Researchers at UC Berkeley have developed a method of sorting and separating microparticles and cells using little more than a bright light and a photosensitive surface. The system, referred to as "optoelectronic tweezers," can control literally thousands of particles in parallel, and allows for the differentiation between living and dead cells. The system can even be used to move particles around like conveyor belts.
The idea is similar to that used in the ubiquitous office copier machine. In xerography, a document is scanned and transferred onto a photosensitive drum, which attracts dyes of carbon particles that are rolled onto a piece of paper to reproduce the image.
In this case, the researchers use a photosensitive surface made of amorphous silicon, a common material used in solar cells and flat-panel displays. Microscopic polystyrene particles suspended in a liquid were sandwiched between a piece of glass and the photoconductive material. Wherever light would hit the photosensitive material, it would behave like a conducting electrode, while areas not exposed to light would behave like a non-conducting insulator. Once a light source is removed, the photosensitive material returns to normal.
Depending upon the properties of the particles or cells being studied, they will either be attracted to or repelled by the electric field generated by the optoelectronic tweezer. Either way, the researchers can use that behavior to scoot particles where they want them to go.
The research paper, published in the July 21 Nature, is available to read for free online. The press release from Berkeley is more oriented to a non-specialist audience, and explains the process involved fairly well. This graphic shows the general structure of the optoelectronic tweezer system, but the movie of micrometer-scale polystyrene particles being shuffled and sorted by projected light is just amazing: 1.4MB streaming Quicktime; 9.3MB MPEG-4. Better still, particles encased in projected light "cages," and moved around almost like Tetris blocks: 1.3MB streaming Quicktime; 6.4MB MPEG-4.
Current techniques for separating and moving cells and particles require painstaking efforts and, in the case of previous "optical tweezer" designs, 100,000 times as much energy. Traditional sorting technologies also tend to disrupt the medium in which the particle/cell sits, and can damage the particle/cell itself. Optoelectronic tweezers leave both the medium and the target item untouched. Separation and isolation of cells and particles is standard practice in a number of scientific studies; the optoelectronic tweezers will give an immediate boost to both biomedical and material science research.