3D Printing Better Soft Robots

Any Star Wars geek can tell you—often in excruciating detail—how the franchise’s resident metal trash can on wheels, R2-D2, survived fried electronics, deep space adventure, and blaster fire with aplomb. Yet, outside the realm of science fiction, traditional robots with their metal joints and electrical motors do not fare well in unpredictable environments. This has led some researchers to suggest that cell-based, soft robots could do better, overcoming the limitations of traditional robotic hardware.

“Such cell-based robots have more degrees of freedom,” said Collin Kaufman, a graduate student in neuroengineering who builds biological robots, or biobots, at the Beckman Institute at the University of Illinois, Urbana-Champaign. “They can move in more directions in 3D space than traditional robots. They’re much more adaptable to the world around them. And they can be less susceptible to damage when they encounter things like radiation, oil spills, or water, too.”

While the research is still early, Kaufman’s work combining biological muscles with structural frames could open up new possibilities for robots and perhaps new types of prosthetics as well.

Several laboratories have come up with fascinating examples of the biobot concept. In 2012, bioengineers at Caltech unveiled a jellyfish-based biohybrid robot powered by cardiac muscle. With the telltale ga-gung of the heartbeat, the small device propelled itself through water with exquisite precision.

Read more about biobots on AABME.org.

Harvard University’s Wyss Institute showcased a more sophisticated heart-cell powered robotic stingray in 2016. Stretched over a gold exoskeleton, the bot also swims with a regulated frequency. Since then, other prototypes, mimicking the mechanics of everything from sperm to inchworms, have appeared in the scientific literature. But Kaufman, hoping to create more useful autonomous devices, wanted to develop something more complex.

“Cardiac muscle beats at an intrinsic frequency,” he explains. “It’s a constant beating and hard to modulate, and that’s useful for some types of movement. But if we want to achieve complex motor control, movements like walking, jumping, or eating an ice cream cone, you need to think beyond cardiac muscle and think about muscle tissue. We wanted to create a motile biological machine based on skeletal muscle.”

Humans are able to move when muscle fibers extend and contract, spurred by the activation of specialized neurons. To create such a biological machine, Kaufman and colleagues created a U-shaped skeleton out of hydrogel using 3D printing technologies and then seeded it with muscle cells that wrapped around it like a rubber band.

The researchers then laid an intact spinal cord from a rat to control the device. When the neurons in the spinal cord are activated, that rubber band of muscle cells contracts, slithering the skeleton forward a bit like an inchworm.

It may sound fairly simple butut Kaufman says that developing a working prototype posed a series of biological and engineering challenges that were overcome only after a lot of trial and error.

“We needed a skeleton with the appropriate stiffness, so when the muscle contracts, it can move it forward without breaking the frame,” Kaufman says. “We had an idea of how much force muscle can generate, but creating something that would bend with the movement but not break was a challenge.

“That’s where the 3D printing was invaluable. We knew that we’d have to iterate and change the design as we went,” he adds. “Our first few attempts were far too stiff—more like bone, really—and didn’t allow the flection we needed. By just changing a few cross-linkers, we could manipulate the stiffness pretty easily.”

Another challenge was seeding the muscle tissue on to the skeleton. At first, the muscle cells would not stick. “Muscle has multiple developmental stages,” Kaufman says. “If you just try to put the cells on the skeleton, they slide right off. We had to find a way to make the cells stick.

“So we turned to biology and asked, ‘How does Mother Nature do this?’ And she helps those muscles stick by this goop called the extracellular matrix. We fabricated our own extracellular matrix gel using collagen. And by doing so, we were able to get the whole thing to compact around the skeleton the way we needed it to.”

Kaufman presented the first working prototype, all 6 mm of it, at Neuroscience 2017, the annual meeting for the Society for Neuroscience. But he’s excited to continue working on this biobot, with its cell-based soft approach, and find new ways to increase its complexity and autonomy—by layering multiple muscles on their skeleton and finding ways to have them work together.

“Don’t get me wrong, traditional robots are doing some impressive things,” he says “But I think these kind of biobots, in many ways, are the future. They will be able to go to places and do things that metal robots can’t. This fusion of biology and engineering is pushing us forward into the realm of science fiction. Except, it’s not fiction. It’s real. And it’s really exciting.”

Kayt Sukel is an independent technical writer based in Houston, TX.