Two pairs of oppositely charged PEO-b-poly(amino acid) copolymers with neutral poly(ethylene oxide) block and polypeptide block composed of the hydrophobic l-phenylalanine (Phe) amino acid mixed with either negative l-glutamic acid (Glu) or positive l-lysine (Lys) units were synthesized. N-carboxyanhydride (NCA) ring opening polymerization (ROP) was performed with either PEO46-NH2 or PEO114-NH2 macroinitiators, leading respectively to PEO46-b-P(Glu100-co-Phe65) and PEO46-b-P(Lys100-co-Phe65), and PEO114-b-P(Glu60-co-Phe40) and PEO114-b-P(Lys60-co-Phe40). Polyion complexes (PIC) formed at near charge equilibrium led to vesicle formation (PICsomes), as shown by DLS, zetametry and TEM. The good stability of PICsomes, even in high salinity media, was interpreted by π-π stacking hydrophobic interactions between the Phe residues, playing the role of “physical cross-linking”. These PICsomes were successfully loaded with siRNA directed against firefly luciferase enzyme expression. They also exhibit minimal cell cytotoxicity while superior silencing efficacy was shown by cell bioluminescence assay as compared to free siRNA and a standard lipofectamine-siRNA complex. As such, self-assembly of oppositely charged PEO-b-poly(amino acids) block copolymers enabled forming PICsomes of high stability thanks to π-π interactions of the Phe co-monomer in the polypeptide block, with high potential as biocompatible nanocarriers for RNA interference.Read less <