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RESEARCH PROFILES Dynamic array cytometer: Grabbing the bull by the horns In flow cytometry, cells are driven through microfabricated channels like cattle being guided into a holding pen. Until now, being able to corral a particular cell in the midst of this herd has been almost impossible. However, Joel Voldman and colleagues at the Massachusetts Institute of Technology may have developed a new way to “grab the bull by the horns.” In the Aug. 15 issue of Analytical Chemistry (3984–3990), Voldman describes a new microfabricated dynamic array cytometer (µDAC) that allows researchers to sort and hold individual cells for analysis over longer periods of time. Although scientists can sort cells relatively quickly using conventional flow cytometry, its inherent flowing nature only allows them to examine cells one at a time. On the other hand, current microscopy instruments can hold a group of cells for observation but cannot sort them. However, with the µDAC, re(a)
searchers may finally be able to do both techniques at the same time. Other researchers are also trying to overcome the limitations of conventional cytometry and microscopy. For example, Stephen Quake and colleagues at the California Institute of Technology recently introduced an integrated microfluidic cytometer that uses reverse cell sorting to trap particular cells (Anal. Chem. 2002, 74, 2451–2457). As a target cell passes through the detection window, valves stop the flow of fluid, and reverse pumping brings the cell back. When the cell is detected again, the cycle of forward and reverse pumping can continue, or the cell can be sorted by directing the flow toward the waste chamber or the collection chamber. Although effective, this is an intensive, multistep process that doesn’t hold the cell exactly in place. With Voldman’s one-step µDAC system, cells are loaded into a cytometer and introduced into microfabricated
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(a) A schematic representation of the µDAC that shows single cells being loaded and sorted. Scanning electron micrographs depict (b) a single trap, which consists of four gold electrodes, and (c) a 1 8 trap array. Scale bars: 20 µm 458 A
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chambers—2 mm wide 8 mm long 150 µm high—via a buffered salt solution. There, the cells are collected in ion traps, each made of four tapered, cylindrical gold electrodes, which measure 18.5 or 19.0 µm in diameter at the top and 50 µm in height. Instead of using pumps and valves to trap the cells, the ion traps use dielectrophoresis (DEP)— the force on charged dipoles in a nonuniform electric field. “The electric fields, in essence, make a prison cell, or cage, for the cells,” says Voldman. The initial flow of cells is allowed to settle for ~1–2 minutes before another round is released into the chamber. This process continues until each trap contains some cells. Then, fluid is once again released into the chamber, but the flow rate is increased from 10 to 12 µL/min, which distorts the potential energy wells of the traps so that only one cell can be held in each trap. The central feature of DEP traps is the competition between the dipole moment of the cell and medium inside the traps. “It doesn’t matter that the cell is charged. What matters is that the cell’s electrical parameters are different than the medium it is in,” says Voldman. “The cell then migrates to the middle of the trap in its efforts to get away from the electric field,” he says. The idea for the µDAC came when Voldman and colleagues realized that there were no devices for moving cells around at the microscale. Voldman believes the µDAC has a promising future for studying the dynamic relationship between cell genotype and phenotype. “If you do screens for dynamics, then you need to look at cells more than once and isolate them to figure out what . . . gave you that behavior of interest,” he says. However, he cautions against any immediate expectations of his technology hitting the market. “What you see is a proof of concept; the next level is taking it to production for real-world functions,” says Voldman. a —Wilder D. Smith
