At first glance, a shark's skin may seem smooth and sleek. Upon closer inspection however, shark skin has tiny scales called denticles that can help it swim faster in the water.

An adult shark has millions of very small denticles all over its body. These little scales overlap each other and scientists have known for a while that these specialized scales can help reduce drag while the shark is moving underwater. By disrupting the flow of water over a shark's skin, drag is reduced. Engineers have been studying the way these denticles reduce drag in the hopes of using the same technique on artificial materials. Researchers from Harvard University have finally managed to use the same technique to create a synthetic membrane that can help swimmers move through the water much easier.

The researchers first headed over to a local fish market to procure a freshly caught mako shark. The scientists then analyzed the mako shark's skin by putting a sample under a microscope. By taking high-resolution images of the microscopic denticles, the scientists were able to zoom in and isolate a single denticle. After gathering enough data, the researchers were able to construct a 3D model of the denticle. The scientists were then able to reproduce the single denticle several thousand times to create a 3D model of a patch of shark's skin. The next step was to use a 3D printer.

"After considering a number of approaches, we decided that the only way to embed hard denticles in a flexible substrate was the 3D printer," said Harvard researcher George Lauder, but this proved to be easier said than done. 'We had to figure out how to print them with multiple materials... The denticles are embedded into the membrane and overlap, which posed a key challenge."

It took the scientists a while to find the perfect mix between printing techniques and materials. The researchers had to experiment with a wide number of materials as well as various configurations of the denticles with different spacing intervals. After some experimenting, they were finally able to create a synthetic shark skin complete with microscopic denticles.

"Seeing the [scanning electron micrograph] SEM of the curved membrane with the denticles was a great moment for us," said Lauder.

After testing the synthetic shark skin, the scientists found that it could reduce drag on a stationary foil by as much as 8.7 percent under low flow speeds. At high flow rates however, they found that the synthetic membrane actually increased drag on the stationary foil by around 15 percent. When the scientists tried moving the foil similar to a fish's fins however, they were able to record a 5.9 percent reduction in energy expenditure and a 6.6 percent increase in swimming speed.

"Artificial shark skin needs to have rigid denticles/scales on a flexible substrate so that the biomimetic skin can flex and bend like real shark skin," said Lauder.

The researchers published their findings in The Journal of Experimental Biology.

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