New ‘snail slime’ adhesive could allow for gravity-defying boots

18 Jun 2019178 Views

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Engineers have managed to create a reversible superglue that has accurately recreated the same adhesive found in ‘snail slime’.

Superglue is an amazing product when we want something to stay stuck for a long time. The only problem is that once it’s stuck, it’s stuck for good. But why does it have to be?

In a paper published to the Proceedings of the National Academy of Sciences, engineers from the University of Pennsylvania and the Korea Institute of Science and Technology have revealed a breakthrough adhesive that can be as durable as superglue but also reused and repositioned multiple times. This could allow for a sticky revolution with reusable envelopes, gravity-defying boots and various industry applications.

The new glue uses the same mechanisms seen in something familiar to many garden explorers: the snail. The creature’s familiar ‘slime’ – otherwise known as its epiphragm – is a layer of moisture that can harden to protect itself from dryness. This allows it to cement itself to the side of walls for long periods of time, but can be switched off when it wants to move.

The same researchers have been looking at nature to find potential reversible glue, but the predominant model for reversible adhesives in the natural world, the gecko, just wasn’t up to the task.

“A gecko is 50g, and a human is at least 50kg,” said Shu Yang, who led the study. “If you want to hold a human on a wall, it’s not possible using the same adhesive. You could use a vacuum, but you have to carry a cumbersome vacuum pump.”

The ultimate test

This latest breakthrough came one day when the team was working on another project involving a hydrogel made from a polymer called polyhydroxyethylmethacrylate (PHEMA) and noticed something unusual.

When wet, PHEMA is rubbery, but rigid when dry. This makes it useful for contact lenses but, as the researchers discovered, also adhesives.

“It’s like those childhood toys that you throw on the wall and they stick,” Yang said. “That’s because they’re very soft. Imagine a plastic sheet on a wall – it comes off easily. But squishy things will conform to the cavities.”

This alone doesn’t make it a good adhesive, but what does is the fact that though it becomes rigid when dry, it doesn’t shrink. Instead, the material hardens into the cavities, fastening itself securely to the surface.

Putting the PHEMA hydrogel to the test, they found it was 89 times stronger than gecko adhesion, but easy to break when wet. Incredibly, one of the researchers volunteered to suspend himself from a harness held up only by a postage stamp-sized patch of their adhesive. This alone was able to hold his entire body, making it the strongest known reversible adhesive discovered to date. However, they stressed that any future applications of the glue would have to be water-free.

The researchers hope to eventually find or engineer adhesives that could respond to cues such as pH, specific chemicals, light, heat or electricity, broadening the potential applications of reversible adhesion.

Colm Gorey is a journalist with Siliconrepublic.com

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