A modular worm robot and jellyfish demonstrate the benefits of "embodied energy" -- an approach that incorporates power sources into the body of a machine, to reduce its weight and cost.

The worm and jellyfish are direct descendants of an aqueous soft robot, inspired by a lionfish and unveiled in 2019, that could store energy and power its applications via a circulating hydraulic fluid -- i.e., "robot blood." Similar blood sustains the new species, but with an improved design for greater battery capacity and power density.

"The jellyfish has much more capacity for its weight, so the duration it can travel is even longer than the fish," said Rob Shepherd, professor of mechanical and aerospace engineering, who led both projects. "The worm is the first version we've done above ground. When it's underwater, you get buoyantly supported, so you don't need a skeleton. It doesn't need to be rigid."

The key innovation of the worm robot was its compartmentalized design. The worm's body is a series of interconnected pods, each containing a motor and tendon actuator so the worm can compress and expand its shape, as well as a stack of anolyte pouches immersed in catholyte.

"There are a lot of robots that are powered hydraulically, and we're the first to use hydraulic fluid as the battery, which reduces the overall weight of the robot, because the battery serves two purposes, providing the energy for the system and providing the force to get it to move," Shepherd said. "So then you can have things like a worm, where it's almost all energy, so it can travel for long distances."

The researchers tested two modes of movement. The worm can inch along the ground, with each pod contracting and then pushing itself forward; the worm can also push its way up and down a vertical pipe like a caterpillar, a technique known as two-anchor crawling.

The robot is not exactly speedy -- it would take 35 hours to travel 105 meters in a single charging -- but it is faster than other hydraulically powered worm bots. As for potential applications, the worm is particularly well suited for exploring long and narrow passageways, such as pipes, and possibly conducting repairs.

Likewise, the jellyfish robot would be an ideal low-cost tool for ocean exploration because it can be carried along with the current, then swim up to the surface to send and receive communications, before sinking back down.

The research was supported by the Office of Naval Research and the Department of Energy Basic Energy Sciences Program.