Researchers have developed a
carbon-nanotube-based tape that could prove useful for creating robots that
climb walls and special adhesive gloves for astronauts. Unlike ordinary tape, which
eventually loses its stickiness, this new material sticks like a permanent
glue, but it can be removed and reused. It can also stick to a wider variety of
materials, including glass and Teflon.
Dubbed “gecko tape” by researchers, the material works
by imitating the nano- and microscale structures on geckos’ feet that allow
them to quickly scale walls and run across ceilings. The tape is reusable and
will not dry up or slide off the wall because, unlike ordinary tape, it does
not use viscoelastic glues. Instead, it employs carbon nanotubes to make use of
microscale van der Waals forces that occur at very short ranges between
surfaces. Bundles of nanotubes conform to the slightest microscopic variations
on a surface, the same way that the bundles of nanoscopic keratin fibers that
make up the hairs on gecko feet allow them to conform to walls.
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Like ordinary tape, gecko tape clings strongly when pulled
parallel to a surface; it can support just under 10 pounds per square
centimeter. But the tape can be peeled off relatively easily when pulled
perpendicular to a surface.
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As nanoscale control over
materials has grown, so has the number of groups developing versions of gecko
tape. But the new tape, made by researchers at the University of Akron and
Rensselaer Polytechnic Institute (RPI), is much stronger than those versions and
is able to support four times more weight per area than gecko feet. Other
researchers have tried using carbon nanotubes in the past, in part because they
are very strong and can withstand much higher temperatures than plastic, hence
would be useful in more applications. But those nanotubes were too stiff to be
practical, says Manoj Chaudhury, a professor
of chemical engineering at Lehigh University, who has worked on similar
projects. So the Akron and RPI researchers modified the nanotubes by exploiting
the fact that they can be chemically grown to have various numbers of walls.
The researchers struck a balance between having too many walls, which makes the
nanotubes too rigid, and having too few, which makes them weak, Chaudhury says.
To make the tape even stronger while maintaining
flexibility, the researchers took a tip from the geckos. They grew nanotubes in
distinct bundles, much like the bundles of fibers that make up the hairs on
gecko feet. Nanotubes are made by exposing catalysts to hydrocarbon feedstocks:
gradually, carbon builds up on the catalysts in a distinctive atomic
arrangement that forms tubes. The researchers grew nanotubes in bundles by
depositing the catalyst material in a pattern of separate squares. They then
transferred these bundles of nanotubes to a flexible plastic to make the tape.
The tape can still be improved in a number of ways. Right
now, it takes a significant amount of pressure to get good contact between the
nanotubes and the surface, says Metin Sitti, a professor of
mechanical engineering at Carnegie Mellon University, who is developing similar
materials. Chaudhury says that ultimately, it would be nice to have a material
that sticks with little or no pressure, particularly for use with wall-climbing
robots. Ali
Dhinojwala, a professor of polymer science at the University of Akron, who
led the work, says that the attachment pressure might be decreased by doing
things like softening the tape’s backing.
The researchers also want to make the tape stronger. Right
now, when pulled parallel to a surface, the tape releases not because the
carbon nanotubes come off the surface, but because the nanotubes themselves
break. The researchers are currently working on a number of ways to strengthen
the nanotubes to take advantage of this strong adhesion. They are also working
to make the tape reusable thousands of times, rather than the dozens of times
it can be used now. To achieve this goal, Dhinojwala and his team will have to
make the tape self-cleaning, like gecko feet.
To commercialize the tape, the researchers will also need to
make bigger patches of tape. So far, they’ve only made pieces of tape that are
far smaller than a dime.