A Natural Attraction

A mussel attached to a sheet of Teflon. Image courtesy of Haeshin Lee and Phillip Messersmith, Northwestern Univers

Friday, October 19th, 2007--

Scientists looking to make better adhesives are studying sticky or sticking organisms for clues. Ron Fearing, a professor in the department of electrical engineering and computer sciences at University of California, Berkeley, studies gecko feet, for example.

Geckos can stick, but they aren't exactly sticky. "Gecko toes are covered with tens of thousands of these really fine hairs," Fearing says. The hairs are about 1/20th the diameter of human hair and split at the tips into even finer hairs. But they aren't sticky. Fearing says, "There was this conundrum--how can the gecko climb up the wall when the hairs aren't sticky?"

Fearing and colleagues discovered that when the hairs brush against a surface in a certain direction, they form strong chemical bonds that allow the animal to stick, Fearing says. This directional stickiness allows the gecko to stick and unstick itself when it takes a step. Fearing is trying to replicate this adhesion for robots.

Mussels are also known for their superior sticking ability. Mussels excrete a liquid that will attach to anything, says Phillip Messersmith, a biomedical engineer at Northwestern University. Messersmith developed an adhesive that combines gecko and mussel properties. By coating gecko-like hairs with a synthetic sticky mussel-juice, Messersmith and colleagues created a strong new adhesive, dubbed "geckel."

Messersmith is also working on other mussel-inspired coatings. Coatings that coat everything are rare: "Very few coating strategies work well on all types of materials," says Messersmith.

The proteins in the mussel adhesive are primarily made of the amino acids DOPA and lysine, says Messersmith. DOPA can form strong bonds, even in water, which is unusual. Dopamine, a chemical that helps neurons communicate in the brain, happens to have similar chemical properties to lysine and DOPA.

Messersmith and colleagues dissolved dopamine in alkaline water (with a pH similar to sea water). "What we found was that the dopamine spontaneously undergoes chemical changes that result in polymer film formation on a surface you immerse in that solution. You could take anything really--a pen, or a piece of paper," Messersmith says. This initial coating can serve as the base for more coatings of different substances, according to Messersmith.

The coating dopamine produced stuck to 25 different materials. The researchers couldn't find a material it didn't stick to. Messersmith looked to the mussel for a recipe--he and his colleagues reported the results in the journal Science this week.

--Flora Lichtman