Apparatus comprising: a blood pump configured to be placed inside a body of subject, the blood pump comprising: an impeller comprising: proximal and distal bushings (64, 58); at least one helical elongate element that extends from the proximal bushing to the distal bushing; and a film of material; a frame configured to be disposed around the impeller, the frame comprising proximal and distal bearings (116, 118); an axial shaft configured to pass through the proximal and distal bearings (116, 118) of the frame and the proximal and distal bushings (64, 58) of the impeller, the proximal bushing of the impeller being coupled to the axial shaft, such that the proximal bushing is held in an axially-fixed position with respect to the axial shaft, and the distal bushing of the impeller not being coupled to the axial shaft, such that the distal bushing is not held in an axially-fixed position with respect to the axial shaft, and the impeller defining a radially-constrained configuration in which the impeller is introduced into the subject's body and a non-radially-constrained configuration in which the impeller is configured to pump blood within the subject's body, the impeller being configured to change from its radially-constrained configuration to its non-radially constrained configuration by the distal bushing sliding over the axial shaft; characterized in that the impeller further comprises a spring that is disposed inside of the helical elongate element, and along an axis around which the helical elongate element winds; with the film of material supported between the helical elongate element and the spring. a blood pump configured to be placed inside a body of subject, the blood pump comprising: an impeller comprising: proximal and distal bushings (64, 58); at least one helical elongate element that extends from the proximal bushing to the distal bushing; and a film of material; a frame configured to be disposed around the impeller, the frame comprising proximal and distal bearings (116, 118); an axial shaft configured to pass through the proximal and distal bearings (116, 118) of the frame and the proximal and distal bushings (64, 58) of the impeller, the proximal bushing of the impeller being coupled to the axial shaft, such that the proximal bushing is held in an axially-fixed position with respect to the axial shaft, and the distal bushing of the impeller not being coupled to the axial shaft, such that the distal bushing is not held in an axially-fixed position with respect to the axial shaft, and the impeller defining a radially-constrained configuration in which the impeller is introduced into the subject's body and a non-radially-constrained configuration in which the impeller is configured to pump blood within the subject's body, the impeller being configured to change from its radially-constrained configuration to its non-radially constrained configuration by the distal bushing sliding over the axial shaft; an impeller comprising: proximal and distal bushings (64, 58); at least one helical elongate element that extends from the proximal bushing to the distal bushing; and a film of material; proximal and distal bushings (64, 58); at least one helical elongate element that extends from the proximal bushing to the distal bushing; and a film of material; a frame configured to be disposed around the impeller, the frame comprising proximal and distal bearings (116, 118); an axial shaft configured to pass through the proximal and distal bearings (116, 118) of the frame and the proximal and distal bushings (64, 58) of the impeller, the proximal bushing of the impeller being coupled to the axial shaft, such that the proximal bushing is held in an axially-fixed position with respect to the axial shaft, and the distal bushing of the impeller not being coupled to the axial shaft, such that the distal bushing is not held in an axially-fixed position with respect to the axial shaft, and the impeller defining a radially-constrained configuration in which the impeller is introduced into the subject's body and a non-radially-constrained configuration in which the impeller is configured to pump blood within the subject's body, the impeller being configured to change from its radially-constrained configuration to its non-radially constrained configuration by the distal bushing sliding over the axial shaft; characterized in that the impeller further comprises a spring that is disposed inside of the helical elongate element, and along an axis around which the helical elongate element winds; with the film of material supported between the helical elongate element and the spring.