Nav: Home

Artificial muscles show more flex

October 31, 2016

WASHINGTON, D.C., October 31, 2016 -- Artificial muscles made significant gains when a literal twist in the development approach uncovered the tensile -- or stretchy -- abilities of polymer fibers once they were twisted and coiled into a spring-like geometry. In a similar manner to the powerful climbing tendrils of cucumber plants, the unique geometry gives the coil a flexing motion when fiber material shrinks -- a reaction that can be controlled with heat. Now, researchers have improved these tensile properties even further by focusing on the thermal properties of the polymer fiber and the molecular structure that makes best use of the chiral configuration.

In the cover article appearing this week in Applied Physics Letters, from AIP Publishing, Guoqiang Li and his team in the Department of Mechanical and Industrial Engineering at Louisiana State University discuss how they have developed a new fiber that offers higher tensile stroke and is triggered -- or actuated -- at temperatures more than 100 degrees Celsius cooler than its predecessors.

"We analyzed the principle behind why the polymer fiber, through twisting and coiling, can behave so remarkably," said Li, explaining their methodology. According to Li, they found two driving factors: the untwisting nature of the fiber during actuation and the negative coefficient of thermal expansion (NCTE). The two-way shape memory polymer (2W-SMP) fiber Li and his team developed addressed both of these factors.

When it comes to the untwisting that drives this chiral-upon-chiral architecture to flex and contract, Li's group focused on this issue at the molecular level. The reversible responses of the 2W-SMP polymer that make them ideal come from a stable molecular network of chemical cross links. The network provides chains of oriented molecules in the polymer whose melting and recrystallization gives rise to the important memory characteristics of the fiber.

The reversible melt/crystallization transition also provided better thermal expansion properties compared to standard fibers, where actuation comes from the intrinsic contraction of the polymer components in the presence of heat (and relaxation when the heat is removed). The 2W-SMP fiber demonstrates thermal expansion/contraction an order of magnitude higher than the NCTE of its predecessors.

By addressing these two characteristics, the fibers Li produced and tested in their twisted-then-coiled muscle configurations showed greater tensile actuation, but they also brought down the temperature needed to actuate these artificial muscle fibers.

"The actuation temperature is very high in the polymer fibers used previously, for example they can go to 160 degrees C," said Li. "For some applications, like medical devices, [the] actuation temperature is too high. So you need to find a way to lower it." That is exactly what the group did, reporting maximum actuation temperatures of 67 C.

The low temperature is significant when considering a host of applications related to human body temperature beyond just medical devices, including breathable textiles and self-healing materials whose structures adapt to environmental changes.

Li and his team still face challenges with the performance of the fiber's specific work as well as efficiency in converting thermal energy into actuation, and look to address these issues in future work. One potential approach may be to incorporate conductive reinforcement into the material with carbon nanotubes.

"Our polymer is very soft. So by adding some reinforcement, like carbon nanotubes, we'd have two benefits," Li said. "The first one makes it into a conductor, that means we can also use electricity and have it trigger the muscle behavior. The other is that the carbon nanotube will increase the stiffness." Greater stiffness means better energy storage for the fiber, which in turn increases the energy conversion efficiency.
-end-
The article, "Artificial muscles made of chiral two-way shape memory polymer fibers," is authored by Qianxi Yang, Jizhou Fan and Guoqiang Li. The article will appear in the journal Applied Physics Letters on October 31, 2016 (DOI: 10.1063/1.4966231). After that date, it can be accessed at http://scitation.aip.org/content/aip/journal/apl/109/18/10.1063/1.4966231.

ABOUT THE JOURNAL

Applied Physics Letters features concise, rapid reports on significant new findings in applied physics. The journal covers new experimental and theoretical research on applications of physics phenomena related to all branches of science, engineering, and modern technology. See http://apl.aip.org.

American Institute of Physics

Related Polymer Articles:

World first: New polymer goes for a walk when illuminated
Scientists have developed a new material that can undulate and therefore propel itself forward under the influence of light.
Polymer-coated silicon nanosheets -- an alternative to graphene
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene.
New polymer additive could revolutionize plastics recycling
Only 2 percent of the 78 million tons of manufactured plastics are currently recycled into similar products because polyethylene (PE) and polypropylene (PP), which account for two-thirds of the world's plastics, have different chemical structures and cannot be efficiently repurposed together.
Responsive filtration membranes by polymer self-assembly
Polymer self-assembly is a crucial tool for manufacturing membranes using scalable methods, enabling easier commercialization.
Biodegradable polymer coating for implants
Medical implants often carry surface substrates that release active substances or to which biomolecules or cells can adhere better.
Praise for polymer science
Engineer Glenn Fredrickson receives the William H. Walker Award for Excellence in Contributions to Chemical Engineering Literature.
When it comes to polymer fragility, size does matter
By combining a number of tools and techniques, a team of researchers from the US, Italy and China was able to find a more complete picture of the glass transition phenomenon in polymers and to point out where the polymers differ from small molecular liquids.
Better, stronger: Polymer breakthrough to improve things we use everyday
Medicine, mobile phones, computers and clothes could all be enhanced using the process for making paint, according to research by the University of Warwick.
CWRU researcher scaling up knotty polymer research
Researchers at Case Western Reserve University developed a technique that produces a long chain molecule in the shape of a trefoil knot.
New 3-D printed polymer can convert methane to methanol
Lawrence Livermore National Laboratory scientists have combined biology and 3-D printing to create the first reactor that can continuously produce methanol from methane at room temperature and pressure.

Related Polymer Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Jumpstarting Creativity
Our greatest breakthroughs and triumphs have one thing in common: creativity. But how do you ignite it? And how do you rekindle it? This hour, TED speakers explore ideas on jumpstarting creativity. Guests include economist Tim Harford, producer Helen Marriage, artificial intelligence researcher Steve Engels, and behavioral scientist Marily Oppezzo.
Now Playing: Science for the People

#524 The Human Network
What does a network of humans look like and how does it work? How does information spread? How do decisions and opinions spread? What gets distorted as it moves through the network and why? This week we dig into the ins and outs of human networks with Matthew Jackson, Professor of Economics at Stanford University and author of the book "The Human Network: How Your Social Position Determines Your Power, Beliefs, and Behaviours".