Octopus suckers are extraordinary. They can move and grasp objects independently. They can "taste" the water around them. They can even form a seal on rough surfaces underwater. And as a many a diver, biologist and intrepid eater can attest, these little suckers are strong.
This strength is astounding, especially considering that their tissue is similar in softness to jellyfish jelly. But we know little about the stuff these awesome suckers are really made of.
Previous studies have described the basic anatomy of octopus suckers, which rely on a cavity toward the top and flexible sides to create pressure and form a seal. But the new study finds that these suckers have been hiding some surprising features.
Last fall, researchers in Livorno, Italy bought a load of common octopuses (Octopus vulgaris) from local fishermen. The scientists removed suckers from the expired animals and examined them under a microscope and with micro-CT (microcomputed tomography) scans. The researchers discovered that the sides and edges of the suckers were rimmed with tiny, concentric groves, key for forming a seal on uneven surfaces underwater.
The upper section was grooved as well, which might also aid in creating--and holding--an effective low-pressure seal. The results were published this week in the open-access Journal of the Royal Society: Interface.
These new insights "may serve as a model for the creation of a new generation of attachment devices" (i.e. sticky robots), the authors wrote. (Some researchers are already 3-D printing octopus-inspired suckers.) "Artificial material that mimics octopus-sucker tissue is crucial for the design of innovative artificial suction cups."
So far, however, our attempts at making faux suckers have fallen flat.
The researchers examined three different materials used in making octopus-esque soft robots. Each of these was smooth, lacking the crucial gripping ridges of a real octopus sucker.
In addition to the superficial landscape of these suckers, the scientists also tested their resilience under pressure using a micro-force tester called a microindentation unit (which measures a material's resilancy and firmness by pushing on it with a spring).
The sides and edges of the suckers (the infundibulum) were quite squishy, much like the rest of the octopus's arm (but were still less elastic than, say, human skin or even our heart valves). The top of an octopus's sucker (known as the acetabular protuberance) was much stiffer than the rest of the sucker. (It withstood 3.4 micronewtons of compression—versus 7.4 micronewtons for the edges). This lesser elasticity is likely crucial for the local generation of the low-pressure that makes the adhesion possible.
Under pressure, both tissues became stiffer, which also helps insure a good hold once the octopus sucker has made contact with a surface. The artificial materials, on the other hand (or arm), by in large did not vary their elasticity under different pressures.
Further study of these surfaces and their elasticity will likely help us develop more effective soft-bodied robots. For instance, the stiffer upper section of the octopus sucker has considerably more connective tissue. And although they don't yet know for sure why, researchers hypothesize that this might help the animal "store elastic energy to generate attachment for long periods of time without muscle contractions"—a huge energy saver.
Until then, however, the amazing cephalopod sucker continues to be quite the engineering sticking point.
Hat tip to Lucas Laursen for spotting this gripping study.
To learn more about the octopus's suckers and the rest of its bizarre body, check out my new book Octopus! The Most Mysterious Creature In the Sea.
Illustration courtesy of Ivan Phillipsen