A powerful new microscale actuator that can flex like a miniature beckoning finger has been developed by researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California, Berkeley.

It is based on a material that expands and contracts dramatically in response to a small (15 degrees C) temperature variation. It is smaller than the width of a human hair and is promising for microfluidics, drug delivery, and artificial muscles.

The amazing vanadium dioxide

“We believe our microactuator is more efficient and powerful than any current microscale actuation technology, including human muscle cells,” says Berkeley Lab and UC Berkeley scientist Junqiao Wu. “What’s more, it uses this very interesting material — vanadium dioxide (VO2).

Vanadium dioxide is a “strongly correlated” material, meaning the behavior of each electron is inextricably tied to its neighboring electrons. When heated past 67 degrees 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.

Better than piezoelectric actuators

The researchers fabricated a free-standing 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 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 with piezoelectric technology, and may even replace it.”

Piezoelectric actuators are the industry-standard for mechanical actuation on micro scales, but they need large voltages for small displacements, typically involve toxic materials such as lead, and the crystals are complicated to grow.

“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.”

Drug delivery and microrobot muscles

The researchers envision using the new microactuators as tiny pumps for drug delivery or as mechanical muscles in micro-scale robots. In those applications, the actuator’s exceptionally high work density (the power it can deliver per unit volume) offers a great advantage.

Ounce for ounce, the vanadium-dioxide actuators deliver a force 1000 times 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 team’s next goal is to create a torsion (twist) actuator, which is a much more challenging prospect. 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.”

This research was supported in part by the DOE Office of Science; theory and synthesis research was supported by the National Science Foundation.

More information about Junqiao Wu’s research is here.