A blood pump (20) is described that includes an impeller (50) having proximal and distal bushings (64, 58), at least one helical elongate element (52), a spring (54) that is disposed inside of the helical elongate element and along an axis around which the helical elongate element (52) winds, and a film (56) of material supported between the helical elongate element (52) and the spring (54). A frame (34) is disposed around the impeller (50). A flexible elongate element (67) extends radially from the spring (54) to the helical elongate element (52), and maintains the helical elongate element (52) within a given distance from the spring (54), to thereby maintain a gap (G) between an outer edge of a blade of the impeller (50) and an inner surface of the frame (34), during rotation of the impeller. Other applications are also described.