Henry Fountain, New York Times 12 Apr 10;
SALT LAKE CITY — Along one wall of 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 a home of sorts, 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.
STICK-TO-IT-IVENESS Clockwise from top left, the sandcastle worm builds its home by using tentacles to grab sand and shell bits and glues them with adhesive from an organ on its head; its tube-shaped dwelling; two beads of a worm’s home, microscopically enlarged; a section of a sandcastle worm colony.
P. californica is a master mason, fashioning its tube, a shelter that it never leaves, from grains of sand and tiny bits of scavenged shell. But it doesn’t slather on the mortar like a bricklayer. Rather, using a specialized 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 Dr. Stewart’s lab, far from their native habitat — is that it does all this underwater.
“Man-made adhesives are very impressive,” said Dr. 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.”
Dr. Stewart is one of a handful of researchers around the country who are developing adhesives that work in wet conditions, with worms, mussels, barnacles and other marine creatures as their guide. While there are many possible applications — the Navy, for one, has a natural interest in the research, and finances some of it — the biggest goal is to make glues for use in the ultimate wet environment: the human body.
It is too early to declare the researchers’ work a success, but they are testing adhesives on animal bones and other tissues and are optimistic that their approaches will work. “I would have moved on to something else if I didn’t think so,” said Phillip B. Messersmith, a Northwestern University professor who is developing adhesives based on those made by mussels and is testing whether they can be used to repair tears in amniotic sacs, among other applications.
While some skin sealants — mostly of the cyanoacrylate, or superglue, variety — are on the market, their effectiveness is limited. They often cannot be used, for example, on incisions where the skin is pulled or stretched, or must be used in tandem with sutures or staples. 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 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. But researchers should view nature’s approach as a general guide, he said, rather than a precise pathway.
“In my view of bioinspired research or materials, I almost always don’t think it’s safe to be slavishly wed to the specific chemistry,” Dr. Waite said, “but rather to distill the important concepts that can then be mimicked.”
So the goal of these researchers is not to duplicate natural adhesives that work well underwater, but to imitate them and make glues that are even better suited for humans. “We want to take elements of the structural adhesives that chemists have made and combine them with the unique elements that nature has used,” Dr. Stewart said.
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 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.
But there are several hurdles to making glues that work underwater, Dr. 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, Dr. Messersmith said, as with any glue, “adhesion is a complicated thing, even when it appears very simple.”
“There are events going on at the interface of adhesive and surface, and there’s the strength of the adhesive itself,” he said. “If you have one but not the other, you’re nowhere, really, because somewhere you’ll have a weak point in the system and it will break.”
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,” Dr. 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, Dr. Stewart said. Over the next six hours, the adhesive hardens completely as cross-links form between the proteins. “It turns into this thing that has the consistency of shoe leather,” he said. “It’s still flexible but very tough.”
Like other researchers, Dr. Stewart decided to use synthetic polymers as the backbone for his adhesive, and to ignore many other aspects of the worm’s chemistry. “Who says the exact amino acids are important?” he said, citing one example. “That’s just something the worm is stuck with.
“On the other hand, if we just decide maybe the real important part is the side chains, that’s very simple to copy with a synthetic polymer.”
Dr. Stewart’s 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. “Perfect for a water-borne underwater adhesive,” he said. But unlike the worm, he can tweak the chemistry to make it cure faster or slower depending on the application.
Dr. 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.
Dr. Stewart has worked with sandcastle worms since 2004, and recently began studying another group of tube-building creatures, caddisfly larvae. Fly fishermen are familiar with these organisms, which inhabit the bottom of freshwater streams until the flies hatch.
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. “Except it’s a sticky, underwater silk,” Dr. Stewart said.
He is just beginning to characterize the silk and understand how the caddisflies produce it, but the eventual goal is the same as with the sandcastle worm.
“We want to try to mimic it someday soon, and spin fibers underwater,” he said. “Waterborne polymers underwater, which might have some medical application.”
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,” Dr. Messersmith said, “but then we confront the biological problems.”
There are medical superglues that do form strong bonds, he said, “but those materials are highly immunogenic.”
Dr. Stewart said that so far he has seen little inflammation in the rat studies, and little if any evidence of toxicity or inhibition of bone healing.
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. So in degradable versions of his synthetic polymer glues, Dr. Stewart actually adds back proteins that can be attacked and broken down by specialized cells.
“You wouldn’t want some plastic glue in your bones for the rest of your life,” he said.
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