Apparatus comprising: an impeller comprising: at least one helical elongate element; a spring, the spring being disposed inside of, and coaxially with, the helical elongate element; and a film of material supported between the helical elongate element and the spring; an elongate tube configured to traverse an aortic valve of a subject, such that a proximal end of the tube is disposed within an aorta of the subject and a distal end of the tube is disposed within a left ventricle of the subject, the elongate tube comprising: a frame formed from a shape-memory alloy; and a blood impermeable material that is disposed upon the frame, the elongate tube being configured to be disposed around the impeller, and the impeller being configured to pump blood from the left ventricle to the aorta, by rotating, characterized in that: the apparatus is configured such that, during rotation of the impeller, a gap of at least 0.05 mm and less than 1 mm is maintained between an outer edge of the impeller and an inner surface of the elongate tube; and the impeller is configured to be stabilized with respect to the elongate tube, such that, during rotation of the impeller, the gap between the impeller and the inner surface of the elongate tube is maintained. an impeller comprising: at least one helical elongate element; a spring, the spring being disposed inside of, and coaxially with, the helical elongate element; and a film of material supported between the helical elongate element and the spring; at least one helical elongate element; a spring, the spring being disposed inside of, and coaxially with, the helical elongate element; and a film of material supported between the helical elongate element and the spring; an elongate tube configured to traverse an aortic valve of a subject, such that a proximal end of the tube is disposed within an aorta of the subject and a distal end of the tube is disposed within a left ventricle of the subject, the elongate tube comprising: a frame formed from a shape-memory alloy; and a blood impermeable material that is disposed upon the frame, the elongate tube being configured to be disposed around the impeller, and the impeller being configured to pump blood from the left ventricle to the aorta, by rotating, characterized in that: the apparatus is configured such that, during rotation of the impeller, a gap of at least 0.05 mm and less than 1 mm is maintained between an outer edge of the impeller and an inner surface of the elongate tube; and the impeller is configured to be stabilized with respect to the elongate tube, such that, during rotation of the impeller, the gap between the impeller and the inner surface of the elongate tube is maintained. a frame formed from a shape-memory alloy; and a blood impermeable material that is disposed upon the frame, the elongate tube being configured to be disposed around the impeller, and the impeller being configured to pump blood from the left ventricle to the aorta, by rotating, characterized in that: the apparatus is configured such that, during rotation of the impeller, a gap of at least 0.05 mm and less than 1 mm is maintained between an outer edge of the impeller and an inner surface of the elongate tube; and the impeller is configured to be stabilized with respect to the elongate tube, such that, during rotation of the impeller, the gap between the impeller and the inner surface of the elongate tube is maintained. the apparatus is configured such that, during rotation of the impeller, a gap of at least 0.05 mm and less than 1 mm is maintained between an outer edge of the impeller and an inner surface of the elongate tube; and the impeller is configured to be stabilized with respect to the elongate tube, such that, during rotation of the impeller, the gap between the impeller and the inner surface of the elongate tube is maintained.