Label-free microscope for rheological imaging of cells
Many essential cellular functions depend on the viscoelastic properties of the cytoplasm. While techniques such as optical tweezers and atomic force microscopy can measure these properties, their reliance on localized probes prevents intracellular imaging and perturbs native cellular behaviour. Label-free microscopy offers non-invasive alternatives that are capable of imaging. However, bandwidth limitations often confine these techniques to the assessment of static mechanical properties or to measurements at gigahertz frequencies, which both lie outside the interesting frequency range typically associated with cellular viscoelasticity. Here, we introduce a label-free microscope capable of imaging the viscoelastic properties of cells at frequencies relevant to biology. The microscope measures intracellular viscoelasticity -- twenty times faster than previous label-free approaches -- and does this with diffraction limited resolution. The measurements reveal characteristic viscoelastic features that were previously inaccessible, allowing quantitative rheology of the cellular cytoskeleton. We apply the microscope to live cancer cells. The rheological images produced identify spatial variations in cellular mechanics, allow active and thermal processes to be distinguished pixel-by-pixel, and enable the state of the cell to be visualised over time and in the presence of stress. The microscope is also able to resolve cellular structures that are invisible to regular phase-sensitive imaging, and do this with high contrast. The ability to image both intracellular viscoelasticity and activity offers a powerful tool to advance fundamental cell biology, cancer research, clinical diagnostics, and drug development.