Fig 1: Formation of cardiac µ-tissues: A) Generation of uniformly sized spheroids in µ-Wells. B) Flow cytometry analysis of CM and FB cell populations prior to spheroid formation. C) Fusing of individual cardiac spheroids to an aligned tissue over the timespan of 24 ?h after loading. The underlying integrated O2 sensor spot is visualized via widefield fluorescence microscopy (green, Cy5 filterset). D) Characterization of the structure of the generated cardiac muscle fiber in fibronectin-coated HoC by immunostaining of cTnT (green) and DAPI (blue). The shaft region is characterized by aligned fibers (xy slice). The perpendicular z-stack view reveals stacked nuclei, indicating the generation of a multilayered 3D tissue (yz slice). E) Characterization of beating motion of a fiber. Motion vectors at tissue contraction reveal a collective displacement. F) Optically determined beating kinetics display pronounced peaks of contraction (red circles) and relaxation (red triangles), amenable for beating frequency extraction.