To make fluid flow in one direction down a pipe, it helps to be a shark

Researchers from the University of Washington have discovered a new way to help liquid flow in only one direction -- but without flaps. In a paper published Sept. 24 in the Proceedings of the National Academy of Sciences, they report that a flexible pipe -- with an interior helical structure inspired by shark intestines -- can keep fluid flowing in one direction without the flaps that engines and anatomy rely upon.

Human intestines are essentially a hollow tube. But for sharks and rays, their intestines feature a network of spirals surrounding an interior passageway. In a 2021 publication, a different team proposed that this unique structure promoted one-way flow of fluids -- also known as flow asymmetry -- through the digestive tracts of sharks and rays without flaps or other aids to prevent backup. That claim caught the attention of UW postdoctoral researcher Ido Levin, lead author on the new paper.

"Flow asymmetry in a pipe with no moving flaps has tremendous technological potential, but the mechanism was puzzling," says Levin. "It was not clear which parts of the shark's intestinal structure contributed to the asymmetry and which served only to increase the surface area for nutrient uptake."

To answer these questions, Levin led a team that included co-authors Sarah Keller and Alshakim Nelson, both UW professors of chemistry, and Naroa Sadaba, a fellow UW postdoctoral researcher. They 3D-printed a series of "biomimetic pipes," all with interior helices inspired by the layout of shark intestines. They varied the geometrical parameters among these prototype pipes, such as the pitch angle of the helix or the number of turns. Their first pipes were printed from rigid materials, and they found that some showed a strong preference for unidirectional flow.

"The first measurement of flow asymmetry was a 'Eureka' moment," said Levin. "Until that instant, we didn't know if our idealized structures could reproduce the flow effects seen in sharks."

By further tuning the geometrical parameters and printing new designs, the researchers increased the flow asymmetry until it rivaled and even exceeded designs of famed inventor Nikola Tesla, who more than a century ago patented the Tesla valve, a one-way fluid flow device with no moving parts.

"You don't get to beat Tesla every day!" said Levin.

But shark intestines -- like human intestines -- aren't rigid. The team suspected that so-called "deformable structures," which are made from more flexible materials, might perform even better as Tesla valves. They 3D-printed a second series of prototypes made from the softest polymer that is both printable and commercially available. These flexible pipe designs, which are better mimics for shark intestines through both their "deformability" and their interior helices, performed at least seven times better compared to all previously measured Tesla valves.

"Chemists were already motivated to develop polymers that are simultaneously soft, strong and printable," said Nelson, an expert in developing new types of polymers. "The potential use of these polymers to control flow in applications ranging from engineering to medicine strengthens that motivation."

"Actual intestines are still about 100 times softer than our soft material, so there is plenty of room for improvement," said Sadaba.

Keller credits the project's success to the team's interdisciplinary ideas from biology, chemistry and physics, and to the sharks themselves.

"Biomimicry is a powerful way of discovering new designs," said Keller. "We never would have thought of the structures ourselves."

The research was funded by the National Science Foundation, the Washington Research Foundation and the Fulbright Foundation.