Snail-like reversible adhesive strong enough to hold a human
By Anthony Caggiano
A polymer-based product can be hard when dried out, yet soft again when rewetted. Image credit: Max Pixel.
Scientists have accidentally discovered a hydrogel that can stick to objects and release itself, quite like a common mollusc.
Researchers at Penn Engineers, USA, were working on another project that involved a hydrogel made of a polymer called polyhydroxyethylmethacrylate (PHEMA) and noticed its unusual adhesive properties.
PHEMA is rubbery when wet but rigid when dry, a quality that makes it useful for contact lenses but also, adhesives, as discovered by the team under the firm’s Department of Materials Science and Engineering professor, Shu Yang.
When PHEMA is wet, it conforms to all of the small grooves on a surface, from a tree trunk's distinct ridges to the invisible microporosity of a seemingly smooth wall. This conformal contact is what allows PHEMA to stick to a surface.
As PHEMA dries, it becomes as rigid as a plastic bottle cap, but, uniquely, it doesn't shrink. Instead, the material hardens into the cavities, fastening itself securely to the surface.
These properties that helped Yang's team identify PHEMA as a unique candidate for reversible, strong adhesion are the same properties found in a snail's epiphragm. 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.
That reversibility between wet flexibility and dry adhesion was tested by the group. The team ran several tests on their PHEMA hydrogel, evaluating its ability to hold weight and the time it takes for water to infiltrate the adhesive and reverse its grip. They found that PHEMA acted remarkably similar to the snail epiphragm. It was 89 times stronger than gecko adhesion, but its hold was easily broken when it got wet. A postage-size amount of the material was able to support an adult suspended from a harness.
‘When it's conformal and rigid, it's like super glue. You can't pull it off. But, magically, you can rewet it, and it slips off effortlessly,’ Yang said.
‘Additionally, PHEMA doesn't lose its strong adhesion when scaled up. Usually, there's a negative correlation between adhesion strength and size. Since PHEMA is not dependent on a fragile structure, it doesn't have that problem.’
Yang sees durable, reversible adhesives like her PHEMA hydrogel as having massive potential for household products, robotics systems, and industrial assembly. Despite its promise in applications like heavy manufacturing, PHEMA is not a fit for most industries because its reversibility is controlled by water.
"With a lot of things, you don't want to use water. Water takes time to diffuse. In the future, we want to find the right material that can switch the property like that," says Yang.
The researchers hope to eventually find or engineer adhesives that could respond to cues like pH, specific chemicals, light, heat, or electricity, broadening the potential applications of reversible adhesion.
The research was supported by the National Science Foundation through grants DMR-1410253 and EFRI-1331583, and the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-FG02-07ER46463.
The study was published in Proceedings of the National Academy of Sciences.
