Cell culture and in situ DNA origami self-assembly HEK cells were cultured in 1× DMEM HG supplemented with 10 vol% FBS and 100 U.mL -1 Penicillin-Streptomycin, at 37°C and 5% CO2. The cells were passaged every 3 to 4 days at ~80% confluence using 2× Trypsin solution and were seeded at 1.10 5 cell.mL -1 . For the in situ DNA origamis selfassembly experiments, we used HEK cells at 20% confluence obtained the day after their seeding in 100 µL of culture medium. A volume of medium corresponding to the total concentrated DNA volume to be added was first withdrawn, before addition of 10 µL (triangle, final concentration 40 nM) or 27.8 µL (toroid, final concentration 100 nM) concentrated staple solution and gentle mixing by pipetting. 1 µL (triangle, final concentration 1 nM) or 2.5 µL (toroid, final concentration 10 nM) of concentrated scaffold was rapidly introduced and gently mixed by pipetting and tapping the cell plate, before being re-incubated at 37°C. hiPSCs were cultured at 37°C and 5% CO2, on vitronectin-coated dishes in Essential 8 TM , or in mTeSR TM (from STEMdiff TM Cerebral organoid kit) for the differentiation into brain organoid. They were detached every 3 to 4 days at 80% confluence using 0.5 mM ethylenediaminetetraacetic acid (EDTA) solution, and seeded in their cell culture medium supplemented with 10 µM Y-27632 after 10 or 5-fold dilution. For the formation of embryoid bodies, 9 000 iPSCs in 100 µL medium were first seeded in an ultra-low attachment V-bottom 96 well plate (Greiner Bio-One) and kept in culture for 5 days, as indicated in the STEMdiff TM Cerebral Organoid kit protocol. At day 2 after the seeding, a mixture of 78 µL of fresh medium, 20 µL concentrated staple solution and 2 µL of concentrated scaffold solution was added. This was repeated at day 4 with a mixture of 89 µL of fresh medium, 10 µL of concentrated staple solution and 1 µL of concentrated scaffold solution. In both cases, the final concentration of scaffold and the staples in the resulting cell medium was 1 nM and 40 nM, respectively.