Three-dimensional Isotropic Imaging of Live Suspension Cells Enabled by Droplet Microvortices

Three-dimensional (3D) imaging of non-adherent cells in suspension media is challenging due to their propensity to drift when not fixed to a substrate, as required by optical sectioning technologies. Resolution differences in the lateral versus depth directions typically present in those systems further complicates single-cell morphometry of cellular features indicative of effector functions, such as cytosol and organelle volumetric distribution, and cell membrane topography. Here, we present a method for 3D fluorescent isotropic imaging of live, non-adherent single cells encapsulated in picoliter droplets using Optical Projection Tomography (OPT) enabled by droplet microvortices. Our microfluidic platform features a droplet trap array that leverages flow-induced droplet interfacial shear to generate intra-droplet microvortices, which in turn are modulated to rotate single-cells on their axis to enable OPT-based imaging. This strategy allows observation of cells encapsulated inside non-toxic isotonic buffer droplets and facilitates scalable OPT acquisition by the simultaneously spinning of hundreds of cells. Specifically, we demonstrate 3D imaging of live myeloid and lymphoid cells in suspension, including K562 cells, as well as naive and activated T cells - small cells prone to movement in their suspended phenotype. In addition, morphometry of primary T cells under different immunological activation states allowed us to identify six distinct nuclear content distributions, which differ from the conventional 2D images depicting spheroid and bean-like nuclear shapes commonly associated with lymphocytes. This Arrayed-Droplet Optical Projection Tomography (ADOPT) technology is capable of isotropic, single live-cell 3D imaging and has the potential to perform large-scale morphometry of immune cell effector function states, while providing compatibility with microfluidic droplet operations. ### Competing Interest Statement B.C. B., X. L., and A. P. L. are inventors of a US patent application related to this work.