Nature's super glue

Along one wall of Dr Russell J Stewart’s laboratory at the University of Utah sits a saltwater tank containing a strange object: a rock-hard lump the size of a soccer ball, riddled with hundreds of small holes.

It has the look of something that fell from outer space, but its origins are earthly, the intertidal waters of the California coast. It’s occupied by a colony of Phragmatopoma californica, otherwise known as the sandcastle worm. Actually, it’s more of a condominium complex. Each hole is the entrance to a separate tube, built one upon another by worm after worm.

P. californica is a master mason, fashioning its tube, a shelter that it never leaves, from grains of sand and bits of scavenged shell. But it doesn’t slather on the mortar like a bricklayer. Rather, using a specialised organ on its head, it produces a microscopic dab or two of glue that it places, just so, on the existing structure. Then it wiggles a new grain into place and lets it set.

What is most remarkable, and the reason these worms are in Stewart’s lab, far from their native habitat, is that it does all this underwater. “Man-made adhesives are very impressive,” said Stewart, an associate professor of bioengineering at the university. “You can glue airplanes together with them. But this animal has been gluing things together underwater for several hundred million years, which we still can’t do.”

Glues in wet conditions

Stewart is one of a handful of researchers developing adhesives that work in wet conditions, with worms, mussels, barnacles and other marine creatures as their guide. It is too early to declare the researchers’ work a success, but they are testing adhesives on animal bones and tissues and are optimistic that their approaches will work. “I would have moved on to something else if I didn’t think so,” said Dr Phillip B Messersmith, a Northwestern University professor, developing adhesives based on those made by mussels.

Adhesives strong enough to hold skin together under tension, or repair bone or other internal tissues, without inviting attack by the body’s immune system, have eluded researchers. Nature shows how it can be done, said Dr J Herbert Waite, a professor at the University of California, Santa Barbara, who did much of the early work of identifying the adhesives that mussels use to stick to rocks and other surfaces. The goal of these researchers is not to duplicate natural adhesives that work well underwater, but to imitate them and make glues even better suited for humans.

Synthetic adhesives might not only work better, but they should also be able to be produced in large quantities. Marine organisms make their glues in very small amounts, the typical dollop from a sandcastle worm, for example, is on the order of 100 picoliters. Even if it could somehow be collected before it set, it would take roughly 50 million dollops to make a teaspoon.

“At the end of the day, the single biggest reason to do this is you can get more stuff,” said Dr Jonathan Wilker, an associate professor of inorganic chemistry at Purdue University who works on analogues of mussel adhesives and studies oysters, barnacles and other organisms as well.

Several hurdles

But there are several hurdles to making glues that work underwater, Wilker said. “One is that whenever the surface is really wet, you’re going to be bonding to the surface layer of water, rather than the surface itself. So it’s going to lift off.”

Another is that in order to cure, glues need a little water or none at all, they need to dry out. Most will not cure underwater, but those that do tend to set as soon as they are out of the container, overwhelmed by all the water. Beyond that, Messersmith said, as with any glue, “adhesion is a complicated thing, even when it appears very simple.”

Worm has adhesion promoters

The sandcastle worm resolves the underwater issues neatly. The proteins that are the basis of its adhesive contain phosphate and amine groups, molecular fragments that are well-known adhesion promoters. “Those side chains are probably what helps it wet the surface in the first place,” Stewart said. The worm produces the glue in two parts, with different proteins and side groups in each.

The two are made separately in a gland, and, like an epoxy, come together only as they are secreted. When they mix they form a compound that, even though water based, does not dissolve. The glue sets initially in about 30 seconds, probably triggered by the abrupt change in acidity, it is far more acidic than seawater, Stewart said. Over the next six hours, the adhesive hardens as cross-links form between the proteins. Stewart decided to use synthetic polymers for his adhesive. His adhesive forms what chemists call a complex coacervate, a kind of molecular circling of the wagons against water. So it’s an injectable, immiscible liquid.

Strong enough to repair fractures

Stewart says the glue appears to be strong enough to repair fractures in craniofacial bones, an application he is studying with rats. He also thinks it may be useful for repairing corneal incisions, and for setting other bone fractures more precisely, by anchoring small pieces that cannot be secured with pins or screws. “But we don’t have any fantasies about gluing femurs back together,” he said.

Stewart has worked with sandcastle worms since 2004, and recently began studying another group of tube-building creatures, caddisfly larvae.  Caddisflies build their tubes in the same way as P. californica, but with a much different glue, strands of silk that attach to the bits of sand, tying them all together. At some evolutionary point tens of millions of years ago the flies were related to silkworms, so the fact that they spin silk is not too surprising.

A big concern with any synthetic glue, no matter how closely it mimics one from a living creature, is biocompatibility. “We might be able to solve the adhesion problems,” Messersmith said, “but then we confront the biological problems.” But he noted that since one goal would be to have the glue eventually degrade, some response by the body would seem to be necessary. With a bone glue, for example, “you want it to degrade roughly at the same rate as the bone regrows,” he said.

Bioengineering

* The sandcastle worm’s colony is a condominium complex, with separate tubes built one upon another by worm after worm.

* Marine organisms make their glues in small amounts, the typical dollop from a sandcastle worm is on the order of 100 picoliters.

* The proteins that are the basis of its adhesive contain phosphate and amine groups, molecular fragments that are well-known adhesion promoters.

New York Times News Service