Tissue engineers are ambitious. If they had their way, a dialysis patient could receive a new kidney made in the lab from his own cells, instead of waiting for a donor organ that his immune system might reject. Likewise, a diabetic could, with grafts of lab-made pancreatic tissue, be given the ability to make insulin again. But tissue engineering has stalled in part because bioengineers haven’t been able to replicate the structural complexity of human tissues. Now researchers have taken an important first step toward building complex tissues from the bottom up by creating what they call living Legos. These building blocks, biofriendly gels of various shapes studded with cells, can self-assemble into complex structures resembling those found in tissues.