A team of engineers at Cornell's Organic Robotics Lab has built a soft-bodied robotic lionfish powered by electric blood, which not only serves as an energy source, but acts hydraulically to create propulsion as well. This bio-inspired approach addresses one of the great challenges for small, untethered robots – mass vs. power.

Batteries add significant weight to robots, and this weight impacts range, maneuverability, speed and size. So, if engineers can store energy in a component which also serves a secondary purpose, then that, as they say, is a problem halved.

Designed by study co-author James Pikul, a former postdoctoral researcher (now an assistant professor at the University of Pennsylvania), the robotic lionfish is 40 cm (16 in) long, and made of molded silicon.

Onboard are two hydraulic pumps, each activated by interconnected zinc-iodide flow cell batteries. One pump moves the the tail by moving fluid from one side of the tail to the other, while the other pumps fluid stored in the dorsal fins into the corresponding pectoral fins. It's not what you'd call speedy though. Currently, it swims at a rate of one and a half body-lengths per minute, but these are early days for the project.

Hydraulics in soft-bodied robots isn't a new idea, but tasking hydraulic fluid with the extra job of supplying energy is. By employing the liquid (an electrolyte solution dubbed robot blood) as both electrical and mechanical energy, the mass of the fish-shaped robot is significantly reduced, increasing the relative energy payload. This means the robot is able to swim about autonomously for 36 hours before requiring a recharge.

"In nature we see how long organisms can operate while doing sophisticated tasks," says Rob Shepherd, associate professor of mechanical and aerospace engineering. "Robots can't perform similar feats for very long. Our bio-inspired approach can dramatically increase the system's energy density while allowing soft robots to remain mobile for far longer."

The hybrid battery/hydraulic synthetic vascular system was modeled after redox flow batteries. While this form of battery isn't particularly powerful compared to say, lithium-ion batteries, it has the advantage of being able to be crammed into pretty much any space or shape. This flexibility can be particularly useful when designing robots for specific tasks or for navigating awkward spaces, as traditional batteries can unduly influence the physical proportions of a design, limiting its scope.

"We want to take as many components in a robot and turn them into the energy system," says Shepherd. "If you have hydraulic liquids in your robot already, then you can tap into large stores of energy and give robots increased freedom to operate autonomously."

Developments such as this mark yet another step towards better, more efficient, autonomous aquatic robots. The potential for marine exploration, inspection of pipelines and underwater cables and the like is huge. As for flexible, soft-bodied robots, these could play an important role in delicate environments – such as coral reefs – where deploying a hard-bodied robot might be too risky.

The study was published yesterday in the journal Nature.

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