Automated monitoring and positioning of single microparticles via ultrasound-driven microbubble streaming.

We present a novel technique that uses non-invasive and non-destructive ultrasound-driven microbubble streaming for automated positioning or isolating of single microparticles or rare biological cells in microchannels. We introduce a microfluidic bead sorter with an accuracy of up to tens of micrometers in single-particle positioning, which does not require additional structures to precondition the flow for particle sorting (e.g., pre-focusing of particles by using sheath flows). By using the controlled microbubble streaming (activation and deactivation of the piezo transducer at the resonant frequency of the microbubbles) as a sorting operator, a variety of small particles with arbitrary initial positions can be precisely positioned across the width of the microchannel in a live mode, which cannot be readily achieved in applications using just an external on-off force. Lab-VIEW is used to simultaneously control image acquisition, image analysis, particle tracking, and positioning by a custom-designed feedback system in a live mode. The particles can be distinguished by their facial characters or the intensity of their emitted lights. The final position of the particle can be defined manually by an operator or by the movement of a mouse pointer in a live mode. This method is universally applicable in all areas of microfluidics, for particles of different sizes, shapes, densities, and compressibility.