Animal goo inspires better glue
Finding a great glue is a sticky task — especially if you
want it to attach to something as slick as the inside of the human body. Even the strongest human-made adhesives don’t work well on wet surfaces like tissues and organs. For surgeons closing internal incisions, that’s more than an annoyance. The right glue could hold wounds together as effectively as stitches and staples with less damage to the surrounding soft tissue, enabling safer surgical procedures.
A solution might be found under wet leaves on a forest floor, recent research suggests. Jianyu Li of McGill University in Montreal and colleagues have created a surgical glue that mimics the chemical recipe of goopy slime that slugs exude when they’re startled. The adhesive stuck to a pig heart even when the surface was coated in blood, the team reported in the July 28 Science. Using the glue to plug a hole in the pig heart worked so well that the heart still held in liquid after being inflated and deflated tens of thousands of times. Li, who did the research while at Harvard University, and colleagues also tested the glue in live rats with liver lacerations. It stopped the rats’ bleeding, and the animals didn’t appear to suffer any bad reaction from the adhesive.
J. LI, ADAM D. CELIZ, DAVID J. MOONEY
The glue has “excellent, excellent properties,” says Andrew Smith, a biologist at Ithaca College in New York.
And slugs aren’t the only biological inspiration for new adhesives. Clues to better glues have long been hiding out in damp, soggy and downright wet places. For slugs, mussels, marine worms and a cadre of other critters, secreting sticky substances that attach strongly to soaked surfaces is just a fact of life. That’s why scientists are studying the structures of those substances to design new and better surgical adhesives.
“There’s really a big need to develop new ways of sealing tissues, of affixing devices to tissues — in particular, for minimally invasive procedures,” says Jeff Karp, a biomedical engineer at Brigham and Women’s Hospital in Boston. While existing medical-grade superglue is great at sealing up fingertip cuts, it is too toxic to use inside the body. Other alternatives just aren’t sticky enough to fully replace stitches. With a better glue, surgeons could also make snips that are too tiny to be stitched or stapled closed. Smaller incisions speed healing time and decrease risk of complications, Karp says.
Smith says he isn’t surprised that slug slime could lead to a big advance. For several years, he’s been trying to understand how the slug Arion subfuscus builds its ooze. For his research, Smith prods slugs gently with the tip of a metal spatula to startle them, and scoops up the slime as it’s released. “If you get it on your hands, it’ll set within seconds into an extremely sticky material,” he says.
The goo, Smith and others have found, overcomes a major challenge that adhesive designers face. It seems obvious that glue should be sticky. Yet the molecules in glue need to adhere not just to the things you’re trying to stick together, but also to each other. And that stickiness can’t come at the expense of flexibility, especially for medical applications. Soft, squishy organs are going to jiggle; skin is going to stretch. Without some bendiness, the glue might attach securely to each of the surfaces being stuck together, but the glob of glue itself might snap or shear under stress.
Slug defense slime solves that problem with two interwoven networks of molecules, tangled together like strings of holiday lights. One network is rigid, with chemical bonds that break easily, Smith says. The other is deformable, stretching substantially without breaking. This combo makes the goo simultaneously tough, flexible and sticky.
Li’s slug-inspired adhesive takes a similar approach. One layer of the material is a polymer, a type of material made from long molecules built from many repeated subunits, like a string of beads. Positively charged appendages dangling off the polymers are drawn to wet tissue surfaces by the same forces underlying static electricity. This first layer weaves into another layer, a water-based gel. The gel layer acts like a shock absorber in a car, Li says. It soaks up energy that might otherwise dislodge or snap the adhesive.
Despite being 90 percent water, the material is both sticky and tough, Li says. The fact that it’s mostly water makes it more likely to be nontoxic to humans.
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