Micropillar arrays are used to measure traction forces of individual cells and of cells in tissue. For this technique single cells are placed on poly-di-methyl siloxane (PDMS) micropillar arrays...Show moreMicropillar arrays are used to measure traction forces of individual cells and of cells in tissue. For this technique single cells are placed on poly-di-methyl siloxane (PDMS) micropillar arrays and observed on an inverted high-resolution optical microscope. As of the requirements for a high-resolution inverted microscopy, the images of the micropillar array are taken through the cell.A refractive index change between the cell and cell medium results in refraction of light at their interface. Here we introduce a method to characterize astigmatism caused by this varying refractive index. We found that that astigmatism can lead to aberrations up to 400 nm. Further, based on the same method we were able to estimate the refractive index of the nucleus to be $1.366\pm0.004$. Our results demonstrate that astigmatism should be taken into account during traction force measurements, especially when measuring forces close to the nucleus.Show less
