When illuminated by laser light shining through a prism, a silicon chip coated with a gold film (center of apparatus) can pull particles out of a liquid solution flowing over the top. (Photo: Kenneth Crozier, Harvard University.)
Tiny chip-based optical devices that can attract particles out of a liquid using the force of photons could enable scientists to image and identify disease cells without the use of microscopes and lasers. Developed by a team of physicists at Harvard University (Cambridge, MA) led by Kenneth Crozier, associate professor of electrical engineering, these optical traps are designed to be integrated with microfluidic devices, some of which are currently in clinical trials for diagnosing cancer and monitoring patient response to therapies.
Usually costing tens of thousands of dollars, traditional optical traps require powerful lasers and microscopes to focus light onto particles as small as a single atom. In contrast, photons can transfer their momentum to atoms, molecules, or cells, enabling physicists to control the particle’s movement by holding it steady or by pulling on it to monitor its response. The Harvard group hopes to integrate these optical traps into microfluidic devices for sorting and imaging disease cells in the blood.
The optical traps developed by Crozier and Harvard researchers Ethan Schonbrun and Kai Wang can trap particles as strongly as more-complex systems. Microfluidic chips shuttle cells around in a fluid and typically control their movements using physical barriers and variations in pressure and voltage. The Harvard team’s optical traps can pull cells down to the surface of a chip for observation and then use them to sort the cells based on their identity.
Using manufacturing techniques common to the semiconducting industry, the Harvard researchers patterned chips with two different designs. One design is a silicon chip patterned with a ring with a radius of five µm. When illuminated by a laser, light resonates around the ring, generating an optical force that can pull particles from liquid flowing above the chip. The other design consists of a chip patterned with arrays of 64 bull’s-eye patterns. When illuminated, each of these can trap a flowing particle. Each pattern has the function of a confocal microscope and could be used to get a 3-D picture of a cell, Crozier explains.
Crozier’s team has developed a third design based on gold structures that can generate a form of light energy, or surface waves, called plasmons. When a smooth gold film is illuminated, the light couples to the surface in the form of plasmons, which generate forces that are very localized and strong. The Harvard researchers have shown that when long tapered gold films patterned on silicon chips are illuminated by light shining through a small prism, they can used to pull a particle down and then push it along the gold surface. By changing the angle of the light, the particle’s speed can be controlled. This type of structure will be particularly useful for cell sorting, Crozier remarks.
These types of systems might eventually replace clinical-laboratory devices called flow cytometers, says Holger Schmidt, professor of electrical engineering and director of the W. M. Keck Center for Nanoscale Optofluidics at the University of California, Santa Cruz. Today’s flow cytometers use bulky optical systems to separate cells in blood samples based on their size and shape. Chip-scale optics could do the same thing, but as portable devices, they could be brought to a patient’s bedside. These compact optical traps might be on the market in three to five years, notes Schmidt.
