The invention relates to a fluid pump or rotary cutter having at least one first element (9′″, 10′″) which can be brought from a transport state into an operating state by changing at least one mechanical property. Such a pump can, for example, be a blood pump for the medical, microinvasive area. The object of achieving a transition between the transport state and the operating state which is as comfortable as possible and in so doing leaving a freedom in the design of corresponding apparatus, in particular of a pump, which is as large as possible, is achieved using the means of the invention in that the first element at least partly comprises a material (24, 25, 26, 27) or can be filled with a material or material mixture which passes through a chemical reaction, in particular cross-linking, or a crystallization for transition into the operating state.