Fine-tuned PEGylation of chitosan to maintain optimal siRNA-nanoplex bioactivity

•Chitosan/siRNA nanoplexes can be modified with defined degrees of PEG.•PEG has little influence on physicochemical characteristics of nanoplexes.•PEG has strong influence on biological function of nanoplexes.•Biological functionality cannot be predicted only from physicochemical studies.•Degree of PEG grafting has to be fine-tuned and verified in biological systems.