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MEETING NEWS
Silica sol–gels for microchannels
COURTESY OF QIRONG WU
Silica beads, although widely used in HPLC, have been difficult to incorporate into microfluidics because structures must be fabricated to keep them in place. Now, Qirong Wu and colleagues at the University of Virginia present a silica sol–gel that can be applied to a microchannel and used for solid-phase extraction (SPE) or as a chromatography column. One of the main advantages of using a sol–gel is that it stays in place better than conventional beads. “Since it is a single big piece, a greater pressure is needed to
The integrated glass chip with silica sol–gel solid-phase extraction channel (white) and PCR channel (gray) to separate and analyze DNA from biological fluids.
force it from the channel,” says Wu. In addition, she says that the microchip surface (currently glass) is preconditioned to hold onto the sol–gel. Others have used organic polymer sol–gels in microchannels; however, their porosity changes when they are exposed to organic solvents, and high temperatures are often required to polymerize these gels. “If we were using polymeric chips, we would have to address these issues” says Wu. “As for the silica gel, you only need to heat it to 40 °C, and it is resistant to organic solvents.” She adds, “I like the fact that we have so much experience with silica materials; we can do a lot of chemistry with these. It is easy to add functionalities.” Wu and colleagues have tested the silica sol–gel for SPE of DNA from biological samples. Wu said that the sys290 A
Symposium on Capillary Chromatography and
tem’s most impressive ability was isolating DNA from whole blood because a ~10-µL sample contains only 5–10 ng of DNA and much more protein and other components. In the new process, not only is the patient’s DNA extracted, but DNA from any viruses or bacteria present are isolated, as well. The SPE has been integrated with PCR and electrophoresis on one chip to amplify and detect DNA. The system was tested on the extraction and amplification of herpes simplex virus DNA from spinal fluid. It was also compared with a commercial DNA extraction system for the detection of anthrax, and the detection limit was found to be lower using the microSPE method than with the commercial system. The researchers are currently working on integration issues as well as using the silica sol–gels for microchip-based chromatography.
COURTESY OF DANIEL CHIU
Michael Felton reports from the 26th International Electrophoresis—Las Vegas, Nev.
Unusual microfluid flow
(a) A ring of fluorescent beads forms in a microvortex created in a diamond-shaped void. (b) One such vortex spins a -lymphocyte cell.
At small scales, fluids often exhibit unique behaviors, such as laminar flow, which have posed a challenge to microfluidic design. Instead of trying to overcome these properties, Daniel Chiu and colleagues at the University of Washington are investigating how unconventional structures on microchips will affect fluid flow and, in the process, are finding some unusual properties. The researchers added a diamondshaped void that is open on one corner to a microchannel to investigate what would happen to the flow. The liquid flow detached at the opening of the void and created a microvortex. At certain flow speeds in the microchannel, a single strong vortex is created in the diamond, according to Chiu. Using fluid dynamics and a new flow mapping technique, the researchers determined that particles in the vortex were exposed to very high g-forces. At certain flow speeds, micrometersized beads formed dramatic patterns of
rings of different diameters within the microvortex. Objects placed in the center of this microvortex are spun around. In lymphocyte cells, the nucleus was clearly enlarged after spinning. Large, intentionally swelled mast cells were also centrifuged. The granules within the cell collected at the center of the cell after the first two spins, and after the cell was spun twice more, granules were concentrated in one chunk that migrated to the cell wall. “I really don’t know what this could be used for,” says Chiu, “but I think there are enough unique and interesting things that happen in these microfluidic systems that some of these phenomena may turn out to be extremely useful.” He points out that this centrifugation was very different than large-scale centrifuges. “Regular centrifuges force cells against a wall, but this microfluidic centrifuge spins a single cell around its own center of rotation.”
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