It was no small challenge facing a team of scientists at Johannes Kepler University Linz:
Could they make superglue even more super?
The researchers had been wrestling with a particularly thorny problem: When it came to bonding materials to hydrogels—soft, squishy objects composed of polymers suspended in water—no adhesive was very effective. If the hydrogel was stretched, the bond became brittle and pulled apart. (Imagine trying to glue two Jell-O cubes together.) It was a dilemma in the burgeoning fields of “soft” electronics and robotics that rely on hydrogels.
While they’ve been used for many years to dress wounds or in soft contact lenses, hydrogels have more recently become a key component of quite a few innovative products, ranging from electronic “Band-Aids” that can deliver medicine, to stretchable electronics to tiny, jelly-like robots that can be implanted inside a person’s body.
Scientists can attach hydrogels to other objects with an ultraviolet light treatment, but the process can take as long as an hour. That’s just not very efficient, says Martin Kaltenbrunner, one of the Austrian researchers.
“This bridging the gap between soft and hard materials really is a big challenge for everyone in the field,” he said. “We really were looking for a rapid prototyping, a do-it-at home method of bonding hydrogels to diverse materials that is fast and universal. What was out there was a little too impractical to implement in our labs and use on a day-to-day basis.”
The team thought a lot about what might work. Someone suggested superglue. Why not, since hydrogels are mainly water, and superglue bonds things together because water triggers the reaction.
But it wasn’t that easy. When Kaltenbrunner and the other researchers tried using off-the-shelf superglue, it didn’t work very well. Once it dried, and the hydrogel was stretched, the bond again cracked and failed.
Then, someone came up with the idea to add a nonsolvent, which would not dissolve into the glue and would keep it from hardening. That could help the adhesive actually disperse down into the hydrogel.
And that, it turned out, was the answer.
Mixing the cyanoacrylates—the chemicals in superglue—with a nonsolvent kept the adhesive from dissolving, and when materials were pressed together, the glue diffused into the outer layers of the hydrogel. “Water triggers the polymerization of the cyanoacrylates,” Kaltenbrunner explained, “and it gets entangled with the polymer chains of the gel, which leads to a very tough bond.” In other words, the glue was able to seep down below the surface of the hydrogel and connect with its molecules, forming a strong attachment within a few seconds.
It was clear the researchers were on to something when they bonded a hydrogel piece to an elastic, rubbery material called an elastomer. “The first thing we recognized,” Kaltenbrunner said, “is that the bond was still transparent and stretchable. We really tried a lot of other methods before, but turns out sometimes the simplest is the best.”
Here's their how-to video on hydrogel gluing:
The scientists put their new adhesive to the test by creating a strip of “electronic skin,” a hydrogel band onto which they glued a battery, a processor, and temperature sensors. It could provide data to a smartphone through a wireless connection.
They also produced a prototype of artificial vertebrae with which hydrogel was used to repair deteriorating discs in the spine. With the glue, the vertebrae could be assembled much more quickly than normal, according to a report on the research, recently published in Science Advances.
Kaltenbrunner said he sees much potential for the adhesive as part of the “soft robotics revolution.” It could, for instance, be incorporated into upgrades to the “octobot,” the first autonomous, entirely soft robot unveiled by Harvard scientists last year. About the size of your hand, the octobot has no hard electronic components—no batteries or computer chips. Instead, hydrogen peroxide interacts with flecks of platinum inside the robot, which produces gas that inflates and flexes the octobot’s tentacles, propelling it through water.
For now, that movement is largely uncontrolled, but scientists hope to be able to add sensors that would allow it to maneuver toward or away from an object. That’s where the new adhesive could come in handy.
But the future of the new type of superglue is still taking shape. Kaltenbrunner estimates that it could be another three to five years before it’s available in the marketplace. Still, he’s feeling pretty optimistic.
“Since our method is easy to reproduce,” he said, “we hope others will join us in finding even more applications.”