Acoustofluidic interferometric device : for optomechanical cytometry

Abstract

In the past years a growing demand for label-free cell analysis has emerged. This demand answers the need for cell analysis in its developing stages and, perhaps more importantly, to study cell physiological state in a simpler way than using fluorescence-based analyses. Mechanical and optical properties of cells are emerging as powerful biomarkers to discriminate cells. The cell deformation induced by acoustic pressure is measured with the Acoustofluidic Interferometric Device, developed in this Thesis, and allows for studying the cells deformability in a way similar to what is done for the analysis of the Young modulus. Deformability is an integral biomarker that summarizes cell gene expression, while the cell refractive index is related to the density of proteins in the cytoskeleton. The Acoustofluidic Interferometric Device, developed for the measurement of optomechanical cell properties on a cytometric basis, is detailed described and characterized in this thesis. The device enables the assessment of size, deformability and refractive index (or a combination of them) of non-adherent cells by means of a low finesse Fabry Perot resonator and acoustic manipulation.When an acoustically focussed cell (or another micro-sized particle) crosses the axis of the Fabry-Perot cavity it will perturb the resonator’s fringe pattern governed by the Airy’s transmission function. Such perturbation can be characterized and analyzed by means of the parameters ρ (radii of the circular interference fringes), Full Width at Half Maximum of the individual fringe and by the distance between fringes (Free Spectral Range). The analysis of the perturbation enables the assessment of the cell’s optomechanical properties. Measurement of the deformability of Algae and Yeast cells has been carried out to test the instrument’s performance and compared to the equivalent perturbation introduced by Microgel beads and Polystyrene spheres as controls. The experiment is based on the cell-induced fringe pattern perturbation images analysis. Images are acquired under two different conditions; 1) acoustic focussing and 2) acoustically induced deformation. 180 independent intensity profiles are retrieved and analyzed for each image, allowing for statistical analysis of the parameters: cell focal length and perturbed resonator Finesse. The results show a change in the optomechanical properties of the Algae, Yeast and Microgel while the Polystyrene sample remains virtually unchanged, as expected since Polystyrene is much stiffer than a cell and cannot be deformed by the pressure field of the instrument. These results show that the acoustofluidic technique presented here is useful to detect and measure different optomechanical properties which, potentially, can be used as label free biomarkers in clinical diagnosis.