Apparatus comprising: a left-ventricular assist device configured to assist left-ventricular functioning of a subject, the left-ventricular assist device comprising: a 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; a frame disposed within at least a portion of the tube, the frame defining a plurality of cells, the frame being configured such that, in a non-radially-constrained configuration of the frame, the frame comprises a generally cylindrical portion; an inner lining that lines at least some of the cylindrical portion of the frame; and an impeller disposed inside the frame such that, in a non-radially-constrained configuration of the impeller, at a location at which a span of the impeller is at its maximum, the impeller is disposed within the cylindrical portion of the frame, such that a gap between an outer edge of the impeller and the inner lining is less than 1 mm, the impeller being configured to rotate such as to pump blood from the left ventricle to the aorta, the apparatus characterized in that: a width of each of the cells within the cylindrical portion as measured around a circumference of the cylindrical portion is less than 2 mm, and the impeller is configured to be stabilized with respect to the frame, such that, during rotation of the impeller, the gap between the outer edge of the impeller and the inner lining is maintained and is substantially constant. a 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; a frame disposed within at least a portion of the tube, the frame defining a plurality of cells, the frame being configured such that, in a non-radially-constrained configuration of the frame, the frame comprises a generally cylindrical portion; an inner lining that lines at least some of the cylindrical portion of the frame; and an impeller disposed inside the frame such that, in a non-radially-constrained configuration of the impeller, at a location at which a span of the impeller is at its maximum, the impeller is disposed within the cylindrical portion of the frame, such that a gap between an outer edge of the impeller and the inner lining is less than 1 mm, the impeller being configured to rotate such as to pump blood from the left ventricle to the aorta, the apparatus characterized in that: a width of each of the cells within the cylindrical portion as measured around a circumference of the cylindrical portion is less than 2 mm, and the impeller is configured to be stabilized with respect to the frame, such that, during rotation of the impeller, the gap between the outer edge of the impeller and the inner lining is maintained and is substantially constant. a width of each of the cells within the cylindrical portion as measured around a circumference of the cylindrical portion is less than 2 mm, and the impeller is configured to be stabilized with respect to the frame, such that, during rotation of the impeller, the gap between the outer edge of the impeller and the inner lining is maintained and is substantially constant.