A library of 1,400 biodegradable lipidoids was synthesized combinatorially through the (a) conjugate addition of alkyl-amines (in red) to alkyl-acrylate tails (in blue). (b) A subset of the 280 amines used (complete listing in Supplementary Fig. 1) are shown here. (c) Lipidoids were formulated with cholesterol, the phospholipid DSPC, PEG2000-DMG and siRNA to form nanoparticles. (d) A cryo-transmission electron microscopy image of lipidoid nanoparticles. Scale bar, 100 nm.