Head of a young remora (Remora osteochir) [Image: Dave Johnson, Smithsonian National Museum of Natural History]

These days, you don’t often see a person standing alone on the side of the highway with their arm out and their thumb up, looking to hitch a ride – by now, we all know the many potential dangers of both offering or accepting a ride. I think Phoebe says it best in an episode of Friends when she and Joey are driving from Las Vegas to New York. While she’s sleeping in the backseat, Joey picks up a hitchhiker and gives him control of the wheel so he, too, can catch some shut-eye; upon seeing the stranger driving the car when she wakes up, Phoebe yells at Joey, “He could be a rapist or a killer or something!” But not all hitchhikers are bad – in fact, some can actually be helpful, like the remora fish.

The remora has a specially adapted dorsal fin called a suction disc that it uses to cling to larger sea creatures, like whales, dolphins, and sharks. The disc is made up of a soft, circular lip and linear rows of tissue called lamellae, which have small, needle-like spinules – using muscles around the disc, the remora can actually change its shape to hang on to its host in order to feed off of its feces and dead skin. The spinules add friction, so the fish has pretty good suction gripping power.

Li Wen, a robotics and biomechanics researcher at Beihang University in China, said, “Biologists say that it represents one of the most extraordinary adaptions within the vertebrates.”

Morphology structure of remora’s adhesive disc and biomimetic prototype design: (A) Dorsolateral view of the remora. (B) 3D reconstructed model of the remora disc based on microCT data. (C) CAD model of the biomimetic remora disc. (D) Photo of the adhesive disc prototype. (E) Drive mechanism for lamella pitch motions. (F) Pitch angle θ of the three sections of the biomimetic disc lamellae vs. the input air pressure P. (G) ESEM image of higher-magnification views of a single biological spinule. (H) Optical microscopy image of a single carbon fiber spinule fabricated by laser machining.

I’m sure by now you’re wondering what this little hitchhiker fish has to do with 3D printing technology. As is the case with so many experiments these days, researchers are getting their ideas from nature, and the remora has inspired a robot, created through 3D printing, that can cling to surfaces underwater with a force that’s over 340 times its own weight.

Wen is the lead author of a paper describing the remora bot, titled “A biorobotic adhesive disc for underwater hitchhiking inspired by the remora suckerfish” and published in Science Robotics; co-authors include Yueping Wang, Xingbang Yang, Yufeng Chen, Dylan K. Wainwright, Christopher P. Kenaley, Zheyuan Gong, Zemin Liu, Huan Liu, Juan Guan, Tianmiao Wang, James C. Weaver, and Robert J. Wood.

The paper describes why the remora’s suction abilities are so special: “The greatest advantage of this behavior is the reduced energy expenditure associated with movement: By attaching to swimming hosts, remoras can be transported over large distances with minimal effort.”

Wen explained that he first got the idea for the robot during his time as a postdoctoral researcher at Harvard, when he was working with his advisor to design 3D printed sharkskin. While looking through images for the paper, Wen noticed the remoras in photos of sharks and realized that no one had ever attempted to make a biorobotic remora disc before. So he and his team decided to take on the project, and began working out a way to make a disc with sections that ranged from rigid to skin-soft.

They used microcomputed tomography (microCT) to scan, segment, and reconstruct a preserved remora, turning to an environmental scanning electron microscope (ESEM) to measure the geometries of over 30 spinule samples. Then, they created a CAD model to help fabricate a multi-material 3D printed prototype with composite lamellae, which are lined with roughly 1,000 laser machined, at-scale carbon fiber spinules. The disc moves just like the real thing, thanks to the six embedded pneumatic actuators that are able to “inflate and deflate on cue.”

The remora bot measures about 5″ end to end, and the team attached it underwater to multiple surfaces that ranged from smooth to rough, rigid to flexible. They determined that the robot would cling better to smooth surfaces – it took about 436 newtons of force to pull it from plexiglass, versus the 167 newtons to remove it from rough surfaces, like real mako shark skin.

The team finally attached the disc to a remotely operated underwater vehicle (ROV) and after testing its clinging ability to the same range of surfaces they used out of the water, had a 100% success rate, “with an average time to attach of less than 4 seconds,” according to the study.

Wen said, “The rigid spinules and soft material overlaying the lamellae engage with the surface when rotated, just like discs of live remora.”

Attachment of an underwater vehicle using the biorobotic remora disc.

There have been other 3D printed robots inspired by fish, and even adhesive robots aren’t an entirely new concept. But the remora bot is one of the first robots that is able to be fully submerged and work as underwater attachments. Wen said that the remora bot is able to attach all sorts of broad underwater surfaces, like boats, and applications include tags for tracking the movements of marine animals.

[Images: Wang et al., Sci. Robot. 2, eaan8072 (2017)]

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