Sweating the small stuff Home

Holographic Optical Trapping

By Charles Choi
United Press International

Scientists have found a simple way to use light to manipulate one of the most important building blocks of future technologies: carbon nanotubes.

Experts said the technique could lead to the mass manufacture of a new generation of novel devices.

"It's like having hands in the microscopic world," said researcher David Grier, a physicist at New York University, one of the participating institutions. "It's a new platform for doing things on small materials on a large scale."

Nanotubes are pipes only nanometers -- or billionths of a meter -- in diameter. Carbon nanotubes, which are chemical relatives of diamonds -- comprise some of the strongest materials known. In addition, the microscopic strands can conduct electricity better than copper.

Industrial giants such as Intel, Motorola, Samsung and DuPont -- along with academic institutions and government agencies worldwide -- are researching carbon nanotubes as key ingredients for tomorrow's nanotechnology devices.

For example, IBM is working on transistors made with the nanotubes that could prove far smaller and more powerful than current devices. Analysts at MindBranch, a research firm in Williamstown, Mass., predict the global market for carbon nanotubes could reach $700 million as early as 2005, impacting multi-billion dollar industries developing advanced, super-strong composites and high-definition video displays.

One crucial obstacle to progress, however, has been manipulating the nanotubes, which are smaller than a wavelength of visible light, making them extremely hard to grasp and difficult to structure or assemble into devices.

Now researchers find they might be able to tinker with nanotubes and other microscopic objects -- including cells -- using lasers and holograms.

"We can move however we want things from hundreds of micrometers to only a few nanometers in size -- grab them, turn them, almost feel them, chemically transform them, reorganize them -- all under a conventional light microscope," Grier told United Press International. "It's as simple to use as a point and click interface on a computer."

When a ray of light passes through a transparent object, it often gets bent. This bending imparts a little push on the object. Although the force involved is extraordinarily small, it can be quite useful for moving things of commensurate size, such as cells.

The concept is not really new. For nearly two decades, scientists have been using lasers as so-called optical tweezers. Researchers use wavelengths of light to which their targets are transparent, since objects that absorb wavelengths will heat and eventually burn up. "Most biological things are fortunately transparent over at least some wavelengths of light. We're sitting here bathed by light from the sun, and if we absorbed it all, we'd fry," Grier explained.

Originally scientists were limited to generating only one or two tweezers at a time in a given microscopic space. Grier and colleagues at Arryx Inc., the Chicago firm partnering with NYU in the project, developed a way to generate up to hundreds of optical tweezer rays at once by sending laser beams through computer-generated, animated holograms -- three-dimensional recordings of an image.

The holographic optical trap device they patented fits inside a microscope and can steer and experiment with cells with remarkable control.

"In a real sense, the difference between optical tweezers and holographic optical trapping is the difference between grabbing something with physical tweezers or reaching in with your hands," Grier said. "Put another way, a single beam of light is like Star Trek's tractor beam, while the holographic optical trap is more like Star Trek's holodeck -- a completely immersive 3-D experience -- if you're E. coli bacteria."

In findings submitted to international physics journal Optics Express, Grier and colleagues reported their device can steer bundles of carbon nanotubes as well using green lasers. This is surprising because nanotubes are dozens of times smaller than the light's wavelength. Moreover, nanotubes experience a great deal of jostling from molecules in their surroundings. They are buffeted from all sides.

"Grabbing a nanotube with light is a lot like trying to snatch a hair in a hurricane while wearing an oven mitt. You wouldn't expect to be able to do that -- and yet it works," Grier said. "Holographic trapping is like being able to reach into this tiny world with your hands and scissors and screwdrivers and move things around as you will. It's early days yet, but I anticipate this unique and unprecedented capability to be revolutionary in many areas."

As far as Grier knows, no one has trapped anything this size with optical tweezers before.

"The fact that you can is surprising and interesting and important," physicist David Weitz at Harvard University, Cambridge, Mass., told UPI.

Still, Grier said this new technique cannot trap single nanotubes, only bundles. Though the technique might not replace alternate ways of assembling nanotubes into delicate structures, he added it could complement these other methods by acting like a dump truck for nanotubes in the future electronics industry.

"We have yet to actually assemble a nanotube-based device using optical techniques. That's in the works," Grier said. "There's a huge interest in nanotubes."

Arryx is developing commercial and lab applications for the device. Because different wavelengths of light affect different sized objects, now that researchers have shown holograms can manipulate objects as small as nanotubes, Grier suggested the gadget could sort chemicals in a mixture from one another for a multimillion-dollar impact.

Conventional industrial sorting techniques -- used, for example, to purify drugs -- often require step after step to extract one size of molecules from another. "This can do this on a continuous flow," Grier said.

Arryx already has sold its devices to government researchers at the National Institute of Standards and Technology in Gaithersburg, Md., academic investigators at Emory University in Atlanta, and Japanese scientists.

"We've had companies express interest in the electronics industry, food industry, agriculture and pharmaceutical industries. All of those industries are involved in manipulating things on the microscopic scale, most obviously with pharmaceuticals and electronics," Arryx president and Chief Executive Officer Lewis Gruber told UPI.

Arryx also is finalizing a partnership for large-scale use of their device to sort cattle sperm, dividing batches into X-chromosome-carrying cells that would lead to female cows and Y-chromosome-loaded sperm that would birth male bulls.

Similarly, the food and agriculture industries look for microscopic contaminants such as bacteria and toxins. "We sample them to detect problems or sort them," Gruber said. "A product we're developing sorts sperm so the agriculture industry can determine the sex of cattle to make more money. This way dairy farmers can have cows and beef farmers can have bulls that have more meat." X chromosomes are larger than Y chromosomes, so the cell sorters can tell apart sperm based on sized.

Although conventional devices can sort cells a maximum of 300,000 cells per second, "we're at 3 million cells per second," Gruber said, adding Arryx expects by next year to have a cell sorter that works at 10 million cells per second.

Grier said he envisions the device assembling such devices as microscopic sensors to detect pollutants, toxins, or germs, with hundreds of different sensors fitting on the head of a pin. "You could bolt them to a Palm Pilot to monitor the air or water," he said.

Grier also said he hopes to improve the precision of the device with new arrays of microscopic mirrors under development from the U.S. Defense Advanced Research Projects Agency, which invented such arrays for use in fighter jet lasers. Such mirrors would allow the device to create holograms with five or 10 times higher resolutions.

Charles Choi covers research and technology for UPI Science News.
E-mail sciencemail@upi.com