Blood pumps

A blood flow assist system comprising:

• an impeller assembly comprising a rotor assembly and an impeller coupled with the rotor assembly, the rotor assembly comprising a concave bearing surface, an impeller shaft and a rotor magnet coupled to the impeller shaft, the impeller disposed on the impeller shaft;
• a drive unit proximal the impeller assembly, the drive unit comprising a drive magnet and a drive bearing between the drive magnet and the impeller assembly, the drive bearing comprising a convex bearing surface shaped to fit within the concave bearing surface, the convex bearing surface comprising a plurality of distally-projecting segments, the plurality of distally-projecting segments spaced apart circumferentially to define at least one channel between adjacent segments, the at least one channel extending outwardly from a void;
• and
• a sleeve bearing disposed about the impeller shaft at a location distal the impeller;
• wherein, in a cross-section taken perpendicular to an axis of rotation of the impeller shaft, a support surface of the sleeve bearing is disposed about only a portion of a perimeter of the impeller shaft at a selected axial location, such that, when the impeller shaft is rotated about the axis of rotation, an exterior surface of the impeller shaft at the selected axial location is cyclically exposed to blood during operation of the blood flow assist system.