Among the numerous extrudable GelMA/bPE ink formulations, those with the lowest possible bPE concentration within the window of printability were chosen to reduce the exposure of cells to charged polymers. Hence, we considered 7.5 wt% TbPEs and 20 wt% DbPEs as suitable polymer concentrations to supplement the GelMA ink in the following cytocompatibility studies. Changes in the GelMA microstructure after photocrosslinking, which were induced by incorporation of the bPEs, were visualized by scanning electron microscopy (SEM) of the lyophilized hydrogel samples (Figure ). In the GelMA/DbPE hydrogel, the high PEC micelle content led to polymer accumulation around the GelMA pores, although, without influencing the micropore sizes significantly. In comparison, interpenetration with PEC network composed of TbPEs revealed a less uniform GelMA network structure with noticeable changes in pore size distribution. Cytocompatibility of the bPEs and their complexes was evaluated in two-and three-dimensional (2D and 3D) toxicity studies using the human hepatocarcinoma cell line HepG2. HepG2 is a fastgrowing cell line which is a well-established model system for liver cell function in vitro and shares many of the characteristics of primary hepatocytes, including metabolism and processing of xenobiotics . A colorimetric assay for assessing metabolic activity (MTT) was conducted to determine the viability of HepG2 cells, which were grown in 2D on standard cell culture plasticware and exposed to the bPEs for 72 hours (Figure ). bPEs were tested both in individual solutions (only sulfonate or guanidinium bPEs) and in the complexed state to investigate the impact of excess charge and macromolecular charge complexation on cell fate. As oppositely charged bPEs self-assemble into hydrogels even at concentrations as low as 5 wt% and would have thus hindered diffusion and potential cellular uptake in the toxicity studies, a concentration of 0.5 wt% for the individual bPEs and 1 wt% for the oppositely charged complexes was chosen, that did not result in any apparent viscosity increase. The cytotoxicity of the bPEs was found to be highly dependent on the type of bPEs, with high survival rates for the sulfonate bPEs and complete cell death for the guanidinium bPEs (Figure ). This strikingly detrimental impact of positive charge on cell fate has been previously reported, especially in the context of cellpenetrating peptides (CPP) rich in lysine or arginine . Their strong affinity to the negatively charged cell membrane as well as cellular internalization can cause cell membrane leakage and other toxic side effects . However, complexation of the positively and negatively charged bPEs led to remarkably increased cell viability (Figure ). Following self-assembly, the guanidinium groups were hypothesized to be primarily confined in the PEC domains, preventing interaction with the cells in the water-filled inter-domain space. We note that despite the use of identical polymer concentrations of DbPEs and TbPEs, their respective cytotoxicity varied marginally due to their different chemical structure (i.e., varying lengths of the PEO block and the charged blocks).