Substrate-dependent cell elasticity measured by optical tweezers indentation

In the last decade, cell elasticity has been widely investigated as a potential label free indicator for cellular alteration in different diseases, cancer included. Cell elasticity can be locally measured by pulling membrane tethers, stretching or indenting the cell using optical tweezers. In this paper, we propose a simple approach to perform cell indentation at pN forces by axially moving the cell against a trapped microbead. The elastic modulus is calculated using the Hertz-model. Besides the axial component, the setup also allows us to examine the lateral cell–bead interaction. This technique has been applied to measure the local elasticity of HBL-100 cells, an immortalized human cell line, originally derived from the milk of a woman with no evidence of breast cancer lesions. In addition, we have studied the influence of substrate stiffness on cell elasticity by performing experiments on cells cultured on two substrates, bare and collagen-coated, having different stiffness. The mean value of the cell elastic modulus measured during indentation was 2679 Pa for the bare substrate, while for the collagencoated substrate it diminished to 1977 Pa. The same trend was obtained for the elastic modulus measured during the retraction of the cell: 23710 Pa and 1377 Pa, respectively. These results show the cells adapt their stiffness to that of the substrate and demonstrate the potential of this setup for lowforce probing of modifications to cell mechanics induced by the surrounding environment (e.g. extracellular matrix or other cells)