Solitons could power molecular electronics and artificial muscles
18 July 2006
Solitary waves travelling through organic polymers could
be harnessed to supply power in medical devices.
|A solitary electron wave (ripples of red and
blue, centre) travels along a polymer chain, causing the chain to
bend in the middle. Image: Ohio State University.
The polymer chains tend to bend and twist as solitons pass through them.
This has led to the possibility of them being used to power artificial
muscles for high-tech robots and devices to aid human mobility. Such muscles
would be made of organic polymers, and flex in response to light or
Solitons were first discovered in water in
the 1840's by a Scottish Engineer, John Scott Russell, while observing
a barge in a canal near Edinburgh (see Heriot Watt University's web page on
www.ma.hw.ac.uk/~chris/scott_russell.html and Wikipedia for a
description of solitons:
http://en.wikipedia.org/wiki/Soliton). It was only in the 1960's when it
was discovered that the theory of solitons could be applied to
hydrodynamics, optics, plasmas, shock waves, tornados, elementary particles
of matter and even the elementary particles of thought, that their
importance was appreciated.
Since the 1980s, scientists have known that solitons can carry an
electrical charge when travelling through certain organic polymers. A new
study now suggests that solitons have intricate internal structures.
Scientists may one day use this information to put the particles to work in
molecular electronics and artificial muscles, said Ju Li, Assistant
Professor of materials science and engineering at Ohio State University.
Li explained that each soliton in the organic polymers is made up of an
electron surrounded by other particles called phonons. Just as a photon is a
particle of light energy, a phonon is a particle of vibrational energy.
The new study suggests that the electron inside a soliton can attain
different energy states, just like the electron in a hydrogen atom.
we know that such internal electronic structures exist in all atoms, this is
the first time anyone has shown that such structures exist in a soliton," Li
The soliton's quantum mechanical properties — including these newly
discovered energy states — are important because they affect how the
particle carries a charge through organic materials such as conducting
polymers at the molecular level.
"These extra electronic states will have
an effect — we just don't know right now if it will be for better or worse,"
Li and his longtime collaborators from MIT published their
findings in a recent issue of the Proceedings of the National Academy of
The name "soliton" is short for "solitary wave." Though
scientists often treat particles such as electrons as waves, soliton waves
are different. Ordinary electron waves spread out and diminish over time,
and soliton waves don't.
"It's like when you make a ripple in water — it
quickly spreads and disappears," Li said. "But a soliton is a strange kind
of object. Once it is made, it maintains its character for a long time."
In fibre optics, normal light waves gradually flatten out; unless the signal
is boosted periodically, it disappears. In contrast, solitonic light waves
retain their structure and keep going without assistance. Some
telecommunication companies have exploited that fact by using solitons to
cheaply send signals over long distances.
Before solitons can be fully
exploited in a wider range of applications, scientists must learn more about
their basic properties, Li said. He's especially interested in how solitons
carry a charge through conducting polymers, which consist of long, skinny
chains of molecules.
The tiny chains are practically one-dimensional, and
this calls some strange physics into play, Li said.
In their PNAS paper,
Li and MIT colleagues Xi Lin, Clemens Först, and Sidney Yip describe a
detailed calculation of what happens to solitons at a quantum-mechanical
level as they travel along a chain of the organic polymer polyacetylene.
Their mathematical model builds upon a 1979 model called the
Su-Schrieffer-Heeger (SSH) model. Alan Heeger, a University of California,
Santa Barbara physicist who won the Nobel Prize in 2000 for his pioneering
work on conducting polymers.
Li said the new work extends the SSH model by
including the full flexibility of the polymer chain, as well as interactions
The finding will likely affect the development of
molecular electronics — devices built from individual molecules.
understood, solitons may also be harnessed to drive molecular motors in
nanotechnology," Li said.
This work was mainly funded by Honda R&D Co.,
Ltd., with computing resources provided by the Ohio Supercomputer Center.