To verify the biocompa bility of the 3D printed microalgae-based materials, rat embryonic fibroblasts (REF) were cultured on 5x5 arrays of 3D printed square scaffolds (bounding box 100x100 µm, z-height: 5 µm) in vitro for roughly 24 h (Figure and Figure ). A LIVE/DEAD™ Viability/Cytotoxicity Kit was used to dis nguish between living and dead cells. Living cells were stained with calcein-AM (λex = 493 nm) and dead cells with ethidium homodimer-1 (EthD1) (λex = 577 nm). In addi on, the nuclei of cells were stained with Hoechst (λex = 405 nm). The confocal fluorescence images displayed in Figure show that cells were distributed evenly on the surface as well as on the scaffolds while maintaining their viability for both microalgae-based materials. Furthermore, the reproducibility of the results was demonstrated by repea ng the experiments three mes (Figure ). The immunostaining of the cells and the orthogonal projec on in the YZ-direc on display that the cells are well spread and located on top of the scaffolds (Figure ). The posi on of the cell nuclei on top of the scaffolds is indicated by the white arrows in Figure . The orthogonal projec on visualizes that the ac n cytoskeleton is spanning across the whole scaffold. In addi on, the scaffolds could be coated with fibronec n (Figure ), which allows modifica on of the scaffolds with desired proteins for cell typespecific preference. There was no significant difference in the cell viability of the inks, and the absence of dead cells suggests that the microalgae-based inks have good cytocompa bility for fibroblasts.