secondly, the robot's combustion-based leaping style is one of the few ways researchers have found to imbue a largely flexible, rubbery robot with the explosive power required to jump. Bartlett's robot can bound more than 6 times its height, and even when landing at odd angles, the soft robot bounces away without being harmed.

Bartlett fabricated the robot on a Stratasys Connex 500 3D printer using two different materials "that were mixed at different ratios to form a 9-layer spectrum from completely rigid to completely soft," he says. To see how much the gradient of stiffness truly helped the robot, Bartlett created two other prototypes to compare. One prototype had a rigid top to its inflatable body, and another had a totally soft and flexible one. When Bartlett and his team did stress tests, they found that the blended-stiffness robot was much better at surviving awkward falls without cracking or breaking—unlike the hard robot. And unlike the entirely soft version, it wouldn't simply transfer the force of a fall to the robot's delicate, inflexible components.

Wyss Institute for Biologically Inspired Engineering and the Harvard John A. Paulson School of Engineering and Applied Sciences

If you just look at nature, Bartlett says, it should come as no surprise that a gradual blend of rigid materials could benefit a largely-soft robot so much. "If you take something like an octopus, an animal with an entirely soft body and rigid beak, you find that there's not an abrupt change between the two materials, but a gradual transition," he says.

Print your own robot

Bartlett's robot was inspired by another combustion-powered, soft jumper created by Robert Shepherd, a roboticist at Cornell University, back in 2012. Shepherd, who was not involved in the new research, says that the new bot "is a really elegant use of 3D printing, and I was very excited to see all the improvements they've made through utilizing this interesting [material-gradient] technique," he says.

To Shepherd, the most exciting aspect of Bartlett's new take on the jumping robot is that it's 3D printed. "There's a very low barrier to entry for anyone who wants to play around with and create these soft robots easily and rapidly" he says. "That's a big deal, because when something that previously required an expensive process is suddenly accessible to a large amount of interested people, you can expect huge improvements in the technology," he says.

"There's a very low barrier to entry for anyone who wants to play around with this."

Yong-Lae Park, a leading soft roboticist at Carnegie Mellon University, says that even in the near future, scientists worldwide will be using 3D printers to make various types of soft robots like Bartlett's.

"Right now the biggest limitation is that you don't have that much of a choice in regards to materials you use when printing. Even the soft material [Dr. Bartlett] uses here isn't very elastic," says Park. "But as we're are able to use more and more materials, that will totally change the paradigm of manufacturing."

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