Peer into any fishbowl, and you鈥檒l see that pet goldfish and guppies have nimble fins. With a few flicks of these appendages, aquarium swimmers can turn in circles, dive deep down or even bob to the surface.
New research led by the University of Colorado Boulder has uncovered the engineering secrets behind what makes fish fins so strong yet flexible. The team鈥檚 insights could one day lead to new designs for robotic surgical tools or even airplane wings that change their shape with the push of a button.听
Close-up image of a ray in a fish fin in a relaxed, top, and bended, bottom, state. (Credit: Fracois Barthelat)
The researchers Aug.听11 in the journal Science Robotics.
Francois Barthelat, senior author of the study, noted that fins are remarkable because they can achieve feats of dexterity even though they don鈥檛 contain a single muscle. (Fish move these structures by twitching sets of muscles located at the base of the fins).听
鈥淚f you look at a fin, you鈥檒l see that it鈥檚 made of many stiff 鈥榬ays,鈥欌�� said Barthelat, professor in the Paul M. Rady Department of Mechanical Engineering. 鈥淓ach of those rays can be manipulated individually just like your fingers, but there are 20 or 30 of them in each fin.鈥�
In their latest research, Barthelat and his colleagues drew on a range of approaches, including computer simulations and 3D-printed materials, to dive deep into the biomechanics of these agile structures. They report that the key to fish fins may lie in their unique design. Each ray in a fin is made up of multiple segments of a hard material that stack on top of much softer collagen, making them the perfect balance between bouncy and stiff.听
鈥淵ou get this dual capability where fins can morph, and yet they鈥檙e still quite stiff when they push water,鈥� he said.听
Armor and airplanes
Barthelat is no stranger to looking into aquariums. He previously studied how fish scales can help engineers to design better body armor for humans, and how seashells might inspire tougher glasses.
Fins may be just as useful. When it comes to engineering, Barthelat explained, materials that are both stiff and flexible are a hot commodity. Airplane designers, for example, have long been interested in developing wings that can morph on command, giving planes more ability to maneuver while still keeping them in the air.听
鈥淎irplanes do this now, to some extent, when they drop their flaps,鈥� Barthelat said. 鈥淏ut that鈥檚 in a rigid way. A wing made out of morphing materials, in contrast, could change its shape more radically and in a continuous manner, much like a bird.鈥�
To understand how ordinary run-of-the-mill goldfish achieve similar feats every day, take a close look at these structures under the microscope. Each of the rays in a fin has a layered structure, a bit like a bakery 茅clair: The spikes include two layers of stiff and mineralized materials called hemitrichs that surround an inner layer of spongy collagen.听
But, Barthelat said, those layers of hemitrichs aren鈥檛 solid. They鈥檙e divided into segments, as if someone had听cut up the 茅clair into bite-sized pieces.听
鈥淯ntil recently, the function of those segments hadn鈥檛 been clear,鈥� he said.
Swimming, flying and walking
The engineer and his team decided to use computer simulations to examine the mechanical properties of fins. They discovered that those segments can make all the difference.
Pretend for a moment, Barthelat explained, that fish fins are made up entirely of collagen. They could bend easily, but wouldn鈥檛 give fish much traction in the water because hydrodynamic forces would collapse them. Rays made up of solid, non-segmented hemitrichs, in contrast, would have the opposite problem鈥攖hey鈥檇 be way too stiff.听
鈥淎ll of the segments, essentially, create these tiny hinges along the ray,鈥� Barthelat said. 鈥淲hen you try to compress or pull on those bony layers, they have a very high stiffness. This is critical for the ray to resist and produce hydrodynamic forces that push on water. But if you try to bend individual bony layers, they鈥檙e very compliant, and that part is critical for the rays to deform easily from the base muscles.鈥�
The researchers further tested the theory by using a 3D printer to produce model fish fins made from plastic, some with those hinges built in and some without. The idea panned out: The team found that the segmented design provided better combinations of stiffness and morphing capabilities.听
Barthelat added that he and his colleagues have only scratched the surface of the wide diversity of fins in the fish world. Flying fish, for example, deploy their fins to glide above the water, while mudskippers use their fins like legs to walk on land.
鈥淲e like to pick up where the biologists and zoologists have left off, using our background in the mechanics of materials to further our understanding of the amazing properties of the natural world,鈥� Barthelat said.
Coauthors of the new study include Floren Hannard at the Catholic University of Louvain in Belgium, Mohammad Mirkhalaf at the University of Sydney in Australia and Abtin Ameri at MIT.
