A tiny motorized positioning device has twice the dexterity of similar devices being developed for applications that include biological sensors and more compact, powerful computer hard drives. The monolithic comb drive, designed by Purdue University assistant professor of electrical and mechanical engineering Jason Clark, might be used as a “nanoscale manipulator” that precisely moves or senses movement and forces. In conventional comb drives, the applied voltage positively charges fingers on one comb and negatively charges those on the other, inducing an attraction between the opposite charges. Removing the voltage lets the spring-loaded comb sections return to an original position. By comparison, the new device has a single structure (monolithic) with two perpendicular comb drives. Clark says the devices also can be used in watery environments for probing biological molecules. Furthermore, Clark calls the device monolithic because it contains comb-drive components that are not mechanically and electrically separate. Conventional comb drives are structurally “decoupled” to separate opposite charges.

Piezoelectric actuators, a competing design, typically deflect, or move just a fraction of a micrometer. Comb drives, on the other hand, can deflect tens to hundreds of micrometers. “And unlike conventional comb drives, which only move in one direction, the new device moves in two - left to right, forward and backward - an improvement that could open doors to many applications,” says Clark. In addition, conventional small sensors detect objects using a probe and a platform to hold a specimen.

Comb drives use a pair of comb-like sections with meshed finger, which are drawn toward each after applying a voltage. The recent design replaces both components with a single unit, which could make it possible to improve a class of probe-based sensors that detect viruses and biological molecules.

The development could let sensors work faster, at higher resolution, and be small enough to fit on a microchip. Higher resolution might be used to design future computer hard drives capable of high-density data storage and retrieval. Another possible use: fabricate or assemble micro and nanoscale machines.

Clark has also found a way to determine the precise deflection and force of such microdevices while reducing heat-induced vibrations that could interfere with measurements. Current probe-based biological sensors have a resolution of about 20 nm.

“Twenty nanometers is about the size of 200 atoms, so if scanning for a particular molecule, it may be hard to find,” he says. “The higher atomic-scale resolution in our design should make it easier to find.”

Proper use of such small devices requires precisely knowing how much force is being applied to comb-drive sensors and how far they are moving. The new design, based on a what Clark calls electro-micro metrology, lets engineers determine a precise displacement and force applied by or to a comb drive. Forces are measured by comparing changes in electrical properties, such as capacitance or voltage.