All three morphologies were further analysed by cryo-TEM. Similar structures to those imaged by dry-state TEM were found (Figure ). However, the spherical objects showed internal structures resembling spherically entangled worm-like micelles, similar to those reported by Lee et al. for mixtures of copolymers with high-and low-molecular weight hydrophobic blocks. The polymersome membrane can also be clearly identified in the cryo-TEM images, and an average thickness of 44 ± 10 nm was measured. In order to interface bioPISA with biological systems it is important that bioPISA can not only be carried out in simple buffers, but also in complex biological media. To this end, bioPISA was conducted in three different cell culture media: Luria-Bertani (LB), Minimum Essential Medium (MEM) and RPMi-1640, aiming for spheres, worms and vesicles. The reactions yielded high monomer consumption along with self-assemblies of the same morphology and similar sizes as the reactions in PBS-Br (Figure , Figure , Figure , Figure , and Table ). Moreover, when bioPISA was carried out in the presence of Escherichia coli, the bacteria remained viable (Figure , Figure ). Taken together, these results prove the robustness of bioPISA to synthesise structures in biologically relevant aqueous solutions and opens the road to an easy, enzyme-catalysed synthesis of selfassemblies for biomaterials, living matter encapsulation or in situ formation of biomimetic nano-objects and synthetic cells. As GUVs are the required morphology for artificial cells, all further bioPISA experiments were carried out under conditions targeting a DP of 400 and with a monomer concentration of 25 wt% (Table ).