Apparatus and methods are described including placing within a blood vessel of a subject an impeller (50), the impeller including at least one helical elongate element (52), and a spring (54), the spring being disposed inside of, and coaxially with, the helical elongate element. A film of material (56) is supported between the helical elongate element and the spring. Blood is pumped through the subject's blood vessel, using the impeller. Other applications are also described.
