Measuring Acoustic Mobility of Particles by Particle Tracking Velocimetry

Sammanfattning: Acoustophoresis is a method to manipulate and move cells or particles suspended in a medium using acoustic waves. The speed of the acoustophoretic motion depends on the cells' acoustic mobility, i.e. their size, density and compressibility relative to the medium. Differences in acoustic mobility can be used to separate particles from each other. Separation is an important tool within biomedical and clinical applications. In this thesis, we develop a method to measure the acoustic mobility of a particle or cell with Particle Tracking Velocimetry (PTV) software. The acoustic mobility determines the migration velocity, thus it is possible to determine the acoustic mobility by measuring the migration velocity. As several particles' acoustic mobilities are determined, they can be used to assess optimal buffer conditions to maximise the relative acoustic mobility between two particles or cells, improving their separation by acoustophoresis. In this thesis, a simple rectangular microfluidic chip was filled with particles suspended in a solution. The migration of particles due to exposure to an acoustic standing wave field was then repeatedly imaged and the procedure was repeated 20 times collect sufficient data. The particle paths and migration velocities can be extracted by PTV software. By fitting this velocity to a theoretical model the acoustic mobility can be derived. The experimental set-up and computational model were already present at the Thomas Laurell group at BMC, Lund University where this thesis was conducted. They were, however, lacking sufficient precision and reliability, yielding a wide spread of data points for each experiment and resulting in large variations in measurements between experiments. Thus, the main focus of this thesis primarily revolved around improving the experimental procedures, data processing and the computational model. The long-term goal of this project is to create an easy to use, reliable and rapid method to determine the acoustic properties of any given cell and propose a suitable medium composition for ideal separation. One improvement of the model is the division of the images into thinner slices to achieve more local and accurate measurements. Another change was the introduction of a velocity filter, excluding data that stemmed from particles with only minimal movement unrelated to the migration effects induced by the acoustic field. The thesis also improved and evaluated the experimental set-up in itself, focusing on the usage of fluorescent Polystyrene beads as reference particles as well as evaluating the accuracy of the model and detecting errors in it. This resulted in the discovery of significant differences in the acoustic properties of differently coloured polystyrene particles which were confirmed by an independent experiment. Finally, the experimental model was used in preliminary measurements of three different breast cancer cell lines' acoustic mobility.