Focusing power: This chip, mounted with paperclips on a microscope objective for observation, is patterned with gold films 500 nanometers wide. When light shines on the gold lines through a prism underneath the chip, it forms surface energy waves that can trap particles and push them along.
“If you want to do cell sorting, silicon optics is a good path,” says Tom Perkins, a physicist at the National Institute of Standards and Technology in Boulder, CO. The advantage of silicon systems over conventional optical traps, Perkins says, is compatibility both with microfluidics and with the manufacturing methods already in place for making computer chips.
A third design of Crozier’s is based on gold structures that can generate a form of light energy called plasmons. When a smooth gold film is illuminated, the light couples to the surface in the form of surface waves called plasmons; the forces generated by these waves are very localized and very strong. Crozier has demonstrated that long, tapered gold films patterned on silicon chips can, when illuminated by light shining through a small prism, be used to pull a particle down and then push it along the gold surface. By changing the angle of the light, it’s possible to control a particle’s speed. This type of structure will be particularly useful for cell sorting, Crozier says.
These types of systems might eventually replace clinical-laboratory devices called flow cytometers, says Holger Schmidt, professor of electrical engineering and director of the Keck Center for Nanoscale Optofluidics at the University of California, Santa Cruz. Today’s flow cytometers use bulky optical systems to separate cells in, say, a blood sample based on their size and shape. Chip-scale optics could do the same thing but would cost much less and might be portable, allowing them to be brought to a patient’s bedside. Schmidt, who’s developed compact, sensitive optical systems for trapping cell organelles and detecting single virus particles, says these compact optical traps might be on the market in as few as three to five years.