It gives "auto mechanic" a new meaning. Engineers have devised gears and cogs that make themselves, saving the expense of machining the tiny parts that keep microscopic motors ticking over.
The components are formed from discs that buckle into shape after a change in temperature, and the team behind the study says the technique can produce complicated bevels or curves that are difficult to produce with traditional methods.
For a decade, engineers have been experimenting with materials that spontaneously bend into shape. A thin metal film deposited on top of a heat-expanded polymer such as PDMS will buckle into an ordered array of wrinkles when the polymer is cooled down.
Xi Chen's team at Columbia University in New York City realised the technique could be used to produce the regularly spaced teeth of a gear without having to rely on costly etching and grinding methods.
The team heated a disc of PDMS until it expanded, and then coated it with a thin copper film. When they cooled the disc, the polymer shrank more than the metal, causing the copper to buckle into an array of teeth. Cooling the disc further increased the height of the teeth.
"Theoretically, as long as the film thickness is uniform and the substrate properties are homogeneous, the resulting pattern should be highly regular with uniform teeth spacing and height," says Chen. Using stiffer films that are less likely to buckle can help control the number of teeth formed, he adds.
A hardening agent can be added to firm up the polymer substrate once the teeth have formed, and gears made this way should be durable and strong enough for practical use, Chen says. "Polymer-based microgears are already used in some applications."
Chen thinks, though, the real power of the technique is that it can be modified to produce complicated gears for more sophisticated gear trains. The key is to adjust the polymer's properties so that it shrinks preferentially in one or more directions when it is cooled.
"As long as one can create a preferred wrinkle direction, all kinds of teeth patterns are possible," he says. "For example, complicated gears with zigzag-shaped teeth."
So far the researchers have tested their technique at the macroscale, producing simple and bevelled gears 6 to 25 millimetres across, but Chen says it can be readily extended to the sub-micrometer scale – something he plans to do soon.
"I think this is a creative work and as far as I know, this is the first demonstration of building gears with this type of self-assembly," says Babak Parviz, a nanotechnologist at the University of Washington in Seattle, US, who specialises in self-assembly. "Building angled structures on flat surfaces is difficult with controlled etching, so this method could help in building these kinds of structures."
Journal reference: Soft Matter (DOI: 10.1039/b904635f)
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