Patent Publication Number: US-2023137473-A1

Title: Centrifugal and mixed-flow impellers for use with a blood pump

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Patent Application 63/003,955 to Zipory, entitled “Ventricular assist device,” filed Apr. 2, 2020, which is incorporated herein by reference. 
    
    
     FIELD OF EMBODIMENTS OF THE INVENTION 
     Some applications of the present invention generally relate to medical apparatus. Specifically, some applications of the present invention relate to a ventricular assist device and methods of use thereof. 
     BACKGROUND 
     Ventricular assist devices are mechanical circulatory support devices designed to assist and unload cardiac chambers in order to maintain or augment cardiac output. They are used in patients suffering from a failing heart and in patients at risk for deterioration of cardiac function during percutaneous coronary interventions. Most commonly, a left-ventricular assist device is applied to a defective heart in order to assist left-ventricular functioning. In some cases, a right-ventricular assist device is used in order to assist right-ventricular functioning. Such assist devices are either designed to be permanently implanted or mounted on a catheter for temporary placement. 
     SUMMARY OF EMBODIMENTS 
     In accordance with some applications of the present invention, a distal end of a ventricular assist device is placed inside a subject&#39;s left ventricle, such that one or more centrifugally-pumping impellers of the ventricular assist device are disposed within the subject&#39;s left ventricle. For some applications, a plurality of centrifugally-pumping impellers are placed within the left ventricle. Each of the centrifugally-pumping impellers is typically disposed inside a respective impeller housing. Typically, the impeller housing defines at least one blood inlet opening (via which blood flows into the impeller housing), and a blood-outlet portion. Typically, at least one volute is disposed at least partially around the blood-outlet portion. Typically, the volute spirals and curves through 90 degrees and then leads into a pump-outlet tube, which is disposed such that blood-outlet openings defined by a proximal portion of the pump-outlet tube, are disposed in the subject&#39;s aorta. Typically, the centrifugally-pumping impellers are configured to assist the functioning of the left ventricle, by pumping blood from the left ventricle, through the volutes, into the pump-outlet tube, and then out of the blood-outlet openings and into the aorta. 
     For some applications, each of the impeller housings includes a plurality of blood inlet openings, e.g., an upper blood-inlet opening and a lower blood-inlet opening. Similarly, for some applications, each of the impeller housings includes a plurality of volutes, as described in further detail hereinbelow. 
     For some applications, the ventricular assist device includes two or more centrifugally-pumping impellers, that are disposed in series with one another along the axis of the device. For some such applications, each of the two or more centrifugally-pumping impellers is disposed on a single rigid axial shaft. Pumping blood using a plurality of centrifugally-pumping impellers typically increases the volume of blood that is pumped by the ventricular assist device relative to if only one centrifugally-pumping impeller is used, ceteris paribus. It is noted that although some applications of the present invention are described with reference to a ventricular assist device that includes two or more centrifugally-pumping impellers, the scope of the present application includes practicing some of the applications described herein with a ventricular assist device that includes a single centrifugally-pumping impeller. 
     For some applications, a left-ventricular assist device includes a pump-outlet tube configured such that a proximal portion of the pump-outlet tube traverses the subject&#39;s aortic valve, and a distal portion of the pump-outlet tube is disposed within the subject&#39;s left ventricle. The pump-outlet tube typically at least one blood inlet opening that is configured to be disposed within the left ventricle and at least one blood outlet opening that is configured to be disposed within the subject&#39;s aorta. For some applications, a mixed-flow impeller is configured to pump blood through the pump-outlet tube from the subject&#39;s left ventricle to the subject&#39;s aorta. For some such applications, the mixed-flow impeller includes an expandable portion disposed along an axis of the impeller and shaped such that, in its expanded configuration, a diameter of the expandable portion increases from a distal end of the expandable portion to a proximal end of the expandable portion. The mixed-flow impeller is thereby configured to impart radial flow components to blood as the blood flows from the distal end to the proximal end of the mixed-flow impeller. 
     For some applications, the expandable portion of the impeller is inflatable. Typically, the left-ventricular assist device includes a purging system that is configured to purge portions of the left-ventricular assist device by pumping purging fluid through the left-ventricular assist device. For some applications, the expandable portion of the impeller is configured to be inflated with the purging fluid. For some such applications, the purging system is configured to control the inflation of the expandable portion of the impeller, by modulating the pressure at which the purging fluid is pumped into the ventricular assist device. 
     For some applications, the impeller includes one or more impeller blades which are shaped to transition from being spiral shaped within a distal portion of the impeller to being substantially radially-extending within a proximal portion of the impeller. Typically, the distal portion of the impeller is configured primarily to impart axial flow to blood that it pumps, and the proximal portion of the impeller is configured to impart substantial radial flow to blood that it pumps. 
     For some applications, the pump-outlet tube defines a widened region in the vicinity of the impeller, which is wider than a portion of the pump-outlet tube proximal thereto. Typically, the pump-outlet tube is configured to redirect the radial blood flow components axially, by the pump-outlet tube narrowing within the portion of the pump-outlet tube that is proximal to the widened portion. For some applications, one or more spiral flow rectifiers are disposed within the pump-outlet tube, proximally with respect to impeller. Typically, the spiral flow rectifiers are configured to reduce radial flow components from blood flow by converting radial flow components to axial flow prior to the blood flowing out of the at least one blood outlet opening. For some applications, the one or more spiral flow rectifiers have spiral shapes with a pitch of the spiral shapes increasing from the distal ends of the flow rectifiers to their proximal ends, such that the spiral flow rectifiers are configured to gradually covert radial flow to axial flow. 
     For some applications, an expandable flow rectifier is configured to reduce radial flow components from blood flow through the pump-outlet tube prior to the blood flowing from the at least one blood outlet opening. Typically, the expandable flow rectifier is disposed along an axis of the pump-outlet tube and, in its expanded configuration, is shaped such that its diameter decreases from a distal end of the expandable flow rectifier to a proximal end of the expandable flow rectifier. Typically, the expandable flow rectifier is inflatable. For some such applications, the left-ventricular assist device includes a purging system configured to purge portions of the left-ventricular assist device by pumping purging fluid through the left-ventricular assist device. For some applications, the expandable flow rectifier is configured to be inflated with the purging fluid. For some such applications, the purging system is configured to control the inflation of the expandable flow rectifier, by modulating the pressure at which the purging fluid is pumped into the ventricular assist device. 
     There is therefore provided, in accordance with some applications of the present invention, apparatus including: 
     a left-ventricular assist device configured to assist left-ventricular functioning of a subject, the left-ventricular assist device including:
         a pump-outlet tube configured such that a proximal portion of the pump-outlet tube traverses an aortic valve of the subject, and a distal portion of the pump-outlet tube is disposed within a left ventricle of the subject, the pump-outlet tube defining at least one blood inlet opening that is configured to be disposed within the left ventricle and at least one blood outlet opening that is configured to be disposed within an aorta of the subject;   a mixed-flow impeller configured to pump blood through the pump-outlet tube from the subject&#39;s left ventricle to the subject&#39;s aorta, the mixed-flow impeller including an expandable portion disposed along an axis of the impeller and being shaped such that, in its expanded configuration, a diameter of the expandable portion increases from a distal end of the expandable portion to a proximal end of the expandable portion,   the mixed-flow impeller thereby being configured to impart radial flow components to blood as the blood flows from the distal end to the proximal end of the mixed-flow impeller.       

     In some applications, the pump-outlet tube defines a widened region in the vicinity of the impeller, which is wider than a portion of the pump-outlet tube proximal thereto, and pump-outlet tube is configured to redirect the radial blood flow components axially, by the pump-outlet tube narrowing within the portion of the pump-outlet tube that is proximal to the widened portion. 
     In some applications, in its expanded configuration, the expandable portion of the impeller has a three-dimensional teardrop shape. In some applications, in its expanded configuration, the expandable portion of the impeller has a conical shape. In some applications, in its expanded configuration, the expandable portion of the impeller has a frustoconical shape. 
     In some applications, the expandable portion of the impeller includes a shape memory alloy that is shape set such that the expandable portion of the impeller is self-expandable. 
     In some applications, the apparatus further includes an expandable flow rectifier that is configured to reduce radial flow components from blood flow through the pump-outlet tube prior to the blood flowing from the at least one blood outlet opening, the expandable flow rectifier being disposed along an axis of the pump-outlet tube and, in its expanded configuration, being shaped such that its diameter decreases from a distal end of the expandable flow rectifier to a proximal end of the expandable flow rectifier. 
     In some applications, the expandable portion of the impeller is inflatable. In some applications, the left-ventricular assist device is configured for use with a purging fluid, the left-ventricular assist device includes a purging system configured to purge portions of the left-ventricular assist device by pumping the purging fluid through the left-ventricular assist device, and the expandable portion of the impeller is configured to be inflated with the purging fluid. In some applications, the purging system is configured to control the inflation of the expandable portion of the impeller, by modulating the pressure at which the purging fluid is pumped into the ventricular assist device. 
     In some applications, the impeller includes one or more impeller blades which are shaped to transition from being spiral shaped within a distal portion of the impeller to being substantially radially-extending within a proximal portion of the impeller. In some applications, the distal portion of the impeller is configured primarily to impart axial flow to blood that it pumps, and the proximal portion of the impeller is configured to impart substantial radial flow to blood that it pumps. 
     In some applications, the apparatus further includes one or more spiral flow rectifiers disposed within the pump-outlet tube, proximally with respect to impeller, the spiral flow rectifiers being configured to reduce radial flow components from blood flow by converting radial flow components to axial flow prior to the blood flowing out of the at least one blood outlet opening. In some applications, the one or more spiral flow rectifiers have spiral shapes with a pitch of the spiral shapes increasing from the distal ends of the flow rectifiers to their proximal ends, such that the spiral flow rectifiers are configured to gradually covert radial flow to axial flow. 
     There is further provided, in accordance with some applications of the present invention, apparatus including: 
     a left-ventricular assist device configured to assist left-ventricular functioning of a subject, the left-ventricular assist device including:
         a pump-outlet tube configured such that a proximal portion of the tube traverses an aortic valve of the subject, and a distal portion of the pump-outlet tube is disposed within a left ventricle of the subject, the pump-outlet tube defining at least one blood inlet opening that is configured to be disposed within the left ventricle and at least one blood outlet opening that is configured to be disposed within an aorta of the subject;   an impeller configured to pump blood through the pump-outlet tube from the subject&#39;s left ventricle to the subject&#39;s aorta;   an expandable flow rectifier that is configured to reduce radial flow components from blood flow through the pump-outlet tube prior to the blood flowing from the at least one blood outlet opening, the expandable flow rectifier being disposed along an axis of the pump-outlet tube and, in its expanded configuration, being shaped such that its diameter decreases from a distal end to a proximal end of the expandable flow rectifier.       

     In some applications, the pump-outlet tube defines a widened region in the vicinity of the impeller, which is wider than a portion of the pump-outlet tube proximal thereto, the impeller is configured to impart centrifugal flow to blood such as to cause the blood to flow into the widened region of the pump-outlet tube, and pump-outlet tube is configured to redirect the blood axially by the pump-outlet tube narrowing within the portion of the pump-outlet tube that is proximal to the widened portion. 
     In some applications, in its expanded configuration, the expandable flow rectifier has a three-dimensional teardrop shape. In some applications, in its expanded configuration, the expandable flow rectifier has a conical shape. In some applications, in its expanded configuration, the expandable flow rectifier has a frustoconical shape. 
     In some applications, the expandable flow rectifier includes a shape memory alloy that is shape set such that the expandable flow rectifier is self-expandable. 
     In some applications, the expandable flow rectifier is inflatable. In some applications, the left-ventricular assist device is configured for use with a purging fluid, the left-ventricular assist device includes a purging system configured to purge portions of the left-ventricular assist device by pumping the purging fluid through the left-ventricular assist device, and the expandable flow rectifier is configured to be inflated with the purging fluid. In some applications, the purging system is configured to control the inflation of the expandable flow rectifier, by modulating the pressure at which the purging fluid is pumped into the ventricular assist device. 
     In some applications, the apparatus further includes one or more spiral flow rectifiers disposed within the pump-outlet tube proximally with respect to impeller, the spiral flow rectifiers being configured to reduce radial flow components from blood flow by converting radial flow components to axial flow prior to the blood flowing out of the at least one blood outlet opening. In some applications, the one or more spiral flow rectifiers have spiral shapes with a pitch of the spiral shapes increasing from the distal ends of the flow rectifiers to their proximal ends, such that the spiral flow rectifiers are configured to gradually covert radial flow to axial flow. 
     There is further provided, in accordance with some applications of the present invention, apparatus including: 
     a left-ventricular assist device configured to assist left-ventricular functioning of a subject, the left-ventricular assist device including:
         a pump-outlet tube configured such that a proximal portion of the pump-outlet tube traverses an aortic valve of the subject, and a distal portion of the pump-outlet tube is disposed within a left ventricle of the subject;   a plurality of centrifugally-pumping impellers, disposed in series with each other, each of the centrifugally-pumping impellers being disposed within a respective impeller housing, and at least one volute being disposed around each of the impeller housings, each of the volutes leading to the pump-outlet tube,   each of the impellers being configured to pump blood from the left ventricle to an aorta of the subject by pumping blood into the impeller housing within which the impeller is disposed, into the at least one volute that is disposed around the impeller housing, and into the pump-outlet tube.       

     There is further provided, in accordance with some applications of the present invention, apparatus including: 
     a left-ventricular assist device configured to assist left-ventricular functioning of a subject, the left-ventricular assist device including:
         a pump-outlet tube configured such that a proximal portion of the pump-outlet tube traverses an aortic valve of the subject, and a distal portion of the pump-outlet tube is disposed within a left ventricle of the subject;   at least one centrifugally-pumping impeller disposed within an impeller housing, the impeller housing defining an upper blood-inlet opening and a lower blood inlet opening;   at least one volute disposed around the impeller housing, the volute leading to the pump-outlet tube,   the impeller being configured to pump blood from the left ventricle to an aorta of the subject by pumping blood into the impeller housing via both the upper blood-inlet opening and the lower blood-inlet opening, into the at least one volute that is disposed around the housing, and into the pump-outlet tube.       

     There is further provided, in accordance with some applications of the present invention, apparatus including: 
     a left-ventricular assist device configured to assist left-ventricular functioning of a subject, the left-ventricular assist device including:
         a pump-outlet tube configured such that a proximal portion of the pump-outlet tube traverses an aortic valve of the subject, and a distal portion of the pump-outlet tube is disposed within a left ventricle of the subject;   at least one centrifugally-pumping impeller disposed within an impeller housing, the impeller housing defining at least one blood-inlet opening;   at least one volute disposed around the impeller housing, the volute leading to the pump-outlet tube;   at least one blood-inflow tube that passes through at least one of the volute and pump-outlet tube, the blood-inflow tube defining a blood-inlet-tube opening, which is an opening through at least one of the volute and pump-outlet tube that is fluid communication with the ventricular blood stream;   the impeller being configured to pump blood from the left ventricle to an aorta of the subject by pumping blood into the impeller housing via the blood-inflow tube, into the blood-inlet opening, into the at least one volute that is disposed around the housing, and into the pump-outlet tube.       

     There is further provided, in accordance with some applications of the present invention, a method including: 
     manufacturing a multi-layered biconical centrifugal impeller, by:
         cutting a tube of a shape-memory alloy, such that the tube defines a plurality of elongate struts;   curving respective sets of the elongate struts such that the respective sets of elongate struts defines respective layers of the multi-layered biconical centrifugal impeller; and       

     covering the respective sets of elongate struts with respective layers of a covering material. 
     The present invention will be more fully understood from the following detailed description of embodiments thereof, taken together with the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  are schematic illustrations of a ventricular assist device that includes one or more centrifugally-pumping impellers, in accordance with some applications of the present invention; 
         FIG.  2    is a graph showing pressure-flow curves, respectively, for axially-pumping impellers, and for centrifugally-pumping impellers, such as centrifugally-pumping impellers that are used in accordance with some applications of the present invention; 
         FIG.  3    is a schematic illustration of an impeller housing, in a non-radially-constrained configuration, in accordance with some applications of the present invention; 
         FIG.  4    is a schematic illustration of a centrifugally-pumping impeller, in a non-radially-constrained configuration, in accordance with some applications of the present invention; 
         FIGS.  5 A,  5 B,  5 C, and  5 D  are schematic illustrations of respective views of a centrifugally-pumping impeller, in accordance with some applications of the present invention; 
         FIGS.  6 A,  6 B, and  6 C  are schematic illustrations of respective views of a centrifugally-pumping impeller, in accordance with some alternative applications of the present invention; 
         FIGS.  7 A,  7 B, and  7 C  are schematic illustrations of respective views of a centrifugally-pumping impeller, in accordance with some further alternative applications of the present invention; 
         FIGS.  8 A,  8 B, and  8 C  are schematic illustrations of respective views of a centrifugally-pumping impeller, in accordance with some still further alternative applications of the present invention; 
         FIGS.  9 A,  9 B, and  9 C  are schematic illustrations of blood-flow channels of an impeller in respective states of radial constraint, in accordance with some applications of the present invention; 
         FIGS.  10 A,  10 B, and  10 C  are schematic illustrations of a blood-flow channel of an impeller in respective states of radial constraint, in accordance with some applications of the present invention; 
         FIGS.  11 A and  11 B  are schematic illustrations of a biconical centrifugally-pumping impeller, in accordance with some applications of the present invention; 
         FIGS.  12 A,  12 B, and  12 C  are schematic illustrations a multi-layered, biconical centrifugally-pumping impeller, in accordance with some applications of the present invention; 
         FIGS.  13 A,  13 B,  13 C, and  13 D  are schematic illustrations of a pump-head portion of a ventricular assist device, in accordance with some applications of the present invention; 
         FIGS.  14 A,  14 B,  14 C, and  14 D  are schematic illustrations of a pump-head portion of a ventricular assist device, in accordance with some alternative applications of the present invention; 
         FIGS.  15 A,  15 B,  15 C, and  15 D  are schematic illustrations of a pump-head portion of a ventricular assist device that includes one or more blood inlet tubes that pass through a volute and/or a pump-outlet tube, in accordance with some applications of the present invention; 
         FIGS.  16 A,  16 B,  16 C, and  16 D  are schematic illustrations of a pump-head portion of a ventricular assist device that includes one or more blood inlet tubes that pass through a volute and/or a pump-outlet tube, in accordance with some alternative applications of the present invention; 
         FIG.  17    is a graph showing pressure-flow curves for respective centrifugally-pumping impellers, in accordance with some applications of the present invention; 
         FIG.  18    is a graph showing pressure-flow curves for respective centrifugally-pumping impellers, in accordance with some applications of the present invention; 
         FIGS.  19 A,  19 B, and  19 C  are schematic illustrations of a pump-head portion of a ventricular assist device that includes a mixed-flow impeller, in accordance with some applications of the present invention; 
         FIGS.  20 Ai  and  20 Aii are schematic illustrations of respective views of an impeller, in accordance with some applications of the present invention; 
         FIGS.  20 Bi  and  20 Bii are schematic illustrations of respective views of an impeller, in accordance with some alternative applications of the present invention; 
         FIGS.  20 Ci  and  20 Cii are schematic illustrations of respective views of an impeller, in accordance with some further alternative applications of the present invention; 
         FIGS.  21 Ai  and  21 Aii are schematic illustrations of respective views of a pump-head portion of a ventricular assist device that includes an expandable flow rectifier, in accordance with some applications of the present invention; 
         FIGS.  21 Bi  and  21 Bii are schematic illustrations of respective views of a pump-head portion of a ventricular assist device that includes an expandable flow rectifier, in accordance with some alternative applications of the present invention; 
         FIGS.  21 Ci  and  21 Cii are schematic illustrations of respective views of a pump-head portion of a ventricular assist device that includes an expandable flow rectifier, in accordance with some further alternative applications of the present invention; 
         FIGS.  21 Di  and  21 Dii are schematic illustrations of respective views of a pump-head portion of a ventricular assist device that includes an expandable flow rectifier, in accordance with some further alternative applications of the present invention; 
         FIGS.  22 Ai  and  22 Aii are schematic illustrations of respective views of a pump-head portion of a ventricular assist device that includes spiral flow rectifiers, in accordance with some applications of the present invention; and 
         FIGS.  22 Bi  and  22 Bii are schematic illustrations of respective views of a pump-head portion of a ventricular assist device that includes spiral flow rectifiers, in accordance with some applications of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is now made to  FIGS.  1 A-B , which are schematic illustrations of a ventricular assist device  20 , a distal end of which is configured to be disposed in a subject&#39;s left ventricle  22 , in accordance with some applications of the present invention. The ventricular assist device includes one or more centrifugally-pumping impellers  24  that are configured to be disposed within the subject&#39;s left ventricle  22 . For some applications, the ventricular assist device includes a plurality of centrifugally-pumping impellers  24  that are configured to be disposed within left ventricle  22 . Each of the centrifugally-pumping impellers  24  is typically disposed inside a respective impeller housing  26 . Typically, the impeller housing defines at least one blood inlet opening  42 ,  44  (via which blood flows into the impeller housing, shown in  FIG.  3   ), and a blood-outlet portion  46  (shown in  FIG.  3   ). Typically at least one volute  27  is disposed at least partially around the blood-outlet portion. Typically, the volute spirals and curves through 90 degrees and then leads into a pump-outlet tube  28 , which is disposed such that blood-outlet openings  30 , defined by a proximal portion of the pump-outlet tube, are disposed in the subject&#39;s aorta  32 . By being shaped in this way, the volute gradually converts radial flow to axial flow. Typically, the centrifugally-pumping impellers are configured to assist the functioning of the left ventricle, by pumping blood from the left ventricle, through volutes  27 , into pump-outlet tube  28 , and then out of blood-outlet openings  30  and into aorta  32 . 
     For some applications, each of the impeller housings includes a plurality of blood-inlet openings, e.g., an upper blood-inlet opening  42  and a lower blood-inlet opening  44 , as described in further detail hereinbelow. Similarly, for some applications, each of the impeller housings includes a plurality of volutes  27 , as described in further detail hereinbelow. 
     With reference to  FIG.  1 A , for some applications, a motor  10  is disposed inside a motor unit  11  that is disposed outside the subject&#39;s body. Typically, a computer processor  12 , which is part of a control console  14 , drives the motor to rotate. The motor typically conveys rotational motion to the impeller via a drive cable  15 , which is typically disposed within an outer tube  17  (with the outer tube acting as a bearing within which the drive cable rotates). For some applications, the drive cable is directly coupled to the centrifugally-pumping impeller. Alternatively, the drive cable is coupled to a rigid axial shaft  19  (shown in  FIG.  4   , for example), and the centrifugally pumping impeller is disposed on the rigid axial shaft. For some applications, the apparatus includes a purging system  16  and a plurality of purging fluid ports  18  (e.g., purging fluid inlet ports, and/or purging fluid outlet ports), via which a purging fluid (e.g., a glucose-based solution and/or saline) is pumped into the ventricular assist device, in order to purge portions of the ventricular assist device. Typically, the purging fluid is configured to be pumped into the apparatus such that the purging fluid flows between drive cable  15  and outer tube  17  and thereby purges the interface between the drive cable and the outer tube. Further typically, the purging fluid is configured to purge interfaces between axial shaft  19  and radial bearings within which the axial shaft rotates. For some applications, ventricular assist device  20  includes two or more centrifugally-pumping impellers  24 , that are disposed in series with one another along the axis of the device, as shown in  FIG.  1 B . (It is noted that, in  FIG.  1 B , the ventricular-assist device is not shown to scale relative to the subject&#39;s anatomy. Similarly, the disposition of portions of the ventricular assist device relative to portions of the left ventricular anatomy is typically different from that shown in  FIG.  1 B .) For some such applications, each of the two or more centrifugally pumping impellers is disposed on a single rigid axial shaft  19 . Pumping blood using a plurality of centrifugally-pumping impellers typically increases the volume of blood that is pumped by the ventricular assist device relative to if only one centrifugally-pumping impeller is used, ceteris paribus. It is noted that although some applications of the present invention are described with reference to a ventricular assist device that includes two or more centrifugally-pumping impellers, the scope of the present application includes practicing some of the applications described herein with a ventricular assist device that includes a single centrifugally-pumping impeller. 
     For some applications, the ventricular assist device is used to assist the functioning of a subject&#39;s left ventricle during a percutaneous coronary intervention. In such cases, the ventricular assist device is typically used for a period of up to 10 hours (e.g., up to six hours), during a period in which there is risk of developing hemodynamic instability (e.g., during or immediately following the percutaneous coronary intervention). Alternatively or additionally, the ventricular assist device is used to assist the functioning of a subject&#39;s left ventricle for a longer period (e.g., for example, 2-20 days, e.g., 4-14 days) upon a patient suffering from cardiogenic shock, which may include any low-cardiac-output state (e.g., acute myocardial infarction, myocarditis, cardiomyopathy, post-partum, etc.). For some applications, the ventricular assist device is used to assist the functioning of a subject&#39;s left ventricle for yet a longer period (e.g., several weeks or months), e.g., in a “bridge to recovery” treatment. For some such applications, the ventricular assist device is permanently or semi-permanently implanted, and the impellers of the ventricular assist device are powered transcutaneously, e.g., using an external antenna that is magnetically coupled to the impeller. 
     Typically the distal end of the ventricular assist device is guided to the left ventricle over a guidewire (not shown). During the insertion of the distal end of the device to the left ventricle, a delivery catheter  34  (shown in  FIG.  1 B ) is disposed over the distal end of the device. Once the distal end of the device is disposed in the left ventricle, the delivery catheter is typically retracted to the aorta, and the guidewire is withdrawn from the subject&#39;s body. Typically, centrifugally-pumping impellers  24  and impeller housings  26  are radially expandable (e.g., radially self-expandable), as described in further detail hereinbelow. During the insertion of the distal end of the device to the left ventricle, delivery catheter  34  typically maintains the centrifugally-pumping impellers  24  and impeller housings  26  in radially-constrained (also known as, crimped) configurations. The retraction of the delivery catheter typically causes the centrifugally-pumping impellers  24  and impeller housings  26  to assume non-radially-constrained configurations, due to the centrifugally-pumping impellers  24  and impeller housings  26  radially self-expanding. Alternatively or additionally, upon having been delivered to the left ventricle, the centrifugally-pumping impellers  24  and impeller housings  26  are manually radially expanded. Typically, the ventricular assist device is inserted into the subject&#39;s body in order to provide an acute treatment to the subject. For some applications, in order to withdraw the left ventricular device from the subject&#39;s body at the end of the treatment, the delivery catheter is advanced over the distal end of the device, which causes the centrifugally-pumping impellers  24  and impeller housings  26  to assume radially-constrained configurations. Alternatively or additionally, the distal end of the device is retracted into the delivery catheter which causes centrifugally-pumping impellers  24  and impeller housings  26  to assume radially-constrained configurations. 
     For some applications (not shown), the ventricular assist device and/or delivery catheter  34  includes an ultrasound transducer at its distal end and the ventricular assist device is advanced toward the subject&#39;s ventricle under ultrasound guidance. 
     Typically, the pump-outlet tube and the one or more volutes are continuous with respect to each other. That is to say that, typically, the pump-outlet tube and the one or more volutes are made of the same material as each other, and the volutes merge with the blood-outlet tube to form a continuous blood flow channel. Pump-outlet tube  28  and/or volute  27  are typically made of a blood-impermeable collapsible material. For example, pump-outlet tube  28  and/or volute  27  may include polyurethane, polyester, and/or silicone. Alternatively or additionally, pump-outlet tube  28  and/or volute  27  are made of polyethylene terephthalate (PET) and/or polyether block amide (e.g., PEBAX®). For some applications (not shown), pump-outlet tube  28  and/or volute  27  are reinforced with a reinforcement structure, e.g., a braided reinforcement structure, such as a braided nitinol tube. 
     Typically, the proximal portion of pump-outlet tube  28  is configured to be placed such that it is at least partially disposed within the subject&#39;s ascending aorta. For some applications, the proximal portion of pump-outlet tube  28  traverses the subject&#39;s aortic valve, passing from the subject&#39;s left ventricle into the subject&#39;s ascending aorta, as shown in  FIG.  1 B . As described hereinabove, for some applications, the proximal portion of pump-outlet tube  28  defines one or more blood-outlet openings  30 , via which blood flows from the tube into the ascending aorta, during operation of the impeller. Typically, pump-outlet tube  28  defines a plurality of blood-outlet openings  30 , for example, between two and eight blood-outlet openings (e.g., between two and four blood-outlet openings). During operation of the impeller, the pressure of the blood flow through pump-outlet tube  28  typically maintains the proximal portion of pump-outlet tube  28  in an open state. For some applications, in the event that, for example, the impellers malfunction, the proximal portion of pump-outlet tube  28  is configured to collapse inwardly, in response to pressure outside of the proximal portion of the pump-outlet tube exceeding pressure inside the proximal portion of the tube. In this manner, the proximal portion of the pump-outlet tube acts as a safety valve, preventing retrograde blood flow into the left ventricle from the aorta. 
     Reference is now made to  FIG.  2   , which is a graph showing pressure-flow curves, respectively, for axially-pumping impellers and for centrifugally-pumping impellers, such as impellers  24  of the present application. The curves schematically illustrate the flow that is generated by the impellers when they are pumping against a given pressure gradient, with the dashed lines added to the graph to illustrate the pressure gradients, respectively, at diastole and systole. As shown, it is typically the case the that the pressure-flow curve of a centrifugally-pumping impeller is flatter than that of an axially-pumping impeller. Typically, this results in the flow (and arterial pressure) that is generated by a centrifugally-pumping impeller being more pulsatile than that generated by an axially-pumping impeller, since there is a greater change in flow between diastole and systole. In some patients, it may be desirable for the patient&#39;s arterial blood pressure to maintain pulsatility even while a treatment is being provided to the patient to assist with the patient&#39;s left-ventricular functioning. Therefore, in accordance with some applications, centrifugally-pumping impellers  24  are used in ventricular assist device  20 . Alternatively or additionally, ventricular assist device  20  includes centrifugally-pumping impellers  24  due to other considerations. 
     Reference is now made to  FIG.  3   , which is a schematic illustration of impeller housing  26 , in a non-radially-constrained configuration, in accordance with some applications of the present invention. Typically the impeller housing is radially expandable (e.g., radially self-expandable), such that the impeller housing may be delivered while it is disposed in a radially-constrained configuration inside delivery catheter  34 , and may subsequently be deployed in a non-radially-constrained configuration inside the left ventricle. For some applications, the impeller housing includes a self-expandable frame  40 , which is typically made of a shape-memory alloy, such as nitinol. Typically, the frame is a stent-like frame, in that it comprises struts that typically, in turn, define cells. For some applications, at least a portion of the frame is covered with a covering material  41 , such as silicone, polyurethane, polyester, polyethylene terephthalate (PET), and/or polyether block amide (e.g., PEBAX®). For some applications, the covering is disposed around the frame, such as to define an upper blood-inlet opening  42  and a lower blood-inlet opening  44 . During operation of the centrifugally-pumping impeller, blood is pumped into impeller housing  26  by the impeller via both the upper blood-inlet opening and the lower blood-inlet opening. Typically, this increases the volume of blood that is pumped into the housing by the impeller relative to if the impeller housing only defined a single blood-inlet opening. For some applications, covering material  41  is disposed around the frame, such as to define a blood-outlet portion  46 . Typically, the blood-outlet portion is a circumferential portion of the frame that is not covered by covering material  41 . Further typically, volute  27  surrounds the blood outlet portion, as described in further detail hereinbelow. For some applications, the impeller housing is radially constrained by axially elongating the frame, such that the diameter of the frame is decreased. 
     Reference is now made to  FIG.  4   , which is a schematic illustration of centrifugally-pumping impeller  24 , in a non-radially-constrained configuration, in accordance with some applications of the present invention. For some applications, in its non-radially-constrained configuration, the impeller defines a plurality of blood-flow channels  50 U or  50 L, each of which channels blood from an axially-facing, blood-inlet opening  52  that is disposed at an upper or lower end of the impeller, to a radially-facing, blood-outlet opening  54  that is disposed between the upper and lower ends of the impeller, e.g., toward or at the axial center of the impeller. Typically, the impeller includes at least one upper blood-flow channel  50 U that defines a blood-inlet opening at the upper end of the impeller and at least one lower blood-flow channel  50 L that defines a blood-inlet opening at the lower end of the impeller. For some applications, each of the blood-flow channels is individually constructed. Alternatively, the blood-channels are defined by an element, such as a frame (e.g., a shape-memory alloy frame, such as a nitinol frame) that is dipped in a material (e.g., a polymer, such as an elastomeric polymer, silicone, polyurethane, polyester, polyethylene terephthalate (PET), and/or polyether block amide (e.g., PEBAX®)), with the material being made to cover the frame such as to define the blood-flow channels (e.g., via the dipping process, and/or by cutting the material). 
     It is noted that, for some applications, the blood-inlet openings are disposed at an angle with respect to the axial direction, and/or the blood-outlet openings are disposed at an angle with respect to the radial direction. However, it is typically the case that the blood-inlet openings are substantially axially facing, and the blood-outlet openings are substantially radially facing. 
     The blood-flow channels are typically coupled to axial shaft  19  in such a manner that during insertion of the impeller via delivery catheter  34  (shown in  FIG.  1 B ), the blood-flow channels are radially constrained by one or both ends of the blood-flow channels sliding axially with respect to the axial shaft. An example of this is described in further detail hereinbelow with reference to  FIGS.  10 A-C . 
     Typically, during operation of the centrifugally-pumping impeller, the impeller is disposed in its non-radially-constrained configuration. While disposed in its non-radially-constrained configuration, the impeller is driven to rotate, by the motor rotating the drive cable, and the drive cable rotating axial shaft  19 , to which the blood-flow-channels are coupled. The rotation of the impeller causes the impeller to draw blood in through the axially-facing blood-inlet openings  52  of the blood-flow channels (and through blood-inlet openings  42 ,  44  of the impeller housing, shown in  FIG.  3   ), and to pump blood out of radially-facing blood outlet openings  54  (and out of blood-outlet portion  46  of the impeller housing, shown in  FIG.  3   ). 
     Reference is now made to  FIGS.  5 A,  5 B,  5 C, and  5 D  which are schematic illustrations of respective views of centrifugally-pumping impeller  24 , in accordance with some applications of the present invention.  FIG.  5 A  shows a side view of the impeller,  FIG.  5 B  shows a top view,  FIG.  5 C  shows a bottom view, and  FIG.  5 D  shows a side view of the impeller disposed inside frame  40  of impeller housing  26 . (For illustrative purposes, frame  40  is shown in the absence of covering material  41  in  FIG.  5 D .) As described hereinabove, typically, the impeller includes at least one upper blood-flow channel  50 U that defines a blood-inlet opening at the upper end of the impeller and at least one lower blood-flow channel  50 L that defines a blood-inlet opening at the lower end of the impeller. As shown, for some applications, blood-outlet openings defined by the upper and lower blood-flow channels are disposed around the circumference of the impeller at the same axial location as each other, and alternate with each other around the circumference of the impeller. For some applications, the blood-flow channels of the impeller do not define any circumferential curvature. Rather, as shown in  FIGS.  5 A-D , each of the blood-flow channels curves through 90 degrees from the axially-facing blood-inlet opening  52  to the radially-facing blood-outlet opening  54 . 
     Reference is now made to  FIGS.  6 A,  6 B, and  6 C , which are schematic illustrations of respective views of centrifugally-pumping impeller  24 , in accordance with some applications of the present invention.  FIG.  6 A  shows a side view of the impeller,  FIG.  6 B  shows a top view, and  FIG.  6 C  shows a bottom view. As described hereinabove, typically, the impeller includes at least one upper blood-flow channel  50 U that defines a blood-inlet opening at the upper end of the impeller and at least one lower blood-flow channel  50 L that defines a blood-inlet opening at the lower end of the impeller. As shown, for some applications, blood-outlet openings defined by the upper and lower blood-flow channels are disposed around the circumference of the impeller at the same axial location as each other, and alternate with each other around the circumference of the impeller. For some applications, the blood-flow channels of the impeller define a circumferential curvature. For example, as shown in  FIGS.  6 A-C , each of the blood-flow channels curves through 90 degrees from the axially-facing blood-inlet opening  52  to the radially-facing blood-outlet opening  54 , and additionally curves circumferentially around the axial shaft. 
     Reference is now made to  FIGS.  7 A,  7 B, and  7 C , which are schematic illustrations of respective views of centrifugally-pumping impeller  24 , in accordance with some applications of the present invention.  FIG.  7 A  shows a side view of the impeller,  FIG.  7 B  shows a top view, and  FIG.  7 C  shows a bottom view. As described hereinabove, typically, the impeller includes at least one upper blood-flow channel  50 U that defines a blood-inlet opening at the upper end of the impeller and at least one lower blood-flow channel  50 L that defines a blood-inlet opening at the lower end of the impeller. As shown, for some applications, blood-outlet openings defined by the upper and lower blood-flow channels are disposed around the circumference of the impeller at different axial locations from each other, with the blood-outlet openings of the upper blood-flow channels typically disposed above those of the lower blood-flow channels. For some applications, the blood-outlet openings at the upper and lower axial locations are staggered with respect to each other, as shown in  FIGS.  7 A-C . For some applications, the blood-flow channels of the impeller define a circumferential curvature. For example, as shown in  FIGS.  7 A-C , each of the blood-flow channels curves through 90 degrees from the axially-facing blood-inlet opening  52  to the radially-facing blood-outlet opening  54 , and additionally curves circumferentially around the axial shaft. 
     Reference is now made to  FIGS.  8 A,  8 B, and  8 C , which are schematic illustrations of respective views of centrifugally-pumping impeller  24 , in accordance with some applications of the present invention.  FIG.  8 A  shows a side view of the impeller,  FIG.  8 B  shows a top view, and  FIG.  8 C  shows a bottom view. As described hereinabove, typically, the impeller includes at least one upper blood-flow channel  50 U that defines a blood-inlet opening at the upper end of the impeller and at least one lower blood-flow channel  50 L that defines a blood-inlet opening at the lower end of the impeller. As shown, for some applications, blood-outlet openings defined by the upper and lower blood-flow channels are disposed around the circumference of the impeller at the different axial locations from each other, with the blood-outlet openings of the upper blood-flow channels typically disposed above those of the lower blood-flow channels. For some applications, the blood-outlet openings at the upper and lower axial locations are not staggered with respect to each other. Rather, a single blood-outlet opening is disposed at the upper axial location above a single corresponding blood-outlet opening disposed at the lower axial location, as shown in  FIGS.  8 A-C . For some applications, the blood-flow channels of the impeller define a circumferential curvature. For example, as shown in  FIGS.  8 A-C , each of the blood-flow channels curves through 90 degrees from the axially-facing blood-inlet opening  52  to the radially-facing blood-outlet opening  54 , and additionally curves circumferentially around the axial shaft. 
     With reference to each of the examples of the blood-flow channels shown in  FIGS.  5 A- 8 C , it is noted that it is typically the case that the blood-flow channel widens from the blood-inlet opening to the blood-outlet opening, i.e., the cross-sectional area of the channel increases from the blood-inlet opening to the blood-outlet opening. 
     Reference is now made to  FIGS.  9 A,  9 B, and  9 C , which are schematic illustrations of blood-flow channels  50 U and  50 L of an impeller in respective states of radial constraint, in accordance with some applications of the present invention. As described hereinabove, typically, the impeller includes at least one upper blood-flow channel  50 U that defines a blood-inlet opening at the upper end of the impeller and at least one lower blood-flow channel  50 L that defines a blood-inlet opening at the lower end of the impeller. For some applications, each of the blood-flow channels comprises a tube, and the upper and lower blood-flow channels alternate with each other around the circumference of the impeller. Each of the tubes that defines the blood-flow channels typically defines a first hole at its first end that the defines blood-inlet opening  52  and a second hole at its second end that defines blood-outlet opening  54 . Typically, in the non-radially-constrained configuration of the impeller (shown in  FIG.  9 C ), the blood-flow channels are disposed at an angle with respect to the axis of the impeller, such that the blood outlet openings are substantially radially facing. In the radially-constrained configuration of the impeller (shown in  FIG.  9 A ), the blood-flow channels are substantially aligned with the axis of the impeller.  FIG.  9 B  shows the blood-flow channels during the transition from the radially-constrained configuration to the non-radially-constrained configuration of the impeller. 
     Reference is now made to  FIGS.  10 A,  10 B, and  10 C , which are schematic illustrations of a blood-flow channel  50  of an impeller in respective states of radial constraint, in accordance with some applications of the present invention. For some applications, a single blood-flow channel  50  defines an upper blood-inlet opening  52 U at the upper end of the impeller and a lower blood-inlet opening  52 L at the lower end of the impeller, in addition to a blood-outlet opening  54  between the upper and lower ends. Typically, in the non-radially-constrained configuration of the impeller (shown in  FIG.  10 C ), the blood-flow channel forms a laterally-facing V-shape or U-shape with respect to axial shaft  19 , with the blood-outlet opening facing radially outwardly at the center of the V-shape or U-shape. In the radially-constrained configuration of the impeller (shown in  FIG.  10 A ), the blood-flow channel is substantially aligned with the axial shaft.  FIG.  10 B  shows the blood-flow channel during the transition from the radially-constrained configuration to the non-radially-constrained configuration of the impeller. 
     Reference is now made to  FIGS.  11 A and  11 B , which are schematic illustrations of centrifugally-pumping impeller  24  in a non-radially-constrained configuration, the impeller being a biconical centrifugally-pumping impeller, in accordance with some applications of the present invention.  FIG.  11 A  shows just the impeller, and  FIG.  11 B  shows the impeller disposed inside impeller housing  26 . For some applications, the impeller has a generally biconical shape. Typically, the impeller is constructed from a self-expandable frame  60 , which is typically made of a shape-memory alloy, such as nitinol. For some applications, at least a portion of the frame is covered with a covering material  62 , such as silicone, polyurethane, polyester, polyethylene terephthalate (PET), and/or polyether block amide (e.g., PEBAX®). For some applications, frame  60  includes struts that extend between a proximal bushing  64  and a distal bushing  66 . Typically, one of the bushings is coupled to axial shaft  19 , and the other bushing is axially slidable with respect to axial shaft  19 . For some applications, during insertion of the impeller via delivery catheter  34  (shown in  FIG.  1 B ), the impeller is radially constrained by the slidable bushing sliding axially with respect to the axial shaft. 
     Typically, during operation of the centrifugally-pumping impeller, the impeller is disposed in its non-radially-constrained configuration (shown in  FIGS.  11 A-B ). While disposed in its non-radially-constrained configuration, the impeller is driven to rotate, by the motor rotating the drive cable, and the drive cable rotating axial shaft  19 , to which frame  60  is coupled. The rotation of the impeller causes the impeller to draw blood in through blood-inlet openings  42 ,  44  of the impeller housing (shown in  FIG.  11 B ), and to pump blood out of blood-outlet portion  46  of the impeller housing (shown in  FIG.  11 B ). 
     Reference is now made to  FIGS.  12 A,  12 B, and  12 C , which are schematic illustrations of centrifugally-pumping impeller  24  in a non-radially-constrained configuration, the impeller being a multi-layered biconical centrifugally-pumping impeller, in accordance with some applications of the present invention. The impeller shown in  FIGS.  12 A-C  typically functions in a generally-similar manner to that described with reference to the impeller shown in  FIGS.  11 A-B , except for the differences described hereinbelow. 
       FIGS.  12 A and  12 B  show, respectively, a 3D view and a cross-sectional view of an illustrative example of a multi-layered biconical centrifugally-pumping impeller that is made out of a solid material. In practice, impeller  24  is typically constructed from a frame  60  and a covering material  62 , as described with reference to  FIGS.  11 A-B . Typically, each layer of the impeller is configured to pump blood from the ends of the impeller and radially outwardly from the center of the impeller, as is schematically illustrated in  FIG.  12 B .  FIG.  12 C  is a schematic illustration of a slice of frame  60  of a multi-layered biconical centrifugally-pumping impeller. For some applications, a single tube of a shape-memory alloy (such as nitinol) is cut into struts and the struts are shaped such as to define each of the layers of the multi-layered biconical centrifugally-pumping impeller. For example, in the slice shown in  FIG.  12 C , strut  70  (in combination with additional struts that are not shown) defines an outer layer, strut  72  (in combination with additional struts that are not shown) defines an intermediate layer and strut  74  (in combination with additional struts that are not shown) defines an inner layer. Typically the covering material is applied to the struts that define each of the layers of the multi-layered biconical centrifugally-pumping impeller. For some applications, the struts extend from proximal bushing  64  to distal bushing  66 , with the bushings typically being defined by the same tube from which the struts were cut, and with the bushings typically functioning as described hereinabove with reference to  FIGS.  11 A-B . 
     Reference is now made to  FIGS.  13 A-D , which are schematic illustrations of ventricular assist device  20 , in accordance with some applications of the present invention. As described hereinabove, the ventricular assist device includes one or more centrifugally-pumping impellers  24  that are configured to be disposed within the subject&#39;s left ventricle. Each of the centrifugally-pumping impellers  24  is typically disposed inside an impeller housing  26 . Typically, the impeller housing defines at least one blood inlet opening  42 ,  44  (via which blood flows into the impeller housing) and blood-outlet portion  46 . Typically at least one volute  27  is disposed at least partially around the blood-outlet portion. Typically, the volute spirals and curves through 90 degrees and then leads into pump-outlet tube  28 , which is disposed such that blood-outlet openings  30 , defined by a proximal portion of the pump-outlet tube (shown in  FIG.  1 B ), are disposed in the subject&#39;s aorta  32 . Typically, by being shaped in the above-described shape, the volute gradually coverts radial flow to axial flow. As described hereinabove, typically, the pump-outlet tube and the volute are continuous with respect to each other. That is to say that, typically, the pump-outlet tube and the volute are made of the same material as each other, and the volute merges with the blood-outlet tube to form a continuous blood flow channel. 
     Typically, the centrifugally-pumping impellers are configured to assist the functioning of the left ventricle, by pumping blood from the left ventricle, through volute  27 , into pump-outlet tube  28 , and then out of blood-outlet openings  30  and into aorta  32 . For some applications, each of the impeller housings includes a plurality of blood inlet openings, e.g., an upper blood-inlet opening  42  and a lower blood-inlet opening  44 , as shown. 
     Reference is now made to  FIGS.  14 A-D , which are schematic illustrations of pump-head portion  80  of ventricular assist device  20 , in accordance with some applications of the present invention. As described hereinabove, the ventricular assist device includes one or more centrifugally-pumping impellers  24  that are configured to be disposed within the subject&#39;s left ventricle. Each of the centrifugally-pumping impellers  24  is typically disposed inside an impeller housing  26 . Typically, the impeller housing defines at least one blood inlet opening  42 ,  44  (via which blood flows into the impeller housing) and blood-outlet portion  46 . For some applications, a plurality of volutes  27  are disposed at least partially around the blood-outlet portion, with the inlets to each of the volutes typically being spaced from each other around the circumference of the blood-outlet portion. Typically, each of the volutes spirals and curves through 90 degrees and then leads into pump-outlet tube  28 , which is disposed such that blood-outlet openings  30 , defined by a proximal portion of the tube (shown in  FIG.  1 B ), are disposed in the subject&#39;s aorta  32 . As described hereinabove, typically, the pump-outlet tube and the volutes are continuous with respect to each other. That is to say that, typically, the pump-outlet tube and the one or more volutes are made of the same material as each other, and the volutes merge with the blood-outlet tube to form a continuous blood flow channel. 
     Typically, the centrifugally-pumping impellers are configured to assist the functioning of the left ventricle, by pumping blood from the left ventricle, through volutes  27 , into pump-outlet tube  28 , and then out of blood-outlet openings  30  and into aorta  32 . For some applications, each of the impeller housings includes a plurality of blood inlet openings, e.g., an upper blood-inlet opening  42  and a lower blood-inlet opening  44 , as shown. 
     Reference is now made to  FIGS.  15 A-D , which are schematic illustrations of pump-head portion  80  of ventricular assist device  20 , in accordance with some applications of the present invention. Reference is also made to  FIGS.  16 A-D , which are schematic illustrations of pump-head portion  80  of ventricular assist device  20 , in accordance with some alternative applications of the present invention. 
     As described hereinabove, the ventricular assist device includes one or more centrifugally-pumping impellers  24  that are configured to be disposed within the subject&#39;s left ventricle. Each of the centrifugally-pumping impellers  24  is typically disposed inside an impeller housing  26 . Typically, the impeller housing defines at least one blood inlet opening  42 ,  44  (via which blood flows into the impeller housing) and blood-outlet portion  46 . For some applications, volute  27  is disposed at least partially around the blood-outlet portion and the volute spirals and curves through 90 degrees and then leads into pump-outlet tube  28 , which is disposed such that blood-outlet openings  30 , defined by a proximal portion of the pump-outlet tube (shown in  FIG.  1 B ), are disposed in the subject&#39;s aorta  32 . As described hereinabove, typically, the pump-outlet tube and the one or more volutes are continuous with respect to each other. That is to say that, typically, the pump-outlet tube and the one or more volutes are made of the same material as each other, and the volutes merge with the blood-outlet tube to form a continuous blood flow channel. For some applications, the continuous blood flow channel is shaped to define a fold  78 , with the fold defining the spiraling and the curvature of the volute, and defining respective levels of the spiral of the volute, as shown in  FIGS.  15 A-D  and  16 A-D. Alternatively (not shown), a closed blood-flow channel defines the spiraling and the curvature of the volute, with the closed blood-flow channel forming respective levels of the volute, and with respective levels of the volute having one or more layers of material separating between them, rather than the fold defining the levels of the volute. 
     For some applications blood flow into the upper blood inlet opening is via one or more blood inlet tubes  82 , which pass through volute  27  and/or pump-outlet tube  28 . For example, as shown in  FIGS.  15 A-D  and  16 A-D, pump head portion  80  includes a plurality of blood inlet tubes  82 , each of which defines a blood-inlet-tube opening  84 , which is an opening through volute  27  and/or pump-outlet tube  28  that is fluid communication with the ventricular blood stream. Blood flows into impeller housing  26  via blood-inlet-tube openings  84 , through the blood-inlet tubes and into blood-inlet opening  42 . For some applications, blood additionally flows into impeller housing  26  via lower blood-inlet opening  44 . Blood typically flows out of the impeller housing via outlet portion  46  of the impeller housing, into volute  27  and then into pump-outlet tube  28 . As described hereinabove, the blood typically then flows out of pump-outlet tube  28 , via blood-outlet openings  30  (shown in  FIG.  1 B ) and into aorta  32 . 
     It is noted that in  FIGS.  13 A- 16 D , the proximal end of pump-outlet tube  28  (which defines blood-outlet openings) is not shown, for illustrative purposes. 
     Reference is now made to  FIG.  17   , which is a graph showing pressure-flow curves for respective centrifugally-pumping impellers, in accordance with some applications of the present invention. Curve A is the pressure-flow curve for a first impeller having a height of 5 mm that is disposed inside an impeller housing having blood-inlet openings that are 8.4 mm in diameter, whereas curve B is the pressure-flow curve for a second impeller having a height of 2.5 mm and that is disposed inside an impeller housing having blood-inlet openings that are 5 mm in diameter. Both curves correspond to a rotation rate of 6,500 RPM. As may be observed, the flow generated by the second impeller is lower than that generated by the first impeller at all pressure gradients, and the flow generated by the second impeller drops to zero at a lower pressure gradient than that of the first impeller. Therefore, in accordance with some applications of the present invention, an impeller is used that has a minimum height of 4 mm (e.g., 5 mm). For some applications, the diameter of the blood-inlet opening (e.g., the upper blood-inlet opening, and/or the lower blood-inlet opening) of the impeller housing is more than 5 mm (e.g., more than 7 mm). 
     Reference is now made to  FIG.  18   , which is a graph showing pressure-flow curves for respective centrifugally-pumping impellers, in accordance with some applications of the present invention. Curve A is the pressure-flow curve for a forward-curved impeller (i.e., an impeller the blades of which curve circumferentially in the same direction as the direction in which the impeller is rotated), and curve B is for a generally similar impeller but one that is backward-curved (i.e., an impeller the blades of which curve circumferentially in the opposite direction from the direction in which the impeller is rotated). Both curves correspond to a rotation rate of 6,500 RPM. As may be observed, the flow generated by the second impeller is lower than that generated by the first impeller at all pressure gradients, and the flow generated by the second impeller drops to zero at a lower pressure gradient than that of the first impeller. Therefore, in accordance with some applications of the present invention, a forward-curved impeller is used. 
     Reference is now made to  FIGS.  19 A,  19 B, and  19 C , which are schematic illustrations of pump-head portion  80  of a ventricular assist device  20  that includes a mixed-flow impeller  100 , in accordance with some applications of the present invention. For some applications, a mixed-flow impeller that is configured to pump blood both axially and centrifugally is used within the ventricular assist device. For some applications, the mixed-flow impeller is disposed inside a frame  110 , and pump-outlet tube  28  extends over the frame or within the frame such as to cover the portion of the frame within which the impeller is disposed. It is noted that in  FIG.  19 C , the pump-outlet tube is shown inside frame  110  for illustrative purposes, but typically, the pump outlet-tube is disposed outside of frame  110 . For some applications (not shown), an additional inner lining lines at least a portion of frame  110 , such that blood flow into the frame is exposed to a smooth inner surface. 
     Typically, the pump-outlet tube defines one or more blood inlet openings  108  via which blood is pumped into a distal end of pump-outlet tube  28  (which is configured to be placed within the ventricle). As described hereinabove, the blood typically then flows out of pump-outlet tube  28 , via blood-outlet openings  30  (shown in  FIG.  1 B ) and into aorta  32 . It is noted that in  FIGS.  19 A-C , the proximal end of pump-outlet tube  28  (which defines blood-outlet openings) is not shown, for illustrative purposes. For some applications, a widened region  106  of the pump-outlet tube around the impeller is widened relative to a region proximal thereto. The impeller is configured to pump blood entering through blood inlet openings  108  both axially (in the proximal direction) and radially. The radial flow that is imparted to the blood causes the blood to flow into the widened region of the pump-outlet tube, and the blood is then redirected axially by the narrowing of the pump-outlet tube. 
     For some applications, spiral flow rectifiers  102  are disposed within pump-outlet tube  28  proximally with respect to impeller  100 . The spiral flow rectifiers are configured to reduce radial and/or rotational flow components from the blood flow by converting these components to axial flow prior to the blood flowing out of the proximal end of the pump-outlet tube. For some applications, the flow rectifiers have spiral shapes with a pitch of the spiral increasing from the distal ends of the flow rectifiers to their proximal ends. By being shaped in this way, the flow rectifiers gradually covert radial flow to axial flow. For some applications, the flow rectifiers extend radially from a tube within which the drive cable is disposed (e.g., tube  17 ) to pump-outlet tube. In accordance with respective applications, the flow rectifiers are made of a flexible material (e.g., an elastomeric material, silicone, polyurethane, polyester, polyethylene terephthalate (PET), and/or polyether block amide (e.g., PEBAX®)), or comprise a structure that is made of a shape memory alloy (such as nitinol) that is covered with a flexible material (e.g., an elastomeric material, silicone, polyurethane, polyester, polyethylene terephthalate (PET), and/or polyether block amide (e.g., PEBAX®)). 
     Reference is now made to  FIGS.  20 Ai  and  20 Aii, which are schematic illustrations of respective views of impeller  100 , in accordance with some applications of the present invention.  FIG.  20 Ai  shows the impeller disposed on axial shaft  19 , in the absence of any surrounding portions of pump-head portion  80 , for illustrative purposes. FIGS.  20 Aii shows a cross-sectional view of the impeller disposed within frame  110 , but in the absence of pump-outlet tube  28 , again for illustrative purposes. For some applications, the impeller has a generally similar structure to that described in US 2020/0237981 to Tuval, which is incorporated herein by reference. Typically, the impeller includes a plurality of outer elongate elements  101 , which are helical and which wind around a central axial spring  103 , such that the helices defined by the helical elongate elements are coaxial with the central axial spring. Typically, the impeller includes two or more helical elongate elements (e.g., three helical elongate elements). For some applications, the helical elongate elements and the central axial spring are made of a shape-memory material, e.g., a shape-memory alloy such as nitinol. Typically, each of the helical elongate elements and the central axial spring support a film  105  of a material (e.g., an elastomer, such as polyurethane, and/or silicone) therebetween. For some applications, the film of material includes pieces of nitinol embedded therein, for example in order to strengthen the film of material. 
     As shown in  FIGS.  20 Ai -ii, for some applications, the shapes of the helical elongate elements are axially symmetrical, i.e., the shapes of the helical elongate elements at their proximal ends is the same as that of their distal ends. Typically, when deployed within a pump head that is as shown in  FIGS.  21 Bi -Bii, such an impeller is configured to pump blood both axially and radially, as described in further detail hereinbelow. 
     Reference is now made to  FIGS.  20 Bi  and  20 Bii, which are schematic illustrations of respective views of impeller  100 , in accordance with some applications of the present invention.  FIG.  20 Bi  shows the impeller disposed on axial shaft  19 , in the absence of any surrounding portions of pump-head portion  80 , for illustrative purposes. FIGS.  20 Bii shows a cross-sectional view of the impeller disposed within frame  110 , but in the absence of pump-outlet tube  28 , again for illustrative purposes. For some applications, the impeller has a generally similar structure to that described in US 2020/0237981 to Tuval, which is incorporated herein by reference, except for differences described hereinbelow. Typically, the impeller includes a plurality of outer elongate elements  101 , which wind around a central axial spring  103 . For some applications, toward a distal end of the impeller, the elongate elements are helical, such that the helices defined by the helical elongate elements are coaxial with the central axial spring. As the elongate elements transition toward the proximal end of the impeller, the elongate elements transition to defining a paddle shape that extends radially from the central axial spring. Typically, the impeller includes two or more elongate elements (e.g., three elongate elements). For some applications, the elongate elements and the central axial spring are made of a shape-memory material, e.g., a shape-memory alloy such as nitinol. Typically, each of the elongate elements and the central axial spring support a film  105  of a material (e.g., an elastomer, such as polyurethane, and/or silicone) therebetween. For some applications, the film of material includes pieces of nitinol embedded therein, for example in order to strengthen the film of material. 
     Typically, by virtue of the change in the shapes of the elongate elements, the impeller blades transition from being spiral shaped within a distal portion of the impeller to being substantially radially-extending and paddle shaped within a proximal portion of the impeller. The distal portion of the impeller (by virtue of its shape) is configured primarily to impart axial flow to blood that it pumps, whereas the proximal portion of the impeller is configured to impart substantial radial flow to blood that it pumps. For some applications, this increases the flow and/or pressure that impeller is able to add to blood that it pumps (relative to an impeller that is shaped as shown in  FIGS.  20 Ai -Aii, for example) because as blood flows from the distal end of the impeller to the proximal end of the impeller the impeller continues to impart additional flow and/or pressure to the blood. 
     Reference is now made to  FIGS.  20 Ci  and  20 Cii, which are schematic illustrations of respective views of impeller  100 , in accordance with some applications of the present invention.  FIG.  20 Ci  shows the impeller disposed on axial shaft  19 , in the absence of any surrounding portions of pump-head portion  80 , for illustrative purposes. FIGS.  20 Cii shows a cross-sectional view of the impeller disposed within frame  110 , but in the absence of pump-outlet tube  28 , again for illustrative purposes. For some applications, the impeller has a generally similar structure to that described with reference to  FIGS.  20 Bi -Bii, but the impeller includes an expandable portion  116  disposed between the central axial spring  103  and elongate elements  101 . Alternatively, the impeller has a different configuration but has expandable portion  116  disposed along its axis. In its expanded configuration, the expandable portion is typically shaped such that its diameter increases from the distal end of the expandable portion to the proximal end of the expandable portion. For example, the expandable portion may have a conical shape, a frustoconical shape, or a three-dimensional teardrop shape (i.e., with a semispherical distal end, and a conical or frustoconical proximal end). For some applications, the expandable portion is self-expandable. For example, the expandable portion may be made of a shape-memory alloy (e.g., nitinol) that is covered with a material (e.g., an elastomeric material, and/or a flexible plastic material). Alternatively or additionally, the expandable portion is inflatable. 
     As described hereinabove, for some applications, the apparatus includes a purging system  16  and a plurality of purging fluid ports  18  (e.g., purging fluid inlet ports, and/or purging fluid outlet ports), via which a purging fluid (e.g., a glucose-based solution and/or saline) is pumped into the ventricular assist device, in order to purge portions of the ventricular assist device. Typically, the purging fluid is configured to be pumped into the apparatus such that the purging fluid flows between drive cable  15  and outer tube  17 , and thereby purges the interface between the drive cable and the outer tube. Further typically, the purging fluid is configured to purge interfaces between axial shaft  19  and radial bearings within which the axial shaft rotates. For some applications, expandable portion  116  of impeller  100  is configured to be inflated by the purging fluid. Typically, in such applications, in order to control the inflation of the expandable portion of the impeller, the pressure at which the purging fluid is pumped into the ventricular assist device is modulated. 
     Typically, by virtue of the shape of the expandable portion of the impeller, the impeller is configured to impart additional radial flow to the blood as it flows from the distal end to the proximal end of the impeller. For some applications, this increases the flow and/or pressure that impeller is able to add to blood that it pumps (relative to an impeller that is shaped as shown in  FIGS.  20 Ai -Aii, for example) because as blood flows from the distal end of the impeller to the proximal end of the impeller the impeller imparts additional flow and/or pressure to the blood. 
     Reference is now made to  FIGS.  21 Ai  and  21 Aii, which are schematic illustrations of respective views of pump-head portion  80  of ventricular assist device  20 , the pump-head portion including an expandable flow rectifier  120 , in accordance with some applications of the present invention. For some applications, the general configuration of the pump-head portion is similar to that described in US 2020/0237981 to Tuval, which is incorporated herein by reference, except for differences described hereinbelow. As shown, the impeller is disposed inside frame  110 , within a distal portion of pump-outlet tube  28 . The distal end of pump-outlet tube defines an axially-facing blood-inlet opening  108 , via which blood flow into the pump-outlet tube. In the example shown in  FIGS.  21 Ai -Aii, the impeller has a configuration as shown and described with reference to  FIGS.  20 Bi -Bii. As described hereinabove, in addition to imparting axial flow to the blood, the impeller imparts radial flow to the blood. Typically, the flow rectifier is configured to reduce radial and/or rotational components of the blood flow by converting these components to axial flow prior to the blood flowing out of blood-outlet openings  30  at the proximal end of the pump-outlet tube, and into the aorta. 
     Typically, expandable flow rectifier is disposed along the axis of pump-outlet tube  28 . For some applications, the expandable flow rectifier extends radially from around the drive cable (e.g., from around outer tube  17 , which surround the drive cable, or from outside an additional outer tube disposed around the drive cable). Typically, in its expanded configuration, the expandable flow rectifier is shaped such that its diameter decreases from its distal end to its proximal end. For example, the expandable flow rectifier may have a conical shape, a frustoconical shape, or a three-dimensional teardrop shape (i.e., with a semispherical distal end, and a conical or frustoconical proximal end). For some applications, the expandable flow rectifier is self-expandable. For example, the expandable flow rectifier may be made of a shape-memory alloy (e.g., nitinol) that is covered with a material (e.g., an elastomeric material, silicone, polyurethane, polyester, polyethylene terephthalate (PET), and/or polyether block amide (e.g., PEBAX®)). Alternatively or additionally, the expandable flow rectifier is inflatable. 
     As described hereinabove, for some applications, the apparatus includes a purging system  16  and a plurality of purging fluid ports  18  (e.g., purging fluid inlet ports, and/or purging fluid outlet ports), via which a purging fluid (e.g., a glucose-based solution and/or saline) is pumped into the ventricular assist device, in order to purge portions of the ventricular assist device. Typically, the purging fluid is configured to be pumped into the apparatus such that the purging fluid flows between drive cable  15  and outer tube  17 , and thereby purges the interface between the drive cable and the outer tube. Further typically, the purging fluid is configured to purge interfaces between axial shaft  19  and radial bearings within which the axial shaft rotates. For some applications, expandable flow rectifier  120  is configured to be inflated by the purging fluid. Typically, in such applications, in order to control the inflation of the expandable flow rectifier, the pressure at which the purging fluid is pumped into the ventricular assist device by the purging system is modulated. 
     Reference is now made to  FIGS.  21 Bi  and  21 Bii, which are schematic illustrations of respective views of pump-head portion  80  of ventricular assist device  20 , the pump-head portion including expandable flow rectifier  120 , and pump-outlet tube  28  including a widened region  122  in the vicinity of impeller  100 , in accordance with some applications of the present invention. As shown in  FIGS.  21 Bi - 21 Bii, for some applications, pump-outlet tube  28  includes widened region  122  in the vicinity of impeller  100  (e.g., immediately proximally to, and/or surrounding, the impeller), which is wider than a portion of the pump-outlet tube proximal thereto. The impeller is configured to pump blood entering through blood inlet opening  108  both axially (in the proximal direction) and centrifugally. The centrifugal flow that is imparted to the blood causes the blood to flow into the widened region of the pump-outlet tube, and the blood is then redirected axially by the narrowing of the pump-outlet tube. In the example shown in  FIGS.  21 Bi -Bii, the impeller is configured as shown in  FIGS.  20 Ai -Aii. Typically, when deployed within a pump-outlet tube that has a widened portion as shown in  FIGS.  21 Bi -Bii, such an impeller is configured impeller to pump blood both axially and centrifugally. As shown in  FIGS.  21 Bi -Bii, for some applications, pump-head portion  80  includes expandable flow rectifier  120 , which is generally as described hereinabove with reference to  FIGS.  21 Ai -Aii. 
     Reference is now made to  FIGS.  21 Ci  and  21 Cii as well as  FIGS.  21 Di  and  21 Dii, which are schematic illustrations of respective views of pump-head portion  80  of ventricular assist device  20 , the pump-head portion including expandable flow rectifier  120 , and pump-outlet tube  28  including a widened region  122  in the vicinity of impeller  100 , in accordance with some alternative applications of the present invention. Pump-head portion  80  as shown in  FIGS.  21 Ci -Dii is generally as shown and described with reference to  FIGS.  21 Bi -Bii. However, in addition to expandable flow rectifier  120 , spiral flow rectifiers  102  are disposed within pump-outlet tube proximally with respect to impeller, spiral flow rectifiers  102  extending radially from the expandable flow rectifier to the pump-outlet tube. The spiral flow rectifiers are configured to reduce rotational flow components from the blood flow prior to the blood flowing out of the pump-outlet tube. For some applications, the flow rectifiers have spiral shapes with a pitch of the spiral increasing from the distal ends of the flow rectifiers to their proximal ends. By being shaped in this way, the flow rectifiers gradually covert radial flow to axial flow. In accordance with respective applications, the flow rectifiers are made of a flexible material (e.g., an elastomeric material, silicone, polyurethane, polyester, polyethylene terephthalate (PET), and/or polyether block amide (e.g., PEBAX®)), or comprise a structure that is made of a shape memory alloy (such as nitinol) that is covered with a flexible material (e.g., an elastomeric material, silicone, polyurethane, polyester, polyethylene terephthalate (PET), and/or polyether block amide (e.g., PEBAX®)). 
     As shown in  FIGS.  21 Ci -Cii, for some applications, a pump-head portion  80  that is configured as described in the above paragraph is used with an impeller as is generally described in  FIGS.  20 Ai -Aii. Alternatively, as shown in  FIGS.  21 Di -Dii, a pump-head portion  80  that is configured as described in the above paragraph is used with an impeller as is generally described in  FIGS.  20 Ci -Cii. Further alternatively (not shown), a pump-head portion  80  that is configured as described in the above paragraph is used with an impeller as is generally described in  FIGS.  20 Bi -Bii, or any other type of impeller. In general it is noted that the scope of the present application includes combining any of the components of any of the impellers described with reference to  FIGS.  20 Ai - 20 Cii and any component of any of the pump-head portions described with reference to  FIGS.  21 Ai - 21 Dii with each other. For example, any one of the impellers described herein may be combined with a pump-outlet tube having widened region  122  in the vicinity of the impeller, expandable flow rectifier  120 , and/or spiral flow rectifiers  102 . 
     Reference is now made to  FIGS.  22 Ai  and  22 Aii and to  FIGS.  22 Bi  and  22 Bii, which are schematic illustrations of respective views of pump-head portion  80  of ventricular assist device, in accordance with some applications of the present invention. As shown in  FIGS.  22 Ai -Aii, for some applications, spiral flow rectifiers  102  are disposed within a portion of pump-outlet tube that is immediately distal to blood-outlet openings  30 . Alternatively or additionally, for some applications spiral flow rectifiers  102  are disposed within a portion of pump-outlet tube that is immediately proximal to impeller  100 , as shown in  FIGS.  22 Bi -Bii. The spiral flow rectifiers are generally as described hereinabove. 
     It is noted that in  FIGS.  21 Ai,  21 Bi,  21 Ci,  21 Di,  22 Ai, and  22 Bi , the pump-outlet tube is shown inside frame  110  for illustrative purposes, but typically, the pump outlet-tube is disposed outside of frame  110 . For some applications (not shown), an additional inner lining lines at least a portion of frame  110 , such that blood flow into the frame is exposed to a smooth inner surface. 
     With regards to all aspects of ventricular assist device  20  described with reference to  FIGS.  1 A - 22 Bii, it is noted that, although  FIG.  1 B  show ventricular assist device  20  in the subject&#39;s left ventricle, for some applications, device  20  is placed inside the subject&#39;s right ventricle, such that the device traverses the subject&#39;s pulmonary valve, and techniques described herein are applied, mutatis mutandis. For some applications, components of device  20  are applicable to different types of blood pumps. For example, aspects of the present invention may be applicable to a pump that is used to pump blood from the vena cava and/or the right atrium into the right ventricle, from the vena cava and/or the right atrium into the pulmonary artery, and/or from the renal veins into the vena cava. For example, any one of the impeller configurations, and/or any configuration of volutes, pump-outlet tubes, and/or flow rectifiers described herein, may be used in conjunction with any of the aforementioned pumps, mutatis mutandis. 
     The scope of the present invention includes combining any of the apparatus and methods described herein with any of the apparatus and methods described in one or more of the following applications, all of which are incorporated herein by reference: 
     US 2020/0237981 to Tuval, entitled “Distal tip element for a ventricular assist device,” filed Jan. 23, 2020, which claims priority from: 
     U.S. Provisional Patent Application 62/796,138 to Tuval, entitled “Ventricular assist device,” filed Jan. 24, 2019; 
     U.S. Provisional Patent Application 62/851,716 to Tuval, entitled “Ventricular assist device,” filed May 23, 2019; 
     U.S. Provisional Patent Application 62/870,821 to Tuval, entitled “Ventricular assist device,” filed Jul. 5, 2019; and 
     U.S. Provisional Patent Application 62/896,026 to Tuval, entitled “Ventricular assist device,” filed Sep. 5, 2019. 
     US 2019/0209758 to Tuval, which is a continuation of International Application No. PCT/IB2019/050186 to Tuval (published as WO 19/138350), entitled “Ventricular assist device,” filed Jan. 10, 2019, which claims priority from:
         U.S. Provisional Patent Application 62/615,538 to Sohn, entitled “Ventricular assist device,” filed Jan. 10, 2018;   U.S. Provisional Patent Application 62/665,718 to Sohn, entitled “Ventricular assist device,” filed May 2, 2018;   U.S. Provisional Patent Application 62/681,868 to Tuval, entitled “Ventricular assist device,” filed Jun. 7, 2018; and   U.S. Provisional Patent Application 62/727,605 to Tuval, entitled “Ventricular assist device,” filed Sep. 6, 2018;       

     US 2019/0269840 to Tuval, which is the US national phase of International Patent Application PCT/IL2017/051273 to Tuval (published as WO 18/096531), filed Nov. 21, 2017, entitled “Blood pumps,” which claims priority from U.S. Provisional Patent Application 62/425,814 to Tuval, filed Nov. 23, 2016; 
     US 2019/0175806 to Tuval, which is a continuation of International Application No. PCT/IL2017/051158 to Tuval (published as WO 18/078615), entitled “Ventricular assist device,” filed Oct. 23, 2017, which claims priority from U.S. 62/412,631 to Tuval filed Oct. 25, 2016, and U.S. 62/543,540 to Tuval, filed Aug. 10, 2017; 
     US 2019/0239998 to Tuval, which is the US national phase of International Patent Application PCT/IL2017/051092 to Tuval (published as WO 18/061002), filed Sep. 28, 2017, entitled “Blood vessel tube,” which claims priority from U.S. Provisional Patent Application 62/401,403 to Tuval, filed Sep. 29, 2016; 
     US 2018/0169313 to Schwammenthal, which is the US national phase of International Patent Application PCT/IL2016/050525 to Schwammenthal (published as WO 16/185473), filed May 18, 2016, entitled “Blood pump,” which claims priority from U.S. Provisional Patent Application 62/162,881 to Schwammenthal, filed May 18, 2015, entitled “Blood pump;” 
     U.S. Pat. No. 10,583,231 to Schwammenthal, which is the US national phase of International Patent Application PCT/IL2015/050532 to Schwammenthal (published as WO 15/177793), filed May 19, 2015, entitled “Blood pump,” which claims priority from U.S. Provisional Patent Application 62/000,192 to Schwammenthal, filed May 19, 2014, entitled “Blood pump;” 
     U.S. Pat. No. 10,039,874 to Schwammenthal, which is the US national phase of International Patent Application PCT/IL2014/050289 to Schwammenthal (published as WO 14/141284), filed Mar. 13, 2014, entitled “Renal pump,” which claims priority from (a) U.S. Provisional Patent Application 61/779,803 to Schwammenthal, filed Mar. 13, 2013, entitled “Renal pump,” and (b) U.S. Provisional Patent Application 61/914,475 to Schwammenthal, filed Dec. 11, 2013, entitled “Renal pump;” 
     U.S. Pat. No. 9,764,113 to Tuval, issued Sep. 19, 2017, entitled “Curved catheter,” which claims priority from U.S. Provisional Patent Application 61/914,470 to Tuval, filed Dec. 11, 2013, entitled “Curved catheter;” and 
     U.S. Pat. No. 9,597,205 to Tuval, which is the US national phase of International Patent Application PCT/IL2013/050495 to Tuval (published as WO 13/183060), filed Jun. 6, 2013, entitled “Prosthetic renal valve,” which claims priority from US Provisional Patent Application 61/656,244 to Tuval, filed Jun. 6, 2012, entitled “Prosthetic renal valve.” 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.