Power-law rheology of adherent cells by local optical stretching and implications for mechanical modelling.

The lack of quantitative agreement between different techniques and publications has been an open issue in cell rheology for many years. Major differences in experimental design - and thus potential sources of the discrepancy - include the magnitude and lateral length scale of force application. To address these issues, we have developed an optical stretcher capable of extracting viscoelastic parameters from adherent cells while applying very low forces of a few pN and deformations of a few nm in a contact-free manner. This paper outlines the potential and limitations of such a setup. The latter include the necessity of reference measurements with the cells in an index-matched medium. It is to our knowledge the first time that adherent cells have been characterized with an optical stretching setup capable of quantitative mechanical measurements. Furthermore, a meta-analysis is conducted with papers measuring the same cell lines by AFM indentation and evaluating the results with the same power-law model. The apparent stiffness values obtained vary by two orders of magnitude, but turn out to be strongly correlated with contact radius as well as applied mechanical stress. The values from optical stretching obtained at low stress and deformation fit into that picture.