When an ultrasonic wave scatters from a solid object it exerts a small force on that object. This phenomena is known as the acoustic radiation force (or ARF). These forces have been used on a microscopic scale to move cells, for applications such as tissue engineering. Much of the effort here has been on so called Lab-on-a-chip microfluidic devices. The concept is now being used to perform a diverse range of functions such as transport, sorting, counting, cleaning, and analysis, all on a small “chip” (think here computer chip). So, the laboratory is shrunk and can be easily transported and cheaply reproduced.
Back to the physics. The ARF results from a nonlinearity of the fluid through which the ultrasonic wave travels. As most fluids are actually quite linear this means that the ARF is small compared to the oscillatory acoustic pressure and so the approach is not particularly energy efficient. However, the ARF exists for almost any combination of host liquid and particle material which makes it an incredibly versatile technique. The use of the ARF to manipulate cells is particularly compelling as it is both completely non-contact and harmless. It is able to move cells suspended in a liquid media into desired configurations in a few seconds.
My work on micro-particle manipulation has focused on the use of ultrasonic array devices. These have large numbers of elements/sources and this enables the creation of almost any acoustic field, which in turn means almost any ARF field. In particular I have been at the forefront of the development of dynamic and reconfigurable devices. The idea being that one device can perform many functions, for example, it could transport a blood or urine sample in a small device, concentrate them, e.g. over a sensor, sort them into different types and even measure their mechanical properties. This is very much in the spirit of the Lab-on-a-chip ideal. As well as device development I am studying applications such as the manipulation of small fibres to form composite materials with increased strength, the seeding of cells to create new 2D and 3D structures for tissue engineering as well as helping to develop new biomedical assays (e.g. new low-cost devices that can test for disease).
Bruce Drinkwater 