While “sticky-fingered” might be a felonious insult for humans, it’s an apt description for geckos, whose hairy feet give them phenomenal powers to hang by one toe on even the slipperiest vertical surfaces.
Researchers from Rensselaer Polytechnic Institute and the University of Akron have used their knowledge of what makes geckos stick to create a carpet of super-sticky carbon nanotubes that could form the basis for future types of adhesives. In this case, science has even surpassed nature by producing bundles of nanotubes with an adhesive power 200 times greater than that of the gecko foot hairs.
“The reason these materials (nanotubes) are so exceptional is that they form very unique structures,” said Ali Dhinojwala, who led the research team. “Usually, it’s the defects that prevent us from achieving properties we want, but when nanotubes assemble they are relatively defect-free, and that determines their strength and how they perform,” Dhinojwala said.
Dhinojwala and his crew aren’t the only ones with gecko-foot fascination. In 2002, a team of scientists studying the creatures explained to the world just how geckos stay stuck. By fabricating synthetic gecko hairs from different materials, they found that the geckos’ adhesive powers came not from chemistry, but from geometry — the size and shape of the tips of the gecko foot hairs.
Geckos have very hairy feet. Each gecko foot is covered by half a million setae, tiny hairs 50,000 nanometers long. The length is often compared to the width of a human hair. Each setae branches off into hundreds of even more miniscule hairs, called spatulae, just 200 nanometers wide.
The scientists discovered that an appropriate arrangement of setae and spatulae held the geckos to the wall by means of a type of an intermolecular attraction known as a van der Waals force. The same force that holds geckos to walls has been used to explain everything from snowflake formation to spider acrobatics.
Early attempts to create synthetic gecko hair-like adhesive structures involved plastic pillars arranged through a process known as photolithography. That approach had its limitations due to the relative fragility of the plastic pillars and the inherent size difference between nanometer-size gecko foot hairs and plastic pillars measured in microns (1 micron equals 1,000 nanometers).
The bottom-up, nanotube-based approach to building synthetic gecko feet employed by Dhinojwala’s team had advantages over the earlier plastic-pillar technique both in terms of mechanical strength and size. Nanotubes are similar in size to the actual gecko setae and thus more likely to exhibit the same van der Waal properties.