Micromachines in a spin

Minuscule mills the size of a single red blood cell and driven by light beams are turning the cogs of some of the World’s smallest machines.

Péter Galajda and Pál Ormos of the Biological Research Centre of the Hungarian Academy of Sciences have used light to create, hold and power tiny rotors that could potentially have a big future as pumps and switches.

These ‘microelectromechanical’ systems (MEMS) are typically just a few millimetres across, and their microscopic components might include strips that bend or resonate to detect pressure or sound, or cogs and gears that manoeuvre little mirrors for guiding light beams.

What marks the rotors devised by Galajda and Ormos is that they are five thousandths of a millimetre across, making them as small as a single tooth on a MEMS cog.

MEMS components are generally carved from silicon wafers using masks and etching acids, or electron beams but the Hungarian researchers used a resin, which hardens when exposed to light.

Galajda and Ormos ‘wrote’ the shape of the rotor into liquid resin using a laser beam. The liquid solidified only where the laser spot passed.

With computer control of the laser, they generated complex shapes — precise to half a micron, the size of the laser spot. They then washed away the unhardened resin.

The researchers also created screw-like spoked rotors, which are driven by light in a single direction.

Galajda and Ormos suspended the rotors in a solvent on a glass slide, and held onto them with another laser beam. In a very bright beam, small objects may be immobilised by the light’s electromagnetic field.

In this case, the light did not only trap the rotors; it made them spin. As light bounces off the hard resin, it pushes the spokes around. The twist ensures that all of these impulses induce rotation in the same direction. Held in the laser’s fingers, the rotor spins.

Galajda and Ormos have made a series of cogs using the same light-hardening method, and fixed them onto a glass slide so that their teeth intermesh. One of the cogs, a twisted-spoke rotor, drove the others into motion. Galajda and Ormos suggest that this set-up could be used to make a cell-sized pump or switch.