On/Off Functionality for Fast, Sensitive, Ultra-Small Technologies

Osaka University, Osaka, Japan

Bright-field microscopy image of a VO₂ chevron-type planar actuator. Superposition in false color of the tip of the shuttle at low and high temperature (bar, 1 μm).

To turn an ultra-small component on and off, one requires an actuator — a device that transmits an input, such as electricity, into physical motion. Actuators in small-scale technologies to date have critical limitations; for example, if it's difficult to integrate the actuator into semiconductor electronics, real-world applications of the technology will be limited. An actuator design that operates quickly, has precise on/off control, and is compatible with modern electronics would be immensely useful.

Researchers have developed an actuator with high sensitivity, fast on/off response, and nanometer-scale precision that is based on vanadium oxide crystals. Many current technologies use a property of vanadium oxide known as the phase transition to cause out-of-plane bending motions within small-scale devices; for example, such actuators are useful in ultra-small mirrors. Using the phase transition to cause inplane bending is far more difficult but would be useful in ultra-small grippers in medicine.

At 68 °C, vanadium oxide undergoes a sharp monoclinic-to-rutile phase transition that's useful in microscale technologies. The team used a chevron-type (sawtooth) device geometry to amplify in-plane bending of the crystal. Using a two-step protocol, they fabricated a 15-micrometer-long vanadium oxide crystal attached by a series of 10-micrometer arms to a fixed frame. By means of a phase transition caused by a readily attainable stimulus — a 10 °C temperature change — the crystal moves 225 nanometers in-plane. The expansion behavior is highly reproducible over thousands of cycles and several months.

The actuator also was moved in-plane in response to a laser beam. The on/off response time was a fraction of a millisecond near the phase transition temperature, with little change at other temperatures.

Small-scale technologies such as advanced implanted drug delivery devices wouldn't work without the ability to rapidly turn them on and off. The underlying principle of the actuator — a reversible phase transition for on/off, in-plane motion — will dramatically expand the utility of many modern technologies such as microrobotics.

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