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Bead diameter (|m)

figure 7.5

Measurement sensitivity to particle size using 5-, 7-, 10-, 15-, and 20-|m-diameter beads; mean displacement shows 5- and 7-|m-diameter beads were too light and also noisier; for diameters between 10 and 20 |im, the beads show a slowly increasing trend in displacement; a relatively low size-sensitivity results for particles in this size range is advantageous for measuring cells that often have widely varying sizes even in otherwise homogenous populations.

Bead diameter (|m)

figure 7.5

Measurement sensitivity to particle size using 5-, 7-, 10-, 15-, and 20-|m-diameter beads; mean displacement shows 5- and 7-|m-diameter beads were too light and also noisier; for diameters between 10 and 20 |im, the beads show a slowly increasing trend in displacement; a relatively low size-sensitivity results for particles in this size range is advantageous for measuring cells that often have widely varying sizes even in otherwise homogenous populations.

smaller. For diameters between 10 and 20 |im, the beads stayed on the surface during measurement, and we see a slowly increasing trend in displacement. This increase is a result of the increasing optical interaction mass for larger particles; however, the rate of increase is low because this increasing optical force is being counteracted by an also-increasing drag force due to the larger diameter of the particle. A relatively low size sensitivity results for particles in this size range, which is advantageous for measuring cells that often have widely varying sizes even in otherwise homogenous populations. We have observed experimentally with cells that usually only a shallow dependence on cell size results for cells within a single population.

In general, measuring beads requires less laser power than measuring cells because of their higher index of refraction (n ~ 1.5 for polystyrene beads vs. n ~ 1.37 for cells).15 The optical force imparted to a particle scales with the difference in index of refraction between the particle and the fluidic medium.16 For bead measurements, we typically operate at a laser power of 2.5 W, whereas for cell measurements the laser is operated at 10 W to obtain similar displacements. These relative power levels are in line with the comparative refractive index differences between the two different particle types and water.

7.3 Biological Examples of the Uses of Optophoresis

To demonstrate the utility of optophoresis in the preclinical stages of drug discovery, we screened several cell lines for their response to three standard chemotherapy drugs: flu-darabine, vincristine, and Gleevec (imatinib mesylate, Novartis Pharmaceuticals, Basel, Switzerland) (Figure 7.6). To perform these measurements with the fast-scan technique, cells were suspended in a 9-mm-diameter well in a sample holder. The bottom surface of

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