Researchers describe creating powerful, unipolar electrochemical yarn muscles that contract more when driven faster, thereby solving important problems that have limited the applications for these muscles.
This polymer coating converts the normal bipolar actuation of carbon nanotube yarns to unipolar actuation, where the muscle actuates in one direction over the entire stability range of the electrolyte. This long-sought behavior has surprising consequences that make electrochemical carbon nanotube muscles much faster and more powerful.
The advances provide electrochemical unipolar muscles that contract to generate a maximum average output mechanical power per muscle weight of 2.9 watts/gram, which is about 10 times the typical capability of human muscle and about 2.2 times the weight-normalized power capability of a turbocharged V-8 diesel engine.
Above – This scanning electron microscope image shows a coiled unipolar muscle made from carbon nanotubes and coated with poly(sodium 4-styrenesulfonate). The outer coil diameter is approximately 140 microns, about twice that of a human hair.
For more than 15 years, researchers at The University of Texas at Dallas and their collaborators in the U.S., Australia, South Korea and China have fabricated artificial muscles by twisting and coiling carbon nanotube or polymer yarns. When thermally powered, these muscles actuate by contracting their length when heated and returning to their initial length when cooled. Such thermally driven artificial muscles, however, have limitations.
Electrochemically driven carbon nanotube (CNT) muscles provide an alternative approach to meet the growing need for fast, powerful, large-stroke artificial muscles for applications ranging from robotics and heart pumps to morphing clothing.
Wang said the team also discovered that unipolar behavior, without scan-rate enhanced strokes, could be obtained when graphene oxide nanoplatelets were incorporated within the yarn muscle using a biscrolling process that UT Dallas researchers created and patented.
“Use of this guest to provide the dipolar fields needed for unipolar behavior increased the maximum average contractile mechanical power output from the muscle to a remarkable 8.2 watts/gram, which is 29 times the maximum capability of the same weight human muscle and about 6.2 times that of a turbocharged V-8 diesel engine,” Wang said.
Pump it up
Carbon nanotube yarns can be used as electrochemical actuators because infiltration with ions causes a contraction in length and an expansion in diameter. Either positive or negative ions can cause this effect. Chu et al. constructed an all-solid-state muscle that eliminated the need for an electrolyte bath, which may expand the potential for its use in applications. By infiltrating the yarns with charged polymers, the fibers start partially swollen, so the length can increase through the loss of ions. It is thus possible to increase the overall stroke of the muscle. Further, these composite materials show a surprising increase in stroke with scan rate.
Success in making artificial muscles that are faster and more powerful and that provide larger strokes would expand their applications. Electrochemical carbon nanotube yarn muscles are of special interest because of their relatively high energy conversion efficiencies. However, they are bipolar, meaning that they do not monotonically expand or contract over the available potential range. This limits muscle stroke and work capacity. Here, we describe unipolar stroke carbon nanotube yarn muscles in which muscle stroke changes between extreme potentials are additive and muscle stroke substantially increases with increasing potential scan rate. The normal decrease in stroke with increasing scan rate is overwhelmed by a notable increase in effective ion size. Enhanced muscle strokes, contractile work-per-cycle, contractile power densities, and energy conversion efficiencies are obtained for unipolar muscles.
SOURCES- University of Texas at Dallas , Journal Science
Written by Brian Wang, Nextbigfuture.com
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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