A microscale actuator that flexes like a tiny beckoning finger under a burst of laser light may point toward practical applications in artificial muscles, microfluidics and drug delivery. The Lawrence Berkeley National Laboratory (Berkeley Lab) microactuator is more efficient and powerful than current microscale actuation technology, including human muscle cells, according to Berkeley Lab and University of California, Berkeley, scientist Junqiao Wu. It uses an oxide material that expands or contracts based on the temperature, providing more information about the fundamental materials science of phase transitions, Wu said. The actuators use vanadium dioxide, a strongly correlated material with exotic electronic behaviors such as an unusual pair of phase transitions. When heated above 67 °C, 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. A pulse of laser light can induce the Berkeley Lab microactuator to flex. In this microscope image, a palm-like array of actuators flexes one at a time (top panel) or all at once (middle panel). The lower panel shows individual fingers flexing underwater — a capability that makes the device suitable for biological applications. Images courtesy of Lawrence Berkeley National Lab. “At the transition, a 100-µm-long wire shrinks by about 1 µm, which can easily break the contact,” Wu said. “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.” Taking advantage of the shrinkage, the scientists fabricated a free-standing strip of vanadium dioxide topped with a chromium layer. When the strip is heated with a burst of laser light or a small electrical current, the material contracts, and the whole strip bends like a finger. "The displacement of our microactuator is huge," Wu said. "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." Currently, piezoelectric actuators are the standard for mechanical actuation on microscales, but they are difficult to grow, require large voltages and often use toxic materials such as lead. A single actuator curls and extends as the temperature is changed by 15 °C, as shown in this micrograph. On right, actuators in a palmlike configuration all curl together, opening and closing like a tiny hand. The scale bar is 50 µm. “Our device is very simple, the material is nontoxic, and the displacement is much bigger at a much lower driving voltage,” Wu said. “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 microactuators could also be used as tiny pumps for drug delivery or as mechanical muscles in microscale robots. Wu and his colleagues have already partnered with the Berkeley Sensing and Actuation Center to integrate the actuators into devices for applications such as radiation-detection robots for hazardous environments. Next, they plan to create a torsion actuator — a more challenging feat because it involves a complicated design of gears, shafts and belts. “Here we see that with just a layer of thin film, we could also make a very simple torsional actuator,” Wu said. The findings were reported in Nano Letters (doi: 10.1021/nl303405g). For more information, visit: www.lbl.gov