The discovery of a cell-membrane water channel protein, aquaporin-1, culminated in the award of half of the 2003 Nobel Prize in Chemistry. The identification of this and numerous other aquaporins has fueled interest in osmosis, diffusion and in membrane permeability research in general. Furthermore, as drugs usually must cross the membrane to exert beneficial effects, membrane permeability analysis is important for medicine and for the pharmaceutical and biotechnology industries. However, it remains difficult to quantify.Comparing the mass of liquids in the cell to the mass of dry components, such as proteins, at various time points could provide information about membrane permeability over time. Although standard methods exist for measuring the dry mass, including transmission electron microscopy and optical interferometry, these techniques cannot measure the wet mass, and they require bulky instruments. Furthermore, transmission electron microscopy requires complex image processing algorithms and preparation methods that usually are incompatible with living cells.In contrast, a micro-optical fluidic chip developed by professor Ai-Qun Liu and colleagues from Nanyang Technological University and from DSO National Laboratories, both in Singapore, enables simultaneous measurement of wet and dry mass without harming living cells. Plus, chip-based devices save lab space.Precision opticsThe micro-optical fluidic chip consists of a fiber-coupled optical trap that holds a cell in place and a fiber-coupled Mach-Zehnder interferometer that measures the mass of the cell. The trap and interferometer beams run perpendicular to the fluidic channel, and the trap captures each cell within a gap between the fiber waveguides. The trap constitutes two counter-directional beams originating from a tunable fiber laser at 1064 nm and 45 mW, whereas the sample beam of the interferometer is a single unidirectional beam that stems from a DenseLight Semiconductors superluminescent LED (SLED) with a 1290-nm center wavelength and a 70-nm bandwidth. The beam wavelengths had to be different to avoid superposition of the light waves. The Mach-Zehnder interferometer used a superluminescent LED (SLED) to generate the sample beam, fiber collimators to introduce a phase delay in the reference beam and an optical spectrum analyzer. The optical trap beam was produced by a fiber laser. Liu said that they empirically discovered that the Mach-Zehnder interferometer performed well, after first evaluating a Fabry-Perot interferometer -- which is incompatible with optical trapping; consequently, the researchers had to use a micropipette, which is more likely to deform cells and which requires precise hand-eye coordination to operate, in contrast to the hands-free laser-based setup.To achieve the clearest interferometric spectrum, the researchers delayed the phase and attenuated the power of the off-chip reference beam to make the optical intensities and optical path lengths equivalent to the on-chip sample beam, using two fiber pigtail collimators from Thorlabs Inc. of Newton, N.J. Vibration isolation also was employed to achieve optimal results. The interferometric spectrum was recorded by an optical spectrum analyzer from Advantest Corp. of Tokyo. The researchers measured the relative change in the optical cavity, which enabled them to ignore uncertainties therein.Know thy cellThey used the micro-optical fluidic chip to measure representative single living mammalian cells. They measured dry, wet, and total mass and cellular volume. The cell remained alive with no noticeable changes in morphology. They found that the device has a precision greater than 5 percent, which is relatively high, according to Liu. These results are detailed in the Nov. 26 issue of Applied Physics Letters.Liu said that, because their setup measures cells suspended in fluid rather than attached to a substrate, it avoids cell deformity and death, as well as restrictions by the type of substrate. Because the device employs an optical trap, it stably aligns the cell the same way every time for high measurement repeatability. It enables cell manipulation via fluidics and optical trapping, it can analyze single cells, and it is high-throughput. Finally, it requires only low volumes of standard buffer solution, which saves money and reagent. A micro-optical fluidic chip employs an optical trap to hold cells in a fluidic channel, as shown in this microscopic image. The cell is then measured by interferometry. Images reprinted with permission of Applied Physics Letters. Limitations include the fact that the device cannot measure more than one cell at a time and that the interferometer requires a relatively high Q-factor for ideal precision and sensitivity. Liu said that they plan to improve the precision and to explore the micro-optical fluidic chip for cellular measurements. Already, his team has used the micro-optical-fluidic-chip to investigate variations in the refractive indices from various cell lines. As this example illustrates, the device could enable measurements beyond wet and dry mass.
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