a polymeric prodrug composition including a hydrogel , a biologically active moiety and a reversible prodrug linker the prodrug linker covalently links the hydrogel and the biologically active moiety at a position and the hydrogel has a plurality of pores with openings on its surface . the diameter of the pores is larger than that of the biologically active moiety at least at all points of the pore between at least one of the openings and the position of the biologically active moiety .