Micomachines All the Rage
By Virginia Heffernan

In a technological age that promises to miniaturize everything from motors to computer chips, nickel is expected to play an increasingly valuable role.

An emerging field known as micro-electro-mechanical systems (MEMs) uses tiny components - some no larger than a grain of sand - to perform complex tasks. A handful of MEMs have already found commercial applications, from pressure transducers for medical monitoring to sensors designed to activate airbags in cars.

Industry observers predict that MEMs will do for mechanical components what microchips did for electronics. As a result, the the MEMs market will grow to more than US$5 billion by 2002, forecasts Cronos Integrated Microsystems, the first commercial enterprise to focus exclusively on MEM products.

Some microstructures, including those applicable to biomedicine (stents and protheses), defense systems (sensors to detect chemical and biological weapons, for example) and portable consumer products, require 3-D metallic components. This is where nickel comes in.

Electrodeposited nickel gives 3-D components structural integrity. In other words, microelectrodeposition is the small-scale equivalent of the casting, forging, and welding processes used to manufacture mechanical structures at the macro level, says George Whitesides, professor of chemistry at Harvard University.

"The actual amount of nickel that's involved is minuscule, but the value that the nickel provides is very high," says Whitesides.

Whitesides is testing nickel for use in 3-D metallic structures ranging from heat exchangers to components of small aircraft. His team combines electrodeposition with soft lithography - a set of techniques used to transfer patterns - to build the microstructures.

"These structures are hard to make by conventional procedures," says Whitesides. "The reason for working with nickel is that it responds to electrochemistry, has good mechanical and corrosion properties and is inexpensive. It is strong and cheap and easily processed in this particular style."

For example, one technique routinely produces metal features at the 1-to-100-micrometer scale. Electrodeposition then transforms the planar metallic structures into miniature 3-D devices by welding the separate 2-D components together.

Nickel could even be used in biomedical applications, such as implants that dispense drugs, though the metal would likely be coated to prevent allergic reactions. Nickel's magnetic properties also make it a natural choice for magnetic applications.

Alternative materials for 3-D applications include welded copper, material that has been machined out of silicon and, for biomedical purposes, stainless steel, titanium, and gold-plated materials.

By testing a number of different materials, the Harvard team hopes to develop a range of microstructures that cannot be made cost-effectively by conventional means.

"We've reached the point where we can demonstrate clearly that one can make interesting structures," says Whitesides. "The question is, are these structures sufficiently interesting that they will be worth someone's effort to commercialize?"

Whitesides says once the components pass applications testing, commercialization should be relatively rapid because the devices are uncomplicated and inexpensive to produce.