Transfection of the NIH3T3 fibroblasts by Nucleofection in order to secrete NanoLUC luciferase (pNL1.3CMV [secNluc] luciferase reporter vector; Promega, Madison, WI) was performed according to Lonza’s manual for the Amaxa 4D-Nucleofector Protocol for NIH/3T3 (Lonza Cologne GmbH, Cologne, Germany) using a Single Nucleocuvette (100 µL). Solution SG The cells were trypsinized and an aliquot of 106 cells was resuspended in 100 µL of 4D-Nucleofector Solution (Lonza Cologne). One microgram of pNL1.3CMV [secNluc] luciferase reporter vector was added, and the suspension was transferred into a Nucleocuvette. The Nucleocuvette was placed into the 4D-Nucleofector X Unit. The cells were transfected with program EN-158. After transfection, the cells were plated in cell culture medium into a T flask. For separation of the dead cells, the flask was incubated overnight, and vital cells were allowed to adhere to the bottom. The vital cells were subsequently used for the microtissue production process.