Patent Publication Number: US-8986254-B2

Title: Medical fluid pump systems and related components and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 12/725,673, now U.S. Pat. No. 8,192,401 filed on Mar. 17, 2010, which claims the benefit under 35 U.S.C. §119(e) of U.S. Application Ser. No. 61/162,134, filed on Mar. 20, 2009. The above-referenced applications are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to medical fluid pump systems and related components and methods. 
     BACKGROUND 
     Dialysis is a treatment used to support a patient with insufficient renal function. The two principal dialysis methods are hemodialysis and peritoneal dialysis. 
     During hemodialysis (“HD”), the patient&#39;s blood is passed through a dialyzer of a dialysis machine while also passing a dialysis solution or dialysate through the dialyzer. A semi-permeable membrane in the dialyzer separates the blood from the dialysate within the dialyzer and allows diffusion and osmosis exchanges to take place between the dialysate and the blood stream. These exchanges across the membrane result in the removal of waste products, including solutes like urea and creatinine, from the blood. These exchanges also regulate the levels of other substances, such as sodium and water, in the blood. In this way, the dialysis machine acts as an artificial kidney for cleansing the blood. 
     During peritoneal dialysis (“PD”), the patient&#39;s peritoneal cavity is periodically infused with sterile aqueous solution, referred to as PD solution or dialysate. The membranous lining of the patient&#39;s peritoneum acts as a natural semi-permeable membrane that allows diffusion and osmosis exchanges to take place between the solution and the blood stream. These exchanges across the patient&#39;s peritoneum result in the removal waste products, including solutes like urea and creatinine, from the blood, and regulate the levels of other substances, such as sodium and water, in the blood. 
     Many PD machines are designed to automatically infuse, dwell, and drain dialysate to and from the patient&#39;s peritoneal cavity. The treatment typically lasts for several hours, often beginning with an initial drain cycle to empty the peritoneal cavity of used or spent dialysate. The sequence then proceeds through the succession of fill, dwell, and drain phases that follow one after the other. Each phase is called a cycle. 
     SUMMARY 
     In one aspect of the invention, a medical fluid pump system includes a movable piston head and a cassette. The cassette includes a base defining an opening and a membrane attached to the base covering the opening. The membrane together with the base define a fluid pump chamber, a flow pathway that leads from the fluid pump chamber to an inlet of the cassette, and a flow pathway that leads from the fluid pump chamber to an outlet of the cassette. The cassette is positioned so that the membrane faces the piston head and can be moved by the piston head to change a volume of the fluid pump chamber. An adhesive is disposed between and in contact with the piston head and the membrane. The adhesive has sufficient affinity for the piston head to allow the piston head to retract and deflect the membrane outward to increase the volume of the fluid pump chamber while maintaining adhesive contact with the membrane. The adhesive has substantially greater affinity for the membrane than for the piston head such that the piston head can be retracted in a manner to separate the piston head from the adhesive without separating the adhesive from the membrane. 
     In another aspect of the invention, a medical fluid delivery cassette includes a base defining an opening and a membrane attached to the base covering the opening. The membrane together with the base define a fluid pump chamber, a flow pathway that leads from the fluid pump chamber to an inlet of the cassette, and a flow pathway that leads from the fluid pump chamber to an outlet of the cassette. A portion of the membrane overlying the fluid pump chamber is moveable relative to the base such that the volume of the fluid pump chamber can be changed by applying a force to the portion of the membrane overlying the fluid pump chamber. An adhesive coating is disposed on an outside surface of the portion of the membrane overlying the fluid pump chamber, and a release layer substantially covers and is releasably attached to the adhesive coating. 
     In an additional aspect of the invention, a medical fluid delivery method includes adhering a first piston head of a medical system to a membrane of a medical fluid delivery cassette by moving the first piston head into contact with adhesive disposed on a portion the membrane overlying a first fluid pump chamber, and, with the first piston head adhered to the membrane, changing a volume of the first fluid pump chamber by flexing the portion of the membrane overlying the first fluid pump chamber with the first piston head. 
     Implementations can include one or more of the following features. 
     In some implementations, the piston head is adapted to be moved away from the cassette with a force sufficient to overcome the affinity between the piston head and the adhesive such that the piston head can be separated from the membrane. 
     In certain implementations, the piston head is arranged to be moved a distance of at least 1.5 centimeters away from a plane in which the membrane lies when the membrane is not deformed by the piston head. 
     In some implementations, the system includes a wall adjacent the cassette, and the piston head can be retracted beyond an outer surface of the wall. 
     In certain implementations, the piston head includes (e.g., is formed of) polyoxymethylene and the membrane includes (e.g., is formed of) a low density polyolefin. 
     In some implementations, the adhesive includes (e.g., is formed of) synthetic rubber (e.g., a double coated synthetic rubber tape). 
     In certain implementations, an adhesion strength of the adhesive to polyester is about 89 Oz./in. (about 97 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 72 hour RT). 
     In some implementations, an adhesion strength of the adhesive to polypropylene is about 85 Oz./in. (about 93 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 72 hour RT). 
     In certain implementations, an adhesion strength of the adhesive to polycarbonate is about 101 Oz./in. (about 110 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 72 hour RT). 
     In some implementations, an adhesion strength of the adhesive  161  to stainless steel is about 97 Oz./in. (about 106 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 72 hour RT). 
     In certain implementations, an adhesion strength of the adhesive to the membrane is substantially greater than an adhesion strength of the adhesive to the piston head. 
     In some implementations, the adhesive includes a first layer of adhesive in contact with the membrane and a second layer of adhesive in contact with the piston head. 
     In certain implementations, the medical fluid pump system further includes a base layer (e.g., a substantially liquid impermeable base layer) disposed between the first and second layers of adhesive. 
     In some implementations, the first layer of adhesive is biocompatible and the second layer of adhesive is bioincompatible. 
     In certain implementations, the piston head is movable in a direction substantially perpendicular to the cassette. 
     In some implementations, the piston head can be separated from the adhesive by moving the piston head in the direction substantially perpendicular to the cassette. 
     In certain implementations, the piston head is configured to be rotated relative to the cassette. 
     In some implementations, the piston head can be separated from the adhesive by rotating the piston head relative to the cassette. 
     In certain implementations, the base of the cassette is a molded tray-like base. 
     In some implementations, the adhesive is disposed on a portion of the membrane overlying the fluid pump chamber. 
     In certain implementations, the adhesive is substantially uniformly disposed on the portion of the membrane overlying the fluid pump chamber. 
     In some implementations, the medical system includes first and second movable piston heads, and the membrane together with the base defines first and second fluid pump chambers, flow pathways that lead from the first and second fluid pump chambers to the inlet of the cassette, and flow pathways that lead from the first and second fluid pump chambers to the outlet of the cassette. The cassette is positioned so that the membrane faces the first and second piston heads and can be moved by the first and second piston heads to alter volumes of the first and second fluid pump chambers, and adhesive is disposed between and in contact with the first and second piston heads and the membrane. The adhesive has sufficient affinity for the first and second piston heads to allow the first and second piston heads to retract and deflect the membrane outward to increase the volumes of the first and second fluid pump chambers while maintaining adhesive contact with the membrane. The adhesive has substantially greater affinity for the membrane than for the first and second piston heads such that the first and second piston heads can be retracted in a manner to separate the first and second piston heads from the adhesive without separating the adhesive from the membrane. 
     In certain implementations, the medical fluid pump system is a dialysis system (e.g., a peritoneal dialysis system). 
     In some implementations, the adhesive coating is produced by applying the release layer carrying adhesive to the membrane. 
     In certain implementations, the release layer is release paper (e.g., a wax-coated paper). 
     In some implementations, the adhesive coating is disposed on a portion of the membrane that is contacted by a piston head of a medical fluid pump system during use. 
     In certain implementations, the release layer is removable from the adhesive coating to expose at least a portion of the adhesive coating. 
     In some implementations, the release layer includes a pull tab that extends beyond an outer boundary of the adhesive. 
     In certain implementations, the membrane includes a low density polyolefin. 
     In some implementations, the medical fluid delivery cassette is configured for use with a dialysis machine (e.g., a peritoneal dialysis machine). 
     In certain implementations, the medical fluid delivery cassette is disposable. 
     In some implementations, the adhesive coating has a greater affinity for the membrane than for a piston head of a medical fluid pumping system when the cassette is in use with the medical fluid pumping system such that the piston head can be retracted in a manner to separate the piston head from the adhesive without separating the adhesive from the membrane. 
     In certain implementations, the adhesive coating includes first and second layers of adhesive, and the first layer of adhesive is in contact with the membrane. 
     In some implementations, the adhesive has a greater affinity for the membrane than for the first piston head such that the first piston head can be retracted in a manner to separate the first piston head from the adhesive without separating the adhesive from the membrane. 
     In certain implementations, flexing the portion of the membrane overlying the first fluid pump chamber includes moving the first piston head away from the cassette with a force that does not exceed the affinity between the membrane and the adhesive. 
     In some implementations, the medical fluid delivery method further includes decoupling the first piston head from the membrane by moving the first piston head relative to the cassette. 
     In certain implementations, the adhesive remains attached to the membrane after decoupling the first piston head from the membrane. 
     In some implementations, the medical fluid delivery method further includes decoupling the first piston head from the membrane by moving the piston head in a direction substantially perpendicular to the cassette. 
     In certain implementations, the medical fluid delivery method further includes decoupling the first piston head from the membrane by rotating the piston head relative to the cassette. 
     In some implementations, the medical fluid delivery method further includes adhering a second piston head to the membrane by moving the second piston head into contact with adhesive disposed on a portion of the membrane overlying a second fluid pump chamber and, with the second piston head adhered to the membrane, changing a volume of the second fluid pump chamber by flexing the portion of the membrane overlying the second fluid pump chamber with the second piston head. 
     In certain implementations, the medical fluid delivery method further includes disposing the adhesive on the membrane by applying a release layer carrying adhesive to the membrane. 
     In some implementations, the medical fluid delivery method further includes removing the release layer from the adhesive. 
     In certain implementations, removing the release layer includes pulling on a pull tab of the release paper, where the pull tab extends beyond an outer boundary of the adhesive prior to removing the release layer from the adhesive. 
     Implementations can include one or more of the following advantages. 
     In some implementations, the PD system includes an adhesive disposed between the piston head and the membrane. This arrangement permits the piston head to be moved in a direction away from the cassette to draw dialysate into the cassette without extensive use of a vacuum system. For example, while the PD system may include a vacuum system to deflate inflatable valves that are used to direct fluid through desired pathways of the cassette, the membrane of the cassette in the region of the pump chamber can be attached to the piston head using only adhesive. By using adhesive, as opposed to vacuum pressure, to secure the membrane of the cassette to the piston head, the likelihood of fluid being pulled through the membrane (e.g., through very small holes in the membrane) is substantially reduced or eliminated. Additionally or alternatively, the level of noise produced by the PD system during operation can be substantially reduced relative to certain prior PD systems that use vacuum pressure retract the cassette membrane in the region of the pump chamber as the piston head is retracted. In addition, the use of adhesive to secure the membrane of the cassette to the piston head can reduce the overall cost and complexity of manufacturing the PD system. 
     In certain implementations, the piston head is adhesively attached to the membrane of the cassette and can draw the membrane away from the cassette to draw dialysate into the cassette. The adhesive attachment between the piston head and the membrane can result in a substantially direct correlation between the piston head position and the volume drawn into the cassette. Thus, adhesively attaching the piston head to the membrane can improve the speed and accuracy of volumetric calculations of dialysate drawn into the cassette. Additionally or alternatively, if any holes (e.g., pinholes) were to develop in the portion of the membrane to which the adhesive is attached, the adhesive could act as a seal and thus reduce the likelihood that fluid will leak out of the cassette. 
     In certain implementations, the adhesion strength or affinity of the adhesive to the membrane of the cassette is greater than the adhesion strength or affinity of the adhesive to the piston head. In such a configuration, the piston head can be moved away from the membrane by a sufficient distance and with a sufficient force to detach the piston head from the adhesive without detaching the adhesive from the membrane. The piston head can, for example, be retracted with a force greater than the adhesion strength of the adhesive to the piston head but less than the adhesion strength of the adhesive to the membrane of the cassette to detach the piston head from the adhesive while retaining the attachment between the adhesive and the membrane of the cassette. The particular material properties of the adhesive, cassette membrane, and piston head inhibit or eliminate adhesive from remaining attached to the piston head upon detachment of the piston head from the membrane and can thus reduce or prevent adhesive build-up in the PD system over time. 
     In some implementations, the cassette is disposable. In such implementations, the cassette, along with adhesive retained on the cassette, can be discarded after use. Such a disposable cassette can reduce the need for the user to remove or otherwise handle the adhesive. 
     In certain implementations, a release layer (e.g., a release paper) covers and is releasably attached to adhesive on the cassette. The release layer can be removed to expose the adhesive before use. The release layer can help to prevent debris and contaminants from collecting on the adhesive prior to use of the cassette. 
     Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a PD system. 
         FIG. 2  is a perspective view of a PD cycler and PD cassette of the PD system of  FIG. 1 . A door of the PD cycler is in the open position to show the inner surfaces of the PD cycler that interface with the PD cassette during use. 
         FIG. 3  is a perspective view of a cassette compartment of the PD cycler of  FIGS. 1 and 2 . 
         FIG. 4  is a perspective view from a rigid base side of the PD cassette of the PD system of  FIG. 1 . 
         FIG. 5  is a perspective view from a flexible membrane side of the PD cassette of the PD system of  FIG. 1 . 
         FIG. 6  is a plan view from the membrane side of the PD cassette of the PD system of  FIG. 1 . 
         FIG. 7  is a side view of the PD cassette of the PD system of  FIG. 1 , showing adhesive disposed on the portion of the membrane overlying a pump chamber of the cassette. 
         FIG. 8  is a perspective view from the membrane side of the PD cassette of the PD system of  FIG. 1  with eyeglass-shaped release paper covering and adhered to the adhesive regions on the membrane. 
         FIG. 9  is a plan view from the membrane side of the PD cassette of the PD system of  FIG. 1  with eyeglass-shaped release paper covering and adhered to the adhesive regions on the membrane. 
         FIG. 10  is a plan view of the PD cassette of  FIGS. 8 and 9  showing the eyeglass-shaped release paper being removed from the adhesive regions on the membrane. 
         FIG. 11  is a partial perspective view of the PD cassette in the cassette compartment of the PD cycler of the PD system of  FIG. 1 . 
         FIGS. 12A-12C  are diagrammatic cross-sectional views of the PD cassette in the cassette compartment of the PD cycler of the PD system of  FIG. 1 , during different phases of operation. 
         FIGS. 13A-13C  illustrate various fluid flow paths through the PD cassette of the PD system of  FIG. 1  during a PD treatment. 
         FIGS. 14A-14C  illustrate a method of making eyeglass-shaped composites of adhesive regions disposed between and adhered to two release papers. 
         FIG. 15  is a perspective view of an eyeglass-shaped composite made using the method illustrated in  FIGS. 14A-14C , showing one of the release papers of the composite being removed from the adhesive regions. 
         FIG. 16  is a perspective view of the eyeglass-shaped composite of  FIG. 15  (with one of the release papers removed) being applied to the membrane of the PD cassette of the PD system of  FIG. 1 . 
         FIG. 17  is a plan view of a PD cassette with discrete release papers including pull tabs covering and adhered to the adhesive regions. 
         FIG. 18  is a plan view of a PD cassette with a release paper covering and adhered to the adhesive regions. The release paper is sized to cover substantially the entire surface of the cassette and includes printed text on its surface. 
         FIG. 19  is a side view of a piston head of the PD system of  FIG. 1  and a PD cassette with a three-layer adhesive composite adhered to a membrane of the cassette. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a peritoneal dialysis (“PD”) system  100  includes a PD cycler (also referred to as a PD machine)  102  seated on a cart  104 . Referring also to  FIG. 2 , the PD cycler  102  includes a housing  106 , a door  108 , and a cassette interface  110  that mates with a disposable PD cassette  112  when the cassette  112  is disposed within a cassette enclosure  114  formed between the cassette interface  110  and the closed door  108 . A heater tray  116  is positioned on top of the housing  106 . The heater tray  116  is sized and shaped to accommodate a bag of PD solution (e.g., a 5 liter bag of PD solution). The PD cycler  102  also includes a touch screen  118  and additional control buttons  26  that can be operated by a user (e.g., a patient) to allow, for example, set-up, initiation, and/or termination of a PD treatment. 
     PD solution bags  122  are suspended from fingers on the sides of the cart  104 , and a heater bag  124  is positioned in the heater tray  116 . The PD solution bags  122  and the heater bag  124  are connected to the cassette  112  via PD solution bag lines  126  and a heater bag line  128 , respectively. The PD solution bag lines  126  can be used to pass PD solution from PD solution bags  122  to the cassette  112  during use, and the heater bag line  128  can be used to pass PD solution back and forth between the cassette  112  and the heater bag  124  during use. In addition, a patient line  130  and a drain line  132  are connected to the cassette. The patient line  130  can be connected to a patient&#39;s abdomen via a catheter and can be used to pass PD solution back and forth between the cassette  112  and the patient during use. The drain line  132  can be connected to a drain or drain receptacle and can be used to pass PD solution from the cassette  112  to the drain or drain receptacle during use. 
       FIG. 3  shows a more detailed view of the cassette interface  110  and the door  108  of the PD cycler  102 . As shown, the PD cycler  102  includes pistons with substantially hemispherical piston heads  134 A,  134 B that can be axially moved within piston access ports  136 A,  136 B formed in the cassette interface  110 . The piston heads  134 A,  134 B are made of polyoxymethylene (marketed under the tradename Delrin available from Dupont of Wilmington, Del.). The hemispherical shape of the piston heads  134 A,  134 B can be achieved using machining techniques. Alternatively or additionally, the hemispherical shape of the piston heads  134 A,  134 B can be formed using molding or casting techniques. The pistons include piston shafts that are coupled to motors that can be operated to move the piston heads  134 A,  134 B axially inward and outward within the piston access ports  136 A,  136 B. As discussed below, when the cassette  112  (shown in FIGS.  2  and  4 - 7 ) is positioned within the cassette enclosure  114  with the door  108  closed, the piston heads  134 A,  134 B of the PD cycler  102  align with pump chambers  138 A,  138 B of the cassette  112 . The piston heads  134 A,  134 B, as described in greater detail below, can be adhered to portions of a membrane  140  overlying the pump chambers  138 A,  138 B of the cassette  112 . As a result, the piston heads  134 A,  134 B can be moved in the direction of the cassette  112  to force the membrane  140  into the volume defined by the pump chambers  138 A,  138 B, causing the volume defined by the pump chambers to decrease and forcing fluid out of the pump chambers  138 A,  138 B. The piston heads  134 A,  134 B can also be retracted away from the cassette  112  and out of the volume defined by the pump chambers  138 A,  138 B such that the volume defined by the pump chambers  138 A,  138 B increases and fluid is drawn into the pump chambers  138 A,  138 B. 
     The PD cycler  102  also includes multiple inflatable members  142  positioned within inflatable member access ports  144  in the cassette interface  110 . The inflatable members  142  align with depressible dome regions  146  of the cassette  112  when the cassette  112  is positioned within the cassette enclosure  114 . While only one of the inflatable members  142  is labeled in  FIG. 3 , it should be understood that the PD cycler  102  includes an inflatable member associated with each of the depressible dome regions  146  of the cassette  112  (shown in  FIG. 5 ). The inflatable members  142  act as valves to direct fluid through the cassette  112  in a desired manner during use. In particular, the inflatable members  142  bulge outward beyond the surface of cassette interface  110  and into contact with the depressible dome regions  146  of the cassette  112  when inflated, and retract into the inflatable member access ports  144  and out of contact with the cassette  112  when deflated. By inflating certain inflatable members  142  to depress their associated dome regions  146  on the cassette  112 , certain fluid flow paths within the cassette  112  can be blocked off. Thus, fluid can be pumped through the cassette  112  by actuating the piston heads  134 A,  134 B, and can be guided along desired flow paths within the cassette  112  by selectively inflating and deflating the inflatable members  142 . 
     Still referring to  FIG. 3 , locating pins  148  extend from the cassette interface  110 . When the door  108  is in the open position, the cassette  112  can be loaded onto the cassette interface  110  by positioning the top portion of the cassette  112  under the locating pins  148  and pushing the bottom portion of the cassette  112  toward the cassette interface  110 . The cassette  112  is dimensioned to remain securely positioned between the locating pins  148  and a lower ledge  150  extending from the cassette interface  110  to allow the door  108  to be closed over the cassette  112 . The locating pins  148  help to ensure that the pump chambers  138 A,  138 B of the cassette  112  are aligned with the piston heads  134 A,  134 B when the cassette  112  is positioned in the cassette enclosure  114  between the closed door  108  and the cassette interface  110 . 
     The door  108  defines recesses  152 A,  152 B that substantially align with the piston heads  134 A,  134 B when the door  108  is in the closed position. When the cassette  112  is positioned within the cassette enclosure  114 , hemispherical projections  154 A,  154 B of the cassette  112  (shown in  FIG. 4 ), which partially define the pump chambers  138 A,  138 B, fit within the recesses  152 A,  152 B. In this configuration, the portions of the door  108  forming the recesses  152 A,  152 B can support the hemispherical projections  154 A,  154 B while the planar surface of the door  108  can counteract the force of the inflatable members  142  and thus allow the inflatable members  142  to actuate the depressible dome regions  146  on the cassette  112 . 
     The PD cycler  102  includes various other features not described here. Further details regarding the PD cycler  102  and its various components can be found in U.S. Patent Application Publication No. 2007/0112297, which is incorporated by reference herein. 
     Referring to  FIGS. 4-7 , the cassette  112  includes a tray-like rigid base  156  and the flexible membrane  140 , which is attached to (e.g., thermally bonded to, adhered to) the periphery of the base  156 . The base is made of polypropylene, and can be formed using machining, molding, and/or casting techniques. As shown in  FIGS. 4 and 7 , the hemispherical, hollow projections  154 A,  154 B of the base  156  are substantially symmetrically positioned with respect to the center vertical axis of the cassette  112 . The hemispherical projections  154 A,  154 B extend away from the membrane  140  and are sized and shaped to fit within the recesses  152 A,  152 B in the door  108  of the PD cycler  102 . The membrane  140  cooperates with the base  156  to form the pump chambers  138 A,  138 B. In particular, the volume between the membrane  140  and the projections  154 A,  154 B define the pump chambers  138 A,  138 B. The base  156  further includes raised structural features that extend toward and into contact with the inner surface of the membrane  140 . The membrane  140  cooperates with the raised structural features extending from the base  156  to form a series of fluid pathways  158  and the multiple, depressible dome regions  146 , which are widened (e.g., substantially circular) portions of the fluid pathways. At each depressible dome region  146 , the membrane  140  can be deflected to contact the base  156 . Such contact can substantially impede (e.g., prevent) the flow of PD solution along the pathway  158  associated with that dome region  146  during use. Thus, as described in further detail below, the flow of PD solution through the cassette  112  can be controlled through the selective depression of the depressible dome regions  146  by selectively inflating the inflatable members  142  of the PD cycler  102 . 
     The rigidity of the base  156  helps to hold the cassette  112  in place within the cassette enclosure  114  of the PD cycler  102  and to prevent the hemispherical projections  154 A,  154 B and the other raised features of the cassette  112  from flexing and deforming in response to changes in pressure within the cassette  112  during use. The rigidity of the base  156  also allows the hemispherical projections  154 A,  154 B to resist deformation by the piston heads  134 A,  134 B during use. 
     The fluid pathways  158  in the cassette  112  lead from the pumping chambers  138 A,  138 B to connectors  160  positioned along the bottom edge of the cassette  112 . The connectors  160  are positioned asymmetrically along the width of the cassette  112 . The asymmetrical positioning of the connectors  160  helps to ensure that the cassette  112  will be properly positioned in the cassette enclosure  114  with the membrane  140  and of the cassette  112  facing the cassette interface  110 . The connectors  160  are configured to receive fittings on the ends of the PD solution bag lines  126 , the heater bag line  128 , the patient line  130 , and the drain line  132 . These fittings are double male fittings. One end of the fitting can be inserted into and bonded to its respective line and the other end can be inserted into and bonded to its associated connector  160 . By permitting the PD solution bag lines  126 , the heater bag line  128 , the patient line  130 , and the drain line  132  to be connected to the cassette, as shown in  FIGS. 1 and 2 , the connectors  160  allow PD solution to flow into and out of the cassette  112  during use. 
     Still referring to  FIGS. 4-7 , the membrane  140  of the cassette  112  includes three layers. The inner and outer layers of the laminate are formed of a compound that is made up of 60 percent Septon® 8004 thermoplastic rubber (i.e., hydrogenated styrenic block copolymer) and 40 percent ethylene. The middle layer is formed of a compound that is made up of 25 percent Tuftec® H1062 (SEBS: hydrogenated styrenic thermoplastic elastomer), 40 percent Engage® 8003 polyolefin elastomer (ethylene octene copolymer), and 35 percent Septon® 8004 thermoplastic rubber (i.e., hydrogenated styrenic block copolymer). The thickness of the membrane  140  is selected so that the membrane  140  has sufficient flexibility to flex toward the base  156  in response to the force applied to the membrane  140  by the piston heads  134 A,  134 B. For example, the membrane  140  can be about 0.100 inch to about 0.150 inch in thickness. 
     Adhesive  161  is disposed on regions  162 A,  162 B of the membrane  140  that overlie the pump chambers  138 A,  138 B. The adhesive  161  is a double coated synthetic rubber adhesive tape manufactured by 3M as part number 9443NP. The properties of the adhesive  161  allow the adhesive  161  to achieve a temporary adhesive attachment with the piston heads  134 A,  134 B when the piston heads  134 A,  134 B are brought into contact with the adhesive regions during use. The attachment between the adhesive  161  and the piston heads  134 A,  134 B is sufficiently strong to allow the adhesive to remain attached to the piston heads  134 A,  134 B as the piston heads  134 A,  134 B are reciprocated during use. At the same time, the attachment between the adhesive  161  and the piston heads  134 A,  134 B can be readily broken by retracting the piston heads  134 A,  134 B by a greater distance than they are retracted during treatment and with a force that exceeds the adhesion force between the adhesive  161  and the piston heads  134 A,  134 B. 
     The adhesion strength of the adhesive  161  to polyester is about 89 Oz./in. (about 97 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 72 hour room temperature (RT)). The adhesion strength of the adhesive  161  to polypropylene is about 85 Oz./in. (about 93 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 72 hour RT). The adhesion strength of the adhesive  161  to polycarbonate is about 101 Oz./in. (about 110 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 72 hour RT). The adhesion strength of the adhesive  161  to stainless steel is about 93 Oz./in. (about 101 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 15 minute RT). The adhesion strength of the adhesive  161  to stainless steel is about 97 Oz./in. (about 106 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 72 hour RT). The adhesion strength of the adhesive  161  to stainless steel is about 126 Oz./in. (about 137 N/100 mm), as tested using the ASTM D3330 test (90 degree, 2 mil Al foil, 72 hour 158° F. (70° C.)). The adhesion strength of the adhesive  161  to stainless steel is about 206 Oz./in. (about 224 N/100 mm), as tested using the ASTM D3330 test (180 degree, 2 mil Al foil, 72 hour RT). 
     During use, the cassette  112  is secured within the cassette enclosure  114  by positioning the adhesive-carrying side of the cassette  112  adjacent to the cassette interface  110  of the PD cycler  102  and closing the door  108  over the cassette  112 . As noted above, the piston heads  134 A,  134 B align with the pump chambers  138 A,  138 B of the cassette  112  when the cassette  112  is positioned in the cassette enclosure  114  between the cassette interface  110  and the closed door  108 . Thus, with the cassette  112  secured in the cassette enclosure  114 , the piston heads  134 A,  134 B can extend into the enclosure  114  to contact the adhesive  161  disposed on regions  162 A,  162 B of the flexible membrane  140  and become temporarily adhesively attached to the membrane  140  of the cassette  112 . The adhesive  161 , the material of the piston heads  134 A,  134 B, and the material of the membrane  140  are selected so that the adhesive  161  has a greater adhesion or affinity to the membrane  140  than to the piston heads  134 A,  134 B. The adhesive attachment between the piston heads  134 A,  134 B and the membrane  140  causes the regions  162 A,  162 B of the membrane  140  overlying the pump chambers  138 A,  138 B to move along with the piston heads  134 A,  134 B, and thus allows PD solution to be drawn into or forced out of the pump chambers of the cassette  112  in response to piston head movement during treatment. Movement of those regions  162 A,  162 B of the membrane  140  overlying the pump chambers  138 A,  138 B (e.g., through movement of piston heads  134 A,  134 B) changes the internal volume of the pump chambers  138 A,  138 B. For example, movement of the membrane  140  toward the rigid base  156  decreases the fluid volume stored in each of the pump chambers  138 A,  138 B, and thus forces some of the PD solution out of the cassette  112  during treatment. Similarly, movement of the membrane  140  away from the base  156  increases the fluid volume stored in the pump chambers  138 A,  138 B, and thus draws PD solution into the cassette  112  during treatment. 
     By retracting the piston heads  134 A,  134 B farther than they are retracted during treatment (i.e., farther than they are retracted to draw fluid into the pump chambers  138 A,  138 B during treatment) and with a force that exceeds the adhesion force between the piston heads  134 A,  134 B and the adhesive  161 , the piston heads  134 A,  134 B can be detached from the adhesive  161  and the membrane  140 . The piston heads  134 A,  134 B can, for example, be fully retracted into the piston access ports  136 A,  136 B to detach the piston heads  134 A,  134 B from the adhesive  161  and the membrane  140 . In some cases, the piston heads  134 A,  134 B are retracted at least about 1.0 centimeters (e.g., at least about 1.5 centimeters, about 1.0 centimeters to about 2.5 centimeters, about 1.5 centimeters) away from the plane in which the membrane  140  lies in its undeformed state with the cassette  112  positioned in the cassette enclosure  114 . Retracting the piston heads  134 A,  134 B to this position can help to ensure that the piston heads  134 A,  134 B detach from the adhesive  161 . 
     The movement of the piston heads  134 A,  134 B away from the cassette  112  exerts a tensile stress on the adhesive  161 . The piston heads  134 A,  134 B remain substantially adhered to the adhesive  161  if the tensile forces exerted by the piston heads  134 A,  134 B are less than the adhesion strength of the adhesive  161  to the piston heads  134 A,  134 B and/or the piston heads  134 A,  134 B are retracted by a distance less than the distance required to detach the piston heads  134 A,  134 B from the adhesive  161 . The piston heads  134 A,  134 B detach from the adhesive  161  if the tensile forces exerted by the piston heads  134 A,  134 B are greater than the adhesion strength of the adhesive  161  to the piston heads  134 A,  134 B and the piston heads  134 A,  134 B are retracted by a sufficient distance to detach the piston heads  134 A,  134 B from the adhesive  161 . 
     To permit the piston heads  134 A,  134 B to be detached from the adhesive  161  while the adhesive  161  remains attached to the membrane  140  of the cassette  112 , the adhesive  161 , the piston heads  134 A,  134 B, and the membrane  140  are formed of materials such that the adhesion strength or affinity of the membrane  140  to the adhesive  161  is greater than the adhesion strength or affinity of the piston heads  134 A,  134 B to the adhesive  161 . To reduce the likelihood that the adhesive  161  will detach from the membrane  140  (and remain attached to the piston heads  134 A,  134 B) while attempting to detach the piston heads  134 A,  134 B from the adhesive (i.e., by retracting the piston heads  134 A,  134 B), the adhesive  161 , the piston heads  134 A,  134 B, and the membrane  140  are formed of materials such that the adhesion strength or affinity of the adhesive  161  to the membrane  140  is substantially greater (e.g., at least about two times greater (e.g., about two to about three times greater)) than the adhesion strength or affinity of the adhesive  161  to the piston heads  32 A,  23 B. 
     As a result of the selected materials of the adhesive  161 , the piston heads  134 A,  134 B, and the membrane  140 , the piston heads  134 A,  134 B can be detached from the adhesive  161  in a manner similar to that in which a 3M Post-It® note is detached from a surface (e.g., desktop or sheet of paper). The material selection can, for example, ensure that an insignificant amount of adhesive (e.g., no adhesive) remains attached to the piston heads  134 A,  134 B after the piston heads  134 A,  134 B are detached from the adhesive  161  and the membrane  140 . 
     In some implementations, the adhesive  161  is capable of maintaining contact with the piston heads  134 A,  134 B for up to 24 hours at a pump speed of about 200 ml/min to about 600 ml/min. 
     In certain implementations, the adhesive  161  is disposed substantially uniformly over regions  162 A,  162 B. Such a substantially uniform distribution of the adhesive  161  can reduce the likelihood that the membrane  140  will separate or decouple from the piston heads  134 A,  134 B during normal operation. Additionally or alternatively, such a substantially uniform distribution of adhesive can improve the accuracy in calculating the volume of pump chambers  138 A,  138 B based on the position of the piston heads  134 A,  134 B, which can be used to closely track the volume of PD solution pumped out of and drawn into the pump chambers  138 A,  138 B during treatment. 
     In addition to securing the piston heads  134 A,  134 B to the membrane  140  of the cassette  112 , the adhesive  161  can reduce the likelihood of fluid intrusion into the PD cycler  102  during use. For example, because the adhesive  161 , rather than vacuum pressure, is used to retract the portions of the membrane  140  overlying the pump chambers  138 A,  138 B in order to draw fluid into the pump chambers  138 A,  138 B, the possibility of fluid being drawn through the membrane  140  as a result of excessive vacuum pressure is eliminated. In addition, in some cases, the adhesive  161  acts as a substantially impermeable layer that restricts (e.g., prevents) PD solution from passing through the cassette  112  into the PD cycler  102  in regions  162 A,  162 B. For example, to the extent that the membrane  140  is semi-permeable or becomes semi-permeable (e.g., through repeated flexing during use), the adhesive  161  can form a substantially liquid tight seal with the membrane  140  across regions  162 A,  162 B of the membrane  140 . 
     As shown in  FIGS. 8 and 9 , when initially provided to the user, the cassette  112  includes a generally eyeglass-shaped release paper  164  (e.g., wax-coated paper) that covers the adhesive  161  overlying the pump chambers  138 A,  138 B. Such a release paper can, for example, prevent debris and contaminants from collecting on the adhesive  161  during storage and handling of the cassette  112 . The release paper  164  can be peeled off of the cassette  112  to expose the adhesive  161  prior to loading the cassette  112  into the cassette enclosure  114  of the PD cycler  102 . 
     The release paper  164  includes two circular portions  166 A,  166 B and a connector portion  168  that extends between the two circular portions  166 A,  166 B. The circular portions  166 A,  166 B cover the adhesive laden regions  162 A,  162 B of the cassette membrane  140 . The release paper  164  also includes a pull tab  170  that extends beyond the outer boundary of the adhesive  161  such that the pull tab  170  is not attached to the adhesive  161 . A user can remove the release paper  164  to expose the adhesive  161  by pulling the pull tab  170 . As the pull tab  170  is pulled away from the cassette  112 , the connector portion  168  of the release paper  164  facilitates removal of both circular portions  166 A,  166 B of the release paper  164  through a single, continuous motion. 
     Referring to  FIG. 10 , to prepare the PD cassette  112  for use, the release paper  164  is first peeled away from the membrane  140  of the cassette  112  by grasping and pulling the pull tab  170 . This exposes the adhesive  161  initially positioned beneath the release paper  164 . The release paper  164  can be discarded after removing it from the cassette  112 . 
     As shown in  FIG. 11 , the door  108  of the PD cycler  102  is then opened to expose the cassette interface  110 , and the cassette  112  is positioned adjacent to the interface  110  such that the adhesive  161  on the regions  162 A,  162 B of the membrane  140  overlying the pump chambers  138 A,  138 B of the cassette  112  is facing and is aligned with the piston heads  134 A,  134 B. In order to ensure that the adhesive  161  aligns with the piston heads  134 A,  134 B, the cassette  112  is positioned between the locating pins  148  and the lower ledge  150  extending from the cassette interface  110 . The asymmetrical positioning of the connectors  160  of the cassette act as a keying feature that reduces the likelihood that the cassette  112  will be installed with the adhesive facing in the wrong direction (e.g., facing outward toward the door  108 ). Additionally or alternatively, the locating pins  148  can be dimensioned to be less than the maximum protrusion of the hemispherical projections  154 A,  154 B such that the cassette  112  cannot contact the locating pins  148  if the adhesive covered regions  162 A,  162 B of the membrane  140  are facing outward toward the door  108 . 
     While loading the cassette  112  into the PD cycler  102 , the piston heads  134 A,  134 B are completely retracted within the piston access ports  136 A,  136 B. This positioning of the piston heads  134 A,  134 B can reduce the likelihood of damage to the piston heads  134 A,  134 B during installation of the cassette  112 . This positioning of the piston heads  134 A,  134 B can also facilitate positioning the cassette  112  against the cassette interface  110  before closing the door  108 . For example, this positioning can help to ensure that the adhesive  161  is not inadvertently attached to the piston heads  134 A,  134 B prior to properly positioning the cassette  112  within the cassette enclosure  114 . 
     Referring to  FIG. 12A , with the cassette  112  positioned adjacent to the cassette interface  110 , the door  108  is closed over the cassette  112  such that the cassette  112  is substantially contained within the cassette enclosure  114  between the door  108  and the cassette interface  110 . As shown, the hemispherical projections  154 A,  154 B of the cassette  112  fit within the recesses  152 A,  152 B in the door  108 . Because the PD system  100  does not require a vacuum system to move the portions  162 A,  162 B of the membrane  140  overlying the pump chambers  138 A,  138 B, a substantially airtight seal between the door  108  and the cassette interface  110  is typically not required. Thus, as compared to systems including a vacuum system adapted to retract portions of the cassette membrane overlying pump chambers, the door sealing mechanism of the PD cycler  102  can be simpler and more cost effective. 
     As shown in  FIG. 12B , with the cassette  112  secured within the enclosure  114 , the piston heads  134 A,  134 B are moved outward (e.g., to a substantially fully extended position) to contact the adhesive  161  disposed on the regions  162 B,  162 B of the cassette membrane  140 . In this fully extended position, the inner surface of the membrane  140  comes into contact or near contact with the inner surface of the hemispherical projections  154 A,  154 B of the rigid base  156  of the cassette  112 . The contact between the piston heads  134 A,  134 B and the adhesive  161  causes the adhesive  161  to adhere to the piston heads  134 A,  134 B. Because the adhesive  161  is also adhered to the membrane  140  of the cassette  112 , the adhesive  161  secures the piston heads  134 A,  134 B to the membrane  140 . 
     With the piston heads  134 A,  134 B adhesively attached to the membrane  140 , PD solution can be drawn into the pump chambers  138 A,  138 B of the cassette  112  by retracting the membrane  140  along with the piston heads  134 A,  134 B to increase the volume of the pump chambers  138 A,  138 B, as shown in  FIG. 12C . The fluid can then be forced out of the pump chambers  138 A,  138 B by again returning the piston heads  134 A,  134 B to the position shown in  FIG. 12B , causing the membrane  140  to move toward the rigid base  156  and thus decreasing the volume of the pump chambers  138 A,  138 B. As noted above, while forcing PD solution into and out of the pump chambers  138 A,  138 B, certain inflatable members  142  of the PD cycler  102  can be selectively inflated to direct the pumped PD solution along desired pathways in the cassette  112 . 
     Referring back to  FIGS. 1 and 2 , during PD treatment, the patient line  130  is connected to a patient&#39;s abdomen via a catheter, and the drain line  132  is connected to a drain or drain receptacle. The PD treatment typically begins by emptying the patient of spent PD solution that remains in the patient&#39;s abdomen from the previous treatment. To do this, the pump of the PD cycler  102  is activated to cause the piston heads  134 A,  134 B to reciprocate and selected inflatable members  142  are inflated to cause the spent PD solution to be drawn into the pump chambers  138 A,  138 B of the cassette  112  from the patient and then pumped from the pump chambers  138 A,  138 B to the drain via the drain line  132 . This flow path of the spent PD solution through the fluid pathways  158  in the cassette  112  is shown in  FIG. 13A . 
     After draining the spent PD solution from the patient, heated PD solution is transferred from the heater bag  124  to the patient. To do this, the pump of the PD cycler  102  is activated to cause the piston heads  134 A,  134 B to reciprocate and certain inflatable members  142  of the PD cycler  102  are inflated to cause the spent PD solution to be drawn into the pump chambers  138 A,  138 B of the cassette  112  from the heater bag  124  via the heater bag line  128  and then pumped from the pump chambers  138 A,  138 B to the patient via the patient line  130 . This flow path of the PD solution through the fluid pathways  158  in the cassette  112  is shown in  FIG. 13B . 
     Once the PD solution has been pumped from the heater bag  124  to the patient, the PD solution is allowed to dwell within the patient for a period of time. During this dwell period, toxins cross the peritoneum into the PD solution from the patient&#39;s blood. As the PD solution dwells within the patient, the PD cycler  102  prepares fresh dialysate for delivery to the patient in a subsequent cycle. In particular, the PD cycler  102  pumps fresh PD solution from one of the four full PD solution bags  122  into the heater bag  124  for heating. To do this, the pump of the PD cycler  102  is activated to cause the piston heads  134 A,  134 B to reciprocate and certain inflatable members  142  of the PD cycler  102  are inflated to cause the PD solution to be drawn into the pump chambers  138 A,  138 B of the cassette  112  from the selected PD solution bag  122  via its associated line  126  and then pumped from the pump chambers  138 A,  138 B to the heater bag  124  via the heater bag line  128 . This flow path of the PD solution through the fluid pathways  158  in the cassette  112  is shown in  FIG. 13C . 
     After the PD solution has dwelled within the patient for the desired period of time, the spent PD solution is pumped from the patient to the drain. The heated PD solution is then pumped from the heater bag  124  to the patient where it dwells for a desired period of time. These steps are repeated with the PD solution from two of the three remaining PD solution bags  122 . The PD solution from the last PD solution bag  122  is typically delivered to the patient and left in the patient until the subsequent PD treatment. 
     The PD cycler is typically used in an alternating pumping mode in which one piston head is protracted while the other piston head is retracted. Thus, as fluid drawn into one pumping chamber, fluid is simultaneously expelled from the other pumping chamber. 
     The adhesive attachment between the piston heads  134 A,  134 B and the membrane  140  can result in a substantially direct correlation between the position of the piston heads  134 A,  134 B and the volume of fluid drawn into and pumped out of the pump chambers  138 A,  138 B of the cassette  112 . Such a direct correlation can improve the speed and accuracy of volumetric calculations of PD solution drawn into and pumped out of the cassette  112 . 
     After completion of the PD treatment, the piston heads  134 A,  134 B are retracted away from the cassette  112  (e.g., perpendicular to the cassette  112 ) to a distance sufficient to allow the adhesive  161  to detach from the piston heads  134 A,  134 B and with a force that exceeds the adhesion strength or affinity of the adhesive  161  to the piston heads  134 A,  134 B. With this motion, the piston heads  134 A,  134 B become completely detached from the adhesive  161 . Because the adhesion strength or affinity of the adhesive  161  to the membrane  140  is greater than the adhesion strength or affinity of the adhesive  161  to the piston heads  134 A,  134 B, the adhesive  161  remains adhered to the cassette  112  after the piston heads  134 A,  134 B have been detached. This helps to reduce or eliminate adhesive build-up on the piston heads  134 A,  134 B through repeated use. With the piston heads  134 A,  134 B detached from the adhesive  161 , the door  108  of the PD cycler  102  is opened to expose the cassette  112 , and the cassette  112 , including the adhesive thereon, is removed from the cassette enclosure  114  by moving the bottom portion of the cassette  112  away from the cassette interface  110  and disengaging the top portion of the cassette  112  from the locating pins  148 . In some cases, the cassette  112  is then discarded along with the fluid lines attached to the cassette  112 . Because the adhesive  161  detaches from the piston heads  134 A,  134 B, it will generally be unnecessary for the user to clean the piston heads  134 A,  134 B prior to a subsequent use with a new cassette. 
       FIGS. 14A-14C  illustrate a method of preparing eyeglass-shaped composites of adhesive and release paper that are used to apply the adhesive  161  and release paper  164  to the membrane  140  of the cassette  112 . Referring to  FIG. 14A , the adhesive  161  is first sprayed onto the top surface of a sheet of release paper (e.g., a sheet of wax-coated paper)  200  by a sprayer  202  including two spray nozzles  204 . The adhesive  161  is sprayed onto the sheet of release paper  200  in pairs of circular adhesive regions  206  that are longitudinally spaced from each other along the sheet of release paper  200 . The sprayer  202  is intermittently activated while the sheet of release paper  200  is continuously passed under the nozzles  204  to create the longitudinally spaced adhesive regions  206 . The circular adhesive regions  206  within each pair are sized to correspond to the size of the pump chambers  138 A,  138 B of the cassette  112  (i.e., the largest diameter of the pump chambers  138 A,  138 B), and the circular adhesive regions  206  within each pair are laterally spaced from one another across the sheet of release paper  200  by substantially the same distance that the pump chambers  138 A,  138 B of the cassette  112  are laterally spaced from one another. As an alternative to using two separate nozzles configured to create the circular adhesive regions, a large surface area sprayer can be equipped with a stencil with circular shaped openings such that only adhesive passing though the openings will reach the sheet of release paper  200  while the remaining adhesive accumulates on the stencil. In addition, as an alternative to spraying the adhesive onto the sheet of release paper  200 , any of various other techniques for applying the adhesive to the sheet of release paper  200  can be used. Examples of other techniques include dip coating, pouring, painting, etc. 
     As shown in  FIG. 14B , after applying the adhesive circular regions to the top surface of the sheet of release paper  200 , another sheet of release paper  208  is disposed over the circular adhesive regions  206  and secured to the first sheet of release paper  200  by the adhesive regions  206 . The resulting composite sheet includes a discontinuous layer of circular adhesive regions  206  sandwiched between the two sheets of release paper  200 ,  208 . 
     Referring to  FIG. 14C , a generally eyeglass-shaped cutter  210  is then forced through the composite sheet at multiple, longitudinally spaced positions along the sheet to form multiple generally eyeglass-shaped composites. The cutter  210  includes a sharp cutting edge that extends around its periphery on its lower surface. The cutter  210  is manipulated to cut through the composite along a path that encompasses each of the pairs of adhesive regions  206 . To do this, the composite sheet is passed under the cutter  210  until one of the pairs of adhesive regions  206  within the composite sheet lies directly beneath the cutter  210  with cutting edges of the cutter  210  surrounding the pair of adhesive regions  206 . At this point, the movement of the composite sheet is paused, and the cutter  210  punches through the composite sheet to form an eyeglass-shaped composite. This process is repeated to produce multiple eyeglass-shaped composites. Each eyeglass-shaped composite includes two circular adhesive shaped regions  206  sandwiched between two eyeglass-shaped layers of release paper. Each of the two layers of release paper includes a pull tab  212  that extends beyond the adhesive regions  206  to facilitate removal of the release paper by a user. 
       FIG. 15  shows one of the eyeglass-shaped composites produced from the above described method. As shown in  FIG. 15 , to permit the adhesive regions  206  to be secured to the regions  162 A,  162 B of the membrane  140  overlying the pump chambers  138 A,  138 B of the cassette  112 , one of the release papers is peeled away from the adhesive by pulling on its pull tab  212 . 
     Referring to  FIG. 16 , the exposed adhesive regions are then applied to the portions  162 A,  162 B of the membrane  140  overlying the pump chambers  138 A,  138 B of the cassette  112 . A set of fluid lines is then attached to the cassette, and the assembly of the cassette  112  and the attached fluid lines is packaged in a container (e.g., a bag and a box) for delivery to a user. At this point, the cassette  112 , including the adhesive regions and release paper thereon, the fluid lines, and the packaging is sterilized with ethylene oxide (ETO). The release paper advantageously limits contact between the adhesive regions and the ETO during this sterilization process, which helps to maintain the integrity of the adhesive. Alternatively or additionally, the cassette  112  can be sterilized using other sterilization techniques, such as gamma or e-beam sterilization. 
     While certain implementations have been described, other implementations are possible. 
     While the release paper  164  has been described as having a generally eyeglass shape, other types of release papers can be used. In some implementations, as shown in  FIG. 17 , for example, two substantially circular shaped release papers  364  are used to cover the adhesive regions. Each of the release papers  364  includes a pull tab  370  that extend beyond the outer boundary of the adhesive region to facilitate removal of the release paper  364  from the adhesive. 
     While the release papers described above are shaped to cover substantially only those regions of the cassette membrane  140  that include adhesive thereon, the release paper can be dimensioned to cover any of various different areas of the cassette membrane  140 . In some implementations, as shown in  FIG. 18 , for example, a release paper  464  covers substantially the entire membrane  140  of the cassette  112 . This can help protect the cassette membrane  140  from damage and/or contamination prior to use. One of the corners of the release paper  464  can be used as a pull tab to facilitate removal of the release paper  464  from the adhesive. As shown, the release paper  464  bears printed text that reminds the user to remove the release paper  464  before use. Any of the various other release papers described herein could include similar text. 
     While the adhesive  161  has been described as being initially disposed on the membrane  140  of the cassette  112 , other arrangements are possible. In some implementations, for example, the adhesive  161  is initially disposed on the piston heads  134 A,  134 B. In certain implementations, the adhesive  161  remains on the piston heads  134 A,  134 B after the cassette  112  has been removed from the cassette enclosure  114  of the PD cycler  102 . For example, the adhesive  161 , the membrane  140  of the cassette  112 , and the piston heads  134 A,  134 B can be formed of materials such that the adhesive  161  has a greater adhesion or affinity with the piston heads  134 A,  134 B than with the membrane  140 . Because the adhesive  161  remains on the piston heads  134 A,  134 B, the adhesive  161  can be reused through multiple PD treatments, or the user can remove the adhesive  161  from the piston heads  134 A,  134 B between treatments. In other implementations, the materials of the adhesive  161 , the membrane  140  of the cassette  112 , and the piston heads  134 A,  134 B can be selected such that the adhesive  161  has a greater adhesion or affinity with the membrane  140  than with the piston heads  134 A,  134 B. In such cases, the adhesive  161  would ultimately remain adhered to the cassette  112  that is removed from the PD cycler  102  and discarded. 
     While the adhesive differential between the piston heads  134 A,  134 B and the membrane  140  has been described as being achieved through a combination of materials including forming the piston heads  134 A,  134 B of polyoxymethylene and the membrane  140  from the above-described multi-layer laminate while using a synthetic rubber adhesive, other material combinations that provide levels of adhesion between the adhesive and the piston heads and between the adhesive and the cassette membrane to allow the piston heads to retract the membrane during treatment and to allow the piston heads to be detached from the membrane after treatment without detaching the adhesive from the cassette membrane can be used. 
     In certain implementations, for example, the adhesive can be formed of any of various other types of materials that have adhesion properties similar to the synthetic rubber adhesive described above. For example, the adhesive can be a natural rubber adhesive or an acrylic adhesive. The piston heads  134 A,  134 B can be formed of one or more polyvinyl chlorides (PVC), polyamides, polycarbonates, ethyl vinyl acetates, and/or polysulfones. The membrane  140  can be formed of any of various types of polyolefins (e.g., high density polyethylenes (HDPE), low density polyethylenes (LDPE), or combinations of HDPE and LDPE) and/or polyvinylchlorides (PVC). 
     In certain implementations, the adhesive  161  is formed of natural rubber adhesive, the piston heads  134 A,  134 B are formed of polyoxymethylene plastic, and the membrane  140  is formed of polyolefin. In other implementations, the adhesive  161  is formed of acrylic co polymer, the piston heads  134 A,  134 B are formed of polycarbonate, and the membrane  140  is formed of PVC. 
     While the membrane  140  of the cassette  112  has been described as including three layers, the membrane of the cassette can alternatively include fewer than three layers. For example, the membrane can include two layers or only a single layer. Alternatively, the membrane of the cassette can include more than three layers. 
     While the base  156  of the cassette  112  has been described as being formed of polypropylene, the base  156  can be formed of any of various different rigid materials that are capable of being securely attached (e.g., thermally bonded, adhesively bonded) to the cassette membrane  140 . In some implementations, for example, the base  156  is formed of polyvinyl chloride, polycarbonate, polysulfone, or other medical grade plastic materials. 
     While the piston heads  134 A,  134 B have been described as being formed of a single material that has a desired affinity for the adhesive, other types of piston head constructions can be used. For example, in some implementations, the piston head includes a core on which an outer layer or coating is applied. In such implementations, the outer layer or coating can be formed of a material that has the desired affinity for the adhesive while the core of the piston head can be formed of a different material that does not have the desired affinity for the adhesive. The outer layer or coating can, for example, be formed of any of the materials described above with regard to piston heads  134 ,  134 B, and the core can be formed of any of various other materials, including polymers, metals, and/or alloys. 
     While the adhesion or affinity differential between the piston heads  134 A,  134 B and the membrane  140  has been described as being achieved through material selection, other methods of achieving the adhesion differential can alternatively or additionally be used. For example, the piston heads  134 A,  134 B can be roughened (e.g., through etching, through roughness built into a mold) to increase the surface area of the piston heads  134 A,  134 B. As compared to piston heads having smooth surfaces, such an increase in surface area can increase the adhesion between the adhesive  161  and the piston heads  134 A,  134 B and thus increase the types of materials that can be used for the piston heads  134 A,  134 B. 
     While the adhesive has been described as being applied in circular regions to the sheet of release paper before cutting the release paper into a desired shape (e.g., a generally eyeglass shape), in certain implementations, the adhesive is applied to (e.g., extruded onto) the sheet of release paper in a continuous layer. In such implementations, the adhesive would extend across the entire surface of the release paper/adhesive composite that is ultimately produced. In some implementations, the adhesive is not applied to an edge region of the sheet of release paper such that pull tabs of the release paper/adhesive composite can be cut from the edge region to facilitate removal of the release paper from the adhesive. 
     In certain cases, ready-to-use adhesive/release paper composites can be purchased from a supplier. In such cases, the release paper on one side of the adhesive would be removed from the composite and the exposed adhesive (with the other release paper secured to its opposite side) would be attached to the cassette membrane. 
     In some implementations, multiple layers of adhesive are used to releasably attach the piston heads  134 A,  134 B to the membrane  140 . For example, as shown in  FIG. 19 , a double-sided tape  500  is disposed between the piston heads  134 A,  134 B and the membrane  140 . For improved clarity of the cassette, no components of the PD cycler  102  other than the piston head  134 A and its related piston shaft are shown in  FIG. 19 . The double-sided tape  500  includes a first adhesive layer  502  and a second adhesive layer  504  disposed on opposite sides of a base  506 . The first adhesive layer  502  is affixed to portions  162 A,  162 B of the membrane  140  overlying the pump chambers  138 A,  138 B of the cassette  112 . With the first adhesive layer  502  adhered to the membrane  140 , the second adhesive layer  504  is exposed toward the piston heads  134 A,  134 B. During use, the piston heads  134 A,  134 B can be moved into contact with the second adhesive layer  504  to adhere the second adhesive layer  504  to the piston heads  134 A,  134 B. The first adhesive layer  502  in contact with the membrane  140  can be formed of a biocompatible adhesive to reduce the likelihood of contaminating the PD solution contained in the cassette  112 . The second adhesive layer  504  in contact with the piston heads  134 A,  134 B can be formed of either a biocompatible or bioincompatible adhesive capable of achieving the desired adhesion to the material of the piston heads  134 A,  134 B. In implementations in which the first adhesive layer  502  is formed of a biocompatible adhesive and the second adhesive layer  504  is formed of a bioincompatible adhesive, the base  506  can be formed of low density polyethylene (LDPE). The LDPE base  506  can resist permeation of the bioincompatible adhesive of the second layer  504  and, thus, reduce the likelihood of contamination of the biocompatible adhesive in contact with the membrane  140 . 
     The use of two or more different adhesives allows the cassette to be used with membrane/piston head material combinations different than those discussed above. For example, multiple different adhesive combinations can be used for different dialysis systems. 
     While double-sided tape  500  has been described as including a base  96 , the two different adhesives can be directly adhered to one another, particularly if each adhesive is biocompatible. 
     While the adhesive has been described as being uniformly distributed across those regions  162 A,  162 B of the cassette membrane  140  that overlie the pump chambers  138 A,  138 B, other arrangements are possible. For example, in some implementations, the adhesive  161  is distributed over the regions  162 A,  162 B in a pattern. 
     While the piston heads  134 A,  134 B have been described as being axially moved to break the attachment between the adhesive and the piston heads  134 A,  134 B, other types of movements of the piston heads  134 A,  134 B can alternatively or additionally be used to break the attachment between the adhesive and the piston heads  134 A,  134 B. In some implementations, each piston head is at least partially rotatable about an axis perpendicular to a membrane to detach the piston head from the adhesive through a substantially shear force. In certain implementations, each piston head is moveable in a direction substantially parallel to the membrane to detach the piston head from the adhesive through a substantially shear force. By allowing the release of the piston head using a different type of force than the one used to move the membrane, the likelihood of inadvertent detachment of the piston head can be reduced. 
     While the piston heads  134 A,  134 B have been described as being moved (e.g., retracted, rotated, and/or laterally displaced) by a distance sufficient to completely detach the piston heads  134 A,  134 B from the adhesive, the piston heads  134 A,  134 B can alternatively be moved by a distance that causes the piston heads  134 A,  134 B to only partially detach from the adhesive. In such implementations, the user can complete the detachment of the piston heads  134 A,  134 B from the adhesive when the user pulls the cassette out of the cassette enclosure of the PD cycler. 
     While the adhesive has been described as being exposed through the removal of a release paper, other methods of exposing the adhesive are possible. For example, the adhesive can be formed on the cassette  112  through the chemical reaction of two materials on the cassette  112 . In such a configuration, a first non-adhesive material can be initially disposed on regions  162 A,  162 B of the cassette  112 , and a second material can be placed into contact with the first material to form an adhesive. 
     While the cassette  112  has been described as being positioned between the locating pins  148  and the lower ledge  150  extending from the cassette interface  110  in order to hold the cassette  112  in a position such that the piston heads  134 A,  134 B align with the pump chambers  138 A,  138 B, other techniques for ensuring that the piston heads  134 A,  134 B align with the pump chambers  138 A,  138 B can alternatively or additionally be used. In some implementations, for example, the cassette  112  is placed against the door  108  of the PD cycler  102  with the hemispherical projections  154 A,  154 B of the cassette  112  disposed in the recesses  152 A,  152 B of the door  108 . The cassette  112  is held in this position by retainer clips attached to the door  108 . Upon closing the door  108 , the piston heads  134 A,  134 B of the PD cycler  102  align with the pump chambers  138 A,  138 B of the cassette  112 . This technique helps to prevent the adhesive  161  from inadvertently sticking to the piston heads  134 A,  134 B or the cassette interface  110  when loading the cassette  112  into the PD cycler  102 . 
     While the PD cycler  102  has been described as including a touch screen and associated buttons, the PD cycler can include other types of screens and user data entry systems. In certain implementations, for example, the cycler includes a display screen with buttons (e.g., feathertouch buttons) arranged on the console adjacent the display screen. Certain buttons can be arranged to be aligned with operational options displayed on the screen during use such that the user can select a desired operational option by pressing the button aligned with that operational option. Additional buttons in the form of arrow buttons can also be provided to allow the user to navigate through the various display screens and/or the various items displayed on a particular screen. Other buttons can be in the form of a numerical keypad to allow the user to input numerical values in order, for example, to input operational parameters. A select or enter button can also be provided to allow the user to select an operational option to which the user navigated by using the arrow keys and/or to allow the user to enter values that the user inputted using the numerical keypad. 
     While the adhesive laden cassettes described above have been described as being part of a PD system, these types of cassettes can be used in any of various other types of cassette-based medical fluid pumping systems, including hemodialysis systems.