What physicists can learn from shark intestines

In 1920, inventor Nikola Tesla patented a type of pipe called a “valvular conduit”, built to draw fluid in one direction without any moving parts or additional energy, and has applications from soft robotics to medical implants.

In 2021, scientists discovered that the spiral intestine of a shark works much the same way, favoring the flow of fluid in one direction – from the head to the pelvis. Ido Levin, a physicist in Sarah Keller’s lab at the University of Washington, became interested in the flow of fluid physics through these shark spirals. He will present how 3D printed models of shark intestines are helping them learn how these spirals work on Monday, February 20 at the 67th Annual Meeting of the Biophysical Society in San Diego, California.

Levin explained that “the researchers of the 2021 study attached a tube to the shark’s intestines, and they added water with glycerin – a very viscous fluid – through these pipes. And they showed that if you connect these intestines in the same direction as the digestive tract, you get fluid flows faster than if you connect them the other way.”

“We thought this was very interesting from a physics point of view… One of the theorems in physics actually says that if you take a pipe, and if you flow a fluid very slowly through it, you will have the same flow if you invert you it. So we were very surprised to see experiments that contradict the theory. But then you remember that the intestines are not made of steel – they are made of something soft, so even though fluid flows through the pipe, he deformed it.”

To study fluid dynamics through spiral pipes, Levin and Keller collaborated with colleagues in the Nelson Group at the University of Washington to create soft, 3D structures that mimic features of shark intestines. “15 or 20 years ago, trying to recreate these shapes in artificial materials was impossible,” Levin said.

When they used a rigid material to 3D print the shapes, there was no difference in fluid flow in one direction or the other. However, printing the shapes using a softer elastomer resulted in faster fluid flow in one direction. Using these 3D printed structures, the team is studying how the radius, pitch and thickness of the inner structure affects fluid flow.

With the softer materials, they can also study the coupling between the flow rate and how the pipe deforms. Understanding these parameters will help engineer similar structures that can be used for things like soft robotics.

Until recently, robots were made with rigid materials and hinges. But using soft materials that can be deformed in different ways, as an octopus does, opens up a whole world of possibilities, explains Levin, “this is one step forward to try to understand the basic mechanics of the interaction between the membranes and the understand flow.” One day, this seemingly simple system could control industrial or medical devices.