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Putting a spin on microdevices T
nique as a cheap and effective way to here’s now a way to spin a microtum to an object. The slots in the center spin any microscale object. “The angular gear, microvalve, or almost anything are key—they cause the object’s index of else at the microscale level without the refraction to change with direction. When momentum acts like an optical wrench,” says Dholakia. “Normally, you need maneed for special materials or for creating the light beam “looks” along the slots, terials with a special property to asymmetrically shaped objects. do this. But if you have something Steven Neale and colleagues at St cheap, we thought, ‘Wouldn’t it Andrews University (U.K.) can robe great if any laboratory in the tate an object of arbitrary shape world could take a simple beam of with just a beam of laser light. light and make anything spin?’” Their technique promises a whole The innovation of etching slots in toolbox of light-spun microgears, the object means spinnable micro-valves, and -pumps (Nat. Mater. devices can be made of almost any 2005, 4, 530–533). material, as long as it doesn’t abNeale and colleagues made the sorb the laser light. microgears out of SU-8, a polyThe novelty of the microgear mer commonly used in photoliis “marrying areas that have nevthography. They used electron er been married together,” says beam lithography for fabrication, Halina Rubinsztein-Dunlop at the carefully inscribing slots ~0.5 µm University of Queensland (Auswide through the center of the Scanning electron micrograph of a microgear sitting on a tralia), complimenting the combimicrogears. They then transferred substrate. The ribbed structure and the polarization of the nation of the photonics technique each gear to a drop of water on a light are enough to spin the gear. (Adapted with permisof etching slots with the optical glass slide so it could spin freely. sion. Copyright 2005 Nature Publishing Group.) tweezing of a microfluidics device. All it takes to spin the microHowever, Rubinsztein-Dunlop gear is a tightly focused laser. “It’s which are filled with water, it “sees” a dif- cautions that the spinning microgear like a microscopic tractor beam,” says ferent index of refraction than when it isn’t trapped by the laser in all three diKishan Dholakia, one of Neale’s thesis mensions. If the laser is too intense, the advisers. The technique is related to op- looks across the slots and sees the polygear gets pushed against the glass slide, tical tweezing, in which a laser shines on mer material of the gear. If the light is and this limits how fast it can spin. But a small, transparent object with a refrac- polarized, as most lasers are, it “fits” through the slots better in one direction Neale and Dholakia claim their techtive index higher than that of the surthan the other. Linearly polarized light nique isn’t necessarily for spinning obrounding water or air. When the laser will squeeze through the slots and trans- jects continuously. “A lot of people just beam hits the object, the light bends fer a little angular momentum as it does want to open or shut a valve” very pretoward the normal and transfers some so, making the gear rotate a tick. As cisely, says Dholakia. momentum to the object. The part of more light squeezes through, the gear “Imagine needing to handle an obthe beam farthest away from the laser’s rotates more, until the slots line up perject the size of a single red blood cell focus bends the most and transfers the fectly with the linear polarization and within a drop of water—to put it where most momentum, whereas the part of you want it and make it work. That’s the beam directly at the focus bends the the light doesn’t have to squeeze. If the light is circularly polarized, the the advantage of the optical technique,” least. Therefore, the object tends to be gear will continue rotating, spinning as pushed to the laser focus; this means says David Marr at the Colorado School the polarization of the light spins. The that the object can be dragged around of Mines. “It’s really a nice way of solvmore intense the laser, the faster the by moving the laser focus. ing the problem” of how to manipulate gear rotates. micromachines, he says. a The St Andrews team’s technique also The researchers envision their tech—Kim Krieger uses a laser to transfer angular momen376 A
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