Abstract:
A pump for a dialysis machine, the pump having a pump chamber and a deformable membrane actuable to pump a fluid from the pump chamber, the pump chamber being substantially conical such that the membrane is actuated to extend into the conical chamber in order to pump the fluid from the chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from PCT/GB/2010/001667 filed on Sep. 3, 2010, and from GB 0915327.1 filed Sep. 3, 2009, all of which are hereby incorporated by reference in their entireties. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to membrane pumps, and in particular, but not exclusively, to a membrane pump for a dialysis machine. 
     2. State of the Art 
     Membrane pumps are used in a number of medical applications, for example in hemodialysis for the extracorporeal circulation of blood and the preparation and delivery of dialysate to and from the dialyser. Commonly, the membrane covers a dome shaped pump chamber into which the membrane is actuated to draw fluid into, and pump the fluid from, the chamber. 
     A known pump is described with reference to  FIG. 1  below. This form of pump has a number of drawbacks as follows. 
     The sharp angle between the concave recess of the drive chamber and the flat portion of the pump to which the membrane is attached causes high stresses in the membrane as it folds and stretches over the edge of the flat portion and into the recess upon actuation. This can lead to reduced life of the membrane. 
     Furthermore, the sharp angle between the flat portion and the recess can also cause the membrane to fold over the edge leaving a dead spot between the membrane and the recess wall where the membrane has not been actuated sufficiently to touch the chamber wall and expel fluid therebetween. This can lead to pumping inaccuracies and even to blood damage where the blood pools in this area for an unacceptable length of time. 
     The dome shaped profile of the pump chamber also presents a number of problems. The substantially hemispherical shape means that the membrane descends rapidly into the chamber upon actuation. Thus a large volume of fluid is pumped at the beginning of the stroke compared to the end of the stroke. Thus the volume of dispensed fluid can be difficult to control on the length of stroke. This can also lead to a hammer action of the pump which can lead to pumping inaccuracies and potential blood damage. 
     SUMMARY OF THE INVENTION 
     It is the purpose of the present invention to mitigate at least some of the above problems. 
     According to one aspect of the invention there is provided a pump for a dialysis machine, the pump having a pump chamber and a deformable membrane actuable to pump a fluid from the pump chamber, the pump chamber being substantially conical such that the membrane is actuated to extend into the conical chamber in order to pump the fluid from the chamber. 
     By providing a conical pump chamber the membrane descends into the chamber in a more controlled and predictable manner than the prior art pump. The displacement of fluid is more evenly distributes through the stroke of the membrane which improves the volumetric performance of the pump. 
     In a further aspect of the invention there is provided a pump for a dialysis machine, the pump having a pump chamber and an upper surface for supporting a deformable membrane actuable to pump a fluid, the pump chamber having an inner surface which extends downwardly from the upper surface, wherein arranged between the upper surface and the inner surface is a transition surface which has a curved profile so as to provide a smooth transition of the membrane within the pump chamber upon actuation of the pump. 
     The invention will now be described, by way of example only, and with reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representation of a prior art pump; 
         FIG. 2  is a representation of a pump according to the present invention; 
         FIG. 3  is a detailed representation part of the pump of  FIG. 2 ; and 
         FIG. 4  is a representation of the deflection of the membrane within the chamber of the pump of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  a blood pump  2  is shown comprising a disposable pump cartridge  4  comprising a rigid plastic shell  6  having concave recess  8  covered by a flexible membrane  10 . The recess  8  and the flexible membrane  10  form a pump cavity  12  having an inlet  14  and an outlet  16  leading into and out of the cavity respectively. The cartridge  4  abuts a pump driver  18  comprising a platen  20  having a recessed surface  22  therein and a fluid port  24 . In use the platen  20  sealingly engages with the cartridge  4  such that the recessed surface  22  and the flexible membrane  10  form a drive chamber  26 . The fluid port  24  is connectable with a source of positive fluid pressure  28  and a source of negative fluid pressure  30  via a valve  32 , controlled by a controller  34  to allow fluid to flow into or out of the drive cavity  26 . The positive  28  and negative  30  fluid sources are a pressure pump and a vacuum pump respectively. When the valve  32  is operated to allow fluid to flow into the drive chamber  26  from the source of positive fluid pressure  28 , the membrane  10  is moved towards the recessed surface  8  and any blood that is in the pump cavity  12  is expelled via the outlet  16 . When the valve  32  is operated to allow fluid to flow out of the drive chamber  26  to the source of negative fluid pressure  30 , the membrane  10  is moved away from the recessed surface  8  and towards surface  22  and blood is drawn into the pump cavity  12  from the inlet  14 . 
     In order to pump blood through the pump  2  the inlet  14  has an inlet valve (not shown) and the outlet  16  has an outlet valve (not shown) associated therewith. In operation, when the valve  32  is operated to allow fluid to flow into the drive chamber  26  from the source of positive fluid pressure  28  the inlet valve is closed and the outlet valve is open so the blood within the pump cavity  12  exits the outlet  16  via the outlet valve, and when the valve  32  is operated to allow blood to flow out of the drive chamber  26  to the source of negative fluid pressure  30 , the inlet valve is open and the outlet valve is closed such that blood is drawn into the pump cavity  12  through the inlet  14  via the open inlet valve. 
     Referring to  FIG. 2  a blood pump  102  according to the present invention is shown comprising a disposable pump cartridge  104  comprising a rigid plastic shell  106  having conical recess  108  covered by a flexible membrane  110 . The flexible membrane  110  is attached to a flat upper surface  117  of the rigid shell  106 . The upper surface  117  takes the form of an annular surface which surrounds the circular profile of the recess  108 . The recess  108  and the flexible membrane  110  form a pump cavity  112  having an inlet  114  and an outlet  116  leading into and out of the cavity respectively. The cartridge  104  abuts a pump driver  118  comprising a platen  120  having a recessed inner surface  122  therein and a fluid port  124 . In use the platen  210  sealingly engages with the cartridge  104  such that the inner surface  122  and the flexible membrane  110  form a pump chamber  126 . The fluid port  124  is connectable with a source of positive fluid pressure  28  and a source of negative fluid pressure  30  via a valve  32 , controlled by a controller  34  to allow fluid to flow into or out of the pump chamber  126 . The membrane  110  is operated in a similar way to the prior art device of  FIG. 1 . 
     Referring to  FIG. 3 , the upper surface  117  and inner surface  122  are shown in further detail. The upper surface  117  and inner surface  122  are separated by the transition surface  123  which is curve so as to provide a smooth transition of the membrane  110  between the upper and inner surfaces upon actuation. The effect of this transition surface is to eliminate the dead spot observed between the membrane  10  and the recessed surface  22  of the prior art pump of  FIG. 1  as the membrane is able to deflect in a more controlled manner upon actuation. Furthermore the smooth transition of the membrane reduces the stress observed in the membrane  110 . 
     This process is shown in more detail in  FIG. 4  which shows the pump chamber membrane  110  (not to scale for illustrative purposes) in a series of position of increasing actuation from  110 A to  110 F. This illustrates the gradual folding of the membrane over the transition surface  123  and the inner surface  122 . This provides increased predictability of the pumped volume for a given stroke position, reduces fluid damage and increases the accuracy of the dispensed volume. Additionally the membrane life is improved by a less severe deflection profile, and in particular reduced stress concentration when compared with the part of the prior art membrane situated at the top of the recessed surface  22  of the prior art. 
     It will be appreciated that the pump of the present invention could be used to pump extracorporeal blood, water, partly or fully formed dialysate solution, or other suitable medical liquid used in dialysis or similar medical processes requiring the control of fluid.