Would you believe jellyfish are actually the most energy efficient swimmers in the ocean? It’s true, and it's why the Navy has funded efforts to design robots based on jellyfish.It’s tempting to think of jellyfish as motionless blobs, floating passively with the currents. For all that they’re constantly pulsing, they never seem to get anywhere.
Actually, that's a common misperception, according to Dr. Jack Costello, professor of biology at Providence College and a Whitman visiting investigator at MBL. It's based on a very small sample of the diversity of jellyfish - the hundred or so species with which we tend to have contact. In fact, there are jellyfish who are quite proficient swimmers, able to navigate amongst mangrove roots or evade fish looking for a salty Jell-o fix.
But it's the pulsing, drifting variety of jellyfish that engineers are trying to imitate with robots. Why?
That gentle pulsing is actually the most energy-efficient of all propulsion mechanisms used by animals. It's just that many jellyfish are more interested in eating than swimming. As they pulsate, jellyfish suck water into their bell and squirt it out. Each jellyfish creates a pattern of flow and turbulence that is uniquely suited to capture that species' particular prey.
Robots may not need to eat, but being energy efficient sure is useful.
So what would jellyfish-inspired robots look like?
Perhaps the most obvious possibility is Virginia Tech's Cryo - a 5 1/2 foot-wide, 170 lb robot with eight mechanical arms and a silicone bell.
In designing Cryo, Costello and the Virginia Tech students stumbled onto another surprising and intriguing aspect of jellyfish locomotion. An initial version with a rigid bell was a flop - it just couldn't swim. So they devised the flexible silicone bell .. and it worked.
That got Costello and his students thinking, wondering: if the flexible edge of a jellyfish's bell is so fundamental to its swimming, might other swimmers and flyers also make use of flexible edges. Turns out, the answer is a resounding "yes." In fact, Costello says he was surprised at just how universal the so-called bending rules they discovered turned out to be.
Which brings us to the idea of flying jellyfish of the robotic variety:
Costello says this robot misses some key features of jellyfish. In particular, it has four wings, as opposed to a single bell.
To be honest, the similarity between jellyfish and this robot was recognized after the fact. Designer Leif Ristroph of New York University’s Courant Institute drew on mathematic principals, not the natural world, when devising a means for keeping aerial robots upright and stable.
The result, nonetheless, bears striking similarity to jellyfish propulsion - a testament, perhaps, to the simple effectiveness of the jellyfish and the power of natural selection.