1Engineering Sciences, University of Southampton, Southampton, Hants, United Kingdom
The ability to trap, move and position biological cells is of fundamental importance in a wide variety of life sciences applications. One means of manipulating cells is through the use of ultrasonic standing wave (USW) fields. Within such a field, gradients of pressure and velocity interact with small scatterers, such as cells, to generate time-averaged forces, in addition to the oscillatory acoustic forces. These steady-state radiation forces have a component towards the acoustic velocity maximum for a dense scatterer (relative to the surrounding fluid) and a component towards the acoustic pressure minimum for a relatively stiff particle. The resultant of these components will move the majority of scatterers, such as cells in aqueous suspension, towards the pressure nodes of a plane standing wave.
This paper discusses the origin of the second order terms that lead to the radiation forces and describes different approaches to modelling the forces, both numerical and analytical. It is shown that the magnitude and scale of the potential wells that can be created within USWs complement alternative approaches such as optical traps and dielectrophoresis, and that USWs are particularly suitable for integration into lab-on-a-chip devices. Data relating to cell viability is also presented and it is shown that the fields are suitable for biological cell handling.
Several applications are discussed including:
Ultrasonic radiation forces are able to levitate and manipulate cells without impairing their viability and their use is highly compatible with microfluidic devices. The paper concludes with a discussion of emerging areas of importance for the technology, including: