[0001]     The present application incorporates by reference the entire disclosure of U.S. Application entitled “Pressure Activated Safety Valve With High Flow Slit” filed on even day herewith naming Karla Weaver and Paul DiCarlo as inventors, U.S. Application entitled “Pressure Activated Safety Valve With Anti-Adherent Coating” filed on even day herewith naming Karla Weaver and Paul DiCarlo as inventors, and U.S. Application entitled “Pressure Actuated Safety Valve With Spiral Flow Membrane” filed on even day herewith naming Paul DiCarlo and Karla Weaver as inventors, and U.S. Application entitled “Dual Well Port Device” filed on even day herewith naming Katie Daly, Kristian DiMatteo and Eric Houde as inventors.  
       BACKGROUND OF THE INVENTION  
       [0002]     Many medical procedures require repeated and prolonged access to a patient&#39;s vascular system. For example, during dialysis treatment blood may be removed from the body for external filtering and purification, to make up for the inability of the patient&#39;s kidneys to carry out that function. In this process, the patient&#39;s venous blood is extracted, processed in a dialysis machine and returned to the patient. The dialysis machine purifies the blood by diffusing harmful compounds through membranes, and may add to the blood therapeutic agents, nutrients etc., as required before returning it to the patient&#39;s body. Typically the blood is extracted from a source vein (e.g., the vena cava) through a catheter sutured to the skin with a distal needle of the catheter penetrating the source vein.  
         [0003]     It is impractical and dangerous to insert and remove the catheter for each dialysis session. Thus, the needle and catheter are generally implanted semi permanently with a distal portion of the assembly remaining within the patient in contact with the vascular system while a proximal portion of the catheter remains external to the patient&#39;s body. The proximal end is sealed after each dialysis session has been completed to prevent blood loss and infections. However, even small amounts of blood oozing into the proximal end of the catheter may be dangerous as thrombi can form therein due to coagulation. These thrombi may then be introduced into the patient&#39;s vascular system when blood flows from the dialysis machine through the catheter in a later session.  
         [0004]     A common method of sealing the catheter after a dialysis session is to shut the catheter with a simple clamp. This method is often unsatisfactory because the repeated application of the clamp may weaken the walls of the catheter due to the stress placed on the walls at a single point. In addition, the pinched area of the catheter may not be completely sealed allowing air to enter the catheter which may coagulate any blood present within the catheter. Alternatively, valves have been used at the opening of the catheter in an attempt to prevent leaking through the catheter when the dialysis machine is disconnected. However, the unreliability of conventional valves has rendered them unsatisfactory for extended use.  
         [0005]     One type of valve that is often used in sealing catheters is the Pressure Activated Safety Valve (PASV), which opens in response to a prescribed pressure being applied to the fluid flowing in the catheter. When no pressure is applied, the PASV remains closed, thus sealing the catheter. These valves are subject to great mechanical stress due to the high number of open-close cycles they undergo, and to the high flow of fluids (such as blood) they must pass. The valves are designed to reliably seal the catheter opening when not in use, while retaining a simple and relatively inexpensive configuration.  
       SUMMARY OF THE INVENTION  
       [0006]     In one aspect, the present invention is directed to a pressure activated valve for medical applications comprising a housing having a lumen extending therethrough from a proximal end to a distal end thereof and a flow control membrane extending across the lumen to control flow therethrough, the flow control membrane including a mounting portion at which the flow control membrane is coupled to the housing and a lumen occluding portion having a slit extending therethrough so that, when the lumen occluding portion is subjected to a pressure of at least a predetermined threshold level, the lumen occluding portion moves from a closed configuration in which flow through the lumen is prevented to an open configuration in which flow is permitted and wherein a thickness of the mounting portion is greater than a thickness of the lumen occluding portion.  
         [0007]     The present invention is further directed to a method of forming a membrane for a pressure activated valve, comprising the steps of forming a substantially planar flow control membrane dimensioned to fit in a housing of the pressure activated valve, wherein a mounting portion of the flow control membrane is adapted to engage the housing and forming at least one slit in the flow control membrane, the slit being openable by pressure of a fluid in the pressure activated valve of at least a predetermined threshold level in combination with the steps of forming an annular base membrane dimensioned to substantially overlie the mounting portion of the flow control membrane and stacking the base membrane on the mounting portion of the flow control membrane. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is diagram showing a pressure activated safety valve in a closed configuration and in two opposite flow configurations;  
         [0009]      FIG. 2  is a perspective exploded view showing a stacked valve membrane according to an embodiment of the present invention;  
         [0010]      FIG. 3  is a top plan view of a flow control portion of the stacked valve membrane according to an embodiment of the present invention; and  
         [0011]      FIG. 4  is a top plan view of a base membrane portion of the stacked valve membrane according to the embodiment shown in  FIG. 3 .  
     
    
     DETAILED DESCRIPTION  
       [0012]     The present invention may be further understood with reference to the following description and the appended drawings, wherein like elements are referred to with the same reference numerals. The invention is related to the field of medical valves, and more specifically to the field of pressure activated valves that are used to seal a lumen of a medical catheter.  
         [0013]     Semi-permanently placed catheters may be useful for a variety of medical procedures which require repeated access to a patient&#39;s vascular system in addition to the dialysis treatments mentioned above. For example, chemotherapy infusions may be repeated several times a week for extended periods of time. For safety reasons, as well as to improve the comfort of the patient, injections of these therapeutic agents may be better carried out with an implantable, semi-permanent vascular access catheter. Many other conditions that require chronic venous supply of therapeutic agents, nutrients, blood products or other fluids to the patient may also benefit from implantable access catheters, to avoid repeated insertion of a needle into the patient&#39;s blood vessels. Thus, although the following description focuses on dialysis, those skilled in the art will understand that the invention may be used in conjunction with any of a wide variety of procedures which require long term implantation of catheters within the body.  
         [0014]     Examples of such implantable catheters include those manufactured by Vaxcel™, such as the Chronic Dialysis Catheter and the Implantable Vascular Access System. These devices typically are inserted under the patient&#39;s skin, and have a distal end which includes a needle used to enter a blood vessel. The devices also have a proximal end extending outside the body for connection with an outside line. These semi-permanent catheters may be sutured to the patient&#39;s skin to maintain them in place while the patient goes about his or her normal occupations. These catheters may include two or more lumens which are used respectively to remove fluids from and to reintroduce to the blood vessel. When the catheter is disconnected from the dialysis machine, it can be left within the patient, connected to the patient&#39;s vascular system. In that case, it is important to securely seal the hub of the catheter to prevent fluids from escaping and contaminants from entering the patient&#39;s body. If a seal is not obtained, the patient incurs serious risks such as the possibility of infections, the risk of embolisms due to air entering the blood stream and the risk of venous thrombosis due to coagulation of blood in and near the catheter. Leakage from an improperly sealed catheter can expose the attending medical staff to a risk of infection by blood borne pathogens. Thus a mechanism is necessary to ensure that the proximal end of the catheter can be sealed when it is not in use.  
         [0015]     A conventional clamp or a clip can be used to seal the catheter between medical sessions, but several drawbacks exist with this method. For example, the material of the catheter wall can be damaged by the clamp, since the sealing force is exerted on a very small surface area of the catheter. Repeatedly clamping and releasing the catheter may, over time, weaken the material sufficiently to cause a failure of the catheter. This is dangerous for the patient, since it can lead to loss of blood and to infection. A sealing clamp may also become dislodged during activities of the patient, thus increasing the risk of leaks, infections, and various other problems listed above. Placing a clamp on the catheter&#39;s proximal end may also increase the bulk of the device which is exposed outside the patient&#39;s body, and may adversely affect patient comfort.  
         [0016]     An alternative to clamping the catheter&#39;s proximal end is to include self sealing valves near the entrance of the flow passages of the catheter, to seal those passages when not used. For example, one or more valves may be placed in the catheter (e.g., near a proximal end thereof. The valves are preferably designed to seal the catheter&#39;s lumen under certain conditions, and to allow passage of a fluid under other conditions. In an exemplary case applicable to a dialysis catheter, the system of valves may seal the catheter when it is not connected to an operating dialysis machine, and may allow both an outflow of non-purified blood and an inflow of purified blood to the patient when an operating dialysis machine is connected thereto. The valves used in dialysis thus selectively allow bidirectional flow into the patient and out of the patient, depending on the pressures applied thereto.  
         [0017]     Pressure activated safety valves (PASV&#39;s) are one type of flow control device that is often used to seal such semi-permanently implanted vascular catheters when not in use. These valves function by opening when subject to a pre-determined flow pressure, and by remaining closed when subject to pressures less than the predetermined flow pressure. In the exemplary case of a PASV used in a dialysis catheter, the valve may be set to open when subject to a pressure which would be applied thereto by an operating dialysis machine and to remain closed when subject to lesser pressures (e.g., pressures applied by the vascular system itself. When a dialysis machine is not connected thereto (or when a connected dialysis machine is not operating), the pressure in the lumen is insufficient to open the PASV, and the dialysis catheter remains sealed. The PASV may be calibrated such that the pressure due to the blood flowing naturally through the patient&#39;s vascular system is not sufficient to open it and unseal the catheter.  
         [0018]      FIGS. 1A-1C  show cutaway views of a PASV  20  (a three-way safety valve), depicting three flow conditions. In condition A, a fluid is introduced into the catheter  22  via a hub  18 . In condition C, fluid is removed from the catheter  22  through the hub  18 , and in condition B the valve is closed and no fluid flows therethrough. In the context of a dialysis catheter, condition A is infusion, i.e., blood is traveling from the dialysis machine to the patient and condition C is aspiration, i.e., blood is traveling from the patient to the dialysis machine. Condition B corresponds to a closed state where the valve membrane is in the closed position allowing no fluid travel, e.g., a condition in which no dialysis treatment is ongoing so that the valve  20  remains in the closed configuration. The closed position prevents bleedback. According to one exemplary embodiment of the present invention, the valve  20  comprises a valve housing  30  forming a body of the device and a flow control membrane  32  disposed within the housing  30 . The hub  18  may define the valve housing  30  or, alternatively, the housing  30  and the hub  18  may be formed as separate units. A fluid flow chamber  36  extends through the housing  30  so that fluid (e.g., blood) may flow therethrough into and out of the catheter  22 . As would be understood by those of skill in the art, although the exemplary flow chamber  36  is shown as substantially cylindrical, in different applications the flow chamber  36  may be of any other shape suitable for the efficient flow of a fluid. The housing  30  may be connected to a medical device (for example a dialysis machine) on a proximal side and to a patient line (for example a dialysis catheter) on the distal side.  
         [0019]     A flow control membrane  32  may be disposed in the flow chamber  36  positioned to selectively impede the passage of fluid though flow chamber  36 . The flow control membrane may, for example, be located adjacent the proximal end of the housing  30 . One or more slits  34  are extend through the membrane  32  so that under predetermined conditions, the slit  34  opens. When the membrane  32  is not subject to the predetermined conditions, the slit  34  remains closed. For example, the flow control membrane  32  may be constructed so that the slit  34  opens when subject to a flow pressure of at least a predetermined threshold level, but remains securely closed when a flow pressure impinging thereon is less than this threshold level. This threshold valve opening pressure may correspond, for example, to pressures to which the valve would be subjected if an operating dialysis machine were connected thereto and will preferably be substantially greater than pressure to which the membrane  32  would be generated by the patient&#39;s vascular system or which would be induced in the housing  30  due to patient activity.  
         [0020]     The valve  20  is therefore expressly designed so that the pressure imparted thereto through the operation of a dialysis machine will open the slit  34  and allow a desired volumetric flow of blood to pass through between the proximal and distal ends of the housing to and from the dialysis machine and the patient&#39;s vascular system. For example, membrane  32  is formed so that, when not subject to a pressure of at least the threshold level, edges of the slit  34  remain joined. A first exemplary embodiment may be a 0.010 inch membrane sandwiched between two membranes having a thickness of 0.0025 inches. A second exemplary embodiment may be two 0.010 inch membranes stacked together. An exemplary durometer hardness may be  55 A and an exemplary slit length of 9 mm. Those skilled in the art will understand that the above are only exemplary and that the valve performance may be varied by altering the choice of materials for the membrane  32  (e.g., varying durometer and compression), a thickness of the materials and a size of the slit  34 . In addition, if desired, additional stiffening members may be used for that purpose.  
         [0021]     As shown in  FIG. 1 , the flow control membrane  32  is securely held in place within the housing  30 , so that it will not be displaced by the force of the fluid flowing through the valve  20 . For example, the flow control membrane  32  may be placed on a membrane seat  50  of the housing  30 . The membrane  32  may, for example, be sandwiched between separable portions of the housing  30 , so that a membrane retention portion  52  of the housing  30  applies a compressive force to a periphery of flow control membrane  32  thereby maintaining it in a desired position within the housing. However, if too strong a retention force is applied to a small surface portion of the membrane  32 , the membrane  32  may be deformed possibly resulting in puckering or other deformation thereof. This may prevent the slit  34  from closing completely resulting in an insufficient seal and the many disadvantages associated therewith.  
         [0022]     Too strong a compressive force applied to the membrane  32  may also cause stress cracks to form thereon at locations where the force is applied. Specifically, cracks may appear near a seating portion of the membrane  32  at which it is compressed between the membrane seat  50  and the membrane retention portion  52  of the housing  30 . In the exemplary embodiment shown in the drawings, the seating portion is an annular region comprising a periphery of the flow control membrane  32 . The housing may be formed by two halves, which are joined together to sandwich the flow control membrane  32  therebetween.  
         [0023]     Furthermore, propagation of the slit (or slits) may result as strain concentrates at ends thereof when a fluid flows through the valve  20 , forcing the slit  34  to the open configuration. If the membrane  32  is too thin, edges of the membrane  32  that are retained by the housing may not completely immobilize the membrane  32 . The additional movement of the thin membrane  32  may cause further strains to be exerted on edges of the slit(s)  34 , resulting in propagation of the slit and possible failure of the valve  20 . This may cause the slit  34  to grow to a size whereby the resilience of the material of the membrane  32  is no longer able to maintain the edges of the slit  34  in contact with one another when subject to pressures below the threshold level.  
         [0024]     Thus, from the standpoint of structural integrity it is desirable to form the flow control membrane  32  as thickly as possible to prevent deformation of the surface of the membrane  32  and to prevent cracks from propagating near the periphery of the membrane  32  and at the edges of the slit  34 . However, the design of the flow control membrane  32  takes into consideration parameters in addition to structural strength. For example, a large flow rate through the valve  20  is desirable to minimize the time necessary to carry out procedures such as dialysis. As would be understood by those skilled in the art, the flow rate through a PASV valve is affected by the thickness of the flow control membrane  32 , among other things. The thinner the membrane is made, the higher the rate of flow through the valve  20  as the slit  34  will open more widely for a given pressure applied thereto.  
         [0025]     To satisfy these contradictory requirements a compromise is generally made by selecting a thickness of the membrane  32  which provides an acceptable structural reliability of the membrane  32  and at the same time allows sufficient flow of fluid therethrough. It is desired to enhance the structural integrity of the valve  20  while maintaining or increasing a flow rate therethrough for a given flow pressure.  
         [0026]     Referring to  FIG. 2  and according to the present invention, a stacked membrane is provided which satisfies in an optimal manner the two competing design goals of long structural life and high fluid flow rate. This result is achieved by using a thin flow control membrane  102  having one or more slits  106  extending therethrough to selectively impede the flow of a fluid through the valve  20 . In this embodiment of the invention, the flow control membrane  102  is made as thin as necessary to achieve a desired flow rate therethrough at the expected pressure (e.g., the pressure applied by an operating dialysis machine). In addition to the thin membrane  102 , a base membrane  104  is stacked thereon. The base membrane  104  is formed as desired to provide the extra strength necessary at the periphery of the thin membrane  102  to withstand the compressive forces exerted thereon. This base membrane  104  may preferably be made as thick as or thicker than the thin membrane  102 . The base membrane  104  is preferably formed with a large opening formed in a center thereof so that a large portion of the thin membrane  102  is exposed while the seating portion to which the compressive forces are to be applied by the housing  30  is reinforced by the base membrane  104 . The stacking order for the membranes  102 ,  104  may be changed, without affecting the properties of the resulting stacked membrane. Alternatively, the thin membrane  102  may be sandwiched between a pair of base membranes  104  for extra support.  
         [0027]     By selecting an appropriate thickness for the base membrane  104 , the resulting stacked membrane will have a greater thickness at locations where the retaining compressive force are exerted by the valve housing  30  while the portion of the stacked membrane surrounding the slits  106  is thinner to allow for a greater flow rate. Exemplary ranges for the thickness b 1  and b 2  as shown in  FIG. 2  may be 0.005-0.1 inches. In addition, the base membrane  104  may be tapered to direct the flow of fluid into the valve.  
         [0028]     More specifically,  FIG. 2  shows a perspective view of a stacked membrane  100  of a PASV according to an embodiment of the present invention. As described above, the stacked membrane  100  comprises a flow control membrane  102  and a base membrane  104 . The flow control membrane  102  is designed to extend across the flow chamber  36  of a valve housing  30 , as shown in  FIG. 1 , in such a way that, when closed, it prevents the flow of fluids through the flow chamber  36 . For example, a surface  110  of the flow control membrane  102  may have dimensions substantially equal to the cross sectional area of housing  30 . Two curved slits  106  are formed in the exemplary flow control membrane  102 , to selectively allow the passage of fluid therethrough. Opposing slit edges  112 ,  114  are joined when the stacked membrane  100  are maintained in the closed configuration through the natural bias of the material, so that no fluid passes therethrough. In one example, the resilient material forming the flow control membrane  102  applies a restoring force which urges edges  112 ,  114  to join. Alternatively, external resilient elements may be added to the membrane  102  in proximity to the slits  106 , to urge the edges to join.  
         [0029]     When the stacked membrane  100  is subject to a pressure via the fluids in the flow chamber  36  (e.g., pressure generated by an operating dialysis machine connected to the housing  30 ), of at least the threshold level, the edges  112 ,  114  are pushed apart against the restoring resilient force applied thereto, and the slits  106  open. As a result, the valve assumes the open configuration and flow is allowed to pass across flow control membrane  102 . It will be apparent to those of skill in the art that different shapes, sizes, and configurations of one or more slits  106  may be used to allow fluid to flow across flow control membrane  102 . The maximum flow through the stacked membrane  100  and the force required to open the slits  106  may be adjusted by adjusting the size and configuration of the slits, as well as by selecting appropriate dimensions of the flow control membrane  102  as would be understood by those of skill in the art. In an alternative embodiment, there may be multiple flow control membranes  102  that are stacked at a constant or variable spacing. In this embodiment, the base membrane  104  may be used as the outer most piece(s) of the multiple stack for the purpose of stiffening and reinforcement. Such an arrangement may be used to create inexpensive complex valves having spiral flows.  
         [0030]      FIG. 3  shows a top elevation view of the flow control membrane  102 . When mounted in a valve housing  30 , the flow control membrane  102  rests on a membrane seat  50  as shown in  FIG. 1 . More specifically, a seating portion  108  of the membrane  102  is seated on the membrane seat  50 , which is generally annular in shape. Accordingly, compression strains exerted by the housing  30  to retain the membrane  102  in place are concentrated on the seating portion  108 , which in this exemplary embodiment is an annular region at the periphery of the membrane  102 . As would be understood by those skilled in the art, different shapes and dimensions of the seating portion  108  may be selected, depending on the size and shape of the housing  30 , and in particular on the size and shape of the membrane seat  50  and of the membrane holder  52 . In one exemplary embodiment, the flow control membrane  102  may be elliptical, with a major axis of approximately 0.44 inches and with a minor axis of approximately 0.24 in. The seating portion  108  in this example takes up the annular periphery of the elliptical membrane in the range of 0.045-0.055 inches. These sizes and ranges are only exemplary and the size and selection of the seating portion is a function of the housing size and the slit size. The purpose is to create enough surface tension to help close the slit.  
         [0031]     A top elevation view of the base membrane  104  is shown in  FIG. 4 . As described above, the base membrane  104  is designed to form a region of the stacked membrane  100  which has a local thickness greater than a thickness of the flow control membrane  102 . In the exemplary embodiment shown, the base membrane  104  has an outer periphery that dimensions that are substantially equal to the dimensions of the seating portion  108  of the flow control membrane  102 . In this manner, the portion of stacked membrane  100  that is sandwiched between the retaining parts of the housing  30  is sufficiently thick to avoid the structural problems associated with thin membranes, as descried above. In the exemplary embodiment, the base membrane  104  overlies the seating portion  108 , which in turn overlies the membrane seat  50  of the housing  30 . In different embodiments, the dimensions of the base membrane  104  may be different from those of the seating portion  108 . However, the extent to which the base membrane  104  extends radially within the seating portion  108  is preferably minimized to prevent the flow rate from being reduced due to the resulting increase in the force acting to bias the flow membrane  102  toward the closed position. Of course, those skilled in the art will understand that this dimension may be altered to achieve a desired threshold pressure as well.  
         [0032]     In the exemplary embodiment of  FIG. 2 , a thickness t 1  of the flow control membrane  102  is substantially the same as a thickness t 2  of base membrane  104 . One advantage of this configuration is that the same initial membrane material may be used for both components of the stacked membrane  100 , resulting in savings in the manufacturing process. For example, starting from the same membrane material, a conventional process may be used to form the slits  106  in the flow control membrane  102 , while a stamping process may be used to cut out a center portion  130  of a similar membrane to form the annular base membrane  104  shown in the exemplary embodiment. Alternatively, the thickness t 2  may be different from the thickness t 1 , and in particular may be greater to provide a greater increase in the structural strength of the stacked membrane  100 . In one embodiment, both the base membrane  104  and the flow control membrane  102  may have a thickness of approximately 0.020 inches. With this configuration, the stacked membrane  100  allows passage of a fluid flow equivalent to that of a 0.020 inch thick membrane, but with the structural strength of a membrane of approximately twice that thickness.  
         [0033]     The base membrane  104  and the flow control membrane  102  may be formed from any suitably strong and resilient material such as, for example, a polymeric material or silicone. Both membranes may be of the same material, or each may be formed of a separate composition. In one embodiment, an adhesive  120  is used to cement the membranes  102  and  104  together. For example, an RTV type polymer may be used for that purpose. It will be apparent to those skilled in the art that other methods of binding the flow control membrane  102  to the base membrane  104  may be used, and that the specific method may depend on the materials forming the two membranes  102 ,  104 . In addition, the stacked membrane  100  may be assembled without adhesive (e.g., solvent bond, sonic weld, etc), so that the compression retentive force exerted by the housing on the stacked membrane maintains the two membranes  102 ,  104  in position relative to one another.  
         [0034]     The stacked membrane  100  according to exemplary embodiments of the invention may be formed from two separate components, one of which is processed to have at least one slit extending therethrough, and the other of which may be annular extending around a periphery of the first component . For example, this annular shape may be obtained by removing a center of a substantially circular or elliptical membrane. Alternatively, the base membrane  104  and the thin membrane  102  may be formed integrally by extrusion of a thin membrane with a thick edge to produce a single piece stacked membrane  100 . However, the extrusion process for a complex membrane may be more demanding and expensive than the above-described two piece process. Quality control for the two piece process may also be less demanding as it may be easier to inspect a flat, uniform membrane (prior to forming the slits or stamping the center) than it is to inspect a more complex membrane with multiple thicknesses.  
         [0035]     The present invention has been described with reference to specific embodiments, more specifically to a stacked membrane having two slits and an annular base membrane, as used in a dialysis catheter PASV. However, other embodiments may be devised that are applicable to other types of pressure actuated valves, which have different configurations of slits and different shapes of the base membrane, without departing from the scope of the invention. Accordingly, various modifications and changes may be made to the embodiments without departing from the broadest spirit and scope of the present invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense.
A pressure activated valve for medical applications comprises a housing having a lumen extending therethrough from a proximal end to a distal end thereof and a flow control membrane extending across the lumen to control flow therethrough, the flow control membrane including a mounting portion at which the flow control membrane is coupled to the housing and a lumen occluding portion having a slit extending therethrough so that, when the lumen occluding portion is subjected to a pressure of at least a predetermined threshold level, the lumen occluding portion moves from a closed configuration in which flow through the lumen is prevented to an open configuration in which flow is permitted and wherein a thickness of the mounting portion is greater than a thickness of the lumen occluding portion. A method of forming a membrane for a pressure activated valve, comprises the steps of forming a substantially planar flow control membrane dimensioned to fit in a housing of the pressure activated valve, wherein a mounting portion of the flow control membrane is adapted to engage the housing, and forming at least one slit in the flow control membrane, the slit being openable by pressure of a fluid in the pressure activated valve of at least a predetermined threshold level in combination with the steps of forming an annular base membrane dimensioned to substantially overlie the mounting portion of the flow control membrane and stacking the base membrane on the mounting portion of the flow control membrane.