A method for releasing viruses includes the steps of: preparing a first negatively charged complex, comprising a plurality of viruses, a plurality of polyethyleneimine particles, and a copolymer; transferring the complex to an acidic region, thereby transforming the complex into a positively charged complex to release a portion of the viruses in the acidic region; and transferring the complex to a non-acidic region, thereby transforming the positively charged complex into a negatively charged complex. One embodiment of the copolymer has the following chemical formula: wherein R1 represents aliphatic compounds or aromatic compounds, and R2 includes at least one negatively charged group.