A water-erodible organic polymer bearing anionic salt groups, at least a proportion of which has as counter ions a biologically active heterocyclic cation having a delocalised positive charge. The heterocyclic cation is preferably a bis-imidazolium cation and the anionic salt group is preferably derived from (meth)acrylic acid or 2-acrylamido-2-methyl-propane sulphonic acid.
