Study of clinging ways may lead to new glues for marine, medical uses

05:54 PM CST on Sunday, January 25, 2004

By SUSAN GAIDOS / Special Contributor to The Dallas Morning News

While scuba diving off the Atlantic coast one blustery day, Jonathan Wilker spotted a large colony of mussels clinging to a canyon wall.

"I was being buffeted around by the surf and couldn't stay in one place," he says, "but the mussels were anchored firmly to the rock." The image stuck in his mind. As a chemistry graduate student at the Massachusetts Institute of Technology, he was curious about how the shellfish stay fixed through pounding waves and changing tides. He began investigating the glues that bind mussels to rocks, docks and other surfaces.

Now, years later, Dr. Wilker, a researcher at Purdue University, may have helped answer his own questions. This month, he published findings that show iron filtered from ocean water could be the key curing agent that transforms mussel proteins into plaster. The research was published in the international chemical journal Angewandte Chemie.

The discovery may be used to create rustproof coatings for cars and bridges, and develop anti-fouling finishes to keep barnacles, algae – and mussels – off ships and oil rigs.

What's more, mussel glues can adhere in water, making them ideal candidates for use in biomedical applications. Medical researchers have worked years to develop nontoxic adhesives that can create strong bonds in a wet environment. Such glues could be used to suture skin, set broken bones, and repair or deliver therapies to the eye.

Mussels are known for their ability to glom onto just about any type of surface – rock, wood, metal, even Teflon.

To attach to a surface, the mussel secretes sticky threads that are soft and rubbery near its body and stiffen to a nylonlike consistency near the surface. The proteins used to make these glues have both adhesive and elastic qualities, giving the glue a combination of strength, stretch and stickum that is hard to emulate in man-made glues.

Researchers have long known the threads that tether mussels to surfaces contain high concentrations of iron and other metals. Numerous studies have characterized the various proteins and metals contained in the glue. Still, getting a grip on the mussel's curing process has not been easy.

Curing, or cross-linking, occurs when bonds are created between the proteins that make up the mussel's glue. The new findings suggest that adding iron to two key mussel proteins can, within minutes, create the cross-links needed to transform them into cement.

Although metals are commonly used in biological functions, Dr. Wilker says this is the first time a metal such as iron has been deemed essential to forming a biological material.

"One question we have is, 'Could this be prevalent in forming natural adhesives or natural materials, for that matter, throughout nature?' " he says.

The iron used in making these glues comes from open-ocean water. Mussels are filter feeders, pumping water through tiny gills to sift out nutrients in plankton and other material. This filtering mechanism also allows the shellfish to accumulate metals from the seawater. Studies show that mussel concentrations of metals such as iron, zinc and copper can be up to 100,000 times higher than those found in seawater.

Dr. Wilker says the high amounts of metals, particularly iron, led him to wonder whether there was a functional significance or the concentrations were just a byproduct of metabolism.

In a series of studies, he examined how metals bind to mussel proteins called Mefp-1 and Mefp-2. He extracted the proteins from mussels and combined them with iron, zinc or copper to analyze the adhesive properties of each mixture. He found that iron worked best in making cross-links from the extracted material.

"We find when we do our studies with the other metals, we don't get the same bonding to occur. So we think it's only iron that's actually bringing about the chemistry," he says.

He also compared the cross-link in the iron-protein combination to the natural glue, using samples of already cured glue collected from mussels in his laboratory, and to synthetic models using laboratory-made proteins. The findings revealed the same cross-link action in all three systems, Dr. Wilker says.

Not all scientists are immediately accepting Dr. Wilker's findings. Herbert Waite, a marine biochemist at the University of California, Santa Barbara, who has studied the chemistry of mussel glue for more than 20 years, says he reported on the iron-binding effects of these proteins in papers published as early as 1994.

His studies showed the iron-bound proteins act as a coating rather than an adhesive.

"It can function like a glue when it's used in vitro, that is, under experimental conditions. But its location in the byssal [anchoring] thread definitely suggests a coating function," he says. "So it's not at the interface, it's on the surface of the thread. They result in a stabilization, like a varnish or lacquer, on the outside of the thread in the plaque."

Dr. Waite's group is now conducting studies to measure how iron changes the chemical and physical properties of the coating as it is added to the system.

Dr. Waite's efforts to decipher the chemistry of blue mussel glue have inspired ongoing interest in developing these adhesives for applications. In 1981, his group made the surprising discovery that dopa, a brain chemical that's the basis for Parkinson's treatments, is the necessary component that allows the animals to adhere to rock and mineralized surfaces.

His studies have focused on the blue mussel Mytilus edulis because of their ability to stockpile large quantities of adhesive molecules in their tissue. These molecules are stored in the mussel's "foot" and released through tiny grooves there.

Overall, the mechanics of mussel glue are akin to those of the two-part epoxy glues available in hardware stores, Dr. Waite says. Glue glands in the mussel's foot produce resinlike proteins in one compartment and hardeners, or curing agents, in another. When released, the agents mix and cure.

Curing is crucial in the development of a cohesive strength. Too much of it can produce an adhesive as brittle as glass, creating weak points that can break. Too little can cause bonded surfaces to slip and slide, like glass plates held together by molasses.

If iron proves to be a key curing agent in mussel glue, researchers may find it easier and cheaper to use the proteins in applications. That's because using metals to cure proteins would offer certain advantages that traditional cross-linking agents, or enzymes, do not, Dr. Waite says.

Namely, it would allow researchers to fine-tune the degree of hardening in a glue, making it possible to match the stiffness of the glue to the stiffness of the tissue being repaired. Such a system could be used to create hard materials to bond bone to bone and more flexible adhesives to suture skin, for example.

Dr. Wilker and his group at Purdue are now working on ways to use these iron complexes to create skin adhesives to close wounds from surgery or some traumatic event.

Other laboratories are working to develop mussellike glues that behave like the real thing, based on research by Dr. Waite and others. For example, Phillip Messersmith, a biomedical engineer at Northwestern University, is attaching dopa to laboratory-made polymers to create gels that fuse firmly to a wide range of surfaces, wet or dry. If the gels prove nontoxic and prompt no immune response, they could be inserted into the body, making them ideal candidates for use in surgery or drug delivery, Dr. Messersmith says.

"We also have aspirations to develop a similar approach that could be used to glue together pieces of mineralized tissue, such as bone or teeth," he says, noting that the mineralized tissues have compositions similar to the rocks and minerals found in nature to which mussels adhere.

Dr. Messersmith also is exploiting the adhesive properties of mussel glue to anchor a nonstick polymer onto a surface, creating a kind of anti-glue. These nonadhesive surfaces could be applied as a coating for metal implants such as stents, heart valves or other devices to prevent blood cells and proteins from accumulating.

In a recent laboratory experiment, Dr. Messersmith's group exposed surfaces treated with the nonstick agent to a fluid containing human blood proteins. The study resulted in low levels of accumulation of cells and proteins. On surfaces without the treatment, the blood proteins accumulated at rates up to 150 times higher, levels high enough to lead to blood clots. He plans to test the device in animals later this year.

Although similar coatings are now used on stents, Dr. Messersmith says using mussellike adhesives to develop such coatings would allow medical researchers to apply these treatments to all types of surfaces.

"What we're trying to take advantage of is the known ability of those mussel adhesive proteins to adhere to virtually all surfaces. Having a single polymer to treat a variety of surfaces could be very useful commercially," he says.

Susan Gaidos is a free-lance writer in Maine.

The mussel foot creates the sticky threads and plaque that keep the shellfish anchored to a surface. The foot inspects the environment for areas with just the right water flow and for slime or debris. Once in a suitable home, the foot goes into a state of paralysis, and a new thread is molded and assembled. When the thread and plaque are fully assembled the foot retracts and moves on to make another thread.