Researchers at Lawrence Berkeley National Laboratory and University of California, Berkeley, have devised a new simple yet powerful microscale actuator that can flex like a tiny finger. Based on a material that abruptly expands and contracts in response to small temperature variations, the actuators are smaller than the width of a human hair and are promising for microfluidics, drug delivery, and artificial muscles.
“We believe our microactuator is more efficient and powerful than any current microscale actuation technology, including human muscle cells,” says Junqiao Wu of UC Berkeley. “What’s more, it uses this very interesting material—vanadium dioxide—and tells us more about the fundamental materials science of phase transitions.”
Wu and his colleagues came upon the microactuator idea by accident, while studying an unrelated problem. Vanadium dioxide is a classic example of a strongly correlated material, one in which the behavior of each electron is tied to its neighboring electrons. The exotic resulting electronic behaviors have made vanadium dioxide a subject of scientific investigation for decades, most of it focused on an unusual pair of phase transitions.
Independent Phase Transitions
When it is heated over 67 degrees Celsius, vanadium dioxide transforms from an insulator to a metal, together with a structural phase transition that shrivels the material in one dimension while expanding in the other two. For decades, 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.
Junqiao Wu clarified this question in earlier work, in which he and colleagues isolated the two phase transitions in single-crystal nanowires of vanadium dioxide and showed 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 1 micron, which can easily break the contact,” says 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.”
Microfluidic and Biological Applications
To benefit from the reduction, researchers manufactured a free-standing strip of vanadium dioxide with a chromium metal layer on top. When the strip is heated by a flash of laser light or a small electrical current, the vanadium dioxide contracts and the whole strip bends like a finger.
“The displacement of our microactuator is huge,” says Wu, “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. I am very optimistic that this technology will become competitive to piezoelectric technology, and may even replace it.”
Piezoelectric actuators are the norm 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,” says 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 hope the microactuators could one day be used as tiny pumps for drug delivery or mechanical muscles in micro-scale robots. In those applications, the actuator’s extraordinarily high work density, the power delivered per unit volume, offers a great advantage.
Ounce for ounce, the vanadium-dioxide actuators deliver a force three orders of magnitude greater than human muscle. Wu and his colleagues are already partnering with the Berkeley Sensing and Actuation Center to integrate their actuators into devices for applications such as radiation-detection robots for hazardous environments.
The next goal for the researchers is to create a torsion actuator, a much more challenging scenario. Wu explains: “Torsion actuators typically involve a complicated design of gears, shafts and/or belts, and so miniaturization is a challenge. But here we see that with just a layer of thin-film we could also make a very simple torsional actuator.”
“Giant-Amplitude, High-Work Density Microactuators with Phase Transition Activated Nanolayer Bimorphs”
Kai Liu, Chun Cheng, Zhenting Cheng, Kevin Wang , Ramamoorthy Ramesh, and Junqiao Wu
Nano Lett., 2012, 12 (12), pp 6302–6308 DOI: 10.1021/nl303405g