Humans and other complex organisms manage life through integrated systems. Humans store energy in fat reserves spread across the body and an intricate circulatory system transports oxygen and nutrients to power trillions of cells. But with an untethered robot, things are much more segmented with batteries, motors, and cooling systems scattered throughout.
A synthetic vascular system was created that is capable of pumping an energy-dense hydraulic liquid that stores energy, transmits force, operates appendages, and provides structure, all in an integrated design. A circulating liquid — “robot blood” — stores energy and powers applications for long-duration tasks.
Engineers rely on lithium-ion batteries for their dense energy-storage potential. But solid batteries are bulky and present design constraints. Alternatively, redox flow batteries (RFB) rely on a solid anode and highly soluble catholyte to function. The dissolved components store energy until it is released in a chemical reduction and oxidation, or redox, reaction.
Soft robots are mostly fluid — up to 90% fluid by volume — and many times use hydraulic liquid. Using that fluid to store energy offers the possibility of increased energy density without added weight.
The concept was tested by creating an aquatic soft robot inspired by a lionfish, which uses undulating fanlike fins to glide through coral-reef environments. Silicone skin on the outside and flexible electrodes and an ion separator membrane within allow the robot to bend and flex. Interconnected zinc-iodide flow cell batteries power onboard pumps and electronics through electrochemical reactions.
The robot swims using power transmitted to the fins from the pumping of the flow cell battery. The initial design provided enough power to swim upstream for more than 36 hours.
Current RFB technology is typically used in large, stationary applications such as storing energy from wind and solar sources. RFB design has historically suffered from low power density and operating voltage. The researchers overcame those issues by wiring the fan battery cells in series and maximized power density by distributing electrodes throughout the fin areas.
Since aquatic soft robots are supported by buoyancy, they don't require an exoskeleton or endoskeleton to maintain structure. By designing power sources that give robots the ability to function for longer stretches of time, autonomous robots could be used in oceans for vital scientific missions and for delicate environmental tasks like sampling coral reefs. These devices could also be sent to extraterrestrial worlds for underwater reconnaissance missions.