THE SCIENTIST- Optical Cell Sorting
Researchers are using light and new image processing tools for label-free cell characterization.
A highly focused laser beam can separate single cells of a specific type from a mixed population. But it’s difficult to integrate these so-called optical tweezers with the high-throughput need of cell-sorting applications. “The laser has to be aligned perfectly” with the particles of interest, says Yanik, which is difficult when it is focused through a conventional objective lens some distance from a stream of flowing cells. He solved this problem by developing a planar lens that can focus white light to generate an optical force throughout a microfluidic channel. This force is strong enough to immobilize bioparticles belonging to a cell subset of interest as they move through the channel.
Approach: The planar lens comprises tiny (5 microns/side) arrays of round subwavelength holes in a thin metal film that forms one side of a microfluidic chip. Light from a standard halogen source is transmitted through these specially engineered holes, which together act as a nanolens. Their arrangement on the film focuses the light as it emerges, thus delivering a well-controlled beam throughout the chip. When the cells are pumped through the channel, the optical force of this beam is countered by the drag force of the flow, thus separating particles with varying size and refractive indices. The balance between optical and fluidic forces can be adjusted via light intensity to selectively sort particles.
Functionality: Yanik has used this technique to isolate bacterial cells of genetically similar species with subtle differences in protein structure, and to separate rare circulating tumor cells (CTCs) from white blood cells based on size. He warns that “you’ll still need a conventional [fluorescent or antibody label] marking scheme to identify the specific type of CTCs (OSA Technical Digest, doi.org/10.1364/FIO.2015.JTu4A.1, 2015).
Tips: “Any nanophotonic engineer could make this chip,” says Yanik. He notes, though, that the flow channel can clog up with cells that have been trapped by the beam. He advises setting up another cross-channel flow to periodically wash away trapped cells. Also, diluting a blood sample to 25 percent can make it easier to separate particles.
Future Plans: Yanik is using nanohole lenses to develop point-of-care infection monitoring tools that can detect rare biomarkers in small concentrations. In particular, he wants to identify a circulating glycoprotein shed from infectious bacteria using a blood sample taken by a finger prick.