Biomimetics is perhaps the oldest form of
scientific plagiarism – science plagiarizing nature. It is "the science of systems which have
some function copied from nature, or which represent characteristics of natural
systems or their analogues." It is also one of the most fascinating,
most fertile areas of engineering and applied science (from airplane wings to
velcro or echolocation). Living organisms have evolved particularly
well-adapted structures and
materials over
geological time by means of natural selection. Especially in the area of
micro- and nanotechnology, it is difficult to imagine operating without the
remarkable opportunities of biomimetic methodology. Harvard established the Wyss Institute for Biologically Inspired
Engineering precisely with that objective in mind. Biomimetic synthesis
is an entire field of organic chemistry.
Geckos, an ectothermic infraorder of lizards,
are one of nature’s most inspiring evolutionary mysteries: they reach in length
from 1.6 cm to 60 cm. Some species are parthenogenic, perhaps
one reason why they occur throughout the world in warm climates, even on remote
islands.
Geckos are capable of running on almost any surface
smooth or rough, wet or dry, clean or dirty. They do this at any orientation:
up a wall as well as inverted along a ceiling. But not very well on Teflon,
and not very
well under water. Miraculously, their toes are covered by millions of
micron-scale bristles-like structures (setae) that constitute a self-cleaning
dry adhesive. On their foot pads, the micrometer-scale setae branch out into
nanometer-scale projections (spatulae). It is generally assumed that the
exceptional adhesive power – that does not rely on any “sticky” substance –
exploits molecular attraction (Van
der Waals forces) between the gecko’s toe pads and the surface it is
walking on. But very recent research suggests that electrostatic
forces may be primarily responsible.
At many research institutions including Oxford, UC Berkeley, Stanford,
Northwestern,
Carnegie
Mellon, UMass,
UAlberta,
UManchester
and numerous other places, Gecko research has become a very exciting topic for
biomimeticists, especially in nanotechnology and adhesives research.
No wonder DARPA
became interested early as well in its potential military applications for
scaling vertical surfaces, but so is NASA, the NIH
and BAe.
Millions of micron scale setae on each toe form combine to a dry adhesive that
is self-cleaning and does not involve a glue-like substance. The microfibers or
setae are activated by dragging or sliding the toe parallel to the surface. The
tip of a seta ends in 100 to 1000 spatulae measuring just 100 nanometers in
diameter and 0.2 μm in length. In the gecko, evolution has formed intricate
nanostructures that work together in a hierarchy
of spatula, spatula stalks, setal stalks, setal arrays, and toe mechanics.
Each square mm contains about 14,000 setae with a diameter of 5 μm. An average gecko’s
setae can support a weight of 133 kg, while its body weight is 70 g. The adhesion force of a spatula varies with the
surface energy of the substrate to which it adheres. The surface energy
that originates from long-range forces, e.g., van der Waals forces, follows
from the material's structure below the outermost atomic layers at up to
100 nm beneath the surface. The setae are lubricated by phospholipids
generated by the gecko’s body that also enable it to detach its feet prior to
each subsequent step without slowing down.
Principal commercial applications of Gecko
research have focused to date on biomimetic
adhesives, a kind of superglue that attaches equally well to wet
surfaces and to dry
ones. “Geckel,”
as a first product is called, has combined
a coating of fibrous silicone with a polymer mimicking the “glue”
employed by mussels that allows them to stick to rocks while they and the
surfaces they adhere to are being
pounded by giant ocean waves in perfect storms.
“Gecko
tape” was developed already in 2003 at the University of Manchester
but only produced in small quantities
while scaling up production has proved commercially difficult. In the life
sciences, sheets of elastic, sticky polymers could soon replace sutures and
staples, including use in laparoscopic surgeries, and provide long-term drug
delivery patches to expanding and contracting areas such as cardiac tissue and provide stem
cell attracting factors for tissue regeneration.
The
gecko’s ability to” run up and down a tree in any way”, as Aristoteles observed
in his History
of Animals (Περὶ τὰ Ζῷα Ἱστορίαι),
continues to inspire even in the robotic age: Stanford’s “Stickybot”
has likely applications primarily in outer space and security. One common
characteristic of visionary biomimetic technology is, however, that its
potential becomes plausible and sometimes obvious far sooner than its
commercial viability. It is – and will likely remain for considerable time –
one of the central challenges of technology
assessment.
