Few people have gotten close enough to a shark to pet it. If you could run your hand from a shark’s head to its tail—not that you should—it
would feel smooth, almost like suede. Reverse direction and it’s rough
like sandpaper. Viewed under a microscope, shark skin is composed of
ribbed, dragonesque scales layered over each other like shingles on a
roof. These structures, called dermal denticles, are more like teeth
than skin. Made of dentin and enamel, they are innervated, and their
ribbed and layered pattern guides water across the shark’s back,
reducing friction and drag. Sharks’ impressive skin helps them glide
through the water, with some species reaching speeds as fast as 50
kilometers per hour.
Shark denticles are the envy of engineers. To mimic sharks’
impressive hydrodynamic prowess, materials scientists have designed
shark-inspired surfaces for the hulls of boats, wind turbines, and even
high-end swimsuits, all in an attempt to maximize efficiency.
But in a new study,
researchers from Harvard University in Massachusetts, led by
ichthyologist Molly Gabler-Smith, have for the first time compared
materials that attempt to mimic shark skin with the real thing. As it
turns out, the engineered materials have a long way to go.
Previously, scientists have examined shark denticles in impressive
detail using scanning electron microscopes, a technology that can take
images of structures just a few nanometers wide. But the images scanning
electron microscopes put out are two-dimensional. And if you’ve ever
seen a car in a wind tunnel, you’ll know that when it comes to reducing
drag and friction, an object’s 3D structure is incredibly important.
So using a technique called surface profilometry, an imaging
technology in which a scientist essentially uses a thin layer of gel to
make a mold of the surface to be studied, Gabler-Smith and her team
viewed shark skin in 3D. “It’s almost like looking at a topographic
map,” says Gabler-Smith. “You can see where there’s peaks and valleys.”
Gabler-Smith used the technique on skin samples from 17 different
shark species. She also looked at two Speedo swimsuits marketed as
mimicking shark skin—the FS Fastskin II swimsuit and the Lzr Racer Elite
2—as well as a 3D printed surface created in her lab. She compared the
proportions of the denticles and the height and spacing of the riblets
and found substantial differences between real shark denticles and the
artificial materials.
A
close-up analysis shows the differences between real shark skin (top
right) and several synthetic materials including a 3D-printed material
(bottom right), the Speedo FS Fastskin II (bottom left), and the Speedo
Lzr Racer Elite 2 (top left). Photos courtesy of Molly Gabler-Smith
“I hesitate to say that [the swimsuits are] really mimicking shark
skin, because they really aren’t at all,” Gabler-Smith says. One of the
biggest differences, she says, is that unlike a ribbed fabric, real
shark skin is made of hard enamel structures on a flexible surface. When
asked to rate the swimsuit materials out of 10, Gabler-Smith gave the
FS Fastskin II and Lzr Racer Elite 2 a three and a seven, respectively,
and her lab-made engineered surface an eight or nine.
“[The swimsuit designers are] doing a pretty good job of taking all
of the information that biologists are measuring from actual shark skin,
but there’s still so much to do,” she adds. In theory, she says, adding
riblets or other bumpy textures to swimsuits or the surfaces of
watercraft should reduce drag, but these surfaces are still a way off
from functioning like the real thing.
Amy Lang, an aeronautical engineer at the University of Alabama who
studies shark skin and was not involved in the research, says that
replicating the drag-reducing properties of shark skin is even more
difficult than just having denticle-like riblets. To actually decrease
drag instead of increasing it, she says, the riblets must be the right
size and depth. She adds that while it is interesting to use surface
profilometry to directly compare shark skin with engineered materials,
it’s equally important to test how synthetic materials actually work in
the water.
But now that scientists have updated information on the 3D structure
of shark denticles across a variety of different species, engineers may
be one step closer to mimicking one of nature’s most efficient swimmers.
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