Three-dimensional (3D), label-free, and long-term imaging in organ-on-a-chip research is necessary to assess the dynamic development of multicellular structures inside a microfluidic chip and relevant physiological functions. However, imaging on-chip multicellular landscapes generally relies on the use of exogenous labels, which limits the long-term imaging of live cells due to photobleaching and phototoxicity. Here, we demonstrate the 3D label-free observation of 5-day on-chip vasculogenesis of human umbilical vein endothelial cells using optical diffraction tomography. The proposed method enables us to quantify both morphological and biochemical alterations during the vasculogenesis.
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