Aug 17, 2005
Carbon nanotubes mimic gecko foot-hairs
Geckos have an impressive capacity to walk upside down on almost any surface, using just the attractive forces created by their feet to hold on. With this in mind, scientists have attempted to copy the structure of gecko feet to create strongly adhesive materials. The latest version, developed at the University of Akron and Rensselaer Polytechnic Institute, US, uses multiwalled carbon nanotubes attached to a polymer backing.
The researchers claim that the structure creates adhesion forces 200 times higher than gecko foot-hairs do and could have applications in microelectronics and space.
"It is well known that insects, such as beetles, and reptiles, such as geckos, have evolved and developed this most effective adhesive system in order to survive," said Ali Dhinojwala of the University of Akron. "The biological system in these creatures has perfected not only the mechanism to attach to steep vertical surfaces but also to detach at will."
Geckos have five-toed feet covered in minute hairs known as setae. The ends of the setae split into spatulas that contact surfaces and adhere to them through the van der Waals effect. It's because the hairs can deform to fit a surface exactly that the contact is close enough for van der Waals forces to come into play.
Dhinojwala and colleagues mimicked the structure by depositing multiwalled carbon nanotubes by chemical vapour deposition onto a quartz or silicon substrate. The nanotubes were typically 10-20 nm in diameter and around 65 μm long. They then encapsulated the vertically aligned nanotubes in PMMA polymer before exposing the top 25 μm of the tubes by etching away some of the polymer. The nanotubes tended to form entangled bundles about 50 nm in diameter because of the solvent drying process used after etching.
Testing the tubes with a scanning probe microscope revealed their adhesion behaviour. Force-distance curves showed weak repulsive forces as the microscope probe approached the nanotubes and high adhesion as the probe was retracted. The team calculated the minimum force per unit area as 1.6 ± 0.5 × 10-2 nN nm-2. This is much larger than the figure the team estimated for the typical adhesive force of a setae, which was 10-4 nN nm-2.
"We already have strong adhesives that can support large forces and we have weak adhesives such as sticky notes that can be used many times but are not strong enough to support large forces," said Dhinojwala. "It will be a challenge to figure out how to design an adhesive that can provide a strong attachment to support a large force but at the same time have the capability of detaching itself from the surface with ease."
Now the team plans to experiment on larger surface areas and to try using elastomeric rather than glassy polymers in order to maximize the adhesion of the structures.
The researchers reported their work in Chemical Communications.
About the author
Liz Kalaugher is editor of nanotechweb.org.