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Electrostrictive polymer twitches and wrinkles to combat marine biofouling

Duke University Pratt School Of Engineering : 22 April, 2013  (Technical Article)
By twitching their skin, ships may soon be able to shed the unwanted accumulation of bacteria and other marine growth with the flick of a switch. Duke University engineers have developed a material that can be applied like paint to the hull of a ship and will literally be able to dislodge bacteria, keeping it from accumulating on the ship’s surface. This buildup on ships increases drag and reduces the energy efficiency of the vessel, as well as blocking or clogging undersea sensors.

Keeping bacteria from attaching to ship hulls or other submerged objects can prevent a larger cascade of events that can reduce performance or efficiency. Once they have taken up residence on a surface, bacteria often attract larger organisms, such as seaweed and larva of other marine organisms, such as worms, bivalves, barnacles or mussels.

Biofouling – the accumulation of microorganisms, plants, algae, or animals on wet surfaces – has been a long-term problem for the global shipping industry. Coating ships in a new material that shakes itself on command can eliminate several problems associated with keeping ships clean.

Apart from being unsightly, barnacles and other biofouling substances can inhibit a ship’s ability to function. A small layer of slime can significantly increase drag. While for ships sailing worldwide, there is a risk invasive species will be transferred from their native habitat to ones where they can be damaging. Ridding ships of biofouling materials is also of military importance: the more barnacles and bacteria a ship carries, the noisier the vessel is, making it easier to detect.

Traditionally, the shipping industry has used anti-fouling paint, but though though there are alternatives in the pipeline, most antifouling paints currently in use are toxic – the ones based on copper particularly so. An international treaty banned those containing tributyltin in 2007. Another approach has been a polymer coating that reduces biofouling substances’ ability to adhere to the boat. It’s a temporary fix, though, because bacteria and barnacles eventually adapt to the polymer and attach themselves anyway.

The new material works by physically moving at the microscopic level, knocking the bacteria away. This avoids the use of bacteria-killing paints, which can contain heavy metals or other toxic chemicals that might accumulate in the environment and unintentionally harm fish or other marine organisms.

Running voltage through the flat polymer turns it into a capacitor and generates an electric field. This then causes electrostriction, a property that allows them to change shape when exposed to electricity. There are patterned channels – air channels – beneath the polymer, and blowing air into these increases the hydrostatic pressure and buckles up the polymer surface. It forms a wrinkle on the surface of the polymer, and the biofouling substances simply detach.

“We have developed a material that ‘wrinkles,’ or changes it surface in response to a stimulus, such as stretching or pressure or electricity,” says Duke Xuanhe Zhao, assistant professor in Duke’s Pratt School of Engineering. “This deformation can effectively detach biofilms and other organisms that have accumulated on the surface.” Zhao has already demonstrated the ability of electric current to deform, or change, the surface of polymers.

“Nature has offered many solutions to deal with this buildup of biological materials that we as engineers can try to recreate,” said Gabriel López, professor of biomedical engineering and mechanical engineering and materials science. “For example, the hair-like structures known as cilia can move foreign particles from the lungs and respiratory tract,” Lopez said. “In the same manner, these types of structures are used by molluscs and corals to keep their surfaces clean. To date, however, it is been difficult to reproduce the cilia, but controlling the surface of a material could achieve the same result.”

The researchers tested their approach in the laboratory with simulated seawater, as well as on barnacles. These experiments were conducted in collaboration with Daniel Rittschof the Duke University Marine Lab in Beaufort, N.C.

The Duke researchers also say that similar types of materials could be used in other settings where the buildup of bacteria – known as biofilms -- presents problems, such as on the surfaces of artificial joint implants or water purification membranes.

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