Sorting tiny particles with light
Novel microfluidic chip design provides several advantages.
David L. Shenkenberg
Various laboratory instruments can sort cells or molecules, such as viruses and proteins, by fluorescent labels or by their inherent properties. By grouping cells and molecules, these instruments can identify disease states, report drug effectiveness and enable forensic analysis or classification of cells and molecules for basic research.
Microfluidic chips can sort numerous cells and molecules rapidly, yet they occupy much less space and cost much less to produce than other high-throughput sorting technologies. Although laser optical trapping using free-space optics can serve as an effective mechanism for sorting in microfluidic chips, free-space optics occupy benchtop space and add to the cost, defeating the purpose of the chips. Therefore, optics such as waveguides have been integrated inside microfluidic chips.
Researchers from the laboratories of David Erickson and Michal Lipson at Cornell University in Ithaca, N.Y., have used a novel design to integrate polymer waveguides inside microfluidic chips. In their design, the waveguide is perpendicular to the fluidic channel, and an evanescent field that surrounds the waveguide serves as the optical trap (Figure 1).
Figure 1. Researchers integrated waveguides inside a microfluidic chip and connected the chip to a fiber-coupled diode laser. The light in the waveguide generated an evanescent field that surrounded the chip, which acted as an optical trap. Images reprinted with permission of Optics Express.
Erickson said that, compared with previous designs, their version more intimately integrates the optical trap with the fluid flow because particles can flow and be trapped in the same channel, whereas previous designs have depended on the particles to drop onto the trap. Therefore, their design enables greater control over the particles, and it improves sorting efficiency and sensitivity, enabling the manipulation of smaller particles while using lower laser power than before, important in part because too much laser power can damage delicate cells and molecules.
The waveguides were made from the polymer SU-8 because it is transparent to the wavelength the scientists employed for optical trapping. Because of this transparency, the evanescent field could enter deeply enough into the channel to manipulate particles. “We could have used any other material in principle,” Lipson said. She added that silicon is another good choice because it has been in widespread industrial use for years, so it can be mass-produced. Also, most tools for waveguides are made for it, and protocols exist for shaping and etching it.
The researchers used an ILX Lightwave 975-nm fiber-coupled laser diode module to couple light into the waveguide, which then generated the evanescent field. They used the field to sort spherical polystyrene beads of various sizes and concentrations. The particles have shapes, sizes and transparencies similar to cells, but are slightly smaller and have a more perfectly spherical shape. The laser light was polarized to accommodate the waveguides, which are made of polarization-dependent material. The researchers monitored the beads with an Olympus upright microscope and a Sony CCD camera.
They demonstrated that the optical trap can move the beads against or with the current or can hold them in place. They showed that raising the laser power trapped the beads and lowering it released them (Figure 2). The optical trap also could manipulate single particles without disturbing others. Although optical trap waveguides sometimes have a critical bend radius at which the beam can no longer control particles, the investigators observed no such limitation in their experiment because the waveguide they used did not have a significant bend.
Figure 2. The researchers were able to hold polystyrene beads in place or to move them with or against the direction of flow.
Sorting by size
The optical trap could move larger particles faster than smaller ones, meaning that it could sort particles or, potentially, cells and biological molecules, by size. It could move 3-μm-diameter beads as fast as 28 μm/s with an estimated 53.5 mW of power at the trapping location. Such high velocities promise to enable rapid sorting of cells and molecules. The greater the optical power and the lower the pressure of the fluid flow, the faster the particles moved.
Lipson said that their design can be expanded to create hundreds or even thousands of channels that can sort particles simultaneously, increasing the throughput by orders of magnitude. Furthermore, the device works with very small volumes of liquid, which is important because large volumes sometimes are not available in biochemical or cellular experiments. The results of the experiment are detailed in the Oct. 29 issue of Optics Express.
Manipulating cells with the device is the next step. “We would need to make sure that all the materials and wavelengths are compatible, that the channels are the right size and that it is completely sensitive to the symmetry of cells,“ Lipson said. The researchers also plan to experiment with various materials for building the device and to make waves that are more “sophisticated” so that they can create greater trapping forces.
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