Acoustophoretic manipulation of sub-micron particles

Sammanfattning: Microchannel acoustophoresis is a technique which uses acoustically induced forces to translate particles in a microchannel. Its use in biological applications is increasing. Using acoustophoresis to precisely manipulate bioparticles such as yeast, blood cells and cancer cells gives the technique great potential. However, manipulation of sub-micron particles such as bacteria, platelets and micro-vesicles is not possible in continuous flow due to acoustic streaming. Changing the acoustic properties of the medium may permit to suppress acoustic streaming due to induced acoustic forces on the medium. The goals of this study were to (1) determine the minimal size of particles which can be manipulated in homogeneous medium, (2) establish acoustophoretic focusing using a medium with inhomogeneous acoustic properties for particles below the minimal size from the first aim, (3) separate particles with different sizes below the minimal size from the first aim, and (4) grow \textit{E. coli} bacteria and investigate if they can be manipulated independent of acoustic streaming. A microchannel, etched in silicon, sealed by a glass lid and glued onto a piezoelectric crystal, was used for the experiments. Ficoll PM70 was used to change the acoustic properties of the medium. The experiments showed that acoustoporetic focusing was not possible for particles with a diameter of 1.0 \textmu m in a homogeneous medium. However, a density gradient, created by laminating water and a solution with Ficoll, enabled focusing of 1.0 \textmu m particles for a short period. After flattening out of the gradient due to diffusion, the particles were again dominated by acoustic streaming. The results showed that it is also possible to focus \textit{E. coli} with this method. To conclude, introducing an inhomogeneous medium in a microchannel allows to suppress acoustic streaming for a period of time. This permits to manipulate and separate (bio)particles of smaller sizes with high purity. To continue this study, it would be interesting to investigate the possibility to separate bacteria with different acoustic properties. Further, the minimal difference in properties in the medium necessary to suppress acoustic streaming should be investigated.