Remora fish, known commonly as suckerfish, are technically not parasites. They’re just along for the ride. You’ll often see them stuck onto a large marine animal like a shark or a whale and sometimes even small boats, getting free transportation across miles of the ocean while spending very little energy.
These odd creatures have flat foreheads that look like the bottom of a shoe. But it’s this strange suction-cup forehead that allows remora fish to cling onto dolphins even as they twirl into the air and slam back down into the water. In fact, scientists have long wondered whether the remora’s ribbed head held the secrets to better adhesives. Recently, this feature has become the intrigue of a group of international engineers from Beihang University, Imperial College London, and Swiss Federal Laboratories for Materials Science and Technology, who have sought to fabricate a version of this sucker structure to help drones get a better grip.
The result is a remora-inspired aerial-aquatic hitchhiking robot. A paper out in Science Robotics this week details how they made and tested this drone, which can fly, swim, and stick onto surfaces in air and water. It can also easily move between the two mediums, like a flying fish.
To make a drone like a remora, first the researchers had to observe the real fish. They used a camera to track a remora as it attached to the sides of an aquarium, and they saw that the suction disc could still hold on to the aquarium wall even if some parts of it were not in contact with the surface. They also used micro-computed tomography (micro-CT) to scan the head of a remora and look at the different bony and soft tissue structures inside the disc.
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The technique showed them that the remora disc had a gill-like membrane of soft tissue, under which was a layer of bony structures. The membrane can rotate or tilt at an angle, which could help it stick. Both these membranes are joined by connective tissue situated between them and the disc lip, or the edge of the suction cup. The muscles that moved the two membranes sat beneath them and are interspersed with blood vessels.
Then, the team used 3D printing to construct an oval prototype disc with a gill-like grid structure that was 87 mm long and 46 mm wide. The prototype had four functional layers, a soft layer mimicking the connective tissue, a main disc mimicking the gilly membranes, and fluid-controlled channels that act as a motor for rotating the membrane as well as erecting and depressing each row in the membrane. There’s also another fluid-controlled motor that’s used for bending the disc. The disc lip forms the seal, and as the disc moves and rotates, it creates pressure differences between the various compartments and the external environment, resulting in adhesion.
The team then made a hybrid aerial-aquatic robot to which they added the remora-like disc. On the modified quadcopter robot, the disc was accompanied by two motor components, including the hydraulic systems that pump fluid to manipulate the membrane and bend the disc, and a cable system that curls the disc lip to detach. The control system on the robot itself includes a flight control module, a speed regulator, a communications system, a remote control, and a battery. “Passive morphing propellers” were also custom-made for the robot. These propellers will fold underwater (the blades go inward when in contact with water) and unfold in the air (centrifugal force from increasing rotation speed unfolds blades).
The resulting remora-like robot can attach to flat and curved surfaces, wet or dry. In swimming pool tests, the robot was able to steer to, attach to, and detach from a larger underwater robot. During attachment, the robot can cut power to its propellers and switch to “standby mode,” traveling with its host. In field tests, out in the ocean, the robot can take underwater videos and retrieve submerged objects.
“The robot’s air-water transition (per cycle) consumed 1.9 times the power of hovering in the air. Notably, the robot’s hitchhiking state can reduce power consumption up to 51.7 times (in air) and 19.2 times (under water) compared with a hovering state,” the paper’s authors wrote. “Such robotic forms may be promising for several open-environment applications, including long-term air and water observations, cross-medium operations, submerged structure inspections, marine life surveys, and iceberg detections.”
