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Techniques for manipulating, separating, and trapping particles and cells are highly desired in today's bioanalytical and biomedical field. The microfluidic chip-based acoustic noncontact trapping method earlier developed within the group now provides a flexible platform for performing cell-and particle-based assays in continuous flow microsystems. An acoustic standing wave is generated in etched glass channels (600 × 61 µm 2) by miniature ultrasonic transducers (550 × 550 × 200 µm 3). Particles or cells passing the transducer will be retained and levitated in the center of the channel without any contact with the channel walls. The maximum trapping force was calculated to be 430 (135 pN by measuring the drag force exerted on a single particle levitated in the standing wave. The temperature increase in the channel was characterized by fluorescence measurements using rhodamine B, and levels of moderate temperature increase were noted. Neural stem cells were acoustically trapped and shown to be viable after 15 min. Further evidence of the mild cell handling conditions was demonstrated as yeast cells were successfully cultured for 6 h in the acoustic trap while being perfused by the cell medium at a flowrate of 1 µL/min. The acoustic microchip method facilitates trapping of single cells as well as larger cell clusters. The noncontact mode of cell handling is especially important when studies on nonadherent cells are performed, e.g., stem cells, yeast cells, or blood cells, as mechanical stress and surface interaction are minimized. The demonstrated acoustic trapping of cells and particles enables cell-or particle-based bioassays to be performed in a continuous flow format.

Acoustophoresis is getting more attention as an effective and gentle non-contact method of manipulating cells and particles in microfluidic systems. A key to a successful assembly of an acoustophoresis system is a proper design of the acoustic resonator where aspects of fabrication techniques, material choice, thickness matching of involved components, as well as strategies of actuation, all have to be considered. This tutorial covers some of the basics in designing and building microfluidic acoustic resonators and will hopefully be a comprehensive and advisory document to assist the interested reader in creating a successful acoustophoretic device.

This part of the Acoustofluidics tutorial series reviews applications in acoustic trapping of micron-sized particles and cells in microfluidic systems. Acoustic trapping enables non-invasive and non-contact immobilisation of cells and particles in microfluidic systems. Acoustic trapping has been used for reducing the time needed to create 3D cell clusters, enhance particle-based bioassays and facilitated interaction studies of both cells and particles. An area that is increasingly interesting is the use of acoustic trapping for enriching low concentration samples and the washing or fractioning of cell populations prior to sensitive detection methods (MALDI-MS, PCR etc.) The main focus of the review is systems where particles can be retained against a flow while applications in which particles are positioned in a stationary fluid will be addressed in part 21 of the Acoustofluidics tutorial series (M. Wiklund, S. Radel and J. J. Hawkes, Lab Chip, 2012, 12, ).