We describe a novel platform for dynamic, parallel screening of individual cell-cell interactions based on ultrasonic aggregation and positioning of cells in a multi-well microplate. Upon ultrasound actuation, clusters containing one or a few cells are within seconds formed and retained in a precise location synchronously in each of the 100 wells on the microplate for up to 17 h. The aggregation principle is based on acoustic radiation forces driving suspended particles or cells into the pressure nodes of a two-dimensional ultrasonic standing wave field. By combining the acoustic cell handling tool with high-resolution confocal fluorescence microscopy, detailed time-lapse monitoring of individual cell-cell interactions in a highly parallel manner is possible. Of particular interest in our group is to study the long-term interaction between Natural Killer (NK) cells and tumor cells at the level of single cells.
The traditional design of an ultrasonic standing wave device for particle manipulation is to use a high-quality-factor, one-dimensional multilayer structure and careful selection of layer thicknesses and material properties. However, this design is more difficult to implement in a microchip, in particular in chips with fluid channels/chambers deviating from a simple one-dimensional fluid layer. As a consequence, the pattern of trapped and aggregated particles most often has a complex three-dimensional shape which is difficult to theoretically predict.
Our strategy to overcome these problems and accurately control the micro-scaled ultrasonic fields inside the chips is to employ frequency-modulated actuation around 2.5 MHz using broad-banded wedge- or ring-transducers [1]. In this talk we demonstrate how to design a microchip capable of trapping and positioning individual cells by ultrasound in a highly parallel manner, and with a spatial accuracy of the order of a cell diameter. We quantify the cell cluster motility with and without retained ultrasound exposure during 17 h, and we report on the viability of cells when exposed to continuous ultrasound for up to three days. Finally, we quantify the heterogeneity of NK cells’ cytotoxicity against tumor cells. We show that approximately two thirds of the NK cells display cytotoxicity against MHC class I deficient tumor cells, with some NK cells being particularly active, killing up to six target cells during the assay [2]. We also report that simultaneous interaction with several target cells increases the cytotoxic responsiveness of NK cells [2].