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Development of an ultrasonic particle trap incorporating 3-dimensional geometric features

This paper describes the development of a device designed to collect a concentrate of cells for environmental monitoring and bio-hazard detection using ultrasonic radiation forces. As an alternative to a continuous flow-through design it is proposed to use batch trapping, periodically diverting the trapped cells through a high concentration outlet. Trapping against the flow is possible if acoustic radiation forces oppose fluid drag forces, but the challenge is to generate these strong lateral forces. This device uses a series of machined pegs which promote lateral variations within the acoustic field, thus generating lateral forces to oppose fluid drag forces. It has been shown that 20 and 1 µm diameter polystyrene beads can be trapped against a flow and then eluted in order to collect a 10 times concentrate of the particles. The pegs promote lateral trapping forces generated by a combination of enclosure and structural modes, especially of the pegs themselves. The fabrication method allows a large number of geometries to be explored and therefore there is much scope to optimize peg design and position. In summary, the concept of this device (geometric design and operation) offers the potential for practical advances for sample processing in sensor instruments.