The main feature of all of these is the fact that gluing something is largely irreversible, which is exactly what you want when you are repairing something. This is also a drawback, as anyone who has stuck something on skew will know. Even the new types of adhesive strips don’t allow a do-over, as they lose their stickiness once they have been used.
Now snails have inspired the development of a reversible glue that is strong enough to stick a man to the wall, but can be taken off with water. Engineers at the University of Pennsylvania in the US developed the adhesive while working on another project that involved a hydrogel made of a polymer called polyhydroxyethylmethacrylate (PHEMA).
They noticed its unusual adhesive properties, and realised it worked the same way as a snail’s epiphragm, which is a slimy layer of moisture that the snail uses to protect itself from dryness but which also allows it creature to cement itself in place temporarily. On a sunny day, a snail's slimy epiphragm, initially wet, conforms to the surface it's on and hardens, barricading the snail from the dry environment and holding the snail firmly in place. At night, when the environment becomes moist, the epiphragm softens, allowing the snail to move freely again.
PHEMA is rubbery when wet but rigid when dry. When it is wet, it conforms to all of the small grooves on a surface, allowing it to stick to a surface. As PHEMA dries, it becomes as rigid as a plastic bottle cap, but it doesn't shrink. Instead, the material hardens into the cavities, fastening itself securely to the surface. When it's conformal and rigid, it's like super glue, the researchers found. It can’t be pulled off. But if it is moistened, it slips off effortlessly.
Most importantly, it’s an extremely strong glue. A volunteer engineering student weighing 87kg was stuck to a wall with only a postage stamp sized piece of the adhesive holding up the harness from which he was suspended.
With that kind of power, the new adhesive could have an impact on the scientific field as well as in industry. The researchers believe that durable, reversible adhesives like the PHEMA hydrogel have massive potential for household products, robotics systems, and industrial assembly. For example, car assembly uses adhesives, employing a room-sized oven to host the car and cure the adhesives. An adhesive that's strong and reversible could completely change the process of car assembly, and save money because mistakes wouldn't be costly – there would be no wasted parts as there are now if a mistake is made.
However, PHEMA is currently not a fit for most industries because its reversibility is controlled by water. The team is now planning on investigating or engineering adhesives that could respond to things like pH, specific chemicals, light, heat, or electricity, broadening the potential applications of reversible adhesion.