Thomas Laurell, Lund University, Sweden, explains how ultrasound can be used to separate and move cells and particles in microfluidic devices.

Parallel channel acoustic separator offering increased through-put. Channel widths are 350 microns

Ultrasonic standing wave technology enables a new mode of non-invasive and non-contact cell manipulation and separation integrated in microfluidic systems. With the rapid progress in the field of microtechnology and microfluidics the ability to design high performance microscale acoustic resonators opens new possibilities to employ acoustic standing wave technology to the development of advanced particle and cell separating microfluidic systems. Spatial control and manipulation of biological particulate matter in fluids by means of ultrasonic standing waves is thus an area that currently is gaining increased attention. Ultrasonic standing wave manipulation offers a non-contact mode of particle handling, which makes it an attractive tool in cell-based microsystems. Since a minimum of mechanical stress is induced on the cells by the applied acoustic forces, ultrasonic standing wave manipulation allows long-term acoustic field exposure without displaying any cell damage.

All types of cells can be affected by ultrasonic standing wave forces as long as they have different acousto-physical properties from the surrounding medium. Fundamental unit operations can thereby be realised in a microfluidic format, including cell positioning, concentration, separation, carrier medium exchange/carrier medium washing, sizing and binary addressing, enabling a fully integrated approach in the microsystem development.

Microchip-based acoustic standing wave technology also offers attractive features, such as a reasonable throughput (between ten and 100 microlitres per minute) and the ability to separate particles in a size domain ranging from hundreds of nanometres to tens of micrometres.

A recent review describes different continuous flow particle separation modes enabled in microfluidic chips, utilizing ultrasonic standing wave technology. Several applications in life science research and in the medical clinic are also possible. A specific problem has been targeted in a clinical application where blood recovered during thoracic surgery is heavily contaminated by lipid microemboli (LME) leaking from adipose tissue undergoing surgery. Increased awareness now emphasises LME as a major source of brain capillary microembolisation and subsequent cognitive dysfunction, following autologous blood transfusion in thoracic surgery. By employing acoustic standing wave separation technology it is shown that 90-95% of LME can be removed from blood, providing a clean blood fraction for subsequent autologous retransfusion to the patient. An important factor is that the microchip separation process can be scaled up to meet the volume requirements set by clinical needs.

Performing on-line washing of cells or particles is also possible, where cells are translated from a medium containing a contaminant substance into a laminated stream of clean carrier medium by means of acoustic standing wave forces. Wash efficiencies of red blood cells approaching 98% (95% cell recovery) was demonstrated.

It has also been shown that the ultrasonic platform can be designed to perform acoustic binary valving of cells, enabling a binary mode of addressing cells to twice the number of positions in a microchip. Also, acoustic standing wave frequency switching is demonstrated to perform cell/particle sizing, enabling a new mode to separate a complex mixture of cells in a continuous flow format.

Although chip integrated acoustic standing wave separation technology is still in its infancy, it can be anticipated that new laboratory standards very well may emerge from on-going research in this field.

Read more in Thomas Laurell's tutorial review 'Chip integrated strategies for acoustic separation and manipulation of cells and particles' in the March issue of Chemical Society Reviews.