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.
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.