Based on an oxide material that expands and contracts in response to a small temperature variation, the actuators are said to be smaller than the width of a human hair and are promising for microfluidics, drug delivery and artificial muscles.
‘We believe our micro-actuator is more efficient and powerful than any current micro-scale actuation technology, including human muscle cells,’ said Berkeley Lab and UC Berkeley scientist Junqiao Wu in a statement. ‘What’s more, it uses this very interesting material — vanadium dioxide — and tells us more about the fundamental materials science of phase transitions.’
Wu is corresponding author of a paper appearing in Nano Letters that reports these findings, titled Giant-Amplitude, High-Work Density Microactuators with Phase Transition Activated Nanolayer Bimorphs.
When heated past 67ºC, vanadium dioxide transforms from an insulator to a metal, accompanied by a structural phase transition that shrinks the material in one dimension while expanding in the other two. For many years, researchers have debated whether one of these phase transitions drives the other or if they are separate phenomena that coincidentally occur at the same temperature.
Wu is said to have shed light on this in earlier work published in Physical Review Letters, in which he and his colleagues isolated the two phase transitions in single-crystal nanowires of vanadium dioxide and demonstrated that they are separable and can be driven independently.
The team ran into difficulty with the experiments, however, when the nanowires broke away from their electrode contacts during the structural phase transition.
‘At the transition, a 100-micron-long wire shrinks by about one micron, which can easily break the contact,’ said Wu. ‘So we started to ask the question: this is bad, but can we make a good thing out of it? And actuation is the natural application.’
To take advantage of the shrinkage, the researchers are said to have fabricated a freestanding strip of vanadium dioxide with a chromium metal layer on top. When the strip is heated via a small electrical current or a flash of laser light, the vanadium dioxide contracts and the whole strip bends like a finger.
‘The displacement of our micro-actuator is huge: tens of microns for an actuator length on the same order of magnitude — much bigger than you can get with a piezoelectric device — and simultaneously with very large force,’ said Wu. ‘I am very optimistic that this technology will become competitive with piezoelectric technology and may even replace it.’
Piezoelectric actuators are the industry standard for mechanical actuation on micro scales, but they’re complicated to grow, need large voltages for small displacements and typically involve toxic materials such as lead.
‘But our device is very simple, the material is non-toxic and the displacement is much bigger at a much lower driving voltage,’ said Wu. ‘You can see it move with an optical microscope. And it works equally well in water, making it suitable for biological and microfluidic applications.’
The researchers envision using the micro-actuators as tiny pumps for drug delivery or as mechanical muscles in micro-scale robots.
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