Multidepth screening of living cells using optical waveguides

The use of planar optical waveguides as substrata for label-free, non-invasive monitoring of cells growing on them is demonstrated. Different submicrometre depths (measured from and perpendicular to the substratum surface) can be selected for monitoring. The so-called symmetry waveguide configuration with a low refractive index waveguide support (nanoporous silica with refractive index approximately 1.2) and a polystyrene waveguiding film with a heat-embossed grating coupler is exploited to obtain practically useful differences between the penetration depths of different waveguide modes. Robust data processing techniques are developed to obtain quantitative information about the cell refractive index profile perpendicular to the substratum from the measured effective refractive indices of the modes. In particular, a method is introduced with which cell refractive index variations above and below a predefined and tunable depth can be separated using two modes. The technique can be extended to more modes to gain even more comprehensive information from predefined submicrometre slices of the cell layer. The introduced methods are also suitable for monitoring the kinetics of changes in cell refractive index profiles.