Twist, bend, or squeeze piezoelectric materials, and they produce electricity–an effect that is used in microphones and telephones. Now, taking advantage of the piezoelectric effect in zinc-oxide nanowires, researchers at Georgia Institute of Technology have made tiny devices that can measure extremely small forces–in the nano-to-piconewton range. That’s about the force involved in interactions between two molecules, says Zhong Lin Wang, the materials-science and engineering professor at Georgia Tech who led the research.
Being able to measure such small forces might lead to devices that could be implanted in the body to measure minute blood-pressure changes continuously, Wang says. The sensors could also be installed on the wings of airplanes and spacecraft to monitor very small pressure fluctuations. And because the current flow through the nanowire responds quickly, it could be used to make a sensitive trigger for car air bags. “If it’s subject to an external force, then it turns off within a microsecond,” Wang says.
The idea of using the piezoelectric effect to make pressure sensors is not new, says Toh-Ming Lu, a professor of applied physics at Rensselaer Polytechnic Institute (RPI). “What’s interesting, really, is doing it at such a small scale–at the nanoscale,” he says.
By connecting the two ends of a zinc-oxide nanowire to electrodes, Wang’s group has made devices similar to the transistors in electronic devices. In an electronic transistor, applying a voltage to the gate electrode controls the flow of current between the source and drain electrodes. In the new pressure-sensing transistor, the two electrodes that the nanowire is connected to act as the source and drain, but there is no gate. Instead of applying a voltage at the gate, one simply bends the wire.
When the nanowire bends, the stretched outer side of the bent wire becomes positively charged, while the compressed inner surface becomes negatively charged. The difference in charges creates a voltage that substitutes for the gate voltage.
Zinc oxide is biocompatible, so one could implant the nanowire pressure sensor in the arm to monitor blood pressure continuously, Wang says. The sensor could transmit the pressure reading to a receiver on one’s watch that displays the data.
Because the device is based on the deformation in a single nanowire, “one could think that the sensitivity can be very high,” says Yi Cui, professor of materials science and engineering at Stanford University.
An advantage of the pressure sensor is that it could be made into a totally self-powered device by combining it with a nanogenerator, which Wang’s group has previously demonstrated. (See “Free Electricity from Nanogenerators.”) The nanogenerator would harness the mechanical energy from pulsing blood vessels and generate electricity that would power the pressure sensor.
The concept could also be applied to other types of sensing. One use for the device could be as a biosensor, Cui says. The principle is that molecules striking or sticking to the nanowires would deform the wire and change the current through it. Researchers could also develop a chemical sensor, in which the chemical reaction disturbs the nanowire, says RPI’s Lu.
The idea is at a laboratory stage right now, and the researchers still need to come up with a design for a self-powered pressure-sensor device. This engineering challenge might not be easy, Lu says. “The basic idea is pretty good,” he says. “Exactly how you would do it–putting it in the body and getting the response, figure out what’s the signal, what’s the noise–that’s always challenging.”