Using neutron beams and atomic-force microscopes, a team of university researchers
working with the National Institute
of Standards and Technology (NIST) may have resolved a 10-year-old question
about an exotic class of "artificial muscles"-how do they work? Their results*
could influence the design of future specialized robotic tools.
These "artificial muscles," first demonstrated in the early 1990s,
are "ionic polymer metal composite" (IPMC) actuators, a thin polymer
strip plated on both surfaces with conducting metal. The basic unit of the polymer
molecule has a charged component attached to it (hence, "ionic"),
and it forms a sort of open, permeable structure that can be soaked with water
molecules and oppositely charged ions. A modest electric charge across the metalized
surfaces will cause the strip to flex in one direction; an alternating charge
will make it wiggle like a fish's tail. But why?
Flexing muscle: IPMC actuator bending under an applied electrical voltage. As the polarity of the 3-volt potential is switched, the actuator bends back and forth. Credit: R. Moore, Virginia Polytechnic Institute and State University
"There has been a lot of debate as to the mechanism of actuation in these
kinds of systems," says NIST materials scientist Kirt Page. One possibility
was that the electric charge on the metalized faces causes the polymer and the
free ions to reorient themselves next to the metal, stretching one side and
contracting the other. But using a neutron beam at the NIST Center for Neutron
Research (NCNR) to watch an IPMC in action as it wiggled back and forth, the
team found something very different. Neutrons are particularly good for mapping
the locations of water molecules, and they showed that a major force in the
actuator is hydraulics. "The water and ions move to one electrode swelling
one side and dehydrating the other, causing that to contract, and it bends in
that direction," explains Virginia Tech professor Robert Moore, who directed
the research. "Then you flip the potential, the ions come screaming back-positive
ions again moving towards the new negative electrode-and you can go back
and forth."
It happens surprisingly fast, according to Page. "People weren't
quite convinced that water could actually move over these distances that quickly,"
he says, "This paper is the first to show that in fact, this gradient
in the water concentration is established almost instantaneously."
A better understanding of just how IPMC actuators work could allow researchers
to engineer better materials of this type with improved performance. Current
actuators can be small and light-weight, and they can flex over relatively large
distances, but the force they can generate is low so these "muscles"
are not very strong, according to Moore. They could be used in microfluidic
systems as pumps or valves, as tiny robotic grippers in applications where other
actuators are impractical or even, says Moore, "as actual artificial muscles
in living tissues. I think we're still in the infancy stage of using these.
There are still quite a number of details about the mechanism that we need to
unlock."
* J.K. Park, P.J. Jones, C. Sahagun, K.A. Page, D.S. Hussey, D.L. Jacobson,
S.E. Morgan and R.B. Moore. Electrically stimulated gradients in water and counterion
concentrations within electroactive polymer actuators Soft Matter. 2010. 6.
1444-1452. DOI: 10.1039/b922828d.