"They're very good at what they do, says ONR's Paul Armistead. Powerful even. They work fast, possess a tenacity that is remarkable, make it look easy, and they've been at it since the beginning of time. What's more, they do it underwater. Billions of them glue themselves to ship hulls and cost the U.S. Navy over $50 million a year in fuel costs alone due to friction and drag. What's worse is that each of the Navy's ships is obliged to be cleaned in port yearly – sometimes even more frequently.
We're talking the humble barnacle. Historically, everything from pitch to pesticides has been used to try to deter it, but nothing seems to repel completely this lowly, but determined crusty fouler.
Now consider the sleek, smooth dolphin, which can spend its entire life in the water and never host a single barnacle, while a ship – also designed with a smooth, sleek hull – can develop a bad case of them in less than a month.
"Much can be gained by studying and mimicking biological solutions that have evolved over eons," said Armistead. As manager of ONR's Polymer Chemistry program, Armistead supports Karen Wooley, a polymer chemist and professor at Washington University in St. Louis who has dedicated her young life to a study of finding ways to provoke interactions between biological systems and synthetic materials. She designs chemical "functionalities," groups of atoms that either promote or discourage binding between them.
"Over the years, Navy research into antifouling hull coatings has increasingly focused on non-toxic approaches* or ‘fouling release' coatings. An obvious first choice in this approach is Teflon-like coatings, but barnacles are able to get a good grip on these fluoropolymers," says Armistead. For ONR, Wooley decided that rather than look traditionally at smooth fluoropolymer surfaces to keep fouling critters from attaching themselves to surfaces, she would create surfaces that were a complex and rough pattern of nanometer-scale dimensions. She based this upon the hypothesis that interruption of the attachment of adhesin proteins – which are secreted by many organisms to serve as glues – would result in fewer organisms settling on these surfaces in the first place, and would promote organism release, i.e., not many would be able to stay attached long if they did manage to get a foothold. She thought that since the adhesin proteins are of nanometer-scale dimensions, then the surface features should be as well.
Meanwhile, Wooley noted – as others had – that at the nanometer level the sleek dolphin's skin was not smooth at all, but rippled. This might be why nothing could adhere to it – it was too rough at this size for a sticky critter to gain even a tiny foothold.
Wooley thought that if she could take her polymers and mix two contrary types, one smooth, linear, and water-loving (polyethylene glycol – the substance found in hand lotions and shampoos, known to inhibit protein adsorption), and one hyperbranched, water-avoiding, and highly fluorinated (like Telflon), she could allow them to phase-separate into separate domains, but intersperse into the other. She did it, and when the mixture solidified, the heterogeneous coating created was a nano-scale terrain of mountains and valleys.
It did not differ greatly from a dolphin's skin.
"Karen's approach is really quite novel in that it combines surface physical textures and chemical heterogeneity both on the size scale of a barnacle's secreted glue – a coiled protein chain," says Armistead. "Variations of her coatings are being evaluated in the labs of Navy investigators. Early experiments show an exceptionally low settlement rate for some critters."
"Different formulas give us different compound surface properties, so if we tweak the size of the surface features to what's needed, perhaps we'll finally have outwitted the barnacle," says Wooley.
* The world-wide use of tin based paints – used now to combat the problem but highly toxic to the environment – has been banned by the UN's International Maritime Organization. The U.S. Navy evaluated tin-based paints in the 1980's but never adopted them because of early environmental concerns. In 2008, any ship not complying with the UN ban will be denied access to main port states.