Superglue needs clean, dry surfaces to work. Yet mussels can hold fast to wet rocks treacherously slicked with sea spray. University of California researchers have uncovered their secret – and the discovery, published in Science in August, may lead to glues that mend broken bones or repair gashes in boats at sea.
“We’ve learned how the animals can achieve a trick people can’t do yet,” says Purdue University chemist Jonathan Wilker, who also works on bioadhesives.
Rocks drenched in seawater provide a particularly tricky challenge for adhesives. Rocks have a slight negative charge – and that attracts a coating of positive ions from the seawater, further blocking a glue’s bonding ability.
In some ways the inside of your body also resembles a beach. “Physiological fluids are very salty,” says Michael Rapp, co-lead author of the study, “and bones are minerals just like the rocks you find in the ocean”. Should you get a nasty cut or badly break a bone, surgeons use threads, staples and screws to cobble you back together. But some stitches have to be removed and screwing bones together requires drilling them first. The procedures inflict a little more damage and potentially leave the injury open to infection. So a glue that could be applied to ripped tissues or cracked bones would be a boon.
Enter the humble mussel. Mussels have evolved sticky feet with special protein soles that cement them to a surface. Scientists have extracted these mussel proteins, but they’re not great candidates for glues – you need to milk around 10,000 mussels to extract a gram of adhesive protein.
So lead author Greg Maier and his colleagues sought out a related glue-like molecule from bacteria. Called cyclic trichrysobactin, bacteria use it like a sticky glove to grab iron nutrients from the plants they feed upon. Its structure is similar to the mussel protein but easier to cook up in the lab.
When the team spread cyclic trichrysobactin on the surfaces of two submerged silicate mineral plates and brought the two surfaces within a few atoms’ distance of each other, the plates stuck.
By adding and removing different chemical groups on the cyclic trichrysobactin molecule, the researchers discovered the secret of the bacteria’s (and the mussel’s) sticky grip – pairs of chemical fingers that work together to grab on to rock. One finger, a lysine amino acid, sweeps away the surface’s ionic layer like a broom. With the surface clear, another finger – catechol – is free to form strong hydrogen bonds with the minerals’ surface.
With the mussel’s trick figured out, the team could modify the chemical groups hanging off the cyclic trichrysobactin to make a synthetic version of the natural form. The adhesive they made was strong enough to suspend a 100-kilogram weight from a patch of glue the size of a 10-cent coin.
The team has plans to make the glue even stronger. There’s a big market for underwater adhesives, Wilker says – to repair bridges, oil rigs and ships. “If you’ve got a big ship with a hole, dry-docking it is expensive and time-consuming – you want to be able to just patch it up while it’s still in the water.”