Green sea turtle gets relief from “bubble butt” syndrome thanks to 3D printing

Charlotte, a green sea turtle, was hit by a boat back in 2008. This left it with an affliction colloquially referred to as the “bubble butt,” a kind of floating syndrome that makes it impossible for a turtle to dive. Most sea turtles suffering from issues like this simply die at sea, since the condition leaves them stranded at the surface where they can’t forage, sleep, and avoid predators like sharks. But fate had other plans for Charlotte.

Charlotte didn’t end up as a shark’s lunch and didn’t starve to death floating helplessly in the ocean. Instead, it got rescued shortly after the boat accident and eventually found a home at Mystic Aquarium in Stonington, Connecticut, where it received professional care. That was the first time Charlotte got lucky. The second time came when a collaboration formed: Adia, a company specializing in 3D-printing solutions; Formlabs, one of the world’s leading manufacturers of 3D printers; and New Balance Athletic, a sportswear giant based in Boston. This team chose Charlotte as a technology showcase, which basically turned the turtle into an Oscar Pistorius of the sea—just without the criminal conviction.

Weights and diet

Sea turtles are marine reptiles, which means they don’t have gills like fish—they need air to breathe. The lungs also play a key role in their buoyancy regulation system, which allows them to rest for extended periods of time at the sea floor or float at a precisely chosen depth. A sea turtle can precisely choose the depth at which it achieves neutral buoyancy by inhaling the exactly right volume of air.

Their lungs therefore serve a dual-purpose—they are an oxygen storage, but they also enable a turtle to maintain a chosen position under water. “The issue with Charlotte and many other turtles is a bubble butt syndrome, where there are gas pockets causing added buoyancy,” says Alex Pestana, a Formlabs manager involved in helping Charlotte.

Two main reasons those gas pockets appear in turtles are plastics and boat strikes.

When a turtle consumes something it can’t digest—like parts of fishing nets, plastic bottles, or even rubber gloves (yes, there was a sea turtle found with a rubber glove in its intestines)—it sometimes gets stuck somewhere along its gastrointestinal tract. This, in turn, causes gases to gather there, which throws the turtle’s buoyancy out of balance.

Those gases usually gather in the parts of the gastrointestinal tract located near the rear of the turtle, so the animal is left floating bum-up at an unnatural angle. Conditions like that are sometimes curable with dietary modifications, assisted feeding, fluid therapy, and other non-invasive means to the point where afflicted animals can be safely released back into the wild. Boat strikes, on the other hand, often lead to permanent damage.

Sea turtles’ shells are tough but not tough enough to withstand a boat impact, especially when the shell gets hit by a propeller blade. This often leaves a shell deformed, with air bubbles trapped underneath it. In more severe cases, the spinal cord under the shell also gets damaged, which leads to complete or partial paralysis.“

The most popular approach to rehabilitating these injuries relies on gluing Velcro patches to the shell at carefully chosen spots and attaching weights to those patches to counteract the buoyancy caused by the air bubbles. This is a pretty labor-intensive task that has to be done repeatedly every few months for the rest of the turtle’s life. And green sea turtles can live as long as 80 years.

Harnessing advanced manufacturing

Charlotte, as a boat strike victim with air bubbles trapped under its deformed shell, was considered non-releasable and completely dependent on human care. Since full recovery was not an option, Mystic Aquarium wanted to make everyday functioning more bearable for both the turtle and its caretakers. It got in touch with Adia, which in turn got New Balance and Formlabs onboard. Their idea was to get rid of the Velcro and replace them with a harness fitted with slots for weights.

The work started with a 3D scan of Charlotte’s shell. The scan was the starting point for a New Balance computational design team. And the task proved quite hard.

For starters, turtles, as all reptiles, grow all their life—their growth rate slows down dramatically when they reach mature sizes, but it never stops. Turtle shells also change over time, with individuals shedding some features and replacing them with new ones. So permanently attaching weights was out of the question.

The air bubbles beneath Charlotte’s shell also shifted at times, which meant the harness had to make repositioning the weights possible. Finally, through years of swimming at an awkward 45-degree angle, the turtle mostly used its fore flippers, which led to atrophy of the hind limbs.

The goal then was to gradually get Charlotte used to using its hind flippers again by leveling its position gradually using small weights at the start and moving to larger weights later on.

When New Balance finally designed a harness that checked all the boxes, the CAD files went to Formlabs for 3D printing. Pestana’s team prepared those files for 3D printing in a process called slicing, where a digital model is divided into very thin layers. Those sliced models were then fed into the Formlabs Fuse 1 SLS (Selective Laser Sintering) 3D printer. “In this process, we take powderized plastic, finer than sand, and we are heating that powder to just about its melting point,” Pestana explained. “Then we are using a laser to melt it—the laser kind of draws the layer’s image on the powder, starting from the bottom of the model. Then we recoat the powder for the next layer, and we do that over and over, till the model is printed.”  To get the right mix of flexibility, stiffness, and impact-resistance, Formlabs made Charlotte’s harness with carbon fiber-reinforced nylon. And it worked.

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