The good, the bad, and the tiny: a review of microflow cytometry

Abstract

Recent developments in microflow cytometry have concentrated on advancing technology in four main areas: (1) focusing the particles to be analyzed in the microfluidic channel, (2) miniaturization of the fluid-handling components, (3) miniaturization of the optics, and (4) integration and applications development. Strategies for focusing particles in a narrow path as they pass through the detection region include the use of focusing fluids, nozzles, and dielectrophoresis. Strategies for optics range from the use of microscope objectives to polymer waveguides or optical fibers embedded on-chip. While most investigators use off-chip fluidic control, there are a few examples of integrated valves and pumps. To date, demonstrations of applications are primarily used to establish that the microflow systems provide data of the same quality as laboratory systems, but new capabilities-such as automated sample staining-are beginning to emerge. Each of these four areas is discussed in detail in terms of the progress of development, the continuing limitations, and potential future directions for microflow cytometers.

Fig. 1

Schematic diagram of the detection region in a common flow cytometer. A particle is detected in the interrogation volume which is defined by the union of the sample stream, the volume illuminated by the excitation light, and the volume from which the collection optics detect light

Fig. 2

Schematic illustration of the hydrodynamic focusing device developed by Chang et al. [50]. (a) 3-D view, (b) top view, (c) side view, and (d) cross-section perspective depicting core stream centered in channel. Reproduced with permission from the Institute of Physics Publishing Limited

Fig. 3

Schematic diagram of the operating principles for sample focusing. (a) Top view and (b) side view. Cells or particles are focused at the center of the sample stream using DEP and hydrodynamic forces. Reproduced from Lin et al. [55] with permission from IEEE

Fig. 4

A 15 mm² microfluidic dielectrophoretic cell sorter. The device combines dual digital cytometer sensors, in-channel electrokinetic electrodes, and hydrodynamic fluid focusing. Reproduced from Hartley et al. [70] with permission from the IEEE

Fig. 5

Photograph of a lab-on-a-chip device with integrated microfluidic dye laser, optical waveguides, microfluidic network, and photodiodes. The metallic contact pads for the photodiodes are seen on the far right. The chip footprint is 15 mm by 20 mm. The photograph, which was taken before a lid was bonded to the structures, is reproduced from Balslev et al. [75] with permission from the Royal Society of Chemistry