Cross a gecko with a mussel and what comes out is a new type of adhesive tape that can repeatedly stick and restick, even underwater.
Geckos and mussels both have astounding abilities to stick. The feet of many gecko species are padded with millions of tiny hairs that enable them to ascend smooth vertical walls. Mussels glue themselves to wet rocks, unmoved by crashing surf.
Researchers at Northwestern University, led by Phillip B. Messersmith, a professor of biomedical engineering, have made a small piece of adhesive, about 2 square millimeters, that mimics the structure of the gecko foot and then coated it with a polymer inspired by mussel glue.
While other researchers have made gecko-inspired adhesives, this is the first that adheres well when wet, which could make it useful for bandages and underwater robots.
"Under wet conditions, our adhesive is equally as adhesive as a gecko foot is under dry conditions," Messersmith said.
The researchers describe their invention, which they have named "geckel," in the current issue of the journal Nature.
"I think it's very original," said Ralph Spolenak, a professor of nanometallurgy at the Swiss Federal Institute of Technology who was not involved in the research. "It combines two completely different concepts and creates something new out of it."
Gecko feet & Mussel protein
Gecko feet employ a weak attraction known as the van der Waals force to hold on. Multiplied across millions of hairs, that is enough to hold up the gecko. Even more remarkable, when the gecko lifts its foot in a certain direction, the hairs effortlessly detach. And the gecko foot is self-cleaning. It does not pick up dirt and dust as the gecko walks along.
Mussels, meanwhile, excrete a protein that can displace water molecules and very strongly bind several oxygen atoms in the protein to an atom in a metal or mineral.
Mimicking the gecko foot hairs, the adhesive consists of tiny silicone pillars that were coated with the mussel-inspired polymer. Because the researchers wanted the adhesive to be reusable, they lowered the number of metal-oxygen bonds.