Sharks are famous for their sandpaper skin. Its roughness is caused by thousands of microstructures called dermal denticles ("tiny skin teeth"). To call them bumps is to miss the elegance of their forms, for they are shaped like so many ribbed delta-wings and overlap in a precise and beautiful geometry. These denticles maintain a smooth flow of water by creating micro-vortices at each placoid scale. The micro-votices are a turbulence management system, if you will, and obviate the creation of large swirls of liquid against the shark's skin by creating many smaller swirls. Less turbulence means less drag, and therefore faster and quieter movement.
This efficiency of fluid flow appears to have an added benefit: algae and bacteria can't anchor on its skin. The micro-roughness of the surface that reduces drag also makes it inhospitable to attachment by single algae or colonies. The energy required to form a biofilm with other algae is apparently too great and they seek other, easier surfaces.
Anthony Brennan, a materials science and engineering professor at the University of Florida, was touring the Pearl Harbor shipyard when he was asked by the U.S. Office of Naval Research (ONR) to come up with a solution to the fouling of ships' hulls, a major maintenance and operational problem for the world's fleets. Brennan searched for a swimming organism that did not acquire the algae film and found the shark, specifically the Galapagos Shark. After taking a cast of the shark's skin and examining it in a scanning electron microscope, he reproduced the dimensions and proportions of the surface topography and manufactured a material that reduced green algae growth by 86 percent. ONR has funded his research since 1999.
Brennan realized that the same surface geometry could work against bacteria as well as algae and began development in 2002. In 2007, he founded the privately held company Sharklet Technologies, the first commercial antibiotic strategy based on surface structure.
The company makes films based on a microtopographical surface. It is testing the surface on urinary catheters with plans to expand into medical devices and OEM (original equipment manufacturer) of consumer products. The films are applied to high-touch surfaces such as counters, doorknobs and restroom surfaces. Sharklet claims to reduce bacterial colonization of MRSA by 86 percent, Enterococcus faecalis by 99 percent and Pseudomonas aeruginosa by 100 percent after one hour of air exposure. Each Sharklet diamond measures nearly 25 microns across (about 1/5th the thickness of a human hair) and nearly three microns deep. Each diamond contains seven ribs of varying length.
Pathogenic bacteria by Lukiyanova Natalia / frenta via Shutterstock.
Algae's biofilm defense
The common alga, Delsea pulchra, has a different approach to avoiding being covered by biofilm. It releases chemicals called furanomes that interfere with the communication between different bacteria. This communication is called "quorum signaling" because of its initiation of the grouping that eventually forms the biofilm.
Furanomes are molecules that bind to the protein-covered receptor sites on bacteria and block the reception of signaling molecules from the neighboring bacteria (N-acyl homoserine lactone).
Professors Staffan Kjelleberg and Peter Steinberg of the University of New South Wales in Sydney began research in this area in the early 1990s, and started making synthetic furamones for antifouling applications. They formed BioSignal Ltd. of Australia to also solve the boat hull antifouling challenge in 1999.