Carbon nanotube/biopolymer composites show promise as artificial muscles

(Nanowerk Spotlight) The development of artificial muscles is one of the key areas for bionic enhancements or replacements. The quest to build artificial muscles using inherently conducting polymers can be traced to Ray H. Baughman's paper "conducting polymer electromechanical actuators" at the 1989 NATO Advanced Research Workshop "Conjugated polymeric materials: opportunities in electronics, optoelectronics and molecular electronics". The discovery of the electromechanical actuation properties of single-walled carbon nanotubes and the complex behavior of multi-walled carbon nanotubes has led to the development of various carbon nanotube (CNT) actuators. Besides artificial muscles, potential applications include microelectro-mechanical systems (MEMS), biomimetic micro-and nanorobots, and micro- and nanofluidic devices.
"Recently, a new class of active system, carbon nanotube/polymer composite actuators, has received great attention with regard to macroscopic artificial muscle applications," Wei Chen tells Nanowerk. "It has been demonstrated that successful introduction of the highly conductive CNTs could significantly enhance the polymer nanocomposite’s electrical, thermal, mechanical, and interface properties, thus providing a suitable material for novel artificial muscle-like actuator investigations."
Chen, a professor at the Suzhou Institute of Nano-tech and Nano-bionics together with his team, has now demonstrated the electromechanical behavior of natural biopolymer due to introducing a carbon nanotube conductive network. The biopolymer used by the team is chitosan (CS), the second most abundant naturally occurring biomolecule after cellulose, which has been shown to be a biocompatible, low-cost, and smart polymer material with useful biological and chemical properties. In addition, chitosan has also proved to be a good dispersant which can debundle CNTs at very high concentration.
A biopolymer is a biocompatible and low cost polymer material with useful biological and chemical properties. However, it is an insulator in air, which greatly limits its usage as a smart material under electrical stimulus. CNTs, on the other hand, have good electrical properties which make them excellent candidates for the conductive nano-filler in insulating materials. Combining biopolymers with CNTs to form a conductive composite is therefore a promising strategy.
Reporting their findings in the May 13, 2010 online edition of ACS Nano ("Electromechanical Actuation with Controllable Motion Based on a Single-Walled Carbon Nanotube and Natural Biopolymer Composite"), the team demonstrated that the energy conversion in the CNT/CS composite material results in a controllable actuation and a new electromechanical property of the composite which doesn't exist in either individual component.
artificial biopolymer/carbon nanotube muscles
Side view optical images of the suspended strip with the voltage off (a) and on (b). The lower part is the glass substrate, and the black line is the 25 wt % SWCNT/CS composite. The applied voltage is 0.1 Hz alternating sine wave voltage of 5 V. (Reprinted with permission from American Chemical Society)
Chen explains that, without sophisticated fabrication and configuration, the materials they used are fabricated by simple mixture of the two components (single-walled carbon nanotubes and the biopolymer chitosan). The electrical actuation of these materials is performed by applying an electrical stimulus to the composite.
"Frankly, this electrical actuation process that we found is simple, but useful and interesting" says Chen. "When an electrical voltage is applied to the composite, the electrical power is converted into thermal energy through the CNT-formed conductive network in the biopolymer matrix. The thermal expansion of the matrix leads to electrical actuation. The actuated vibrational motion, including the frequency and waveform, can be controlled by the applied low alternating voltages."
In order to understand the function of the single-walled CNTs in the composite actuator, the team subjected CS samples with different CNT loadings to the same 0.1 Hz positive sine wave voltage (0-5 V) electromechanical characterization.
"We found that, with reduced nanotube content, smaller displacements take place" says Chen. "For the pure CS matrix we observed no displacement at all. The higher the CNT doping, the higher the electrical conductivity, and therefore a larger current can pass through the sample under the same applied voltage. When an alternating current passes through a thin conductor, periodic heating takes place following the variation in the current strength. The generated temperature waves were then propagated into the surrounding medium, which caused the thermal expansion and contraction of the layer near the conductor. Depending on the medium’s properties, the thermal response will be different."
Chen notes that there are several areas of this work which the team hasn't studied yet. "For example, how is the force output of the composite during the actuation? What about the high frequency response by using a better measuring instrument to observe it?"
The team will therefore focus future research on investigating the mechanical properties of the composite; improving its electromechanical performance; and designing prototype devices for practical applications.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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