The tiny glass knifefish does not dazzle on first impression. The South American native likes to hide in submerged vegetation to avoid being eaten. If you manage to spot one, you probably will not notice anything unusual. "When you watch them from maybe three feet away, they are just sitting still," says Shahin Sefati, a doctoral candidate in the Whiting School of Engineering.
But look closely at the ribbonlike fin that runs along its underbelly and you will see something odd. Even when the fish is stationary, this fin is in constant motion. Two opposing waves run along it. One starts at the head and undulates along the fin toward the tail, and the other starts at the tail and undulates toward the head. When the fish is at a standstill, those waves meet in the middle, canceling each other out. When the fish wants to move forward, the nodal point where the two waves meet moves backward, and vice versa if the fish is in reverse. It seems a design worthy of Rube Goldberg. If your goal is to remain in one place, why waste energy creating opposing waves?
"If they hadn't seen this fish, there's a very low chance that engineers would come up with this design," Sefati says. "They would look at you like you're crazy." But Sefati works in the Locomotion in Mechanical and Biological Systems Laboratory, an interdisciplinary lab whose members study animal movement, sometimes as inspiration for robot design. A LIMBS team led by director Noah J. Cowan has found that the glass knifefish is on to something: It gains stability and maneuverability through its unusual movements, allowing it to stick to its hiding places even in unpredictable currents. This finding is a challenge to a basic engineering rule of thumb—to gain agility, you must sacrifice stability, and vice versa.
The LIMBS researchers were simply curious about why and how the fish moved the way it did. "The puzzle turned out to be a lot more interesting and complicated than we realized," says Cowan, who is an associate professor of mechanical engineering in the Whiting School. First, the team filmed the fin from below at 100 frames per second as the fish maneuvered to remain within a tube, counteracting varying rates of water flow. Sefati then derived a mathematical model from the observations. The model suggested that opposing waves traveling along the fin, while less energy-efficient than a single wave, give the fish more control and stability. To fight fluctuations in a current, the glass knifefish simply moves its nodal point slightly. To counter even a minor fluctuation, a fish with a single wave coursing through its fin would have to make drastic adjustments such as reversing the motion of its fin.
Malcolm MacIver, a biomedical engineer at Northwestern University, then designed and built a biomimetic robot based on the fish. It is about three times longer than the real thing and a good deal heavier. Thirty-two rays are spaced along its Lycra ribbon fin, each one independently controlled by a tiny motor, allowing researchers to program any wave they like. When they tested the robot, the team found that its performance matched the mathematical model.
The implications go far beyond the humble Amazonian fish. The team suspects that many other animals, including hummingbirds, cockroaches, and even human beings, employ forces that oppose one another. So even though it means using a bit more energy, engineers should take note, Cowan says. "If, for example, you're building a legged robot that's designed to scurry along on rough terrain, and you don't want it to fall over all the time, and you want it to be able to agilely move around obstacles as they come up, this would be a reasonable thing to consider in your design," he says. MacIver agrees. "I think this framework has legs that could generalize to a great number of cases," he says. "But this is a fish paper, so let's say it has fins."