Two postage stamp-sized patches of a removable, reusable super glue that mirrors the properties of snail slime are strong enough to suspend a 192-pound man from a harness, a new study published in the Proceedings of the National Academy of Sciences reports.
The hydrogel, polymer-based substance—developed by researchers from the University of Pennsylvania, the Korea Institute of Science and Technology, and Lehigh University—circumvents the problem posed by most glues. As Matt Kennedy explains for New Atlas, adhesives tend to be either removable, somewhat reusable and fairly weak, or very strong but completely irreversible. The snail-inspired glue represents the best of both worlds, offering impressive strength and reusability, or reversibility.
According to the Times’ Tom Whipple, snails secrete a sticky mucus when traveling from one spot to another. This slime enables the mollusks to stay firmly attached to surfaces, including rocks, ceilings and walls, as they glide along. It hardens whenever the snails stop for an extended period of time, locking the animals into place with immense force, but easily softens once they are ready to continue their journey. In addition to keeping the snails safely secured to surfaces, this temporary adhesive, known as the epiphragm, retains moisture and prevents their bodies from drying out.
Gizmodo’s Ryan F. Mandelbaum writes that the scientists’ polyhydroxyethylmethacrylate (PHEMA) hydrogel mimics the epiphragm by hardening when dried and softening, conforming to the “microscopic nooks and crannies of a surface,” when rehydrated with water.
“It’s like those childhood toys that you throw on the wall and they stick,” study lead author Shu Yang of the University of Pennsylvania says in a press release. “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.”
Crucially, Yang adds, PHEMA—unlike most materials—doesn’t shrink as it dries. Instead, the glue simply hardens into the cavities found on a surface, staying “conformal” and maintaining its grip.
As Ian Sample notes for the Guardian, the team tested the substance with the help of tiny, PHEMA-coated wafers equipped with channels that allow water in and out. The glue acted as a strong adhesive when air-dried naturally or heated to hasten the process but easily detached with “a well-placed squirt of water.” Impressively, the researchers found that PHEMA was durable enough to attach and detach butterfly wings without inflicting any damage.
According to the press release, University of Pennsylvania graduate student and study co-first author Jason Christopher Jolly volunteered to demonstrate PHEMA’s strength by suspending himself from a harness held up by two tiny adhesive patches. As seen in a video accompanying the paper, the glue easily held the 192-pound Jolly, supporting his full weight for several seconds.
Overall, PHEMA proved to be 89 times stronger than gecko adhesion, a technique inspired by the lizard’s clinging feet and, until now, nature’s dominant model for reversible adhesives. It was also capable of withstanding forces seven times the limit of the strongest Velcro.
Moving forward, the researchers hope to use their creation as a starting point for developing non-water based glues. Although PHEMA could one day be used in industrial assembly, robotics systems and even everyday objects such as reusable envelopes, the fact that its reversibility is controlled by water means it likely won’t be used in heavy manufacturing anytime soon. Still, it’s possible that the snail-inspired glue could pave the way for reversible adhesives that respond to such cues as pH, chemicals, light, heat and electricity.