Abstract:
An improved pump is provided for controlled delivery of fluids wherein the pump includes a reservoir and a movable piston. A plunger slide is in removable contact with the movable piston. A motor, is operably coupled to a drive member, such as a drive screw. The motor is disposed in-line with the drive member and the plunger slide. The drive member is operably connected to the plunger slide and is disposed to be substantially enclosed by the plunger slide when the plunger slide is in at least one position. The drive member is adapted to advance the plunger slide in response to operation of the motor.

Description:
This application claims priority from provisional patent application No. 60/106,237, filed Oct. 29, 1998 and which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to improvements in infusion pumps such as those used for controlled delivery of medication to a patient. More specifically, this invention relates to an improved infusion pump having a modified and space-efficient drive system. 
     2. Description of the Related Art 
     Infusion pump devices and systems are relatively well-known in the medical arts, for use in delivering or dispensing a prescribed medication such as insulin to a patient. In one form, such devices comprise a relatively compact pump housing adapted to receive a. syringe or reservoir carrying a prescribed medication for administration to the patient through infusion tubing and an associated catheter or infusion set. 
     The infusion pump includes a small drive motor connected via a lead screw assembly for motor-driven advancement of a reservoir piston to administer the medication to the user. Programmable controls can operate the drive motor continuously or at periodic intervals to obtain a closely controlled and accurate delivery of the medication over an extended period of time. Such infusion pumps are utilized to administer insulin and other medications, with exemplary pump constructions being shown and described in U.S. Pat. Nos. 4,562,751; 4,678,408; 4,685,903; 5,080,653 and 5,097,122, which are incorporated by reference herein. 
     Infusion pumps of the general type described above have provided significant advantages and benefits with respect to accurate delivery of medication or other fluids over an extended period of time. The infusion pump can be designed to be extremely compact as well as water resistant, and may thus be adapted to be carried by the user, for example, by means of a belt clip or the like. As a result, important medication can be delivered to the user with precision and in an automated manner, without significant restriction on the user&#39;s mobility or life-style, including in some cases the ability to participate in water sports. 
     These pumps often incorporate a drive system which uses a lead screw coupled to motors. The motors can be of the DC, stepper or solenoid varieties. These drive systems provide an axial displacement of the syringe or reservoir piston thereby dispensing the medication to the user. Powered drive systems are advantageous since they can be electronically controlled to deliver a predetermined amount of medication by means well known in the art. 
     In the operation of these pump systems, the reservoir piston will be fully advanced when vitually all of the fluid in the reservoir has been dispensed. Correspondingly, the axial displacement of the motor lead screw is also typically fully displaced. In order to insert a new reservoir which is full of fluid, it is necessary to restore the lead screw to its original position. Thus the lead screw will have to be rewound or reset. 
     DC motors and stepper motors are advantageous over solenoid motors in that the former are typically easier to operate at speeds that allow rewinding the drive system electronically. Solenoid based drive systems, on the other hand, often must be reset manually, which in turn makes water resistant construction of the pump housing more difficult. 
     Lead screw drive systems commonly use several gears which are external to the motor. FIG. 1 shows such a lead screw arrangement which is known in the art. A motor  101  drives a lead screw  102  which has threads which are engaged with a drive nut  103 . Thus the rotational force of the lead screw  102  is transferred to the drive nut  103  which causes it to move in an axial direction d. Because the drive nut  103  is fixably attached to a reservoir piston  104 , it likewise will be forced in an axial direction d′, parallel to direction d, thus dispensing the fluid from the reservoir  105  into the infusion set  106 . The entire assembly can be contained in a water resistant housing  107 . 
     FIG. 2 shows a different lead screw arrangement which also is known in the art. In this arrangement, a motor  201  (or a motor with an attached gear box) has a drive shaft  201   a  which drives a set of gears  202 . The torque is then transferred from the gears  202  to a lead screw  203 . The threads of the lead screw  203  are engaged with threads [not shown] in a plunger slide  204 . Thus the torque of the lead screw  203  is transferred to the slide  204  which causes it to move in an axial direction d′, parallel to the drive shaft  201   a  of the motor  201 . The slide  204  is in contact with a reservoir piston  205  which likewise will be forced to travel in the axial direction d′ thus dispensing fluid from the reservoir  206  into the infusion set  207 . The assembly can be contained in a water resistant housing  208 . 
     As previously noted, these lead screw drive systems use gears which are external to the motor. The gears are in combination with a lead screw with external threads which is used to drive the reservoir&#39;s piston. This external arrangement occupies a substantial volume which can increase the overall size of the pump. Moreover, as the number of drive components, such as gears and lead screw, increases, the torque required to overcome inherent mechanical inefficiencies can also increase. As a result, a motor having sufficient torque also often has a consequent demand for increased electrical power. 
     Yet another known drive is depicted in FIGS. 3 a  and  3   b.  A reservoir  301  fits into the unit&#39;s housing  302 . Also shown are the piston member  303  which is comprised of an elongated member with a substantially circular piston head  304  for displacing the fluid in the reservoir  301  when driven by the rotating drive screw  305  on the shaft (not visible) of the drive motor  306 . 
     As is more clearly shown in FIG. 3 b,  the reservoir  301 , piston head  304  and piston member  303  comprise an integrated unit which is placed into the housing  302  (FIG. 3 a ). 
     The circular piston head  304  displaces fluid in the reservoir upon axial motion of the piston member  303 . The rearward portion of the piston member  303  is shaped like a longitudinal segment of a cylinder as shown in FIG. 3 b  and is internally threaded so that it may be inserted into a position of engagement with the drive screw  305 . The drive screw  305  is a threaded screw gear of a diameter to mesh with the internal threads of the piston member  303 . Thus the motor  306  rotates the drive screw  305  which engages the threads of the piston member  303  to displace the piston head  304  in an axial direction d. 
     While the in-line drive system of FIG. 3 a  achieves a more compact physical pump size, there are problems associated with the design. The reservoir, piston head and threaded piston member constitute an integrated unit. Thus when the medication is depleted, the unit must be replaced. This results in a relatively expensive disposable item due to the number of components which go into its construction. 
     Moreover the design of FIG. 3 a  is not water resistant. Because the reservoir, piston head and threaded piston member are removable, the drive screw  305  and motor  306  are exposed to the atmosphere. Any water which might come in contact with the drive screw  305  and motor  306  will result in corrosion and probable motor failure. 
     The design of FIG. 3 a  further gives rise to problems associated with position detection of the piston head  304 . The piston member  303  can be decoupled from the drive screw  305 . However, when another reservoir assembly is inserted, it is not known by the system whether the piston head  304  is in the fully retracted position or in some intermediate position. Complications therefore are presented with respect to providing an ability to electronically detect the position of the piston head  304  in order to determine the extent to which the medication in reservoir  301  has been depleted. 
     The construction of pumps to be water resistant give rise to operational problems. As the user travels from various elevations, such as might occur when traveling in an air plane, or as the user engages in other activities which expose the pump to changing atmospheric pressures, differential pressures can arise between the interior of the air tight/water-resistant pump housing and the atmosphere. Should the pressure in the housing exceed external atmospheric pressure, the resulting forces could cause the reservoir piston to be driven inward thus delivering unwanted medication. 
     Thus it is desirable to have an improved, compact, water resistant drive system which permits safe user activity among various atmospheric pressures. Moreover it is desirable that such a system employ inexpensive medication reservoirs. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     An improved pump is provided with a reservoir for accommodation of a liquid and a movable piston for varying the size of the reservoir and adapted to discharge the liquid from the reservoir through the outlet. In a certain aspect of the present inventions, a plunger slide is releasably coupled with the movable piston and has at least two positions. A driving device, such as a motor, is operably coupled to a drive member, such as a drive screw. The motor is disposed in-line with the drive screw and the plunger slide. The drive screw is operably connected to the plunger slide and is disposed to be substantially enclosed by the plunger slide when it is in at least one position. The drive screw is adapted to advance the plunger slide in response to operation of the motor. 
     In one alternative, a housing for the reservoir, the movable piston, the plunger slide, the drive screw and the motor is provided along with a sealing device, such as an O-ring, that separates the portion of the housing which encloses the movable piston from the portion of the housing which encloses the drive screw and the motor. 
     In another preferred embodiment, a coupler is attached to the plunger slide. The coupler is removably attached to the movable piston to prevent separation of the movable piston from the plunger slide when the air pressure in the housing exceeds the pressure external to the water resistant housing. 
     In still another embodiment, the housing includes a vent port between the exterior and interior of the housing. The vent port contains a hydrophobic material or a relief valve, either of which will permit air to pass through the vent, but will prevent water from passing. 
     In another alternative, the driving device is a motor which is attached to the housing with a compliance mount. In another embodiment, the plunger slide comprises a telescoping lead screw formed from at least two segments. 
     In yet another embodiment , the pump includes a key which is coupled with the plunger slide and which is operable to permit movement of the plunger slide in the direction of the at least two positions but prevent movement of the plunger slide in any other direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side plan view of a conventional lead-screw drive mechanism. 
     FIG. 2 is a side plan view of a another conventional lead-screw drive mechanism. 
     FIG. 3 a  is a perspective view of another conventional lead-screw drive mechanism. 
     FIG. 3 b  shows the details of a disposable reservoir with the piston and drive member withdrawn of the lead-screw drive mechanism of FIG. 3 a.    
     FIG. 4 is a side plan, cut-away view of a drive mechanism in a retracted position in accordance with an embodiment of the present invention. 
     FIG. 5 is a perspective view of the in-line drive mechanism of FIG. 4 outside of the housing. 
     FIG. 6 is a cut-away perspective view of the drive mechanism of FIG. 4 in a retracted position. 
     FIG. 7 a  is a side plan, cut-away view of the drive mechanism of FIG. 4 in an extended position. 
     FIG. 7 b  is a cut-away perspective view of the drive mechanism of FIG. 4 in an extended position. 
     FIG. 8 is a cut-away perspective view of an anti-rotation device for use with the drive mechanism shown in FIG.  4 . 
     FIG. 9 is a cross-sectional view of a segmented (or telescoping) lead screw in accordance with an embodiment of the present invention. 
     FIGS. 10 a,    10   b  and  10   c  are cross-sectional views of various embodiments of venting ports for use with the drive mechanism of FIG.  4 . 
     FIG. 11 is a partial, cross-sectional view of a reservoir and plunger slide assembly. 
     FIG. 12 is a partial, cross sectional view of a reservoir and a reservoir connector. 
     FIGS. 13 a  and  13   b  are plunger slide force profile diagrams. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, reference is made to the accompanying drawings which form a part hereof and which illustrate several embodiments of the present inventions. It is understood that other embodiments may be utilized and structural and operational changes may be made without departing from the scope of the present inventions. 
     As shown in the drawings for purposes of illustration, some aspects of the present inventions are directed to a drive mechanism for an infusion pump for medication or other fluids. In preferred embodiments, a releasable coupler couples an in-line drive to a plunger or piston of a reservoir to dispense fluids, such as medications, drugs, vitamins, vaccines, hormones, water or the like. However, it will be recognized that further embodiments of the invention may be used in other devices that require compact and accurate drive mechanisms. 
     In addition, other embodiments use a telescoping drive member (or lead screw) to minimize the packaging dimensions of the drive mechanism and the overall configuration of the medication pump. Still further, a ventilation feature using hydrophobic materials or a relief valve can be employed to equalized any pressure differentials which might otherwise exist between the atmosphere and the interior of the pump housing. As a back up to this ventilation feature, a threaded attachment permits a secure coupling between the reservoir piston and the in-line drive. 
     FIG. 4 shows a side plan, cut-away view of an infusion pump drive mechanism according to a preferred embodiment of the inventions, in which a housing  401 , containing a lower section  402  for a power supply  420  and electronic control circuitry  422 , accommodates a driving device, such as a motor  403  (e.g., a solenoid, stepper or d.c. motor), a first drive member, such as an externally threaded drive gear or screw  404 , a second drive member, such as an internally threaded plunger gear or slide  405 , and a removable vial or reservoir  406 . The reservoir  406  includes a plunger or piston  407  with O-rings or integral raised ridges for forming a water and air tight seal. The reservoir  406  is secured into the housing  401  with a connector  431  which also serves as the interface between the reservoir  406  and the infusion set tubing (not shown). In a preferred embodiment, the reservoir piston  407  is coupled to the plunger slide  405  by a releasable coupler. In the illustrated embodiment, the coupler includes a female portion  424  which receives a male portion  426  carried by the plunger slide  405 . The female portion  424  is positioned at the end face  428  of the piston  407  and includes a threaded cavity which engages the threads of a male screw extending from the end  430  of the plunger slide  405 . 
     While preferred embodiments of the present inventions are directed to disposable, pre-filled reservoirs, alternative embodiments may use refillable cartridges, syringes or the like. The cartridge can be pre-filled with insulin (or other drug or fluid) and inserted into the pump. Alternatively, the cartridge could be filled by the user using an adapter handle on the syringe-piston. After being filled, the handle is removed (such as by unscrewing the handle) so that the cartridge can be placed into the pump. 
     Referring again to FIG. 4, as the drive shaft  432  of the motor  403  rotates, the drive screw  404  drives the plunger slide  405  directly to obtain the axial displacement against the reservoir piston  407  to deliver the predetermined amount of medication or liquid. When using a DC or stepper motor, the motor can be rapidly rewound when the reservoir is emptied or as programmed by the user. A sealing device, such as an O-ring seal  409  is in contact with the plunger slide  405  thus allowing it to move axially while maintaining a water resistant barrier between the cavity holding the reservoir  406  and the motor  403 . This prevents fluids and other contaminants from entering the drive system. 
     An anti-rotation key  410  is affixed to the plunger slide  405  and is sized to fit within a groove (not shown) axially disposed in the housing  401 . This arrangement serves to prevent motor and plunger slide rotation which might otherwise result from the torque generated by the motor  403  in the event that the friction of the O-ring seal  409  is not sufficient alone to prevent rotation. 
     The motor  403  is a conventional motor, such as a DC or stepper motor, and is journal mounted in the housing  401  by a system compliance mounting  412 . A system compliance mount can be useful in aiding motor startup. Certain types of motors, such as stepper motors, may require a great deal of torque to initiate rotor motion when the rotor&#39;s initial at-rest position is in certain orientations with respect to the motor&#39;s housing. A motor which is rigidly mounted may not have enough power to develop the necessary starting torque. Including system compliance mounting permits the motor housing to turn slightly in response to high motor torque. This alters the orientation between the rotor and the housing such that less torque is required to initiate rotor motion. A compliance mount can include a rubberized mounting bracket. Alternatively, the mounting could be accomplished using a shaft bearing and leaf spring or other known compliance mountings. 
     FIG. 5 shows a perspective view of the in-line drive mechanism of FIG. 4 outside of the housing. The plunger slide  405  (internal threads not shown) is cylindrically shaped and has the screw-shaped male portion  426  of the coupler attached to one end thereof. The anti-rotation key  410  is affixed to the opposite end of the slide  405 . The drive screw  404  is of such a diameter as to fit within and engage the internal threads of the plunger slide  405  as shown in FIG. 4. A conventional gear box  501  couples the drive screw  404  to the drive shaft  432  of the motor  403 . 
     FIGS. 4 and 6 show the infusion pump assembly with the plunger slide  405  in the retracted position. The reservoir  406  which may be full of medication or other fluid is inserted in a reservoir cavity  601  which is sized to receive a reservoir or vial. In the retracted position, the plunger slide  405  encloses the gear box  501  (not visible in FIG. 6) while the drive screw  404  (not visible in FIG. 6) remains enclosed within the plunger slide  405  but is situated close to the coupler. 
     The motor  403  may optionally include an encoder (not shown) which in conjunction with the system electronics can monitor the number of motor rotations. This in turn can be used to accurately determine the position of the plunger slide  405  thus providing information relating to the amount of fluid dispensed from the reservoir  406 . 
     FIGS. 7 a  and  7   b  show the infusion pump assembly with the plunger slide  405  in the fully extended position. In this position, the plunger slide  405  has withdrawn from over the gear box  501  and advanced into the reservoir  406  behind the reservoir piston  407 . Accordingly, the plunger slide  405  is sized to fit within the housing of the reservoir  406 , such that when the reservoir piston  407  and the plunger slide  405  are in the fully extended position as shown, the reservoir piston  407  has forced most, if not all, of the liquid out of the reservoir  406 . As explained in greater detail below, once the reservoir piston  407  has reached the end of its travel path indicating that the reservoir has been depleted, the reservoir  406  may be removed by twisting such that the threaded reservoir piston  407  (not shown in FIG. 7 b ) disengages from the male portion  426  of the coupler. 
     In a preferred embodiment, the motor drive shaft  432 , gear box  501 , drive screw  404 , and plunger slide  405  are all coaxially centered within the axis of travel  440  (FIG. 4) of the reservoir piston  407 . In certain of the alternative embodiments, one or more of these components may be offset from the center of the axis of travel  440  and yet remain aligned with the axis of travel which has a length which extends the length of the reservoir  406 . 
     FIG. 8 is a cut away perspective view of an anti-rotation device. The anti-rotation key  410  consists of a ring or collar  442  with two rectangular tabs  436  which are spaced 180° apart. Only one tab is visible in FIG.  8 . The ring portion  442  of the key  410  surrounds and is attached to the end of the plunger slide  405  which is closest to the motor. Disposed in the housing  401  are two anti-rotation slots  434 , only one of which is visible in FIG.  8 . The anti-rotation slots  434  are sized to accept the rectangular tabs of the key  410 . As the plunger slide  405  moves axially in response to the motor torque as previously described, the slots  434  will permit the key  410  to likewise move axially. However the slots  434  and the tabs  436  of the key  410  will prevent any twisting of the plunger slide  405  which might otherwise result from the torque generated by the motor. 
     FIG. 9 illustrates a split lead-screw (or plunger slide) design in accordance with an embodiment of the present inventions. The use of a split lead-screw or telescoping lead screw allows the use of an even smaller housing for the drive mechanism. A telescoping lead-screw formed from multiple segments allows the pump to minimize the dimensions of the drive mechanism, in either in-line or gear driven drive mechanisms. 
     In preferred embodiments, an interior shaft  901  is rotated by a gear  906  which is coupled to a drive motor (not shown). This in turn extends a middle drive segment  902  by engaging with the threads of an internal segment  904 . The middle segment  902  carries an outer segment  903  forward with it in direction d as it is extended to deliver fluid. When the middle segment  902  is fully extended, the internal segment  904  engages with a stop  905  on the middle segment  902  and locks it down from pressure with the threads between the middle and internal segments. The locked middle segment  902  then rotates relative to the outer segment  903  and the threads between the middle segment  902  and the outer segment  903  engage to extend the outer segment  903  in direction d to its full length. 
     The use of multiple segments is not limited to two or three segments; more may be used. The use of three segments reduces the length of the retracted lead-screw portion of the drive mechanism by half. In alternative embodiments, the outer segment may be connected to the motor and the inner segment may be the floating segment. In preferred embodiments, O-rings  907  are used to seal each segment relative to the other and to form a seal with the housing to maintain water sealing and integrity. 
     As previously noted, the construction of these pumps to be water resistant can give rise to operational problems. As the user engages in activities which expose the pump to varying atmospheric pressures, differential pressures can arise between the interior of the air tight/water-resistant housing and the atmosphere. Should the pressure in the housing exceed external atmospheric pressure, the resulting forces could cause the reservoir piston to be driven inward thus delivering unwanted medication. On the other hand, should the external atmospheric pressure exceed the pressure in the housing, then the pump motor will have to work harder to advance the reservoir piston. 
     To address this problem, a preferred embodiment of the inventions includes a venting port which resists the intrusion of moisture. Referring to FIG. 7 b,  venting is accomplished through the housing  401  into the reservoir cavity  601  via a vent port  605 . The vent port can be enclosed by a relief valve (not shown) or covered with hydrophobic material. Hydrophobic material permits air to pass through the material while resisting the passage of water or other liquids from doing so, thus permitting water resistant venting. The preferred embodiment uses a hydrophobic material such as Gore-Tex®, PTFE, HDPE, UHMW polymers from sources such as W.I. Gore &amp; Associates, Flagstaff, Az., Porex Technologies, Fairbum, Ga., or DeWAL Industries, Saunderstown, R.I.. It is appreciated that other hydrophobic materials may be used as well. 
     These materials are available in sheet form or molded (press and sintered) in a geometry of choice. Referring to FIGS. 10 a - 10   c,  preferred methods to attach this material to the housing  401  include molding the hydrophobic material into a sphere  1001  (FIG. 10 a ) or a cylinder  1002  (FIG. 10 b ) and pressing it into a cavity in the pre-molded plastic housing. Alternatively, a label  1003  (FIG. 10 c ) of this material could be made with either a transfer adhesive or heat bond material  1004  so that the label could be applied over the vent port  605 . Alternatively, the label could be sonically welded to the housing. In either method, air will be able to pass freely, but water will not. 
     In an alternative embodiment (not shown), the vent port could be placed in the connector  431  which secures the reservoir  406  to the housing  401  and which also serves to secure and connect the reservoir  406  to the infusion set tubing (not shown). As described in greater detail in copending application Ser. No. 09/428,818 filed contemporaneously herewith (Attorney docket No. 0059-0307), which application is incorporated by reference in its entirety, the connector and infusion set refers to the tubing and apparatus which connects the outlet of the reservoir to the user of a medication infusion pump. 
     An advantage of placing the vent port and hydrophobic material in this location, as opposed to the housing  401 , is that the infusion set is disposable and is replaced frequently with each new reservoir or vial of medication. Thus new hydrophobic material is frequently placed into service. This provides enhanced ventilation as compared with the placement of hydrophobic material in the housing  401 . Material in this location will not be replaced as often and thus is subject to dirt or oil build up which will retard ventilation. In yet another alternative embodiment however, vent ports with hydrophobic material could be placed in both the pump housing and in the connector portion of the infusion set. 
     Regardless of the location of the vent port, there remains the possibility that the vent port can become clogged by the accumulation of dirt, oil, etc. over the hydrophobic material. In another feature of certain embodiments of the present invention, the releasable coupler can act to prevent unintentional medication delivery in those instances when the internal pump housing pressure exceeds atmospheric pressure. Referring to FIG. 11, the coupler includes threads formed in a cavity within the external face of the reservoir piston  407 . The threaded cavity  424  engages the threads of the male portion  426  which in turn is attached to the end  430  of the plunger slide  405 . 
     This thread engagement reduces or prevents the effect of atmospheric pressure differentials acting on the water resistant, air-tight housing  401  (not shown in FIG. 1) from causing inadvertent fluid delivery. The threads of the male portion  426  act to inhibit or prevent separation of the reservoir piston  407  from the plunger slide  405  which, in turn, is secured to the drive screw  404  (not shown in FIG. 1) by engagement of the external threads of the drive screw  404  with the internal threads of the plunger slide  405 . As a result, the coupler resists movement of the reservoir piston  407  caused by atmospheric pressure differentials. 
     When the reservoir  406  is to be removed, it is twisted off of the coupler male portion  426 . The system electronics then preferably cause the drive motor  403  to rapidly rewind so that the plunger slide  405  is driven into a fully retracted position (FIGS.  4  and  6 ). A new reservoir  406 , however, may not be full of fluid. Thus the reservoir piston  407  may not be located in the furthest possible position from the reservoir outlet. Should the reservoir piston  407  be in such an intermediate position, then it may not be possible to engage the threads of the male portion  426  of the coupler (which is in a fully retracted position) with those in the female portion  424  of the coupler in the reservoir piston  407  upon initial placement of the reservoir. 
     In accordance with another feature of certain embodiments, the illustrated embodiment provides for advancement of the plunger slide  405  upon the insertion of a reservoir into the pump housing. The plunger slide  405  advances until it comes into contact with the reservoir piston  407  and the threads of the coupler male portion  426  of the coupler engage the threads in the female portion  424  in the reservoir piston  407 . When the threads engage in this fashion in the illustrated embodiment, they do so not by twisting. Rather, they rachet over one another. 
     In the preferred embodiment, the threads of the coupler male portion  426  have a 5 start, 40 threads per inch (“TPI”) pitch or profile while the threads of the coupler female portion  424  have a 2 start, 40 TPI pitch or profile as illustrated in FIG.  11 . Thus these differing thread profiles do not allow for normal tooth-to-tooth thread engagement. Rather, there is a cross threaded engagement. 
     The purpose of this intentional cross threading is to reduce the force necessary to engage the threads as the plunger slide  405  seats into the reservoir piston  407 . In addition, the 2 start, 40 TPI threads of the coupler female portion  424  are preferably made from a rubber material to provide a degree of compliance to the threads. On the other hand, the 5 start, 40 TPI threads of the male coupler portion  426  are preferably made of a relatively hard plastic. Other threading arrangements and profiles could be employed resulting in a similar effect. 
     If on the other hand, the threads had a common thread pitch with an equal number of starts given the same degree of thread interference (i.e., the OD of the male feature being larger than the OD of the female feature), then the force needed to insert the male feature would be pulsatile. Referring to FIG. 13 a,  as each thread tooth engages the next tooth, the insertion force would be high as compared to the point where the thread tooth passes into the valley of the next tooth. But with the cross threaded arrangement of the preferred embodiment, not all of the threads ride over one another at the same time. Rather, they ratchet over one another individually due to the cross-threaded profile. This arrangement results in less force required to engage the threads when the plunger slide moves axially, but still allows the reservoir to easily be removed by a manual twisting action. 
     While the advantage of utilizing a common thread pitch would be to provide a maximum ability to resist axial separation of the reservoir piston  407  from the plunger slide  405 , there are disadvantages. In engaging the threads, the peak force is high and could result in excessive delivery of fluids as the plunger slide  405  moves forward to seat in the cavity of the reservoir piston  407 . As described in greater detail in copending application Ser. No. 09/428411 filed contemporaneously herewith (Attorney Docket No. 0059-0308), which application is incorporated by reference in its entirety, the pump may have an occlusion detection system which uses axial force as an indicator of pressure within the reservoir. If so, then a false alarm may be generated during these high force conditions. 
     Therefore, the insertion force profile is preferably more flat than that shown in FIG. 13 a.  To accomplish this, the cross threading design of the preferred embodiment causes the relatively soft rubber teeth of the female portion  424  at the end of the reservoir piston  407  to rachet or swipe around the relatively hard plastic teeth of the coupler resulting in a significantly lower insertion force for the same degree of thread interference. (See FIG. 13 b ) This is due to the fact that not all of the thread teeth ride over one another simultaneously. Moreover, the cross-sectional shape of the threads are ramped. This makes it easier for the threads to ride over one another as the plunger slide is being inserted into the reservoir piston. However, the flat opposite edge of the thread profile makes it much more difficult for the plunger slide to be separated from the reservoir piston. 
     Referring to FIGS. 11 and 12, the 5 start, 40 TPI (0.125″ lead) thread profile of the coupler male portion  426  was chosen in consideration of the thread lead on the preferred embodiment of the connector  431 . The connector  431  is secured into the pump housing with threads  433  (FIG. 7 b ) having a 2 start, 8 TPI (0.250″ lead) profile. Therefore the 0.250″ lead on the connector is twice that of the reservoir piston  407  which is 0.125″. This was chosen to prevent inadvertent fluid delivery during removal of the reservoir from the pump housing, or alternatively, to prevent separation of the reservoir piston  407  from the reservoir  406  during removal from the pump housing. When the connector  431  is disengaged from the pump, the connector  431  as well as the reservoir  406  will both travel with the 0.250″ lead. Since the threaded coupler lead is 0.125″, the plunger slide  405  will disengage somewhere between the 0.125″ lead of the threaded coupler and the 0.250″ lead of the infusion set  1103 . Therefore, the rate that the reservoir piston  407  is removed from the pump is the same down to half that of the reservoir  406 /connector  431 . Thus any medication which may be present in the reservoir  406  will not be delivered to the user. Additionally, the length of the reservoir piston  407  is sufficient such that it will always remain attached to the reservoir  406  during removal from the pump. Although the preferred embodiment describes the plunger slide  405  having a coupler male portion  426  with an external thread lead that is different from the connector  431 , this is not necessary. The thread leads could be the same or of an increment other than what has been described. 
     The  2  start thread profile of the coupler female portion  424  on the reservoir piston  407  of the preferred embodiment provides another advantage. Some versions of these reservoirs may be designed to be filled by the user. In such an instance, a handle (not shown) will need to be screwed into the threaded portion of the reservoir piston  407  in order for the user to retract the reservoir piston  407  and fill the reservoir. The number of rotations necessary to fully insert the handle depends upon the distance the handle thread profile travels to fully engage the reservoir piston  407  as well as the thread lead. 
     For example, a single start, 40 TPI (0.025″ lead) thread requires 4 complete rotations to travel a 0.10″ thread engagement. However, a 2 start, 40 TPI (0.050″ lead) thread only requires 2 complete rotations to travel the 0.10″ thread engagement. Therefore, an additional advantage of a 2 start thread as compared to a single start thread (given the same pitch) is that half as many rotations are needed in order to fully seat the handle. 
     In alternative embodiments which are not shown, the end of the plunger slide  405  may include a detente or ridge to engage with a corresponding formation in the reservoir piston  407  to resist unintended separation of the plunger slide  405  from the reservoir piston  407 . In other embodiments, the plunger slide  405  is inserted and removed by overcoming a friction fit. Preferably, the friction fit is secure enough to resist movement of the reservoir piston  407  relative to the plunger slide  405  due to changes in air pressure, but low enough to permit easy removal of the reservoir  406  and its reservoir piston  407  from the plunger slide  405  once the fluid has been expended. In other embodiments, the detente or ridge may be spring loaded or activated to grasp the reservoir piston  407  once the drive mechanism has been moved forward (or extended), but is retracted by a switch or cam when the drive mechanism is in the rearmost (or retracted) position. The spring action could be similar to those used on collets. In other embodiments of the inventions, the threaded coupler may be engaged with the threaded cavity of the reservoir piston by twisting or rotating the reservoir as it is being manually placed into the housing. 
     As set forth above, the reservoir piston  407  is made of rubber. In the illustrated embodiment, an insert  1201  (FIG. 12) which is made of hard plastic may provided in the upper portion of the reservoir piston  407 . The insert  1201  provides stiffness to the rubber reservoir piston  407 . This reduces undesirable compliance which is associated with the reservoir. Without the insert  1201 , the flexibility in the reservoir piston  407  due to its rubber composition could cause it to deform under varying reservoir fluid back pressures. This deformation could in turn vary the internal volume of the reservoir  406 . Such variances may adversely affect the controlled delivery of the fluid from the reservoir  406  via the infusion set to the user. 
     It can be appreciated that the design of FIGS. 4-12 results in an arrangement where; the plunger slide  405  is reliably but releasably coupled to the drive screw  404 . When it is time to replace the reservoir  406 , it can be detached from the male end of the coupler without affecting the plunger/drive screw engagement. Moreover in the preferred embodiment, the plunger slide  405  is shaped as a hollow cylinder with internal threads. Thus it completely encircles and engages drive screw  404 . When the plunger slide  405  is in a relatively retracted position, it encloses any gears which couple the motor  403  with the drive screw  404  thus achieving an extremely compact design. Alternative embodiments include an arrangement where the plunger slide  405  encloses the motor  403  itself. A vent port covered with hydrophobic material as well as a threaded coupler provide redundant means for permitting exposure of the pump to changing atmospheric pressures without the unintended delivery of medication. 
     While the description above refers to particular embodiments of the present inventions, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present inventions. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the inventions being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.