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That idea doesn’t suck: Hitchiking fish inspires a robotic suction disk

The paper: Wang, Y., Yang, X., Chen, Y., Wainwright, D. K., Kenaley, C. P., Gong, Z., … Wen, L. (2017). A biorobotic adhesive disc for underwater hitchhiking inspired by the remora suckerfish. Science Robotics, 8072(September), 1–10.

Introduction:

Figure 1: Several remoras cling to a lemon shark. Image source: wikimedia commons

When you think of hitchhiking, you probably do not think of sucking on to the side of a shark or dolphin – but that is exactly how remoras prefer to get around. There are eight different species of remora fish living in our oceans. They suck on to the sides of animals ranging from fast swimming sharks, jumping and spinning dolphins, and even the unsuspecting diver passing by (Figure 1). By hitching a ride on these larger animals, remoras are able to conserve their energy and can even pick up some spare bits of food that are shredded and left around by their sloppily feeding hosts. While these fish may have taken hitchhiking to the next level, being able to suction to surfaces is not a unique feature among fishes- clingfishes stick to rocks in fast moving currents using a disc made by their pelvic fins and some gobies can even climb up waterfalls using just their mouth! These fishes, however, rely on their body conforming to the structure they are attaching to – much like a suction cup.  Remora discs, on the other hand, are made from a highly modified dorsal fin that forms a more elaborate structure than just a suctioning disc that conforms to the surface the fish is trying to stick on (Figure 2).

The unique structure of the remora suction disc has intrigued scientists for ages. Uncovering how the suction disc works could lead to some exciting new adhesives or other technologies. This concept of turning to nature for inspiration in human technological problems is known as biomimicry or biomimetics and has led to stronger adhesives inspired by gecko skin, glue capable of sticking underwater brought to you by scientists studying mussels, and even Velcro, which was invented based on a plant’s uncanny ability to stick to pant legs.  These are just a few of the many examples of biomimetic devices that scientists have developed by turning to structures in nature that have been fine-tuned by thousands of years of evolution.

Now, a group of scientists has unraveled the mechanism behind the remora’s herculean hitchhiking to bring us a new biomimetic robotic adhesive disc!

Methods:

Before the scientists could build their robotic sucker, they needed to have a detailed understanding of how the remora disc functioned. To do this, they examined specimens of the slender sharksucker (Echeneis naucrates) by using a microCT to visualize the fish skeleton and anatomy of the adhesive disc. This smaller version of the CT or CAT scanner you would sit in at the doctor’s office allows scientists (or your doctor) to see both bones and softer structures like internal organs.

Figure 2 – the head of the sharksucker remora showing it in side view (top) and viewed from the top down (bottom), for a view of the sucking disc. Image source: wikimedia commons

These images revealed that the remora disc is composed of an outer layer of soft tissue that forms a lip around an inner suction disc. This suction disc has numerous rows of spine-like structures, called spinules.  By filming live remoras, the scientists were able to see that these spinules oscillate along the disc so that they can either lay flat, or can be erected and stand upwards at an angle depending on what the remora is trying to attach to. Unique to the suction disc of remoras, these spinules were further examined using a high-powered microscope called an environmental scanning electron microscope (ESEM). This microscope allows for extremely high-resolution images to be taken of specimens, letting the scientists take detailed measurements on each part of the remora sucker disc. Combining these measurements with the analysis of how live remoras swim and position the different parts of the sucker disc to attach to different surfaces gave the scientists an in-depth understanding of how the various parts of the remora disc function together. And with that, they had a blueprint for their robotic remora disc.

They used a 3-D printer to make a soft outer layer mimicking the soft tissue around the disc and used laser machining techniques to make carbon fiber versions of the remora’s spinules. Because ability to oscillate the spinules appeared to be important in live fishes, the scientists mimicked this capability with their robot as well.

Did it suck(tion)?

The short answer – yes!

The scientists tested their robotic remora disc on a variety of surfaces including smooth and rough surfaces made from epoxy resin, a flexible surface made from a silicone elastomer, and even real shark skin! Traditional suction cups fail to stick to rough surfaces, making this new robotic disc pretty incredible. What is even more astounding is that it took up to 45 kilograms of force to remove the robotic remora disc from some of the surfaces – that is 340 times the weight of the robot itself!

The ability to stick to a variety of surfaces is attributed to the soft laminae surrounding the disc that can create suction along with the rotating spinules that can grasp on to rougher surfaces and change their angle for better attachment on different surfaces when necessary. By using both of these structures in combination, the robotic remora disc was able to adhere to both smooth and rough surfaces, a feat not possible using a suction cup alone.

After seeing that their prototype did in fact work, the researchers tested out their robotic sucker disc by attaching it to a small underwater remotely operated vehicle (ROV). With the robotic remora disc attached, the ROV was able to grasp on to surfaces in less than 4 seconds and successfully hitchhike on smooth, rough, and flexible surfaces in addition to shark skin. The hitchhiking robot also conserved a lot of energy which sheds light on the value of the hitchhiking behavior exhibited by remoras and could be an important development in the future of underwater robotic devices in general.

Although this robotic remora disc was only a prototype, it is possible that it could be used in the future to allow under water robots to hitchhike on other vehicles, reducing the energetic demands and possibly prolonging the time that these robots could stay out at sea, or maybe even on animals – perhaps functioning as a new mechanism for tracking animals without having to attach permanent or semi-permanent tags that are used now.

Watch the video below to see the robotic remora disc in action:

Can you think of other ways the robitic remora suction disc could be used in the future?  Write your ideas in the comments below!

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