Placoid scales are found on, among other fish, sharks. Also called 'dermal denticles,' placoid scales are for the most part structurally similar to the teeth found in vertebrates.
With an outer layer composed of vitrodentine (a mostly inorganic and enamel-like substance), placoid scales entirely cover the outside of the shark and we think of them as 'shark skin.'
It's these scales that, by creating tiny vortices around the shark, reduce drag and make the shark a fast and quiet seagoing machine. As a result of the arrangement of these scales, sharks are clothed in a flexible skin of collagenous fibers arranged in a helical pattern which provides their outer skeleton.
Now a group of scientific researchers have created a 3D printed model of the shark's skin to demonstrate functional properties of the arrangement. A flexible and biomemetic surface, the hydrodynamic workings of surface roughness can be studied using the flexible, synthetic membrane.
The three-dimensional model of shark skin denticles was constructed using Micro-CT imaging derived from a shortfin mako shark subject by the team from the Museum of Comparative Zoology, the School of Mechanical Engineering and Automation, Beijing and the Wyss Institute for Biologically Inspired Engineering.
Researchers Li Wen and George V. Lauder planned the experiments and data analysis, Li Wen conducted the experiments and the 3D modeling of shark skin, James C. Weaver printed the models and contributed to planning the project and figure preparation, and Li Wen and George V. Lauder ultimately wrote the paper revealing their findings.
In that research, thousands of rigid synthetic shark denticles were printed on flexible membranes in a linear pattern which was then tested in water using a robotic device which allowed the team to either hold the models in a stationary position, or move them dynamically at their self-propelled swimming speed.
The team found that when compared with a smooth control model which had no denticles, the 3D printed shark skin increased swimming speed and reduced energy consumption under certain motion patterns. Lauder and his colleagues took a detailed scan of a single square of skin from a mako shark, and built a 3D model of a single denticle just 0.15mm long.
The researchers found that speed was increased by 6.6% and the energy 'cost-of-transport' was reduced by 5.9%. Those numbers were due in large part to the fact that a leading-edge vortex with greater vorticity than the smooth control resulted from their 3D printed shark skin.
Lauder said it took the team nearly a year to fabricate the particular shape of the denticles. In order for them to be 3D printed, the surface required support structures which had to be removed later.
While the size of the denticles on the surface of the skin were roughly 10 times larger than those found on a mako shark, the researchers were able to extrapolate their findings to gauge the effect.
Lauder says that although previous studies of the fluid dynamics of samples of actual shark skin have been done before, he believes the 3D printed model behaves in a more realistic manner as it moves and bends like that on a living shark.
"You have a rigid scale structure embedded into a flexible membrane, that can then swim," Lauder said. "It pays us to understand how the natural world works. Millions of years of evolution give us solutions to problems that we may not have thought of."