The invention relates to the use of a fleece composed of plastic filaments, in particular made of a thermoplastic, as a sustained-released system or as a storage system for at least one active substance, in particular for at least one active liquid. The filaments are spun using a spinneret, and the plastic material of the filaments is foamed so that pores are formed at least on the surface or in surface regions of the filaments and at least some of the pores are pores that are open outward, and at least some of the open pores are filled with the active substance, in particular with the active liquid.
