Apparatus comprising: an impeller (50) comprising: at least one helical elongate element (52)； a spring (54), the spring (54) being disposed inside of, and coaxially with, the helical elongate element (52), and the spring defining a lumen (62) therethrough； proximal and distal bushings (64, 58)； and a film of material (56) supported between the helical elongate element (52) and the spring (54)； and a rigid shaft (63) configured to extend from the proximal bushing (64) to the distal bushing (58) of the impeller (50) via the lumen (62) defined by the spring (54).
