Tom Shelley reports on electrically driven actuators of extraordinary flexibility
Octopus-like actuators are now using electrostatics to achieve great speed and flexibility.
And, while unlikely to come close to what the deep-sea creatures can achieve, they’ve led to the invention of a new kind of muscle-like actuator.
Dr Cecilia Laschi, associate professor in biomedical engineering at the Scuola Superiore Sant’Anna in Pisa, Italy, has been explaining how the actuators came about and the principles behind how they might work.
Addressing the recent conference on Biological Approaches for Engineering, organised by the Institute of Sound and Vibration at the University of Southampton, she described how octopus arms are composed of special muscular structures, named hydrostats, whose volumes are constant during contraction. Hence, if they are decreased by muscle contraction, their lengths increase and vice versa. The arms have muscles along their transverse, longitudinal and oblique axes. From biology, it is known that elongation is achieved by contracting the transverse muscles; shortening by contracting the longitudinal muscles; and bending by contracting both the longitudinal muscles on one side and also the transverse muscles, to resist the longitudinal compression forces caused by that contraction. Meanwhile, the external oblique muscles enable twisting.
“We analysed many different technologies for developing an actuator to embed in our muscles,” says Laschi, ”and we chose an electro active polymer, based on dielectric elastomers.”
The way these work is to have sheets of silicone rubber, 300 microns thick, and to sputter a very thin layer of gold, only 90nm thick, on to both surfaces. The sheet is then cut, folded and embedded in a silicone matrix to produce a stack of silicone rubber sheets, 5mm across, interspersed with electrode layers. Applying a voltage difference to the two sets of electrodes causes these to attract each other. The silicone used is ‘Ecoflex’ 00-30 Platinum Cure Silicone Rubber, made by Smooth-On, which is headquartered in Easton, Pennsylvania, in the US. The development of the actuators is still at an early stage, but first results show that applying 2750V to the electrodes resulted in a contraction of 3.4%. According to Laschi, calculations show that, if the thickness could be reduced to 200 microns, the contraction becomes 8.75%, with a useful force of 0.18N. Moreover, if a reduction of 150 microns could be achieved, the contraction would be as much as 20%, with a useful force of 0.35N.
Importantly, the silicone rubber film has to be kept stretched during the sputtering, in order to guarantee the integrity of the electrodes during folding and in service. After the electrodes have been deposited, the film is allowed to relax, to ensure the gold adheres well.
The goal is to develop actuators that are completely compliant, with no rigid structures, which can elongate or bend at high speeds, and with variable stiffness. The voltage demands may be moderately high, but the power and current requirements are said to be microscopic. Also, a major advantage of biologically inspired systems, according to Professor Julian Vincent of the University of Bath, is that they tend to be extremely energy efficient.
The development has been partly supported by the Italian Institute of Technology Network. Other team members involved in the project are head of department Professor Paolo Dario, biologist Dr Barbara Mazzolai and PhD student Matteo Cianchetti.
Pointers
* Actuators inspired by octopus arms combine muscles with constant volume hydrostats, so that contraction in one direction causes elongation in another
* The most promising type of muscles being experimented with so far are made up of thin layers of silicone rubber, interspersed with electrode layers
* Contraction is achieved by applying opposing voltages to the electrodes so they are pulled towards each other by electrostatic force
* Speeds and energy efficiencies are high, but forces are low. Development continues
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