When you turn on a faucet, water flows one way: out into the sink, then down the drain. That one-way flow is important. If the flow reversed, clean water might become dirtied by germs or other contaminants. But keeping the flow moving one direction isn’t always easy; it often requires valves that can break. Researchers have now developed pipes that keep liquids moving one way — and they’re valve-free.

Their inspiration? Shark intestines.

The team published its findings September 24 in Proceedings of the National Academy of Sciences.

Unlike ours, shark intestines have a hidden internal corkscrew shape, or helix. That helix coils around and around. And a 2021 study found that it holds the key to one-way flow of partly digested food through a shark’s intestines.

“I wanted to know if the effect occurred only in sharks,” says Sarah Keller. A chemist, she works at the University of Washington in Seattle. Keller suspected it might also occur “in squishy … tubes that mimicked the biological system.” To find out, she teamed up with two colleagues at Washington: physicist Ido Levin and materials chemist Alshakim Nelson. Working with scientists who have different backgrounds is “the best way to make discoveries in the lab,” she explains.

Explainer: What is 3-D printing?

Her team started by 3-D printing short, rigid tubes using a material called resin. These didn’t bend or flex. Each tube had a different shaped helix inside. Imagine Slinky toys that were more or less stretched out and had wires with different widths.

The team varied how much of the inside of each tube a helix covered. Some helices were skinny and left a large opening in the middle. Others seemed to fill the entire tube when you peered through one end. The researchers also varied the angle of the helix. Sometimes it was nearly perpendicular to the tube wall. Other times it was slanted sharply downward from the tube’s wall.

To test which helices worked best to control flow, the researchers hooked up the tubes to flowing water. Then they measured the rate of flow coming out the end of the pipe. In half of the tests, the tubes were attached with the helix pointing down, just like a spiral slide. This is the way a shark’s intestine is oriented. The other times, this helix pointed up, like an upside-down slide.

When the helix pointed down, water flowed two to three times faster than when it had been pointed upward. The helix had therefore helped keep water flowing in one direction. Effects didn’t seem to change with the various angles and coils of the helices; most of these worked pretty well.

“In all of the different ways they set up their experiment, it worked,” says Rachel Levy. She’s an applied mathematician at North Carolina State University in Raleigh. Levy, who has studied fluid mechanics, did not take part in this study.

It’s important work, she says. “We can get great engineering ideas [from living things] while at the same time understanding new things about nature.”

These rigid pipes are ready to be tested. Water flowing from the top would move quickly through these pipes, since their helices are pointed down.I. Levin/University of Washington

Faster flow

Water pipes are typically rigid. Shark intestines aren’t. So the Washington team’s next step was to test whether a soft tube would work even better.

The researchers 3-D printed a set of tubes that could flex as a fluid moved through them. It’s like how your body might bend when you stand in a stream or current, Keller notes.

Her team again compared the flow of water when the helix was pointing down versus when it was pointing up. Once more, pointing down proved faster — a whopping 15 times faster than in the other direction, Levin says. “That’s a big effect!”

“Water pushes on the deformable pipe, bending parts of it,” Keller notes. This “in turn changes the flow of the water.” Her team doesn’t yet know why bending the pipe sped up the flow. But they’re excited to figure that out.

Such helical pipes might one day be used for a variety of applications. “Most valves have moving parts that open and close” to prevent backward flow, Nelson says. “These valves can wear out over time and must be replaced.” Helical tubes could replace valves in hard-to-reach locations, he says. Those might be in drainage systems or air-flow systems where one-way flow is essential.

“It’s super exciting that studying these everyday things [like shark guts] can lead to big scientific breakthroughs,” Levin says. “I love that my next big idea for a science project might come from just hanging out in my backyard and noticing something new. Science is everywhere, and that makes it so much fun.”