Researchers hope to take a lesson from mussels

The bandages of the future may come from oceanic tidal zones, where creatures who want to stay in one place have developed sophisticated ways of sticking to things. Frustrated by the inadequacies of human-engineered medical adhesives, researchers hope to take a lesson from mussels, barnacles, tubeworms and other animals that can resist the ocean's buffeting currents. "The interface between ocean and land has been an important zone in evolutionary history," said University of Utah biochemist Russell Stewart. "Marine organisms exploit multiple bonding mechanisms. By using multiple chemical bonds, they're able to bond to multiple substrates" — a fancy way of saying they can stick to anything. Chemists recently made prototype bandages with an inkjet printer filled with adhesive proteins taken from mussels, whose remarkable "feet" — a tangle of fibers that anchor them to rocks — have made them the most widely-studied specialist in marine clinging. Mussels can also attach themselves to wood, iron, steel, each other, and even Teflon. The shortcomings of modern medical adhesives are manifold. As anyone who's ever put a Band-Aid on an elbow knows, off-the-shelf medical glues aren't suitable for moving joints. Sutures — which can be thought of as a form of mechanical adhesion — can leave scars and leave bodies open to infection. Sealants made from blood-coagulating compounds are promising, but still prone to contamination. And surgical-grade glues are essentially Krazy Glue with different brand names. As the instructions on Krazy Glue packets make clear, it's a toxic substance not meant to be put inside a body, even if it could seal a tissue under repair by a surgeon — which, often, it can't. The inside of a body, however, poses many of the same challenges as an intertidal zone. Marine glues need to stick to wet surfaces. They do so by employing a variety of chemical bonds to displace the water, right down to the last  molecule. Then they need to keep their glue from dissolving in water. "There are chemical changes and cellular changes within the body, and all sorts of causes" that can dissolve a medical adhesive, said University of North Carolina bioengineer Roger Narayan, coauthor of the inkjet adhesive study in the Journal of Biomedical Materials Research Tuesday. Earlier research by co-author Jonathan Wilker, a chemist at Purdue University, showed that mussels strengthen their glue with molecules of iron, though the mechanical details of this process remain unclear. So do the molecular details of mussel adhesive itself. The glue is made from a mix of proteins that can be harvested and even synthesized — but much of its adhesive power comes from the proteins' structural arrangement. That's lost during harvesting, and can't yet be artificially replicated. "There's a gradient of proteins in that structure," said Stewart. "The proteins have different functions: varnishes, primers, the parts that connect the adhesive" to the threads that compose the mussel's foot. The difficulty in recreating mussel protein structures could explain why mussel-based medical adhesives are not yet on the market, despite nearly two decades of research. Stewart has chosen a less-complicated source of inspiration: the polycheate, a surf-dwelling worm that glues together grains of sand to make a tubular home for itself. "The mussel has to glue a string to a wet rock, whereas a polycheate just has to glue two similar materials together. That's a much simpler bonding problem," said Stewart.  At the point of contact between surface and adhesive, said Stewart, polycheate and mussel glues — though composed of similar proteins — likely rely on a different mix of  molecular bonds. Among them are the van der Waal forces of gecko foot fame, hydrogen bonds, covalent bonds, and salt bridges — a smorgasbord of molecular stickiness. The bonds have been identified, said Stewart, but not their configuration, or their relationship to individual proteins. Researchers need to determine "the proportion of different bonds, and how those might work in some cooperative and unexpected manner."  Meanwhile, barnacles — the least-understood marine adhesive — don't use dopa, a protein central essential for mussel and polycheate glues. The lack of dopa, said Stewart, shows just how many ways nature has found to solve the problem of adherence in the surf. "A lot of these things are not well-understood," said Narayan. "These sorts of studies are the first steps to better understanding these materials."