The DNA self-assemblies were fixed on a plasma-treated 200 mesh copper grid which supports a carbon film (Ted Pella, Inc.): 10 µL of solution was added on the grid and left for 3 min before blotting it with a filter paper. The sample was negatively-stained using uranyl acetate (UA): 5 µL of 2 wt% UA water solution was added and quickly blotted, then 15 µL UA solution was again added and left for 1 minute before a last blotting. The samples were observed using a JEOL microscope at 200 kV (toroid origamis self-assembled in DMEM), or a Tecnai Spirit microscope at 120 kV (toroid origamis self-assembled with HEK cells).