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Soft-yet-tough robots may one day explore the seas

  • A flexible robotic fish resembling a manta ray swims using the power of a small battery and electric charges in the ocean.
  • Its mostly transparent body can carry a video camera to survey fish, monitor corals, or explore caves or shipwrecks.
  • Soft robots are long-lasting and durable – this fish can swim in water temperatures ranging from freezing to 74.2°C, (165°F) – while being kind to the natural environment, giving them an advantage over unknown terrain.

Robot design continues to advance, and now robots have become soft. And quiet. And increasingly independent.

Chinese researchers have created a flexible robotic fish that could someday independently survey aquatic vegetation, fish, and corals, as well as shipwrecks or caves too dangerous or expensive for humans to visit.

Vintage muscles. Creative Commons license.

It’s electric

The robotic fish has no motor. It runs on a lithium battery that powers a soft electroactive structure, meaning its materials change size or shape when stimulated by an electric field. Like artificial muscles, the materials can bend, compress, or expand in response to available electric current.

In this case, the electric current in the salt water causes the fins to bend and move up and down, enabling the fish to swim on its own. The movement is similar to that of rays flapping their pectoral fins.

The “muscles” of the silicone fins of the robo-ray are actually two membranes made of materials (called dielectric elastomers) that convert electric energy into mechanical work, like flapping. In between them is a thin hydrogel film that transports the electric current across the fins through the movement of ions. The ions in the salt water conduct electricity, so the water itself serves as the electric field that allows these fish to move about.


The robot’s fins move like a ray underwater. Here the manta rays, filmed off Hawaii, appear around 37 seconds into the video. Video credit: GoPro.

It’s a softie

Soft robots are made from flexible materials, such as silicone, plastic, or rubber, and tend to be able to withstand impacts. They are often inspired by the flexibility of animals.

“Soft-bodied animals become important inspiration for designing soft robots that are geometrically adaptable and resilient to environments, safe to humans, and biopowered. In particular, mimicking the exceptional agility of insects, fish, octopuses, and snakes has been a longtime pursuit,” explain the researchers.

The body of the robo-ray is made of transparent silicone parts, which the designers lined up and packaged within silicone rubber. This see-through body is fastened under the fish’s silicone frame to avoid interfering with the flapping movement of the fins. The body’s more rigid longitudinal structure minimizes front-to-back bending. Electromagnets are used to both steer the tail and keep the fish moving forward.

The flexible tail and fins of the fish move using electric current. The stiffer body is actually mostly transparent.
The flexible tail and fins of the fish move using electric current. The stiffer body is actually mostly transparent (green added for video contrast). The lower photo shows a movement pattern with a remote-controlled turn. Photo credit: Li et al (2017). Fast-moving soft electronic fish. Science Advances.

This new fish can’t morph into a liquid state to squeeze through crevices and reform in a solid state, but the flexible body is advantageous for certain uses. For example, their soft exterior is less likely to damage the environment and allows for safe contact with living tissue, such as human or animal bodies. Plus this one is mostly transparent, which makes it inconspicuous to any nearby marine life.

The mimic octopus is a shape-shifting expert. The robotic fish still has a long way to catch up. Video from BBC.Wild Indonesia.

Bendable materials and structure can maneuver more easily than rigid ones in difficult or changing environments. Soft robots are also generally durable—this fish can swim in water temperatures ranging from freezing to 74.2°C, (165°F)—and they tend to be cheaper to produce than metal parts of traditional robots. Finally, materials that can bend or compress and then reform have potential to store and release energy, increasing movement efficiency.

Nevertheless, much research is still needed. The softer structure of flexible robots can be difficult to model in the design stage and harder to control. And practical uses for such designs must still be demonstrated.

The new robo-ray can carry a video camera and move under human guidance remote control, making it a potential tool for surveying aquatic vegetation, fish, and corals. With a stronger, longer-lasting power mechanism on its small (9 cm / 4 inch) frame, it could potentially explore underwater caves or crevices too small, dangerous, or expensive for humans to visit. It could be a small underwater spy. A single battery charge operates this new robotic fish for three hours, enough to look around an area, but not yet enough to deep-dive or travel far.

The 9 cm (4-inch) robot moves through near-freezing waters
The 9 cm (4-inch) robot moves through near-freezing waters (0.3 degrees C) inconspicuously. Photo credit: Li et al (2017). Fast-moving soft electronic fish. Science Advances.

The robo-ray doesn’t swim fast yet (just under 4 meters, or 12 feet, in a minute), but a human operator can tell it where to go, including making sharp turns. In their paper, the researchers added, “This design allows us to first connect the electronic fish with wired power to evaluate its performance, and then integrate the onboard system for power and remote control.”

The design principle can be potentially extended to a variety of flexible devices and soft robots. According to the research team, flexible robots powered by stimuli-responsive materials “have unique advantages over conventional rigid robots, especially in their high adaptability for field exploration and seamless interaction with humans. The grand challenge lies in achieving self-powered soft robots with high mobility, environmental tolerance, and long endurance.”

The researchers recently published their findings in the journal Science Advances.

Featured image shows a close-up of an eagle ray. Photo credit: Steve Jurvetson, Wikipedia

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