Performance Assessment of a Liquid Handling Robot

Acoustic Droplet Ejection (ADE)

 

This technique transfers droplets in volumes between 0.1 pL to 1 μL by using a focused acoustic transducer (Ellson, 2002). The transducer converts radio frequency energy in the megahertz range into an acoustic vibration without any physical contact. With the help of acoustic vibration, ADE shoots a droplet from a source plate upward to a receiving plate placed above the source plate in a downward direction (see Figure 5.4). After ejection, the droplets fly towards the receiving plate against the gravity. The plate inversion provides the compatibility with conventional microplates, which eases the integration with laboratory automation. When the droplets reach the destination plate, due to the dominance of the surface tension, neither the gravity nor the inertial force of the droplets dislodges solutions or cause splashing (Ellson et al., 2003).

 

ADE is capable of transferring samples in low-nanolitre volumes precisely and accurately. Experiments performed by Ellson et al., (2003) have shown that ADE enables 5-nL sample transfer with CVs below 8%. Moreover, the direct droplet ejection eliminates any physical contacts in liquid handling. This feature not only allows gentle liquid transfer in nanolitre scale, but also reduces the risk of contamination. However, ADE has its drawback: delivering highly viscous biomaterials in nanolitre scale may be problematic because the acoustic driving force may not be large enough to overcome the high viscosity (Kong et al., 2012).

 

Above all, the author is convinced that the ADE is a promising novel technology of lower liquid volume dispensing. However, this technique may not be as economical as the traditional air- or liquid-displacement technique due to the higher cost of the acoustic equipment. Besides, the plate inversion may be problematic for liquid dispensing in the microlitre or millilitre scale. Also, the acoustic transducer can only eject a stream of droplets into one well each time, so it may be not as high-throughput as the multichannel liquid handling robots integrated with 96-, 384- or 1536-channel heads. Thus, efforts should be make to tackle these problems to achieve greater applicability for the future laboratory automation field.