Interpretation of projection cytometer signals for cell analysis

In this paper the influence of optical absorption on projection cytometer measurements is discussed. Compared to conventional flow cytometry where far field parameters such as forward and side scatter are measured, our system measures physical parameters of biological cells in the optical near field with only one optical detector. This technique has been integrated into a microsystem featuring a microfluidic channel equipped with an integrated photodiode and illuminated by a laser beam. Presenting finite element studies we show how the sensor signal shape is altered by varying parameters such as optical absorption inside a cell and projection distance. The interesting result is that with changing absorption coefficient only a part of the signal shape changes namely the signal peak that results from the cell focusing the incident beam on the photosensitive area. This indicates the ability to extract the optical absorption of a cell from the shape of the signal only. Experimental results with non-transparent and transparent polystyrene beads, yeast, and Chinese hamster ovary cells are presented which confirm the simulations for low and high absorption.