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 (G) 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 (G) 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 (G) 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 (G) 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. <EOS>