Ultrasonic Handling of Living Cells in Microfluidic Systems

Abstract: Microfluidic chips have become a powerful tool in research where biological cells are processed and/or analyzed. One method for contactless cell manipulation in microfluidic chips that has gained an increasing amount of attention the last decade is ultrasonic standing wave (USW) technology. This Thesis explores the biocompatibility of USW technology applied to microfluidic chips, and presents a novel USW-based method for serial processing and accurate characterization of living cells.The biocompatibility has been investigated by measuring the proliferation rate of cells after they had been trapped and aggregated inside a chip by ultrasound. No negative influence was observed after continuous exposure to 0.85 MPa pressure amplitudes for up to 75 min. Furthermore, the heat generation in the fluid channel caused by the ultrasound has been measured and used in a regulation scheme where the temperature can be controlled around any relevant temperature (e.g. 37?) with ±0.1? accuracy for more than 12 hours. The proliferation rate and temperature investigations suggest that USW technology applied to microfluidic chips is a biocompatiblemethod useful for long-term handling of living cells.We have introduced a new concept of contactless ultrasonic ”caging” of single cells or small aggregates of cells. These cages are channel segments in the microfluidic chips that are geometrically designed to resonate at one or several actuation frequencies. The actuation is performed remotely by up to five external frequency specific wedge transducers, where each transducer produces a localized and spatially confined standing wave with a specific orientation of its corresponding radiation force field. By multi-frequency actuation, sophisticated and flexible force fields are realized by both overlapping and separated single fields. The Thesis describes two different cages: A sub-mm ”micro-cage” for tree-dimensional manipulationof single cells, and a 5-mm ”mini-cage” for selective retention of small cell aggregates (up to approx. 10^3 cells) from a continuously feeding sample flow. Finally,our microfluidic chips were also designed to be compatible with high-resolution optical microscopy. We have demonstrated sub-?m-resolution confocal fluorescence and trans-illumination microscopy imaging of ultrasonically caged living cells.