Shark Skin Taught Boats and Planes to Slip Along on Tiny Grooves

Stroke a shark from nose to tail and the skin slides under your fingers. Stroke it the other way and it feels like sandpaper. That one-way roughness comes from a coat of scales far too small to see, and the idea behind it has been borrowed for racing yachts, rowing boats and aircraft.

A skin made of tiny teeth

The scales are called denticles, which means little teeth, and that is what they look like under a microscope: rows of tiny tooth-shaped plates, overlapping, each with fine ridges running from front to back. On a mako shark, one of the quick swimmers of the open sea, a single denticle is about 150 micrometres long, roughly the size of a speck of dust, according to Harvard University's Wyss Institute. The shark wears them from nose to tail, all pointing the same way.

How grooves calm the water

By the 1980s, engineers at NASA's Langley Research Center were looking for ways to make aircraft burn less fuel. Much of the drag on a plane or a hull is skin friction: the rubbing of air or water against the surface. The Langley team found that a surface covered in tiny V-shaped grooves, lined up with the flow and, in NASA's words, no deeper than a scratch, met less friction than a plain one, because the grooves smooth the turbulent layer right next to the skin. NASA called them riblets, and described them as based on the riblet-like projections on a shark's skin.

Cutting grooves into a whole aircraft is hard work, so engineers at the company 3M moulded them into a light plastic film with a sticky back. The film went onto a rowing shell that carried a United States crew to an Olympic silver medal, and in 1987 onto the underside of the yacht Stars & Stripes, which won the America's Cup; its design coordinator called the film the boat's secret weapon. NASA reckoned at the time that riblets might save an aeroplane 5 percent of its fuel.

What people often get wrong

The natural guess is that the fastest surface is the smoothest one. Shark skin says otherwise. In 2012 two Harvard researchers, writing in the Journal of Experimental Biology, fixed pieces of real shark skin to a robotic flapping fin. Flexible pieces with their denticles in place swam 12.3 percent faster on average than the same skin sanded smooth.

The second mistake is to think that any ridged fabric is a shark skin. The same study tested a swimsuit fabric sold as shark-inspired and found no consistent gain in speed, while a man-made riblet film, with its ridges lined up with the flow, gave 7.2 percent. Stiffness mattered too: on a rigid plate, the sanded skin was the quicker one. The benefit appeared only when the skin could bend as it moved. A shark's coat is hard little teeth on a soft, flexing body, and the two work together.

• 150 micrometres the length of one mako denticle
• 12.3% faster with real shark skin on a flexible fin than with sanded skin
• 7.2% faster with man-made riblet film, ridges lined up with the flow

How we know

In 2014 the same Harvard laboratory went a step further. The team took X-ray scans of mako skin, built a computer model of a single denticle, and 3D-printed thousands of copies in rows onto a flexible sheet, each enlarged to about 1.5 millimetres. It took months to stop the tiny teeth from snapping off. In the water tank, the printed skin swam more than 6 percent faster than a smooth sheet and used about 6 percent less energy.

The tank also showed something that grooves alone do not explain. The denticles helped a small whirl of water to hold on along the front edge of the moving sheet, and that whirl, the researchers said, helps to suck the shark forward. So the skin may do two jobs at once: less drag, and a little extra pull. Engineers have copied the first. The second is still mostly the shark's own.