Apparatus comprising: an impeller (152), comprising: an impeller frame that comprises proximal and distal end portions (164) and at least two helical elongate elements (166) that wind from the proximal end portion to the distal end portion； a material (168) that is coupled to the at least two helical elongate elements, such that the helical elongate elements with the material coupled thereto define a blade of the impeller； and a plurality of sutures (170) tied around the at least two helical elongate elements (166), the sutures (170) being configured to facilitate coupling of the material (168) to the at least two helical elongate elements (166).
