NEMS: Machines Get Tiny
Nanoelectromechanical systems begin to flex their muscles.
Ultrasmall machines are everywhere these days. Tiny mechanical devices, so minute that a hundred thousand could sit on a pencil eraser, are responsible for triggering your airbags during an accident, spitting colors out in precise detail on your inkjet printer and projecting light in the newest digital theaters. Made with the same silicon fabrication methods used to crank out computer chips, microelectromechanical systems (or MEMS) have over the last decade become well embedded in the high-tech landscape.
Now engineers and physicists are taking the next step in machine miniaturization, building mechanical devices on the nanometer scale (a billionth of a meter). If the researchers succeed, their work could lead to ultrasensitive sensors that can detect even the most subtle genetic alterations responsible for a disease, or to ultrastrong artificial muscles that might replace damaged human tissue or power tiny robots.
This next frontier in mechanization is called nanoelectromechanical systems (or NEMS). “With MEMS, you could make a mirror and it was still a mirror, just smaller,” says physicist Harold Craighead, who directs the Nanobiotechnology Center at Cornell University. “But with NEMS, the whole interaction of matter with light is different. You get completely new physical properties, and that’s a big opportunity for new devices.”
Why is small so beautiful? For one thing, you can pack more devices into a given space. It’s the same idea behind making smaller and more powerful computers by squeezing tens of millions of transistors onto a semiconductor chip. But nanomachines offer special advantages. For one thing, they operate at the same size scale as biological molecules (a DNA molecule is about two nanometers in diameter), allowing the devices to directly interact with biological systems.
A simple vibrating cantilever underpins many nanomachines. With electron beams and chemical etches, researchers carve away the material around a finger of silicon, leaving one end anchored in the substrate. As any mechanical engineer can tell you, this bar will have a well-defined resonant frequency-it will be, in essence, a Lilliputian tuning fork. Nearly anything that affects the vibration frequency of the cantilever can be detected. Add a little extra mass, and the lever will decrease its resonant frequency, making it a highly sensitive mass balance. “With nanometer cantilevers we estimate we can detect a mass change of a single atom,” says Jim Gimzewski, a leader of the IBM Zrich Research Lab’s program in nanomechanical devices.
Others are building nanomachines out of tiny hollow pipes called carbon nanotubes (see “Wires of Wonder,” TR March 2001). At Honeywell Labs in Morristown, NJ, materials scientist Ray Baughman and his coworkers have fabricated a sheet of aligned nanotubes that bends in response to an electrical charge. In the jargon of mechanical engineering, the sheet can act as an ultrastrong actuator, a mechanical device for moving or controlling something. Nanotubes outperform natural muscle and in theory could top even the most powerful ceramic actuator materials. The scientists also discovered that the nano device can, like other actuators, act in reverse; in that mode, the mechanical bending is transformed into electrical power. Baughman envisions that sheets of aligned nanotubes could be placed in the ocean like fronds of seaweed, with the wave motion used to generate electrical power.
The day when such nanomachines become as prevalent as the microscale versions remains years off. But those who have observed the unfolding of the micromachine revolution say the nano research is well worth keeping an eye on. “Nano today is where MEMS was 10 years ago,” says Al Pisano, a professor of mechanical engineering at the University of California, Berkeley, who headed the U.S. Defense Advanced Research Projects Agency’s MEMS program from 1997 to 1999. “I’ve watched MEMS go from what some people thought was a farce to dead serious startup companies with major investment. Nano is a tougher game to win, but we’ve got a lot more resources and momentum now.”