The sticking power of this adhesive is “probably on the order of 10 times better than what’s currently on the market,” said Phillip Messersmith, a professor of bioengineering at the University of California at Berkeley, who was not involved with a study of the substance that was published Thursday in the journal Science.
In addition to gluing things together, the adhesive could be used to deliver slow-release medications. Most intriguingly, said Nikolay Vasilyev of Boston Children’s Hospital, one of the study authors, the flexibility of the adhesive means it could potentially be used in the hearts of growing children with cardiovascular disease.
In experiments with pig organs, the material stuck strongly to skin, cartilage, arteries, livers and hearts, even when they were wet with blood. In one experiment, the sticky matrix was used to patch a hole in a pig heart. When the heart was inflated to simulate beating, the matrix stretched along with the heart and the hole stayed plugged even after more than 10,000 inflations.
In living rats, the adhesive sealed a liver wound and produced less scar tissue than a clamp-like tool that is commonly used to close blood vessels during surgery.
“This is really what we dreamed of,” said Andy Smith, who was not involved with the research but studies slug slime for its adhesive properties at Ithaca College.
The particular species of bright orange slug that inspired this work oozes sticky goo off its back to defend itself from predators. “Anything that tried to eat this slug would get a mouthful of glue,” Smith said. The slugs are smaller than your pinkie, but can churn out 5 percent of their body weight in slime. Seconds after touching one of these slugs, the sticky slime hardens into a rubber cement that will stretch between your fingers and is tough to remove.
Smith says positively charged protein chains may be involved in the adhesion of snail slime, and in the synthetic adhesive. It contains proteins similar to those from snails as well as positively charged calcium ions. The positive charges in the glue are attracted to the surface of biological tissues.
“A lot of surfaces in the environment have negative charges,” said Smith, partly because everything is coated in negatively charged bacterial biofilms.
After the initial electrostatic attraction, stronger interactions take over. These include electron-sharing covalent bonds like those between the hydrogen and oxygen atoms in a molecule of water and “physical entanglements” between the chains of molecules in the matrix and the spongy tissue surface, like a chemical hook-and-loop fastener. This binding is quick, but not immediate, so the new glue will be much easier for doctors to work with than existing tissue adhesives, which stick almost instantly.
The main ingredient in the adhesive is water, so it can be injected, potentially replacing invasive surgery in some cases. The matrix is a hydrogel, a stretchy web of starchy chains of molecules that is 90 percent water. For people with heart disease, Vasilyev said the adhesive could mean more time between heart surgeries. Because the material is stretchy, it will move with a bending joint or expand with a growing heart, meaning it can also be left behind long term as a prosthetic or to attach devices like pacemakers, even in children.
Jianyu Li, a postdoctoral researcher at Harvard and first author of the study, hopes the team might be able to design adhesives that dissolve over time as a complete alternative to stitches.
Xuanhe Zhao, who studies hydrogels at MIT but was not involved with the study, called their approach “very innovative.” These “tough hydrogels” have been tested for various applications from robotics to condoms, but thanks to slugs, this is the first time hydrogels have been adapted to bind tissue in a biocompatible way.