Acoustic manipulation of cells and microbeads in droplet microfluidics

Droplet microfluidics has emerged as a promising platform for miniaturisation of biological assays on-chip. In droplet microfluidics small water droplets (nL-pL) surrounded by an immiscible carrier oil are generated at high throughput. In these droplets particles such as cells or microbeads can be encapsulated, and the idea is that each of these droplets can be used as small reaction chambers for biological analyses. However, one key bottleneck for the full implementation of droplet microfluidics in biology has been the lack of a method to position and enrich particles inside droplets. In this thesis I present for the first time a microfluidic system where cells and microbeads encapsulated inside droplets can be manipulated using acoustic standing waves (i.e. acoustophoresis). The developed microfluidic systems were fabricated in silicon and sealed with glass lids. In the experiments, water droplets containing particles were generated, and an acoustic standing wave-field was created between the channel walls by actuating a piezoelectric transducer attached to the chip. In the first study it was shown that at application of the ultrasound at the first harmonic (1.8 MHz), the encapsulated particles were focused to the centre of the droplets i.e. the pressure node. It was shown that both red blood cells and polystyrene microbeads could be aligned in the centre of the droplets. The usefulness of the technology was proved by combining acoustophoresis with a trident-shaped droplet split to allow for particle enrichment. At application of the ultrasound at the first harmonic close to 90% of the particles were positioned in the centre daughter droplets when approximately 2/3 of the original droplet volume was removed. To better understand the physics of the system, in the second study a theoretical model was developed where the acoustic field inside droplets was investigated. In the third study, switching of encapsulated particles between different microfluidic pathways was shown. At application of the ultrasound at the first harmonic the encapsulated particles were directed into pathway 1 (the centre daughter droplets) while at application of the ultrasound at the second harmonic the encapsulated particles were directed into pathway 2 (the side daughter droplets). In the fourth study, two-dimensional acoustophoresis was used to increase the detectability of particles encapsulated inside droplets by pre-aligning the particles before the droplet generation site. In the fifth and last study, it was demonstrated that acoustophoresis can be used to separate two different particle species originally encapsulated in the same droplet into different daughter droplets based on the acoustic properties of the particles.This thesis proves that acoustophoresis is a versatile technology that can find various applications in droplet microfluidics. The combination of droplet microfluidics and acoustophoresis opens up for new possibilities for miniaturisation of biological assays on-chip