It has been assumed that Sharks can swim so quickly because of their rough skin. Biomimetics has been around for a long time and now Sharks are the next animal to be mimicked by scientists. Harvard University has created the first realistic shark skin which can increase swimming speeds by up to 6.6 %.
A shark’s skin may look smooth but at the microscopic level, there are overlapping scales similar to teeth. These ‘denticles’ intrude on the smooth flow of salt water, decreasing the drag and boosting speeds. Scientists were unsure how the shark’s skin could increase speed and so an experiment needed to be done. They wanted to test how alterations to their skin would affect the shark’s speed. Since these alterations could not be done on real sharks, George Lauder from Harvard University and his colleagues set about to create artificial shark skin.
Lauder found a mako shark at a fish market and simply used its skin as a starting point. They viewed the skin under a high-resolution microscope then zoomed in on one denticle in order to build a detailed model of it. They then reproduced it many times within a computer model. Once the team had the computer model, they needed to reproduce it physically, in some way. ‘After considering a number of approaches, we decided that the only way to embed hard denticles in a flexible substrate was the 3D printer. 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 for 3D printing,’ says Lauder.
The team tested a variety of materials, printing out protocols and After a year of testing different materials, printing protocols and making the denticles and their spacing more substantial, a sample was produced with the denticles fixed in a flexible support. ‘Seeing the [scanning electron micrograph] SEM of the curved membrane with the denticles was a great moment for us’, explains Lauder. Lauder and his team put samples of the ‘skin’ on both sides of a stationary flexible foil in flowing water then on a foil that flapped like a real fish. The scientist then measured the force exerted on both foils. The results were that the static shark skin reduced the drag on the foil by 8.7% at the lowest flow speeds, although at the highest flow speed – almost 0.6 m s−1 – the shark skin produced 15% more drag than the smooth membrane.
On the other hand, when the team tested the flapping foil it displaced its body by 1 cm as it wriggled at 1.5 Hz, the shark skin’s performance was greatly improved and the swimming speed increased by 6.6% while the energy wasted was reduced by 5.9%. Lauder states, ’This is the first time that anyone has measured the energetic cost of shark skin and the reduction in swimming cost relative to a smooth surface…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’, says Lauder. The next task is testing to see if the arrangement of the denticles haven any effect on performance. We shall have to wait and see!
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[Image via sustainable-nano]