The hydrodynamics of shark skin

The skin of sharks is covered with scales called denticles. Images of 250µm across of the skin of a bonnethead shark shows the details of the typical denticles, with three surface ridges leading to three prongs oriented toward the tail. Related in structure to teeth, denticles have long been suspected of reducing hydrodynamica drag on sharks as they swim.  Indeed, shark skin has inspired a variety of materials engineered to reduce drag in submerged bodies: swimsuits are perhaps the best known example.

Many of the experimental studies of such materials -and of shark itself- have examined the drag on rigid bodies, a scenario that may be relevant for some applications but not for shark or swimmers.  New work by Johannes Oeffner and George Lauder of Harvard University has now look at the effects of undulation.  The pair mechanically flapped sheets of shark skin in a flowing water tank to determine the speed at which each sheet held its position.  Comparing the swimming speed for natural shark skin with that for skin with the denticles sanded off, the team found that the denticles actually decreased the swimming speed for rigid sheets but produced a 12% increase for flexible sheets that mimicked typical shark undulations. The team attributes the increase not just to decreased drag but also to increased thrust arising from the altered flow environment observed near the undulating surface. Surprisingly, the researchers saw no clear speed increase in similar experiment with "shark inspired" swimswuit fabric.

J-Oeffner, G.V. Lauder J. Exp.Biol. 215, 785, 212

Another interesting discovery related to shark skin:

 Sharkskin—The Latest Craze in Catheters

Hospitals are constantly worried about germs. No matter how often doctors and nurses wash their hands, they inadvertently spread bacteria and viruses from one patient to the next. In fact, as many as 100,000 Americans die each year from infections they pick up in hospitals. Sharks, however, have managed to stay squeaky clean for more than 100 million years. And now, thanks to them, infections may go the way of the dinosaur.

Unlike other large marine creatures, sharks don’t collect slime, algae, or barnacles on their bodies. That phenomenon intrigued engineer Tony Brennan, who was trying to design a better barnacle-preventative coating for Navy ships when he learned about it in 2003. Investigating the skin further, he discovered that a shark’s entire body is covered in miniature, bumpy scales, like a carpet of tiny teeth. Algae and barnacles can’t grasp hold, and for that matter, neither can troublesome bacteria such as E. coli and Staphylococcus aureus.

Brennan’s research inspired a company called Sharklet, which began exploring how to use the sharkshin concept to make a coating that repels germs. Today, the firm produces a sharkskin-inspired plastic wrap that’s currently being tested on hospital surfaces that get touched the most (light switches, monitors, handles). So far, it seems to be successfully fending off germs. The company already has even bigger plans; Sharklet’s next project is to create a plastic wrap that covers another common source of infections—the catheter.


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