Patent Publication Number: US-8114055-B2

Title: Implantable pump with infinitely variable resistor

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to implantable devices, and more particularly to a reduced size implantable pump and a programmable implantable pump allowing for variable flow rates in delivering medication or other fluid to a selected site in the human body. 
     Implantable pumps have been well known and widely utilized for many years. Typically, pumps of this type are implanted into patients who require the delivery of active substances or medication fluids to specific areas of their body. For example, patients that are experiencing severe pain may require painkillers daily or multiple times per day. Absent the use of an implantable pump or the like, a patient of this type would be subjected to one or more painful injections of such medication fluids. In the case of pain associated with more remote areas of the body, such as the spine, these injections may be extremely difficult to administer and particularly painful for the patient. Furthermore, attempting to treat conditions such as this through oral or intravascular administration of medication often requires higher doses of medication and may cause severe side effects. Therefore, it is widely recognized that utilizing an implantable pump may be beneficial to both a patient and the treating physician. 
     Many implantable pump designs have been proposed. For example, commonly invented U.S. Pat. No. 4,969,873 (“the &#39;873 patent”), the disclosure of which is hereby incorporated by reference herein, teaches one such design. The &#39;873 is an example of a constant flow pump, which typically include a housing having two chambers, a first chamber for holding the specific medication fluid to be administered and a second chamber for holding a propellant. A flexible membrane may separate the two chambers such that expansion of the propellant in the second chamber pushes the medication fluid out of the first chamber. This type of pump also typically includes an outlet opening connected to a catheter for directing the medication fluid to the desired area of the body, a replenishment opening for allowing for refilling of medication fluid into the first chamber and a bolus opening for allowing the direct introduction of a substance through the catheter without introduction into the first chamber. Both the replenishment opening and the bolus opening are typically covered by a septum that allows a needle or similar device to be passed through it, but properly seals the openings upon removal of the needle. As pumps of this type provide a constant flow of medication fluid to the specific area of the body, they must be refilled periodically with a proper concentration of medication fluid suited for extended release. 
     Although clearly beneficial to patients and doctors that utilize them, one area in which such constant flow implantable pumps can be improved, is in their overall size. Typically, such pumps require rather bulky outer housings, or casings, for accommodating the aforementioned medication and propellant chambers, and septa associated therewith. Often times, implantable pumps are limited to rather small areas within the body. Depending upon the size of the patient for which the pump is implanted, this limited area may be even further limited. For example, a person having smaller body features, or those containing abnormal anatomy, may present a doctor implanting a constant flow pump with some added difficulty. Further, patients may be uncomfortable having standard sized constant flow pumps implanted in them. Such pumps are often times capable of being felt from the exterior of the patient. 
     Implantable pumps may also be of the programmable type. Pumps of this type provide variable flow rates, typically through the use of a solenoid pump or a peristaltic pump. In the solenoid pump, the flow rate of medication fluid can be controlled by changing the stroke rate of the pump. In the peristaltic pump, the flow rate can be controlled by changing the roller velocity of the pump. However, both of these types of programmable pumps require intricate designs and complicated controlling mechanisms. As such, it is more desirable to utilize pumps having designs similar to the aforementioned constant flow pumps. 
     However, the benefit of providing a variable flow rate pump cannot be forgotten. While a constant flow of a medication such as a painkiller may indeed be useful in dulling chronic pain, it is very common for patients to experience more intense pain. At times of this heightened pain, it would be advantageous to be able to vary the flow rate of pain killer to provide for more relief. However, constant flow rate pumps typically may only provide such relief by allowing for direct injections of painkillers or the like through the aforementioned bolus port, which provides direct access to the affirmed area. While indeed useful, this method amounts to nothing more than additional painful injections, something the pump is designed to circumvent. 
     Therefore, there exists a need for an implantable constant flow pump, which allows for a reduced overall size, as well as an implantable pump that combines the simplistic design of a constant flow rate type pump and means for varying its flow rate, without requiring the use of the complex solutions provided by known programmable pumps. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is a reduced size implantable device for dispensing an active substance to a patient. The implantable device of a first embodiment of this first aspect includes a housing defining an active substance chamber in fluid communication with an outlet for delivering the active substance to a target site within the patient and a propellant chamber adjacent the active substance chamber. The implantable device further includes an undulating flexible membrane separating the active substance and propellant chambers, wherein the active substance chamber has an undulating surface including a central convex portion flanked by at least two concave portions, the undulating surface cooperating with the undulating flexible membrane. 
     In accordance with this first embodiment of the first aspect of the present invention, the propellant chamber may contain a propellant capable of expanding isobarically where the propellant cooperates with the flexible membrane to reduce the volume of the active substance chamber upon expansion of the propellant. The cooperating undulating surface of the active substance chamber and the undulating flexible membrane preferably meet upon complete expansion of the propellant. The implantable device may further include a replenishment opening in the housing in fluid communication with the active substance chamber, and a first septum sealing the opening. The replenishment opening may be located within the central convex portion of the undulating surface of the active substance chamber so as to lower the overall height of the housing of the implantable device. Additionally, the housing may include two portion beings constructed so as to screw together. The two portions may be constructed of PEEK. The two portions may be configured so as to capture the membrane therebetween. Finally, the housing may also include a locking portion and/or a septum retaining member. 
     A second embodiment of this first aspect of the present invention is yet another implantable device for dispensing an active substance to a patient. The implantable device according to this second embodiment includes a housing defining a chamber and an outlet in fluid communication with the chamber for delivering the active substance to a target site within the patient, the housing having a first portion and a second portion, where the first and second portions are constructed of PEEK and screwed together. 
     A third embodiment of this first aspect of the present invention is yet another implantable device for dispensing an active substance to a patient. The implantable device according to this third embodiment includes a housing including a top portion, a bottom portion and a locking portion. The housing defines a propellant chamber and an active substance chamber in fluid communication with an outlet. The implantable device preferably also includes a membrane retained between the top and bottom portions, the membrane separating the active substance and propellant chambers. In a fully assembled stated, the top and bottom portions are preferably placed together and the locking portion engages one of the top or bottom portions to retain the top and bottom portions together. 
     A fourth embodiment of this first aspect of the present invention relates to a method of assembling a reduced size implantable pump. The method of this embodiment includes the steps of placing together a top portion and a bottom portion to retain a membrane therebetween, and screwing a locking portion into the top portion or the bottom portion to retain the top and bottom portions together. 
     A second aspect of the present invention includes an implantable device for dispensing an active substance to a patient including a housing defining a chamber, said housing having an outlet for delivering the active substance to a target site within the patient, the outlet in fluid communication with the chamber and means for varying the flow rate of the active substance between the chamber and the outlet. The chamber, in accordance with this second aspect of the present invention, may include an active substance chamber in fluid communication with the outlet and a propellant chamber, the active substance and propellant chambers being separated by a flexible membrane. The propellant chamber may contain a propellant capable of expanding isobarically and cooperating with the flexible membrane to reduce the volume of the active substance chamber upon expansion of the propellant. The housing of the implantable device may include an opening in fluid communication with the active substance chamber and a first septum sealing the opening. The housing may further include an annular opening in communication with the outlet and a second septum sealing the annular opening. 
     In a first embodiment of this second aspect, the means for varying the flow rate of the active substance between the chamber and the outlet may include an elongated polymer filament having a cross sectional dimension. The filament, in accordance with this embodiment, is preferably located in a capillary and is preferably capable of being elongated to reduce the cross sectional dimension. In certain examples, the filament is located centrally within the capillary, in others, it is located eccentrically. The filament may have a uniform cross section, a substantially circular cross section, non-uniform cross section and the like along its length. Further, this first embodiment may further include means for elongating the filament. 
     In a second embodiment of this second aspect, the means for varying the flow rate of the active substance between the chamber and the outlet may include a first hollow cylinder having a threaded exterior surface and a second hollow cylinder having a threaded interior surface. The first hollow cylinder is axially received within the second hollow cylinder, such that the threaded exterior surface of the first cylinder engages the threaded interior surface of the second cylinder. In this embodiment, the axial movement of the first cylinder with respect to the second cylinder varies the flow rate of the active substance. 
     In a third embodiment of this second aspect, the means for varying the flow rate of the active substance between the chamber and the outlet may include a hollow tubular element having a cross section that is capable of being varied. This third embodiment may also include a capillary in fluid communication between the chamber and the outlet, where the tubular element is located therein. The hollow tubular element in accordance with this embodiment may be centrally or eccentrically located within the capillary. 
     In a fourth embodiment of this second aspect, the means for varying the flow rate of the active substance between the chamber and the outlet may include an elongate insert having a longitudinally varying cross section along its length. Movement of this elongate insert may increase or decrease the flow rate of the active substance. 
     A third aspect of the present invention includes an implantable device for dispensing an active substance to a patient including a housing defining a chamber, said housing having an outlet for delivering the active substance to a target site within the patient, the outlet in fluid communication with the chamber. The implantable device also includes a capillary in fluid communication between the chamber and the outlet, the capillary having an inner surface and a flow control element received within the capillary. The element has an outer surface opposing the inner surface of the capillary defining therebetween a passageway for the flow of the active substance therethrough. The outer surface of the element is preferably movable relative to the inner surface of the capillary to alter the flow of the active substance therethrough. The movement of the outer surface of the element may alter the shape and/or size of the passageway. 
     In a first embodiment of this third aspect, the means for varying the flow rate of the active substance between the chamber and the outlet may include an elongated polymer filament having a cross sectional dimension. The filament, in accordance with this embodiment, is preferably located in a capillary and is preferably capable of being elongated to reduce the cross sectional dimension. In certain examples, the filament is located centrally within the capillary, in others, it is located eccentrically. The filament may have a uniform cross section, a substantially circular cross section, non-uniform cross section and the like along its length. Further, this first embodiment may further include means for elongating the filament. 
     In a second embodiment of this third aspect, the means for varying the flow rate of the active substance between the chamber and the outlet may include a first hollow cylinder having a threaded exterior surface and a second hollow cylinder having a threaded interior surface. The first hollow cylinder is axially received within the second hollow cylinder, such that the threaded exterior surface of the first cylinder engages the threaded interior surface of the second cylinder. In this embodiment, the axial movement of the first cylinder with respect to the second cylinder varies the flow-rate of the active substance. 
     In a third embodiment of this third aspect, the means for varying the flow rate of the active substance between the chamber and the outlet may include a hollow tubular element having a cross section that is capable of being varied. This third embodiment may also include a capillary in fluid communication between the chamber and the outlet, where the tubular element is located therein. The hollow tubular element in accordance with this embodiment may be centrally or eccentrically located within the capillary. 
     In a fourth embodiment of this third aspect, the means for varying the flow rate of the active substance between the chamber and the outlet may include an elongate insert having a longitudinally varying cross section along its length. Movement of this elongate insert may increase or decrease the flow rate of the active substance. 
     A fourth aspect of the present invention includes a resistor for varying the flow rate of a fluid from a first point to a second point including a capillary having an inner surface and a flow control element received with the capillary. The element has an outer surface opposing the inner surface of the capillary such that a passageway is defined for the flow of fluid therethrough. The outer surface of the element is preferably moveable relative to the inner surface of the capillary to alter the flow of the fluid therethrough. The movement of the outer surface of the element may alter the shape and/or size of the passageway. It is noted that this aspect may be utilized in conjunction with an implantable device such as an implantable pump for delivering a medicament to a site within a patient. Embodiments in accordance with the third aspect are envisioned that are similar to those discussed above in relation to the first and second aspects of the present invention. 
     A fifth aspect of the present invention includes a method of varying the flow rate of an active substance being dispensed to a patient. This method includes the steps of providing an implantable device including a capillary having an inner surface and a flow control element received within the capillary. The element preferably has an outer surface opposing the inner surface of the capillary such that a passageway for the flow of the active substance therethrough is defined therebetween for dispensing the active substance to a target site within a patient. Further the method includes the step of moving the element relative to the inner surface of the capillary to alter the flow rate of the active substance therethrough. This moving step may alter the size and/or shape of the passageway. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the subject matter of the present invention and the various advantages thereof can be realized by reference to the following detailed description in which reference is made to the accompanying drawings in which: 
         FIG. 1  is a cross sectional front view of a reduced size implantable pump in accordance with one embodiment of the present invention. 
         FIG. 2  is a cross sectional bottom view of a portion of the reduced sized implantable pump shown in  FIG. 1 . 
         FIG. 3  is an enlarged view of an attachment area of the pump shown in  FIG. 1 . 
         FIG. 4  is a cross section front view of a reduced size implantable pump in accordance with another embodiment of the present invention. 
         FIG. 5  is a cross section front view of a reduced size implantable pump in accordance with another embodiment of the present invention. 
         FIG. 6  is a cross section front view of a reduced size implantable pump in accordance with another embodiment of the present invention. 
         FIG. 7  is a cross sectional front view of an implantable constant flow pump for use in accordance with the present invention. 
         FIG. 8  is a cross sectional front view of another implantable constant flow pump for use in accordance with the present invention. 
         FIG. 9  is a cross sectional view of a variable flow resistor in accordance with a first embodiment of the present invention having a filament located concentrically in a capillary. 
         FIG. 10   a  is a longitudinal cross sectional view of the variable flow resistor of  FIG. 9 , in an initial position. 
         FIG. 10   b  is a longitudinal cross sectional view of the variable flow resistor of  FIG. 10   a,  in an extended position. 
         FIG. 11   a  is a cross sectional view of a variable flow resistor of the present invention having a filament located eccentrically in a capillary. 
         FIG. 11   b  is a longitudinal cross sectional view of the variable flow resistor of  FIG. 11   a,  depicting the curvature of the capillary. 
         FIG. 12   a  is a longitudinal cross sectional view of the variable flow resistor of  FIG. 11   a,  in an initial position. 
         FIG. 12   b  is a longitudinal cross sectional view of the variable flow resistor of  FIG. 12   a,  in an extended position. 
         FIG. 13  is a longitudinal cross sectional view of another variable flow resistor in accordance with the present invention. 
         FIG. 14  is a longitudinal cross sectional view of another variable flow resistor in accordance with the present invention. 
         FIG. 15  is a cross sectional view of the driving assembly for use with the flow resistor of  FIG. 14 . 
         FIG. 16  is a cross sectional view of a variable flow resistor in accordance with a second embodiment of the present invention in a high resistance position. 
         FIG. 17  is a cross sectional view of the variable flow resistor of  FIG. 16  in a low resistance position. 
         FIG. 18  is a cross sectional view of a variable flow resistor in accordance with a third embodiment of the present invention with an insert centrally located. 
         FIG. 19  is a cross sectional view of a variable flow resistor in accordance with a third embodiment of the present invention with an insert eccentrically located. 
         FIG. 20  is a longitudinal cross sectional view of the variable flow resistor of  FIG. 18 . 
         FIG. 21  is a cross sectional view of the larger end of a variable flow resistor in accordance with a fourth embodiment of the present invention with an insert centrally located. 
         FIG. 22  is a cross sectional view of the larger end of a variable flow resistor in accordance with a fourth embodiment of the present invention with an insert eccentrically located. 
         FIG. 23  is a longitudinal cross sectional view of the variable flow resistor of  FIG. 21 . 
         FIG. 24  is a longitudinal cross sectional view of the variable flow resistor of  FIG. 22 . 
         FIG. 25  is a cross sectional view of an implantable pump in accordance with another embodiment of the present invention. 
         FIG. 26  is a cross sectional view of the implantable pump shown in  FIG. 25 , taken along a different portion thereof. 
         FIG. 27  is a partial top view of the implantable pump shown in  FIG. 25 . 
     
    
    
     DETAILED DESCRIPTION 
     In describing the preferred embodiments of the subject matter illustrated and to be described with respect to the drawings, specific terminology will be used for the sake of clarity. However, the invention is not intended to be limited to any specific terms used herein, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     Referring to the drawings, wherein like reference numerals refer to like elements, there is shown in  FIGS. 1 and 2 , in accordance with various embodiments of the present invention, a reduced size implantable pump designated generally by reference numeral  1010 . In a preferred embodiment, pump  1010  is a constant flow pump including a housing  1012 , which further defines an interior having two chambers  1014  and  1016 . Chambers  1014  and  1016  are preferably separated by a flexible membrane  1018 . It is noted that membrane  1018  may be of any design known in the art, for example, a membrane like that disclosed in commonly owned U.S. Pat. No. 5,814,019, the disclosure of which is hereby incorporated by reference herein. In a preferred embodiment, chamber  1014  is designed and configured to receive and house an active substance such as a medication fluid for the relief of pain, treatment of spasticity and neuro-mechanical deficiencies and the administration of chemotherapy, while chamber  1016  may contain a propellant that expands isobarically under constant body heat. This expansion displaces member  1018  such that the medication fluid housed in chamber  1014  is dispensed into the body of the patient through an outlet catheter  1015  (best shown in  FIG. 2 ). 
     The design and configuration of housing  1012  is such that manufacturing and assembly of pump  1010  is relatively easy. Housing  1012  further includes separately manufactured top portion  1020 , bottom portion  1022  and locking portion  1024 . It is noted that in certain preferred embodiments, housing  1012  defines a substantially circular pump  1010 . However, the housing may ultimately be a pump of any shape. In addition to the above described elements, pump  1010  also preferably includes replenishment port  1026  covered by a first septum  1028  that is in fluid communication with chamber  1014  through a channel  1029 , an annular ring bolus port  1030  covered by a second septum  1032 , and barium filled silicone o-ring  1033 . Each of these elements will be discussed further below. 
     Referring to both  FIGS. 1 and 2 , where  FIG. 2  is a cross sectional bottom view of locking portion  1024 , the flow path of a medication fluid contained within chamber  1014  is shown. Upon the expansion of propellant contained within propellant chamber  1016  and the necessary displacement of membrane  1018 , fluid contained in chamber  1014  is forced through an opening  1049  and into a cavity  1046 , which will be further described below. As shown in  FIG. 2 , cavity  1046  extends in a circular fashion around pump  1010 . Once in cavity  1046 , the fluid may enter at any point along the length of a filter capillary  1072 . Essentially, filter capillary  1072  is a well known type filter that allows for fluid to enter into its inner fluid path through permutation or the like. Thus, once a certain amount of fluid builds up within cavity  1046 , it is capable of entering into filter  1072 . This filter is preferably fixed and sealed in position by drops of glue or other adhesive located at  1070  and  1074 . The fluid then travels through filter capillary  1072  until it exits into a resistor  1076 . This resistor is preferably a LONG tube having a relatively small diameter, so as to dictate the maximum flow rate that may be achieved therethrough. In other words, the smaller the diameter of resistor  1076 , the slower the flow rate of fluid traveling therethrough. Nevertheless, as more fully discussed below, resistor  1076  may be many different types of designs. The fluid within resistor  1076  then continues to an opening  1078  for a bridge  1080 , which essentially allows resistor  1076  to cross over bolus port  1030 . Thereafter, the fluid may continue through resistor  1076  and ultimately out catheter  1015 . Epoxy or another suitable adhesive or sealant may be utilized to seal end  1070 , end  1074  and opening  1078 . Thus, fluid in cavity  1046  may only follow the path outlined above. 
     It is noted that  FIG. 2  also depicts the flow path that fluid introduced through a bolus injection may take. Fluid may be injected into bolus port  1030  through the use of a device suitable for piercing septum  1032 , such as a needle. Once in port  1030 , which extends around pump  1010 , fluid may enter a channel  1082 . This channel extends at least partially around the above mentioned bridge  1080 , and allows fluid injected into bolus port  1030  to ultimately exit catheter  1015  without passing through any portion of resistor  1076 . As shown in  FIG. 2 , regardless of the path the fluid takes, it ultimately ends up in a passage  1084  just prior to catheter  1015 . Thus, fluid coming from chamber  1014  may have one flow rate, while fluid directly injected into port  1030  may have a different flow rate, the latter preferably being greater. 
     The assembly of pump  1010  will now be discussed. It is noted that each of the individual elements/components of pump  1010  may be individually manufactured and thereafter assembled by hand or by another process, such as an automated process. As an initial step, top portion  1020  and bottom portion  1022  are placed or sandwiched together so as to capture membrane  1018  therebetween in an attachment area  1034  for fixably retaining same. As more clearly shown in the enlarged view of  FIG. 3 , attachment area  1034  comprises a projection  1036  located on bottom portion  1022 , a depression  1038  located on top portion  1020 , and a cavity  1040  formed through the cooperation of the two portions. In operation, the step of sandwiching together portions  1020  and  1022 , with membrane  1018  disposed therebetween, causes projection  1036  to be forced into depression  1038 . The portion of membrane  1018  disposed therebetween is thus also forced into depression  1038  by projection  1036 . This causes a crimp-like connection, which fixably attaches and retains membrane  1018  within housing  1012 . As shown in  FIG. 3 , membrane  1018  may consist of multiple layers, of which all are preferably “crimped” during the attachment process. Prior to pressing together portions  1020  and  1022 , a layer of epoxy or other adhesive may be inserted into cavity  1040 . In such embodiments that employ the use of an adhesive, the design may cause portions  1020  and  1022  to become fixably attached to one another upon the sandwiching of same. Further, the use of an adhesive within cavity  1040  may also aid in the fixation of membrane  1018  between the two portions. The epoxy or other adhesive may be placed into the cavity portion formed on either portion  1020  or portion  1022 , prior to the sandwiching step. 
     Prior or subsequent to the assembly of top portion  1020  together with bottom portion  1022 , o-ring  1033  or the like may be placed into a ring-shaped cavity formed in top portion  1022 . In certain preferred embodiments, o-ring  1033  is a barium filled silicone o-ring, and is disposed around the area defining replenishment port  1026 . Such an o-ring design allows for the area defining replenishment port  1026  to be illuminated under certain scanning processes, such as X-rays. As pump  1010  is implanted within the human body, locating port  1026 , in order to refill the pump with medicament or the like, may be difficult. Providing a barium filled o-ring  1033 , which essentially outlines the area of port  1026 , allows for a doctor to easily locate the desired area under well known scanning processes. Other structures may be utilized, in which same also show up on different scans. The placement of o-ring  1033  is preferably accomplished by pressing the o-ring into an undersized channel that retains the o-ring, thereafter. 
     With o-ring  1033  preferably in place, locking portion  1024  is next attached to the other portions. It is noted that prior to attaching portion  1024 , first septum  1028  should be inserted into locking portion  1024 . Preferably, first septum  1028  is slid into a complimentary cavity formed in portion  1024 , such that it remains within absent a force acting upon same. As first septum  1028  is designed to be captured between locking portion  1024  and top portion  1020 , the septum should be placed prior to the attachment of locking portion  1024 . In addition, as mentioned above, locking portion  1024  may include a second septum  1032  for covering bolus port  1030 . In certain preferred embodiments, as shown in  FIG. 1 , second septum  1032  is ring shaped, and is pressed into locking portion  1024  in a similar fashion to that discussed above with relation to the placement of o-ring  1033 . This may be done prior or subsequent to the attachment of locking portion  1024  to the other portions. 
     With regard to the attachment step, locking portion  1024  preferably includes a threaded area  1042  for cooperating with a threaded extension  1044 . In operation, locking portion  1024  is merely screwed into engagement with bottom portion  1022 . This necessarily causes top portion  1020 , which is disposed between the two other portions, to be retained therebetween. In other words, the screw attachment of locking portion  1024  with bottom portion  1022  not only causes such portions to be fixably attached to one another, but also causes top portion  1020  to be fixably retained therebetween. It is noted that, depending upon how tight locking portion  1024  is screwed into  1022 , portions  1020  and  1022  may be further pressed together, thereby increasing the fixation of membrane  1018  therebetween. Thus, pump  1010  is designed so that minimal connection steps are performed in order to cause all of the components thereof to be retained together. It is further noted that, in addition to the above discussed screw connection of portions  1022  and  1024 , other attachment means may be utilized. For example, such portions may be snap fit together or fixed utilizing an adhesive. Finally, locking portion  1024  may be configured so as to form cavity  1046  between itself and top portion  1020 . This cavity may be designed so as to allow for the injection of adhesive therein, thus increasing the level of fixation between the different portions of housing  1012 . Additionally, cavity  1046  may house a flow resistor or the like, as will be more fully discussed below. 
     As set forth above, pump  1010  is configured and dimensioned to be relatively simplistic in both manufacture and assembly. However, pump  1010  is also configured and dimensioned so as to employ a significantly reduced overall size, while still providing for a useful amount of medicament and propellant to be housed therein. In the preferred embodiments depicted in the figures, top portion  1020  of pump  1010  includes an interior surface  1047  having an undulating or convoluted shape. More particularly, surface  1047  includes a convex central portion flanked by two concave portions. This configuration allows for the centrally located replenishment port  1026  and cooperating septum  1028  to be situated in a lower position with respect to the remainder of pump  1010 . At the same time, the aforementioned flanking concave portions allow for the overall volume of chambers  1014  and  1016  to remain substantially the same as a pump employing an interior surface having one constant concave portion or the like. In other words, the flanking concave portions make up for the volume lost in situating port  1026  and cooperating septum  1028  in a lower position. Membrane  1018  is also preferably configured so as to have an initial undulating shape for cooperation with interior surface  1047 . Thus, with no medicament or other fluid located within chamber  1014 , membrane  1018  preferably rests against surface  1047 . However, upon injection of fluid into chamber  1014 , membrane  1018  adapts to the position shown in  FIG. 1 . 
       FIG. 4  depicts another reduced sized implantable pump designated by reference numeral  1110 . As shown in the figure, pump  1110  includes several elements which are similar in structure and function to that of pump  1010 . These elements are labeled with like references numerals within the  1100  series of numbers. For example, membrane  1118  is similar to the above described membrane  1018 . In addition, pump  1110  operates in a similar fashion to that of pump  1010 . Nevertheless, pump  1110  does include certain additional elements, as well as elements employing different constructions. Most notably, pump  1110  includes an additional component, namely septum retaining member  1125 . This member is preferably adapted to be screwed into top portion  1120 . Pump  1110  also includes a bottom o-ring  1150 , but does not include a barium filled o-ring. 
     The assembly of pump  1110  also differs from that of pump  1010 . As briefly mentioned above, initially, septum retaining member  1125  is first screwed into top portion  1120  in order to retain previously placed septum  1128  in place. Like the above described assembly of pump  1010 , the assembly of pump  1110  then includes the step of sandwiching together portions  1120  and  1122 , where membrane  1118  is likewise captured therebetween in attachment area  1134 . However, in this embodiment, locking portion  1124  is adapted to engage top portion  1120 , so that it is positioned on the bottom side of pump  1110 . As shown in  FIG. 4 , top portion  1120  includes a threaded extension  1152  to cooperate and engage with threaded area  1142  of locking portion  1124 . The screw connection between the two portions is similarly achieved. However, bottom o-ring  1150  is preferably situated between locking portion  1124  and bottom portion  1122 . This o-ring both increases the force exerted on bottom portion  1122  by locking portion  1124 , and also causes housing  1112  to retain a smooth exterior surface. The latter is important in implanting the pump within a patient, as rough or jagged surfaces may cause damage to tissue abutting the pump. Finally, it is noted that second septum  1132  may be pressed into top portion  1120 , at any point during the assembly. 
       FIG. 5  depicts another reduced sized implantable pump designated by reference numeral  1210 . As shown in that figure, pump  1210  includes several elements which are similar in structure and function to that of pumps  1010  and  1110 . Once again, these elements are labeled with like reference numerals within the  1200  series of numbers. Nevertheless, pump  1210  does include certain additional elements, as well as elements employing different constructions. For example, like pump  1110 , pump  1210  includes a septum retaining member  1225 . Similarly, like pump  1010 , pump  1210  utilizes a top mounting locking portion  1224 , although it has a different construction. 
     The assembly of pump  1210  differs from that of the above discussed pumps  1010  and  1110 . Like pump  1110 , septum retaining member  1225  is first screwed into top portion  1220 , in order to retain previously placed septum  1228  in place. Next, portions  1120  and  1222  are sandwiched together, thus capturing member  1218  within attachment  1234 . Finally, locking portion  1224  is screwed into engagement with bottom portion  1222 . Like the design of pump  1010 , locking portion  1224  includes a threaded area  1242  which engages a threaded extension  1244  of bottom portion  1222 . In addition to completing the assembly of pump  1210  by capturing bottom portion  1222  and forcing top portion  1220  towards bottom portion  1222 , locking portion  1224  is configured and dimensioned in this embodiment to also capture second septum  1232 . As shown in  FIG. 5 , locking portion  1224  includes a concave section  1254  for engaging septum  1232  upon the full engagement of portions  1222  and  1224 . 
     Yet another embodiment reduced sized pump  1310  is shown in  FIG. 6 . Like those pumps discussed above, pump  1310  preferably includes several elements which are similar in structure and function, and are thus labeled with like reference numerals within the  1300  series of numbers. Essentially, pump  1310  is akin to the configuration set forth in pump  1210 . However, there are two main distinctions, namely, the cooperation of locking portion  1324  and portions  1320  and  1322 , and the inclusion of a channel  1362  between locking portion  1324  and top portion  1320 . In the embodiment depicted in  FIG. 6 , it is noted that locking portion  1324  includes a threaded extension  1356 , which cooperate and engage threaded areas  1358  and  1360  of portions  1320  and  1322 , respectively. Furthermore, locking portion  1324  preferably includes a channel  1362  formed therein. This channel may be adapted to cooperate with any of the chambers and/or ports discussed above. Additionally, channel  1362  may house other elements, such as a flow resistor or the like, which will be discussed more fully below. 
     A second aspect of the present invention relates to providing a constant flow type implantable pump with infinitely variable flow capabilities. A mentioned above, such a construction may be beneficial to patients requiring more or less medication to be delivered by an implantable pump. While the different embodiments of this second aspect of the present invention may indeed be sized and configured to be utilized with any constant flow type implantable pump, preferred pumps will be described herein. In one preferred pump, as shown in  FIG. 6  of the present application, the basic implantable pump design is designated as reference numeral  20 . Pump  20  includes a housing  22  defining an interior having two chambers  24  and  26 . Chambers  24  and  26  are separated by a flexible membrane  28 . Chamber  24  is designed to receive and house the active substance such as a medication fluid for the relief of pain, treatment of spasticity and neuro-mechanical deficiencies and the administration of chemotherapy, while chamber  26  may contain a propellant that expands isobarically under constant body heat. This expansion displaces membrane  28  such that the medication fluid housed in chamber  24  is dispensed into the body of the patient through the path defined by an outlet opening  30 , a resistor  32 , an outlet duct  34  and ultimately an outlet catheter  36 . 
     Resistor  32  provides a connection between chamber  24  and outlet duct  34 . Thus, as mentioned above, a medication fluid flowing from chamber  24  to outlet catheter  36  must necessarily pass through resistor  32 . This resistor allows for the control of the flow rate of the medication fluid, such that the flow rate is capable of being varied. Resistor  32  may be configured differently in many different embodiments, some of which are discussed below in the detailed description of the present invention. Essentially, resistor  32  defines a passageway for the flow of the medication fluid, where the passageway may be altered to thereby alter the flow rate of the medication fluid. 
     Implantable pump  20  also includes a replenishment port  38  covered by a first septum  40 . Septum  40  can be pierced by an injection needle (such as needle  42  shown in  FIG. 7 ) and, upon removal of such needle, is capable of automatically resealing itself. Septa of this type are well known to those of ordinary skill in the art. As implantable pump  20  is designed to medicate a patient over a limited period of time, replenishment port  38  is utilized for replenishing chamber  24  when empty or near empty. In operation, a physician or other medical professional inserts an injection needle  42  into an area of a patient&#39;s body where pump  20  is located, such that it may pierce septum  40 . Thereafter, operation of the needle causes injection of the solution from the needle to pass into port  38 , through passage  44 , and into chamber  24 . It is noted that the particular dimension and/or the patient&#39;s need may require such a process to be repeated at given intervals, for example, monthly, weekly, etc. 
     In addition to replenishment port  38 , pump  20  also includes an annular ring bolus port  46  covered by a second septum  48 . Essentially, this port allows for direct introduction of a solution into outlet catheter  36  and to the specific target area of the body. This port is particularly useful when a patient requires additional or stronger medication, such as a single bolus injection, and/or when it is desired to test the flow path of catheter  36 . Such an injection is performed in a similar fashion to the above discussed injection into replenishment port  38 . However, an injection into bolus port  46  bypasses passage  44 , chamber  24  and resistor  32 , and provides direct access to catheter  36 . It is also contemplated to utilize bolus port  46  to withdraw fluid from the body. For example, where pump  20  is situated within the body such that catheter  36  extends to the vertebral portion of the spinal column, a needle with a syringe connected may be inserted into bolus portion  46  and operated to pull spinal fluid through catheter  36  and into the syringe. 
     In certain embodiments, septum  40  and septum  48  may be situated so that only specifically designed injection needles may be used to inject into the respective ports. For example, as is also shown in  FIG. 7 , septum  48  may be situated relatively close to the bottom of port  46  and septum  40  may be situated a greater distance away from the bottom of port  38 . In this embodiment, injection needle  42  is provided with an injection eye  43 , which is located above the tip of needle  42 . Alternatively, injection needle  50  is provided with an injection eye  51  located at or near its tip. This arrangement prevents needle  42 , which is typically utilized for replenishing chamber  24  with a long term supply of medication fluid, from being inadvertently used to inject its contents into bolus port  46 . As is shown on the left side depiction of bolus port  46 , needle  42  would have its eye  43  blocked by septum  48  if the needle is inadvertently inserted into this port. Needle  50 , on the other hand, would be capable of injecting into port  46  because of the lower location of its eye  51 . This is an important safety feature, as direct injection of a long term supply of medication fluid into port  46  could be dangerous. It is noted that needle  50  is also capable of injecting a solution into replenishment port  38 , however, the same concerns (i.e.—over-medication) do not exist with respect to the filling of chamber  24 , and as such medication housed in the chamber is slowly released. While this is one example of a possible safety feature with regard to the injection of materials into the pump, it is envisioned that other safety precautions may be utilized. For example, U.S. Pat. No. 5,575,770, the disclosure of which is hereby incorporated by reference herein, teaches a similar multiple injection needle system with additional valve protection. It is noted that such a safety needle system may be employed with regard to any of the various implantable pump embodiments disclosed herein. One of ordinary skill in the art would recognize the modifications required to utilize such a safety feature in the other discussed pump designs. 
     In other embodiments, the basic implantable pump design of the aforementioned &#39;873 patent may also be utilized. As is discussed in its specification and shown in  FIG. 8  of the present application, the &#39;873 patent discloses a housing made up of two parts  1 ,  2  and an interior having two chambers  4 ,  5 , which are separated by a flexible membrane  3 . Chamber  4  is designed to receive and house the medication fluid, while chamber  5  may contain a propellant which, like that discussed in the above description of pump  20 , expands isobarically under constant body heat. This expansion displaces membrane  3  such that the medication fluid housed in chamber  4  is dispensed into the body of the patient through the path defined by an outlet opening  6 , an outlet reducing means  7  and ultimately an outlet catheter  8 . It is noted that reducing means  7  is preferably a tube winding that wraps around part  1  of the housing. The resistor of the present invention, in certain embodiments, is preferably located at or near outlet opening  6 . This will be discussed more fully below. 
     Prior to reaching outlet catheter  8 , the medication fluid is introduced into a chamber  9  which is provided annularly on part  1  of the housing. Chamber  9  is sealed at its upper side by a ring or septum  10 , which can be pierced by an injection needle and which automatically reseals upon withdrawal of the needle. This chamber is similar to the above discussed bolus port  46  of pump  20 . In addition to allowing medication fluid from chamber  4  to pass into outlet catheter  8 , chamber  9  also allows the direct injection of a solution into outlet catheter  8 , the importance of which is discussed above. The aforementioned outlet reducing means  7  prevents a solution injected into the bolus port from flowing into chamber  4 . In a similar fashion, when need be, chamber  4  may be replenished via a further septum  12 . Once again an injection needle may be utilized for this purpose. 
     While two basic designs of implantable pumps are described above, it is noted that other designs may include different or additional elements. Similarly, while the above description teaches two implantable pumps that may be utilized in accordance with the present invention, other implantable pump designs are also capable of being utilized. For example, U.S. Pat. Nos. 5,085,656, 5,336,194, 5,722,957, 5,814,019, 5,766,150, 5,836,915 and 6,730,060, the disclosures of which are all hereby incorporated by reference herein, may be employed in accordance with the present invention. In addition, one specific embodiment will be discussed below. 
     As mentioned above, the capability of varying the flow rate of an implantable pump is desired. In the above discussed constant flow pumps, the flow rate of the medication fluid depends upon the pump pressure, the pressure at the end of the catheter and the hydraulic resistance of any of the capillaries or other passages that the medication fluid must travel through. With regard to the resistance of the capillaries, such resistance depends upon the geometry of the capillary itself, as well as the viscosity of the medication fluid. This viscosity, as well as the pump pressure, may both be influenced by body temperature. As such, one instance in which it is desired to control the flow rate of the pump exists if the patient develops a fever because the flow rate of the infusion device may be affected in an undesired way. 
     Another example of when the variable flow rate of the implantable pump is desired relates to the condition or active status of the patient. For example, especially in the case where painkillers are being administered, it may be advantageous to deliver less medication during the nighttime hours, when the patient is sleeping. Additionally, as discussed above, it may be desirable to be able to increase the dosage of such painkillers or the like when the patient&#39;s symptoms worsen. Increasing of the flow rate of the medication fluid may be necessary in order to diminish the patient&#39;s pain level. In accordance with the present invention, the aforementioned resistor  32  is useful for adjusting the flow rate in order to counteract undesirable flow rate changes due to body temperature changes, and to allow for desired adjustments of flow rate to treat heightened or worsened symptoms. 
     In a first embodiment this adjustment of flow rate is realized by adjusting the cross-sectional geometry of an article of the resistor. It is noted that the first embodiment will be discussed with respect to pump  20 ; however, it may be utilized in combination with any implantable pump. As shown in  FIGS. 9-15 , in accordance with this first embodiment, resistor  32  includes an elastic and resilient filament  52  situated in a resistor capillary  54 , where resistor capillary  54  provides a connection between outlet opening  30  and outlet capillary  34 . Capillary  54  may be situated so as to constitute substantially the entire outlet capillary  34 , or may only be a portion thereof. Essentially, capillary  54  need only require the aforementioned medication fluid to pass therethrough, and thus, may be any length suitable for use in varying the flow rate. 
       FIGS. 9 ,  10   a  and  10   b  show a first example of the first embodiment resistor  32 , where elastic filament  52  is located concentrically in resistor capillary  54 . This configuration forms a ring-shaped flow channel  56  through which fluid flows in a direction shown by arrow F. As is best shown in  FIG. 10   a,  filament  52  includes a first end  58  attached to a stationary attachment  60 , and a second end  62  attached to a movable attachment  64 . Resistor  32  also has an effective length L extending between capillary entrance  66  to exit  68 , and an initial diameter D 1  (i.e.—2 times its radius R 1 ). Additionally, capillary  54  has a diameter D 3  (i.e.—2 times its radius R 3 ). This will be similar throughout in the various other capillaries discussed herein. 
     In this example, movable attachment  64  is capable of moving in the opposite longitudinal directions shown by arrows A and B, while attachment  60  remains stationary. In operation, movement of attachment  64  in the direction of arrow B increases the distance between attachments  62  and  64  and also results in the decrease of the initial diameter D 1  to a lesser diameter D 2  (i.e.—2 times its lesser radius R 2 ). This is best shown in  FIG. 10   b . The decrease of the diameter of filament  52  from D 1  to D 2  increases the size of channel  56  and thus necessarily decreases the hydraulic resistance in capillary  54 . Oppositely, movement of attachment  64  in the direction of arrow A returns filament  52  to the position shown in  FIG. 10   a,  and increases the hydraulic resistance in capillary  54 . A filament of this type may be constructed of silicone rubber, or other suitable polymer materials for providing the required elasticity and resiliency so as to return to its original shape and size after being deformed by stretching. Similarly, although filament  52  is shown in the figures as having a substantially circular cross section, it is envisioned that filaments having other cross sections may be utilized, for example, polygonal, oval, square and the like. 
     As the inner diameter of capillary  54  is typically very small (on the order of several thousands of millimeters), it is often difficult to locate filament  52  directly in the center of the capillary.  FIGS. 11   a,    11   b,    12   a  and  12   b  depict a second example where elastic filament  52  touches the inner wall of capillary  54  (i.e.—an eccentric position). This eccentrically placed filament  52  creates a sickle-shaped flow channel  56 , as opposed to the ring-shaped flow channel of the first example. This second example also differs from the first example discussed above, in that both ends  58 ,  62  of filament  52  are attached to movable attachments  60 ,  64 , respectively. This is useful, as in operation, one movable attachment (or the mechanism moving it) may fail. The two movable attachment design provides a failsafe, thereby allowing filament  52  to be stretched through the movement of the non-failing attachment. Attachment  64  is still capable of moving in the direction depicted by arrows A and B and attachment  60  is capable of moving in the direction depicted by arrows A′ and B′. 
     In operation, movement of either of attachments  60 ,  64  in the directions B′ and B, respectively, decreases the diameter D 1  to a lesser diameter D 2  (once again, these diameters refer to two times the radii R 1  and R 2 , respectively). This position is best shown in  FIG. 12   b . Like that of the above discussed first example, this decrease in the diameter of filament  52  from D 1  to D 2  increases the size of channel  56  and thus necessarily decreases the hydraulic resistance in capillary  54 . Oppositely, movement of either of attachments  60 ,  64  in the direction of arrows A′ and A, respectively, returns filament  52  to the position shown in  FIG. 12   a,  and increases the hydraulic resistance in capillary  54 . 
     Attachment  64  in the first example, and attachments  60 ,  64  in the second example may be moved by any means known to those of ordinary skill in the art. For example, it is well known to utilize micro-motors, magnets, or other hydraulic, electrical or mechanical actuators. One example of a suitable motor assembly is sold under the designation X15G by Elliptec Resonant Actuator of Dortmund, Germany. 
     In accordance with the present invention, it is known to design a capillary with a circular lumen defined by a rigid wall. Essentially, this type of apparatus is a hollow tube having a flow therethrough (i.e.—the present design without filament  52 ). For such a design, the flow rate can be calculated using the well-known Hagen-Poisseuille Equation:
 
 V =(Δ pπR   2   4 )/(8η L )
 
     Where: 
     V=flow rate 
     Δp=pressure difference between entrance  66  and exit  68  of capillary  54 . 
     η=viscosity of fluid. 
     L=effective length L of resistor  32 . 
     R 2 =radius of resistor capillary  54  (see in  FIG. 9 ). 
     As shown in the above equation, small changes in the diameter of a capillary have a profound effect on the flow rate. However, the modification of the R 2 dimension is often technically very difficult to realize. Thus, as discussed above, the design of this first embodiment of the present invention includes implementing elastic filament  52  into resistor capillary  54 , as discussed above. For the first example of the first embodiment (i.e.—concentrically located filament  52 ), the following equation may be utilized in determining the flow rate of this design:
 
 V =[(Δ p π)( R   2   −R   1 ) 3 ( R   2   +R   1 )]/(8η L )
 
     Where: 
     V=flow rate 
     Δp=pressure difference between entrance  66  and exit  68  of capillary  54 . 
     η=viscosity of fluid. 
     L=effective length L of resistor  32 . 
     R 1 =radius of filament  52  (see in  FIG. 9 ). 
     R 2 =radius of resistor capillary  54  (see in  FIG. 9 ). 
     Alternatively, for the second example of the first embodiment (i.e.—eccentrically located filament  52 ), the following equation may be utilized in determining the flow rate of this design:
 
 V =[(Δ p π)( R   2   −R   1 ) 3 ( R   2   +R   1 )2.5]/(8η L )
 
     Where: 
     V=flow rate 
     Δp=pressure difference between entrance  66  and exit  68  of capillary  54 . 
     η=viscosity of fluid. 
     L=effective length L of resistor  32 . 
     R 1 =radius of filament  52  (see in  FIG. 9 ). 
     R 2 =radius of resistor capillary  54  (see in  FIG. 9 ). 
     All three of the above equations are well known in the field of fluid dynamics. Further, while the effective length L of resistor  32 , as best shown in  FIGS. 10   a  and  12   a,  corresponds to the length of capillary  54 , it is noted that the effective length more specifically relates to the length of capillary  54  in which filament  52  resides. Therefore, the effective length L, for use in the above equations, may be less than the length of capillary  54  if filament  52  has a length less than the length of capillary  54 . It is noted that these equations apply to the use of capillaries and filaments having circular cross sections. Other embodiments may utilize differently shaped capillaries and filaments. For these embodiments, separate equations must be utilized. 
     As is clearly shown by the second equation, situating filament  52  in the offset position with relation to the center of capillary  54  of, as shown in  FIG. 11   a,  allows the flow rate to be changed by a factor of 2.5. Therefore, for applications where it is desired to vary the flow rate by such a ratio, it is possible to merely move filament  52  from a central position taught in the first example (as shown in  FIG. 9 ) to the eccentric position taught in the second example (as shown in  FIG. 11   a ). However, often times, it is typically desired to vary the flow rate by a factor of 25 or more. In order to achieve such a flow rate change, one may utilize an elastic filament  52  as discussed above, situated in an offset position. Typically, to ensure that filament  52  remains in the offset position, a curved capillary  54  is utilized. As shown in  FIG. 11   b,  filament  52  remains eccentrically placed within capillary  54  because of the curvature of the capillary. As filament  52  is generally elastic and resilient, it easily conforms to any curvature of capillary  54 . 
     A realistic range for the change in diameter of elastic filament  52  is approximately from its original size to about seventy percent of its original size (i.e.—a 1 to 0.7 ratio). Calculations have been carried out using the above equation relating to the eccentrically positioned filament  52 . For example, with the initial radius R 1  of filament  52  being approximately eighty percent (80%) of the radius R 2  of capillary  54  (i.e.—a 0.8 to 1 ratio) and the maximal elongation of filament  52  giving a radius R 3  that is approximately fifty six percent (56%) of the radius R 2  of capillary  54  (i.e.—a 0.56 to 1 ratio), it was calculated the ratio of flow rate between the non-elongated state and the maximal elongated state is approximately 9.20 to 1. With the initial radius R 1  of filament  52  being approximately eighty five percent (85%) of the radius R 2  of capillary  54  (i.e.—a 0.85 to 1 ratio) and the maximal elongation of filament  52  giving a radius R 3  that is approximately fifty nine point five percent (59.5%) of the radius R 2  of capillary  54  (i.e.—a 0.595 to 1 ratio), it was calculated the ratio of flow rate between the non-elongated state and the maximal elongated state is approximately 17.00 to 1. Finally, with the initial radius R 1  of filament  52  being approximately ninety percent (90%) of the radius R 2  of capillary  54  (i.e.—a 0.9 to 1 ratio) and the maximal elongation of filament  52  giving a radius R 3  that is approximately sixty three percent (63%) of the radius R 2  of capillary  54  (i.e.—a 0.63 to 1 ratio), it was calculated the ratio of flow rate between the non-elongated state and the maximal elongated state is approximately 43.46 to 1. Thus, using a filament  52  having a radius R 1  between approximately eighty five percent (85%) and ninety percent (90%) of the total radius R 2  of capillary  54 , would result in a flow rate variation of approximately 25. From the foregoing, one can calculate the desired flow rate variation based on the known geometry of the flow resistor. 
     A third example of the first embodiment of the present invention is shown in  FIG. 13 . This example includes a capillary  154  that is divided into two sectors by a center wall  155 . Fluid is capable of flowing through capillary  154  by entering through entrance  166  and exiting through exit  168 , as depicted by fluid flow arrow F. An elastic filament  152  is fixed at its ends by fixation points  160  and  164 , and is wrapped around a magnetic element  170  at the approximate central portion of filament  152 . Repulsive magnetic forces are transmitted to magnetic element  170  by a corresponding magnetic counterpart  172 , having a similar polarity. Thus, movement of counterpart  172  results in the like movement of element  170 . Counterpart  172  may be located in a hermetically sealed housing  174 , or the like. Movement of the magnetic element in a direction indicated by arrow B will, as in the above discussed examples, cause the diameter of filament  152  to shrink, thereby allowing for the increase in flow rate. Similarly, movement of element  170  in the direction indicated by arrow A will decrease the flow rate. It is noted that this two sector design includes two capillary and filament relationships for use in varying the flow rate. As such, where both the capillary and the filament have circular cross sections, two separate calculations in accordance with the above discussed equations, must be conducted to determine the overall hydraulic resistance provided by the system. 
     Further, in accordance with this third example of the first embodiment, it is envisioned that magnetic element  170  and magnetic counterpart  172  may be oppositely polarized, such that they are attracted to one another. In this type of design, moving counterpart  172  in a direction closer to element  170  would cause the attraction between them to be greater. Thus, if counterpart  172  is located below element  170  (as opposed to that shown in  FIG. 13 ), movement of counterpart  172  towards element  170  would increase the magnetic attractive force between the two components and necessarily cause the movement of element  170  in the direction indicated by arrow B. As discussed above, this lengthens filament  152 , while at the same time decreasing its diameter. Thus, this would constitute one alternate design. Similarly, it is possible to provide a single magnetic component with a corresponding metallic component, rather than the above discussed two magnet configuration. Clearly, as is well understood, such components would be attracted to one another. Therefore, operation of this magnet/metal configuration would operate in a like manner to the above discussed opposite polarity magnetic configuration. However, it is to be understood that various configurations are envisioned depending upon the polarity of the magnetic components and/or the situation of the metallic element and its corresponding magnetic element. For example, filament  152  may be wrapped around a metallic element, with a magnetic component located in housing  174  or vice versa. 
     A fourth example of the first embodiment of the present invention is shown in  FIG. 14 . This example includes an elastic filament  252  that is fixed at one end by attachment  260  and wrapped around axle  276  on the other. Once again, fluid enters capillary  254  at entrance  266 , and exits at exit  268 . Fluid flow direction is once again indicated by arrow F. Rotation of axle  276 , in a direction depicted by arrow W (i.e.—counter-clockwise), causes filament  252  to lengthen, while its diameter reduces. This, in turn, increases the possible flow rate through capillary  254 . Alternatively, rotation of axle  276  in a clockwise direction causes the opposite effect. As previously mentioned, if filament  252  and filament  254  have circular cross sections, the above equations may be utilized in calculating the hydraulic resistance of the system. Axle  276  may be driven directly by a micro motor, via a reduction gear drive assembly  280  as shown in  FIG. 15 . 
     While other means may be utilized for driving axle  276 , the following sets forth a discussion of the aforementioned reduction gear drive assembly  280 . As shown in  FIG. 15 , assembly  280  presents a solution for the transfer of rotational motion from hermetic enclosure  274  to axle  276 . Assembly  280  includes a motor  282  that is augmented by a gear drive  284  and transferred to disc  286 . The disc includes a shaft  288  which is preferably positioned at an angle which is less than ninety degree relative to the plane of disc  286 . Shaft  288  extends into cylindrical portion  290  of hermetic enclosure  274 . Further, shaft  288  is supported via bearings  292  within cylindrical portion  290 . Finally, cylindrical portion  290  is connected to enclosure  274  by an elastic connection  294  and is capable of transmitting forces via pusher plate  296  to rotate axle  276 . Essentially, the offset nature of the connections between disc  286  and shaft  288 , and portion  290  and plate  296 , coupled with the elastic nature of the connection between enclosure  274  and portion  290  allows for the rotation of axle  276 . It is noted that operation of the motor in different directions causes the rotation of the axle in the clockwise or counter-clockwise direction. 
     Gear drive assembly  280  is useful for allowing a relatively small or weak motor to drive axle  276 . Providing a gear assembly to better utilize a motor is well known. However, any known gear assembly, suitable for use with the present invention, may be employed. Further, it is also contemplated that a suitable motor may be employed that may be capable of directly rotating axle  276 . Essentially, in a design like this, axle  276  may be a continuation of the drive shaft of the motor. 
     Any of the examples set forth in the discussion relating to this first embodiment may include different, additional or fewer elements. Such revisions will be understood by those of ordinary skill in the art. For example, it is envisioned that the various elastic filaments, while shown in the figures having a substantially circular cross section, may include any shaped cross section. Similarly, although shown as substantially straight, the above may be utilized in conjunction with curved capillaries. Additionally, it is to be understood that the inventions set forth in the first embodiment may be utilized with any known implantable pump. The particular pump design may require the use of a resistor that is particularly configured and dimensioned to operate with the pump. Such design requirements are evident to those of ordinary skill in the art. 
     In a second embodiment the adjustment of flow rate is realized by providing a pair of threaded matched cylinders for use as resistor  32 . Once again, the second embodiment will be discussed with respect to pump  20 ; however, it may be utilized in combination with any implantable pump. As shown in  FIGS. 16 and 17 , in accordance with this second embodiment, resistor  32  includes a first threaded member  302  having a hollow interior  304  and a threaded exterior  306 . First threaded member is disposed in second threaded member  308 , which is an oppositely configured hollow member having a threaded interior surface  310  and a closed end  312 . The threaded cooperation between first and second threaded members  302  and  308  allows for the first member to be disposed within the second member at varying levels, therefore, allowing for different overlaps of the two members. For example,  FIG. 16  depicts the first member being substantially disposed within the second member, while  FIG. 17  depicts the first member being only partially disposed within the second member. 
     In operation of this second embodiment, fluid is introduced into hollow interior  304  in the direction indicated by arrow  314 . Upon the sufficient build up of pressure created by the flow of the fluid, the closed end  312  design of second member  308  forces the fluid to move in the direction indicated by arrow  315  (best shown in  FIG. 17 ) and through the flow channel defined by the threaded configuration of the two members  320 ,  308 . The degree of overlap of the two threaded geometries determines the hydraulic resistance, and thus the flow rate of the fluid. Therefore, the high overlap shown in  FIG. 16  would result in a lesser flow rate than that of the low overlap depicted in  FIG. 17 . Nevertheless, the fluid ultimately emerges from the resistor design as illustrated by arrows  316 . It is envisioned that in other examples in accordance with this embodiment of the present invention the shapes of the two members may vary, as can the particular thread design employed. 
     In a third embodiment the adjustment of flow rate is realized by adjusting the cross-sectional geometry of the resistor. However, unlike the above discussed first embodiment where the cross-sectional geometry is adjusted by lengthening filament  52  in order to decrease its diameter, this third embodiment varies the cross-sectional geometry of a tube  402  by changing its internal pressure. Once again, the third embodiment will be discussed with respect to pump  20 ; however, it may be utilized in combination with any implantable pump. As shown in  FIGS. 18-20 , in accordance with this third embodiment, resistor  32  includes an elastic tubular element  402  disposed in a capillary  404 . As best shown in  FIG. 20 , the tubular element  402  extends through capillary  404  and is fixed at its ends by sealing elements  406  and  408 . As shown in  FIGS. 18 and 20 , the tubular element  402  is situated so as to define a ring-shaped flow channel  410  through capillary  404 . However, like the above discussed first embodiment, the tube may be positioned eccentrically, thereby forming a sickle-shaped flow channel  410 , as shown in  FIG. 19 . 
     In operation, fluid flows in the direction indicated by arrows F, and is subjected to the flow channel from entrance  412  to exit  414 . Once again, the effective length of the resistor extends along the portion where tube  402  and capillary  404  overlap. The diameter of tubular element  402  depends upon its internal pressure P 1 . Thus, the flow rate of the fluid can be affected by pressure being applied or reduced to the inside of tube  402 . Rising the pressure will increase the outer diameter of the tubing and thus will have the effect of reducing the flow rate. Similarly, lowering the pressure will decrease the outer diameter of the tubing and increase the flow rate. It is noted that tubular element  402  will have a particular resting diameter (i.e.—with no pressure being applied). The design of this third embodiment will be subject to the flow rate calculations discussed above in relation to the first embodiment. Specifically, in the design shown in  FIG. 19 , adjusting the tubing between approximately eighty five percent (85%) to ninety percent(90%) of the overall inner diameter of capillary  404  will result in an approximate flow rate variation of 1 to 25, which is the desired ratio for an implantable pump. However, it is to be understood that the operation of this third embodiment will be substantially opposite to that of the first embodiment. Clearly, rather than decreasing the diameter of tube  402  from its resting diameter, this third embodiment aims to increase the diameter. Thus, operation of tube  402  will move the system from a state in which the flow rate is greater to a state where the flow rate is lesser. This is contrary to the first embodiment. 
     Any means suitable for rising and lowering the pressure to the inside of tubular element  402  can be utilized. For example, it is envisioned that a piston or bellows assembly may be utilized, or that a chemical reaction may be employed to achieve the pressure differential. 
     In a fourth embodiment the adjustment of flow rate is realized by providing an insert  502  having a longitudinally varying cross section. By moving the insert  504  along the longitudinal axis of a capillary  504 , the hydraulic resistance of resistor  32  is changed. Once again, the fourth embodiment will be discussed with respect to pump  20 ; however, it may be utilized in combination with any implantable pump. As shown in  FIGS. 21-24 , in accordance with this fourth embodiment, resistor  32  includes the aforementioned insert  502  positioned within a capillary  504 . In one example of this fourth embodiment, as is shown in  FIGS. 21 and 23 , insert  502  is depicted as having a conical shape, and is centrally located within capillary  504 . Thus, the cross section of insert  502  varies across its longitudinal axis and the design forms a ring-shaped flow channel  506 . This insert is fixed at its ends to two movable piston-like attachments  508 ,  510 . However, another example is shown in  FIGS. 23 and 24 , in which insert  502  may be positioned eccentrically resulting in a sickle-shaped flow channel  506 . In this example, insert  502  is fixed at its ends to two movable fixations  512 ,  514 . 
     In operation of both examples, fluid flows in the direction indicated by arrows F, and is subjected to the flow channel from entrance  516  to exit  518  (i.e.—the aforementioned effective length). While the above-discussed equations relating to the flow rate do not necessarily apply to this embodiment, it is clear that the width of flow channel  506  may be varied by moving insert  502  in the direction of the axis of capillary  504 . For example, as shown in  FIG. 23 , movement of insert  502  in the direction depicted by arrow A will cause a decrease in the width of flow channel  506 , and thus a decrease in the flow rate of the fluid. Alternatively, movement of insert  502  in the direction depicted by arrow B will cause an increase in the width of flow channel  506 , and thus an increase in the flow rate of the fluid. 
     It is noted that the movement of insert  502  may be achieved in different fashions depending upon the type of design utilized. For example, as shown in  FIG. 23 , piston-like attachments  508 ,  510  are preferably moved by providing a suitable pressure thereto. However, as shown in  FIG. 24 , movable fixations  512 ,  514  may also be utilized that are moved by providing a mechanical force thereto, from source such as a hydraulic, electrical or mechanical source or the like. Various means may be employed for providing movement to insert  502 , including those discussed herein and others that would be well known to those skilled in the art. For example, once again, magnetic forces may be employed for moving insert  502 . Finally, insert  502  may include a varying cross section that creates a substantially smooth longitudinal surface, as shown in the figures, or, insert  502  may be comprised of several non-congruent cross sectional portions. The latter configuration would provide an insert that has several different stepped sections. Thus, moving a first section into capillary  504  having a relatively large cross section would most likely reduce the flow rate, while moving a second section of lesser cross section would increase the flow rate. 
     The various embodiments of resistor  32 , in accordance with the present invention, should be positioned such that fluid housed in the slow release chamber of an implantable pump is forced to pass through it. This configuration allows for the implantable pump to operate in its normal fashion, with resistor  32  controlling the fluid flow rate. However, preferred constructions would situate resistor  32  such that an injection into a bolus port or the like would not be forced to pass through the resistor. It is typically not required to control the flow rate of a bolus injection. Rather, such an injection is often intended to be a quick and direct application of a medication fluid. For example, as shown in  FIG. 7 , resistor  32  is situated so as to capture fluid flowing from chamber  24 , but not fluid directly injected into bolus port  46 . However, other constructions are envisioned. Furthermore, where the implantable pump is utilized to withdraw spinal fluid, it is also contemplated to not force such fluid through resistor  32 . In the pump of  FIG. 7 , withdrawal of spinal fluid would occur through bolus port  46 . As such, the fluid would not be required to pass through the resistor. 
     For each of the embodiments above, providing a controlling mechanism for selectively varying the flow rate of the medication fluid is envisioned. Many different such mechanisms are well known and widely utilized with implantable devices for implantation into a patient&#39;s body. For example, prior art devices have shown that it is possible to utilize dedicated hard wired controllers, infrared controllers, or the like, which controllers could be used in accordance with the present invention to control various elements, such as motor  282 , to selectively vary the flow rate of the medication fluid. U.S. Pat. No. 6,589,205 (“the &#39;205 patent”), the disclosure of which is hereby incorporated by reference herein, teaches the use of a wireless external control. As discussed in the &#39;205 patent, such a wireless control signal may be provided through modulation of an RF power signal that is inductively linked with the pump. The &#39;205 cites and incorporates by reference U.S. Pat. No. 5,876,425, the disclosure of which is also hereby incorporated by reference herein, to teach one such use of forward telemetry or the exchange of information and programming instructions that can be used with the present invention to control the pump and the various aforementioned elements that are varied in order to affect the flow rate. However, it is noted that similar external controllers may also be utilized. Such controllers can send control signals wirelessly (such as by IR, RF or other frequencies) or can be wired to leads that are near or on the surface of the patient&#39;s skin for sending control signals. Furthermore, a pump in accordance with the present invention may include safeguards to prevent the inadvertent signaling or improper programming of the pump. For example, the present invention could utilize a secure preamble code or encrypted signals that will be checked by software or hardware used for controlling the pump or even dedicated only for security purposes. This preamble code would prevent the inadvertent varying of the flow rate of the fluid from the pump, from being caused by outside unrelated remote control devices or signals and by other similar pump controllers. Other safety precautions may be used, such as passwords, hardware or software keys, encryption, multiple confirmation requests or sequences, etc. by the software or hardware used in the programming of the pump. 
     The electronics and control logic that can be used with the present invention for control of the motors and controllably displaceable elements used to vary the flow rate may include microprocessors, microcontrollers, integrated circuits, transducers, etc. that may be located internally with or in the implantable pump and/or externally with any external programmer device to transmit pump programming information to control the pump. For example, any external programmer device used to allowing programming of the pump. The electronics can also be used to perform various tests, checks of status, and even store information about the operation of the pump or other physiological information sensed by various transducers. 
     An external programmer device may also be avoided by incorporating the necessary logic and electronics in or near or in the implantable pump such that control can be accomplished, for example, via control buttons or switches or the like that can be disposed on or below the surface of the skin. Of course, necessary precautions (such as confirmation button pressing routines) would need to be taken so that inadvertent changing of programming is again avoided. 
     A specific implantable pump  700 , which incorporates the above discussed reduced size designs, as well as the above discussed infinitely variable designs of the present invention will now be described. Essentially, pump  700  is an implantable pump having certain novel characteristics. These characteristics allow for both the relative miniaturization and easy construction of the pump. In addition, pump  700  incorporates one of the aforementioned resistor  32  designs into the specific embodiment. While pump  700  is indeed one preferred embodiment for use in accordance with the present invention, it should be clearly understood that the pump could be modified to incorporate each of the resistor  32  designs discussed above in many different configurations. 
     As shown in  FIGS. 25 and 26 , pump  700  includes a housing constructed of an upper portion  702  and a lower portion  704 . The housing portions are preferably constructed of a strong polymeric material, such as polyetherehterketone, sold under the designation PEEK by Invibio of the United Kingdom. Other suitable materials may also be employed. Nevertheless, the particular material should be chosen so as to be capable of forming a two part housing that can be safely assembled without the use of a complicated double clinch assembly, a welding process or the like. Clearly, safety is a very big concern in the construction of any apparatus inserted into the body especially one housing an overdose of medication solution. Heretofore, implantable pump housings have either been constructed of a metallic material, wherein a welding process is utilized for attaching the portions of the housing together, or a polymeric material, wherein a complicated clinching assembly is utilized for attaching the portions of the housing together. For example, a metallic pump is typically constructed by welding together two metallic halves of the pump housing. Similarly, as taught in commonly owned U.S. Pat. Nos. 5,814,019 and 5,836,915, a double clinching assembly has been previously proposed for safely attaching the housing halves of a polymeric pump. 
     In accordance with the present invention, it has been discovered that utilizing a material such as PEEK may allow for a polymeric pump housing to be constructed without the use of any of the complicated attachment procedures. The elimination of such extraneous elements allows for pump  700  to be smaller in size. For example, the elimination of the aforementioned double clinch safety feature allows for the overall width of pump  700  to be reduced. Further, in certain embodiments, this may also decrease the overall weight of the pump, as well as the level of complicity required in assembling same. As shown in  FIG. 25 , portions  702  and  704  of the housing of pump  700  are constructed of PEEK and designed so as to be capable of simply screwing together. More particularly, portion  702  includes an interiorly threaded extension  703  for receiving an exteriorly threaded surface  705  of portion  704 . In certain embodiments, in addition to the threaded connection, a layer of glue or other adhesive may be applied to the connection between portions  702  and  704 . Such an application may provide further assurance that the two portions do not inadvertently become detached. It is also contemplated that other less complicated attachment modes may be employed. For example, in addition to the threadable connection between portions  702  and  704 , a single clinch connection may be utilized. In this type of attachment, the two portions may include elements that are designed so as to snap fit together, and thereafter fixably secure the portions together. 
     As with the aforementioned generic pump  20  design, implantable pump  700  further includes an interior having two chambers  724  and  726 , each chamber being separated by a flexible membrane  728 . Chamber  724  is designed to receive and house an active substance such as a medication fluid, while chamber  726  is designed to house a propellant that expands isobarically under constant body temperature. Similar to above discussed generic pump  20 , the expansion of the propellant in pump  700  displaces membrane  728  such that the medication fluid housed in chamber  724  is dispensed into the body of the patient through the path defined by an outlet opening  730  ( FIG. 26 ), a cylindrical recess  764 , a resistor  732  ( FIG. 27 ), a cylindrical recess  766  ( FIG. 25 ), an outlet duct  734  and ultimately an outlet catheter  736 . Also in accordance with pump  20 , pump  700  further includes a replenishment port  738  covered by a first septum  740 , and an annular ring bolus port  746  covered by a second ring shaped septum  748 . The utility of each of these ports is substantially identical to those of pump  20 . For example, a passage  744  allows fluid injected into replenishment port  738  to be introduced into chamber  724 . In addition, like that of pump  20 , it is envisioned that specifically designed injection needles and correspondingly situated septa may be employed to increase safety, as discussed above. 
     Contrary to the aforementioned pump  20 , pump  700  includes an undulating membrane  728  which cooperates with a similarly undulating interior surface  707  of portion  702 . As best shown in  FIGS. 25 and 26 , interior surface  707  of portion  702  has an undulating surface that serves as the top surface of chamber  724 , while membrane  724  has a corresponding undulating surface that serves as the bottom surface of chamber  724 . When chamber  724  is empty, membrane  724  fits flush against the similarly shaped interior surface  707 . This is best shown in  FIG. 25 . However, upon introduction of a fluid into chamber  724 , membrane  728  is capable of flexing and allowing for the expansion of chamber  724 . This is best shown in  FIG. 26 . This undulating configuration of membrane  728  and interior surface  707  of portion  702  allows for replenishment port  738  and septum  740  to be situated at a lower position with respect to the height of the pump. Essentially, a center portion of both interior surface  707  and membrane  728  are a convex shape allowing for portion  738  and septum  740  to be set lower. At the same time, portions to the left and right of this center portion are enlarged, taking substantially concave shapes. This allows for the overall volume of chamber  724  to remain substantially similar in comparison to well-known implantable pumps. Operation of pump  700  also remains substantially similar to prior art implantable pumps being driven by a propellant. While the specific undulating design (i.e.—a convex or lower portion flanked by two concave or higher portions), shown in  FIGS. 25 and 26 , is one suitable embodiment, other embodiments are envisioned. For example, other pumps may include surfaces and membranes that have corresponding shapes having multiple concave and/or convex portions. 
     The specific construction and cooperation of resistor  732  within pump  700  is shown in detail in  FIGS. 25-27 . The resistor shown in this specific embodiment is akin to the above described first embodiment resistor. As best shown in  FIG. 27 , resistor  732  includes an elastic and resilient filament  752  situated in a capillary  754 . Filament  752  extends through capillary  754  and is attached on its ends to two spools  760  and  762 . Spool  760  resides within cylindrical recess  764  in fluid communication with opening  730  in portion  702 , while spool  762  resides within a cylindrical recess  766  in portion  702 . Recess  764  is in fluid communication with outlet opening  730  and hence chamber  724  (best shown in  FIG. 26 ). Similarly, recess  766  is in fluid communication with outlet duct  734 , and hence outlet catheter  736  (best shown in  FIG. 25 ). Thus, fluid will flow from chamber  724  through resistor  732 , and out of catheter  736  to a target site within the body. 
     As best shown in  FIG. 27 , capillary  754  is preferably curved so as to force filament  752  to one side thereof. Spools  760  and  762  are adapted to wind filament  752  thereon and thus vary its cross section. As more specifically discussed above, this varying in cross section varies the flow rate of fluid through capillary  754 . In the embodiment shown in  FIGS. 25-27 , spool  760  is adapted to remain in a fixed position, while spool  762  is adapted to be rotated. However, in other embodiments, both spools may be adapted to be rotated. As best shown in the cross sectional view of  FIG. 25 , spool  762  is mechanically coupled to several actuation components including being coupled via an axle  770  to a wheel  772 . A motor  774 , like that of the above mentioned X15G, is employed to provide rotation to wheel  772 . A bearing  776  or the like may aid in the rotation of axle  770 , by guiding and providing smooth motion to axle  770 . In the embodiment shown in the figure, motor  774  receives electrical energy and control from an electronic unit  778 , which, as discussed above, controlled from either internally or externally of the body. 
     The aforementioned actuation components are held together and within pump  700  through a specific cooperation that is best shown in  FIG. 25 . Essentially, ring septum  748  and an elastic element  780  are designed to hold the actuation components to pump  700 . The actuation elements are preferably housed so as to be a single module encompassing spool  762 , axle  770 , wheel  772 , motor  774 , bearing  776  and electronic unit  778 . During assembly, this module is placed into a recess on pump  700  so that one side abuts ring septum  748 . With the module in place, septum  740  is attached to portion  702  by screwing a holder  782 , which holds septum  740 , to portion  702  of pump  700 , so as to form a threaded connection  783 . Holder  782  is preferably constructed of PEEK material like portions  702  and  704 . It is also contemplated that other modes of attachment may be employed, such as, by adhesive or a combination of adhesive and threads. Ring  780  of elastomeric material is preferably placed between holder  782  and electronic unit  778 , and the cooperation thereof holds the aforementioned module between septum  748  and ring  780 . Essentially, one side of the module is designed to cooperate with septum  748  (i.e.—curved cooperation), while the other side is designed to cooperate with ring  780  (i.e.—sloped cooperation). Thus, in the fully constructed state, the module of actuation components is essentially frictonally attached to pump  700 . 
     The specific embodiment shown in  FIGS. 25-27  also allows for an easy conversion from a variable flow rate pump to a fixed flow rate pump. In operation, the manufacturer or user of the pump would simply remove the aforementioned module of actuation components. A spacer, insert or the like may inserted into any cavity formed in the housing of pump  700 , after the removal of the module. Filament  752  is also removed from capillary  754  and replaced with a small tube (not shown), constructed of a material such as glass. The tube preferably has an outer diameter slightly smaller than the inside diameter of capillary  754 , so as to allow a snug fit therein. Further, the tube may have any suitable inner diameter, it being noted that the particular inner diameter size dictates the flow rate of fluid through capillary  754 . Thus, depending upon the desired fixed flow rate, a particular tube having a suitable inner diameter should be selected. Finally, the tube should be capable of conforming to the preferable curved shape of capillary  754 . With these simple modifications to pump  700 , a relatively inexpensive fixed flow rate pump may be produced. This simple conversion allows for the use of the majority of the components of pump  700  without requiring the modification of any. This is beneficial, because new molds or the like would not be needed to change between pump designs. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.