[tt] MIT develops 'tractor beam' for cells | Science Blog

[Brian Atkins]

http://www.scienceblog.com/cms/mit-develops-tractor-beam-cells-14659.html

In a feat that seems like something out of a microscopic version of Star Trek, 
MIT researchers have found a way to use a “tractor beam” of light to pick up, 
hold, and move around individual cells and other objects on the surface of a 
microchip.

The new technology could become an important tool for both biological research 
and materials research, say Matthew J. Lang and David C. Appleyard, whose work 
is being published in an upcoming issue of the journal Lab on a Chip. Lang is an 
assistant professor in the Department of Biological Engineering and the 
Department of Mechanical Engineering. Appleyard is a graduate student in 
Biological Engineering.

The idea of using light beams as tweezers to manipulate cells and tiny objects 
has been around for at least 30 years. But the MIT researchers have found a way 
to combine this powerful tool for moving, controlling and measuring objects with 
the highly versatile world of microchip design and manufacturing.

Optical tweezers, as the technology is known, represent “one of the world's 
smallest microtools,” says Lang. “Now, we're applying it to building [things] on 
a chip.”

Says Appleyard, “We've shown that you could merge everything people are doing 
with optical trapping with all the exciting things you can do on a silicon 
wafer…There could be lots of uses at the biology-and-electronics interface.”

For example, he said, many people are studying how neurons communicate by 
depositing them on microchips where electrical circuits etched into the chips 
monitor their electrical behavior. “They randomly put cells down on a surface, 
and hope one lands on [or near] a [sensor] so its activity can be measured. With 
[our technology], you can put the cell right down next to the sensors.” Not only 
can motions be precisely controlled with the device, but it can also provide 
very precise measurements of a cell's position.

Optical tweezers use the tiny force of a beam of light from a laser to push 
around and control tiny objects, from cells to plastic beads. They usually work 
on a glass surface mounted inside a microscope so that the effects can be observed.

But silicon chips are opaque to light, so applying this technique to them not an 
obvious move, the researchers say, since the optical tweezers use light beams 
that have to travel through the material to reach the working surface. The key 
to making it work in a chip is that silicon is transparent to infrared 
wavelengths of light - which can be easily produced by lasers, and used instead 
of the visible light beams.

To develop the system, Lang and Appleyard weren't sure what thickness and 
surface texture of wafers, the thin silicon slices used to manufacture 
microchips, would work best, and the devices are expensive and usually available 
only in quantity. “Being at MIT, where there is such a strength in 
microfabrication, I was able to get wafers that had been thrown out,” Appleyard 
says. “I posted signs saying, 'I'm looking for your broken wafers'.”

After testing different samples to determine which worked best, they were able 
to order a set that were just right for the work. They then tested the system 
with a variety of cells and tiny beads, including some that were large by the 
standards of optical tweezer work. They were able to manipulate a square with a 
hollow center that was 20 micrometers, or millionths of a meter, across - 
allowing them to demonstrate that even larger objects could be moved and 
rotated. Other test objects had dimensions of only a few nanometers, or 
billionths of a meter. Virtually all living cells come in sizes that fall within 
that nanometer-to-micrometers range and are thus subject to being manipulated by 
the system.

As a demonstration of the system's versatility, Appleyard says, they set it up 
to collect and hold 16 tiny living E. coli cells at once on a microchip, forming 
them into the letters MIT.

-- 
Brian Atkins
Singularity Institute for Artificial Intelligence
http://www.singinst.org/