John Dabiri's Jellyfish Life
by Lori Dajose (BS ‘15)
The humble jellyfish certainly would not win in a race against other sea creatures. They would, however, win a gold medal in a contest for the most efficient way to travel the ocean.
Energy efficiency is measured by how much energy it takes to go a certain distance. A person flailing their arms in a pool might get somewhere but would expend a lot of energy to travel a short distance. They would travel farther and waste less energy, and therefore would be more energy efficient, if they used a traditional freestyle swimming stroke. But neither human nor fish can compare to the jellies, which, although they appear to be merely drifting, have the most energy-efficient motion in the ocean. They swim by pulsing their circular bodies, closing like a flower then opening up again.
Why is this motion so efficient? Biophysicists had hypothesized that the jellyfish pulse creates a kind of jet propulsion, like a rocket of water to propel the animal forward. In the early 2000s, John Dabiri (MS '03, PhD '05), who has made a career of learning from this sea creature's secrets, set out to answer this question via experiment.
To visualize the flow, Dabiri, now Caltech’s Centennial Professor of Aeronautics and Mechanical Engineering, and his collaborators added colored dye—and, later, tiny spherical particles illuminated by lasers—to water as jellyfish swam. To his surprise, he discovered that jellyfish do not use jet propulsion at all. Rather, they create an intricate system of spiraling vortex rings behind them, as if they were blowing smoke rings. The experiments and visualizations showed that a jellyfish creates a swirling ring of water as it closes its body in its characteristic pulse, pushing it forward. Then, when it opens its body, a second vortex ring is created that gives the jelly an additional boost.
"It's kind of counterintuitive, but in both halves of the jellyfish propulsive cycle, the closing and opening, they're able to move," Dabiri says. "Their seemingly lazy motion here is actually quite energy efficient."
It turns out that transportation wasn’t even the reason jellyfish evolved their signature swimming stroke. The pulsing motion serves as a way to draw food into the vortex rings and thus into the jelly's mouth located in the center of its body; that the flow also creates propulsion is a happy accident. Nevertheless, for Dabiri, who considers himself to be a "bioinspired" engineer, jellyfish motion is so remarkable that it has sparked new ways of thinking about tackling problems in the human world.
In 2000, Dabiri paid a visit to an aquarium that would change his career. A visiting undergraduate student in Caltech's Summer Undergraduate Research Fellowship (SURF) program, Dabiri was studying in the laboratory of Mory Gharib, Hans W. Liepmann Professor of Aeronautics and Bioinspired Engineering and director of the Center for Autonomous Systems and Technologies, whose research focuses on fluid mechanics. Gharib suggested that Dabiri drive 30 miles south to the Aquarium of the Pacific in Long Beach and choose an animal with which to study the fluid mechanics of motion. Dabiri chose the jellyfish, becoming (to his knowledge) the first person on the Caltech campus to conduct jellyfish studies and leading him down the path of discovering unlikely insights from this ancient sea creature.
One way jellyfish have fueled unexpected discoveries is in the case of the human heart. When Dabiri returned to Caltech to pursue his PhD in 2001, he, along with his graduate advisor, Gharib, applied the fluid dynamics of vortex rings to understand the vortices of blood that flow through the heart as it pumps. Using noninvasive devices to assess blood flow, the team showed that vortex flows in the heart can indicate abnormalities in the flow. After Dabiri joined the Caltech faculty in 2005, he continued research on fluid mechanics and flow physics; Gharib's research continues to include a focus on developing medical devices to monitor cardiovascular vortices.
Jellyfish motion has also inspired advances in the more obvious field of underwater propulsion, where their efficient means of transportation could inspire better ways to explore the ocean and measure its parameters. Around 2005, the Dabiri lab began to study how submarines might propel themselves using pulses of vortex rings. The researchers developed a submarine that was in some ways even more efficient than a traditional propeller-driven vehicle. However, the machinery to create vortex rings was itself still inefficient, especially compared to jellyfish muscle tissue.
Then Dabiri had an idea: What if you could build a propulsion device out of muscle? Harvard professor Kit Parker had pioneered a technology to pattern rat muscle cells on a surface to create a thin film of muscle and had reached out to Dabiri to see if the technology could be applied to jellyfish muscle cells. The team was able to develop a sheet of jellyfish muscle that could be induced with electrical signals to contract and swim like a jellyfish.
However, these Frankenstein cellular systems weren't viable outside of the lab. They could not survive in the ocean, for example, defeating the purpose of developing new methods of undersea exploration.
At this point, Dabiri had another idea: Instead of trying to design a system to match or surpass the performance of a jellyfish, why not use the animals themselves? This led to the design of tiny electric devices that can be attached to a jellyfish and steer its motion, like a harness and reins on a horse. Now, these bionic jellyfish are being developed to explore the oceans and take measurements of the deep underwater climate.
"I originally picked jellyfish for my SURF project because I thought, 'Well, they're simple enough. I could probably come up with a mathematical model,'" Dabiri says. "And here we are, this is literally 20 years later, still learning from them."
Caltech researchers are discovering many other surprising things these little creatures can teach us about the planet and ourselves. Read more in the Caltech Magazine cover story, The Wonders of Jellyfish.