Nanoglue Sticks Underwater

Geckos and mussels inspire a better glue.

Bandages might stay put even after a swim, thanks to a new adhesive developed by researchers at Northwestern University. The glue not only works well on wet surfaces, but it can also be pulled off and reused more than a thousand times.

Underwater Glue: Researchers, inspired by the sticking skills of the gecko and the mussel, have made a new type of glue that remains sticky on wet surfaces. Credit: H. Lee, W. Lim, and A. J. Kane

The nanoglue is made of 400-nanometer-wide silicone pillars covered with a polymer that mimics the adhesive proteins found in mussels. In addition to bandages, the new material could be used in drug-delivery patches and in adhesive tapes to close surgical wounds, says Phillip Messersmith, a biomedical-engineering professor at Northwestern University, who reported the glue in Nature this week.

Many researchers are working on glues that mimic the tiny hairlike structures on geckos' feet, which give the lizard the ability to run up walls and across ceilings, and even hang by one toe. Carbon-nanotube pillars have led to one of the strongest "gecko tapes" yet, but the adhesives, just like a gecko's feet, lose their grip on wet surfaces. (See "Climbing Walls with Carbon Nanotubes.") Ali Dhinojwala, a professor of polymer science at the University of Akron, who developed the gecko tape made of carbon nanotubes, says that the pillars, which are thousands of nanometers tall, simply "collapse in water because of pressure."

The new adhesive is based on a similar pillar design, but it works better underwater for two reasons. Messersmith and his colleagues make the pillars shorter so that they don't collapse. And they coat the nanopillars with a thin layer of a polymer that imitates a mussel's extremely strong adhesive protein. The result is a glue that stays attached to wet surfaces just as well as a gecko or a sticky note sticks to dry surfaces.

Right now, the material covers up to two square millimeters. The biggest challenge to making the adhesive practical will be creating larger swaths. "To make this a viable adhesive, you have to be able to make square yards, not just a few millimeters," Messersmith says.

On larger areas, it becomes more difficult to get every pillar to stick to a surface, according to Metin Sitti, a mechanical-engineering professor at Carnegie Mellon University who is working on similar adhesives. The short pillars in the new material make the problem especially hard. "If the surface is rougher than the pillar height, then most of them will be in the air, so you need to press down a lot," Sitti says.

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Unlike previous plastic and carbon-nanotube pillar designs, the new material doesn't depend on physical van der Waals forces. Instead, it relies on the chemical interaction of the surface with the chemical hydroxy groups in the synthetic mussel protein. Because of this, Dhinojwala says that the glue might not attach to every type of surface.

But Messersmith believes that the glue will prove to be versatile. "These functional [hydroxy] groups have the ability to adhere to a variety of surfaces," he says. So far, the researchers have tested the glue on silicon nitride, titanium oxide, and gold, all of which are used in electronics. But if the glue is to be used in bandages and medical tape, it would need to stick to skin. The researchers have tested other mussel-inspired synthetic proteins that have similar chemical groups and have found that they are adhesive to biological tissue. "Mussels can stick to anything," Messersmith says. "They adhere to a piece of wood, which is organic. They also adhere to the skin of whales."