To make red blood cells in the lab, study leader Samir Mitragotri and colleagues started with spheres of biodegradable polymer. They then exposed the spheres to isopropanol, which collapsed them into the discoid shape characteristic of red blood cells. The researchers then layered proteins — either albumin or hemoglobin — onto the doughnut-shaped disks, cross-linked the proteins to give them extra strength and stability, and finally dissolved away the PLGA template to leave only a strong but flexible shell