Harvard scientists have used 3D printing in order to produce a scaled-up replica of a mako shark’s skin which has demonstrated how microscopic tooth-like scales improve the shark’s swimming efficiency. The study has been published in The Journal of Experimental Biology.
The skin of a shark may appear glossy and smooth as they elegantly glide through water, but closer inspection reveals that it is in fact scattered with millions of tiny, overlapping three-dimensional scales called denticles which disrupt the pattern of water flow over the shark and thus aid swimming. These intriguing structures have inspired scientists over the years in the field of biomimetics, which is the imitation of biological systems in design. In particular, researchers have tried to model shark skin in order to reduce drag, which has even been attempted in racing car design.
Previous efforts to investigate how precisely these denticles bestow advantages in locomotion have fallen down because of difficulties in replicating these complex structures, meaning that models had to be simplified. Models were also placed on rigid structures which don't take into account how the sharks bend and flex during swimming.
In a bid to overcome these limitations, Harvard scientists took a sample of skin from a shortfin mako shark and scanned it using micro-CT imaging. This enabled the team to construct an incredibly detailed 3D model of a single denticile a mere 0.15mm long, which they then repeated thousands of times. Now for the really tricky part- developing a method to accurately replicate the denticles on a supple synthetic skin.
“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 Professor Lauder, lead researcher of the study. “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.”
After around a year of trial and error, the scientists were satisfied with their result. Because of limitations imposed by 3D printers with regards to resolution, the denticles were around 10 times the size of those found on mako sharks.
The team then coated a robotic device with this artificial skin and tested it in water. The device could either be held stationary or moved to mimic swimming. The advantage these denticles bequeathed was immediately apparent; the artificial toothy skin boosted swimming speed by 6.6% when compared to a device with a smooth control coating. Furthermore, the artificial shark skin reduced energy expenditure by 5.9%.
“That’s a huge effect, when factored over the entire lifetime of an animal that is constantly swimming,” Lauder told the BBC.
Interestingly, the scientists also found that the biggest advantages conveyed by the denticles occurred at slower speeds, when the shark is cruising around as opposed to propelling through the water to attack prey.
By imaging the flow of water as the device was in motion, the team were able to discern that the denticles also produced a stronger leading-edge vortex than the smooth control, which is a low pressure whirlpool produced by the movement of the device. According to Lauder, this finding could be very important since it suggests that rather than just reducing drag, the denticles may actually increase the thrust. “It can help suck the fish forward,” he added.
The scientists are currently continuing the work by tweaking the denticles, altering their shape and arrangement in order to get a better idea of what causes this apparent effect. But according to Lauder, it will probably be a while before we see any denticle-clad swimwear.