A Cyborg Stingray Made of Rat Muscles and Gold

Photo: Karaghen Hudson and Michael Rosnach

Robots have advanced an enormous amount over the past few years, in both hardware and software, and the next few years promise even more advancements. It’s exciting, but we’re nowhere close to the efficiency and capability of animals, and it’s going to be a while before humans are able to create anything to match their level of elegance, especially when it comes to powered motion.

One way to avoid playing catch-up with animals all of the time is to simply steal everything you can from them as directly as possible. Want vision like insects? Steal the structure of their eyeballs with a bioinspired camera. Or heck, why stop with bioinspiration when you can instead hijack animals directly by wiring up a cybernetic beetle? These approaches are useful in certain situations, but ideally, you’d want to be able to leverage all of actual animal magic that you get with cybernetics and work it into the kind of bioinspired robots that you can design to do exactly what you want. 

A team of researchers, led by Sung-Jin Park and Professor Kevin Kit Parker at the Wyss Institute for Biologically Inspired Engineering at Harvard, have found a way to meld bioinspiration with robotics and cybernetics with the creation of a fully controllable robotic ray that uses light-activated rat muscle cells to swim. Their research has just been published in Science, and it’s impressive. And also adorable.

On the right of the picture below is a very cute real life batoid fish (a skate in this case, but stingrays are batoids too). Batoids swim quickly and efficiently by bending their fins in an undulating motion, sending a traveling wave from front to back that propels them forward in water. It’s also a simple motion that is nonetheless highly stable while still allowing for maneuverability, which makes a batoids “ideal biological models in robotics,” according to the researchers.

On the left of the picture above, you can see the tiny robotic skate, and if you’d prefer not to squint, here’s a pic of just the robot:

The robot consists of a cast elastomer body with a skeleton of gold, along with a single layer of carefully aligned muscle fibers harvested from neonatal rat hearts. The fibers were genetically modified to respond to pulses of blue light, and structured along the body of the robot in a serpentine pattern such that contractions result in a repetitive undulating motion without the need for any other control systems.

Since there’s only one layer of muscle that contracts to generate a downstroke, the upstroke is taken care of by the gold skeleton, which is asymmetrically stiff, like a spring. The downstroke compresses the skeleton, which then rebounds as the muscle relaxes. It takes about seven days to fabricate the robot, mostly due to the fact that unlike almost every other robot, it needs some time to grow.

About 200,000 live rat heart cells form the muscle layer that powers the robot, which has a body 16.3 mm long and weighs just over 10 grams. At full tilt, it can swim at a speed of 3.2 mm/s, which isn’t bad for such a tiny thing. Since faster light pulses result in faster flapping, steering is simply a matter of pulsing the light a bit faster on one side than the other. Batoids turn the same way. The researchers did a bunch of experimentation to figure out exactly how to structure the muscle layer in concert with the shape of the robot’s body; an asymmetrical design (fins wider at the back than at the front) yielded significantly higher speed and efficiency. Again, batoids have a similar design.

The video below shows the robot being led through a 250 mm long obstacle course; note the timer in the upper left.

You can’t tell from the video, but the robot is powered by the stuff that it’s swimming through. It looks like water, but it’s not: it’s something called “Tyrode’s Solution,” which contains all of the stuff that blood has, including the sugars that muscles cells use as fuel to do their thing. In other words, if you were to submerge this robot in water, it’s not going to move no matter how much you blast it with light, although it would be perfectly happy swimming in a sea of human blood, so feel free to picture that in your head.

While this little guy looks kind of fragile, it’s actually able to keep on flapping away at up to 80 percent efficiency for as long as six days after, uh, birth, as long as you keep it fed and watered (literally). This suggests that it could, at some point, lead to something useful, as opposed to just being a curiosity, although it’s important to reiterate that as-is, it won’t operate outside of its special nutrient bath.

What particularly struck me reading the paper is that the researchers refer to what they created as an “adaptive swimming animal.” They also call it a “tissue-engineered robot,” which seems more accurate, but maybe that’s just because it’s so weird to think of this thing as even a little bit alive, even if does incorporate living cells. The paper suggests that this robot “paves the way for the development of autonomous and adaptive artificial creatures,” and it’s intriguing to think about where we might be able to go from here: perhaps the future will be full of robots that harness biology to make hybrid animals that are far more capable than pure mechanical or biological systems would be on their own.

“Phototactic Guidance of a Tissue-Engineered Soft-Robotic Ray” appears this week in the journal Science.

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IEEE Spectrum’s award-winning robotics blog, featuring news, articles, and videos on robots, humanoids, drones, automation, artificial intelligence, and more.
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