The increasing interest for dedicated analysis of single particles at microscopic scales, such as biological cells, has led researchers to create micro-fluidic systems capable of trapping particles in a liquid flow. The most common trapping mechanism is by physical obstruction, which is simple, but it has its limitations. For instance, in these systems particle selectivity is poor because all the particles that pass through the channel may get pin down against the physical obstruction regardless of the size and shape of the particle [Di Carlo et al., Tan et al., Nilsson et al.]. In addition, releasing the particles that have been trapped presents a problem not only because the flow needs to be stopped or reversed, but because there is a high probability that the released particles are going to be trapped again when the system resumes its operation [Nilsson et al.]. Other devices use pressure differentials to trap the particles but each trap requires a separate flow or a valve mechanism for trap activation. This method requires multiple pumps or complex structures within the device such as micro-valves and actuators.

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