Viewed up close, a shark's skin bristles⁶ with（充满） tiny teeth or "denticles" which aid swimming. 

 

Engineers have tried to mimic⁷ the roughness of shark skin when designing swim suits and even racing⁸ cars.

 

But the denticles have never been so well reproduced before, says a report in the Journal of Experimental Biology. 

 

George Lauder and his colleagues took a detailed⁹ scan of a tiny square of skin from a mako shark, and built a 3D model of a single denticle just 0.15mm long. 

 

The challenge was then to manufacture a synthetic¹⁰ skin, with thousands of these denticles embedded in a smooth, flexible membrane. 

 

"It took us about a year," said Prof Lauder, of Harvard University.

 

3D printing builds up new objects layer-by-layer, following a computer-generated design. To print the shark skin, the scientists had to use two different materials for the hard, tooth-like structures and for the flexible base - much like the different coloured inks used to print a picture. 

 

The particular shape of the denticles also posed difficulties: "Because they're overhung, the 3D printers have to print a supporting material, which you then have to remove," Prof Lauder told the BBC. "It took a while to work out all the tricks."

 

The artificial skin has impressed Oliver Crimmen, a fish expert at the Natural History Museum who has previously¹¹ advised Speedo on swim suit design. 

 

"I used to think, how on earth would you mimic that complex structure accurately¹²?" he said. "3D printing is it - what a marvellous application for it."

 

Because the resolution of even the latest 3D printers is limited, the artificial denticles are about 10 times larger than the real ones seen on the skin of a mako shark. 

 

Nonetheless, when the team stuck the new artificial skin onto a small, flexible paddle and studied it in a water tank, they were able to see the benefit sharks glean¹³ from their unusual scales.

 

A paddle with the new, toothy skin delivered a boost of up to 6.6% in swimming speed, compared to one coated with the smooth membrane alone. The artificial denticles also allowed the paddle to travel the same simulated distance while using 5.9% less energy.

 

"That's a huge effect, when factored over the entire lifetime of an animal that is constantly swimming," said Prof Lauder.

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