The refractive index (RI) of cells and tissues is crucial in pathophysiology as a noninvasive and quantitative imaging contrast. Although its measurements have been demonstrated using three-dimensional quantitative phase imaging methods, these methods often require bulky interferometric setups or multiple measurements, which limits the measurement sensitivity and speed. Here, we present a single-shot RI imaging method that visualizes the RI of the in-focus region of a sample. By exploiting spectral multiplexing and optical transfer function engineering, three color-coded intensity images of a sample with three optimized illuminations were simultaneously obtained in a single-shot measurement. The measured intensity images were then deconvoluted to obtain the RI image of the in-focus slice of the sample. As a proof of concept, a setup was built using Fresnel lenses and a liquid-crystal display. For validation purposes, we measured microspheres of known RI and cross-validated the results with simulated results. Various static and highly dynamic biological cells were imaged to demonstrate that the proposed method can conduct single-shot RI slice imaging of biological samples with subcellular resolution.

A critical requirement for studying cell mechanics is three-dimensional (3D) assessment of cellular shapes and forces with high spatiotemporal resolution. Traction force microscopy (TFM) with fluorescence imaging enables the measurement of cellular forces, but it is limited by photobleaching and a slow 3D acquisition speed. Here, we present refractive-index traction force microscopy (RI-TFM), a high-speed volumetric technique that simultaneously quantifies the 3D morphology and traction force of cells. RI-TFM reconstructs a 3D tomogram of single cells on a planar hydrogel elastomer using a high-speed illumination scheme with 0.5-Hz temporal resolution. This allows for quantitative analyses of 3D dry-mass distributions and shear (in-plane) and normal (out-of-plane) tractions of single cells on the extracellular matrix without labelling. When combined with a constrained total variation-based deconvolution algorithm, the proposed method provides 0.55-Pa shear (in-plane) and 1.59-Pa normal (out-of-plane) traction sensitivity for a 1-kPa hydrogel substrate. We demonstrate the utility of RI-TFM by assessing the effects of compromised intracellular stress due to actin disruption or low temperature and capturing the rapid dynamics of cellular junction formation in the spatiotemporal changes in non-planar traction components.

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