Dye brightens micromachines

By Chhavi Sachdev, Technology Research News

A grain of rice with your name written on it makes a nice present. But you might be able to one-up that gift with a microscopic sculpture that glows.

Researchers at Osaka University in Japan have found a way to to fashion tiny, fluorescent objects using viscous resin and a laser.

The objects could be used to construct micromachines with illuminated innards. This would allow researchers to examine the insides of microstructures -- something not possible with today's optical and scanning tunneling microscopes, said Hong-Bo Sun, a researcher at Osaka University.

The florescence sheds light on the internal joints and mechanisms of micro objects, said Sun. Just as the light emanating from the surface of a glowing structure makes it visible, the absence of light, for instance, could prove that a tiny passageway is clear, he said.

To demonstrate the technique, the researchers shaped a pair of microscopic fluorescent objects: a gear about 10 microns in diameter and 5 microns thick, and a hollow, 20-sided polyhedron about 8 microns across. A micron is a thousandth of a millimeter, and a red blood cell is about 5 microns in diameter.

The tool could be used to make optical switches and laser diodes used for telecommunications, said Sun. It could also eventually be also used to craft miniscule machines that travel in and through blood vessels, said Satoshi Kawata, a professor of applied physics at Osaka University.

The researchers used very short laser pulses to cast acrylic resin injected with a fluorescent dye into different shapes. The laser pulses lasted 150 femtoseconds, or trillionths of a second, and the lightwaves were in the near-infrared range, where light fades to heat.

They focused the laser into the center of the transparent, viscous liquid, causing molecules there to absorb a pair of photons, a process called two-photon photopolymerization.

Some molecules absorb two photons at once when they're hit with intense, highly focused laser beams. When this happens, the pair of photons paradoxically behave like one photon of a shorter wavelength, affecting a smaller area than a single photon.

Photopolymerization is a chain reaction that connects isolated monomers, or small molecules, to form interconnected macromolecules. "In short, small molecules grow into macromolecules," said Sun.

Using this process, the researchers sketched three-dimensional structures inside a drop of liquid resin with a computer-controlled beam scanner. "We use a developer to remove the unpolymerized liquid, and the solid... remains. That is the desired structure," said Sun.

The challenge was to find a dye that would retain its fluorescent nature throughout the fabrication process, he said. Many fluorescent dyes can be bleached away by strong light, he said.

The photopolymerization tool is accurate to 150 nanometers with a spatial resolution of 120 nanometers, which is finer than that of a laser, said Kawata. A nanometer is one millionth of a millimeter and one thousandth of a micron.

Spatial resolution is the size of the smallest voxel that can be made. A voxel, or volume element, is the three-dimensional equivalent of a pixel, or dot on a computer screen. Better spatial resolution means a smaller voxel size and higher fabrication accuracy, said Sun. Commercially available rapid prototyping systems have resolutions of between 20 and 100 microns, said Sun.

Incorporating fluorescent dye into the resin system used for two-photon polymerization is a nice extension of the researchers' work, said Kevin D. Belfield, an associate professor of chemistry and optics at the University of Central Florida.

The novel aspect of the work is the suggestion that microstructures may behave similarly to photonic crystals, and that by this process the researchers might also be able to make microscopic lasers, said Belfield.

The system could be commercially available within two years; nanostructures made by this method could be in use 5 to 6 years from now, Kawata said.

Kawata and Sunís research colleagues were Tomokazu Tanaka and Kenji Takada at the University of Osaka. The project was funded by the university. The researchers published their findings in the September 3 issue of the journal Applied Physics Letters and the August 16 issue of the journal Nature.

Timeline:  2-3 years
Funding:   University
TRN Categories:  Microelectromechanical Systems (MEMS)
Story Type:   News
Related Elements:  Technical papers, "Two-photon photopolymerization and diagnosis of three-dimensional microstructures containing fluorescent dyes," Applied Physics Letters, September 3, 2001; "Finer Features for Functional Microdevices," Nature, August 16, 2001.




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November 7, 2001

Page One

Hubs key to Net viruses

Electrified water spins gold into wire

Virtual reality gets easier

Laser emits linked photons

Dye brightens micromachines

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