Microactuator gives microrobotics a flexible finger

Ashley N. Rice, ashley.rice@photonics.com

A powerful new microscale actuator that flexes like a miniature beckoning finger when heated with laser light can deliver a force three orders of magnitude greater than human muscle. The discovery may point toward practical applications in artificial muscles, microfluidics and drug delivery.

The Lawrence Berkeley National Laboratory (Berkeley Lab) microactuator uses a vanadium dioxide material – a strongly correlated substance with exotic electronic behaviors such as an unusual pair of phase transitions – that expands and contracts dramatically in response to slight temperature variations. When heated above 67 ºC, 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.

Taking advantage of the shrinkage, the researchers fabricated a free-standing strip of vanadium dioxide topped with a chromium layer. 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.

A pulse of laser light can induce the Berkeley Lab microactuator to flex. A single actuator curls and extends as the temperature is changed by 15 ºC, as shown in this micrograph. On the right, actuators in a palmlike configuration all curl together, opening and closing like a tiny hand. Courtesy of Lawrence Berkeley National Lab.

“The displacement of our microactuator is huge,” said Berkeley Lab and University of California, Berkeley, scientist Junqiao 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.”

Although the technology is promising, the team must overcome some hurdles for the microactuators to replace piezoelectric technology.

“There are several challenges for achieving this goal, such as the controllability of actuation and the integration into devices, which may require us to cooperate with some specialists in other research fields,” Dr. Kai Liu of Wu’s lab told BioPhotonics. “Typically, it’s a relatively long time for a new technique to replace a very mature technique, maybe several years or even several decades. At present, it’s hard to say how far or how long it will be, but we believe there is such a possibility.”

The microactuators work well in water, making them suitable for biological and microfluidic applications. They could also be used as tiny pumps for drug delivery, but Liu believes the “technology may be firstly applicable in MEMS [microelectromechanical systems] and microrobotics.”

The researchers 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.

As for what’s next, “I am developing a new type of torsional actuator based on this material,” Liu said, a more challenging feat because it involves a complicated design of gears, shafts and belts. “Kevin [Wang] utilized some polymers and vanadium dioxide nanobeams to fabricate hybrid actuating materials, which are potentially useful in microfluidic and biomedical applications.”

The findings were reported in Nano Letters (doi: 10.1021/nl303405gm).