2017년 6월 14일 Nature News & Views “Materials science: How to suck like an octopus”
How to suck like an octopus
Rubber sheets that reversibly bind and release substrates have been made by copying a subtlety in the shape of octopus suckers. The findings reveal how macro-scale biological structures can influence function.
Subject terms : Materials science
Ask beachgoers how mussels, barnacles and oysters attach themselves to rocks, and they will often guess that suction cups are involved. Actually, these shellfish use adhesives. But go a little deeper into the water, and you will find organisms that do use suckers: octopuses. These soft-bodied animals use suction cups for surface attachment, locomotion and grabbing their next meal. On page 396, Baik et al.1 report adhesive patches that are synthetic mimics of octopus suckers. The authors go beyond simply copying suction cups by discovering a specific architectural feature that enhances adhesion.
Characterizing and mimicking biological attachment strategies is a booming research area. The two most prominent strategies are wet adhesion and dry adhesion. Mussels, seagrasses and bacteria belong to the wet-adhesive community of organisms: they deposit glue, and use it to stay in place for long periods, if not their entire lives. The underwater bonding achieved by such species cannot be matched by most synthetic adhesives, although some biomimetic compounds now exhibit adhesion strengths similar to those of their natural counterparts2.
Dry adhesion is more typical of insects and geckos, which use hardened, hair-like or pad-like structures on their feet to walk up walls. Such adhesion is temporary, used for locomotion and often employed in dry environments. Efforts to mimic natural dry adhesives have also yielded high-performance, hard-structured adhesives in the past few years (see ref. 3, for example).
Insects and geckos cannot always bond as well to wet surfaces as they do to dry ones4. Some organisms solve this problem by using wet adhesives for locomotion, with the starfish being a prime example5 — but starfish move at rates that would make a tortoise look speedy. So how can an animal attach itself to wet surfaces in a way that still allows it to move rapidly?
Octopuses have found the answer in suction cups. The grace of an octopus moving across the sea floor is captivating, combining the advantages of a soft body with water-jet propulsion and an enviable coordination of body motion. Their suckers contribute by enabling fast cycles of attachment and detachment.
An octopus uses muscles to flex, expand and contract its suckers, which makes mimicry a difficult task. Nevertheless, moulded cups shaped roughly like octopus suckers can be useful adhesives — for example, they can attach to and lift weights when a vacuum is generated in the cup6. Suction cups are also used in our everyday lives, as toilet plungers, on toy balls that roll slowly down vertical surfaces and for attaching hooks to surfaces to hang dish towels on. But these cups are crude mimics of the suckers used by octopuses. You would remain hungry and wouldn't get far if you depended on such round pieces of rubber to hunt and move.
Baik et al. set out to make suction cups more true to those of an octopus. The authors focused on a dome-shaped bulge found at the bottom of the suction cup of one species, Octopus vulgaris (Fig. 1), taking pains to examine how such bumps contribute to function. To do this, they made flexible rubber sheets that contained arrays of micrometre-sized holes. Inside each hole, they added material to mimic the bulges. They also tested other surface configurations (such as cylindrical holes without bulges, and raised pillars) for comparison with the octopus-inspired cups, under various wet or dry conditions.
The researchers found that adding bulges in suckers enhanced the adhesion of their sheets to wet surfaces, compared to sheets of holes that didn't contain bulges. Interestingly, this increased adhesion occurred only in wet conditions — when the substrate was dry, the bulges were neither beneficial nor detrimental compared with simple holes in rubber. The authors propose that enhanced wet adhesion occurs because the 'dome-in-a-cup' structure provides capillary forces between the dome and the substrate. This explanation was supported by calculations. Other surface configurations, such as a simpler cylindrical hole without the bulge, could also stick to wet surfaces, but not always as well as the octopus-inspired design.
Baik et al. demonstrated two potential applications for their adhesives. In the first, they used an array of the sucker mimics to transfer silicon wafers from one location to another, by bonding the adhesive to the wafers and then peeling it off. The surfaces of the wafers did not pick up any contamination from the suction cups, making the transfer method potentially useful for moving such substrates between stations during semiconductor manufacturing.
The authors also prepared a wound dressing that could reversibly bond, which enhanced the healing of skin lesions on mice compared with a smooth rubber dressing that lacked the octopus-mimicking structures. The results were most encouraging when the suction cups were first filled with a buffer solution. The increased healing seems to be the result of the moisture trapped in the cups of the dressing. The biomimetic dressing was outperformed by a commercially available adhesive patch (3M Tegaderm), but the authors' results are a promising start down a road that might yield new biomedical devices.
The dome-in-a-cup shapes were cut in flat sheets of rubber, producing holes sunk within the sheet. What would happen if more-faithful sucker mimics were made, in which the suckers are true cups that stand up above the smooth rubber background? And what would happen if the micrometre-sized features used by Baik and co-workers were enlarged to the centimetre scale observed in animals, or to even larger structures? Real octopus suckers need to function more rapidly than the authors' analogues, and under less well-controlled conditions in which forces are applied simultaneously from several angles. It would also be interesting to see how the performance of the biomimetic suckers compares with that of the real thing.
Researchers developing biomimetics often find themselves playing catch-up with evolution. The current work is a starting point — perhaps the addition of further biomimetic features, such as synthetic muscles, would improve the function of octopus mimics. If fully functioning mimics can be made at multiple size scales, it might open up applications such as locomotion strategies for robots and biomedical devices (and maybe even better toys). Applications aside, understanding and mimicking the fundamental science of attachment strategies used by sea creatures can just be plain fun.