Mark A. Skylar‐Scott

Rheological measurements with in situ visualization can elucidate the microstructural origin of complex flow behaviors of an ink. However, existing commercial rheometers suffer from high costs, the need for dedicated facilities for microfabrication, a lack of design flexibility, and cabling that complicates operation in sterile or enclosed environments. To address these limitations, a low‐cost ($300) visual, in‐expensive and wireless rheometer (VIEWR) using 3D‐printed and off‐the‐shelf components is presented. VIEWR measurements are validated by steady‐state and transient flow responses for different complex fluids, and microstructural flow profiles and evolution of yield‐planes are revealed via particle image velocimetry. Using the VIEWR, a wholly‐cellular bioink system comprised of compacted cell aggregates is characterized, and complex yield‐stress and viscoelastic responses are captured via concomitantly visualizing the spatiotemporal evolution of aggregate morphology. A symmetric hyperbolic extensional‐flow geometry is further constructed inside a capillary tube using digital light processing. Such geometries allow for measuring the extensional viscosity at varying deformation rates and further visualizing the alignment and stretching of aggregates under external flow. Synchronized but asymmetric evolution of aggregate orientation and strain through the neck is visualized. Using varying geometries, the jamming and viscoelastic deformation of aggregates are shown to contribute to the extensional viscosity of the wholly‐cellular bioinks.