Ultrasonic fields can be used to trap and manipulate micron-scale particles and second-phase fluids, utilising energy
densities that do not impair cell viability. The technology can be seen as complementary to optical trapping as the size of
the potential wells generated can be relatively large, making ultrasound suitable for the formation and manipulation of
cell agglomerates, but less suitable for the manipulation of individual cells. This paper discusses physical phenomena
associated with ultrasonic manipulation, including radiation forces, cavitation, and acoustic streaming.
The technology is well suited to integration within "Lab on a Chip" devices and can involve excitation by plane,
focussed, flexural, or surface acoustic waves. Example applications of resonators are discussed including particle
filtration and concentration, cell washing, and biosensor enhancement.
A recently developed device that uses both ultrasonic and magnetic forces to enhance the detection of tuberculosis
bacteria using magnetic beads is discussed in detail. This approach uses ultrasonic levitation forces to overcome some of
the issues associated with purely magnetic trapping. The technology has been implemented in a device in which the
main fluidic components are disposable to allow for low production costs and improved control of biohazards.