An electrochemically generated alginate matrix cross-linked with Fe 3+ cations was used to entrap lysozyme and then release it upon application of an electrochemical signal. The switchable behavior of the alginate hydrogel was based on the different interaction of Fe 3+ and Fe 2+ cations with alginate. The oxidized Fe 3+ cations strongly interact with alginate resulting in its cross-linking and formation of the hydrogel, while the reduced Fe 2+ cations weakly interact with alginate and do not keep it in the hydrogel state. Thus, the electrochemical oxidation of iron cations at +0.8 V (Ag/AgCl) in the presence of alginate and lysozyme resulted in the Fe 3+ -cross-linked alginate hydrogel thin-film on the electrode surface with the physically entrapped lysozyme. On the other hand, application of reductive potentials (e.g. À1.0 V) converted the iron cations to the Fe 2+ state, thus resulting in dissolution of the alginate thin-film and lysozyme release. The bactericidal effect of the electrochemically released lysozyme was tested on the Gram-positive bacterium Micrococcus luteus demonstrating the same activity as the unadulterated lysozyme commercially supplied by Sigma-Aldrich. The present result represents the first step towards drug delivering systems (exemplified by the lysozyme release) based on alginate hydrogels and activated by electrochemical stimuli. † Electronic supplementary information (ESI) available: AFM imaging of the Fe 3+ -cross-linked alginate hydrogel thin-film on the electrode surface with the physically entrapped lysozyme. See