Robot Fish Powered by Synthetic Blood Just Keeps Swimming

A liquid battery that doubles as hydraulic fluid helps this robot swim for up to 36 hours

3 min read
Researchers from Cornell and the University of Pennsylvania developed a robotic fish that uses synthetic blood pumped through an artificial circulatory system
The robotic fish uses synthetic blood pumped through an artificial circulatory system to provide both hydraulic power for muscles and a distributed source of electrical power.
Photo: James Pikul

Living things are stupendously complicated, and when we make robots (even bio-inspired robots), we mostly just try and do the best we can to match the functionality of animals, rather than the details of their structure. One exception to this is hydraulic robots, which operate on the same principle as spiders do, by pumping pressurized fluid around to move limbs. This is more of a side effect than actual bio-inspiration, though, as spiders still beat robots in that they use their blood as both a hydraulic fluid and to do everything else that blood does, like transporting nutrients and oxygen where it’s needed.

In a paper published in Nature this week, researchers from Cornell and the University of Pennsylvania are presenting a robotic fish that uses synthetic blood pumped through an artificial circulatory system to provide both hydraulic power for muscles and a distributed source of electrical power. The system they came up with "combines the functions of hydraulic force transmission, actuation and energy storage into a single integrated design that geometrically increases the energy density of the robot to enable operation for long durations," which sounds bloody amazing, doesn’t it?

This fish isn’t going to win any sprints, but it’s got impressive endurance, with a maximum theoretical operating time of over 36 hours while swimming at 1.5 body lengths per, uh, minute. The key to this is in the fish’s blood, which (in addition to providing hydraulic power to soft actuators) serves as one half of a redox flow battery. The blood is a liquid triiodide cathode, which circulates past zinc cells submerged in an electrolyte. As the zinc oxidizes, it releases electrons, which power the fish’s microcontroller and pumps. The theoretical energy density of this power system is 322 watt-hours per liter, or about half of the 676 watt-hours per liter that you’ll find in the kind of lithium-ion batteries that power a Tesla.

Cornell Robot Fish The innards of the robot fish include two pumps, molded silicone shell with fin actuators, a microcontroller, and a synthetic vascular system containing flexible electrodes and a cation-exchange membrane encased in a soft silicone skin. Image: James Pikul

Conventional batteries may be more energy dense, but that Tesla also has to lug around motors and stuff if it wants to go anywhere. By using its blood to drive hydraulic actuators as well, this fish is far more efficient. Inside the fish are two separate pumps, each one able to pump blood from a reservoir of sorts into (or out of) an actuator. Pumping blood from the dorsal spines into the pectoral fins pushes the fins outward from the body, and pumping blood from one side of the tail to the other and back again results in a swimming motion.

In total, the fish contains about 0.2 liter of blood, distributed throughout an artificial vascular system that was designed on a very basic level to resemble the structure of a real heart. The rest of the fish is made of structural elements that are somewhat like muscle and cartilage. It’s probably best to try not to draw too many parallels between this robot and an actual fish, though, and we may have already gone just slightly overboard on the whole “blood” thing. But the point is that combining actuation, force transmission, and energy storage has significant advantages for this particular robot. The researchers say that plenty of optimization is possible as well, which would lead to benefits in both performance and efficiency. 

Electrolytic vascular systems for energy-dense robots,” by Cameron A. Aubin, Snehashis Choudhury, Rhiannon Jerch, Lynden A. Archer, James H. Pikul, and Robert F. Shepherd from Cornell University and the University of Pennsylvania, appears in the current issue of Nature

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Robot with threads near a fallen branch

RoMan, the Army Research Laboratory's robotic manipulator, considers the best way to grasp and move a tree branch at the Adelphi Laboratory Center, in Maryland.

Evan Ackerman

This article is part of our special report on AI, “The Great AI Reckoning.

"I should probably not be standing this close," I think to myself, as the robot slowly approaches a large tree branch on the floor in front of me. It's not the size of the branch that makes me nervous—it's that the robot is operating autonomously, and that while I know what it's supposed to do, I'm not entirely sure what it will do. If everything works the way the roboticists at the U.S. Army Research Laboratory (ARL) in Adelphi, Md., expect, the robot will identify the branch, grasp it, and drag it out of the way. These folks know what they're doing, but I've spent enough time around robots that I take a small step backwards anyway.

The robot, named RoMan, for Robotic Manipulator, is about the size of a large lawn mower, with a tracked base that helps it handle most kinds of terrain. At the front, it has a squat torso equipped with cameras and depth sensors, as well as a pair of arms that were harvested from a prototype disaster-response robot originally developed at NASA's Jet Propulsion Laboratory for a DARPA robotics competition. RoMan's job today is roadway clearing, a multistep task that ARL wants the robot to complete as autonomously as possible. Instead of instructing the robot to grasp specific objects in specific ways and move them to specific places, the operators tell RoMan to "go clear a path." It's then up to the robot to make all the decisions necessary to achieve that objective.

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