In search of a better adhesive

Scientists looking to nature for ways to improve glues

September 08, 2006|By CAROLYN Y. JOHNSON | CAROLYN Y. JOHNSON,The Boston Globe

Glue makes modern life possible, quietly holding together electronics, houses, planes, shoes and more with unseen chemical bonds that are taken for granted - until something breaks.

Bolts held in place with epoxy are the likely culprit in the July collapse of ceiling panels in Boston's Big Dig tunnel.

But adhesives can be superior to conventional nuts and bolts in some circumstances.

Scientists and companies are searching for even better glues - substances that will stick under water or grip even the slipperiest surfaces. Some look to the natural world for inspiration, unraveling the secret of the stuff that mussels secrete to stick to rocks, or the tiny hairs on gecko feet that enable the lizards to walk across a ceiling. Others study the mechanics of stickiness itself.

"It's a very active field," said Phillip Messersmith, an associate professor of biomedical engineering at Northwestern University who recently published a paper on mussel adhesion that could lead to new medical glues for surgery. "It sounds like an age-old problem, and it is," but there's still a lot to be learned about how adhesion works, he said. "Adhesion dominates everyday life."

Adhesives are a multibillion-dollar industry, with dozens of kinds of glues and hundreds of different products. There are Krazy Glue and Super Glue, which are "cyanoacrylates" made famous by the commercial showing a worker dangling from his helmet. There are two-part epoxies that must be mixed together and are said to be stronger than cement, and there are futuristic formulations, such as the super-strong "gecko tape" that researchers crafted in 2003, mimicking the pads of gravity-defying gecko toes.

But even as new formulations emerge, the science of adhesion depends largely on tiny forces between individual molecules.

Glues work in a number of ways. One of the primary mechanisms is Van der Waals forces, atomic-scale electrical attractions that occur at short distances. As electrons swarm in clouds around molecules, they create temporary positive and negative poles, which attract the opposite poles of neighboring molecules. Those minuscule attractions add up, and two surfaces stick.

Liquid glue may also seep into tiny pores and crevices on a surface. When the adhesive hardens, the glue that penetrated the tiny pits on the surface anchors to the surface underneath like a key fitting into a lock.

But much of the strength of a bond depends on the chemical makeup of the adhesive. The trick is in the hardening process. As some adhesives harden, the liquid undergoes a chemical reaction - chains of molecules link together, creating three-dimensional spider webs of molecules that can hold together a childhood craft project or repair cracking cement.

What many see as the next frontier in glue might well be very tiny.

"We can make newer and better materials," said David Fowler, an epoxy expert and engineer at the University of Texas. "I suspect from nanotechnology we'll probably come up with a new generation of adhesives."

A.T. Charlie Johnson, a physicist at the University of Pennsylvania, mixed tiny carbon nanotubes - tubes with walls of carbon atoms - into epoxy. The material got somewhat stronger, and its ability to conduct heat changed, he said. Nanotubes are among the strongest materials known.

The idea, he said, "is that when you put the epoxy under some kind of load of some sort, if you can transfer some of that load to the nanotubes, the combined system will be stronger than the original epoxy."

Another leading-edge example of adhesion on a nanoscale comes from Germany, where scientists studying the jumping spider's feet reported two years ago that the microscopic hairs stick because of slight electrical attraction from Van der Waals forces. Those forces enable the insect to walk with ease across a ceiling.

And in a recent paper in the scientific journal Proceedings of the National Academy of Sciences, Messersmith and his Northwestern University colleagues unraveled the molecular basis of the amazing glue that enables mussels to latch on to rocks underwater. Mussels secrete a unique glue that can form strong bonds with a variety of surfaces, even when wet. This property makes the glue desirable to surgeons because it could potentially hold together human tissues.

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