The self-assembly medium, supplemented by water when concentrated, was first heated at 37°C using a ThermoMixer® (Eppendorf®) prior to direct introduction of the desired volume of a concentrated staple mix without thermal pretreatment. After briefly vortexing the solution, a desired volume of concentrated scaffold solution was added and gently mixed by pipetting and tapping. The solution was shortly centrifuged, and kept at 37°C to start the incubation. Incubation time for DNA self-assembly was stopped when the sample was fixed on a surface for imaging (AFM and TEM) or quickly cooled by introducing the solution into ice and liquid nitrogen for storage at -20°C until use. For all experiments in DMEM (except Fig. ), DMEM 5× was used as a concentrated medium and its final concentration