To avoid such, we also conducted cryogenic SEM (cryo-SEM) analysis. It has been shown that for very small hydrogel samples mounted on a TEM grid, liquid nitrogen (LN) as cooling agent provides sufficient cooling rates despite of the Leidenfrost effect , for bigger samples however, more efficient cooling agents like slush nitrogen (SN) , liquid ethane or high pressure freezing in LN are necessary. Here, we used SN to investigate whether the microporous structure observed by SEM was an artifact from freeze-drying or not (Figure ). The result is rather obvious, clearly, the sample preparation affects the observed morphology. Cryo-SEM images obtained of the gel of A-PPheOzi 15 -A shows a highly ordered porous structure with round pores in the sub-micrometer to low micrometer range, which is in agreement with the rheological data of a strong physical network (Figure ). In comparison, for aqueous solutions of A-PPheOzi 5 -A, which does not form a hydrogel, rather lamellae-like structure are observed (Figure ). Therefore, it appears that even though a 3D network is formed, it is too weak to lead to macroscopic gelation, presumably due to a limited correlation length of the underlying physical network. In addition, we investigated the gel morphology of 15 wt.% A-PPheOzi 15 -A before and after dispense plotting (see below) by cryo-SEM (Figure ). It appears that the microstructural morphology remains unaffected which also corroborates the rheological data, which show rapid recovery of the gel structure after printing. From a rheological and structural point of view, the aqueous solutions/gels of A-PPheOzi 15 -A appear well suited from (bio)printing. Obviously, cytocompatibility represents another critical feature for any hopeful bioink or biomaterial ink. For the present polymers, no dose dependent cytotoxicity (WST-1 assay) was found for HEK and Calu3 cell lines at concentrations of up to 10 wt.% (Figure ). In contrast, we found an increased apparent cell viability, which has been found similarly for other POx block copolymers. 32 Finally, the printability of the novel material was assessed (Figure ). We printed first scaffolds using 10, 15 and 20 wt.% of A-PPheOzi 15 -A and 20 wt.% of A-PPheOzi 30 -A hydrogels using a 3D bioprinter equipped with a conical nozzle with 0.25 mm inner diameter (Figure , Figures ). To evaluate printing conditions such as speed and pressure, we first printed a four layer 10x10 line wood-pile structure (Figure -c; S11b, S12a-c). The strength of a 10 wt.% of A-PPheOzi 15 -A hydrogel is insufficient and strand fusion is observed, which is in line with the low yield point of the system. Nevertheless, a 3D cube could be printed (Figure ).